15
Oct
Report on Pesticide Contamination of Nation’s Groundwater Shows Widespread Exposure and Health Threats
(Beyond Pesticides, October 15, 2025) The latest Scientific Investigations Report for 2025 from the U.S. Geological Survey (USGS), entitled “National Water Quality Program: Multidecadal Change in Pesticide Concentrations Relative to Human Health Benchmarks in the Nation’s Groundwater,” finds moderate concentrations of five pesticides, with the highest percentages in agricultural wells, and concentrations of the carcinogenic soil fumigant DBCP (1,2-dibromo-3-chloropropane), which also causes infertility, that are greater than the maximum containment level, despite being banned over 45 years ago. These results highlight the persistence of pesticides used in agriculture and the elevated risks of pesticide contamination in agricultural areas.
This report monitors concentrations of pesticides in well networks across the U.S. in decadal intervals, with this last one incorporating data ranging from 1993-2023. Additionally, DBCP in one well network in the San Joaquin-Tulare River Basin in California continues to be assessed due to previous levels exceeding the human health benchmark (HHB) established by the U.S. Environmental Protection Agency (EPA).
The limitations of the study are disclosed in the text of the report. As the authors state: “Only pesticides with an HHB were included in the multidecadal pesticide change analysis… The total number of pesticides included in this study is less than the previous national assessment (n=80), because only 21 compounds were included in laboratory analysis from 1993 to 2023 and also had HHBs to evaluate potential risk to human health.” This limitation restricted the study, which only includes 22 pesticides in their analysis from 1993 to 2023. The 22 pesticides in which concentrations were measured in groundwater include: the degradate deethylatrazine (DEA) and the parent compounds acetochlor, alachlor, atrazine, azinphos-methyl, carbaryl, chlorpyrifos, cis-permethrin, diazinon, fonofos, malathion, methyl-parathion, metolachlor, metribuzin, pendimethalin, phorate, prometon, propyzamide, simazine, tebuthiuron, terbufos, and 1,2-dibromo-3-chloropropane (DBCP). This list does not include many pesticides with a history of high use, such as glyphosate, 2,4-D, dicamba, or paraquat.
Furthermore, the limitation of HHBs as a measure of safety has been raised as a concern by Beyond Pesticides. HHBs, like other regulatory measures of risk, do not take into account certain adverse health outcomes, such as endocrine disruption, for which EPA has not established a completed protocol for regulatory review. Additionally, EPA does not fully evaluate the effect of pesticide mixtures with other pesticides and other chemicals like pharmaceuticals. Exposure to mixtures can cause both cumulative and synergistic effects, as captured by the independent peer-reviewed scientific literature.
Overview of Groundwater Contamination
The widespread use of pesticides in both agricultural and urban environments has led to the contamination of groundwater, which threatens human health when the contaminated groundwater is a source for drinking water. As the authors state, “In the United States, groundwater from domestic supply wells is used as a drinking-water source for 13 percent of the U.S. population.” Since domestic supply wells are not regulated by state or federal law, this leaves residents accountable for not only maintenance but for monitoring these drinking-water sources. (See here, here, and here.)
Fifty-nine well networks, each including 20 to 30 wells, are monitored for pesticide concentrations by USGS. These well networks are “distributed throughout eight aggregated ecoregions (Pacific Northwest, Pacific Coast, Arid West, Semiarid West, Mountain West, Midcontinent, South Atlantic Gulf, and Northeast)… [and] represent a range of soils, climate, and landforms in the conterminous United States.” The USGS National Water Quality Network for Groundwater (NWQN-GW), according to the report, is the largest spatially distributed groundwater-quality monitoring network in the world.
This monitoring is a result of the 2009 SECURE Water Act, where the U.S. Congress tasked USGS to perform regular, comprehensive water availability assessments. The USGS National Water Quality Network-Groundwater (NWQN-GW) began in 1991, operating as a part of the larger National Water Quality Assessment (NAWQA) Project, while the National Ground-Water Monitoring Network (NGWMN) is a separate network specifically for monitoring groundwater that began with a pilot network in 2009 and began full implementation in 2015.
“Groundwater quality is a key water resources domain that can affect water availability trends, and the purpose of this multi-decadal groundwater pesticide trends study is to assess changes in concentrations within the NWQN-GW,” the authors note. Changes in relative concentrations are noted when the percentage of wells with pesticide contamination exceeds human health benchmarks (HHBs). The HHBs in this report, those available from EPA, incorporate legally enforceable drinking-water standards and nonenforceable drinking water levels.
Study Methodology
The analysis of the 22 pesticides in groundwater is separated into decadal intervals, with decade 1 from 1993–2001, decade 2 from 2002–2012, and decade 3 from 2013–2022. The quality of groundwater is assessed in 24 agricultural wells, 15 urban wells, and 20 domestic supply wells within 25 principal aquifers of the U.S. that have been monitored since 1993. The additional analysis of DBCP in California includes data collected in decades 1-3, as well as decade 4 (2023–onward) within one well network comprised of 36 wells.
The authors state: “Samples collected in decades 1 and 2 were analyzed using gas chromatography/mass spectrometry or high-performance liquid chromatography… Decade 3 samples were analyzed at the NWQL using a broad-spectrum liquid chromatography-tandem mass spectrometry method.” They continue: “The change in laboratory analytical methods between decades 2 and 3 has the potential to introduce bias into the analysis of datasets that span all three decades.” This raises concerns about the ability of the reported data to reflect accurate risks to human health.
After sampling the wells and performing analyses in the lab for concentrations of the 22 pesticides included in the study, the relative concentrations, as compared to HHBs, classified the results into one of four categories. Pesticide concentrations above the HHB are high, while those “that exceeded 0.10 of the HHB but were lower than or equal to the HHB were moderate. Concentrations that exceeded 0.05 of the HHB but were lower than or equal to 0.10 of the HHB were defined as low-moderate. Concentrations lower than or equal to 0.05 of the HHB were low.”
Study Results
In the main sampling for decades 1-3, no pesticides are detected at high concentrations, but five pesticides are detected at moderate concentrations, including alachlor, atrazine, deethylatrazine (DEA), prometon, and simazine. The authors also report: “The percentage of all wells that had pesticide concentrations in the moderate category decreased each decade, from 7 percent in decade 1 to 2 percent in decade 3… The agricultural wells were the well type that had the highest percentages of moderate concentrations, and these percentages decreased each decade.”
This reported decrease raises many questions. Are the numbers lower in the latest decade due to the replacement of the tested pesticides with other pesticides not accounted for? Is there actually a reduction in pesticide use as organic agriculture is being adopted? Are there limitations within the study, in addition to the change in analytical methods mentioned above, that do not accurately reflect the concentrations of pesticides within the wells?
The authors say: “We hypothesize that one of the processes that may have contributed to the reduction of pesticide concentrations in groundwater include degradation through abiotic or biotic processes in soils or groundwater. Alternative explanations for the decrease include reduction in pesticide use or a change in the transport of pesticides to groundwater over time. Changes in pesticide use and soil management, coupled with changes in precipitation and temperature, can vary over time, affecting pesticide transport to groundwater.” (See here and here.)
Additionally, four pesticides, including alachlor, atrazine, DEA, and simazine, are detected at low-moderate concentrations, with the highest percentages of low-moderate concentrations occurring in the agricultural wells and remaining constant for each decade.
The additional analysis of DBCP in California for decade 4, which was conducted since this is the only pesticide that previously exceeded its respective HHB, shows that despite being banned over 45 years ago, DBCP concentrations are greater than the maximum contaminant level of 2 micrograms per liter (µg/L) for all four decades. The number of exceedances decreased from 1993 to 2023, but is still higher than the HHB overall.
In summary, the authors say: “In our study, pesticides were detected at moderate concentrations in domestic supply wells in three aggregated ecoregions: the Arid West, Northeast, and Semiarid West. The domestic-well networks within the NWQN-GW cover areas that supply groundwater to more than 6 million people, or about 13 percent of the total number of people relying on domestic supply in the United States, and these networks cover at least part of the principal aquifers that together represent 99 percent of the withdrawals for domestic supply.” (See here, here, and here.)
Previous Research
Beyond Pesticides extensively covers pesticide contamination throughout air, water, soil, and food, as well as within human and wildlife bodies that are subjected to pesticides through multiple exposure routes. As shared in Daily News, the toxic soup in many U.S. waterways is unsustainable and threatens the foundation of many food chains. Imbalances in aquatic environments can ripple throughout the food web, creating trophic cascades that further exacerbate health and environmental damage. Studies of major rivers and streams find that 90% of fish, 100% of surface water samples, and 41% of major aquifers contain one or more pesticides at detectable levels. Almost 90% of water samples contain at least five or more different pesticides.
Daily News from 2021 reports on a study published by the USGS in which they determined that millions of people are drinking from groundwater reserves riddled with pesticide and pesticide metabolites or breakdown chemicals. More specifically, USGS researchers found that 41% of public drinking water supply wells are contaminated with pesticides. USGS, in a 2023 study published in Environment International, found that nearly half (45%) of U.S. tap water is contaminated with per- and polyfluoroalkyl substances (PFAS). Researchers note that USGS can only detect 32 of the more than 12,000 different types of PFAS and PFAS breakdown chemicals, thus indicating the number is most likely higher. (See Daily News here and additional coverage in the groundwater archive here.)
The Organic Alternative
This USGS report further highlights the need for a holistic solution to pesticide contamination. The limitations highlighted within this study show how the true risks from pesticide exposure to human health are not fully evaluated. The authors themselves say, “Currently, groundwater contaminants from either geogenic or anthropogenic origin are very likely to coexist in groundwater, and there is a scientific gap in understanding about the combined effects of these groundwater contaminants on human health.” The inability to consider the cumulative exposure to pesticides, where additive or synergistic effects can occur, threatens the health of the public, wildlife, and the environment.
Additive effects of pesticide mixtures occur when the combined effect is equal to the sum of the individual effects, while synergistic effects occur when the combined effect is greater than the sum of the individual effects. In a Pesticides and You article, Beyond Pesticides reports that pesticide exposures in the real world are not isolated incidents. Rather, they are a string of incidents marked by combinations of exposures. As a result, scientists have argued for years that toxic exposures to pesticides should be measured as they would normally occur, in combination with one another. Yet, current federal law does not require this type of testing for pesticides on the market, except in very limited instances.
In Daily News titled “Scientific Studies Identify EPA Deficiency in Evaluating Safety of Toxic Chemical Interactions,” Beyond Pesticides references a plethora of scientific literature in calling on Congress to require EPA to incorporate real world science into its evaluation of pesticide safety calculations by recognizing that daily exposure involves multiple chemicals and synergistic interactions— a magnified effect greater than the individual chemical effects added together. As noted by Maricel Maffini, PhD, and Laura Vandenberg, PhD, in a commentary in Frontiers in Toxicology, “Current approaches also rely on the assumption that testing chemicals one at a time is appropriate to understand how chemicals act under real-world conditions. Numerous mixture studies, including ones that demonstrated cumulative effects, have disproven this assumption.” (See additional coverage on synergistic effects here.)
Despite the limitations of the target chemicals in the report and the regulatory standards supporting the HHBs, the authors feel: “The rarity of HHB exceedances and the national-scale decrease in moderate concentrations of pesticides can be viewed as encouraging results from a human-health standpoint. However, continued monitoring and assessment of groundwater pesticides is warranted, as many negative human-health effects have been linked to pesticide exposure, and these negative effects can occur when pesticide concentrations are below the human health benchmarks used in this study.” (See research here, here, and here.)
Taking into consideration all of the adverse health effects that peer-reviewed scientific literature connects to pesticide exposure, as well as all of the effects that have yet to be fully studied, the only way, according to Beyond Pesticides, to ensure that any level of pesticide contamination within groundwater or any other resource causes no harm is to transition fully to organic agricultural and land management practices. The health and environmental benefits of organic methods are widely documented and supported by science. (See here, here, and here.)
Take action today by telling your local officials to make your parks organic. Does your community have a pesticide-free park managed with organic practices? Do you wish it did? The time to take action to protect those parks and create new ones is now. With Beyond Pesticides’ supporters, including the retailer Natural Grocers in the Midwest and west, the Beyond Pesticides’ Parks for a Sustainable Future program provides in-depth training to assist community land managers in transitioning two public green spaces to organic landscape management, while aiming to provide the knowledge and skills necessary to eventually transition all public areas in a locality to these safer practices.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source:
Stackpoole, S., Lindsey, B. and Nell, C. (2025) National Water Quality Program: Multidecadal Change in Pesticide Concentrations Relative to Human Health Benchmarks in the Nation’s Groundwater, U.S. Geological Survey. Available at: https://pubs.usgs.gov/sir/2025/5081/sir20255081.pdf.
