Ground-Water Quality in Quaternary Deposits of the Central High Plains Aquifer, South-Central Kansas, 1999

The mission of the U.S. Geological Survey (USGS) is to assess the quantity and quality of the earth resources of the Nation and to provide information that will assist resource managers and policymakers at Federal, State, and local levels in making sound decisions. Assessment of water-quality conditions and trends is an important part of this overall mission.

One of the greatest challenges faced by water-resources scientists is acquiring reliable information that will guide the use and protection of the Nation's water resources. That challenge is being addressed by Federal, State, interstate, and local water-resource agencies and by many academic institutions. These organizations are collecting water-quality data for a host of purposes that include compliance with permits and water-supply standards; development of remediation plans for specific contamination problems; operational decisions on industrial, wastewater, or water-supply facilities; and research on factors that affect water quality. An additional need for water-quality information is to provide a basis on which regional and national-level policy decisions can be made. Wise decisions must be based on sound information. As a society we need to know whether certain types of water-quality problems are isolated or ubiquitous, whether there are significant differences in conditions among regions, whether the conditions are changing over time, and why these conditions change from place to place and over time. The information can be used to help determine the efficacy of existing water-quality policies and to help analysts determine the need for and likely consequences of new policies.

To address these needs, the U.S. Congress appropriated funds in 1986 for the USGS to begin a pilot program in seven project areas to develop and refine the National Water-Quality Assessment (NAWQA) Program. In 1991, the USGS began full implementation of the program. The NAWQA Program builds upon an existing base of water-quality studies of the USGS, as well as those of other Federal, State, and local agencies. The objectives of the NAWQA Program are:

• Describe current water-quality conditions for a large part of the Nation's freshwater streams, rivers, and aquifers.
  
• Describe how water quality is changing over time.
• Improve understanding of the primary natural and human factors that affect water-quality conditions.

The goals of the NAWQA Program are being achieved through ongoing and proposed investigations of 59 of the Nation's most important river basins and aquifer systems, which are referred to as study units. These study units are distributed throughout the Nation and cover a diversity of hydrogeologic settings. More than two-thirds of the Nation's freshwater use occurs within the 59 study units and more than two-thirds of the people served by public water-supply systems live within their boundaries.

National synthesis of data analysis, based on aggregation of comparable information obtained from the study areas, is a major component of the program. This effort focuses on selected water-quality topics using nationally consistent information. Comparative studies will explain differences and similarities in observed water-quality conditions among study units and will identify changes and trends and their causes. The first topics addressed by the national synthesis are pesticides, nutrients, volatile organic compounds, and aquatic biology. Discussions on these and other water-quality topics will be published in periodic summaries of the quality of the Nation's ground and surface water as the information becomes available.

This report is an element of the comprehensive body of information developed as part of the NAWQA Program. The program depends heavily on the advice, cooperation, and information from many Federal, State, interstate, tribal, and local agencies and the public. The assistance and suggestions of all are greatly appreciated.

Robert M. Hirsch
Chief Hydrologist

Water samples from 20 randomly selected domestic water-supply wells completed in the Quaternary deposits of south-central Kansas were collected as part of the High Plains Regional Ground-Water Study conducted by the U.S. Geological Survey's National Water-Quality Assessment Program. The samples were analyzed for about 170 water-quality constituents that included physical properties, dissolved solids and major ions, nutrients and dissolved organic carbon, trace elements, pesticides, volatile organic compounds, and radon. The purpose of this study was to provide a broad overview of ground-water quality in a major geologic subunit of the High Plains aquifer.

Water from five wells (25 percent) exceeded the 500-milligrams-per-liter of dissolved solids Secondary Maximum Contaminant Level for drinking water. The Secondary Maximum Contaminant Levels of 250 milligrams per liter for chloride and sulfate were exceeded in water from one well each. The source of these dissolved solids was probably natural processes.

Concentrations of most nutrients in water from the sampled wells were small, with the exception of nitrate. Water from 15 percent of the sampled wells had concentrations of nitrate greater than the 10-milligram-per-liter Maximum Contaminant Level for drinking water. Water from 80 percent of the sampled wells showed nitrate enrichment (concentrations greater than 2.0 milligrams per liter), which is more than what might be expected for natural background concentrations. This enrichment may be the result of synthetic fertilizer applications, the addition of soil amendment (manure) on cropland, or livestock production.

Most trace elements in water from the sampled wells were detected only in small concentrations, and few exceeded respective water-quality standards. Only arsenic was detected in one well sample at a concentration (240 micrograms per liter) that exceeded its proposed Maximum Contaminant Level (5.0 micrograms per liter). Additionally, one concentration of iron and two concentrations of manganese were larger than the Secondary Maximum Contaminant Levels of 300 and 50 micrograms per liter, respectively. Some occurrences of trace elements may have originated from human-related sources; however, the generally small concentrations that were measured probably reflect mostly natural sources for these constituents.

A total of 47 pesticide compounds from several classes of herbicides and insecticides that included triazine, organophosphorus, organochlorine, and carbamate compounds and three pesticide degradation products were analyzed in ground-water samples during this study. Water from 50 percent of the wells sampled had detectable concentrations of one or more of these 47 compounds. The herbicide atrazine and its degradation product deethylatrazine were detected most frequently (in water from eight and nine wells, respectively); other pesticides detected were the insecticides carbofuran (in water from one well) and diazinon (in water from one well), and the herbicide metolachlor (in water from two wells). However, all concentrations of these compounds were small and substantially less than established Maximum Contaminant Levels.

The use of pesticides in crop production probably is largely responsible for the occurrence of pesticides in the ground-water samples collected during this study. Although concentrations of detected pesticides were small (relative to established Maximum Contaminant Levels), the synergistic effect of these concentrations and long-term exposure to multiple pesticides on human health are unknown.

Water samples from the Quaternary deposits were analyzed for 85 volatile organic compounds. Water from two wells (10 percent) had a detectable concentration of a volatile organic compound. Chloroform was detected at concen-trations of 0.18 and 0.25 microgram per liter, substantially less than the 100-microgram-per-liter Maximum Contaminant Level for total trihalomethanes. In general, the occurrence and detection of volatile organic compounds were infrequent, detectable concentrations were substantially less than established Maximum Contaminant Levels, and the areal distribution of volatile organic compounds in ground water was extremely limited.

Concentrations of radon were detected in water from every well sampled and ranged from 200 to 590 picocuries per liter with a median concentration of 260 picocuries per liter. Radon concentrations in water from 30 percent of the wells were larger than the proposed 300-picocuries-per-liter Maximum Contaminant Level.

Knowledge of the quality of the Nation's water resources is important because of implications for human and aquatic health and because of the substantial costs associated with land and water management, conservation, and regulation. In 1991, the U.S. Geological Survey (USGS) began full implementation of the National Water-Quality Assessment (NAWQA) Program. The long-term goals of the NAWQA Program are to describe the status and trends in the quality of the Nation's surface- and ground-water resources and to determine the natural and human-related factors affecting water quality (Gilliom and others, 1995). More than 50 major river basins or aquifer systems have been identified for investigation by the NAWQA Program. Together, these basins and aquifer systems include water resources available to more than 60 percent of the population and cover about one-half of the land area in the conterminous United States.

NAWQA ground-water studies include a component designed to assess the occurrence and distribution of water-quality constituents for regionally important aquifer systems. Individual studies of ground-water quality are completed in smaller hydrogeologic subunits within a regional aquifer. Completion of multiple subunit surveys within a single regional aquifer systematically evaluates the quality of water resources for an entire study area.

The cooperation of many individuals and organizations was essential for the completion of this study. The authors appreciate the permission granted by well owners for collection of water samples. Also acknowledged are: the Kansas Geological Survey (Lawrence, Kansas) for sharing drillers' logs and water-well data; Mike Dealy, Equus Beds Groundwater Management District No. 2 (Halstead, Kansas), and Sharon Falk, Big Bend Groundwater Management District No. 5 (Stafford, Kansas), for sharing their information and experience pertaining to the study area; Mike Carlson and Carol Carlson (USGS) for collection of water-quality samples; Lesley Red Boy (USGS) for data processing; and Lanna Combs, Mike Kemppainen, Jeff Hartley, and Kristi Hartley (USGS) for editorial review, preparation of illustrations, and processing of this report.

The hydrogeologic setting of the study area has been described in detail by Williams and Lohman (1949), Fader and Stullken (1978), Gutentag and others (1984), and Stullken and others (1987). Generally, Quaternary deposits in the study area consist of unconsolidated sand and gravel with thin, interspersed layers of silt and clay.

Water in the Quaternary deposits is derived mainly from precipitation and irrigation return flow although the density of irrigation wells in south-central Kansas is much less than in other areas of the High Plains aquifer (Gutentag and others, 1984). The Quaternary deposits were deposited by fluvial (stream-deposited) and eolian (windblown) processes during the past 2 million years. The ground water is unconfined, and regional flow is from west to east. The thickness of saturated material ranges from less than 100 to about 400 ft with potential well yields ranging from less than 250 to more than 750 gal/min (Gutentag and others, 1984).

The study design of ground-water quality in Quaternary deposits in south-central Kansas followed the NAWQA protocols described by Gilliom and others (1995) and Lapham and others (1995). Delineation of the study-area boundaries was based on work completed as part of the USGS Regional Aquifer-System Analysis (RASA) Program (Sun and Johnston, 1994). The High Plains RASA, which described the quantity and movement of ground-water resources in the High Plains, determined the boundaries of the High Plains aquifer and the various hydrogeologic subunits included in the aquifer (Gutentag and others, 1984). The boundary of the Quaternary deposits in south-central Kansas was taken from Gutentag and others (1984), with the boundary drawn to include nonglacial deposits in areas where they do not overlie aquifer units of Tertiary age.

Within the study area, potential ground-water sampling sites were selected using a grid-based random site-selection computer program (Scott, 1990). Twenty potential sampling sites were required for that part of the Quaternary deposits where saturated sediment existed (about 7,000 miî). One primary and one alternate site were randomly selected within the equal-area polygons containing each of the 20 sites. The alternate site was identified in the event that no well suitable for sampling could be identified within a 4-mi radius of the primary site. However, none of the alternate sites were needed for this study. The distribution of potential sampling locations represented a density of one primary site for each 350 miî. The average distance between primary sites was about 19 mi.

Domestic wells suitable for sampling were identified near each primary site to achieve a distribution of sampling wells throughout the study area. Domestic wells were chosen for this study because domestic wells (1) provide excellent areal and depth coverage in the Quaternary deposits; (2) are, generally, easily accessible under all weather conditions; and (3) have low-capacity pumping rates that limit the potential for withdrawal of water from formations other than the Quaternary deposits. A well was considered suitable for sampling if (1) it was within a 4-mi radius of the randomly selected site; (2) it was a domestic well that was actively being used as a household drinking-water supply; (3) it could be determined from a drilling log or other means that the well was indeed completed in Quaternary deposits; (4) substantial information could be obtained indicating the construction details of the well, including well depth, screened interval, and casing information; and (5) it was possible to collect a water sample prior to any type of water treatment or storage, such as filtration or residence in a pressure tank or cistern.

The 20 domestic water-supply wells were sampled from May 26 to June 29, 1999, for this assessment of ground-water quality. Each well was sampled once. Ground-water samples were collected and processed in a mobile water-quality laboratory mounted on a four-wheel-drive truck. Existing submersible pumps, permanently installed in each domestic well, were used to deliver water to the land surface. In most cases, a permanent spigot or hydrant near the wellhead was used as the water-quality sampling point. Occasionally, plumbing connections near the wellhead were modified temporarily for sampling purposes. All materials in contact with the water sample after the existing plumbing consisted of either stainless steel or Teflon.

Sampling protocols followed during this study are described in detail in Koterba and others (1995). To minimize the risk of sample contamination, all sample collection and preservation took place in dedicated environmental chambers consisting of clear polyethylene bags supported by tubular polyvinyl chloride (PVC) frames. Sampling equipment extending from the permanent sampling point near the wellhead to the sampling chamber inside the mobile laboratory, was decontaminated thoroughly between each sample collection using a progression of nonphosphate detergent wash, tap-water rinse, methanol rinse, and final deionized-water rinse. Polyethylene bags forming the sample and preservation chambers were replaced between each sample collection.

Quality-control data to test sample collection, processing, and analysis were collected at a frequency of about 30 percent of the environmental ground-water samples collected from wells. Quality-control samples included field blank samples, replicate environmental samples, and field-matrix spike samples. Field-blank samples verified that decontamination procedures were adequate and that onsite and laboratory protocols did not contaminate the samples. Replicate samples assess the combined effects of onsite and laboratory procedures on measurement variability. Matrix-spiked samples were analyzed to test for bias and variability from ground-water matrix interference or degradation of constituent concentration during sample processing, storage, and analysis.

Field-blank samples were analyzed for concentrations of major ions, nutrients and dissolved organic carbon, trace elements, pesticides, and VOC's. The source solution for field-blank samples was specially prepared organic-free or inorganic-free water provided by the USGS NWQL for the NAWQA Program. Field-blank solution was passed through all sampling equipment, and samples were collected using the same protocols as for environmental samples.

Ancillary data were collected for all wells from which water-quality data were obtained. The ancillary data included selected features or conditions of the sampling site, the well, the subsurface at the well, and the landscape and land-management activities in the vicinity of the well. These data subsequently become part of the NAWQA National Data Base Archive. Nationally consistent and quantitative methods for the collection, documentation, and compilation of ancillary data for this study are presented in Koterba (1998).

Ancillary data were divided into two parts and are maintained in two separate data bases. Site, well, and subsurface data are a part of the USGS National Water Information System-Ground-Water Site Inventory (GWSI) data base. Land-use, land-management, and other required landscape data are a part of a new Land-Use and Land-Cover Field Sheet (LULCFS) data base of the NAWQA Program.

The required GWSI data for NAWQA wells are listed in Koterba (1998, table 1, p. 5-8). These data include precise location information, well-construction details, and descriptions of the hydrogeologic unit and lithologic materials in which the well is installed. Because the study of Quaternary deposits in south-central Kansas used existing domestic water-supply wells, the GWSI data base was populated to the extent possible using information recorded on drillers' logs and well-construction records.

Data requirements for the LULCFS data base were designed to document land-surface activities that might affect ground-water quality. These data characterize potential point sources of ground-water contamination and areally extensive land-use practices with possible nonpoint-source effects. Nonpoint-source activities might include large areal applications of water or chemicals such as used in agricultural areas, or extensive industrial, commercial, or residential urban environments. These data generally were recorded onsite at the time of water-sample collection through observations of the area surrounding the well site.

The USEPA has established drinking-water standards for physical properties and chemical constituents that may have adverse effects on human health or that may affect the odor, appearance, or desirability of water (U.S. Environmental Protection Agency, 2000a). A Maximum Contaminant Level (MCL) is the maximum permissible level for a contaminant in drinking water that is delivered to any user of a public-water system. A Secondary Maximum Contaminant Level (SMCL) is a nonenforceable Federal guideline regarding aesthetic effects of drinking water. Ground-water quality in the Quaternary deposits of the central High Plains aquifer in south-central Kansas is discussed in the following sections in relation to these USEPA standards.

Dissolved solids are an important indicator of water quality and, in uncontaminated ground water, are the result of natural dissolution of rocks and minerals. Dissolved solids are also an important indicator of the suitability of water for drinking, irrigation, and industrial use. Although drinking water containing more than 500 mg/L dissolved solids (USEPA SMCL) is undesirable, such water is used in many areas where less mineralized water is not available. Dissolved solids in irrigation water may adversely affect plants directly by the development of high osmotic conditions in the soil solution and by the presence of phytoxins.

Nutrients, containing the elements nitrogen and phosphorus, are essential for the growth and reproduction of plants. Compounds of nitrogen and phosphorus, such as synthetic fertilizers and manure, commonly are applied to cropland and some pastures to stimulate plant growth and increase crop yields and hay production. Synthetic fertilizers include anhydrous ammonia, ammonium nitrate, urea, and mono- and diammonium phosphates.

Synthetic fertilizer use in Kansas more than tripled from 1965 to 1998, from about 640,000 to about 2,100,000 tons (Kansas Department of Agriculture and U.S. Department of Agriculture, 1999), and similar increasing trends are likely in the area underlain by Quaternary deposits in south-central Kansas. Farm livestock also produce large quantities of phosphorus- and nitrogen-rich organic wastes (urine and manure) that contribute to nonpoint sources of nutrients. Leachate from these organic wastes can infiltrate into the shallow ground-water system and potentially contaminate drinking-water sources. Other sources of nutrients include human waste, nitrogen-containing organic compounds, and industrial waste. Large concentrations of nutrients (particularly nitrates) in drinking water may be physiologically damaging to humans.

Human health-based regulations have been established for nitrate concentrations in drinking water because of the potential adverse health effects on infants. Consumption of drinking water with nitrate concentrations larger than 10 mg/L can cause methemoglobinemia (blue-baby syndrome) in infants, a sometimes fatal illness related to the impairment of the oxygen-carrying ability of the blood (U.S. Environmental Protection Agency, 1986). Accordingly, an MCL of 10 mg/L of nitrate as nitrogen has been established by the USEPA and implemented at the State level by the Kansas Department of Health and Environment (1994).

Nitrite and nitrate are inorganic ions produced during various stages of the nitrogen cycle. In most oxygenated water, nitrate is the predominate ion because of rapid oxidation of nitrite (Reid and Wood, 1976, p. 235). Nitrite and nitrate usually occur in relatively small concentrations in uncontaminated water, and concentrations in the range of several milligrams per liter suggest contamination from human activities. Mueller and Helsel (1996) estimated a national average background concentration of nitrite in ground water of 2.0 mg/L as nitrogen, which indicates that concentrations larger than 2.0 mg/L may be affected by contamination from human activities.

Pesticides are a group of compounds used to control unwanted plants or animals. Pesticides are applied primarily to cropland in rural areas, but also to lawns, rights-of-way, and gardens in urban areas. The widespread use of pesticides creates the potential for the movement of pesticides or their degradation products into shallow, unconfined aquifers. The presence of pesticides in ground water is a human-health concern for those using ground water as a drinking-water supply. In sufficiently large concentrations and (or) prolonged exposure, pesticides can cause human-health problems ranging from kidney and nerve damage to leukemia and other cancers (U.S. Environmental Protection Agency, 1989).

Volatile organic compounds (VOC's) are a group of relatively low molecular-weight hydrocarbons characterized by the ability to volatilize at low environ-mental temperatures, with high aqueous solubility, mobility, resistance to degradation, and the potential for some to have carcinogenic effects. The use or production of these compounds in industrial, manufacturing, and agricultural activities is the greatest potential source for VOC contamination of ground-water resources.

VOC's are by-products or components in the production of food, drugs, paints and varnishes, deodorants, pesticides, fumigants, glues and adhesives, rubber, cleaning agents, degreasers, disinfectants, dyes, perfumes, and many other materials. One of the major sources of VOC's is as components in fuels and motor oil. Because of the diverse uses and distribution of VOC's, the potential for ground-water contamination is greatest in industrial or commercial areas as a result of improper disposal of industrial waste, accidental chemical spills, leaking gasoline storage tanks, or seepage from toxic-waste dumps or landfills. However, the land application of some pesticides in crop production and the use of fumigants during grain storage create the potential for movement of associated VOC's to shallow, unconfined aquifers in agricultural areas.

The presence of VOC's in ground water may pose a human-health concern for those using ground water as a drinking-water supply. In sufficiently large concentrations and (or) prolonged exposure, VOC's can cause human-health problems ranging from kidney and nerve damage to leukemia and other cancers (U.S. Environmental Protection Agency, 2000c).

Radon is a naturally occurring radioactive gas produced by the decay of the element radium. Radon, which emits ionizing radiation, occurs in drinking water and indoor air. Radon emitted by soil under homes is the primary source of radon in indoor air. Breathing radon from indoor air in homes poses a substantial public-health risk, and as a result of radon exposure, an estimated 20,000 lung-cancer deaths occur each year in the United States (U.S. Environmental Protection Agency, 1999).

Radon from drinking water is a lesser source of radon to indoor air. Only about 1 to 2 percent of radon in indoor air comes from drinking water (U.S. Environmental Protection Agency, 1999). However, this additional source of radon to indoor air may, over a lifetime, increase the risk of lung cancer. Consumption of radon in contaminated drinking water also may increase the risk of cancers of internal organs, primarily stomach cancer. Currently (2000), no MCL has been established for radon in drinking water; however, a proposed MCL of 300 pCi/L (picocuries per liter) for radon is under review by USEPA (U.S. Environmental Protection Agency, 1999).

Knowledge of the quality of the Nation's water resources is important because of the implications to human and aquatic health and because of substantial costs associated with land and water management, conservation, and regulation. In 1991, the U.S. Geological Survey began full implementation of the National Water-Quality Assessment (NAWQA) Program. The long-term goals of the NAWQA Program are to describe the status and trends in the quality of the Nation's surface- and ground-water resources and to determine the natural and human-related factors affecting water quality.

The High Plains Regional Ground-Water Study was begun in June 1998 and represents a modification of the traditional NAWQA design in that the ground-water resource is the primary focus of investigation. The High Plains aquifer is a nationally important water resource that underlies about 174,000 miî in parts of eight Western States. About 20 percent of agricultural land in the United States is in the High Plains, and about 30 percent of all the ground water used for irrigation in the United States is pumped from this aquifer.

The Quaternary deposits of south-central Kansas constitute a 7,000-miî hydrogeologic subunit of the High Plains aquifer and provide water for irrigation and serve as a major drinking-water supply for the area. Generally, the deposits consist of unconsolidated sand and gravel with thin, interspersed layers of silt and clay. Water in the deposits is derived mainly from recharge of precipitation and irrigation return flow, although the density of irrigation withdrawals from Quaternary deposits in south-central Kansas is much less than from other areas of the High Plains aquifer. Water in the Quaternary deposits is vulnerable to effects from land-surface activities.

Land use overlying the Quaternary deposits of south-central Kansas is typical of this major agricultural region in the Midwestern United States. Cropland and grassland (pasture and rangeland) dominate the area. Major crops include corn, sorghum, soybeans, sunflowers, and wheat. Grassland in the area generally is used, directly or indirectly, for livestock production. Much of the grassland is pasture or rangeland used for cattle and calf production. Areas not used for pasture typically are used for hay production, which is subsequently used as winter feed for cattle on rangeland or in confined feeding operations.

Water samples from 20 randomly selected domestic water-supply wells were analyzed for about 170 water-quality constituents including physical properties, dissolved solids and major ions, nutrients and dissolved organic carbon, trace elements, pesticides, volatile organic compounds (VOC's), and radon. Many of the constituents from these compound classes are regulated in public drinking-water supplies by the U.S. Environmental Protection Agency (USEPA). Sampled well depths ranged from 50 to 160 ft. Depths to water ranged from about 12 to about 93 ft below land surface, with a median depth of about 30 ft.

Physical properties were measured in water from each of the 20 domestic water-supply wells sampled during this study. Measurements made at the time of sample collection included specific conductance, pH, water temperature, turbidity, dissolved oxygen, and alkalinity. Water from most wells had specific conductance values less than 800 ÎS/cm (microsiemens per centimeter at 25 ÷C). Water from only three wells had specific conductance values greater than 800 ÎS/cm, the largest of which was 1,760 ÎS/cm. The pH of water from all sampled wells was within the Secondary Maximum Contaminant Level (SMCL) range of 6.5 to 8.5 standard units with the exception of water from one well which had a pH of 6.4. The turbidity of water from all wells was low, which indicated acceptable aesthetic qualities, a lack of obvious contamination, and that the wells were developed properly and the samples were representative of the aquifer water. Water from most of the wells was well oxygenated as indicated by a median dissolved-oxygen concentration of 5.1 mg/L (milligrams per liter). Water from two wells, however, had unusually small dissolved-oxygen concentrations (0.2 and 0.1 mg/L). Alkalinity as CaCO3 ranged from 60 to 320 mg/L.

Concentrations of dissolved solids in water from the 20 sampled wells ranged from 143 to 1,060 mg/L. The median dissolved-solids concentration was 318 mg/L, substantially less than the 500-mg/L SMCL. However, water from five wells did exceed this secondary standard and probably is the result of dissolved solids from natural processes.

Generally, the data collected for this study indicate that calcium bicarbonate type water is the dominant ground-water type in the Quaternary deposits of south-central Kansas. However, this water chemistry can be modified locally by the introduction of sodium chloride type water, a mixed-water type of calcium bicarbonate sulfate and sodium chloride, or a calcium and magnesium sulfate type water. The SMCL's (250 mg/L) for chloride and sulfate were exceeded in water from one well each. The source of this chloride and sulfate was probably from natural processes.

Concentrations of most nutrients in water from the 20 sampled wells were small, with the exception of nitrate. Concentrations of nitrate as nitrogen ranged from less than 0.05 to 15 mg/L, with a median concentration of 6.2 mg/L. Water from 15 percent of the sampled wells had concentrations of nitrate greater than the 10-mg/L drinking-water Maximum Contaminant Level (MCL). Generally, there is an overall enrichment of nitrate in ground water throughout the Quaternary deposits in the study area. This enrichment probably is the result of human-related activities. Water from 80 percent of the sampled wells showed nitrate enrichment, which is more than what might be expected for natural background concentrations (2.0 mg/L). This enrichment may be the result of synthetic fertilizer applications, the addition of soil amendments (manure) on cropland, or livestock production.

Concentrations of dissolved organic carbon (DOC) were small in water from the 20 sampled wells. Concentrations ranged from 0.3 to 1.5 mg/L. However, DOC was detected at a mean concentration of 0.4 mg/L in two field-blank samples, which indicates that either sample-collection and (or) processing procedures contaminated the samples or there was an inherent bias in the analytical methods.

Most detected trace elements in water from the 20 sampled wells occurred only in small concentrations and were substantially less than established MCL's. The trace elements antimony, beryllium, cadmium, cobalt, lead, and silver were not detected in water from any of the wells sampled during this study. Infrequent detections of chromium (two detections), manganese (two detections), and nickel (three detections) were noted. Copper, molybdenum, selenium, uranium, and zinc were detected more frequently, but none of the detected concentrations were larger than established or proposed drinking-water standards. In contrast, aluminum and barium were detected in water from all 20 wells.

Few trace-element concentrations exceeded respective drinking-water standards. Only arsenic was detected (in one well sample) at a concentration (240 Îg/L, micrograms per liter) that exceeds the proposed MCL (5.0 Îg/L). All other detected concentrations of arsenic were less than the proposed MCL. Additionally, one concentration of iron and two concentrations of manganese were larger than the respective SMCL's of 300 and 50 Îg/L. Exceedances of drinking-water standards for arsenic, iron, and manganese were associated with well locations where water in the Quaternary deposits was under reducing (nonoxygenated) conditions.

Some occurrences of trace elements in ground water in the study area could originate from human-related sources. However, the generally small concentrations that were measured probably reflect mostly natural sources for these constituents.

A total of 47 pesticide compounds from several classes of herbicides and insecticides that included triazine, organophosphorus, organochlorine, and carbamate compounds and three pesticide degradation products were analyzed in ground-water samples. Of the 20 wells sampled, water from 10 (50 percent) had detectable concentrations of one or more of these 47 compounds. The herbicide atrazine and its degradation product deethylatrazine were detected the most frequently (in water from eight and nine wells, respectively); other detected pesticides included the insecticides carbofuran (in water from one well) and diazinon (in water from one well), and the herbicide metolachlor (in water from two wells). All detected concentrations of atrazine and carbofuran were substantially less than their MCL's of 3.0 and 40 Îg/L, respectively; MCL's have not been established for deethylatrazine, diazinon, or metolachlor.

Agricultural activities, particularly those associated with crop production, probably are responsible for the occurrence of pesticides in ground water in the study area. Although concentrations of detected pesticides were small, the synergistic effect of these concentrations and long-term exposure to multiple pesticides on human health are unknown.

Water samples were analyzed for 85 volatile organic compounds (VOC's). Of the 20 wells sampled, water from three had detectable concentrations of one or more VOC's. Chloroform was the most frequently detected VOC (in two samples); styrene was detected in one sample. Analytical results from field-blank samples indicated that styrene was probably a contaminant associated with sample collection, processing, and (or) analytical procedures. In general, the occurrence and detection of VOC's in water from Quaternary deposits in south-central Kansas were infrequent, detectable concentrations were substantially less than established MCL's, and the areal distribution of VOC's in ground water was extremely limited.

Concentrations of radon were detected in water from every well sampled and ranged from 200 to 590 pCi/L (picocuries per liter), with a median concentration of 260 pCi/L. Although the median concentration of radon is less than the proposed MCL of 300 pCi/L, water from six wells (30 percent) contained radon in concentrations larger than this proposed standard.

 

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