At least one pesticide or degradate was detected in just over 40% of samples of untreated groundwater collected from 1,204 wells that tap major drinking-water aquifers. These aquifers produce about 70% of the groundwater used for drinking in the U.S.

The study is the first to assess a large number of pesticide degradates—more than 100—in groundwater at the national level. The results reveal the prevalence of degradates in groundwater. About 30% of the samples contained at least one pesticide degradate. The study also found that pesticide mixtures, that is, more than one pesticide, were present in about one-quarter of samples.

The pesticide concentrations measured are unlikely to be a potential human-health concern. To evaluate results for their potential relevance to human health, individual pesticide compounds were compared to their human-health benchmarks. For the many degradates without benchmarks, concentrations were compared to the benchmark for the “parent” pesticide. And for mixtures of pesticides, the ratio of each compound to its benchmark was computed and the results summed. Overall, pesticides—singly or in a mixture—were found to be of potential concern for human health in 1.6% of wells, where potential concern was defined as a concentration exceeding one-tenth of the human-health benchmark (or 0.1 for mixtures). No pesticide exceeded its benchmark, and no sum of mixture ratios exceeded 1.0.

Citation: Bexfield, L., Belitz, K., Lindsey, B., Toccalino, P., Nowell, L. 2020. Pesticides and pesticide degradates in groundwater used for public supply across the United States: Occurrence and human-health context. Environmental Science and Technology, https://dx.doi.org/10.1021/acs.est.0c05793