« Back to Resource Library

Species-Specific Refined Endangered Species Risk Assessment for Static Aquatic Habitats: Part 1, Exposure Modeling

Authors: Lauren Padilla and Michael Winchell, Stone Environmental; Paul Whatling, FMC Corporation; Scott Teed, Intrinsik

Abstract:

The current USEPA approach to static aquatic exposure modeling at Step 2 of their interim process for national endangered species assessments uses simplified assumptions about water body characteristics, the surrounding landscape, and agronomic practices. The USEPA approach considers generalized aquatic habitat dimensions, a homogeneously cropped watershed with 100% cropped area, up to two weather time series, one soil profile, and one pesticide application date pattern per crop group and HUC2. The same predictions of pesticide concentrations derived from these generalized assumptions are assigned to all of the species within a given HUC2 regardless of the location of that species range with respect to different agricultural crops, weather, and soils. The refined exposure modeling approach presented here for malathion focuses on quantitatively accounting for observed variability in environmental and agronomic factors that impact the potential pesticide exposures to endangered species. For each species, probability distributions of application timing, weather, soil and slope conditions, and crop configurations around ponds are sampled to generate on the order of 1000 30-year Pesticide Root Zone Model (PRZM)/Variable Volume Water Model (VVWM) realizations. Each realization is composed of multiple PRZM simulations, with the number of simulations being determined by the cropping pattern complexity around a given pond. The refined approach led to more realistic predictions of pesticide concentrations for a wide range of environmental conditions in unique species ranges. Assuming 100% treated area, concentrations for many species were two to three orders of magnitude lower than suggested by the USEPA Step 2 analysis. Accounting for actual treated area based on eight years of recent malathion use data resulted in more realistic concentrations.This methodology is readily reproducible and extensible to assess aquatic species across the United States. Risk results corresponding to the exposure concentrations generated by the probabilistic approach are the topic of a companion presentation.