Water Quality Data for Multidecadal Change in Pesticide Concentrations Relative to Human Health Benchmarks in the Nation’s Groundwater

In the United States, the widespread use of pesticides has led to contamination of groundwater resources. Three decades ago, the U.S. Geological Survey (USGS) established a national water quality monitoring network for groundwater (NWQN-GW). This network provides an overview of groundwater conditions across a wide range of hydrogeologic settings and capture the impacts of pesticide use practices in both agricultural and urban environments as well as areas where groundwater is used as a source of domestic supply. This data release provides groundwater pesticide concentrations from 58 well networks for 21 pesticides for 3 decades. The first decade of sampling was 1988–2001, the second was 2002–2012, and the third was 2013–2021. In decades 1 and 2, 20–30 wells in 8 to 10 networks were sampled in a given year, so that all the wells in a network were sampled once every 10 years on a rotating basis. In the third decade, the sampling was reduced to 8 wells within a network.
Human health benchmarks (HHBs) were used to classify pesticide concentrations. Pesticide concentrations above an HHB are defined as high. Concentrations that exceeded 0.10 of the HHB but were lower than the benchmark were moderate. Concentrations that exceeded 0.05 but were lower than 0.10 of the benchmark were defined as low moderate. Concentrations less than 0.05 of the benchmark were low. If available, the HHBs were the USEPA Drinking Water Standards Maximum Contaminant Levels (MCLs). MCLs, established through the Safe Drinking Water Act, are legally enforceable for public water supplies. The USEPA Human Health Benchmarks for Pesticides (HHBPs) and the USGS Health Based Screening Levels (HBSLs) were used as supplemental source of benchmarks. The HHBPs and HBSLs are for pesticides for which the US EPA has not issued a drinking water health advisory or set an enforceable federal drinking water standard. Only pesticides with at least 1 of these types of HHBs were included in the multidecadal pesticide change analysis.
Data for a fumigant, 1,2-Dibromo-3-chloropropane (DBCP), a volatile organic compound (VOC), are also included. The DBCP results presented herein are for data from the USGS NWQN-GW and the Enhanced Trends Network (ETN) representing shallow groundwater in the Central Valley aquifer of California. The VOC data expands the sampling into a fourth decade. Wells from the NWQN-GW network were sampled once per decade, in 1993, 2001, 2013, and 2023. Wells from the USGS ETN were sampled bimonthly from 2014 to 2019.

For more in-depth information on the methodology used to these collect the data, calculate the metrics, and interpretat the results as they relate to national-scale changes in water availability, please refer to: Stackpoole, S.M., Lindsey, B.D., and Nell, C.S., 2025, Multidecadal change in pesticide concentrations relative to human health benchmarks in the Nation’s groundwater: U.S. Geological Survey Scientific Investigations Report 2025–5081, 21 p., https://doi.org/ 10.3133/ sir20255081.

This work was completed as a part of the USGS Pesticides, PFAS, and Geogenic Constituents Data in Surface Water and Groundwater Nationwide to support Integrated Water Availability Assessments Reports Project, which is supported by the Water Mission Area Water Quality Processes Program with the objective to assess changes in water quality that may affect water availability for human and ecological use (https://pubs.usgs.gov/fs/2020/3044/fs20203044.pdf).