A case study simulation of DBCP groundwater contamination in Fresno County, California 1. Leaching through the unsaturated subsurface

This paper is the first installment of a multi-paper series concerned with simulating the potential vulnerability of groundwater in Fresno County (California) to contamination resulting from long-term, agriculture related, applications of the nematocide DBCP. In this paper our focus is on the surface and the unsaturated subsurface. Using PRZM-2, we quantitatively estimate, for a 35 year period, the potential fate and transport of DBCP between the surface and the water table for multiple non-point source applications, related to different and changing land-use, between 1960 and 1977. Our transport simulations include consideration for advection, decay, sorption, and volatilization. The results presented here indicate tremendous spatial and temporal variability in DBCP loading at the water table at the regional scale. Our simulations also suggest that the DBCP loading to the water table, resulting from the legacy of the chemical's use, is currently well below the detectable limit. The characterization of spatially variable DBCP loadings to the saturated subsurface through time, which are presented in this paper, is used as input for the 3-D groundwater models of DBCP fate and transport in Fresno County between 1960 and 1994 that are reported in the companion paper (Loague et al., 1997).