EPA Releases Final Analysis of Model Used to Estimate Pesticide Concentrations in Groundwater

For Release February 28, 2022

The U.S. Environmental Protection Agency (EPA) is releasing its final Analysis of Subsurface Metabolism in Groundwater Modeling (Analysis), along with a response to comments document, the results of an independent peer review, and an implementation memo.

EPA first implemented its current model for estimating pesticide concentrations in groundwater in 2012. The model is part of the Pesticides in Water Calculator, which EPA uses to estimate exposure to pesticides in drinking water when developing human dietary risk assessments.

Since implementing the model in 2012, it became apparent to EPA that estimates provided using this model are often orders of magnitude higher than the pesticide concentrations observed in real-world monitoring data. As a result, EPA developed a draft Analysis to explore possible revisions to its model. The Analysis evaluates possible revised assumptions that EPA could use in its methodology for estimating pesticide concentrations in groundwater.

In April 2021, EPA released the draft Analysis for public comment. In response to public comments and a peer review by experts in soil science, hydrology, environmental fate of pesticides, and groundwater modeling, EPA has updated and included additional clarification of its scientific analyses in the final Analysis released today.

Based on the Analysis, EPA is updating the assumption about the depth at which pesticides may undergo aerobic soil metabolism (i.e., the microbial degradation of pesticides in soil). EPA’s modeling of pesticide concentrations in groundwater currently assumes that pesticides may undergo aerobic soil metabolism up to one meter beneath the surface. The Agency is updating this assumption by extending the depth at which soil metabolism occurs to two meters below the surface.

This change is supported by the Analysis, which includes a literature review and a comparison of modeled versus measured pesticide concentrations in groundwater. This is the only assumption that EPA is updating in the model. The assumption that hydrolysis — the chemical breakdown of a compound due to a reaction with water — may occur throughout the soil profile remains the same.

See docket EPA-HQ-OPP-2021-0241 at www.regulations.gov to read the final Analysis of Subsurface Metabolism in Groundwater Modeling, response to comments document, summary of independent peer review, and implementation memo.

Learn more about EPA’s models for pesticide concentrations in water.