Pesticides are chemicals or biological agents that control plant or animal pests and include herbicides, insecticides, fungicides, and rodenticides. More than a billion pounds of pesticides (measured as pounds of active ingredient) are used in the U.S. each year to control weeds, insects, and other organisms that threaten or undermine human activities. About 80 percent of this total is used for agricultural purposes (U.S. EPA, 2011). Although pesticide use has resulted in increased crop production and other benefits, pesticide contamination of ground water can cause unintended adverse effects on drinking water, irrigation, and other uses.

This indicator examines the presence of pesticides in ground water in selected regions of the contiguous U.S., representing a variety of hydrogeologic conditions. Data come from analyses of water samples collected by the U.S. Geological Survey (USGS) as part of the National Water-Quality Assessment (NAWQA) Project. USGS collected data during two study periods that were both roughly a decade in length: 1993–2001 and 2002–2011. Samples were taken from 1,271 wells in 58 well networks across a variety of land-use types. The 58 well networks are distributed across 23 regionally extensive principal aquifers that are important sources of ground water supply. One untreated ground water sample was collected from each well during each of the two decades, and samples were analyzed for as many as 83 different pesticides and pesticide degradation products.

This indicator tracks the percentage of wells in which pesticides were detected in ground water (Exhibits 1 and 2) and the percentage of wells in which pesticide concentrations exceeded levels that are estimated to be safe for human health in drinking water (Exhibits 3 and 4). Exceedances were determined based on three types of benchmarks:

1. For contaminants that EPA regulates under the Safe Drinking Water Act, concentrations were compared with EPA Maximum Contaminant Levels (MCLs).
2. For unregulated contaminants, concentrations were compared with EPA Human Health Benchmarks for Pesticides (HHBPs) in cases where HHBPs have been derived.
3. For unregulated contaminants for which HHBPs have not been derived, concentrations were compared with Health-Based Screening Levels (HBSLs) that USGS derived using the same approach that EPA uses to develop HHBPs.

These benchmarks are concentrations below which pesticides are not anticipated to cause adverse human health effects from a lifetime of exposure, based on the best available chronic toxicity data from EPA. The use of chronic health benchmarks is appropriate in this case because ground water chemistry changes relatively slowly compared with surface water, so even a single sample from a well is likely to be fairly representative of the condition of the water over a long period of time (potentially years or decades, depending on the hydrogeology of a region). For contaminants with multiple HHBPs or HBSLs (i.e., one for noncancer effects and one for cancer), this analysis used the lower value for screening. MCLs are enforceable standards for drinking water, while HHBPs and HBSLs are not. Altogether, MCLs, HHBPs, or HBSLs were available for 73 of the 83 pesticides and degradation products analyzed.

Exhibits 1 and 3 display the aggregated results for all the pesticides analyzed during the most recent decade of sampling (2002–2011), providing the most current assessment available. Exhibits 2 and 4 show changes over time by comparing the first and second decades of sampling for certain common pesticides that were measured during both decades. In all four exhibits, results are grouped according to the dominant land use in the vicinity of the wells: agricultural, urban, or mixed use. Well networks consisted of shallow wells (typically 20 to 100 feet deep) in agricultural and urban land-use areas and deeper wells (typically 100 to 300 feet deep) in major aquifers in mixed land-use areas. For more information about the sampling design, see Toccalino et al. (2014).

Pesticide Detections

During the 2002–2011 study period, pesticides were detected in 73 percent of wells in agricultural land-use areas, 55 percent of wells in urban land-use areas, and 38 percent of wells in areas of mixed land use (Exhibit 1). Exhibit 2 shows the 20 pesticides that were detected in more than 2 percent of ground water samples collected during at least one of the decades of sampling. The five most frequently detected pesticide compounds were the herbicides atrazine (and its degradation product, deethylatrazine), simazine, metolachlor, and prometon. Atrazine and deethylatrazine were detected in about 20 to 50 percent of wells across all land uses in both decades.

Pesticide Exceedances

From 2002 to 2011, about 6 percent of wells in urban land-use areas contained at least one pesticide at a concentration greater than the MCL, HHBP, or HBSL (Exhibit 3). Human health benchmarks were exceeded in less than 2 percent of wells in agricultural land-use areas and in less than 1 percent of wells in areas of mixed land use. Dieldrin, a highly persistent organochlorine insecticide, accounted for nearly all exceedances in the first decade of sampling and all exceedances in the second decade (Exhibit 4). Dieldrin exceedances occurred predominantly in ground water from urban areas, a finding that may be attributable to the fact that urban application of dieldrin for termite control was permitted until 1987, whereas EPA banned all other uses more than a decade earlier (Agency for Toxic Substances and Disease Registry, 2002).

• These data represent ground water from a set of well networks sampled by USGS in the contiguous U.S. While these wells were chosen to be representative of a wide range of land-use, climatic, and hydrologic conditions, they are not intended to statistically represent the distribution of concentrations in all the nation’s ground water. In particular, undeveloped areas are not included in this indicator.
• This indicator provides only a simple comparison of results across decades. It does not provide information about changes that occur at time scales shorter than 10 years, nor does it provide statistical information about changes in pesticide concentrations over time, which are evaluated elsewhere (Toccalino et al., 2014).
• This indicator accounts for only 83 different types of pesticides and pesticide degradation products, while more than 400 pesticides may be in use during any given year (Stone et al., 2014a,b).
• These data represent ground water quality conditions in untreated (source) water. Pesticide concentrations greater than MCLs, HHBPs, or HBSLs in source water are of potential human health concern, but they do not necessarily indicate that adverse effects will occur, because 1) the benchmarks are conservative (protective), 2) source-water samples were collected prior to any treatment or blending that could alter contaminant concentrations in finished drinking water (some drinking water treatment processes can reduce concentrations of certain pesticides, but they can also create harmful byproducts in other cases), and 3) about half (651) of the 1,271 wells in this study are observation wells that are not used for drinking water. Thus, the pesticide concentrations used for this indicator do not represent the exposure associated with consumption of drinking water from a public water system. Some Americans do get their home drinking water from private wells with little to no treatment, but the levels of contaminants reported in this indicator are not necessarily representative of what most Americans might be exposed to in drinking water—particularly “finished” (i.e., treated) drinking water.
• None of the ground water samples were collected for regulatory compliance purposes. Pesticide concentrations greater than MCLs do not represent MCL violations because MCLs do not apply to source waters.
• No MCL, HHBP, or HBSL is available for 10 of the pesticides and pesticide degradation products analyzed, so the occurrence of those compounds could not be compared with these benchmarks.
• MCLs, HHBPs, and HBSLs do not account for the effect of mixtures of pesticides, which may be a concern when multiple pesticides are present at concentrations approaching these benchmarks. Various combinations of pesticides can interact in different ways.

Summary data for this indicator were provided by USGS. Pesticide data are based on a report by Toccalino et al. (2014), which provides additional analysis and interpretation. For more information on the MCLs, HHBPs, and HBSLs used in Exhibits 3 and 4 of this indicator, see https://water.usgs.gov/water-resources/hbsl/.