HR: 16:15h
AN: H54D-02    [Abstracts]
TI: Probabilistic Modeling for Risk Assessment of California Ground Water Contamination by Pesticides
AU: * Clayton, M
EM: mclayton@cdpr.ca.gov
AF: Department of Pesticide Regulation, California Environmental Protection Agency, 1001 I Street, Sacramento, CA 95812, United States
AU: Troiano, J
EM: jtroiano@cdpr.ca.gov
AF: Department of Pesticide Regulation, California Environmental Protection Agency, 1001 I Street, Sacramento, CA 95812, United States
AU: Spurlock, F
EM: fcspurlock@cdpr.ca.gov
AF: Department of Pesticide Regulation, California Environmental Protection Agency, 1001 I Street, Sacramento, CA 95812, United States
AB: The California Department of Pesticide Regulation (DPR) is responsible for the registration of pesticides in California. DPR's Environmental Monitoring Branch evaluates the potential for pesticide active ingredients to move to ground water under legal agricultural use conditions. Previous evaluations were primarily based on threshold values for specific persistence and mobility properties of pesticides as prescribed in the California Pesticide Contamination Prevention Act of 1985. Two limitations identified with that process were the univariate nature where interactions of the properties were not accounted for, and the inability to accommodate multiple values of a physical-chemical property. We addressed these limitations by developing a probabilistic modeling method based on prediction of potential well water concentrations. A mechanistic pesticide transport model, LEACHM, is used to simulate sorption, degradation and transport of a candidate pesticide through the root zone. A second empirical model component then simulates pesticide degradation and transport through the vadose zone to a receiving ground water aquifer. Finally, degradation during transport in the aquifer to the well screen is included in calculating final potential well concentrations. Using Monte Carlo techniques, numerous LEACHM simulations are conducted using random samples of the organic carbon normalized soil adsorption coefficients (Koc) and soil dissipation half-life values derived from terrestrial field dissipation (TFD) studies. Koc and TFD values are obtained from gamma distributions fitted to pooled data from agricultural-use pesticides detected in California ground water: atrazine, simazine, diuron, bromacil, hexazinone, and norflurazon. The distribution of predicted well water concentrations for these pesticides is in good agreement with concentrations measured in domestic wells in coarse, leaching vulnerable soils of Fresno and Tulure Counties. The leaching potential of a new pesticide is evaluated by substituting it's sorption and persistence data into the model. Because such Koc and TFD data are often sparse, model inputs are typically derived from sampling of a fitted simple triangular distribution. A new product is considered to be a potential ground water contaminant if the 95th percentile of predicted well water concentrations is greater than 0.05 mg/L.
DE: 1831 Groundwater quality
DE: 1847 Modeling
SC: Hydrology [H]
MN: 2007 Fall Meeting