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PUBLICATIONS—Water-Resources Investigations  Report

Ground-Water Quality in Alluvial Aquifers in the Eastern Iowa Basins, Iowa and Minnesota

Water-Resources Investigations Report 00-4106

By Eric M. Sadorf and S. Michael Linhart

 

U.S. Geological Survey, Iowa City, IA


 

The full report is available in pdf.  Link to the pdf.


FOREWARD

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 based. 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:

This information will help support the development and evaluation of management, regulatory, and monitoring decisions by other Federal, State, and local agencies to protect, use, and enhance water resources.

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


CONTENTS

FIGURES

TABLES

CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS
Multiply By To obtain
inch (in) 2.54 square centimeter
feet (ft) 0.3048 meter
mile (mi) 1.609 kilometer
square mile (mi²) 2.590 square kilometer
pound (lb) 0.4536 kilogram
gallon (gal) 3.785 liter

Temperature in degrees Celsius (°C) can be converted to degrees
Fahrenheit (°F) as follows:
°F = 1.8(°C) + 32.
Sea level: In this report, "sea level" refers to the National Geodetic
Vertical Datum of 1929 (NGVD of 1929)-a geodetic datum derived
from a general adjustment of the first-order level nets of the United
States and Canada, formerly called Sea Level Datum of 1929.
Abbreviated water-quality units: Chemical concentrations are
given in metric units of milligrams per liter (mg/L) and micrograms
per liter (mg/L). Milligrams per liter and micrograms per liter are units
expressing the concentration of chemical constituents in solution as
mass (milligrams or micrograms) of solute per unit volume (liter) of
water. For concentrations less than 7,000 mg/L, the numerical value
of milligrams per liter is the same as for concentrations in parts per
million. The numerical value of micrograms per liter is the same as for
concentrations in parts per billion.
Radioactivity is expressed in picocuries per liter (pCi/L). A picocurie
is the amount of radioactivity that yields 2.22 radioactive disintegrations
per minute.
Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey.

Abstract

The quality of shallow alluvial ground water that is used for domestic supplies in the Wapsipinicon, Cedar, Iowa, and Skunk River Basins (Eastern Iowa Basins) is described. Water samples from 32 domestic-supply wells were collected from June through July 1998. This study of ground-water quality in alluvial aquifers in the Eastern Iowa Basins is part of the U.S. Geological Survey's National Water-Quality Assessment Program.

Calcium and bicarbonate were the dominant ions in solution, likely derived from the dissolution of carbonate minerals in the alluvial aquifer material. Concentrations of iron exceeded the U.S. Environmental Protection Agency Secondary Maximum Contaminant Level (300 micrograms per liter) for drinking water in 53 percent of the samples, and 50 percent of the samples exceeded the Secondary Maximum Contaminant Level for manganese (50 micrograms per liter). pH and alkalinity increased and sulfate concentrations decreased with increasing well depth.

Nitrite plus nitrate nitrogen was detected in 53 percent of the samples and exceeded the U.S. Environmental Protection Agency Maximum Contaminant Level of 10 milligrams per liter for drinking water in 13 percent of the samples. Nitrite plus nitrate nitrogen concentrations were negatively correlated with well depth and positively correlated with percentage of oxygen saturation. Ammonia plus organic nitrogen concentrations were positively correlated with well depth, and ratios of nitrite plus nitrate to ammonia were positively correlated with percentage of oxygen saturation.

The majority of samples, 72 percent, contained water recharged since the early 1950's. The recharge date of water was earlier in deeper wells. Nitrite plus nitrate and total pesticide concentrations were greater in more recently recharged water.

Eight pesticides and eight pesticide metabolites were detected in ground-water samples. Atrazine was the most commonly detected pesticide, and metolachlor ethanesulfonic acid was the most commonly detected metabolite. No pesticide detections exceeded U.S. Environmental Protection Agency drinking-water Maximum Contaminant Levels.

The effects of land use on ground-water quality also were examined. There was a positive correlation between percentage of land used for soybean production and concentrations of metolachlor, metolachlor ethanesulfonic acid, and metolachlor oxanilic acid in ground-water samples.

Data from this study and from previous studies in the Eastern Iowa Basins were compared statistically by well type (domestic, municipal, and monitoring wells). Well depths were significantly greater in domestic and municipal wells than in monitoring wells. pH, calcium, sulfate, chloride, and atrazine concentrations were significantly higher in municipal-well samples than in domestic-well samples. pH and sulfate concentrations were significantly higher in municipal-well samples than in monitoring-well samples. Ammonia was significantly higher in domestic-well samples than in monitoring-well samples, chloride was significantly higher in monitoring-well samples than in domestic-well samples, and fluoride was significantly higher in domestic-well samples than in municipal-well samples.

INTRODUCTION

Within the framework of the U.S. Geological Survey's (USGS) National Water-Quality Assessment (NAWQA) Program, study-unit surveys of ground water provide a broad assessment of the water quality of the major aquifer systems within a hydrologic basin (Gilliom and others, 1995, p. 26). The Eastern Iowa Basins NAWQA study unit (fig. 1) consists of four major river basins that drain approximately 19,500 mi² in eastern Iowa and southern Minnesota (Kalkhoff, 1994). The major rivers are the Wapsipinicon, Cedar, Iowa, and Skunk, which drain into the Mississippi River. The focus of the first study-unit survey of the Eastern Iowa Basins was an assessment of the water quality in the Silurian-Devonian and Upper Carbonate aquifers in the eastern part of the study unit (Savoca and others, 1999). Alluvial aquifers are the focus of this, the second study-unit survey. Forty-five percent of the ground-water withdrawals in the Eastern Iowa Basins originate from alluvial aquifers (E. Fischer, USGS, oral commun., June 15, 1999). Alluvial aquifers consist of varying thicknesses of sand, gravel, silt, and clay deposits that occur along most of the major streams and rivers. The presence of permeable materials and shallow depth to water in these aquifers increase the potential for contamination from surface activities.

Purpose and Scope

This report presents the results of an assessment of the water quality in alluvial aquifers that are used as sources of domestic water supplies within the Eastern Iowa Basins study unit and examines relations between ground-water quality and land use. Ground-water samples were collected from 32 domestic wells during June and July 1998. Onsite measurements were obtained for specific conductance, pH, water temperature, dissolved oxygen, and alkalinity. The samples were analyzed at USGS laboratories to determine concentrations of major ions, nutrients, trace metals, dissolved organic carbon (DOC), tritium, radon, pesticides and pesticide metabolites, and volatile organic compounds (VOC's). Additionally, data from previous studies were combined with data from this study to compare water quality from different types (domestic, municipal, and monitoring) of wells.

Previous Studies

Kross and others (1990) conducted the State-Wide Rural Well-Water Survey (SWRL) to assess the number of domestic-supply wells in Iowa affected by various environmental contaminants. They found that mean concentrations for most major ions increased or remained fairly constant with depth and that nitrate reduction and denitrification occurred with depth in ground-water systems in Iowa. Concentrations of pesticides and nitrate were relatively high in wells less than 100 ft deep. Wells less than 100 ft in depth accounted for 50 percent of the domestic-supply wells statewide, for 64 percent of the wells containing water with pesticide detections, and 89 percent of the wells with nitrate concentrations greater than the U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level (MCL) for drinking water (Kross and others, 1990).

Detroy and others (1988) reported a relation between the presence of detectable concentrations of nitrate and pesticides and decreasing well depth, principally in water from Iowa's surficial unconsolidated aquifers. Detroy and Kuzniar (1988) reported similar findings in the Iowa River alluvial aquifer. They found nitrate and herbicide concentrations were higher in water from shallower rather than deeper wells. They also suggested that surface water sometimes can be a source of nitrate and herbicides to underlying alluvial aquifers.

Kalkhoff and others (1992) suggested that agricultural chemical variation in alluvial aquifers may be the result of chemical input some distance from the well rather than leaching from the soil directly above the sampling point. Kelley and Mehrhoff (1993) investigated radon in municipal ground-water supplies in Iowa. Of 60 samples from alluvial aquifers, 73 percent exceeded the USEPA proposed MCL for radon in drinking water. Buchmiller (1994) indicated that small creeks may be possible sources of herbicide contamination to some alluvial aquifers in Iowa.

Kolpin and others (1996a) described water-quality data collected from wells in near-surface aquifers of the midcontinental United States from 1992 through 1994. Many of these wells were within the Eastern Iowa Basins study unit. About 63 percent of wells with water samples containing significant increases in herbicide concentrations during the period from 1992 through 1994 were located in areas that were affected by stream flooding. Herbicide metabolites were the most frequently detected human-related compounds. In near-surface aquifers of the midcontinental United States, pesticide metabolites were more prevalent than their parent compounds (Kolpin and others, 1996b).

Savoca and others (1997) investigated water quality in the Iowa River alluvial aquifer prior to conversion of agricultural land to wetlands as part of the Wetland Reserve Program. They found lower concentrations of pesticides in water from the deepest wells than in water from the shallowest wells.

Description of Study Unit

Areas within the Eastern Iowa Basins study unit underlain by alluvial aquifers constitute approximately 22 percent of the study unit. These aquifers in the study unit cover 4,260 mi⊃2. Agriculture is the dominant land use/land cover and is present across 93 percent of the study unit. Forests cover 5 percent of the study unit, and urban areas occupy 2 percent. Wetlands and water account for less than 1 percent of the study unit (U.S. Geological Survey, 1990). The population of the study unit is approximately 1,169,000 (Bureau of Census, 1990).

Mean annual precipitation from 1961 through 1990 ranged from 30 in. in the northwestern part of the study unit to 36 in. in the southeastern part (Wendland and others, 1992), with most of the precipitation occurring in the spring and summer (April-August). Snowfall has been recorded from September to May, with accumulations rarely exceeding 10 in. in 1 day.

Alluvial deposits, along with other unconsolidated (glacial-outwash and buried channel) deposits overlying the bedrock, have been referred to collectively as the surficial aquifer system (Olcott, 1992). Alluvial aquifers are in alluvial deposits located along the major water courses and adjacent flood plains (fig. 1). They consist mainly of fine- to coarse-grained sand and gravel, interbedded with less permeable silt and clay that have been deposited by streams (Wahl and others, 1978). Alluvial aquifers range in thickness from 30 to 100 ft, are thickest where they coincide with buried-channel aquifers, and are thinnest where bedrock is near the surface. Movement of ground water is usually toward the streams and accounts for most of the flow in streams (Anderson, 1983). Recharge to the alluvial aquifers typically occurs by infiltration of precipitation. Streamflow also can recharge the aquifers during times when river levels are higher than the adjacent ground-water levels. Because of their large permeability, alluvial aquifers usually have the largest yields of the surficial aquifer system. However, they also have great potential for aquifer contamination (Karsten and Burkart, 1984; Hoyer and Hallberg, 1991). More complete descriptions of the hydrogeology of the surficial aquifer system are given by Steinhilber and Horick (1970), Wahl and others (1978), Hoyer and Hallberg (1991), and Olcott (1992).

Acknowledgments

The authors thank the residents within the study unit for allowing the USGS to sample their wells. USGS employees Denise Montgomery and Mark Savoca collected much of the data presented in this report. Steve Kalkhoff, project chief for the Eastern Iowa Basins NAWQA study unit, and Mark Savoca provided guidance and support throughout the study.

STUDY DESIGN AND METHODS

This study was designed to provide a broad assessment of the ground-water quality in the alluvial aquifers within the Eastern Iowa Basins study unit. Ground-water samples were collected from June through July 1998 from 32 randomly selected domestic wells. Domestic wells were selected instead of municipal or monitoring wells to maintain consistency with other NAWQA study units that had completed their alluvial ground-water studies using domestic wells. Monitoring wells were not used due to the cost of installation.

Well Selection

Potential sampling sites were identified using a stratified-random-selection process as described by Scott (1990). For statistical purposes, at least 30 wells were required for the sampling effort. Onsite reconnaissance was conducted within a 1-mi radius of each potential site to determine if a suitable well could be found. Potential alternate sampling sites also were selected using the same random selection process in the event that wells near the primary site did not meet well-selection criteria. Well-selection criteria required that: (1) wells be for domestic use; (2) wells be completed in an alluvial aquifer; (3) permission to sample the wells could be obtained; (4) well depths are known; (5) wells are equipped with a submersible pump; and (6) a water sample could be obtained before a pressure tank or any treatment system. If a suitable well could not be found near the primary site, an onsite reconnaissance was conducted around the closest alternate site. Information about each well was obtained from interviews with well owners and from driller's logs provided by the driller.

In some instances, wells suitable for sampling could not be found at either the primary or secondary sites. This was typically due to difficulty in locating wells with submersible pumps. In this instance, wells were selected where a portable submersible pump could be used. In other instances, suitable wells completed in alluvial aquifers could not be located within the vicinity of either the primary or secondary sites. It then became necessary to look for wells along flood plains of nearby streams until a suitable well was located. Figure 1 and table 1 show the location and describe wells sampled for this study.

Sample Collection

Ground-water samples were collected from 32 wells during June and July 1998. Sample collection followed NAWQA protocols (Koterba and others, 1995). Static water levels were measured with an electric tape to the nearest 0.01 ft, when possible, before pumping each well. Most wells were sampled after three well-casing volumes were purged and onsite measurements of specific conductance, pH, water temperature, and dissolved oxygen (recorded 3 to 5 minutes apart) stabilized for five consecutive readings. In wells that had large diameters (large volume) and where a portable submersible pump was being used, three volumes of purge water were not always obtained. In those cases, the well was purged until the onsite measurements stabilized. Samples were collected for the analysis of major ions, nutrients, trace metals, DOC, tritium, radon, pesticides, pesticide metabolites, and VOC's.

Alkalinity, major ion, and nutrient samples were filtered onsite using 0.45-micron cartridge filters. Pesticide samples were filtered using 0.7-micron baked glass-fiber filters. DOC samples were filtered using 0.45-micron silver membrane filters. Tritium, radon, and VOC samples were not filtered. Major ion and VOC samples were acidified prior to shipping. Samples were chilled and shipped overnight to USGS analytical laboratories in Denver, Colorado, Menlo Park, California, and Lawrence, Kansas. All sampling and filtering equipment was decontaminated after each use using methods described in Koterba and others (1995).

Water-Quality Analysis

Major ions, nutrients, trace metals, DOC, radon, pesticides, and VOC's were analyzed at the USGS National Water-Quality Laboratory (NWQL) in Denver, Colorado. Tritium was analyzed at the USGS Isotope Tracers Project Laboratory in Menlo Park, California. Selected pesticides and pesticide metabolites were analyzed at the USGS Organic Geochemistry Research Laboratory in Lawrence, Kansas. Analytical methods and reporting limits used for each constituent are listed in tables 2, 3, and 4.

Results were reported relative to method detection limits (MDL's) or to minimum reporting levels (MRL's). MDL's are the minimum concentration of a constituent that can be measured and that can be reported as being greater than zero with a 99-percent confidence level (Wershaw and others, 1987). MRL's are the minimum concentrations of a constituent that can be reported reliably using a given analytical method (Timme, 1995). MRL's are generally higher than MDL's because MRL's are not statistically determined. MRL's are used more commonly (Timme, 1995). Commonly, MDL's are reported for pesticides and pesticide metabolites, and MRL's are reported for other constituents. Estimated values are used when (1) constituents are detected but concentrations are less than the lowest calibration standard or greater than the highest calibration standard, (2) when the quality-control data indicate low constituent recovery, or (3) when quantitative analysis was compromised by analytical interference. Table 2 lists the MRL's and references for analytical techniques used in this study for major ions, nutrients, trace metals, carbon, and radiochemical isotopes, including tritium and radon. Table 3 lists the MDL's and references for analytical techniques used for pesticides and pesticide metabolites, and table 4 lists the MRL's and reference for analytical techniques used for VOC's. Some of the MRL's listed for a constituent appear as a range because those constituents were analyzed at different times using different MRL values.

Quality Assurance

Several quality-assurance/quality-control samples were collected to ensure that sampling and analysis procedures were not responsible for the presence of constituents. These samples included one equipment blank (major ion, nutrient, pesticide and metabolite, VOC), two major ion and nutrient field blanks, two DOC field blanks, two pesticide field blanks, two VOC field blanks, two replicate (sequential) samples for major ions, nutrients, DOC, pesticides, and VOC's, and one pesticide and VOC field-spiked sample.

Equipment blank samples were collected by running water of known quality through the sampling and processing equipment, and collecting the blank sample in the same type of bottles used for the ground-water sampling. Equipment blank samples were collected in the USGS office in Iowa City, Iowa. This type of blank sample was used to determine if any constituent was potentially added from the equipment during sample collection and processing. Field blank samples were collected in the same manner onsite to determine if onsite conditions potentially added any constituent during the sampling process.

Replicate (sequential) samples were collected onsite immediately after the initial ground-water sample was collected. Replicate samples were used to determine the precision of onsite and laboratory procedures for the detection of ground-water constituents. Field-spiked samples also were collected. These were samples collected onsite and spiked with known amounts of selected pesticides and VOC's to determine the accuracy of constituent recovery by the analyzing laboratory and potential degradation of analytes during the time between collection and analysis.

Field and equipment blank samples showed that, for most constituents, sampling and laboratory procedures did not introduce constituents into the samples. However, several constituents were present in blank samples at detectable concentrations. These constituents are listed in table 8 in the "Supplemental Information" section at the end of this report. Some of these were estimated values near or less than the detection limits. Acetone, carbon disulfide, and toluene had estimated concentrations less than the detection-limit range. Dichlorodifluoromethane had an estimated value within the detection-limit range and an estimated value greater than the range. Carbon disulfide had a concentration within the range of detection limits. Calcium, magnesium, silica, and DOC were detected in blank samples at concentrations greater than the MRL, but their concentrations were substantially less than those found in the ground-water samples. Fluoride, nitrate, ammonia, phosphorus, orthophosphorus, and iron were detected in blank samples at concentrations greater than the detection limits, but the concentrations were less than most of the ground-water-sample concentrations.

An acceptable level of precision in laboratory procedures was evaluated by analysis of replicate (sequential) samples. Relative percentage differences (RPD's) were calculated for each constituent that had different concentrations between the replicate samples. Constituents that had RPD's greater than 10 percent are listed in table 9 in the "Supplemental Information" section. Small concentration differences for many of these constituents led to large RPD's because ground-water and replicate-sample concentrations were also small. The constituents that are in this category were bromide, ammonia, carbon disulfide, chloroform, meta/paraxylene, and toluene.

Surrogate recoveries of the ground-water samples were performed at NWQL in Denver, Colorado. The percentage recoveries for the VOC ground-water sample surrogate compounds (1,4-bromofluorobenzene, 1,2-dichloroethane, and toluene) ranged from 69 to 129 percent (median values ranged from 83 to 108 percent), and the pesticide ground-water sample surrogate compounds (diazinon-d10, tertbutylazine, alpha-HCH-d6, and BDMC) recoveries ranged from 0 to 123 percent (median values ranged from 83 to 106 percent). If the single value of zero was omitted, the range would be 63 to 123 percent for the pesticide ground-water samples. These results indicate little matrix-interference problems and that the analytical methods were effective.

Recovery for field-spiked samples ranged from 0 to 151 percent for pesticides (table 10 in "Supplemental Information" section) and 0 to 96 percent for VOC's (table 11 in "Supplemental Information" section). The formulas used to calculate spike recovery percentage are as follows: Percentage recovery = [(spike concentration) - (sample concentration)] X 100 / expected concentration. The expected concentration in micrograms per liter = [(spike solution) X amount in milliliters added] / spiked-sample volume in liters. Where concentrations of the sample were less than the MDL or MRL, the spike recovery percentage was given as a range that covered the possible percentage value. Where concentrations of the spike were less than the MDL or MRL (except for estimated values), the spike recovery percentage was not calculated. Of the compounds detected in ground-water samples, only 3-hydroxycarbofuran (spike recovery, 38 percent) and picloram (spike recovery, 43-47 percent) had spike recoveries less than 75 percent.

Statistical Analysis

Wilcoxon rank-sum tests (Ott, 1993) were performed using SAS software (SAS, 1990) to evaluate statistical differences between groups of data. Univariate procedures were used to determine means and percentiles. Spearman and Pearson correlation tests (Ott, 1993) were performed to find the degree of correlation between variables. On certain variables that had high probability of correlation, regression analyses were performed to find the equation of the linear relationship. A 95-percent confidence level was used in the statistical analyses for this report. A probability (p) value of 0.05 indicates a 95-percent confidence that observed differences are not the result of chance occurrence. Differences between groups with probability values of 0.05 or less were considered significant. For statistical-analysis purposes, the value used for constituents with concentrations less than their MDL or MRL was one-half the value of the MDL or MRL, except in the comparison of domestic-, municipal-, and monitoring-well data where a value of 0 was used.

Land-Use Classification

Land use was classified using low-altitude aerial photographs obtained from USGS's Earth Resources Observation System Data Center and onsite ground surveillance for the 32 well locations. The aerial photographs were taken in the spring of 1994, and onsite ground surveillance was conducted in the fall of 1998. Transparent mylar was taped to each aerial photograph, and a circle was drawn around the well location representing a circular area 1,640 ft in diameter. Land-use types (such as corn field, residential housing, or cemetery) that occurred within the circular area were traced as polygons and labeled on the mylar. Land-use percentages were calculated from the mylar tracings using the method described in Harvey and others (1996).

GROUND-WATER QUALITY

Selected physical properties and chemical constituent concentrations for each water sample collected for this study are reported in Akers and others (2000). A statistical summary of selected well measurements and ground-water-quality data is given in table 12 in the "Supplemental Information" section.

In this report, the quality of water in alluvial aquifers in the Eastern Iowa Basins is discussed in terms of physical properties and chemical constituents. Under the Safe Drinking Water Act of 1986, the USEPA has established three sets of regulations that set maximum levels for certain physical properties of and chemical constituents in finished (treated) drinking water-Maximum Contaminant Levels (MCL's), Secondary Maximum Contaminant Levels (SMCL's), and Health Advisory Levels (HAL's). These regulations apply to properties and constituents that, if present in drinking water, may cause adverse human health effects. MCL's are enforceable, health-based standards. SMCL's are established for properties or constituents that can adversely affect the aesthetic quality of the water (taste, odor, appearance) and may result in discontinued use of the water. HAL's are nonregulatory levels that establish acceptable constituent concentrations for different exposure periods-1-day, 10-day, long-term, and lifetime. Lifetime HAL's are estimates of concentrations that would result in no known or anticipated adverse health effects (U.S. Environmental Protection Agency, 1999).

Table 5 lists MCL's, SMCL's, and HAL's for physical properties and chemical constituents measured or analyzed in ground-water samples collected from domestic wells completed in alluvial aquifers in the Eastern Iowa Basins for June and July 1998. Table 5 also indicates the number of samples exceeding the established USEPA regulation.

Physical Properties

Specific conductance is the ability of a substance to conduct an electric current (Hem, 1985). It provides an indication of ion concentration in a solution. The sample range of specific conductance was from 331 to 1,150 µS/cm (microsiemens per centimeter at 25 °C).

pH is controlled by interrelated chemical reactions that produce or consume hydrogen ions (Hem, 1985). Temperature greatly affects pH. Most ground water in the United States has a range of 6.0 to 8.5 (Hem, 1985). The sample range for pH in the alluvial aquifers was from 5.9 to 7.4. The pH value of 5.9 was less than the typical range for ground water. This sample also had a low value for alkalinity and was from a well with a relatively shallow depth of 20 ft.

Temperature can have an effect on physical properties and chemical equilibria in ground water. The sample range for temperature was from 9.7 to 15.8 °C.

Alkalinity is the capacity for solutes to react with and neutralize acid (Hem, 1985). In natural water that has a pH less than 9.5, as in this study, the alkalinity can be assigned almost entirely to bicarbonate and carbonate. The alkalinity of ground water reflects its passage through the hydrologic cycle. The main source of the carbon dioxide species that produce alkalinity is CO2 (carbon dioxide) in the atmosphere and atmospheric gases in the soil (Hem, 1985). Much of the bedrock in the Eastern Iowa Basins study unit is carbonate, which can be dissolved by CO2-enriched water, increasing alkalinity in ground water. The CO2 species that contribute to alkalinity are important participants in reactions that control the pH of natural water (Hem, 1985). The sample range for alkalinity was from 58 to 423 mg/L.

There was a significant (p = 0.0191) increase in pH with increasing well depth, and a significant (p = 0.0312) increase in alkalinity with increasing well depth (fig. 2). These two positive correlations probably are due to longer residence times of the ground water at greater depths. Increased residence time of ground water increases the time ground water has to dissolve soluble minerals, leading to higher ion concentrations.

Major Ions and Trace Metals

Major ions and trace metals found in ground water can occur naturally and through ground-water contamination. Natural occurrences are usually through dissolution of minerals in the aquifer materials. The alluvial aquifer materials contain minerals from both local sedimentary rocks and transported sedimentary, igneous, and metamorphic rocks. The dissolution of carbonate rocks can add calcium, magnesium, and manganese to ground water. Dissolution of gypsum and fluorite can add sulfate, calcium, and fluoride to ground water. Dissolution of feldspars in igneous rock and various salts contained in sedimentary rock can release sodium, potassium, and chloride to ground water. Silica, iron, and manganese are released to ground water through the dissolution of minerals contained in igneous, metamorphic, and sedimentary rocks. The solubility of iron in water is strongly affected by pH (Hem, 1985).

Ground water from wells sampled during this study was a calcium bicarbonate type (fig. 3), probably resulting from dissolution of carbonate minerals in the alluvial deposits that were derived from reworked glacial drift and loess. Most samples had 50 to 70 percent of the total cations represented by calcium and 20 to 40 percent of the total cations represented by magnesium. Sodium commonly accounted for 0 to 20 percent of total cations.

Calcium, magnesium, sodium, potassium, chloride, and silica were detected in all samples. Bromide was detected in 97 percent of all samples, sulfate and fluoride in 91 percent, manganese in 72 percent, and iron in 69 percent. Concentration ranges for these constituents are listed in table 12 in the "Supplemental Information" section.

Sulfate decreased significantly with increasing well depth (p = 0.0137), possibly through sulfate reduction, a bacteria-mediated anaerobic process whereby sulfate is converted to H2S and HS¯. There appeared to be a general trend of decreasing oxygen with increasing well depth, but it was not statistically significant. Concentrations of iron exceeded the USEPA SMCL (300 µg/L) in 53 percent of the samples. Fifty percent of the samples exceeded the SMCL for manganese (50 µg/L).

Nutrients and Dissolved Organic Carbon

Nitrite and nitrate can occur in ground water through natural processes. Small amounts are produced in the atmosphere through a reaction of molecular N2 and lightning and can be absorbed in rainwater (Hem, 1985). Under aerobic conditions, soil bacteria convert nitrogen to nitrate through the process of nitrification. Nitrite plus nitrate concentrations in ground water greater than 3.0 mg/L commonly are attributed to human activities (Madison and Brunett, 1984). In general, agricultural activities are the main source of nitrogen and are the primary cause of widespread ground-water contamination in shallow alluvial aquifers (Hallberg, 1986). Corn and soybeans are the main crops grown in the Eastern Iowa Basins study unit. More nitrogen fertilizers are used for corn than for any other crop (U.S. Department of Agriculture, 1999).

Nitrite was detected in 38 percent of the samples, and nitrite plus nitrate was detected in 53 percent of the samples. Sample concentrations of nitrite ranged from less than 0.01 to 0.12 mg/L, and concentrations of nitrite plus nitrate ranged from less than 0.05 to 22 mg/L. Nitrite plus nitrate exceeded the MCL (10 mg/L) in 13 percent of the samples (table 5). Nitrite did not exceed the MCL (1.0 mg/L) in any of the samples.

Ammonia and ammonium are the most common nitrogen compounds in fertilizers (Anderholm, 1996). These compounds also occur in human and animal waste, through ammonification of organic nitrogen, and in precipitation (Freeze and Cherry, 1979; Hem, 1985). Ammonia plus organic nitrogen is associated with animal-produced fertilizers, livestock runoff, and septic systems. Ammonia was detected in 91 percent of the samples and ammonia plus organic nitrogen in 72 percent of the samples. Sample concentrations of ammonia ranged from less than 0.02 to 6.3 mg/L, and concentrations of ammonia plus organic nitrogen ranged from less than 0.1 to 6.4 mg/L.

Phosphorus is a common element in igneous and sedimentary rock but has a low solubility for most of its compounds (Hem, 1985). Elevated concentrations of phosphorus compounds in natural water can be caused by human activities. Phosphorus is a component in sewage and in some fertilizers and pesticides. Orthophosphorus was detected in 94 percent of the samples. Sample concentrations of orthophosphorus ranged from less than 0.01 to 0.77 mg/L.

DOC in ground water can occur naturally from organic debris along flow paths or can be a synthetic contaminant (Hem, 1985). All ground-water samples had detectable amounts of DOC. Sample concentrations of DOC ranged from 0.5 to 5.8 mg/L.

There was a positive correlation between ammonia plus organic nitrogen and well depth (p = 0.0260). There was a negative correlation (p = 0.0149) between nitrite plus nitrate concentration and well depth, and a positive correlation (p = 0.0001) between nitrite plus nitrate concentration and percentage of oxygen saturation (fig. 4). Oxygen is supplied to ground water through oxygen-rich recharge water and movement of air through the unsaturated zone (Hem, 1985). Depletion of dissolved oxygen in ground water can occur rapidly if there is oxidizable minerals or organic matter in the aquifer material. The sample range for dissolved oxygen was from 0.1 to 8.3 mg/L. Denitrification, the reduction by bacteria of nitrate to nitrogen gases, occurs in low-oxygen environments. The ratio of nitrite plus nitrate to ammonia had a positive correlation with percentage of oxygen saturation (p = 0.0010).

Tritium

Tritium samples were analyzed as a means to determine the relative age of ground water. Natural ground-water concentrations of tritium are less than 2.6 pCi/L (Plummer and others, 1993). Atmospheric nuclear testing beginning in the early 1950's enriched the atmosphere with tritium, thus enriching ground water through infiltration of precipitation. Ground-water samples with tritium concentrations greater than 2.6 pCi/L are considered to represent ground water that contains at least some portion of water recharged after the early 1950's. Because most pesticides were developed and used after 1950, ground-water samples with tritium values less than 2.6 pCi/L are not likely to contain pesticides or pesticide metabolites. Tritium was detected in 78 percent of the ground-water samples collected for this study; 72 percent of the samples contained tritium concentrations greater than 2.6 pCi/L. None of the ground-water samples with tritium values less than 2.6 pCi/L had detectable pesticide or pesticide metabolite concentrations.

Wilcoxon rank-sum tests were performed to compare ground-water samples representing recharge before the early 1950's (tritium concentrations less than 2.6 pCi/L) and ground-water samples representing recharge after the early 1950's (tritium concentration equal to and greater than 2.6 pCi/L). There was a significant difference (p = 0.0002) in well depth (fig. 5) between the two groups of samples (older and younger water). Ground-water samples from greater well depths generally had lower concentrations of tritium due to longer residence time. There were significant differences, between the older and younger water, in concentrations of nitrite plus nitrate (p = 0.0103) and total pesticide plus metabolite (p = 0.0014). Ground-water samples from younger water had higher concentrations of nitrite plus nitrate and total pesticide plus metabolite.

Radon

Radon is produced naturally from the radioactive decay of radium226, is water soluble, and has a half-life of 3.8 days (Hem, 1985). The source of radon is uranium-rich rock and sediment. Radon has been implicated in the development of lung cancer in people exposed over long periods to high concentrations of airborne radon (Zapecza and Szabo, 1988; Robillard and others, 1991). The USEPA previously proposed MCL for radon is 300 pCi/L. A higher alternative MCL was being considered as required by the 1996 Safe Drinking Water Act amendments (U.S. Congress, 1996).

All samples in this study had detectable concentrations of radon. Fifty-seven percent of the samples exceeded the USEPA previously proposed drinking-water MCL for radon (table 5).

Pesticides and Pesticide Metabolites

Crop yields are improved through the application of pesticides to control weeds, insects, and fungus. These pesticides may reach ground water by downward movement after application, through accidental spills, by back-siphoning accidents, and through improper disposal of formulation or rinse water (Kross and others, 1990). The frequency of pesticide detections in Iowa rural wells exceeds the national level (Glanville and others, 1995).

Atrazine was the most commonly detected pesticide (excluding pesticide metabolites) in ground-water samples collected for this study and was detected in 38 percent of the samples (fig. 6). Sample concentrations of atrazine ranged from less than 0.001 to 0.26 µg/L. Prometon was detected in 16 percent of the samples and had a concentration range from less than 0.018 to 0.19 µg/L. Metolachlor was detected in 9 percent of the samples and had a range from less than 0.002 to 0.02 µg/L; bentazon was detected in 6 percent and had a range from less than 0.014 to 0.22 µg/L; and tebuthiuron, 3-hydroxycarbofuran, dichlorprop, and picloram were detected in 3 percent and had ranges from less than 0.01 to 0.01 µg/L, less than 0.003 to 0.07 µg/L, less than 0.032 to 0.10 µg/L, and less than 0.05 to 0.17 µg/L, respectively. Concentrations of pesticide and pesticide metabolites are graphically represented in figure 7. No pesticide exceeded its MCL (table 5).

Atrazine, bentazon, and metolachlor are herbicides frequently used on row crops. Dichlorprop, picloram, and tebuthiuron are used to control broadleaf weeds and brush in noncrop areas, commonly on utility rights-of-way. Prometon is a nonselective herbicide frequently mixed into asphalt for road construction to prevent plant growth, and 3-hydroxycarbofuran is an insecticide.

Pesticide metabolites are formed when pesticides break down into different compounds in the environment. Metabolites can be more persistent and mobile than their parent compound (Kolpin and others, 1996b), which can lead to more frequent detections and higher concentrations in ground water (figs. 6 and 7). Of the 10 most commonly detected herbicides and metabolites, 7 were metabolites. Metolachlor ethanesulfonic acid (metolachlor ESA) was the most commonly detected metabolite and was present in 45 percent of the samples. Sample concentrations of metolachlor ESA ranged from less than 0.20 to 20 µg/L. Acetochlor ESA, alachlor ESA, alachlor oxanilic acid (alachlor OA), cyanazine amide, deethylatrazine, deisopropylatrazine, and metolachlor OA also were detected. Pesticide metabolite detections are listed in table 12 in the "Supplemental Information" section. Figure 8 graphically shows concentrations of alachlor and its metabolites, atrazine and its metabolites, and metolachlor and its metabolites, in water from each well.

Volatile Organic Compounds

The VOC carbon disulfide was detected in 9 percent of the ground-water samples collected for this study. Dibromochloromethane, chloroform, dichlorobromomethane, and toluene were detected in 3 percent of the samples. No other VOC's were detected. No VOC exceeded its respective MCL (table 5). The detections of dibromochloromethane, chloroform, and dichlorobromomethane all occurred in a single sample from a 14-ft deep, 48-in. diameter, hand-dug well. The data suggest this well may have been disinfected with chlorine.

RELATIONS BETWEEN GROUND-WATER QUALITY AND LAND USE

The concentration and distribution of water-quality constituents determined in this study were compared statistically with land-use type in a 1,640-ft radius around the well to evaluate relations between ground-water quality and land use in the Eastern Iowa Basins. Major and secondary land-use types for each well site are listed in table 1. There was a positive correlation (p = 0.0465) between metolachlor and percentage of land used for soybean production, between metolachlor ESA and percentage of land used for soybean production (p = 0.0075) (fig. 9), and between metolachlor OA and percentage of land used for soybean production (p = 0.0036). These were the only significant relations. These relations are difficult to explain as metolachlor is used more commonly in corn rather than soybean production. Nationally, metolachlor is the second most commonly used pesticide on corn in total pounds applied, but it is only the fourth most used pesticide on soybeans (U.S. Department of Agriculture, 1999). There was no significant relation between metolachlor and percentage of land used for corn production. Perhaps landowners have rotated plantings of corn and soybeans on land near the sampled wells. Metolachlor may have been applied to these fields and later affected the underlying ground water.

RELATIONS BETWEEN GROUND-WATER QUALITY AND TYPE OF WELL SAMPLED

Well construction and pumping capacity may affect the quality of water withdrawn from wells. Data from several different studies were analyzed to provide a comparison between water-quality data collected from domestic, municipal, and monitoring wells. All data represent water from alluvial aquifers in the Eastern Iowa Basins study unit.

In 1988-89, Iowa domestic wells were sampled as part of the State-Wide Rural Well-Water Survey (SWRL) (Kross and others, 1990). Eleven wells from the SWRL data set occurred within the Eastern Iowa Basins study unit and were completed in alluvial aquifers. Data from these 11 wells were combined with the data from the 32 domestic wells sampled during this study to make comparisons with municipal and monitoring wells completed in alluvial aquifers in the Eastern Iowa Basins study unit. Data from 58 municipal wells from the Iowa Ground-Water Monitoring network (GWM) (Detroy, 1985; Schaap and Linhart, 1998) and from 61 monitoring wells sampled in a NAWQA land-use study (Savoca and others, 2000) also were used in this comparison. Municipal wells from the GWM network have been sampled annually since 1982. Data from the GWM network used for the comparison described herein were collected from 1982 through 1996. Monitoring wells for the land-use study were installed by the USGS in agricultural and urban settings and were sampled during the summer of 1997. Results of the comparison of data from domestic, municipal, and monitoring wells are listed in table 6, and summary statistics are presented in table 7. The physical properties and chemical constituents in table 6 are those common to the three data sets. Wilcoxon rank-sum tests (SAS, 1990) were used to compare the data sets. Some of the data are displayed in the form of box plots in figure 10.

Well depth was significantly less for monitoring wells than for domestic wells (p = 0.0001) and municipal wells (p = 0.0001). There was a significant difference in pH between domestic- and municipal-well samples (p = 0.0001), and between municipal- and monitoring-well samples (p = 0.0001). The pH in the municipal-well samples was higher in both instances.

Calcium concentrations were significantly higher in municipal-well samples than in domestic-well samples (p = 0.0010). Sulfate concentrations were significantly higher in municipal-well samples than in domestic-well samples (p = 0.0001) and monitoring-well samples (p = 0.0001). Chloride concentrations were significantly lower in domestic-well samples than in municipal-well samples (p = 0.0003) and monitoring-well samples (p = 0.0006). Fluoride concentrations were significantly higher in domestic-well samples than in municipal-well samples (p = 0.0200).

Ammonia concentrations were significantly higher in domestic-well samples than in monitoring-well samples (p = 0.0087). Atrazine concentrations were significantly higher in municipal-well samples than in domestic-well samples (p = 0.0361).

The differences in the three types of wells may explain the differences in water quality. Generally, both domestic and municipal wells are deeper than monitoring wells. Major ion concentrations tend to increase with depth due to the greater residence time with increasing depth of ground water. This may explain the higher pH and concentrations of sulfate in the municipal wells. These had positive correlations with well depth (p = 0.0001 for pH and p = 0.0417 for sulfate). Chloride did not follow this pattern and had higher concentrations in ground-water samples from the monitoring wells than in samples from domestic wells. A possible explanation could be the application of road salt. Many of the monitoring wells were constructed on public rights-of-way alongside roads in both urban and rural areas.

Municipal wells have large screens and pumps that enable them to withdraw both deep and shallow water from a large area of the aquifer. This may explain the higher pH and concentrations of calcium, sulfate, chloride, and atrazine in ground-water samples from the municipal wells than in samples from the domestic wells. The reason for higher fluoride concentrations in samples from domestic wells than in samples from municipal wells is not known.

The higher ammonia concentrations in samples from domestic wells than in samples from monitoring wells possibly could be due to the reduction of nitrate to ammonia in low-oxygen environments that occur in deeper wells. There was a positive correlation between concentrations of ammonia and well depth (p = 0.0001) and a negative correlation between well depth and concentrations of oxygen (p = 0.0334).

SUMMARY

Ground-water samples from 32 domestic wells completed in alluvial aquifers in the Eastern Iowa Basins were collected from June through July 1998 and analyzed to determine concentrations of major ions, trace metals, nutrients, DOC, tritium, radon, pesticides and pesticide metabolites, and VOC's. Onsite measurements of specific conductance, pH, water temperature, dissolved oxygen, and alkalinity were obtained for each well. Forty-five percent of ground water pumped in the Eastern Iowa Basins originates from alluvial aquifers. The alluvial aquifers consist of varying thicknesses of sand, gravel, silt, and clay deposits, and occur along most of the major streams and rivers.

There were significant increases in pH and alkalinity with increasing well depth. These increases can be attributed to longer ground-water residence time associated with the increased depths. The dominant major ions in most samples were calcium and bicarbonate, likely derived from the dissolution of carbonate minerals in the alluvial aquifer material. Concentrations of iron exceeded the USEPA SMCL (300 µg/L) in 53 percent of the samples, and 50 percent of the samples exceeded the SMCL for manganese (50 µg/L). Sulfate concentrations decreased significantly with increasing well depth, possibly through sulfate reduction.

Ammonia and orthophosphorus were the most commonly detected nutrients. Nitrite plus nitrate and ammonia plus organic nitrogen had the highest nutrient concentrations. Nitrite plus nitrate exceeded its MCL (10 mg/L) in 13 percent of the samples. There was a positive correlation between nitrite plus nitrate concentration and percentage of oxygen saturation, and a negative correlation between nitrite plus nitrate concentration and well depth. There was a positive correlation between ammonia plus organic nitrogen and well depth. The ratio of nitrite plus nitrate to ammonia had a positive correlation with percentage of oxygen saturation.

Tritium data indicate the majority of samples, 72 percent, contained water recharged since the early 1950's. Ground-water samples from greater well depths had lower concentrations of tritium due to longer residence time. Ground-water samples from younger water had higher concentrations of nitrite plus nitrate and total pesticides plus metabolites. These relations are due to recharge of young tritium-enriched water transporting pesticides and fertilizers to the ground water. Older ground water recharged before the 1950's would not contain pesticides or relatively high concentrations of nitrite plus nitrate. All samples had detectable concentrations of radon. Fifty-seven percent of the samples exceeded USEPA previously proposed drinking-water MCL for radon (300 pCi/L).

Atrazine and prometon, found in 38 and 16 percent of the samples, respectively, were the most common pesticides detected in ground-water samples. Median concentrations were higher for pesticide metabolites than for pesticides. Metolachlor ESA was the most commonly detected metabolite.

Carbon disulfide was the most common VOC detected in 9 percent of the samples. Dibromochloromethane, chloroform, dichlorobromomethane, and toluene were detected in 3 percent of the samples. No VOC exceeded its MCL.

There were positive correlations between percentage of land used for soybean production and metolachlor, metolachlor ESA, and metolachlor OA. Metolachlor is more commonly used for corn than soybeans. Perhaps landowners have rotated plantings of corn and soybeans on land near the sampled wells. Metolachlor may have been applied to these fields and later affected the underlying ground water.

A comparison was made using previously collected data and data from this study to determine differences in water quality between domestic, municipal, and monitoring wells completed in alluvial aquifers in the Eastern Iowa Basins study unit. Well depth was significantly greater for domestic and municipal wells than for monitoring wells. pH, calcium, sulfate, chloride, and atrazine concentrations were significantly higher in municipal-well samples than in domestic-well samples. pH values and sulfate concentrations were significantly higher in municipal-well samples than in monitoring-well samples. Ammonia concentrations were significantly higher in domestic-well samples than in monitoring-well samples, chloride concentrations were significantly higher in monitoring-well samples than in domestic-well samples, and fluoride concentrations were significantly higher in domestic-well samples than in municipal-well samples.

Major ion concentrations tend to increase with depth due to the greater residence time with increasing depth of ground water. This would explain the higher pH and concentrations of sulfate in the municipal wells. These had positive correlations with well depth. Chloride concentrations did not follow this pattern and had higher concentrations in ground-water samples from the monitoring wells than in samples from domestic wells. A possible explanation could be the application of road salt.

Municipal wells have large screens and pumps that enable them to withdraw both deep and shallow water from a large area of the aquifer. This can explain the higher pH and concentrations of calcium, sulfate, chloride, and atrazine in water samples from the municipal wells than in samples from the domestic wells.

The higher ammonia concentrations in ground-water samples from domestic wells than in samples from monitoring wells could possibly be due to the reduction of nitrate to ammonia in low oxygen environments that occur in deeper wells. There was a positive correlation between ground-water sample concentrations of ammonia and well depths and a negative correlation between well depths and ground-water sample concentrations of oxygen.

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Sadorf, E.M, and Linart, S.M., 2000, Ground-water quality in Alluvial Aquifers in the Eastern Iowa Basins, Iowa and Minnesota:  U.S. Geological Survey Water-Resources Investigations Report 00-4106 46 p

 


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