A geo-referenced modeling environment for ecosystem risk assessment: organophosphate pesticides in an agriculturally dominated watershed.

A geo-referenced modeling system was developed in this study to investigate the spatiotemporal variability of pesticide distributions and associated ecosystem risks. In the modeling system, pesticide fate and transport processes in soil-canopy system were simulated at field scale by the pesticide root zone model (PRZM). Edge-of-field mass fluxes were up-scaled with a spatially distributed flow-routing model to predict pesticide contaminations in surface water. The developed model was applied to the field conditions of the Orestimba Creek watershed, an agriculturally-dominated area in California's Central Valley during 1990 through 2006, with the organophosphate insecticides diazinon and chlorpyrifos as test agents. High concentrations of dissolved pesticides were predicted at the watershed outlet during the irrigation season of April through November, due to the intensive pesticide use and low stream flow. Concentration violations, according to the California aquatic life criteria, were observed for diazinon before 2001, and for chlorpyrifos during the entire simulation period. Predicted pesticide exposure levels showed potential adverse effects on certain genera of sensitive aquatic invertebrates in the ecosystem of the Orestimba Creek. Modeling assessments were conducted to identify the factors governing spatial patterns and seasonal trends on pesticide distribution and contamination potentials to the studied aquatic ecosystem. Areas with high pesticide yields to surface water were indicated for future research and additional studies focused on monitoring and mitigation efforts within the watershed. Improved irrigation techniques and management practices were also suggested to reduce the violations of pesticide concentrations during irrigation seasons.