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Pesticides and Pesticide Degradates in Groundwater Used for Public Supply across the United States: Occurrence and Human-Health Context

Cite this: Environ. Sci. Technol. 2020, XXXX, XXX, XXX-XXX
Publication Date (Web):December 14, 2020
https://doi.org/10.1021/acs.est.0c05793
This article not subject to U.S. Copyright. Published 2020 by American Chemical Society
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Abstract

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This is the first assessment of groundwater from public-supply wells across the United States to analyze for >100 pesticide degradates and to provide human-health context for degradates without benchmarks. Samples from 1204 wells in aquifers representing 70% of the volume pumped for drinking supply were analyzed for 109 pesticides (active ingredients) and 116 degradates. Among the 41% of wells where pesticide compounds were detected, nearly two-thirds contained compound mixtures and three-quarters contained degradates. Atrazine, hexazinone, prometon, tebuthiuron, four atrazine degradates, and one metolachlor degradate were each detected in >5% of wells. Detection frequencies were largest for aquifers with more shallow, unconfined wells producing modern-age groundwater. To screen for potential human-health concerns, benchmark quotients (BQs) were calculated by dividing concentrations by the human-health benchmark, when available. For degradates without benchmarks, estimated values (estimated benchmark quotients (BQE)) were first calculated by assuming equimolar toxicity to the most toxic parent; final analysis excluded degradates with likely overestimated toxicity. Six pesticide compounds and 1.6% of wells had concentrations approaching levels of potential concern (individual or summed BQ or BQE values >0.1), and none exceeded these levels (values >1). Therefore, although pesticide compounds occurred frequently, concentrations were low, even accounting for mixtures and degradates without benchmarks.

Introduction

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Multiple pesticides, some of which are known to pose a risk to human health above certain levels of exposure,(1−7) have been detected in groundwater, including in drinking-water aquifers,(8−13) but relatively little is known about the occurrence and human-health relevance of pesticide degradates, defined to include compounds resulting from any biotic or abiotic transformation of pesticides,(14) whether toxic or nontoxic. Based on 2005–2012 estimates, the annual usage of conventional pesticides in the United States, excluding biological and antimicrobial pesticides, is about 400–450 million kg. Agricultural usage exceeds other applications, accounting for about 90% of herbicides, 85% of fungicides, and 60% of insecticides applied.(15) Herbicides are the most commonly used type of pesticide, totaling 62% of pesticide usage for agriculture. The five most used agricultural herbicides in 1992–2017 in the United States were glyphosate, atrazine, metolachlor-(S), 2,4-D, and acetochlor.(16)
The dominant factors controlling the distribution of pesticides and pesticide degradates in the Nation’s groundwater resources are well established, as described in a publication synthesizing findings from multiple studies.(8) In areas where pesticides are applied, recharge from rainfall or irrigation drives transport to groundwater, with susceptibility being higher where soils are more permeable. Detected pesticides generally have high persistence and low tendency to sorb to soils and sediments. Shallow, more recently recharged groundwater tends to have more pesticide contamination than deeper, older groundwater. Also, subsurface drainage systems in agricultural areas can divert shallow groundwater to surface water, thus leading to lesser contamination in deeper groundwater. Pesticide degradates are often detected more frequently and at higher concentrations than their parents due to high transformation rates and because long travel times, common from source to the depth zone used for public supply, support environmental transformation. Compound mixtures are common.
The U.S. Environmental Protection Agency (EPA) has assessed the potential human-health risk associated with the occurrence of individual pesticides (i.e., active ingredients) in drinking water, establishing enforceable primary drinking-water standards—maximum contaminant levels, or MCLs—for 18 pesticides(1) and nonenforceable screening levels—human-health benchmarks for pesticides, or HHBPs—for 394 pesticides that do not have MCLs.(2) The U.S. Geological Survey (USGS) has developed nonenforceable screening levels—health-based screening levels, or HBSLs—for 30 pesticides and 7 degradates that do not have MCLs or HHBPs.(3) As reflected in the range of benchmark concentration values (0.002–30 000 μg/L), pesticide compounds vary widely with respect to types of human-health effects and potential degree of toxicity.
As indicated by EPA resources,(4−7) human-health toxicity information is much more commonly available for the more than 400 pesticides recently used in the United States than for their degradates. Data on the human-health effects of pesticide mixtures also are relatively uncommon, even though the importance of understanding the potential effects of mixtures, particularly for environmentally relevant concentrations, is well recognized.(17−27) EPA cumulative risk assessments(28) provide evaluations of cumulative exposure and risk for certain compounds sharing a common mechanism of toxicity.
Relatively few studies have characterized the occurrence of pesticide degradates in groundwater at regional or national rather than local scales, including in other countries.(29−31) Most regional or national investigations in the United States were conducted between 1991 and 2001 using gas chromatography/mass spectrometry (GC/MS) and (or) high-performance liquid chromatography (HPLC), providing detection limits (DLs) of typically 1–100 ng/L.(8−13,32−43) These have included studies of shallow groundwater and of depths used for public supply.
Studies of shallow groundwater,(32−34) sometimes in combination with deeper groundwater from domestic and public-supply wells,(8,35−37) have generally detected degradates at rates and concentrations equal to or greater than those for parent compounds. In five occurrence studies,(8,32−35) evaluating up to > 80 pesticide compounds and 9 degradates, atrazine was the most commonly detected pesticide and its degradate deethylatrazine (DEA) was detected with similar frequency. In one of those studies,(35) conducted at a national scale, atrazine and DEA were each detected in about 30% of 2,460 wells, with the maximum concentration for atrazine being 4,200 ng/L. Five of the six most frequently detected compounds for shallow wells studied in midwestern states were herbicide degradates.(32,33)
Regional and national studies of groundwater used for drinking supply across the United States have shown similar results. The national studies evaluated 75 pesticides and 8 degradates in up to 904 public-supply wells(9,26) and about 2,000 domestic and (or) other supply wells.(10,11) Atrazine and DEA were the most frequently detected pesticide compounds, commonly in the same samples and at concentrations of a few ng/L to a few hundred ng/L; atrazine was also the most frequently detected pesticide in a 2020 study across the western United States.(44) Results for smaller subsets of public-supply wells sampled for up to 150 pesticide compounds, including as many as 40 degradates, indicate that degradates of other herbicides are common in at least some regions.(12,13,38−43) However, most of these studies included only selected degradates of a small number of pesticides, and none attempted to quantify potential human-health risk for detected degradates that lack benchmarks.
This paper provides a comprehensive assessment of the occurrence of pesticides and their degradates in raw (untreated) groundwater used for public supply across the United States. Samples were collected from 1,204 sites in 23 principal aquifers (PAs)(45) (Table S1) providing groundwater used for drinking by an estimated 73 million people. The samples were analyzed for 109 pesticides and 116 degradates (of >50 pesticides) using direct aqueous-injection liquid chromatography-tandem mass spectrometry (LC-MS/MS), providing low DLs of generally 1–50 ng/L. This is the first national study to include degradates without human-health benchmarks in a process to screen concentrations of individual compounds and mixtures of compounds for potential human-health concerns. The objectives of this study were to (1) evaluate the occurrence of pesticides and their degradates in groundwater used for drinking supply at the scale of principal aquifers and the Nation and (2) assess the potential relevance of detections of all compounds and compound mixtures in a human-health context.

Methods

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Study Design

The 1199 wells and 5 springs (collectively referred to as “wells”) sampled for this study by the USGS National Water-Quality Assessment (NAWQA) Project during 2013–2018 (Table S2) are all sources of public drinking supply. Based on location and depth information, wells were assigned to corresponding principal aquifers (PAs), which are regionally extensive aquifers or aquifer systems that have the potential to be used for drinking water.(45) The wells are in 23 PAs covering more than 5 million km2 in 46 states (Figure 1), together supplying about 70% of the groundwater pumped from PAs for public supply in 2015.(46) For sampling, PAs were grouped into 19 principal aquifer surveys (PASs; Table S1), which are nationally consistent studies designed to assess the quality of the groundwater resource used for public supply across an entire PA.(47) PASs utilize a spatially distributed, randomized approach for well selection that uses equal-area grids, with one well randomly selected for sampling within each grid cell; therefore, a given percentage of wells equates to the same percentage of PAS area (for example, the presence of a compound in 1% of wells equates to its presence in about 1% of aquifer area).(48) The few cases where deviations from the standard study approach resulted in sampling of additional wells and use of cell declustering for PAS analysis are detailed in Section S1 of the Supporting Information (SI). In each PAS, the dataset includes one sample from between 19 and 111 wells (median = 59), with well depths ranging from less than 10–1300 m (median of about 125 m).

Figure 1

Figure 1. Number of pesticide compounds detected at each of 1204 wells sampled in 19 NAWQA principal aquifer surveys (PASs), 2013–2018, indicating where the sum of benchmark quotients (BQEsum) from tier 2 screening of concentrations of pesticide compounds, including degradates without benchmarks, exceeds 0.1 (well symbols outlined in red).

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Sample Collection

Raw samples were collected at or near the wellhead, prior to any treatment or blending, in accordance with USGS procedures.(49,50) Samples were collected using a Teflon sampling line and filtered through a 0.7 μm baked glass-fiber filter. Samples for pesticides were collected in 20-mL amber glass bottles and were shipped on ice overnight to the USGS National Water-Quality Laboratory (NWQL) in Denver, Colorado, for analysis. Groundwater samples also were analyzed for about 300 additional constituents and isotopes.(51−55)

Laboratory Analysis

The NWQL’s analytical method for pesticides and pesticide degradates, detailed in Section S2.2, uses direct aqueous-injection LC-MS/MS to determine 225 compounds in water, representing a broad range of uses (including herbicides, fungicides, and insecticides) and chemical classes (including acetanilides, triazines, and acids).(56) The compounds include 109 pesticides and 116 pesticide degradates (Table S3), hereafter referred to as “pesticides.” The method and instrumentation used for this study are suitable for a wide range of compounds and provide improved sensitivity and selectivity over previous GC/MS and HPLC methods,(56) with a reduction in DLs of an order of magnitude being common for several classes of compounds. The LC-MS/MS method includes the majority of pesticide compounds prioritized by the USGS on the basis of factors including potential persistence and toxicity,(57) although this method cannot analyze for some commonly used pesticides, including glyphosate (considered to be of low toxicity and unlikely to move to groundwater(58)) and fumigants.
The DL (Table S3) is the lowest concentration that can be measured and reported with 99% confidence that the analyte concentration is greater than zero (false-positive risk ≤1%). Despite an increase in false-positive risk below the DL, concentrations that meet identification and relevant quality-control criteria are reported down to a value typically set at about 10% of the DL. To minimize false-negative risk, the reporting limit (RL) typically is set at 2 times the DL.(56) When an analyte is not detected or does not meet qualitative criteria and is below the RL, it typically is reported as “<” the RL. However, some results are reported as less than a raised reporting level (RRL) because of issues resulting in uncertainty in the identification or quantification of the pesticide.(59)

Quality Assurance and Quality Control

Quality-control samples associated with this study have been previously published and evaluated.(59−61) Conclusions regarding potential contamination and measurement bias are briefly summarized here and in Section S3. Field blanks and laboratory set blanks (LBs) were used to examine potential sources of contamination; detections in LBs prompted some censoring (see the Data Preparation Steps section). Laboratory reagent-water spikes indicated acceptable bias for all pesticides (median recovery 70–108%), although median recoveries below 90% indicate a small to moderate bias low for 30 compounds. For field matrix spikes, 19 pesticides had median recoveries below 70%, indicating that degradation and (or) matrix effects could contribute to a substantial bias low, possibly resulting in underreporting of occurrence and (or) concentrations; didealkylatrazine (CAAT) and 2-hydroxy-6-ethylamino-4-amino-s-triazine (OEAT) had median recoveries of 150 and 167%, respectively, indicating a potential bias high and possible overreporting of concentrations in groundwater.

Data Preparation Steps

Three steps, discussed further in Section S4 and a USGS publication,(61) were conducted to prepare the dataset for analysis. First, data affected by field or laboratory error, such as exceedance of the holding time, were removed from the dataset. Second, nondetections reported with an RRL greater than a certain magnitude were excluded from the dataset because of an increase in the potential for false-negative results; the resulting increases in calculated detection frequencies for compounds with many RRL exclusions are addressed in the Results and Discussion section. Third, to reduce the risk of false positives resulting from laboratory contamination, study reporting levels were calculated for pesticides that were detected in LBs, as described in the SI; only quantified results greater than or equal to this level were retained as detections. The final pesticide results used for this study are available in a USGS data release.(60)

Screening Approach for Potential Relevance to Human Health

Individual compounds and mixtures of compounds detected at single wells were screened for potential relevance to human-health by comparison with benchmarks. A tiered approach was developed to incorporate degradate compounds without benchmarks into the screening process. For the 107 pesticides and 12 degradates with existing benchmarks (Table S3), the screening process is the same as that used in previous NAWQA groundwater studies.(9,26) That is, a benchmark quotient (BQ) is calculated as the ratio of the concentration of the compound in groundwater to the benchmark—MCL, HHBP, or HBSL—for that compound, using the lowest available value when a range exists within a benchmark type (Section S5). All benchmark types incorporate an uncertainty factor of typically 2 orders of magnitude or more, either built into the underlying toxicity values (for chronic noncancer effects) or represented as risk ranges (for cancer effects).
A BQ > 1 indicates the concentration exceeds the benchmark (i.e., the concentration is of potential human-health concern) and a BQ > 0.1 indicates the concentration exceeds 10% of the benchmark (i.e., the concentration is approaching a level of potential human-health concern). For wells with multiple detected compounds, the sum of all individual BQs (BQsum = ∑BQ) is calculated and is also evaluated relative to the thresholds of 1 and 0.1 to provide a measure of potential human-health concern resulting from the mixture. This process conservatively assumes that simple dose additivity generally applies, although multiple compounds do not necessarily have the same mode of action.(17,18,20−27) Because response addition may be more appropriate than dose addition to assess combined cancer risk,(17,27) when BQsum > 0.1, the current study identifies any contributing compounds with benchmarks based on potential carcinogenicity rather than chronic noncancer effects.
For the 104 degradates without benchmarks, the screening process has two tiers. Tier 1 makes the generally conservative (protective) assumption that degradates have equimolar toxicity to the most toxic potential parent compound (i.e., the parent with the lowest benchmark), with the exceptions of desethylhydroxyatrazine (OIAT) and OEAT, which were assumed to have equimolar toxicity to 2-hydroxyatrazine (OIET) rather than to atrazine based on their toxicological endpoints.(62) For each degradate, an estimated benchmark quotient (BQE) is determined as followswhere Cdeg is the degradate concentration, MW is the molecular weight of the parent or degradate, and Bparent is the benchmark value of the parent. At single wells, BQE values are summed along with BQs to obtain a BQEsum.
Tier 2 screening is carried out only for degradates that have BQE values >0.1 or that contribute substantially to BQEsum values >0.1 in tier 1 screening. In this refined second screening tier, toxicity information available from EPA documents (such as reregistration eligibility decisions and human-health risk assessments)(6) is used to place each degradate in one of three categories regarding the assumption of equimolar toxicity to the parent: (1) the assumption is likely reasonable, (2) the assumption likely results in an overestimate of toxicity, or (3) validity of the assumption is unknown. The degradate is retained in the refined assessment if it is reasonable to assume that the degradate has comparable toxicity to the parent (1, above), or if information on the toxicity of the degradate is generally unavailable (3, above). The pesticide degradate is removed from the refined assessment if the toxicity information indicates that the equimolar assumption overestimates toxicity (i.e., the toxicity of the degradate is less than the toxicity of the parent; 2, above).
This screening approach has known limitations. It assesses contributions to toxicity from only the 225 pesticide compounds included in laboratory analysis and does not consider other pesticides or contaminant classes. Also, concentrations of up to 19 pesticides may have been underestimated due to low recovery bias (see the Quality Assurance and Quality Control section). For degradates with little or no toxicity information, equimolar toxicity to the parent has been assumed and could potentially overestimate or underestimate potential human-health effects. Although studies of toxicity in aquatic life and (or) mammals have indicated that most degradates are about as toxic or less toxic than their parents, some degradates (perhaps about 30%) can be more toxic.(8,39,63,64) Finally, this approach does not address the potential for synergistic or antagonistic effects between parents and degradates or between compounds from different chemical classes (for example, triazine, carbamate, organophosphate) in a mixture; toxicity is most likely to be additive (as assumed in this study) for pesticides within the same class that have similar structures and a common mode of action, whereas pesticides from different classes can have varied effects.(8)

Ancillary Datasets

In the Results and Discussion section, general patterns of pesticide occurrence among PASs are discussed in relation to previously identified explanatory factors for pesticide occurrence in drinking-water aquifers (statistical analysis of relations of occurrence to these factors across PASs is discussed in Section S7.4). The explanatory factors include well depth, aquifer type (confined or unconfined), lithology, groundwater age classification, and land use (Table S2). Well depth and aquifer type were retrieved from the USGS National Water Information Systems database (https://doi.org/10.5066/F7P55KJN). General lithology was assigned based on the primary lithology of the PAS to which the well was assigned.(45) A tritium age classification of modern (recharged in 1953 or later), premodern (recharged prior to 1953), or mixed was assigned for each sample in accordance with a published classification system.(65) The 30-m National Land Cover Database for 2016(66) was used to assign percentages of major land-use types (agricultural, urban, or natural) across the entire area of each PAS. This study does not specifically examine processes affecting transformation of parent compounds to degradates.

Results and Discussion

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Occurrence of Pesticide Compounds and Mixtures

At least one pesticide compound was detected at 491 of the 1204 wells (41%) (Figure 1), with reported concentrations ranging from about 0.02 to 17 500 ng/L.(60) The area-weighted detection frequency (i.e., areal proportion with detections(67)) across all PASs was 46%, as calculated by applying cell declustering to account for deviations from the standard sampling design that occasionally resulted in two or three wells within an individual grid cell and applying area weighting to account for the differing contributions of individual PAS sampling grids to the total aquifer area sampled (Section S7.1). These results are consistent with the 2010 USGS NAWQA study of 904 public-supply wells, which detected at least one pesticide at 41% of wells;(9) however, results from that study are not fully comparable because it included only eight degradates, had higher DLs for several compounds, and studied wells that were clustered in targeted areas.
Degradates were commonly observed in the current study, as were mixtures of pesticide compounds. Nearly three-quarters of wells with at least one detection had detections of degradates, and 23% had detections of degradates without detections of any pesticide active ingredients. The five pesticide compounds with the largest maximum concentrations (1450–17 500 ng/L; Table S3) are degradates—specifically of 4-hydroxychlorothalonil, alachlor, atrazine, and metolachlor. Degradates that were not included in the analysis of samples for the 2010 study(9) were found at 27% of all sampled wells and were the only pesticide compound(s) detected at 7.3% of sampled wells. Two or more compounds were detected at 63% of wells with at least one detection, and 6 or more compounds (maximum 27 compounds) were detected at 24% of these wells.
The areal proportion with detections differed substantially by PAS (range 12–100%) (Figure 2 and Table S4 [PASs grouped by lithology]); the average number of compounds detected, weighted for multiple wells per cell as needed, showed a similar pattern (range <0.2–9). Given the typical characteristics of the sampled PAS areas, their relative detection frequencies are generally consistent with previously established explanatory factors for pesticide distribution, including well depth, aquifer type, groundwater age, land use, and (to a lesser degree) lithology (see Section S7.4 for results of statistical tests).(8,9)

Figure 2

Figure 2. Areal proportion and average number of detections for each NAWQA principal aquifer survey, grouped by lithology. The number at the top of each blue bar indicates the rank of the areal proportion among all 19 principal aquifer surveys.

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Among the six carbonate PASs are the two PASs with the highest areal proportions (≥72.9%)—the Biscayne and Valley and Ridge and Piedmont and Blue Ridge (VRPBR)—but also two of the five PASs with the lowest areal proportions (≤20.3%)—the Basin and Range carbonates and Ozark Plateaus. The Biscayne and VRPBR PASs have generally shallow wells (medians <80 m) that are predominantly unconfined or mixed and produce modern water, and these PASs have <50% undeveloped land (Table S1); also, the presence of karst features has shown a relation with higher pesticide detection frequency in the Valley and Ridge aquifers.(68) In contrast, the Basin and Range carbonates and Ozark Plateaus have generally deep wells (medians >148 m), many of which are confined (leading to a reasonable expectation of less connection to activities at the land surface) and produce water of mixed or premodern age; also, these PASs have ≥58% undeveloped land. Among carbonate PASs, the areal proportions for the Floridan and Edwards-Trinity PASs are intermediate and are most similar to previous results for carbonate aquifers.(9) The Edwards-Trinity aquifer system includes the predominantly carbonate Edwards aquifer (areal proportion of 32%) and the predominantly sandstone Edwards-Trinity (25%) and Trinity (21%) aquifers.
After the Biscayne and VRPBR PASs, two western unconsolidated/semiconsolidated PASs (the Stream Valley and High Plains) and the Glacial PAS, all located in the intensely agricultural midwestern United States, have the next highest areal proportions (58.4–64.3%), consistent with relatively high detection frequencies previously observed by NAWQA for similar lithologies.(9) All three of these PASs have generally shallow wells (medians <77 m) that are mostly or predominantly unconfined, with two-thirds or more of them producing water of modern or mixed age. By contrast, the Rio Grande, which has the third lowest areal proportion, has 93% undeveloped land, along with the deepest wells (median 181 m) among the western PASs, produces mostly premodern water. The Basin and Range basin-fill aquifers have an intermediate areal proportion among western PASs (37.2%), with somewhat higher percentages of wells producing mixed-age water than the Rio Grande, despite similar well depths, degrees of confinement, and land use.
Of the two metamorphic/igneous PASs, the Piedmont and Blue Ridge crystalline aquifers have a higher areal proportion (53.6%) than the Columbia Plateau basaltic aquifer (35.6%). Wells in the Piedmont and Blue Ridge are generally shallower (median 108 m compared with 142 m) and much more likely to produce modern water; also, some layers of the Columbia Plateau basalt can act as confining layers.(69) Of the two interior sandstone PASs, the areal proportion for the Colorado Plateau (12.1%) is the lowest of any PAS and more consistent with previous NAWQA results(9) than that of the Cambrian–Ordovician (39.5%). Because wells in the Cambrian–Ordovician typically are deeper (median 267 m compared with 147 m), with more common confinement and production of premodern water, this difference in the rate of detection likely is driven by much greater pesticide loading on the predominantly agricultural land (73%) overlying the Cambrian–Ordovician relative to the Colorado Plateaus (2% agricultural land). The four coastal silicate PASs all have relatively low areal proportions (18.2–30.5%) and average number of compounds detected (<0.9), consistent with wells in these PASs being quite deep (median 155 m) and confined, producing predominantly premodern water.
A total of 128 pesticide compounds—60 pesticides and 68 degradates—were detected in groundwater from at least one well. Ten pesticides (all herbicides) and 11 degradates (all of herbicides) were each detected in groundwater from at least 2% of all wells (Figure 3). The four most commonly detected herbicides were atrazine (11%), hexazinone (7.8%), prometon (6.4%), and tebuthiuron (6.0%), with reported concentrations ranging from tenths to tens or hundreds of ng/L. Previous NAWQA studies of public-supply wells or various well types list atrazine, simazine, and metolachlor among the four most commonly detected herbicides,(8,9,12,13) along with the noncrop herbicides prometon or tebuthiuron (hexazinone was not commonly included in these studies). In contrast, the current study detected bromacil and bentazon—both of which were discontinued for some uses in the 1980s or 1990s—more commonly than metolachlor and simazine, which were detected at lower rates than in the previous studies. The detection of propazine at >2% of wells despite relatively low use(16) might be partially related to its presence as an impurity in some atrazine formulations.(70) Dieldrin, an insecticide that was the eighth most commonly detected compound in the 2010 study (3.1%),(9) was not included in the current study, which showed propoxur to be the most commonly detected insecticide (1.8%) and metalaxyl to be the mostly commonly detected fungicide (0.7%).

Figure 3

Figure 3. Detection frequency of pesticide compounds detected in groundwater from at least 2% of sampled wells. An asterisk indicates a compound that generally was not included in previous NAWQA groundwater studies. Colored text identifies degradates and, when present, their corresponding parent (in bold text).

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The atrazine degradates CAAT (14.4%) and DEA (14.3%) and the metolachlor degradate metolachlor sulfonic acid (SA) (11.4%) were detected more often than atrazine and far more often than metolachlor (Figure 3). [If nondetections with high RRLs had not been excluded, the detection frequency for CAAT, a poorly performing compound under the analytical method, would be 7.6%—a lower bound (with an increased risk of false-negative error) that would make it the fifth most frequently detected compound; no other compound in Figure 3 would decrease in detection frequency by more than 0.9% or change rank by more than one place.] Also among the 10 most frequently detected compounds were the atrazine degradates OIET (7.1%), OIAT (6.2%), and deisopropylatrazine (DIA, 4.6%). These degradates plus dechlorometolachlor were detected more frequently than metolachlor and simazine, with tebuthiuron transformation product 108 additionally being more common than simazine. Also detected at more than 2% of wells were the degradates demethyl hexazinone B, hydroxymetolachlor, and alachlor SA, even though alachlor use has declined steeply since the introduction of acetochlor in 1994(16) and alachlor was detected in groundwater from only three wells in the current study. Of the 11 degradates in Figure 3, only DEA was commonly included in previous NAWQA studies.
An atrazine degradate (CAAT, DEA, or OIET) was the most commonly detected pesticide compound among all wells in each of 9 PASs (Table S4); metolachlor or one of its degradates (hydroxymetolachlor or metolachlor SA) was most common in four other PASs. The prevalence of these compounds is consistent with atrazine and metolachlor being the second and third most commonly used agricultural pesticides, respectively, in 2012,(15) in addition to having noncrop uses and (along with several of their degradates) being at least moderately mobile and persistent in the environment.(62,71) Hexazinone, also mobile and persistent,(72) was the most commonly detected compound in the Floridan PAS (nine wells) and the Coastal Lowlands PAS (three wells), where it has agricultural in addition to noncrop uses.(16) The noncrop herbicide prometon was the most commonly detected compound in the Edwards-Trinity (eight wells), despite restrictions on its use in Texas.(73) The carbamate insecticide propoxur, used in a variety of applications and considered highly mobile and moderately persistent,(74) was the most commonly detected compound in the Colorado Plateaus and Ozarks Plateaus but was detected at only three wells in each.
Of the 20 most common mixtures of two or three pesticide compounds detected among the 1204 sampling wells (Section S7.3), 17 consisted only of atrazine, atrazine degradates, and (or) metolachlor SA, with hexazinone or prometon included in three mixtures (Figure S3). Illustrating the prevalence of some degradates and their importance to groundwater assessments, atrazine was detected without any of its degradates at only 12 wells, whereas one or more of its degradates were detected without atrazine at 127 wells; both atrazine and its degradate(s) were detected at 115 wells. Metolachlor also was detected without any of its degradates at only 12 wells, whereas one or more of its degradates were detected without metolachlor at 121 wells; both metolachlor and its degradate(s) were detected at 30 wells.

Screening of Concentrations for Potential Human-Health Concerns

Individual Compounds

Of detected pesticide compounds with benchmarks, none had a BQ > 1 (Table S3). Four compounds (atrazine, alachlor, diuron, and CAAT) had a BQ > 0.1 at a total of five wells (0.4%) (Table 1) distributed among four PASs in midwestern and eastern parts of the United States (Table S4); adjusting for potential bias high for reported CAAT concentrations did not affect these results.
Table 1. Individual Pesticide Compounds (cmpds) and Number (no.) of Wells With a Benchmark Quotient (BQ), Initial Estimated Benchmark Quotient (BQE) from Tier 1 Screening, or Final BQE from Tier 2 Screening >1 or 0.1
 TIER 1: cmpds (no. wells) withFINAL/TIER 2: cmpds (no. wells) with
 BQ or BQE > 1BQ or BQE > 0.1BQ or BQE > 1BQ or BQE > 0.1
compounds with a benchmark alachlor (1) alachlor (1)
 atrazine (1) atrazine (1)
 diuron (1) diuron (1)
  didealkylatrazine/CAAT (2) didealkylatrazine/CAAT (2)
degradates without a benchmarkalachlor SA (1 well)b4-hydroxychlorothalonil (1)a 4-hydroxychlorothalonil (1)a
 deethylatrazine/DEA (3)a deethylatrazine/DEA (3)a
 fipronil amide (1)b  
 alachlor OA (2)b  
 alachlor SA (18)b  
 hydroxyalachlor (1)b  
  pyrimidinol (1)b  
totals1 cmpd (1 well)11 cmpds (27 wells)c0 cmpds (0 wells)6 cmpds (8 wells)c
a

Compound likely has similar toxicity to the parent and so was retained in tier 2 analysis and included in final totals.

b

Compound likely has substantially lower toxicity than the parent and so was dropped in tier 2 analysis and not included in final totals.

c

Some wells had multiple compounds with BQ or BQE > 0.1.

Under tier 1 screening, one degradate without a benchmark—alachlor SA—had a BQE > 1 (Table 1), occurring in modern groundwater from a 35 m well in the Stream Valley PAS. Seven degradates without benchmarks had a BQE > 0.1: alachlor SA (18 wells), alachlor oxanilic acid (OA; 2 wells), and hydroxyalachlor (1 well), mostly in the Glacial and Cambrian–Ordovician PASs; DEA (3 wells), all in the High Plains PAS; and fipronil amide, 4-hydroxychlorothalonil, and pyrimidinol (1 well each). The totals from tier 1 screening indicated that 11 individual compounds had a BQ or BQE > 0.1 at 27 wells.
The BQE exceedances from tier 1 triggered a tier 2 screening evaluation. EPA has indicated concerns about exposure to alachlor SA and OA,(75) which are mobile and persistent, as supported by the results of the current study. However, all three alachlor degradates were dropped from tier 2 analysis because available data indicate that they are much less toxic than alachlor (Table 1).(76) Fipronil amide was dropped because it is not indicated by EPA to be a fipronil degradate of toxicological concern(77) and is not included in drinking-water exposure assessments.(78) Pyrimidinol (a diazinon degradate) was dropped because of evidence that it is essentially nontoxic.(79) In contrast, DEA was retained because atrazine and its chlorinated degradates are considered to be equivalent in toxicity, with a shared neuroendocrine effect.(62) The degradate 4-hydroxychlorothalonil also was retained and has actually been shown to be more toxic to mammals than its parent chlorothalonil.(80) As a result, BQE values for 4-hydroxychlorothalonil could be underestimated, and it may be useful to note that six wells (all in the Biscayne PAS) had BQE > 0.01 for this degradate. The final totals from tier 2 screening indicate that six individual compounds have a BQ or BQE > 0.1 at eight wells.

Individual Wells With Mixtures

The BQsum values for pesticide compounds with benchmarks did not exceed 1 for any wells. Only the same five wells with individual BQs > 0.1 had BQsums > 0.1.
Under tier 1 screening, three wells (in the Cambrian–Ordovician, Glacial, and Stream Valley PASs) had BQEsums >1 (Table S2), primarily due to contributions from alachlor SA and (or) alachlor OA. Forty wells had BQEsums >0.1, 33 of which had contributions primarily from alachlor, alachlor SA, alachlor OA, atrazine, CAAT, and (or) DEA. Most of these 33 wells are in the Cambrian–Ordovician, Glacial, High Plains, or Stream Valley PAS. The remaining seven wells with BQEsums >0.1 are in five PASs and have contributions primarily from 4-hydroxychlorothalonil, diuron, fipronil amide, or pyrimidinol.
After excluding degradates with likely overestimated toxicity (as described in the previous section), final tier 2 screening totals indicate that no wells have BQEsum >1 and 19 wells have BQEsums >0.1 (red-outlined well symbols in Figure 1). Atrazine and (or) its chlorinated degradates account for the BQEsum exceeding 0.1 at 16 of these wells, in the Cambrian–Ordovician, Glacial, High Plains, Stream Valley, and VRPBR (two to six wells each) PASs (Figure S2). At the other three wells, the BQEsum exceeds 0.1 based on contributions from 4-hydroxychlorothalonil (Biscayne PAS), alachlor (Southeastern Coastal Plain PAS), or diuron (Piedmont and Blue Ridge crystalline PAS) alone. Alachlor and diuron are considered likely human carcinogens, at least at high doses, whereas the other compounds contributing substantially to BQEsums >0.1 are evaluated based on chronic noncancer effects.
Atrazine and its chlorinated degradates were the largest contributors to BQEsums >0.1, but the maximum sum of their concentrations at any well was 4.3 μg/L, which is more than 2 orders of magnitude below even the most protective 4-day drinking-water level of comparison (DWLOC) of 700 μg/L for infants established by the EPA in a cumulative risk assessment for these compounds that includes dietary (food and drinking water) and residential exposures.(62) The sum of the concentrations of the total chlorinated triazines, adding propazine and simazine to the above calculation, also does not exceed 4.3 μg/L and is more than 2 orders of magnitude below the most protective 4-day DWLOC for these compounds of 610 μg/L for dietary exposure for infants and of 580 μg/L for dietary and residential exposures for children 1–2 years old.(81)
Some aquifers are more vulnerable to anthropogenic contamination than others, and although the areal proportion with pesticide detections at any concentration is one indicator of this, BQEsum is a potential alternative indicator that emphasizes concentrations that might be considered substantial for some purposes. BQEsum >0.01 was selected here to indicate greater vulnerability. More than half of wells in the Biscayne PAS had BQEsum >0.01 (largely as a result of contributions from 4-hydroxychlorothalonil or diuron), whereas about 21–32% of wells in the High Plains, Piedmont and Blue Ridge crystalline, Stream Valley, and VRPBR carbonate PASs had BQEsum >0.01 (Table S4). These five PASs also were among the six with the highest areal proportion and average number of detections per well (Figure 2).

Implications

This is the first systematic assessment of raw groundwater used for public drinking supply across the United States to include and provide human-health context for a large number of pesticide degradates. Findings indicate that although the occurrence of pesticide compounds (including compound mixtures and degradates not typically included in previous studies) is common, concentrations for only a small number of compounds and wells approach levels of potential human-health concern, and none exceed these levels. It should be noted, however, that the laboratory method could not quantify every type of pesticide. Compared with considering only compounds with benchmarks, adding degradates without benchmarks to the screening process for potential human-health concerns resulted in small increases in the final number of individual compounds with BQ or BQE > 0.1, from 4 (at five wells) to 6 (at eight wells). Similarly, the final number of wells with compound mixtures having BQsum or BQEsum >0.1 rose slightly, from 5 (0.4%) to 19 (1.6%). Atrazine and its chlorinated degradates were the primary contributors to BQEsums >0.1, occurring mainly in the midwestern United States. In this same area, results of initial screening indicated that alachlor SA also might approach levels of concern at several wells, but subsequent screening showed that the initial assumption of equimolar toxicity to alachlor likely overestimated potential health effects of this degradate. Other commonly detected compounds that did not contribute substantially to BQEsums because their concentrations were well below their respective benchmarks included the herbicides metolachlor, hexazinone, prometon, tebuthiuron, and (or) their degradates. This screening process can be refined as future studies address the current lack of detailed toxicological information for many degradates and compound mixtures. However, based on currently available knowledge, this tiered screening process provides reasonable confidence that degradates without benchmarks are unlikely to substantially increase the potential for human-health concerns associated with the occurrence of pesticide compounds in groundwater used for public supply.

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  • Corresponding Author
  • Authors
    • Kenneth Belitz - U.S. Geological Survey, 10 Bearfoot Road, Northborough, Massachusetts 01532, United States
    • Bruce D. Lindsey - U.S. Geological Survey, 215 Limekiln Road, New Cumberland, Pennsylvania 17070, United States; Orcidhttp://orcid.org/0000-0002-7180-4319
    • Patricia L. Toccalino - U.S. Geological Survey, 911 Northeast 11th Avenue, Portland, Oregon 97232, United States; Orcidhttp://orcid.org/0000-0003-1066-1702
    • Lisa H. Nowell - U.S. Geological Survey, 6000 J Street, Placer Hall, Sacramento, California 95819, United States; Orcidhttp://orcid.org/0000-0001-5417-7264
  • Author Contributions

    The manuscript was written through the contributions of all authors. All authors have given approval to the current version of the manuscript.

  • Notes

    The authors declare no competing financial interest.

Acknowledgments

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This work was funded by the U.S. Geological Survey (USGS) National Water-Quality Assessment Project. The study would not have been possible without the dedication of numerous USGS field crews and the generous assistance of participating well owners and operators. We also appreciate the contributions of Mark Sandstrom and other USGS National Water Quality Laboratory personnel, who analyzed samples and assisted in quality-control evaluations; Delicia Beaty, who compiled and examined Quality Systems Branch performance data; Tyler Johnson, who processed ancillary data; and reviewers of this article. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Abbreviations
BQ

benchmark quotient

BQE

estimated benchmark quotient

CAAT

didealkylatrazine

DEA

deethylatrazine

DIA

deisopropylatrazine

DL

detection limit

EPA

U.S. Environmental Protection Agency

LB

laboratory set blank

NAWQA

National Water-Quality Assessment

NWQL

National Water-Quality Laboratory

OA

oxanilic acid

OEAT

2-hyroxy-6-ethylamino-4-amino-s-triazine

OIAT

desethylhydroxyatrazine

OIET

hydroxyatrazine

PAS

principal aquifer survey

RL

reporting limit

SA

sulfonic acid

μg/L

micrograms per liter

USGS

U.S. Geological Survey

VRPBR

Valley and Ridge and Piedmont and Blue Ridge

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    Characterizing all possible chemical mixtures in drinking water is a potentially overwhelming project, and the task of assessing each mixture's net toxicity even more daunting. We propose that analyzing occurrence information on mixtures in drinking water may help to narrow the priorities and inform the approaches taken by researchers in mixture toxicology. To illustrate the utility of environmental data for refining the mixtures problem, we use a recent compilation of national ground-water-quality data to examine proposed U.S. Environmental Protection Agency (EPA) and Agency for Toxic Substances and Disease Registry (ATSDR) models of noncancer mixture toxicity. We use data on the occurrence of binary and ternary mixtures of arsenic, cadmium, and manganese to parameterize an additive model and compute hazard index scores for each drinking-water source in the data set. We also use partially parameterized interaction models to perform a bounding analysis estimating the interaction potential of several binary and ternary mixtures for which the toxicological literature is limited. From these results, we estimate a relative value of additional toxicological information for each mixture. For example, we find that according to the U.S. EPA's interaction model, the levels of arsenic and cadmium found in U.S. drinking water are unlikely to have synergistic cardiovascular effects, but the same mixture's potential for synergistic neurological effects merits further study. Similar analysis could in future be used to prioritize toxicological studies based on their potential to reduce scientific and regulatory uncertainty. Environmental data may also provide a means to explore the implications of alternative risk models for the toxicity and interaction of complex mixtures.
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    Kortenkamp, A.; Faust, M. State of the Art Report on Mixture Toxicity, Final Report: Executive Summary; School of Pharmacy, University of London: London, U.K., 2009.
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  24. 24
    Meek, M. E.; Boobis, A. R.; Crofton, K. M.; Heinemeyer, G.; Van Raaij, M.; Vickers, C. Risk assessment of combined exposure to multiple chemicals: A WHO/IPCS framework. Regul. Toxicol. Pharmacol. 2011, 60, S1– S14,  DOI: 10.1016/j.yrtph.2011.03.010
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    Risk assessment of combined exposure to multiple chemicals: A WHO/IPCS framework
    Meek, M. E.; Boobis, Alan R.; Crofton, Kevin M.; Heinemeyer, Gerhard; Van Raaij, Marcel; Vickers, Carolyn
    Regulatory Toxicology and Pharmacology (2011), 60 (2, Suppl. 1), S1-S14CODEN: RTOPDW; ISSN:0273-2300. (Elsevier B.V.)
    This paper describes a framework for the risk assessment of combined exposure to multiple chems. based on and developed subsequent to the World Health Organization/International Program on Chem. Safety Workshop on Aggregate/Cumulative Risk Assessment (Combined Exposures to Multiple Chems.) held in 2007. It is designed to aid risk assessors in identifying priorities for risk management for a wide range of applications where co-exposures to multiple chems. are expected. It is based on a hierarchical (phased) approach that involves integrated and iterative consideration of exposure and hazard at all phases, with each tier being more refined than the previous one, but more labor and data intensive. It includes ref. to predictive and probabilistic methodol. in various tiers in addn. to tiered consideration of uncertainty. Two case studies that have been developed to test and refine the framework are shown.
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    Price, P. S.; Han, X. Maximum cumulative ratio (MCR) as a tool for assessing the value of performing a cumulative risk assessment. Int. J. Environ. Res. Public Health 2011, 8, 2212– 2225,  DOI: 10.3390/ijerph8062212
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    25
    Maximum cumulative ratio (MCR) as a tool for assessing the value of performing a cumulative risk assessment
    Price Paul S; Han Xianglu
    International journal of environmental research and public health (2011), 8 (6), 2212-25 ISSN:.
    Due to the vast number of possible combinations of chemicals to which individuals are exposed and the resource-intensive nature of cumulative risk assessments, there is a need to determine when cumulative assessments are most required. This paper proposes the use of the maximum cumulative ratio (MCR) as a tool for this evaluation. MCR is the ratio of the cumulative toxicity received by an individual from exposure to multiple chemical stressors to the largest toxicity from a single chemical stressor. The MCR is a quantitative measure of the difference in an individual's toxicity estimated using a chemical-by-chemical approach and using an additive model of toxicity. As such, it provides a conservative estimate of the degree to which individuals' toxicities could be underestimated by not performing a cumulative risk assessment. In an example application, MCR is shown to be applicable to the evaluation of cumulative exposures involving up to 81 compounds and to provide key insights into the cumulative effects posed by exposures to multiple chemicals. In this example, MCR values suggest that individuals exposed to combinations of chemicals with the largest Hazard Indices were dominated by the contributions of one or two compounds.
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    Toccalino, P. L.; Norman, J. E.; Scott, J. C. Chemical mixtures in untreated water from public-supply wells in the U.S.—Occurrence, composition, and potential toxicity. Sci. Total Environ. 2012, 431, 262– 270,  DOI: 10.1016/j.scitotenv.2012.05.044
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    26
    Chemical mixtures in untreated water from public-supply wells in the U.S. - Occurrence, composition, and potential toxicity
    Toccalino, Patricia L.; Norman, Julia E.; Scott, Jonathon C.
    Science of the Total Environment (2012), 431 (), 262-270CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
    Chem. mixts. are prevalent in groundwater used for public water supply, but little is known about their potential health effects. As part of a large-scale ambient groundwater study, the authors evaluated chem. mixts. across multiple chem. classes, and included more chem. contaminants than in previous studies of mixts. in public-supply wells. They (1) assessed the occurrence of chem. mixts. in untreated source-water samples from public-supply wells, (2) detd. the compn. of the most frequently occurring mixts., and (3) characterized the potential toxicity of mixts. using a new screening approach. The U.S. Geol. Survey collected one untreated water sample from each of 383 public wells distributed across 35 states, and analyzed the samples for as many as 91 chem. contaminants. Concns. of mixt. components were compared to individual human-health benchmarks; the potential toxicity of mixts. was characterized by addn. of benchmark-normalized component concns. Most samples (84%) contained mixts. of two or more contaminants, each at concns. greater than one-tenth of individual benchmarks. The chem. mixts. that most frequently occurred and had the greatest potential toxicity primarily were composed of trace elements (including arsenic, strontium, or uranium), radon, or nitrate. Herbicides, disinfection byproducts, and solvents were the most common org. contaminants in mixts. The sum of benchmark-normalized concns. was greater than 1 for 58% of samples, suggesting that there could be potential for mixts. toxicity in more than half of the public-well samples. The findings can be used to help set priorities for groundwater monitoring and suggest future research directions for drinking-water treatment studies and for toxicity assessments of chem. mixts. in water resources.
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    OpenURL UNIV OF NORTH TEXAS
  27. 27
    Framework for Assessing Health Impacts of Multiple Chemicals and Other Stressors (Update); U.S. Department of Health and Human Services: Washington, D.C., 2018. https://www.atsdr.cdc.gov/interactionprofiles/ip-ga/ipga.pdf (accessed April 23, 2020).
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  28. 28
    Cumulative Assessment of Risk from Pesticides; U.S. Environmental Protection Agency: Washington, D.C., 2020. https://www.epa.gov/pesticide-science-and-assessing-pesticide-risks/cumulative-assessment-risk-pesticides (accessed March 20, 2020).
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    Manamsa, K.; Crane, E.; Stuart, M.; Talbot, J.; Lapworth, D.; Hart, A. A national-scale assessment of micro-organic contaminants in groundwater of England and Wales. Sci. Total Environ. 2016, 568, 712– 726,  DOI: 10.1016/j.scitotenv.2016.03.017
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    A national-scale assessment of micro-organic contaminants in groundwater of England and Wales
    Manamsa, Katya; Crane, Emily; Stuart, Marianne; Talbot, John; Lapworth, Dan; Hart, Alwyn
    Science of the Total Environment (2016), 568 (), 712-726CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
    A large variety of micro-org. (MO) compds. is used in huge quantities for a range of purposes (e.g., manufg., food prodn., healthcare) and is now being frequently detected in the aquatic environment. Interest in the occurrence of MO contaminants in the terrestrial and aquatic environments continues to grow, as well as in their environmental fate and potential toxicity. However, the contamination of groundwater resources by MOs has a limited evidence base compared to other freshwater resources. Of particular concern are newly 'emerging contaminants' such as pharmaceuticals and lifestyle compds., particularly those with potential endocrine disrupting properties. While groundwater often has a high degree of protection from pollution due to phys., chem. and biol. attenuation processes in the subsurface compared to surface aquatic environments, trace concns. of a large range of compds. are still detected in groundwater and in some cases may persist for decades due to the long residence times of groundwater systems. This study provides the first national-scale assessment of micro-org. compds. in groundwater in England and Wales. A large set of monitoring data was analyzed to det. the relative occurrence and detected concns. of different groups of compds. and to det. relationships with land-use, aquifer type and groundwater vulnerability. MOs detected including emerging compds. such as caffeine, DEET, bisphenol A, anti-microbial agents and pharmaceuticals as well as a range of legacy contaminants including chlorinated solvents and THMs, petroleum hydrocarbons, pesticides and other industrial compds. There are clear differences in MOs between land-use types, particularly for urban-industrial and natural land-use. Temporal trends of MO occurrence are assessed but establishing long-term trends is not yet possible.
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  30. 30
    Kiefer, K.; Muller, A.; Singer, H.; Hollender, J. New relevant pesticide transformation products in groundwater detected using target and suspect screening for agricultural and urban micropollutants with LC-HRMS. Water Res. 2019, 165, 114972  DOI: 10.1016/j.watres.2019.114972
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    New relevant pesticide transformation products in groundwater detected using target and suspect screening for agricultural and urban micropollutants with LC-HRMS
    Kiefer, Karin; Muller, Adrian; Singer, Heinz; Hollender, Juliane
    Water Research (2019), 165 (), 114972CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    Groundwater is a major drinking water resource, but its quality is threatened by a broad variety of anthropogenic micropollutants (MPs), originating from agriculture, industry, or households, and undergoing various transformation processes during subsurface passage. To det. a worst-case impact of pesticide application in agriculture on groundwater quality, a target and suspect screening for more than 300 pesticides and more than 1100 pesticide transformation products (TPs) was performed in 31 Swiss groundwater samples which predominantly originated from areas with intensive agriculture. To assess addnl. urban contamination sources, more than 250 common urban MPs were quantified. Most of the screened pesticide TPs were exptl. obsd. by the pesticide producers within the European pesticide registration. To cover very polar pesticide TPs, vacuum-assisted evaporative concn. was used for enrichment, followed by liq. chromatog. high-resoln. tandem mass spectrometry (LC-HRMS/MS). Based on intensity, isotope pattern, retention time, and in silico fragmentation, the suspect hits were prioritised and verified. We identified 22 suspects unequivocally and five tentatively; 13 TPs are reported here for the first time to be detected in groundwater. In 13 out of 31 groundwater samples, the total concn. of the 20 identified and quantified suspects (1 pesticide and 19 pesticide TPs) exceeded the total concn. of the 519 targets (236 pesticides and TPs; 283 urban MPs) for which we screened. Pesticide TPs had higher concns. than the parent pesticides, illustrating their importance for groundwater quality. The newly identified very polar chlorothalonil TP R471811 was the only compd. detected in all samples with concns. ranging from 3 to 2700 ng/L. Agricultural MP concn. and detection frequency correlated with agricultural land use in the catchment, except for aquifers, where protective top layers reduced MP transport from the surface. In contrast to agricultural MPs, urban MPs displayed almost no correlation with land use. The dominating entry pathway of urban MPs was river bank filtration.
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  31. 31
    Close, M. E.; Humphries, B.; Northcott, G. Outcomes of the first combined national survey of pesticides and emerging organic contaminants (EOCs) in groundwater in New Zealand 2018. Sci. Total Environ. 2020, 754, 142005  DOI: 10.1016/j.scitotenv.2020.142005
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    Kolpin, D. W.; Goolsby, D. A.; Thurman, M. E. Pesticides in near-surface aquifers: an assessment using highly sensitive analytical methods and tritium. J. Environ. Qual. 1995, 24, 1125– 1132,  DOI: 10.2134/jeq1995.00472425002400060011x
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    32
    Pesticides in near-surface aquifers: an assessment using highly sensitive analytical methods and tritium
    Kolpin, Dana W.; Goolsby, Donald A.; Thurman, E. Michael
    Journal of Environmental Quality (1995), 24 (6), 1125-32CODEN: JEVQAA; ISSN:0047-2425. (American Society of Agronomy)
    In 1992, the U.S. Geol. Survey (USGS) detd. the distribution of pesticides in near-surface aquifers of the midwestern USA to be much more widespread than originally detd. during a 1991 USGS study. The frequency of pesticide detection increased from 28.4% during the 1991 study to 59.0% during the 1992 study. This increase in pesticide detection was primarily the result of a more sensitive anal. method that used reporting limits as much as 20 times lower than previously available and a threefold increase in the no. of pesticide metabolites analyzed. No pesticide concns. exceeded the U.S. Environmental Protection Agency's (USEPAs) max. contaminated levels or health advisory levels for drinking water. However, five of the six most frequently detected compds. during 1992 were pesticide metabolites that currently do not have drinking water stds. detd. The frequent presence of pesticide metabolites for this study documents the importance of obtaining information on these compds. to understand the fate and transport of pesticides in the hydrol. system. It appears that the 56 parent compds. analyzed follow similar pathways through the hydrol. system as atrazine. When atrazine was detected by routine or sensitive anal. methods, there was an increased likelihood of detecting addnl. parent compds. As expected, the frequency of pesticide detection was highly dependent on the anal. reporting limit. The no. of atrazine detections more than doubled as the reporting limit decreased from 0.10 to 0.01 μg/L. The 1992 data provided no indication that the frequency of pesticide detection would level off as improved anal. methods provide concns. below 0.003 μg/L. A relation was detd. between groundwater age and the frequency of pesticide detection, with 15.8% of the samples composed of pre-1953 water and 70.3% of the samples of post-1953 water having a detection of at least one pesticide or metabolite. Pre-1953 water is less likely to contain pesticides because it tends to predate the use of pesticides to increase crop prodn. in the Midwest. Pre-1953 water was more likely to occur in the near-surface bedrock aquifers (50.0%) than in the near-surface unconsolidated (9.1%) sampled.
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    Kolpin, D. W.; Thurman, M. E.; Goolsby, D. A. Occurrence of selected pesticides and their metabolites in near-surface aquifers of the midwestern United States. Environ. Sci. Technol. 1996, 30, 335– 340,  DOI: 10.1021/es950462q
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    Occurrence of Selected Pesticides and Their Metabolites in Near-Surface Aquifers of the Midwestern United States
    Kolpin, Dana W.; Thurman, E. Michael; Goolsby, Donald A.
    Environmental Science and Technology (1996), 30 (1), 335-40CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The occurrence and distribution of selected pesticides and their metabolites were investigated through the collection of 837 water-quality samples from 303 wells across the Midwest. Results of this study showed that 5 of the 6 most frequently detected compds. were pesticide metabolites. Thus, it was common for a metabolite to be found more frequently in groundwater than its parent compd. The metabolite alachlor ethanesulfonic acid (alachlor-ESA; 2-[(2,6-diethylphenyl)(methoxymethyl)amino]-2-oxoethanesulfonic acid) was detected almost 10 times as frequently and at much higher concns. than its parent compd. alachlor (2-chloro-2',6'-diethyl-N-(methoxymethyl)acetamide). The median detectable atrazine (2-chloro-4-ethylamino-6- isopropylamino-s-triazine) concn. was almost half that of atrazine residue (atrazine plus the 2 atrazine metabolites analyzed). Cyanazine amide [2-chloro-4-(1-carbamoyl-1-methylethylamino)-6-ethylamino-s-triazine] was detected almost twice as frequently as cyanazine (2-chloro-4-ethylamino-6-methylpropionitrileamino-s-triazine). Results show that information on pesticide metabolites is necessary to understand the environmental fate of pesticides. Consequently, if pesticide metabolites are not quantified, the effects of chem. use on groundwater quality would be substantially underestimated. Thus, continued research is needed to identify major degrdn. pathways for all pesticides and to develop anal. methods to det. their concns. in water and other environmental data.
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  34. 34
    Kolpin, D. W.; Barbash, J. E.; Gilliom, R. J. Occurrence of pesticides in shallow groundwater of the United States: Initial results from the National Water-Quality Assessment Program. Environ. Sci. Technol. 1998, 32, 558– 566,  DOI: 10.1021/es970412g
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    Occurrence of Pesticides in Shallow Groundwater of the United States: Initial Results from the National Water-Quality Assessment Program
    Kolpin, Dana W.; Barbash, Jack E.; Gilliom, Robert J.
    Environmental Science and Technology (1998), 32 (5), 558-566CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The first phase of intensive data collection for the National Water Quality Assessment (NAWQA) program was completed during 1993-1995 in 20 major hydrol. basins of the USA. Groundwater land use studies, designed to sample recently recharged groundwater (generally within 10 yr) beneath specific land use and hydrogeol. settings, are a major component of the groundwater quality assessment for NAWQA. Pesticide results from 41 land use studies conducted during 1993-1995 indicated that pesticides were commonly detected in shallow groundwater, being obsd. at 54.4% of the 1034 sites sampled in agricultural and urban settings across the USA. Pesticide concns. were generally low, with >95% of the detections at concns. <1 mg/L; of 46 pesticide compds. examd., 39 were detected. Compds. detected most frequently were atrazine (38.2%), deethylatrazine (34.2%), simazine (18.0%), metolachlor (14.6%), and prometon (13.9%). Statistically significant relations were obsd. between frequencies of detection and the use, mobility, and persistence of these compds. Pesticides were commonly detected in agricultural (56.4%; 813 sites) and urban (46.6%; 221 sites) settings. Frequent pesticide detection in urban areas indicated that, as with agricultural pesticide use in agricultural areas, urban and suburban pesticide use significantly contributed to pesticide occurrence in shallow groundwater. Although pesticides were detected in groundwater sampled in urban areas and all 9 agricultural land-use categories examd., significant variations in occurrence were obsd. among these categories. Max. contaminant levels (MCL) established by USEPA for drinking water were exceeded for only 1 pesticide (atrazine, 3 mg/L) at a single location; however, MCL have been established for only 25 of 46 pesticide compds. examd., do not cover pesticide degradates, and, currently, do not account for additive or synergistic effects of combinations of pesticide compds. or potential effects on nearby aquatic ecosystems.
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    OpenURL UNIV OF NORTH TEXAS
  35. 35
    Kolpin, D. W.; Barbash, J. E.; Gilliom, R. J. Pesticides in groundwater of the United States, 1992-96. Ground Water 2000, 38, 858– 863,  DOI: 10.1111/j.1745-6584.2000.tb00684.x
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    Pesticides in ground water of the United States, 1992-1996
    Kolpin, Dana W.; Barbash, Jack E.; Gilliom, Robert J.
    Ground Water (2000), 38 (6), 858-863CODEN: GRWAAP; ISSN:0017-467X. (National Ground Water Association)
    During the 1st cycle of the National Water Quality Assessment (1992-1996), groundwater in 20 of the nation's major hydrol. basins was analyzed for 90 pesticide compds. (pesticides and degradates). One or more of the pesticide compds. examd. were detected at 48.4% of the 2485 groundwater sites sampled. However, ∼70% of the sites where pesticides were detected, ≥2 pesticide compds. analyzed were present-documenting the prevalence of pesticide mixts. in groundwater. The pesticide concns. encountered were generally low, with the median total concn. (summation of concns. for the 90 pesticide compds.) being 0.046 μg/L. Pesticides were commonly detected in shallow groundwater beneath both agricultural (60.4%) and urban (48.5%) areas. This is an important finding because, although agricultural activities have long been assocd. with pesticide contamination, urban areas have only recently been recognized as a potential source of these types of compds. Pesticides with higher frequencies of detection were generally those with more extensive use, greater environmental persistence, and greater mobility in groundwater (lower soil-water partition coeffs.).
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  36. 36
    Bexfield, L. M. Decadal-scale changes of pesticides in ground water of the United States, 1993-2003. J. Environ. Qual. 2008, 37, S-226– S-239,  DOI: 10.2134/jeq2007.0054
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    Decadal-scale changes of pesticides in ground water of the United States, 1993-2003
    Bexfield, Laura M.
    Journal of Environmental Quality (2008), 37 (5, Suppl.), S/226-S/239CODEN: JEVQAA; ISSN:0047-2425. (American Society of Agronomy)
    Pesticide data for groundwater sampled across the US between 1993-1995 and 2001-2003 by the US Geol. Survey National Water-Quality Assessment Program were evaluated for trends in detection frequency and concn. The data anal. evaluated samples collected from a total of 362 wells located in 12 local well networks characterizing shallow groundwater in agricultural areas and 6 local well networks characterizing the drinking water resource in areas of variable land use. Each well network was sampled once during 1993-1995 and once during 2001-2003. The networks provide an overview of conditions across a wide range of hydrogeol. settings and in major agricultural areas that vary in dominant crop type and pesticide use. Of ∼80 pesticide compds. analyzed, only 6 compds. were detected in groundwater from ≥10 wells during both sampling events. These compds. were the triazine herbicides atrazine, simazine, and prometon; the acetanilide herbicide metolachlor; the urea herbicide tebuthiuron; and an atrazine degradate, deethylatrazine (DEA). Obsd. concns. of these compds. generally were <0.12 μg/L. At individual wells, changes in concns. typically were <0.02 μg/L. Data anal. incorporated adjustments for changes in lab. recovery as assessed through lab. spikes. In wells yielding detectable concns. of atrazine, DEA, and prometon, concns. were significantly lower (α =0.1) in 2001-2003 than in 1993-1995, whereas detection frequency of these compds. did not change significantly. Trends in atrazine concns. at shallow wells in agricultural areas were found to be consistent overall with recent atrazine use data.
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  37. 37
    Toccalino, P. L.; Gilliom, R. J.; Lindsey, B. D.; Rupert, M. G. Pesticides in groundwater of the United States: Decadal-scale changes, 1993-2011. Groundwater 2014, 52, 112– 125,  DOI: 10.1111/gwat.12176
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    Pesticides in Groundwater of the United States: Decadal-Scale Changes, 1993-2011
    Toccalino, Patricia L.; Gilliom, Robert J.; Lindsey, Bruce D.; Rupert, Michael G.
    Groundwater (2014), 52 (S1), 112-125CODEN: GRWAAP; ISSN:0017-467X. (Wiley-Blackwell)
    The national occurrence of 83 pesticide compds. in groundwater of the United States and decadal-scale changes in concns. for 35 compds. were assessed for the 20-yr period from 1993-2011. Samples were collected from 1271 wells in 58 nationally distributed well networks. Networks consisted of shallow (mostly monitoring) wells in agricultural and urban land-use areas and deeper (mostly domestic and public supply) wells in major aquifers in mixed land-use areas. Wells were sampled once during 1993-2001 and once during 2002-2011. Pesticides were frequently detected (53% of all samples), but concns. seldom exceeded human-health benchmarks (1.8% of all samples). The five most frequently detected pesticide compds.-atrazine, deethylatrazine, simazine, metolachlor, and prometon-each had statistically significant (p < 0.1) changes in concns. between decades in one or more categories of well networks nationally aggregated by land use. For agricultural networks, concns. of atrazine, metolachlor, and prometon decreased from the first decade to the second decade. For urban networks, deethylatrazine concns. increased and prometon concns. decreased. For major aquifers, concns. of deethylatrazine and simazine increased. The directions of concn. changes for individual well networks generally were consistent with changes detd. from nationally aggregated data. Altogether, 36 of the 58 individual well networks had statistically significant changes in concns. of one or more pesticides between decades, with the majority of changes attributed to the five most frequently detected pesticide compds. The magnitudes of median decadal-scale concn. changes were small-ranging from -0.09 to 0.03 μg/L-and were 35- to 230,000-fold less than human-health benchmarks.
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  38. 38
    Kolpin, D. W.; Kalkhoff, S. J.; Goosby, D. A.; Sneck-Fahrer, D. A.; Thurman, M. E. Occurrence of selected herbicides and herbicide degradation products in Iowa’s ground water, 1995. Ground Water 1997, 35, 679– 688,  DOI: 10.1111/j.1745-6584.1997.tb00134.x
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    38
    Occurrence of selected herbicides and herbicide degradation products in Iowa's ground water, 1995
    Kolpin, Dana W.; Kalkhoff, Stephen J.; Goolsby, Donald A.; Sneck-Fahrer, Debra A.; Thurman, E. Michael
    Ground Water (1997), 35 (4), 679-688CODEN: GRWAAP; ISSN:0017-467X. (Ground Water Publishing)
    Herbicide compds. were prevalent in groundwater across Iowa, being detected in 70% of the 106 municipal wells sampled during the summer of 1995. Herbicide degrdn. products were 3 of the 4 most frequently detected compds. for this study. The degrdn. product alachlor ethanesulfonic acid was the most frequently detected compd. (65.1%), followed by atrazine (40.6%), and the degrdn. products deethylatrazine (34.9%), and cyanazine amide (19.8%). The corn herbicide acetochlor, 1st registered for widespread use in the US in Mar. 1994, was detected in a single water sample. No reported herbicide compd. concns. for this study exceeded current US EPA's max. contaminant levels or health advisory levels for drinking water, although the herbicide degrdn. products examd. have yet to have such levels established. The occurrence of herbicide compds. had a significant, inverse relation to well depth and a significant, pos. relation to dissolved O concn. It is felt that both well depth and dissolved O are acting as rough surrogates to groundwater age, with younger groundwater being more likely to contain herbicide compds. The occurrence of herbicide compds. was substantially different among the major aquifer types across Iowa, being detected in 82.5% of the alluvial, 81.8% of the bedrock/karst region, 40.0% of the glacial-drift, and 25.0% of the bedrock/nonkarst region aquifers. The obsd. distribution was partially attributed to variations in general groundwater age among these aquifer types. A significant, inverse relation was detd. between total herbicide compd. concns. in groundwater and the av. soil slope within a 2-km radius of sampled wells. Steeper soil slopes may increase the likelihood of surface runoff occurring rather than groundwater infiltration, decreasing the transport of herbicide compds. to groundwater. As expected, a significant pos. relation was detd. between intensity of herbicide use and herbicide concns. in groundwater.
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  39. 39
    Kolpin, D. W.; Thurman, M. E.; Linhart, S. M. The environmental occurrence of herbicides: The importance of degradates in ground water. Arch. Environ. Contam. Toxicol. 1998, 35, 385– 390,  DOI: 10.1007/s002449900392
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    The environmental occurrence of herbicides: the importance of degradates in ground water
    Kolpin, D. W.; Thurman, E. M.; Linhart, S. M.
    Archives of Environmental Contamination and Toxicology (1998), 35 (3), 385-390CODEN: AECTCV; ISSN:0090-4341. (Springer-Verlag New York Inc.)
    Numerous studies are being conducted to investigate the occurrence, fate, and effects on human health and the environment from the extensive worldwide use of herbicides to control weeds. Few studies, however, are considering the degradates of these herbicides in their investigations. Our study of herbicides in aquifers across Iowa found herbicide degradates to be prevalent in ground water, being detected in about 75% of the wells sampled. With the exception of atrazine, the frequencies of detection in ground water for a given herbicide increased multifold when its degradates were considered. Furthermore, a majority of the measured concn. for a given herbicide was in the form of its degradates--even for a relatively persistent compd. such as atrazine. For this study, degradates comprised from 60 to over 99% of a herbicide's measured concn. Because herbicide degradates can have similar acute and chronic toxicity as their parent compds., these compds. have environmental significance as well as providing a more complete understanding of the fate and transport of a given herbicide. Thus, it is essential that degradates are included in any type of herbicide investigation.
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  40. 40
    Kolpin, D. W.; Thurman, M. E.; Linhart, S. M. Finding minimal herbicide concentrations in ground water? Try looking for their degradates. Sci. Total Environ. 2000, 248, 115– 122,  DOI: 10.1016/S0048-9697(99)00535-5
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    Finding minimal herbicide concentrations in ground water? Try looking for their degradates
    Kolpin, D. W.; Thurman, E. M.; Linhart, S. M.
    Science of the Total Environment (2000), 248 (2-3), 115-122CODEN: STENDL; ISSN:0048-9697. (Elsevier Science Ireland Ltd.)
    In Iowa groundwater, herbicide degradates were frequently detected. Herbicide degradates were 8 of the 10 most frequently detected compds. A majority of a herbicide's measured concn. was in the form of its degradates, 55 to >99%. The herbicide detection frequencies and concns. varied significantly among the major aquifer types sampled. These differences, however, were much more pronounced when herbicide degradates were included. Aquifer types presumed to have the most rapid recharge rates (alluvial and bedrock/karst region aquifers) were those most likely to contain detectable concns. of herbicide compds. Two indirect ests. of groundwater age (depth of well completion and dissolved O concn.) were used to sep. the sampled wells into general vulnerability classes (low, intermediate, and high). The results show that the herbicide detection frequencies and concns. varied significantly among the vulnerability classes regardless of whether or not herbicide degradates were considered. Nevertheless, when herbicide degradates were included, the frequency of herbicide compd. detection within the highest vulnerability class approached 90%, and the median total herbicide residue concn. increased over an order of magnitude, relative to the parent compds. alone, to 2 μg/L. The results from this study demonstrate that obtaining data on herbicide degradates is crit. for understanding the fate of herbicides in the hydrol. system. The prevalence of herbicide degradates documented in this study suggests that to accurately det. the overall effect on human health and the environment of a specific herbicide its degradates should also be considered.
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    Kolpin, D. W.; Thurman, M. E.; Linhart, S. M. Occurrence of cyanazine compounds in groundwater: Degradates more prevalent than the parent compound. Environ. Sci. Technol. 2001, 35, 1217– 1222,  DOI: 10.1021/es001520x
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    Occurrence of Cyanazine Compounds in Groundwater: Degradates More Prevalent Than the Parent Compound
    Kolpin, Dana W.; Thurman, E. Michael; Linhart, S. Michael
    Environmental Science and Technology (2001), 35 (6), 1217-1222CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    A recently developed anal. method using liq. chromatog./mass spectrometry was used to study the occurrence of cyanazine and its degradates cyanazine acid (CAC), cyanazine amide (CAM), deethylcyanazine (DEC), and deethylcyanazine acid (DCAC) in groundwater. This research represents some of the earliest data on the occurrence of cyanazine degradates in groundwater. Although cyanazine was infrequently detected in the 64 wells across Iowa sampled in 1999, cyanazine degradates were commonly found during this study. The most frequently detected cyanazine compd. was DCAC (32.8%) followed by CAC (29.7%), CAM (17.2%), DEC (3.1%), and cyanazine (3.1%). The frequency of detection for cyanazine or ≥1 of its degradates (CYTOT) was >12-fold over that of cyanazine alone (39.1% for CYTOT vs. 3.1% for cyanazine). Of the total measured concn. of cyanazine, only 0.2% was derived from its parent compd.-with DCAC (74.1%) and CAC (18.4%) comprising 92.5% of this total. Thus, although DCAC and CAC had similar frequencies of detection, DCAC was generally present in higher concns. No concns. of cyanazine compds. for this study exceeded water-quality criteria for the protection of human health. Only cyanazine, however, has such a criteria established. Nevertheless, because these cyanazine degradates are still chlorinated, they may have similar toxicity as their parent compd.-similar to what has been found with the chlorinated degradates of atrazine. Thus, the results of this study documented that data on the degradates for cyanazine are crit. for understanding its fate and transport in the hydrol. system. The prevalence of the chlorinated degradates of cyanazine found in groundwater suggests that to accurately det. the overall effect on human health and the environment from cyanazine its degradates should also be considered. In addn., because CYTOT was found in 57.6% of the samples collected from alluvial aquifers, about 2-5 times more frequently than the other major aquifer types (glacial drift, bedrock/karst, bedrock/nonkarst) under study, this finding has long-term implications for the occurrence of CYTOT in streams. It is anticipated that low-level concns. of CYTOT will continue to be detected in streams for years after the use of cyanazine has terminated (scheduled for the year 2000 in the US), primarily through its movement from groundwater into streams during base-flow conditions.
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    Kolpin, D. W.; Schnoebelen, D. J.; Thurman, E. M. Degradates provide insight to spatial and temporal trends of herbicides in ground water. Ground Water 2004, 42, 601– 608,  DOI: 10.1111/j.1745-6584.2004.tb02628.x
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    Degradates provide insight to spatial and temporal trends of herbicides in ground water
    Kolpin, Dana W.; Schnoebelen, Douglas J.; Thurman, E. Michael
    Ground Water (2004), 42 (4), 601-608CODEN: GRWAAP; ISSN:0017-467X. (National Ground Water Association)
    Since 1995, a network of municipal wells in Iowa, representing all major aquifer types (alluvial, bedrock/karst region, glacial drift, bedrock/nonkarst region), has been repeatedly sampled for a broad suite of herbicide compds. yielding one of the most comprehensive statewide databases of such compds. currently available in the US. This dataset is ideal for documenting the insight that herbicide degradates provide to the spatial and temporal distribution of herbicides in groundwater. During 2001, 86 municipal wells in Iowa were sampled and analyzed for 21 herbicide parent compds. and 24 herbicide degradates. The frequency of detection increased from 17% when only herbicide parent compds. were considered to 53% when both herbicide parents and degradates were considered. The transport of herbicide compds. to groundwater is substantially underestimated when herbicide degradates are not considered. A significant difference in the results among the major aquifer types was apparent only when both herbicide parent compds. and their degradates were considered. Including herbicide degradates greatly improved the statistical relation to the age of the water being sampled. When herbicide parent compds. are considered, only 40% of the wells lacking a herbicide detection could be explained by the age of the water predating herbicide use. However, when herbicide degradates were also considered, 80% of the ground water samples lacking a detection could be explained by the age of the water predating herbicide use. A temporal pattern in alachlor concns. in groundwater could only be identified when Alachlor degradates were considered.
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    Hladik, M. L.; Bouwer, E. J.; Roberts, A. L. Neutral chloroacetamide herbicide degradates and related compounds in Midwestern United States drinking water sources. Sci. Total Environ. 2008, 390, 155– 165,  DOI: 10.1016/j.scitotenv.2007.09.042
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    Neutral chloroacetamide herbicide degradates and related compounds in Midwestern United States drinking water sources
    Hladik, Michelle L.; Bouwer, Edward J.; Roberts, A. Lynn
    Science of the Total Environment (2008), 390 (1), 155-165CODEN: STENDL; ISSN:0048-9697. (Elsevier Ltd.)
    Recent studies have revealed the presence of neutral degradates of chloroacetamide herbicides in the Chesapeake Bay at concns. greatly in excess of the parent compds. As some degradates are being considered for regulation in drinking water, exposure of human populations to such micropollutants is of interest. We report the results of a survey of source waters used by 12 drinking water utilities in the Midwestern US. Analytes included 20 neutral and 6 ionic chloroacetamide degradates, 4 parent chloroacetamide herbicides, 3 triazine herbicides, and 2 triazine degradates. Samples were collected during Fall 2003 and Spring 2004. In the fall samples, 16 of 20 neutral chloroacetamide degradates were detected, while 18 of 20 neutral chloroacetamide degradates were detected in the spring samples. Concns. of most parent chloroacetamides and neutral degradates were somewhat to substantially higher in the spring than in the fall, with median concns. of ∼10-100 ng/L. Groundwater sources tended to have lower concns. of parents and neutral degradates than surface water sources in the fall, although concns. of parents and degradates in groundwater were similar to those in surface water in the spring.
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    Estimation of aquifer scale proportion using equal area grids: Assessment of regional scale groundwater quality
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    Water Resources Research (2010), 46 (11), W11550/1-W11550/14CODEN: WRERAQ; ISSN:0043-1397. (American Geophysical Union)
    The proportion of an aquifer with constituent concns. above a specified threshold (high concns.) is taken as a nondimensional measure of regional scale water quality. If computed on the basis of area, it can be referred to as the aquifer scale proportion. A spatially unbiased est. of aquifer scale proportion and a confidence interval for that est. are obtained through the use of equal area grids and the binomial distribution. Traditionally, the confidence interval for a binomial proportion is computed using either the std. interval or the exact interval. Research from the statistics literature has shown that the std. interval should not be used and that the exact interval is overly conservative. On the basis of coverage probability and interval width, the Jeffreys interval is preferred. If more than one sample per cell is available, cell declustering is used to est. the aquifer scale proportion, and Kish's design effect may be useful for estg. an effective no. of samples. The binomial distribution is also used to quantify the adequacy of a grid with a given no. of cells for identifying a small target, defined as a constituent that is present at high concns. in a small proportion of the aquifer. Case studies illustrate a consistency between approaches that use one well per grid cell and many wells per cell. The methods presented in this paper provide a quant. basis for designing a sampling program and for utilizing existing data.
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    Environmental Science and Technology (2003), 37 (20), 4617-4625CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Once released to the environment, pesticides may be degraded by abiotic and biotic processes. While parent compds. are assessed in detail in many regulatory schemes, the requirements for the assessment of transformation products are less well developed. This study was therefore performed to explore the relationships between the toxicity of transformation products and their parent compds. and to develop a pragmatic approach for use in the risk assessment of transformation products. Data were obtained on the properties and ecotoxicity of transformation products arising from a wide range of pesticides. Generally, transformation products were less toxic to fish, daphnids, and algae than their parent compd. In instances where a product was more toxic, the increase in toxicity could be explained by either (1) the presence of a pesticide toxicophore; (2) the fact that the product is the active part of a propesticide; (3) the product is accumulated to a greater extent than the parent compd.; or (4) the product has a more potent mode of action than the parent. On the basis of the findings, an approach has been proposed to est. the ecotoxicity of transformation products based on chem. structure and data on the toxicity of the parent compd. The assessments can be performed at an early stage in the risk assessment process to identify those substances that require further testing.
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    Metrics for Assessing the Quality of Groundwater Used for Public Supply, CA, USA: Equivalent-Population and Area
    Belitz, Kenneth; Fram, Miranda S.; Johnson, Tyler D.
    Environmental Science & Technology (2015), 49 (14), 8330-8338CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Data from 11000 public supply wells in 87 study areas were used to assess the quality of nearly all of the groundwater used for public supply in California. Two metrics were developed for quantifying groundwater quality: area with high concns. (km2 or proportion) and equiv.-population relying upon groundwater with high concns. (no. of people or proportion). Concns. are considered high if they are above a human-health benchmark. When expressed as proportions, the metrics are area-weighted and population-weighted detection frequencies. On a statewide-scale, ∼20% of the groundwater used for public supply has high concns. for one or more constituents (23% by area and 18% by equiv.-population). On the basis of both area and equiv.-population, trace elements are more prevalent at high concns. than either nitrate or org. compds. at the statewide-scale, in eight of nine hydrogeol. provinces, and in about three-quarters of the study areas. At a statewide-scale, nitrate is more prevalent than org. compds. based on area, but not on the basis of equiv.-population. The approach developed for this paper, unlike many studies, recognizes the importance of appropriately weighting information when changing scales, and is broadly applicable to other areas.
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    • Abstract

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      Figure 1

      Figure 1. Number of pesticide compounds detected at each of 1204 wells sampled in 19 NAWQA principal aquifer surveys (PASs), 2013–2018, indicating where the sum of benchmark quotients (BQEsum) from tier 2 screening of concentrations of pesticide compounds, including degradates without benchmarks, exceeds 0.1 (well symbols outlined in red).

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      Figure 2

      Figure 2. Areal proportion and average number of detections for each NAWQA principal aquifer survey, grouped by lithology. The number at the top of each blue bar indicates the rank of the areal proportion among all 19 principal aquifer surveys.

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      Figure 3

      Figure 3. Detection frequency of pesticide compounds detected in groundwater from at least 2% of sampled wells. An asterisk indicates a compound that generally was not included in previous NAWQA groundwater studies. Colored text identifies degradates and, when present, their corresponding parent (in bold text).

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        Teuschler, L. K. Deciding which chemical mixtures risk assessment methods work best for what mixtures. Toxicol. Appl. Pharmacol. 2007, 223, 139– 147,  DOI: 10.1016/j.taap.2006.07.010
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        21
        Deciding which chemical mixtures risk assessment methods work best for what mixtures
        Teuschler, Linda K.
        Toxicology and Applied Pharmacology (2007), 223 (2), 139-147CODEN: TXAPA9; ISSN:0041-008X. (Elsevier)
        A review. The most commonly used chem. mixts. risk assessment methods involve simple notions of additivity and toxicol. similarity. Newer methods are emerging in response to the complexities of chem. mixt. exposures and effects. Factors based on both science and policy drive decisions regarding whether to conduct a chem. mixts. risk assessment and, if so, which methods to employ. Scientific considerations are based on pos. evidence of joint toxic action, elevated human exposure conditions or the potential for significant impacts on human health. Policy issues include legislative drivers that may mandate action even though adequate toxicity data on a specific mixt. may not be available and risk assessment goals that impact the choice of risk assessment method to obtain the amt. of health protection desired. This paper discusses three important concepts used to choose among available approaches for conducting a chem. mixts. risk assessment: (1) additive joint toxic action of mixt. components; (2) toxicol. interactions of mixt. components; and (3) chem. compn. of complex mixts. It is proposed that scientific support for basic assumptions used in chem. mixts. risk assessment should be developed by expert panels, risk assessment methods experts, and lab. toxicologists. This is imperative to further develop and refine quant. methods and provide guidance on their appropriate applications. Risk assessors need scientific support for chem. mixts. risk assessment methods in the form of toxicol. data on joint toxic action for high priority mixts., statistical methods for analyzing dose-response for mixts., and toxicol. and statistical criteria for detg. sufficient similarity of complex mixts.
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        OpenURL UNIV OF NORTH TEXAS
      22. 22
        Ryker, S. J.; Small, M. J. Combining occurrence and toxicity information to identify priorities for drinking-water mixture research. Risk Anal. 2008, 28, 653– 666,  DOI: 10.1111/j.1539-6924.2008.00985.x
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        22
        Combining occurrence and toxicity information to identify priorities for drinking-water mixture research
        Ryker Sarah J; Small Mitchell J
        Risk analysis : an official publication of the Society for Risk Analysis (2008), 28 (3), 653-66 ISSN:.
        Characterizing all possible chemical mixtures in drinking water is a potentially overwhelming project, and the task of assessing each mixture's net toxicity even more daunting. We propose that analyzing occurrence information on mixtures in drinking water may help to narrow the priorities and inform the approaches taken by researchers in mixture toxicology. To illustrate the utility of environmental data for refining the mixtures problem, we use a recent compilation of national ground-water-quality data to examine proposed U.S. Environmental Protection Agency (EPA) and Agency for Toxic Substances and Disease Registry (ATSDR) models of noncancer mixture toxicity. We use data on the occurrence of binary and ternary mixtures of arsenic, cadmium, and manganese to parameterize an additive model and compute hazard index scores for each drinking-water source in the data set. We also use partially parameterized interaction models to perform a bounding analysis estimating the interaction potential of several binary and ternary mixtures for which the toxicological literature is limited. From these results, we estimate a relative value of additional toxicological information for each mixture. For example, we find that according to the U.S. EPA's interaction model, the levels of arsenic and cadmium found in U.S. drinking water are unlikely to have synergistic cardiovascular effects, but the same mixture's potential for synergistic neurological effects merits further study. Similar analysis could in future be used to prioritize toxicological studies based on their potential to reduce scientific and regulatory uncertainty. Environmental data may also provide a means to explore the implications of alternative risk models for the toxicity and interaction of complex mixtures.
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        Kortenkamp, A.; Faust, M. State of the Art Report on Mixture Toxicity, Final Report: Executive Summary; School of Pharmacy, University of London: London, U.K., 2009.
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      24. 24
        Meek, M. E.; Boobis, A. R.; Crofton, K. M.; Heinemeyer, G.; Van Raaij, M.; Vickers, C. Risk assessment of combined exposure to multiple chemicals: A WHO/IPCS framework. Regul. Toxicol. Pharmacol. 2011, 60, S1– S14,  DOI: 10.1016/j.yrtph.2011.03.010
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        Risk assessment of combined exposure to multiple chemicals: A WHO/IPCS framework
        Meek, M. E.; Boobis, Alan R.; Crofton, Kevin M.; Heinemeyer, Gerhard; Van Raaij, Marcel; Vickers, Carolyn
        Regulatory Toxicology and Pharmacology (2011), 60 (2, Suppl. 1), S1-S14CODEN: RTOPDW; ISSN:0273-2300. (Elsevier B.V.)
        This paper describes a framework for the risk assessment of combined exposure to multiple chems. based on and developed subsequent to the World Health Organization/International Program on Chem. Safety Workshop on Aggregate/Cumulative Risk Assessment (Combined Exposures to Multiple Chems.) held in 2007. It is designed to aid risk assessors in identifying priorities for risk management for a wide range of applications where co-exposures to multiple chems. are expected. It is based on a hierarchical (phased) approach that involves integrated and iterative consideration of exposure and hazard at all phases, with each tier being more refined than the previous one, but more labor and data intensive. It includes ref. to predictive and probabilistic methodol. in various tiers in addn. to tiered consideration of uncertainty. Two case studies that have been developed to test and refine the framework are shown.
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      25. 25
        Price, P. S.; Han, X. Maximum cumulative ratio (MCR) as a tool for assessing the value of performing a cumulative risk assessment. Int. J. Environ. Res. Public Health 2011, 8, 2212– 2225,  DOI: 10.3390/ijerph8062212
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        25
        Maximum cumulative ratio (MCR) as a tool for assessing the value of performing a cumulative risk assessment
        Price Paul S; Han Xianglu
        International journal of environmental research and public health (2011), 8 (6), 2212-25 ISSN:.
        Due to the vast number of possible combinations of chemicals to which individuals are exposed and the resource-intensive nature of cumulative risk assessments, there is a need to determine when cumulative assessments are most required. This paper proposes the use of the maximum cumulative ratio (MCR) as a tool for this evaluation. MCR is the ratio of the cumulative toxicity received by an individual from exposure to multiple chemical stressors to the largest toxicity from a single chemical stressor. The MCR is a quantitative measure of the difference in an individual's toxicity estimated using a chemical-by-chemical approach and using an additive model of toxicity. As such, it provides a conservative estimate of the degree to which individuals' toxicities could be underestimated by not performing a cumulative risk assessment. In an example application, MCR is shown to be applicable to the evaluation of cumulative exposures involving up to 81 compounds and to provide key insights into the cumulative effects posed by exposures to multiple chemicals. In this example, MCR values suggest that individuals exposed to combinations of chemicals with the largest Hazard Indices were dominated by the contributions of one or two compounds.
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      26. 26
        Toccalino, P. L.; Norman, J. E.; Scott, J. C. Chemical mixtures in untreated water from public-supply wells in the U.S.—Occurrence, composition, and potential toxicity. Sci. Total Environ. 2012, 431, 262– 270,  DOI: 10.1016/j.scitotenv.2012.05.044
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        Chemical mixtures in untreated water from public-supply wells in the U.S. - Occurrence, composition, and potential toxicity
        Toccalino, Patricia L.; Norman, Julia E.; Scott, Jonathon C.
        Science of the Total Environment (2012), 431 (), 262-270CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
        Chem. mixts. are prevalent in groundwater used for public water supply, but little is known about their potential health effects. As part of a large-scale ambient groundwater study, the authors evaluated chem. mixts. across multiple chem. classes, and included more chem. contaminants than in previous studies of mixts. in public-supply wells. They (1) assessed the occurrence of chem. mixts. in untreated source-water samples from public-supply wells, (2) detd. the compn. of the most frequently occurring mixts., and (3) characterized the potential toxicity of mixts. using a new screening approach. The U.S. Geol. Survey collected one untreated water sample from each of 383 public wells distributed across 35 states, and analyzed the samples for as many as 91 chem. contaminants. Concns. of mixt. components were compared to individual human-health benchmarks; the potential toxicity of mixts. was characterized by addn. of benchmark-normalized component concns. Most samples (84%) contained mixts. of two or more contaminants, each at concns. greater than one-tenth of individual benchmarks. The chem. mixts. that most frequently occurred and had the greatest potential toxicity primarily were composed of trace elements (including arsenic, strontium, or uranium), radon, or nitrate. Herbicides, disinfection byproducts, and solvents were the most common org. contaminants in mixts. The sum of benchmark-normalized concns. was greater than 1 for 58% of samples, suggesting that there could be potential for mixts. toxicity in more than half of the public-well samples. The findings can be used to help set priorities for groundwater monitoring and suggest future research directions for drinking-water treatment studies and for toxicity assessments of chem. mixts. in water resources.
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        OpenURL UNIV OF NORTH TEXAS
      27. 27
        Framework for Assessing Health Impacts of Multiple Chemicals and Other Stressors (Update); U.S. Department of Health and Human Services: Washington, D.C., 2018. https://www.atsdr.cdc.gov/interactionprofiles/ip-ga/ipga.pdf (accessed April 23, 2020).
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      28. 28
        Cumulative Assessment of Risk from Pesticides; U.S. Environmental Protection Agency: Washington, D.C., 2020. https://www.epa.gov/pesticide-science-and-assessing-pesticide-risks/cumulative-assessment-risk-pesticides (accessed March 20, 2020).
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      29. 29
        Manamsa, K.; Crane, E.; Stuart, M.; Talbot, J.; Lapworth, D.; Hart, A. A national-scale assessment of micro-organic contaminants in groundwater of England and Wales. Sci. Total Environ. 2016, 568, 712– 726,  DOI: 10.1016/j.scitotenv.2016.03.017
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        29
        A national-scale assessment of micro-organic contaminants in groundwater of England and Wales
        Manamsa, Katya; Crane, Emily; Stuart, Marianne; Talbot, John; Lapworth, Dan; Hart, Alwyn
        Science of the Total Environment (2016), 568 (), 712-726CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
        A large variety of micro-org. (MO) compds. is used in huge quantities for a range of purposes (e.g., manufg., food prodn., healthcare) and is now being frequently detected in the aquatic environment. Interest in the occurrence of MO contaminants in the terrestrial and aquatic environments continues to grow, as well as in their environmental fate and potential toxicity. However, the contamination of groundwater resources by MOs has a limited evidence base compared to other freshwater resources. Of particular concern are newly 'emerging contaminants' such as pharmaceuticals and lifestyle compds., particularly those with potential endocrine disrupting properties. While groundwater often has a high degree of protection from pollution due to phys., chem. and biol. attenuation processes in the subsurface compared to surface aquatic environments, trace concns. of a large range of compds. are still detected in groundwater and in some cases may persist for decades due to the long residence times of groundwater systems. This study provides the first national-scale assessment of micro-org. compds. in groundwater in England and Wales. A large set of monitoring data was analyzed to det. the relative occurrence and detected concns. of different groups of compds. and to det. relationships with land-use, aquifer type and groundwater vulnerability. MOs detected including emerging compds. such as caffeine, DEET, bisphenol A, anti-microbial agents and pharmaceuticals as well as a range of legacy contaminants including chlorinated solvents and THMs, petroleum hydrocarbons, pesticides and other industrial compds. There are clear differences in MOs between land-use types, particularly for urban-industrial and natural land-use. Temporal trends of MO occurrence are assessed but establishing long-term trends is not yet possible.
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      30. 30
        Kiefer, K.; Muller, A.; Singer, H.; Hollender, J. New relevant pesticide transformation products in groundwater detected using target and suspect screening for agricultural and urban micropollutants with LC-HRMS. Water Res. 2019, 165, 114972  DOI: 10.1016/j.watres.2019.114972
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        New relevant pesticide transformation products in groundwater detected using target and suspect screening for agricultural and urban micropollutants with LC-HRMS
        Kiefer, Karin; Muller, Adrian; Singer, Heinz; Hollender, Juliane
        Water Research (2019), 165 (), 114972CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
        Groundwater is a major drinking water resource, but its quality is threatened by a broad variety of anthropogenic micropollutants (MPs), originating from agriculture, industry, or households, and undergoing various transformation processes during subsurface passage. To det. a worst-case impact of pesticide application in agriculture on groundwater quality, a target and suspect screening for more than 300 pesticides and more than 1100 pesticide transformation products (TPs) was performed in 31 Swiss groundwater samples which predominantly originated from areas with intensive agriculture. To assess addnl. urban contamination sources, more than 250 common urban MPs were quantified. Most of the screened pesticide TPs were exptl. obsd. by the pesticide producers within the European pesticide registration. To cover very polar pesticide TPs, vacuum-assisted evaporative concn. was used for enrichment, followed by liq. chromatog. high-resoln. tandem mass spectrometry (LC-HRMS/MS). Based on intensity, isotope pattern, retention time, and in silico fragmentation, the suspect hits were prioritised and verified. We identified 22 suspects unequivocally and five tentatively; 13 TPs are reported here for the first time to be detected in groundwater. In 13 out of 31 groundwater samples, the total concn. of the 20 identified and quantified suspects (1 pesticide and 19 pesticide TPs) exceeded the total concn. of the 519 targets (236 pesticides and TPs; 283 urban MPs) for which we screened. Pesticide TPs had higher concns. than the parent pesticides, illustrating their importance for groundwater quality. The newly identified very polar chlorothalonil TP R471811 was the only compd. detected in all samples with concns. ranging from 3 to 2700 ng/L. Agricultural MP concn. and detection frequency correlated with agricultural land use in the catchment, except for aquifers, where protective top layers reduced MP transport from the surface. In contrast to agricultural MPs, urban MPs displayed almost no correlation with land use. The dominating entry pathway of urban MPs was river bank filtration.
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        OpenURL UNIV OF NORTH TEXAS
      31. 31
        Close, M. E.; Humphries, B.; Northcott, G. Outcomes of the first combined national survey of pesticides and emerging organic contaminants (EOCs) in groundwater in New Zealand 2018. Sci. Total Environ. 2020, 754, 142005  DOI: 10.1016/j.scitotenv.2020.142005
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      32. 32
        Kolpin, D. W.; Goolsby, D. A.; Thurman, M. E. Pesticides in near-surface aquifers: an assessment using highly sensitive analytical methods and tritium. J. Environ. Qual. 1995, 24, 1125– 1132,  DOI: 10.2134/jeq1995.00472425002400060011x
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        Pesticides in near-surface aquifers: an assessment using highly sensitive analytical methods and tritium
        Kolpin, Dana W.; Goolsby, Donald A.; Thurman, E. Michael
        Journal of Environmental Quality (1995), 24 (6), 1125-32CODEN: JEVQAA; ISSN:0047-2425. (American Society of Agronomy)
        In 1992, the U.S. Geol. Survey (USGS) detd. the distribution of pesticides in near-surface aquifers of the midwestern USA to be much more widespread than originally detd. during a 1991 USGS study. The frequency of pesticide detection increased from 28.4% during the 1991 study to 59.0% during the 1992 study. This increase in pesticide detection was primarily the result of a more sensitive anal. method that used reporting limits as much as 20 times lower than previously available and a threefold increase in the no. of pesticide metabolites analyzed. No pesticide concns. exceeded the U.S. Environmental Protection Agency's (USEPAs) max. contaminated levels or health advisory levels for drinking water. However, five of the six most frequently detected compds. during 1992 were pesticide metabolites that currently do not have drinking water stds. detd. The frequent presence of pesticide metabolites for this study documents the importance of obtaining information on these compds. to understand the fate and transport of pesticides in the hydrol. system. It appears that the 56 parent compds. analyzed follow similar pathways through the hydrol. system as atrazine. When atrazine was detected by routine or sensitive anal. methods, there was an increased likelihood of detecting addnl. parent compds. As expected, the frequency of pesticide detection was highly dependent on the anal. reporting limit. The no. of atrazine detections more than doubled as the reporting limit decreased from 0.10 to 0.01 μg/L. The 1992 data provided no indication that the frequency of pesticide detection would level off as improved anal. methods provide concns. below 0.003 μg/L. A relation was detd. between groundwater age and the frequency of pesticide detection, with 15.8% of the samples composed of pre-1953 water and 70.3% of the samples of post-1953 water having a detection of at least one pesticide or metabolite. Pre-1953 water is less likely to contain pesticides because it tends to predate the use of pesticides to increase crop prodn. in the Midwest. Pre-1953 water was more likely to occur in the near-surface bedrock aquifers (50.0%) than in the near-surface unconsolidated (9.1%) sampled.
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        Kolpin, D. W.; Thurman, M. E.; Goolsby, D. A. Occurrence of selected pesticides and their metabolites in near-surface aquifers of the midwestern United States. Environ. Sci. Technol. 1996, 30, 335– 340,  DOI: 10.1021/es950462q
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        Occurrence of Selected Pesticides and Their Metabolites in Near-Surface Aquifers of the Midwestern United States
        Kolpin, Dana W.; Thurman, E. Michael; Goolsby, Donald A.
        Environmental Science and Technology (1996), 30 (1), 335-40CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
        The occurrence and distribution of selected pesticides and their metabolites were investigated through the collection of 837 water-quality samples from 303 wells across the Midwest. Results of this study showed that 5 of the 6 most frequently detected compds. were pesticide metabolites. Thus, it was common for a metabolite to be found more frequently in groundwater than its parent compd. The metabolite alachlor ethanesulfonic acid (alachlor-ESA; 2-[(2,6-diethylphenyl)(methoxymethyl)amino]-2-oxoethanesulfonic acid) was detected almost 10 times as frequently and at much higher concns. than its parent compd. alachlor (2-chloro-2',6'-diethyl-N-(methoxymethyl)acetamide). The median detectable atrazine (2-chloro-4-ethylamino-6- isopropylamino-s-triazine) concn. was almost half that of atrazine residue (atrazine plus the 2 atrazine metabolites analyzed). Cyanazine amide [2-chloro-4-(1-carbamoyl-1-methylethylamino)-6-ethylamino-s-triazine] was detected almost twice as frequently as cyanazine (2-chloro-4-ethylamino-6-methylpropionitrileamino-s-triazine). Results show that information on pesticide metabolites is necessary to understand the environmental fate of pesticides. Consequently, if pesticide metabolites are not quantified, the effects of chem. use on groundwater quality would be substantially underestimated. Thus, continued research is needed to identify major degrdn. pathways for all pesticides and to develop anal. methods to det. their concns. in water and other environmental data.
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      34. 34
        Kolpin, D. W.; Barbash, J. E.; Gilliom, R. J. Occurrence of pesticides in shallow groundwater of the United States: Initial results from the National Water-Quality Assessment Program. Environ. Sci. Technol. 1998, 32, 558– 566,  DOI: 10.1021/es970412g
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        Occurrence of Pesticides in Shallow Groundwater of the United States: Initial Results from the National Water-Quality Assessment Program
        Kolpin, Dana W.; Barbash, Jack E.; Gilliom, Robert J.
        Environmental Science and Technology (1998), 32 (5), 558-566CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
        The first phase of intensive data collection for the National Water Quality Assessment (NAWQA) program was completed during 1993-1995 in 20 major hydrol. basins of the USA. Groundwater land use studies, designed to sample recently recharged groundwater (generally within 10 yr) beneath specific land use and hydrogeol. settings, are a major component of the groundwater quality assessment for NAWQA. Pesticide results from 41 land use studies conducted during 1993-1995 indicated that pesticides were commonly detected in shallow groundwater, being obsd. at 54.4% of the 1034 sites sampled in agricultural and urban settings across the USA. Pesticide concns. were generally low, with >95% of the detections at concns. <1 mg/L; of 46 pesticide compds. examd., 39 were detected. Compds. detected most frequently were atrazine (38.2%), deethylatrazine (34.2%), simazine (18.0%), metolachlor (14.6%), and prometon (13.9%). Statistically significant relations were obsd. between frequencies of detection and the use, mobility, and persistence of these compds. Pesticides were commonly detected in agricultural (56.4%; 813 sites) and urban (46.6%; 221 sites) settings. Frequent pesticide detection in urban areas indicated that, as with agricultural pesticide use in agricultural areas, urban and suburban pesticide use significantly contributed to pesticide occurrence in shallow groundwater. Although pesticides were detected in groundwater sampled in urban areas and all 9 agricultural land-use categories examd., significant variations in occurrence were obsd. among these categories. Max. contaminant levels (MCL) established by USEPA for drinking water were exceeded for only 1 pesticide (atrazine, 3 mg/L) at a single location; however, MCL have been established for only 25 of 46 pesticide compds. examd., do not cover pesticide degradates, and, currently, do not account for additive or synergistic effects of combinations of pesticide compds. or potential effects on nearby aquatic ecosystems.
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        OpenURL UNIV OF NORTH TEXAS
      35. 35
        Kolpin, D. W.; Barbash, J. E.; Gilliom, R. J. Pesticides in groundwater of the United States, 1992-96. Ground Water 2000, 38, 858– 863,  DOI: 10.1111/j.1745-6584.2000.tb00684.x
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        Pesticides in ground water of the United States, 1992-1996
        Kolpin, Dana W.; Barbash, Jack E.; Gilliom, Robert J.
        Ground Water (2000), 38 (6), 858-863CODEN: GRWAAP; ISSN:0017-467X. (National Ground Water Association)
        During the 1st cycle of the National Water Quality Assessment (1992-1996), groundwater in 20 of the nation's major hydrol. basins was analyzed for 90 pesticide compds. (pesticides and degradates). One or more of the pesticide compds. examd. were detected at 48.4% of the 2485 groundwater sites sampled. However, ∼70% of the sites where pesticides were detected, ≥2 pesticide compds. analyzed were present-documenting the prevalence of pesticide mixts. in groundwater. The pesticide concns. encountered were generally low, with the median total concn. (summation of concns. for the 90 pesticide compds.) being 0.046 μg/L. Pesticides were commonly detected in shallow groundwater beneath both agricultural (60.4%) and urban (48.5%) areas. This is an important finding because, although agricultural activities have long been assocd. with pesticide contamination, urban areas have only recently been recognized as a potential source of these types of compds. Pesticides with higher frequencies of detection were generally those with more extensive use, greater environmental persistence, and greater mobility in groundwater (lower soil-water partition coeffs.).
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      36. 36
        Bexfield, L. M. Decadal-scale changes of pesticides in ground water of the United States, 1993-2003. J. Environ. Qual. 2008, 37, S-226– S-239,  DOI: 10.2134/jeq2007.0054
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        Decadal-scale changes of pesticides in ground water of the United States, 1993-2003
        Bexfield, Laura M.
        Journal of Environmental Quality (2008), 37 (5, Suppl.), S/226-S/239CODEN: JEVQAA; ISSN:0047-2425. (American Society of Agronomy)
        Pesticide data for groundwater sampled across the US between 1993-1995 and 2001-2003 by the US Geol. Survey National Water-Quality Assessment Program were evaluated for trends in detection frequency and concn. The data anal. evaluated samples collected from a total of 362 wells located in 12 local well networks characterizing shallow groundwater in agricultural areas and 6 local well networks characterizing the drinking water resource in areas of variable land use. Each well network was sampled once during 1993-1995 and once during 2001-2003. The networks provide an overview of conditions across a wide range of hydrogeol. settings and in major agricultural areas that vary in dominant crop type and pesticide use. Of ∼80 pesticide compds. analyzed, only 6 compds. were detected in groundwater from ≥10 wells during both sampling events. These compds. were the triazine herbicides atrazine, simazine, and prometon; the acetanilide herbicide metolachlor; the urea herbicide tebuthiuron; and an atrazine degradate, deethylatrazine (DEA). Obsd. concns. of these compds. generally were <0.12 μg/L. At individual wells, changes in concns. typically were <0.02 μg/L. Data anal. incorporated adjustments for changes in lab. recovery as assessed through lab. spikes. In wells yielding detectable concns. of atrazine, DEA, and prometon, concns. were significantly lower (α =0.1) in 2001-2003 than in 1993-1995, whereas detection frequency of these compds. did not change significantly. Trends in atrazine concns. at shallow wells in agricultural areas were found to be consistent overall with recent atrazine use data.
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      37. 37
        Toccalino, P. L.; Gilliom, R. J.; Lindsey, B. D.; Rupert, M. G. Pesticides in groundwater of the United States: Decadal-scale changes, 1993-2011. Groundwater 2014, 52, 112– 125,  DOI: 10.1111/gwat.12176
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        Pesticides in Groundwater of the United States: Decadal-Scale Changes, 1993-2011
        Toccalino, Patricia L.; Gilliom, Robert J.; Lindsey, Bruce D.; Rupert, Michael G.
        Groundwater (2014), 52 (S1), 112-125CODEN: GRWAAP; ISSN:0017-467X. (Wiley-Blackwell)
        The national occurrence of 83 pesticide compds. in groundwater of the United States and decadal-scale changes in concns. for 35 compds. were assessed for the 20-yr period from 1993-2011. Samples were collected from 1271 wells in 58 nationally distributed well networks. Networks consisted of shallow (mostly monitoring) wells in agricultural and urban land-use areas and deeper (mostly domestic and public supply) wells in major aquifers in mixed land-use areas. Wells were sampled once during 1993-2001 and once during 2002-2011. Pesticides were frequently detected (53% of all samples), but concns. seldom exceeded human-health benchmarks (1.8% of all samples). The five most frequently detected pesticide compds.-atrazine, deethylatrazine, simazine, metolachlor, and prometon-each had statistically significant (p < 0.1) changes in concns. between decades in one or more categories of well networks nationally aggregated by land use. For agricultural networks, concns. of atrazine, metolachlor, and prometon decreased from the first decade to the second decade. For urban networks, deethylatrazine concns. increased and prometon concns. decreased. For major aquifers, concns. of deethylatrazine and simazine increased. The directions of concn. changes for individual well networks generally were consistent with changes detd. from nationally aggregated data. Altogether, 36 of the 58 individual well networks had statistically significant changes in concns. of one or more pesticides between decades, with the majority of changes attributed to the five most frequently detected pesticide compds. The magnitudes of median decadal-scale concn. changes were small-ranging from -0.09 to 0.03 μg/L-and were 35- to 230,000-fold less than human-health benchmarks.
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        OpenURL UNIV OF NORTH TEXAS
      38. 38
        Kolpin, D. W.; Kalkhoff, S. J.; Goosby, D. A.; Sneck-Fahrer, D. A.; Thurman, M. E. Occurrence of selected herbicides and herbicide degradation products in Iowa’s ground water, 1995. Ground Water 1997, 35, 679– 688,  DOI: 10.1111/j.1745-6584.1997.tb00134.x
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        Occurrence of selected herbicides and herbicide degradation products in Iowa's ground water, 1995
        Kolpin, Dana W.; Kalkhoff, Stephen J.; Goolsby, Donald A.; Sneck-Fahrer, Debra A.; Thurman, E. Michael
        Ground Water (1997), 35 (4), 679-688CODEN: GRWAAP; ISSN:0017-467X. (Ground Water Publishing)
        Herbicide compds. were prevalent in groundwater across Iowa, being detected in 70% of the 106 municipal wells sampled during the summer of 1995. Herbicide degrdn. products were 3 of the 4 most frequently detected compds. for this study. The degrdn. product alachlor ethanesulfonic acid was the most frequently detected compd. (65.1%), followed by atrazine (40.6%), and the degrdn. products deethylatrazine (34.9%), and cyanazine amide (19.8%). The corn herbicide acetochlor, 1st registered for widespread use in the US in Mar. 1994, was detected in a single water sample. No reported herbicide compd. concns. for this study exceeded current US EPA's max. contaminant levels or health advisory levels for drinking water, although the herbicide degrdn. products examd. have yet to have such levels established. The occurrence of herbicide compds. had a significant, inverse relation to well depth and a significant, pos. relation to dissolved O concn. It is felt that both well depth and dissolved O are acting as rough surrogates to groundwater age, with younger groundwater being more likely to contain herbicide compds. The occurrence of herbicide compds. was substantially different among the major aquifer types across Iowa, being detected in 82.5% of the alluvial, 81.8% of the bedrock/karst region, 40.0% of the glacial-drift, and 25.0% of the bedrock/nonkarst region aquifers. The obsd. distribution was partially attributed to variations in general groundwater age among these aquifer types. A significant, inverse relation was detd. between total herbicide compd. concns. in groundwater and the av. soil slope within a 2-km radius of sampled wells. Steeper soil slopes may increase the likelihood of surface runoff occurring rather than groundwater infiltration, decreasing the transport of herbicide compds. to groundwater. As expected, a significant pos. relation was detd. between intensity of herbicide use and herbicide concns. in groundwater.
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        OpenURL UNIV OF NORTH TEXAS
      39. 39
        Kolpin, D. W.; Thurman, M. E.; Linhart, S. M. The environmental occurrence of herbicides: The importance of degradates in ground water. Arch. Environ. Contam. Toxicol. 1998, 35, 385– 390,  DOI: 10.1007/s002449900392
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        The environmental occurrence of herbicides: the importance of degradates in ground water
        Kolpin, D. W.; Thurman, E. M.; Linhart, S. M.
        Archives of Environmental Contamination and Toxicology (1998), 35 (3), 385-390CODEN: AECTCV; ISSN:0090-4341. (Springer-Verlag New York Inc.)
        Numerous studies are being conducted to investigate the occurrence, fate, and effects on human health and the environment from the extensive worldwide use of herbicides to control weeds. Few studies, however, are considering the degradates of these herbicides in their investigations. Our study of herbicides in aquifers across Iowa found herbicide degradates to be prevalent in ground water, being detected in about 75% of the wells sampled. With the exception of atrazine, the frequencies of detection in ground water for a given herbicide increased multifold when its degradates were considered. Furthermore, a majority of the measured concn. for a given herbicide was in the form of its degradates--even for a relatively persistent compd. such as atrazine. For this study, degradates comprised from 60 to over 99% of a herbicide's measured concn. Because herbicide degradates can have similar acute and chronic toxicity as their parent compds., these compds. have environmental significance as well as providing a more complete understanding of the fate and transport of a given herbicide. Thus, it is essential that degradates are included in any type of herbicide investigation.
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      40. 40
        Kolpin, D. W.; Thurman, M. E.; Linhart, S. M. Finding minimal herbicide concentrations in ground water? Try looking for their degradates. Sci. Total Environ. 2000, 248, 115– 122,  DOI: 10.1016/S0048-9697(99)00535-5
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        Finding minimal herbicide concentrations in ground water? Try looking for their degradates
        Kolpin, D. W.; Thurman, E. M.; Linhart, S. M.
        Science of the Total Environment (2000), 248 (2-3), 115-122CODEN: STENDL; ISSN:0048-9697. (Elsevier Science Ireland Ltd.)
        In Iowa groundwater, herbicide degradates were frequently detected. Herbicide degradates were 8 of the 10 most frequently detected compds. A majority of a herbicide's measured concn. was in the form of its degradates, 55 to >99%. The herbicide detection frequencies and concns. varied significantly among the major aquifer types sampled. These differences, however, were much more pronounced when herbicide degradates were included. Aquifer types presumed to have the most rapid recharge rates (alluvial and bedrock/karst region aquifers) were those most likely to contain detectable concns. of herbicide compds. Two indirect ests. of groundwater age (depth of well completion and dissolved O concn.) were used to sep. the sampled wells into general vulnerability classes (low, intermediate, and high). The results show that the herbicide detection frequencies and concns. varied significantly among the vulnerability classes regardless of whether or not herbicide degradates were considered. Nevertheless, when herbicide degradates were included, the frequency of herbicide compd. detection within the highest vulnerability class approached 90%, and the median total herbicide residue concn. increased over an order of magnitude, relative to the parent compds. alone, to 2 μg/L. The results from this study demonstrate that obtaining data on herbicide degradates is crit. for understanding the fate of herbicides in the hydrol. system. The prevalence of herbicide degradates documented in this study suggests that to accurately det. the overall effect on human health and the environment of a specific herbicide its degradates should also be considered.
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      41. 41
        Kolpin, D. W.; Thurman, M. E.; Linhart, S. M. Occurrence of cyanazine compounds in groundwater: Degradates more prevalent than the parent compound. Environ. Sci. Technol. 2001, 35, 1217– 1222,  DOI: 10.1021/es001520x
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        41
        Occurrence of Cyanazine Compounds in Groundwater: Degradates More Prevalent Than the Parent Compound
        Kolpin, Dana W.; Thurman, E. Michael; Linhart, S. Michael
        Environmental Science and Technology (2001), 35 (6), 1217-1222CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
        A recently developed anal. method using liq. chromatog./mass spectrometry was used to study the occurrence of cyanazine and its degradates cyanazine acid (CAC), cyanazine amide (CAM), deethylcyanazine (DEC), and deethylcyanazine acid (DCAC) in groundwater. This research represents some of the earliest data on the occurrence of cyanazine degradates in groundwater. Although cyanazine was infrequently detected in the 64 wells across Iowa sampled in 1999, cyanazine degradates were commonly found during this study. The most frequently detected cyanazine compd. was DCAC (32.8%) followed by CAC (29.7%), CAM (17.2%), DEC (3.1%), and cyanazine (3.1%). The frequency of detection for cyanazine or ≥1 of its degradates (CYTOT) was >12-fold over that of cyanazine alone (39.1% for CYTOT vs. 3.1% for cyanazine). Of the total measured concn. of cyanazine, only 0.2% was derived from its parent compd.-with DCAC (74.1%) and CAC (18.4%) comprising 92.5% of this total. Thus, although DCAC and CAC had similar frequencies of detection, DCAC was generally present in higher concns. No concns. of cyanazine compds. for this study exceeded water-quality criteria for the protection of human health. Only cyanazine, however, has such a criteria established. Nevertheless, because these cyanazine degradates are still chlorinated, they may have similar toxicity as their parent compd.-similar to what has been found with the chlorinated degradates of atrazine. Thus, the results of this study documented that data on the degradates for cyanazine are crit. for understanding its fate and transport in the hydrol. system. The prevalence of the chlorinated degradates of cyanazine found in groundwater suggests that to accurately det. the overall effect on human health and the environment from cyanazine its degradates should also be considered. In addn., because CYTOT was found in 57.6% of the samples collected from alluvial aquifers, about 2-5 times more frequently than the other major aquifer types (glacial drift, bedrock/karst, bedrock/nonkarst) under study, this finding has long-term implications for the occurrence of CYTOT in streams. It is anticipated that low-level concns. of CYTOT will continue to be detected in streams for years after the use of cyanazine has terminated (scheduled for the year 2000 in the US), primarily through its movement from groundwater into streams during base-flow conditions.
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        OpenURL UNIV OF NORTH TEXAS
      42. 42
        Kolpin, D. W.; Schnoebelen, D. J.; Thurman, E. M. Degradates provide insight to spatial and temporal trends of herbicides in ground water. Ground Water 2004, 42, 601– 608,  DOI: 10.1111/j.1745-6584.2004.tb02628.x
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        Degradates provide insight to spatial and temporal trends of herbicides in ground water
        Kolpin, Dana W.; Schnoebelen, Douglas J.; Thurman, E. Michael
        Ground Water (2004), 42 (4), 601-608CODEN: GRWAAP; ISSN:0017-467X. (National Ground Water Association)
        Since 1995, a network of municipal wells in Iowa, representing all major aquifer types (alluvial, bedrock/karst region, glacial drift, bedrock/nonkarst region), has been repeatedly sampled for a broad suite of herbicide compds. yielding one of the most comprehensive statewide databases of such compds. currently available in the US. This dataset is ideal for documenting the insight that herbicide degradates provide to the spatial and temporal distribution of herbicides in groundwater. During 2001, 86 municipal wells in Iowa were sampled and analyzed for 21 herbicide parent compds. and 24 herbicide degradates. The frequency of detection increased from 17% when only herbicide parent compds. were considered to 53% when both herbicide parents and degradates were considered. The transport of herbicide compds. to groundwater is substantially underestimated when herbicide degradates are not considered. A significant difference in the results among the major aquifer types was apparent only when both herbicide parent compds. and their degradates were considered. Including herbicide degradates greatly improved the statistical relation to the age of the water being sampled. When herbicide parent compds. are considered, only 40% of the wells lacking a herbicide detection could be explained by the age of the water predating herbicide use. However, when herbicide degradates were also considered, 80% of the ground water samples lacking a detection could be explained by the age of the water predating herbicide use. A temporal pattern in alachlor concns. in groundwater could only be identified when Alachlor degradates were considered.
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        OpenURL UNIV OF NORTH TEXAS
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        Hladik, M. L.; Bouwer, E. J.; Roberts, A. L. Neutral chloroacetamide herbicide degradates and related compounds in Midwestern United States drinking water sources. Sci. Total Environ. 2008, 390, 155– 165,  DOI: 10.1016/j.scitotenv.2007.09.042
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        Neutral chloroacetamide herbicide degradates and related compounds in Midwestern United States drinking water sources
        Hladik, Michelle L.; Bouwer, Edward J.; Roberts, A. Lynn
        Science of the Total Environment (2008), 390 (1), 155-165CODEN: STENDL; ISSN:0048-9697. (Elsevier Ltd.)
        Recent studies have revealed the presence of neutral degradates of chloroacetamide herbicides in the Chesapeake Bay at concns. greatly in excess of the parent compds. As some degradates are being considered for regulation in drinking water, exposure of human populations to such micropollutants is of interest. We report the results of a survey of source waters used by 12 drinking water utilities in the Midwestern US. Analytes included 20 neutral and 6 ionic chloroacetamide degradates, 4 parent chloroacetamide herbicides, 3 triazine herbicides, and 2 triazine degradates. Samples were collected during Fall 2003 and Spring 2004. In the fall samples, 16 of 20 neutral chloroacetamide degradates were detected, while 18 of 20 neutral chloroacetamide degradates were detected in the spring samples. Concns. of most parent chloroacetamides and neutral degradates were somewhat to substantially higher in the spring than in the fall, with median concns. of ∼10-100 ng/L. Groundwater sources tended to have lower concns. of parents and neutral degradates than surface water sources in the fall, although concns. of parents and degradates in groundwater were similar to those in surface water in the spring.
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        Rosecrans, C. Z.; Musgrove, M. Water Quality of Groundwater Used for Public Supply in Principal Aquifers of the United States; Scientific Investigations Report 2020–5078; U.S. Geological Survey, 2020.
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        Belitz, K.; Jurgens, B.; Landon, M. K.; Fram, M. S.; Johnson, T. Estimation of aquifer scale proportion using equal area grids: Assessment of regional scale groundwater quality. Water Resour. Res. 2010, 46, W11550  DOI: 10.1029/2010WR009321
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        Estimation of aquifer scale proportion using equal area grids: Assessment of regional scale groundwater quality
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        Water Resources Research (2010), 46 (11), W11550/1-W11550/14CODEN: WRERAQ; ISSN:0043-1397. (American Geophysical Union)
        The proportion of an aquifer with constituent concns. above a specified threshold (high concns.) is taken as a nondimensional measure of regional scale water quality. If computed on the basis of area, it can be referred to as the aquifer scale proportion. A spatially unbiased est. of aquifer scale proportion and a confidence interval for that est. are obtained through the use of equal area grids and the binomial distribution. Traditionally, the confidence interval for a binomial proportion is computed using either the std. interval or the exact interval. Research from the statistics literature has shown that the std. interval should not be used and that the exact interval is overly conservative. On the basis of coverage probability and interval width, the Jeffreys interval is preferred. If more than one sample per cell is available, cell declustering is used to est. the aquifer scale proportion, and Kish's design effect may be useful for estg. an effective no. of samples. The binomial distribution is also used to quantify the adequacy of a grid with a given no. of cells for identifying a small target, defined as a constituent that is present at high concns. in a small proportion of the aquifer. Case studies illustrate a consistency between approaches that use one well per grid cell and many wells per cell. The methods presented in this paper provide a quant. basis for designing a sampling program and for utilizing existing data.
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        Sinclair, C. J.; Boxall, A. B. Assessing the ecotoxicity of pesticide transformation products. Environ. Sci. Technol. 2003, 37, 4617– 4625,  DOI: 10.1021/es030038m
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        Assessing the Ecotoxicity of Pesticide Transformation Products
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        Environmental Science and Technology (2003), 37 (20), 4617-4625CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
        Once released to the environment, pesticides may be degraded by abiotic and biotic processes. While parent compds. are assessed in detail in many regulatory schemes, the requirements for the assessment of transformation products are less well developed. This study was therefore performed to explore the relationships between the toxicity of transformation products and their parent compds. and to develop a pragmatic approach for use in the risk assessment of transformation products. Data were obtained on the properties and ecotoxicity of transformation products arising from a wide range of pesticides. Generally, transformation products were less toxic to fish, daphnids, and algae than their parent compd. In instances where a product was more toxic, the increase in toxicity could be explained by either (1) the presence of a pesticide toxicophore; (2) the fact that the product is the active part of a propesticide; (3) the product is accumulated to a greater extent than the parent compd.; or (4) the product has a more potent mode of action than the parent. On the basis of the findings, an approach has been proposed to est. the ecotoxicity of transformation products based on chem. structure and data on the toxicity of the parent compd. The assessments can be performed at an early stage in the risk assessment process to identify those substances that require further testing.
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        Lindsey, B. D.; Jurgens, B. C.; Belitz, K. Tritium as an Indicator of Modern, Mixed, and Premodern Groundwater Age; Scientific Investigations Report 2019–5090; U.S. Geological Survey, 2019.
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        Data from 11000 public supply wells in 87 study areas were used to assess the quality of nearly all of the groundwater used for public supply in California. Two metrics were developed for quantifying groundwater quality: area with high concns. (km2 or proportion) and equiv.-population relying upon groundwater with high concns. (no. of people or proportion). Concns. are considered high if they are above a human-health benchmark. When expressed as proportions, the metrics are area-weighted and population-weighted detection frequencies. On a statewide-scale, ∼20% of the groundwater used for public supply has high concns. for one or more constituents (23% by area and 18% by equiv.-population). On the basis of both area and equiv.-population, trace elements are more prevalent at high concns. than either nitrate or org. compds. at the statewide-scale, in eight of nine hydrogeol. provinces, and in about three-quarters of the study areas. At a statewide-scale, nitrate is more prevalent than org. compds. based on area, but not on the basis of equiv.-population. The approach developed for this paper, unlike many studies, recognizes the importance of appropriately weighting information when changing scales, and is broadly applicable to other areas.
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