PESTICIDE OCCURRENCE IN GROUNDWATER IN TULARE COUNTY, CALIFORNIA

Abstract

Geographic Information Systems (GIS) and statistical methods were used to identify the major factors affecting pesticide leaching in groundwater from agricultural fields in Tulare County, California. Residues of bromacil, diuron, and simazine increased in groundwater during the 1980s. Bromacil, diuron, and simazine contamination were positively correlated to crop diversity and water demand. Diuron and simazine were positively correlated to groundwater depth and negatively correlated to soil water-holding capacity. DBCP concentration in groundwater was related to the crop coverage. The Goss model was used to examine soil-pesticide interactions and a Pesticide Contamination Index (PCI) was developed. Areas having high leaching potentials were mainly associated with citrus and orchards and coarse-textured sandy soils along the Sierra Nevada foothills, while areas having low leaching potentials were associated 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; however, wells were not tested for this pesticide during that period. Twelve years after DBCP was banned, it was still the most significant health risk contaminant. Spatial maps showing the distribution of leaching potentials and soil interactions for these pesticides can provide useful information to regulatory and planning agencies for land use planning and pesticide management.