A new U.S. Geological Survey study shows some pesticides, and the chemical compounds they degrade to, are common in groundwater used for public drinking water supply, but mostly at concentrations well below levels of concern for human health.
At least one pesticide or degradate was detected in slightly more than 40 percent of samples of untreated groundwater collected from 1204 wells that tap major drinking water aquifers. These aquifers produce approximately 70 percent of the groundwater used for drinking in the United States.
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 percent of the samples contained at least one pesticide degradate. The study also found that pesticide mixtures (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.
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 percent 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.
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