Regulatory Modeling of Pesticide Aquatic Exposures in California's Agricultural Receiving Waters

For the aquatic exposure assessment of pesticides, the USEPA uses the Variable Volume Water Model (VVWM) to predict the estimated environmental concentrations (EECs) of a pesticide in a water body that receives runoff inputs from the Pesticide Root Zone Model (PRZM). The standard farm pond and additional generalized static and flowing water bodies used in endangered species assessment (aquatic bins) are used by USEPA to model the worst-case aquatic exposure for the nationwide exposure assessment. However, whether or not model results are relevant to state-specific conditions has not been validated. In this study, the USEPA water body scenarios are examined for their capability of providing a conservatively realistic estimate of pesticide aquatic exposures in California's agricultural settings. The sensitivity of modeled EECs to key water body parameters (dimensions, flow, and mass transfer) was explored with a one-at-a-time approach by using the standard farm pond as a baseline. The EECs generated from different USEPA water bodies for the worst-case loading were compared with the monitoring data observed in California's agriculturally influencing water bodies. Results showed that the farm pond EECs well captured the worst-case monitoring data, whereas the aquatic bins EECs, especially the flowing bins, tended to overestimate data. The conceptual model of the standard farm pond was also found to be relevant to the highly vulnerable water bodies in California's agricultural areas. The study confirms that VVWM with the standard farm pond scenario is appropriate for the screening-level regulatory exposure assessment in California's agricultural settings.