Frequently asked questions

Additional information and data sets

1. About the National Assessment of VOCs

What is the purpose of this Web site?

This Web site is a supplement to a National assessment of volatile organic compounds (VOCs) published by the U.S. Geological Survey (USGS). It includes additional information, raw data, and graphics that could not be included in the report. The Web site also includes other USGS reports that explain the assessment’s approach and provides more details on occurrence findings for selected compounds.

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Why is it important to study VOCs?

Volatile organic compounds (VOCs) are contaminants of concern in ground water because (1) some VOCs have been detected in ground water in similar studies in the past, and it is important to examine whether there have been changes in their occurrence, (2) some VOCs are released to the environment in large volumes, and (3) some VOCs, once they have reached ground water, tend to persist and migrate to drinking-water wells.

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Which VOCs were studied?

A total of 55 volatile organic compounds (VOCs) were analyzed in this assessment, including VOCs from 7 usage groups: (1) fumigants, (2) gasoline hydrocarbons, (3) gasoline oxygenates, (4) organic synthesis compounds, (5) refrigerants, (6) solvents, and (7) trihalomethanes. For a complete list of VOCs emphasized in this assessment see Appendix 4 of Circular 1292.

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How many wells were sampled and in what areas?

Samples of ambient ground water from almost 3,500 wells in 98 aquifer studies were analyzed for volatile organic compounds (VOCs). The wells were located throughout the conterminous United States as well as Alaska and Hawaii. In addition, results of samples from about 2,400 domestic wells and more than 1,100 public wells were compiled to assess VOC occurrence. These domestic and public wells also were distributed nationwide.

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What is the scope of the National Assessment?

This national assessment of 55 volatile organic compounds (VOCs) in ground water focuses on VOC occurrence in aquifers that are used as an important supply of drinking water. The assessment of aquifers was designed as a large-scale resource assessment that provides a general characterization of water-quality conditions. A summary of results and conclusions have been outlined in an introductory section, Chapter 1, of the USGS Circular 1292.

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How were NAWQA aquifer studies selected?

Aquifer studies were selected by local National Water-Quality Assessment (NAWQA) Study Unit teams in collaboration with local organizations, States, and Tribes, as well as NAWQA National Synthesis Teams. In general, the aquifer (or portion thereof) was selected for study on the basis of its national and local significance as a source of drinking water. The sampled wells include new monitoring wells installed by NAWQA as well as existing monitoring, domestic, and public supply wells. Stratified random sampling techniques were used to select both new sampling sites and existing wells so that the results would more effectively characterize the aquifers.

The assessment did not focus on specific sites with known water-quality problems or narrowly defined "issues of the day," but rather on the condition of the total ambient resource.

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Did the NAWQA study characterize drinking water?

No, this study does not characterize drinking water. This assessment does characterize the quality of untreated, ambient ground water and ground water captured by drinking-water supply wells, and not the quality of finished, treated drinking water (as would be done by monitoring water delivered from water-treatment plants or from household taps). However, water from many of the wells sampled is used for domestic and public supply. By focusing on the quality of ground water in its present conditions (ambient water quality), the National Water-Quality Assessment (NAWQA) Program complements many Federal, State, and local compliance drinking-water monitoring programs.

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How many samples were collected at each site?

Although duplicate samples were taken for quality-control purposes, only one set of sampling results was used for each well because of the comparatively slow rate of change in most ground-water systems. Future studies will assess the trend of volatile organic compound (VOC) occurrence in selected aquifers.

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How are NAWQA data used by others?

Local, State, Tribal, and national stakeholders use National Water-Quality Assessment (NAWQA) information to design and implement strategies for managing, protecting, and monitoring ground-water resources in many different hydrogeologic and land-use settings across the Nation (http://water.usgs.gov/nawqa/xrel.pdf).

NAWQA data sets make up a unique collection of detailed water-quality information collected using consistent protocols across the Nation, and easily retrieved from a single online portal: the NAWQA Data Warehouse. The U.S. Geological Survey(USGS) has worked closely with the U.S. Environmental Protection Agency (USEPA) during the 10-year study. USEPA uses the data extensively in the development and updating of compounds included in the unregulated contaminant (UCM) program. Furthermore, USGS data have had a significant role in the discussion of national policy concerning gasoline oxygenates, especially methyl tert-butyl ether. As new chemicals are approved for use, USEPA will continue to need monitoring data for ambient water resources to ensure that levels in the environment are safe.

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Did USGS do a risk assessment on VOCs?

No. (See the entry below on Risk Assessment.)

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2. Questions related to VOCs in ground water

How did USGS develop the data sets used to assess VOCs in aquifers?

The study of aquifers is a large component of the National Water-Quality Assessment (NAWQA) Program’s ambient ground-water assessments. Aquifer studies have been completed in nearly every NAWQA Study Unit and have provided a comprehensive picture of the chemical quality of water in locally and regionally important aquifers. In addition, data from comparable studies with a similar design were included to expand the geographic coverage of the assessment. Samples from 3,498 wells in 98 aquifer studies that were collected during 1985-2002 were analyzed for 55 volatile organic compounds (VOCs). These analytical results were used to assess VOCs in aquifers.

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Were VOCs found in the studied aquifers?

Yes, volatile organic compounds (VOCs) were detected, but typically at low concentrations. About 19 percent of the ground-water samples from 3,498 wells in aquifer studies contained one or more VOCs at an assessment level of 0.2 microgram per liter (μg/L). At an assessment level of 0.02 μg/L, about 51 percent of the subset of samples from 1,687 wells contained one or more VOCs.

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How many different kinds of VOCs were detected in ground water?

Forty-two of the 55 volatile organic compounds (VOCs) analyzed for in the assessment were detected in one or more samples at a concentration of 0.2 microgram per liter (μg/L) or greater. The 55 VOCs were assigned to one of seven groups on the basis of their primary usage (or origin): (1) fumigants; (2) gasoline hydrocarbons, (3) gasoline oxygenates, (4) organic synthesis compounds, (5) refrigerants, (6) solvents, and (7) trihalomethanes (THMs), which are chlorination by-products. At least one VOC in each of the seven groups was detected in one or more samples. Solvents, THMs, gasoline hydrocarbons, and less frequently, refrigerants had a widespread distribution throughout the United States. Fumigants, gasoline oxygenates, and organic synthesis compounds were not as widely detected across the Nation.

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Which VOCs were found most frequently in ground water?

The three most frequently detected volatile organic compounds (VOCs) were chloroform, perchloroethene (PCE), and methyl tert-butyl ether (MTBE). Chloroform, which is a known by-product of water chlorination and also used as a solvent and other applications, was detected in 7.4 percent of samples at an assessment level of 0.2 microgram per liter (μg/L) and in 21 percent of samples at an assessment level of 0.02 μg/L. PCE, which is a solvent commonly used in dry cleaning, was detected in 3.7 percent of samples at an assessment level of 0.2 μg/L and in 8.3 percent of samples at an assessment level of 0.02 μg/L. MTBE is an octane enhancer that was first introduced in gasoline in 1979 and has been used in large quantities in reformulated gasoline since the mid-1990s. MTBE was detected in 2.8 percent of samples at an assessment level of 0.2 μg/L and in 7.0 percent of samples at an assessment level of 0.02 μg/L.

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At what concentrations were VOCs detected in ground water?

Concentrations for individual volatile organic compounds (VOCs) and total concentrations for all VOCs analyzed generally were low (less than 1 microgram per liter). See appendix 8 of Circular 1292 for concentration information for each VOC.

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What factors affect VOC occurrence in ground water?

The factors most commonly associated with volatile organic compounds (VOCs) in ground water were septic systems, urban land use, Resource Conservation and Recovery Act hazardous-waste facilities, gasoline storage and release sites, climatic conditions, type of soil, dissolved oxygen in ground water, type of well, and depth to top of well screen. These factors are associated with the source, fate, and transport of VOCs in ground water.

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Where were VOCs detected in ground water in the United States?

Volatile organic compounds (VOCs) were detected in aquifer studies completed across the United States, with detections commonly occurring in California, Nevada, Florida, and the New England and Mid-Atlantic States.

National maps showing the occurrence of VOCs are available online as part of the supplementary information released in support of USGS Circular 1292.

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How often do VOC mixtures occur?

Specific mixtures of volatile organic compounds (VOCs) in the 3,498 samples of ground water from aquifer studies occurred relatively infrequently at an assessment level of 0.2 microgram per liter (μg/L). The two most frequently detected VOC mixtures, perchloroethene–trichloroethene and perchloroethene–chloroform, occurred in 1.5 percent of the samples. Only one other mixture, trichloroethene–chloroform, occurred in more than 1 percent of the samples.

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What is the relation between VOC production and VOC occurrence?

Production of volatile organic compounds (VOCs) alone does not fully explain VOC occurrence. For example, if production rates of VOCs alone were the primary governing factor explaining detection frequencies, the gasoline hydrocarbons would be detected in aquifers much more frequently than the other six VOC groups, which is not the case. Possible reasons for the lack of correspondence between VOC production rates and detection frequency in aquifers include: (1) production data are not available for all VOCs in each group so actual production may be considerably higher than available estimates; (2) production is not necessarily an exact measure of a VOC source that is contributing a VOC to ground water; and (3) other factors such as the hydrogeologic setting, geochemistry of the ground water, and the transport and fate properties of VOCs, can control the occurrence of VOCs in aquifers.

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Do we have complete data on VOC releases?

No, we do not have complete data on volatile organic compound (VOC) releases. National data on total releases in the United States for all VOC groups are not available. Furthermore, there are no National data sets for certain sources such as septic systems and accidental transportation spills. Estimated VOC production data were used in the national assessment, but production data are not necessarily an exact measure of a VOC source that is contributing a VOC to ground water.

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3. Questions related to VOCs in samples from domestic and public supply wells

Are VOCs present in samples from wells used as sources for drinking water supplies?

A large number of drinking-water supply wells sampled prior to treatment or blending had no detections of volatile organic compounds (VOCs). One or more VOCs were detected in 14 percent of the 2,401 domestic well samples, and in 26 percent of the public well samples with concentrations equal to or greater than 0.2 microgram per liter (μg/L). Nearly one-half of 1,208 samples from a subset of the domestic wells had VOC detections using the low-level analytical method, for which an order-of-magnitude lower assessment level (0.02 μg/L) was applied. For more information, refer to USGS Circular 1292, Appendixes 8 and 10.

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What concentrations of VOCs were present in samples from domestic and public wells?

Most of the volatile organic compounds (VOCs) detected in well samples had a wide range of concentrations; however, nearly all of the concentrations were less than  U.S. Environmental Protection Agency’s drinking-water standards, and many concentrations were far below these standards. For more information, refer to USGS Circular 1292, Appendix 7.

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When detected, were there different kinds of VOCs in samples from domestic and public wells?

Forty-four of the 55 volatile organic compounds (VOCs) analyzed for in this assessment were detected in samples from domestic and (or) public wells at concentrations equal to or greater than 0.2 microgram per liter (μg/L). These VOCs were assigned to the following “groups” based on their primary use (or origin): gasoline hydrocarbons, gasoline oxygenates, organic synthesis compounds, refrigerants, solvents, and trihalomethanes (THMs). The 15 most frequently detected VOCs in domestic well samples represented all VOC groups except organic synthesis compounds. VOCs in public well samples were predominantly THMs, solvents, and gasoline oxygenates, primarily methyl tert-butyl ether (MTBE).

For more information, refer to Appendixes 8 and 10.

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Which VOCs were detected most frequently in samples from domestic and public wells?

The three most frequently detected volatile organic compounds (VOCs) were the same for samples from domestic and public wells: chloroform, methyl tert-butyl ether (MTBE), and perchloroethene (PCE)―at concentrations equal to or greater than 0.2 microgram per liter (μg/L). Chloroform, a known disinfection by-product, was detected in 5.2 and 11.4 percent of domestic and public well samples, respectively. The gasoline oxygenate MTBE was detected in 2.9 and 5.4 percent of domestic and public well samples, respectively. PCE, a solvent commonly used in dry-cleaning processes, was detected in 2.0 and 5.3 percent of domestic and public well samples, respectively.

For more information, refer to Appendixes 8 and 10.

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Are VOCs likely to co-occur in samples from domestic wells or public wells?

Volatile organic compounds (VOCs) co-occurred in some samples; however, mixtures occurred less frequently in domestic well samples (3.9 percent) than in public well samples (13.4 percent) at concentrations equal to or greater than 0.2 microgram per liter (μg/L). Furthermore, a single VOC occurred more frequently than multiple VOCs in domestic well samples, whereas detections of single VOCs and multiple VOCs were about equal in public well samples. Solvents, trihalomethanes (THMs), and the gasoline oxygenate methyl tert-butyl ether (MTBE) comprised the 10 most frequently detected VOCs co-occurring in domestic well samples. In contrast, all but 1 of the 10 most frequently occurring VOC mixtures in public well samples were THMs; the exception was the solvent mixture of perchloroethene (PCE) and trichloroethene (TCE). More information on this topic is included in USGS Circular 1292, Sidebar 24.

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What factors are related to the occurrence of VOCs in samples from domestic and public wells?

For both domestic and public wells, hydrogeologic factors associated with the likelihood of detecting volatile organic compounds (VOCs) included high aquifer recharge and high soil permeability. Anthropogenic factors included population density, percentage of urban land use near the sampled wells, and proximity of hazardous-waste facilities. Dissolved-oxygen concentration was an important factor associated with the occurrence of individual VOCs. For public well samples, increased MTBE occurrence was associated with local use of this oxygenate in gasoline. Public wells may be more vulnerable to VOCs than domestic wells due to large withdrawal rates, proximity to developed areas, larger capture zones, greater drawdown, and faster movement of water from the top of the water table to the well screen.

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Are VOCs in drinking-water supply wells more likely to occur in one area of the Nation than another area?

Volatile organic compounds (VOCs) with multiple uses and/or widespread sources, for example VOCs within the solvent, trihalomethane (THM), and gasoline hydrocarbon groups, were detected throughout the Nation. Gasoline oxygenates were detected most frequently in samples in the New England and Mid-Atlantic States. Fumigants primarily occurred in the Central Valley of California and in New Jersey, Arizona, and Washington. VOCs used in organic synthesis were seldom detected.

National maps showing the occurrence of VOCs are available online as part of the supplementary information released in s upport of USGS Circular 1292.

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What is meant by "regulated" VOCs in drinking water?

A “National Primary Drinking Water Regulation” (primary standard) is a legally-enforceable standard that applies to public water systems. Primary standards protect public health by limiting the levels of contaminants in drinking water. U.S. Environmental Protection Agency (USEPA) standards, commonly referred to as "Maximum Contaminant Levels (MCLs)" apply to 29 of the 55 VOCs included in the National Water-Quality Assessment (NAWQA) Program’s assessment. The Federal government generally issues "primacy" to the State; that is, the responsibility is delegated to the State by the USEPA for enforcing primary standards. Additionally, States may also have enforceable drinking-water regulations that are more stringent than Federal standards. These standards may vary among states; specific information can be obtained from state drinking-water programs.

Although the quality of water from domestic wells is generally the homeowner’s responsibility, most states and some local agencies do provide guidance to domestic well owners through Web sites and printed materials.

More information on this topic is included in (link to http://www.epa.gov/safewater/links.html

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What are U.S. Environmental Protection Agency’s Maximum Contaminant Levels?

Under the authority of the Safe Drinking Water Act, the U.S. Environmental Protection Agency (USEPA) establishes drinking-water standards, such as Maximum Contaminant Levels (MCLs), to limit the level of contaminants in the Nation’s drinking water. An MCL is a legally enforceable standard that sets the maximum permissible level of a contaminant in water that is delivered to any use of a public water system. An MCL is set as close to the maximum level of a contaminant in drinking water at which no known or anticipated adverse effect on human health would occur, and which allows an adequate margin of safety. For more information, refer to sidebar 17 of VOC Circular 1292 and http://www.epa.gov/safewater/creg.html.

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What is the purpose of the U.S. Environmental Protection Agency’s Contaminant Candidate List?

The Contaminant Candidate List (CCL), published by the U.S. Environmental Protection Agency (USEPA), is a list of unregulated contaminants that are known or anticipated to occur in public water systems and may require regulation with a national primary drinking-water standard. This list is USEPA's primary source of priority contaminants for the Agency's drinking-water program. The Unregulated Contaminant Monitoring (UCM) Program is the mechanism used to collect occurrence data for unregulated contaminants suspected to occur in drinking water. (More information on this topic is included in Circular 1292 in sidebar 21).

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How can I find out if VOCs are present in my drinking water?

If drinking water is supplied by a public water system (PWS), your water supplier has the results of all the tests that are performed on the water samples from the water system. Furthermore, PWSs may summarize the results of testing in a Consumer Confidence Report that is publicly available. Monitoring schedules differ according to the type of contaminant and the population that the PWS  serves. Volatile organic compounds (VOCs) may be among the contaminants that are monitored by your PWS. If drinking water is supplied by a privately owned domestic well, testing generally is initiated by the homeowner; however, specifications may vary by State. More information generally can be obtained through individual State drinking-water programs. The National Ground Water Association maintains a website, www.wellowner.org, that provides information for owners of domestic wells.

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4. What VOC Findings May Mean to Human Health

Do VOC detections automatically mean that harmful effects will occur?

No. The presence of "detectable" volatile organic compounds (VOCs) does not mean that harmful human-health effects will occur because detections were often at low concentrations that were well below benchmarks that are protective of human health. U.S. Geological Survey's analytical methods are designed to measure low levels of VOCs-sometimes parts per trillion. By this approach, the USGS national assessment helps to identify emerging issues and to establish a framework to track changes in concentrations over time, and it provides a comprehensive screen for the types of VOCs we can expect to see in different settings and areas of the country.

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Did USGS do a risk assessment on VOC occurrence?

No. The U.S. Geological Survey (USGS) report is not a risk assessment study, and findings should not be confused with risk assessments for specific VOCs by the U.S. Environmental Protection Agency. To place findings in a context for understanding what detections of VOCs may mean to human health, however, VOC concentrations measured by the National Water-Quality Assessment (NAWQA) Program were compared to either their regulatory Maximum Contaminant Levels (MCLs) or USGS-derived Health-Based Screening Levels (HBSLs) in a screening-level assessment. These human-health benchmarks are estimates of the concentrations below which adverse effects on humans are not expected to occur. The screening-level assessment is a first step toward understanding VOC occurrence in the context of human health and prioritizing further investigations (see next question).

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How were potential human-health effects assessed?

Potential human-health effects were not directly assessed in this study, but a screening level assessment was performed to identify and prioritize VOCs that may warrant further investigation. To help place occurrence data on volatile organic compounds (VOCs) in a human-health context, measured VOC concentrations were compared to the U.S. Environmental Protection Agency's (USEPA) Maximum Contaminant Levels (MCLs) and the U.S. Geological Survey's (USGS) Health-Based Screening Levels (HBSLs). Concentrations of regulated VOCs (those with MCLs) were compared with their MCLs, and concentrations of unregulated VOCs (those without MCLs) were compared with their HBSLs, when available. HBSLs are non-enforceable guidelines that were developed by the USGS in collaboration with USEPA and others using USEPA methodologies and the most current USEPA peer-reviewed, publicly available human-health risk assessments and toxicity information.

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What were the results of the screening level assessment for placing findings in the context of human health?

One sample from each of 3,497 domestic and public wells was analyzed for 55 volatile organic compounds (VOCs). At least one VOC was detected in about 15 percent of all well samples above a threshold of 0.2 part per billion. Eight of the 55 VOCs had concentrations greater than human-health benchmarks in about 1 percent of all drinking-water supply well samples; these concentrations may be of potential human-health concern if the water were to be ingested without treatment for many years. Most of the detections greater than benchmarks were attributed to the fumigant dibromochloropropane (DBCP) (in domestic wells only) and to the solvents trichloroethene (TCE) and perchloroethene (PCE) in samples from both domestic and public wells. VOC concentrations were less than human-health benchmarks in most of the well samples, indicating that adverse effects are unlikely to occur, even if water with such concentrations were to be ingested over a lifetime. Seventeen VOCs may warrant further investigation because their concentrations were greater than, or approached, human-health benchmarks. For more information, refer to USGS Circular 1292, Appendixes 9 and 11 and USGS Fact Sheet 2006-3043. Also see the web site, USGS Health Based Screening Levels.

What are the uncertainties and limitations to using a screening-level assessment to evaluate what the VOC findings may mean to human health?

Screening-level assessments (See USGS Fact Sheet 2006-3043) have several complexities and limitations. VOC concentrations less than human-health benchmarks such as USEPA Maximum Contaminant Levels and USGS Health-Based Screening Levels indicate that adverse effects are unlikely to occur, even if water with such concentrations were to be ingested over a lifetime. Water containing VOCs with concentrations greater than these benchmarks might be of potential human-health concern if the water were to be ingested as the primary drinking-water source without treatment for many years. The likelihood for adverse effects generally increases as the VOC concentrations increase above their benchmark values. If water containing concentrations greater than their benchmarks is ingested, however, it does not mean that adverse human-health effects will occur because:

• Human-health benchmarks are intentionally conservative (protective) and incorporate safety factors and conservative assumptions to account for uncertainty in the underlying toxicity information.
• Models used to develop human-health benchmarks assume lifetime exposure of 2 liters of tap water per day, whereas actual exposure may be lower and for less than a lifetime.
• The source water may be treated prior to ingestion, potentially reducing VOC concentrations.

A limitation of screening-level assessments is that some compounds analyzed by the National Water-Quality Assessment (NAWQA) Program do not have human-health benchmarks because of a lack of a peer-reviewed health risk assessments and toxicity information. Also, most benchmarks are based on toxicity information for individual chemicals, whereas NAWQA results indicate that mixtures of VOCs may occur in some samples. The long-term cumulative effects of low concentrations of multiple contaminants on human health currently are unknown.

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Were human-health benchmarks available for all VOCs?

No. Of the 55 volatile organic compounds (VOCs) analyzed in this study, 44 were detected in domestic and (or) public well samples. A total of 35 VOCs detected in these well samples have human-health benchmarks (27 Maximum Contaminant Levels and 8 Health-Based Screening Levels).

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How do I learn more about the potential health effects associated with VOCs?

The National Water-Quality Assessment (NAWQA) Volatile Organic Compounds (VOC) team has compiled a table that summarizes health effects of VOCs. The compilation provides existing cancer classifications of many VOCs and summarizes the most recent VOC health-effects information published by the U.S. Environmental Protection Agency (USEPA) or the Agency for Toxic Substances and Disease Registry (ATSDR). Data are available for 42 of the 55 VOCs emphasized in Circular 1292. USEPA’s Office of Ground Water & Drinking Water has developed technical fact sheets on VOCs. For regulated VOCs (those with Maximum Contaminant Levels), USEPA summarizes the potential health effects from ingestion of water at http://www.epa.gov/safewater/mcl.html. For most regulated compounds, this Web site links to consumer fact sheets that include information on why the compound is regulated, health effects information, how much of the chemical is produced, and what happens when the chemical is released into the environment.

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What are Health-Based Screening Levels (HBSLs), and how are they different from existing USEPA drinking-water guidelines?

Health-Based Screening Levels (HBSLs) (see USGS Fact Sheet 2005-3059, and the web site, USGS Health Based Screening Levels) are estimates of benchmark concentrations of contaminants in water below which adverse human-health effects are not expected to occur. HBSLs are non-enforceable guidelines that were developed by the U.S. Geological Survey (USGS) in collaboration with the U.S. Environmental Protection Agency (USEPA), the New Jersey Department of Environmental Protection, and the Oregon Health & Science University. HBSLs were calculated for unregulated compounds that do not have USEPA Maximum Contaminant Levels (MCLs) using a consensus approach that entailed (1) standard USEPA Office of Water methodologies (equations) for establishing Lifetime Health Advisory (LHA) and Risk-Specific Dose (RSD) values, and (2) the most current USEPA peer-reviewed, publicly available human-health risk assessments and toxicity information.

HBSLs were calculated to supplement existing Federal drinking-water standards (MCLs) and guidelines (LHAs and RSDs), which are not available for about one-half of the volatile organic compounds (VOCs), pesticides, and pesticide degradation products measured in water by the National Water-Quality Assessment (NAWQA) Program. HBSLs, therefore, provide a basis for a more comprehensive evaluation of contaminant-occurrence data in a human-health context. Because HBSLs were calculated using standard USEPA methodologies, they are equivalent to USEPA LHA and RSD values (when they exist), except for compounds for which more recent toxicity information has become available.

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5. Additional Research Needs

What new data would be needed to determine trends in the occurrence of VOCs?

Little information on trends of volatile organic compounds (VOCs) in ground water was gathered during the first cycle of sampling for the National Water-Quality Assessment (NAWQA) Program. However, the second cycle of NAWQA contains a component of study that involves re-sampling a subset of wells in many aquifer studies across the country. The re-sampling of wells will yield data on the occurrence of VOCs at specific wells, and within specific aquifers, within a temporal context. When completed, the re-sampling will provide data needed to determine trends in the occurrence of VOCs in ground water.

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What are the next steps in improving our understanding of VOCs in ground water and drinking-water supply wells?

There are many areas of study that would improve our understanding of volatile organic compounds (VOCs) in ground water and drinking-water supply wells. Perhaps the most important is increasing our knowledge of the sources of VOCs to ground water. In this assessment, many of the sources of VOCs to ground water were not clearly identified. Controlling the sources of VOCs to ground water is the most important aspect of managing the human effects  on aquifers and thereby protecting ground-water resources.

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How could the scope of NAWQA’s assessment be expanded to include other groups of contaminants in addition to VOCs?

Currently, the National Water Quality Assessment (NAWQA) Program’s water-quality results are mostly presented by various groups of contaminants, like volatile organic compounds (VOCs). Combining results for all groups together would provide a broader understanding of the quality of ground-water resources, and a more complete description could be obtained of all of the factors that cause or influence contaminants in ground water. This information could be used by well owners, water-resource managers, and scientists to most effectively control sources of contaminants to ground water, mitigate transport of contaminants to ground water, and understand the fate of contaminants in ground water.

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What additional ancillary data would improve the understanding of the occurrence, sources, transport, and fate of VOCs in ground water and drinking-water supply wells?

More data on releases of volatile organic compounds (VOCs) to the air, ground surface, or vadose zone would improve our ability to understand the occurrence and sources of VOCs. Many sources of VOCs in ground water are probably small spills, such as consumer and commercial releases, and many times these releases are not reported or are stored in national databases with minimal information. More data or higher resolution data on the hydrogeologic properties of the aquifers examined, such as recharge, flowpath directions, redox conditions, and soil properties, would improve our understanding of the transport and fate of VOCs. Much of this type of data was available for the initial analysis, but in many cases the information was low resolution, was extrapolated from large-scale surveys, or was estimated from surrogate variables.

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6. How to access more Information on VOCs

Where can the raw data used in this report be accessed?

You can retrieve data on volatile organic compounds (VOCs) associated with U.S. Geological Survey Circular 1292 by accessing: http://water.usgs.gov/nawqa/vocs/national_assessment/. These downloadable data sets include information collected in aquifer studies for the National Water-Quality Assessment (NAWQA) Program as well as other domestic and public well data sets gathered in support of the national assessment.

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Are there other sources of VOC data available online?

You also can download more National Water-Quality Assessment (NAWQA) data on volatile organic compounds (VOCs) and other chemical, biological, and physical characteristics for streams and ground water from the NAWQA Data Warehouse. The NAWQA Data Warehouse provides access to more than 11 million measurements of water quality, ecology, and hydrology from the 51 river basins and aquifers across the Nation studied by the NAWQA program through 2004, and additional data are being added to the site as it becomes available. The U.S. Geological Survey also provides VOC data not collected as part of the NAWQA program through the National Water Information System (http://waterdata.usgs.gov/nwis). The U.S. Environmental Protection Agency also distributes VOC data through STORET (http://www.epa.gov/storet).

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Where can the general public get information on individual chemicals?

Additional information on selected chemicals in the environment, including many volatile organic compounds (VOCs), can be found at the U.S. Environmental Protection Agency’s Chemical Fact Sheets Web site.

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What can I do as a homeowner?

It is important to recognize that homeowners supplied by public wells may receive treated or blended water from multiple sources. Treating and blending can reduce the amount of volatile organic compound (VOC) concentrations in the water before the distribution process. Drinking water from public water systems is monitored for regulated and unregulated VOCs. Homeowners can contact state drinking-water agencies to obtain test results from their public water-supply systems. The quality of water from domestic wells, however, might not undergo the same treatment as that public well supplies because these wells are not regulated by the Safe Water Drinking Act. Therefore, individuals relying on domestic wells for a drinking-water source may choose to have the quality of water periodically monitored for VOCs in their tap water. Treatment through certified point-of-use treatment units is an option available to the owner of a private well. State health agencies can provide assistance concerning drinking-water contamination to both homeowners using domestic wells and those using public-water supplies.

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How do I learn more about how drinking water is regulated?

Drinking water is regulated by many governmental agencies, from the local to Federal level. The U.S. Environment Protection Agency provides a summary of water and drinking-water regulations and related issues on their Web site.

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What is the meaning of a term used in the report?

The national assessment report includes a glossary that  gives a description of many terms used in the report. This glossary also is available online.

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Who can I contact at the U.S. Environmental Protection Agency for more information on VOCs?

Visit the U.S. Environmental Protection Agency’s (USEPA) home page for Ground Water and Drinking Water. This page includes information about volatile organic compounds (VOCs) and other chemicals regulated in drinking water.

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Are NAWQA national assessments available for other types of chemical compounds?

Yes. The National Water-Quality Assessment (NAWQA) Program is conducting similar national assessments for pesticides, nutrients and selected trace elements.

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How can I retrieve more information on NAWQA’s methodology and design?

More detailed information on the approach of this national assessment of volatile organic  compounds (VOCs) has been released in USGS Open-File Report 2005-1452. Please visit the National Water-Quality Assessment (NAWQA) home page for more information on the NAWQA Program on the overall methodology and design of NAWQA studies.

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