PESTICIDE OCCURRENCE IN GROUNDWATER IN TULARE COUNTY, CALIFORNIA

Geographic Information Systems (GIS) and statistical methods were used to identify the major factors affecting pesticide leaching in groundw ater from agricultural fields in Tulare County, California. Residues o f bromacil, diuron, and simazine increased in groundwater during the 1 980s. Bromacil, diuron, and simazine contamination were positively cor related to crop diversity and water demand. Diuron and simazine were p ositively correlated to groundwater depth and negatively correlated to soil water-holding capacity. DBCP concentration in groundwater was re lated to the crop coverage. The Goss model was used to examine soil-pe sticide interactions and a Pesticide Contamination Index (PCI) was dev eloped. Areas having high leaching potentials were mainly associated w ith citrus and orchards and coarse-textured sandy soils along the Sier ra Nevada foothills, while areas having low leaching potentials were a ssociated with field crops and clay soils of the southwest region. The PCI was largest for DBCP during the 1980s, suggesting that it was the most significant contaminant before 1977 when it was widely used; how ever, wells were not tested for this pesticide during that period. Twe lve years after DBCP was banned, it was still the most significant hea lth risk contaminant. Spatial maps showing the distribution of leachin g potentials and soil interactions for these pesticides can provide us eful information to regulatory and planning agencies for land use plan ning and pesticide management.