As part of the pesticide registration process, the U.S. Environmental Protection Agency models human exposure to pesticides through groundwater drinking water sources based on parameter estimates obtained from laboratory studies. One of these studies, abiotic hydrolysis, produces formation and decline time-series data for the test compound and its degradates from multiple pH/temperature combinations to assess variation in degradation rates with water pH and temperature. Because infiltrating rainwater and irrigation water contaminated with pesticides can take years to reach a drinking water well at depth, abiotic hydrolysis, even if slow, can greatly reduce exposure to a pesticide and/or its degradates. Therefore, the ability to reliably estimate abiotic hydrolysis half-lives from laboratory studies can be critical for pesticide registration decisions, especially for those pesticides that degrade slowly (<i>e.g.</i>, half-lives &gt; 100 days). This study evaluates three models that estimate formation and decline parameters from abiotic hydrolysis studies: 1) a mass-balance formation and decline model calibrated for individual pH/temperature combinations; 2) the same mass balance model calibrated across pH based on the mechanics of hydrolysis reactions; and 3) the second (calibrated across pH) model additionally calibrated across temperature based on the Arrhenius equation. To evaluate these models, replicated abiotic hydrolysis studies were split into two separate data sets for each replicate, and each model was calibrated separately to both replicate data sets. By comparing the parameter estimates from these independent calibrations between study replicates, it is shown that the model calibrated across pH and temperature provides the most reliable parameter estimates for pesticides with the longest abiotic hydrolysis half-lives.

Reliable estimation of abiotic hydrolysis formation and decline parameters across pH and temperature for pesticide risk assessment

254th National Meeting (2017)

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