The Varying Volume Water Model (VVWM), the receiving water body model for USEPA regulatory assessment of aquatic pesticide exposures, is comprised of a set of static and quasi-static receiving water body conceptual models, but research comparing the performance of these models to observed concentrations is limited. The water body models included are the constant volume (CV), constant volume with overflow (CVO), and varying volume with overflow (VVO). This work quantified the performance of these three VVWM conceptual models compared to observations in community water systems (CWS), and the effect of alternative conceptual models on estimated environmental concentrations (EECs) using realistic use application inputs and USEPA conservative guidance for other model inputs.The 50 selected CWS most relevant to the static and quasi-static VVWM concepts were small in size, with estimated time to peak flow less than 1.5 days for consistency with the daily runoff assumption in the USEPA landscape Pesticide Root Zone Model (PRZM). The CVO and VVO models parameterization resulted in similar distributions of model bias across CWS with the median results being close to no bias, but the CV model parameterization resulted in overestimation in the majority of CWS with median model bias nearly 3 times the observed values. At present the CV conceptual model parameterized with conservative input assumptions has been the regulatory standard invoked in VVWM, yet our results showed that a more physically correct conceptual model (CVO or VVO) could be invoked in regulatory exposure modeling and would remain appropriately conservative for screening purposes.

Comparison of pesticide concentrations observed in community water systems to predictions from US regulatory aquatic exposure models

2019 Fall National Meeting

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