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USGS1989

Geochemistry of the Floridan aquifer system in Florida and in parts of Georgia, South Carolina, and Alabama

The chemical quality of the ground water in the Floridan aquifer system is determined primarily by mineral-water interaction. However, some changes in water quality have been imposed by development, particularly near coastal pumping centers. A total of 601 chemical analyses, all from different wells, most completed in the upper part of the aquifer system, were used to describe the variations in water chemistry and to study the processes responsible for observed changes. The Floridan aquifer system is a vertically continuous sequence of Tertiary carbonate rocks that are of generally high permeability and are hydraulically connected in varying degrees. The rocks are principally limestone and dolomite, but they grade into limy sands and clays near the aquifer system's updip limits. Major minerals in the aquifer system are calcite, dolomite, and, locally, gypsum or quartz; minor minerals include apatite, glauconite, and clay minerals such as kaolinite and montmorillonite. Trace amounts of metallic oxides or sulfides are present in some areas. The aquifer system consists of the Upper and Lower Floridan aquifers, separated in most places by a less permeable confining unit that has highly variable hydraulic properties. Only the Upper Floridan aquifer is present throughout the study area. Freshwater enters the aquifer system in outcrop areas located primarily in central Georgia and north-central Florida. Discharge occurs chiefly to streams and springs and, to a lesser extent, directly into the sea. Most of the flow into and out of the system takes place where it is unconfined or where the upper confining unit is thin. Secondary permeability developed by dissolution of aquifer material is most prominent in these areas of dynamic flow. Dissolved-solids concentrations in water from the Upper Floridan aquifer generally range from less than 25 milligrams per liter near outcrops to more than 25,000 milligrams per liter along the coasts. The dominant cations in the ground water are Ca2+, Mg2+, and Na+; the dominant anions are HCO3-, Cl-, and SO42-, The concentration of Ca2+ is controlled primarily by calcite saturation. Concentrations of Mg2+, NA+, and Cl- are highest where mixing of freshwater and saltwater occurs. Concentrations of HCO3- reflect the control of calcite solubility. The concentration of SO42- is highest where gypsiferous rock units are present in the aquifer system. The major geochemical processes that occur in the Upper Floridan aquifer, based on water-quality maps and computations using a geochemical model, are (1) dissolution of aquifer minerals toward equilibrium, (2) mixing of ground water with recharge, leakage, or seawater, (3) sulfate reduction, and (4) cation exchange between water and aquifer minerals. Similar processes apparently control minor dissolved constituents, although quantification is difficult with the available data. Statistical tests of available nutrient data indicate that concentrations of N (nitrogen) species in unconfined recharge areas may be increasing over time; more detailed studies of all N species are needed to test this hypothesis, however. Data on trace metals, radionuclides, and man-made organic contaminants are limited. Available data indicate that most freshwater within the Upper Floridan is potable, but detection of pesticides in a few samples indicates that the system is susceptible to contamination from the land surface in some areas, particularly where its upper confining unit is thin or absent. Geochemical models were used to examine changes in major chemical elements along selected ground-water paths within the Upper Floridan aquifer. Water in the Upper Floridan aquifer can be categorized into four hydrochemical facies, whose exact distribution is determined by confined or unconfined conditions of the aquifer and by chloride concentrations. The reaction models are considered plausible based on available chemical, isotopic, and hydrologic information, and they

Alabama, Florida, Georgia, South Carolina↗
USGS2010

Gas, oil, and water production from Jonah, Pinedale, Greater Wamsutter, and Stagecoach Draw fields in the Greater Green River Basin, Wyoming

Gas, oil, and water production data were compiled from selected wells in four gas fields in rocks of Late Cretaceous age in southwestern Wyoming. This study is one of a series of reports examining fluid production from tight-gas reservoirs, which are characterized by low permeability, low porosity, and the presence of clay minerals in pore space. Production from each well is represented by two samples spaced five years apart, the first sample typically taken two years after commencement of production. For each producing interval, summary diagrams of oil versus gas and water versus gas production show fluid production rates, the change in rates during five years, the water-gas and oil-gas ratios, and the fluid type. These diagrams permit well-to-well and field-to-field comparisons. Fields producing water at low rates (water dissolved in gas in the reservoir) can be distinguished from fields producing water at moderate or high rates, and the water-gas ratios are quantified. The ranges of first-sample gas rates in Pinedale field and Jonah field are quite similar, and the average gas production rate for the second sample, taken five years later, is about one-half that of the first sample for both fields. Water rates are generally substantially higher in Pinedale than in Jonah, and water-gas ratios in Pinedale are roughly a factor of ten greater in Pinedale than in Jonah. Gas and water production rates from each field are fairly well grouped, indicating that Pinedale and Jonah fields are fairly cohesive gas-water systems. Pinedale field appears to be remarkably uniform in its flow behavior with time. Jonah field, which is internally faulted, exhibits a small spread in first-sample production rates. In the Greater Wamsutter field, gas production from the upper part of the Almond Formation is greater than from the main part of the Almond. Some wells in the main and the combined (upper and main parts) Almond show increases in water production with time, whereas increases in water production are rare in the upper part of the Almond, and a higher percentage of wells in the upper part of the Almond show water decreasing at the same rate as gas than in the main or combined parts of the Almond. In Stagecoach Draw field, the gas production rate after five years is about one-fourth that of the first sample, whereas in Pinedale, Jonah, and Greater Wamsutter fields, the production rate after five years is about one-half that of the first sample. The more rapid gas decline rate seems to be the outstanding feature distinguishing Stagecoach Draw field, which is characterized as a conventional field, from Pinedale, Jonah, and Greater Wamsutter fields, which are generally characterized as tight-gas accumulations. Oil-gas ratios are fairly consistent within Jonah, Pinedale, and Stagecoach Draw fields, suggesting similar chemical composition and pressure-temperature conditions within each field, and are less than the 20 bbl/mmcf upper limit for wet gas. However, oil-gas ratios vary considerably from one area to another in the Greater Wamsutter field, demonstrating a lack of commonality in either chemistry or pressure-temperature conditions among the six areas. In all wells in all four fields examined here, water production commences with gas production—there are no examples of wells with water-free production and no examples where water production commences after first-sample gas production. The fraction of records with water production higher in the second sample than in the first sample varies from field to field, with Pinedale field showing the lowest percentage of such cases and Jonah field showing the most. Most wells have water-gas ratios exceeding the amount that could exist dissolved in gas at reservoir pressure and temperature.

Wyoming↗
USGS2014

Baseline groundwater quality from 34 wells in Wayne County, Pennsylvania, 2011 and 2013

Wayne County, Pennsylvania, is underlain by the Marcellus Shale, which currently (2014) is being developed elsewhere in Pennsylvania for natural gas. All residents of largely rural Wayne County rely on groundwater for water supply, primarily from bedrock aquifers (shales and sandstones). This study, conducted by the U.S. Geological Survey in cooperation with the Pennsylvania Department of Conservation and Natural Resources, Bureau of Topographic and Geologic Survey (Pennsylvania Geological Survey), provides a groundwater-quality baseline for Wayne County prior to development of the natural gas resource in the Marcellus Shale. Selected wells completed in the Devonian-age Catskill Formation, undifferentiated; the Poplar Gap and Packerton Members of the Catskill Formation, undivided; and the Long Run and Walcksville Members of the Catskill Formation, undivided, were sampled. Water samples were collected once from 34 domestic wells during August 2011 and August and September 2013 and analyzed to characterize their physical and chemical quality. Samples were analyzed for 45 constituents and properties, including nutrients, major ions, metals and trace elements, radioactivity, and dissolved gases, including methane and radon-222. The quality of the sampled groundwater was generally within U.S. Environmental Protection Agency (USEPA) drinking-water standards, although in some samples, the concentrations of a few constituents exceeded USEPA drinking-water standards and health advisories. The pH of water samples ranged from 5.5 to 9.3 with a median of 7.0. The pH was outside the USEPA secondary maximum contaminant level (SMCL) range of 6.5 to 8.5 in water samples from 14 of the 34 wells (41 percent). Eleven samples had a pH less than 6.5, and three samples had a pH greater than 8.5. Dissolved oxygen concentrations ranged from 0.2 to 11.5 milligrams per liter (mg/L) with a median of 4.7 mg/L. The dissolved oxygen concentration was less than 1 mg/L in water samples from 6 wells; 5 of these 6 water samples had a pH greater than 7.7. Concentrations of dissolved methane ranged from less than 0.00006 to 3.3 mg/L. Methane was detectable in 22 of the 34 wells sampled (65 percent). Methane concentrations were greatest in the 5 samples with pH of 7.8 or higher, ranging from 0.040 to 3.3 mg/L. These samples also had among the lowest concentrations of dissolved oxygen. Three water samples, which had sufficient dissolved methane concentrations (greater than 0.9 mg/L), were analyzed for isotopes of carbon and hydrogen in the methane. The isotopic ratio values fell within (two samples) or close to (one sample) the range for a thermogenic natural gas source. The total dissolved solids concentration ranged from 33 to 346 mg/L; the median concentration was 126 mg/L. Sodium concentrations ranged from 1.07 to 116 mg/L; the median concentration was 9.42 mg/L. The sodium concentration exceeded the USEPA health advisory for sodium of 20 mg/L in water samples from 7 of the 34 wells (21 percent). Concentrations of dissolved arsenic ranged from less than 0.06 to 21.8 micrograms per liter (µg/L); the median concentration was 0.59 µg/L. Water samples from 2 of the 34 wells (6 percent) exceeded the USEPA maximum contaminant level (MCL) of 10 µg/L for arsenic. Concentrations of dissolved manganese ranged from less than 0.15 to 61.5 µg/L; the median concentration was 0.42 µg/L. A water sample from 1 of the 34 wells (3 percent) exceeded the USEPA SMCL of 50 µg/L for manganese; the concentration was less than the USEPA lifetime health advisory of 300 µg/L for manganese. Activities of radon-222 in water from the 34 sampled wells ranged from 110 to 7,180 picocuries per liter (pCi/L); the median activity was 2,105 pCi/L. Water samples from 33 of the 34 wells (97 percent) exceeded the proposed USEPA MCL of 300 pCi/L, and 4 water samples (12 percent) exceeded the USEPA proposed alternative MCL of 4,000 pCi/L for radon-222. Differences in groundwater chemistry were related to pH. Water with a pH greater than 7.6 generally had low dissolved oxygen concentrations, indicating reducing conditions in the aquifer. These high pH waters also had relatively elevated concentrations of methane, arsenic, boron, bromide, fluoride, lithium, and sodium but low concentrations of copper, nickel, and zinc. Water samples with a pH greater than 7.8 had methane concentrations equal to or greater than 0.04 mg/L.

Pennsylvania↗
USGS2017

Perspectives on chemical oceanography in the 21st century: Participants of the COME ABOARD Meeting examine aspects of the field in the context of 40 years of DISCO

The questions that chemical oceanographers prioritize over the coming decades, and the methods we use to address these questions, will define our field's contribution to 21st century science. In recognition of this, the U.S. National Science Foundation and National Oceanic and Atmospheric Administration galvanized a community effort (the Chemical Oceanography MEeting: A BOttom-up Approach to Research Directions, or COME ABOARD) to synthesize bottom-up perspectives on selected areas of research in Chemical Oceanography. Representing only a small subset of the community, COME ABOARD participants did not attempt to identify targeted research directions for the field. Instead, we focused on how best to foster diverse research in Chemical Oceanography, placing emphasis on the following themes: strengthening our core chemical skillset; expanding our tools through collaboration with chemists, engineers, and computer scientists; considering new roles for large programs; enhancing interface research through interdisciplinary collaboration; and expanding ocean literacy by engaging with the public. For each theme, COME ABOARD participants reflected on the present state of Chemical Oceanography, where the community hopes to go and why, and actionable pathways to get there. A unifying concept among the discussions was that dissimilar funding structures and metrics of success may be required to accommodate the various levels of readiness and stages of knowledge development found throughout our community. In addition to the science, participants of the concurrent Dissertations Symposium in Chemical Oceanography (DISCO) XXV, a meeting of recent and forthcoming Ph.D. graduates in Chemical Oceanography, provided perspectives on how our field could show leadership in addressing long-standing diversity and early-career challenges that are pervasive throughout science. Here we summarize the COME ABOARD Meeting discussions, providing a synthesis of reflections and perspectives on the field.

Marine Chemistry↗
USGS2016

Groundwater quality, age, and susceptibility and vulnerability to nitrate contamination with linkages to land use and groundwater flow, Upper Black Squirrel Creek Basin, Colorado, 2013

The Upper Black Squirrel Creek Basin is located about 25 kilometers east of Colorado Springs, Colorado. The primary aquifer is a productive section of unconsolidated deposits that overlies bedrock units of the Denver Basin and is a critical resource for local water needs, including irrigation, domestic, and commercial use. The primary aquifer also serves an important regional role by the export of water to nearby communities in the Colorado Springs area. Changes in land use and development over the last decade, which includes substantial growth of subdivisions in the Upper Black Squirrel Creek Basin, have led to uncertainty regarding the potential effects to water quality throughout the basin. In response, the U.S. Geological Survey, in cooperation with Cherokee Metropolitan District, El Paso County, Meridian Service Metropolitan District, Mountain View Electric Association, Upper Black Squirrel Creek Groundwater Management District, Woodmen Hills Metropolitan District, Colorado State Land Board, and Colorado Water Conservation Board, and the stakeholders represented in the Groundwater Quality Study Committee of El Paso County conducted an assessment of groundwater quality and groundwater age with an emphasis on characterizing nitrate in the groundwater. Groundwater-quality samples were collected from 50 randomly selected wells between May and June 2013. The samples were analyzed for major ions, nutrients, dissolved gases, tritium ( 3 H), chlorofluorocarbons (CFC-11, CFC-12, and CFC-113), and fuel products (such as benzene, toluene, ethylbenzene, and xylenes). None of the groundwater samples exceeded the U.S. Environmental Protection Agency (EPA) National Primary Drinking Water Regulations for primary maximum contaminant levels (MCL) for major ions. Secondary maximum contaminant levels, which are not health concerns and affect mainly taste, color, or odor of the water, were observed in rare instances for pH (2 samples), chloride (1 sample), iron (3 samples), and manganese (8 samples). The secondary maximum contaminant level for total dissolved solids was also exceeded for two samples. Nitrate (nitrite plus nitrate as nitrogen in groundwater) was elevated above the estimated background concentration of natural recharge waters of 1 milligram per liter (mg/L) in 44 of the 50 wells sampled and showed a median concentration of 5.4 mg/L. Nitrate concentrations were above the MCL of 10 mg/L in 5 of the 50 wells sampled and above half of the EPA MCL (5 mg/L) in 27 of the 50 wells sampled, which included samples above the MCL. Dissolved-oxygen concentrations exceeded 0.5 mg/L in 95 percent of reported values (40 of 42 samples) and exceeded 2.0 mg/L in 90 percent of reported values (38 of 42 samples). The oxidized conditions observed in most areas indicate that nitrate from fertilizers and animal or human waste was geochemically stable and could persist in the groundwater for decades or perhaps longer. A historical analysis of median nitrate concentrations over nearly three decades showed an increase in nitrate of approximately 1 mg/L from 4.3 to 5.4 mg/L, although the increase was not determined to be significantly different using nonparametric statistical methods. Major-ion data indicate that groundwater representative of the primary aquifer was classified as calcium-sodium bicarbonate type water. Other water samples from wells located mainly along the periphery of the primary aquifer had cation-anion compositions consistent with distinct water sources, including groundwater contributions from the underlying bedrock aquifers. The areas with differentiable water sources were located mainly where alluvial deposits were thin and geologic contacts to the underlying bedrock aquifers were relatively shallow. Nitrate concentrations in the groundwater were evaluated for relations to land use. An agricultural region was defined using a sequence of land satellite imagery. Groundwater flow directions interpreted from median water-table elevations measured from 2000 to 2013 were used in conjunction with cropland locations to define the agricultural region boundaries by encompassing potential pathways of nitrate transport in the groundwater from nitrogen-based fertilizers. A statistically significant higher median nitrate concentration was observed for areas inside the agricultural region (6.7 mg/L) compared to areas outside the agricultural region (2.3 mg/L), although median concentrations in both areas were below the MCL (10 mg/L). Median nitrate concentration was also significantly greater in land parcels with septic use (4.9 mg/L) compared to nonseptic parcels (1.7 mg/L). In general, agriculture or septic use was identified as the primary source of nitrate, depending on location, while commercial, county, grazing, and residential land uses were generally secondary sources of nitrate. Apparent groundwater ages were estimated from chlorofluorocarbons (CFC-11, CFC-12, and CFC-113) and tritium ( 3 H) data using models that assumed piston flow and binary mixing (dilution of a young component with old, tracer-free water). The mean and median groundwater ages were about 30 years and the standard deviation was 6 years, indicating that most groundwater in the primary aquifer was “young” water that had recharged to the aquifer over the last few decades (post-1950s). The median fraction of young water was about 71 percent, and the standard deviation was 29 percent. The remaining water predated the 1950s, which may have originated from deeper geologic formations or may represent slow moving groundwater within the primary aquifer. Some of the oldest groundwater ages (older than 30 years) were observed in the upper reaches of the aquifer to the northwest where the primary aquifer is thin and intersects bedrock, supporting the hypothesis of geochemically distinct groundwater entering the primary aquifer from below. Groundwater that had reached the central part of the aquifer from upgradient areas of the basin was variable in age because of differences in flow paths and travel velocities. The groundwater age analysis showed that current (2013) land-use practices could affect water quality over decades to come, and that responses to remedial actions could be slow, especially for constituents, such as nitrate, that are stable under oxidized conditions. Fuel products (including acetone, benzene, diisopropyl ether, ethylbenzene, methyl acetate, methyl tertiary butyl ether (MTBE), methyl tert-pentyl ether, m- + p-xylene, o-xylene, tert-amyl alcohol, tert-butyl alcohol, tert-butyl ethyl ether, and toluene) were analyzed in groundwater from 49 of the 50 wells. Water from seven sites had detections for fuel compounds; all concentrations were below MCL. The results provided assurance of water quality and a valuable baseline to evaluate future trends of fuel constituents as the region is further developed. Probability maps were developed from logistic regression models to examine the likelihood that nitrate concentrations in groundwater exceeded specified levels. Susceptibility analysis examined relations between mid-level (5.0 mg/L) nitrate concentrations and climatic, hydrologic, and geologic variables; the significant variables were identified as depth to groundwater, soil organic matter, and soil water storage to 25-centimeter (cm) depth. The vulnerability assessments included natural factors driving susceptibility but also human factors related to land use and septic use. Vulnerability to low-level (2.5 mg/L) nitrate was related to depth to groundwater, septic zoning, and soil organic matter. The results highlighted that septic zoning affected low-level nitrate concentrations. Vulnerability to mid-level (5.0 mg/L) nitrate was examined using all 50 samples and also with two data outliers removed, which showed relatively high nitrate concentrations but also anomalous water chemistry or were located beyond the primary study area. Vulnerability to mid-level (5.0 mg/L) nitrate using all 50 samples was related to depth to groundwater, land use, septic use within a 500-meter (m) radius, soil water storage to a 25-cm depth, soil organic matter, and whether a location was within the agricultural region. The mid-level (5.0 mg/L) vulnerability model using 48 samples (two outliers removed) produced the best overall fit and was related to the same variables as when using all samples except septic use. The results for mid-level vulnerability provided additional support that septic use was associated with low levels of nitrate in the groundwater. Soil properties and land use were identified as the main drivers of moderate nitrate concentrations. Probabilities of exceeding low-level nitrate concentrations were high in most areas with the lowest probabilities usually to the northwest along thin geologic deposits in the upper part of the basin. The results of this investigation offer the foundational information needed for developing best management practices to mitigate nitrate contamination, basic concepts on water quality to aid public education, and information to guide regulatory measures if policy makers determine this is warranted. Science-based decision making will require continued monitoring and analysis of water quality in the future.

Colorado↗
USGS2010

Pesticides in groundwater in the Anacostia River and Rock Creek watersheds in Washington, D.C., 2005 and 2008

The U.S. Geological Survey (USGS), in cooperation with the District Department of the Environment, conducted a groundwater-quality investigation to (a) determine the presence, concentrations, and distribution of selected pesticides in groundwater, and (b) assess the presence of pesticides in groundwater in relation to selected landscape, hydrogeologic, and groundwater-quality characteristics in the shallow groundwater underlying the Anacostia River and Rock Creek watersheds in Washington, D.C. With one exception, well depths were 100 feet or less below land surface. The USGS obtained or compiled ancillary data and information on land use (2001), subsurface sediments, and groundwater samples from 17 wells in the lower Anacostia River watershed from September through December 2005, and from 14 wells in the lower Anacostia River and lower Rock Creek watersheds from August through September 2008. Twenty-seven pesticide compounds, reflecting at least 19 different types of pesticides, were detected in the groundwater samples obtained in 2005 and 2008. No fungicides were detected. In relation to the pesticides detected, degradate compounds were as or more likely to be detected than applied (parent) compounds. The detected pesticides chiefly reflected herbicides commonly used in urban settings for non-specific weed control or insecticides used for nonspecific haustellate insects (insects with specialized mouthparts for sucking liquid) or termite-specific control. Detected pesticides included a combination of pesticides currently (2008) in use, banned or under highly restricted use, and some that had replaced the banned or restricted-use pesticides. The presence of banned and restricted-use pesticides illustrates their continued persistence and resistance to complete degradation in the environment. The presence of the replacement pesticides indicates the susceptibility of the surficial aquifer to contamination irrespective of the changes in the pesticides used. A preliminary review of the data collected in 2005 and 2008 indicated that differences in the surficial geology, land use (as a surrogate for pesticide use), and above-average precipitation for most of 2004 through 2008, as well as differences in the number and performance of USGS laboratory methods used, could have led to more pesticides detected in groundwater samples collected in 2008 than in groundwater samples collected in 2005. Thus, although data from both years of collection were used for interpretive analysis, emphasis was placed on the analysis of the data obtained in 2008. The presence of pesticides in shallow groundwater (less than approximately 100 ft (feet), or 30 m (meters), below land surface) indicated at least the upper surficial aquifer in Washington, D.C. was susceptible to contamination. One or more herbicides or insecticides were detected in groundwater samples collected from 50 percent of the shallow wells sampled in 2005, and from 62 percent of the shallow wells sampled in 2008. Differences among types of pesticides in shallow groundwater were apparent. The most frequently detected class of herbicides was the s-triazine compounds—atrazine, simazine, or prometon, or the atrazine-degradate compounds—2-chloro-4-ethylamino-6-amino-s-triazine (desethylatrazine or CIAT) and 2-chloro-4-isopropylamino-6-amino-s-triazine (hydroxyatrazine or OIET). The next most frequently detected classes of herbicides were the chloroacetanilides, including metolachlor and acetochlor, and the ureic herbicides, including diuron (and degradate, 3,4-dichloroaniline), fluometuron, metsulfuron methyl, sulfameturon, bromacil, and tebuthiuron. Insecticides also were detected, but less frequently than herbicides, with one or more insecticides present in groundwater samples from 38 percent of shallow wells sampled in 2008. Detected insecticides included parent or degradate compounds commonly used for either nonspecific or haustellate (sucking) insects, including chlorpyrifos and dichlorodiphenyldichloroethane (p,p’-DDD; a degradate of dichlorodiphenyltrichloroethane, DDT), and for termite control, including dieldrin, chlordane, heptachlor epoxide, (a degradate of heptachlor), fipronil, and the sulfone and sulfide degradates of fipronil. The concentrations of individual pesticides in shallow groundwater in both years were low. Maximum concentrations were no greater than a few tenths of a microgram per liter (μg/L); typical concentrations often were less than 0.1 μg/L. Multiple pesticides, however, commonly were present in groundwater. For example, in 2008, approximately 88 percent (7 of 8) of the wells that yielded a sample with at least one detectable pesticide contained five or more pesticides. The highest number of detections occurred in a groundwater sample from well WE Ca 32, which is located in a highly developed urban area; this sample contained 15 different pesticide residues. In relation to human and aquatic health, no pesticide concentration in either 2005 or 2008 exceeded Federal drinking-water standards. Groundwater samples from a few sites, however, contained levels of chiefly banned or restricted-use pesticides that exceeded other human-health and (or) aquatic-health guidelines. For example, concentrations of dieldrin in 2008 groundwater samples from three wells—WE Ca 32 (0.028 μg/L), WE Ba 11 (0.016 μg/L), and WW Ac 8 (0.014 μg/L)—fell within the range of concern for 2004 Federally approved non-regulatory USGS Health-Based Assessment benchmarks (0.002 to 0.2 μg/L), and exceeded earlier (1999) Federal criteria for drinking water (0.000052 μg/L). Other individual compounds whose concentrations exceeded 1999 Federal guidelines for samples from one or more of these three sites, or another site, included p,p’-DDD, dichlorodiphenyldichloroethylene (p,p’-DDE; another degradate of DDT), chlordane, and heptachlor epoxide. Pesticide concentrations in groundwater also were compared to three aquatic-health guidelines for freshwater (United States, Great Lakes, or Canada). One or more of these guidelines were exceeded in groundwater samples obtained in 2005 or 2008 for one or more of the compounds chlordane, dieldrin, heptachlor epoxide, p,p’-DDE, p,p’-DDD, and chlorpyrifos. The spatial distribution of pesticides in the shallow groundwater appeared to be related, in part, to land use, a surrogate for pesticide use. Although most of the wells sampled in this study are in parklands or other relatively open and accessible space, multiple pesticides most often were detected in 2008 groundwater samples collected from wells where a considerable percentage (in excess of 60 percent) of the land within a 500-m radius is developed space (residential, commercial, or other urban infrastructure). Insecticides were detected in wells surrounded by at least 50 percent, and most commonly by more than 80 percent, development. Well WE Ca 32, the site associated with the highest number of pesticide residues in groundwater (8 herbicides and 7 insecticides), is in a small residential park, where 99 percent of the surrounding land is well-maintained residential and commercial development. The vertical distribution of detected pesticides in shallow groundwater appeared to be related, in part, to depth below land surface, surficial-bedrock type, and differences in the chemistry of shallow groundwater. Pesticides were detected at relatively shallow depths in wells that may not have fully penetrated the shallow aquifer. For wells in which at least one pesticide was detected, the median depth below land surface to the top of the well screen was 5.8 m, and the maximum depth was 8.5 m. Among the types of surficial materials in which wells were completed—alluvium, terrace deposits, or Potomac Formation sub- or outcrops in the Coastal Plain Province, and saprolite or fractured bedrock (Laurel and Sykesville Formations) underlying saprolite in the Piedmont Province—no pesticides were detected in groundwater associated with wells completed in the alluvium or fractured bedrock. Detections occurred in some but not all wells completed in the other surficial materials. Overall, the pattern in occurrence appeared related to the local permeability of these sediments and groundwater chemistry. Groundwater with multiple pesticide detections tended to occur in permeable sediments (absent any appreciable overlying clay, silt, or clay-silt layers), in conjunction with other common urban contaminants (elevated chloride in excess of tens to hundreds of milligrams per liter (mg/L), and oxic, rather than reduced, groundwater as evidenced by elevated (in excess of 5 mg/L) concentrations of nitrate). The results of this investigation were compared to results from two other similar and recent studies on pesticide occurrence in the shallow aquifer. These included a study in the nearby Maryland and Delaware Coastal Plain Physiographic Province and one in the Maryland and Virginia Piedmont Physiographic Province. Results from these studies were similar to the current study in relation to (a) the types, frequencies, concentrations, and mixtures of pesticides detected; (b) compounds that exceeded human-and aquatic-health criteria; and (c) the occurrence and distribution of pesticides within the surficial aquifer in relation to depths, sediment types, and groundwater chemistries.

Washington D.C.↗