Profiling areas of ground water contamination by pesticides in California : Phase II - Evaluation and modification of a statistical model

A well sampling study was conducted to evaluate an empirical approach to classifying areas of land in California as vulnerable to ground water contamination by pesticides (Troiano et al., 1994). Wells were sampled from sections of land that had no previous detections of pesticide residues. The sections had been classified into vulnerable soil clusters or into a not-classified group using a procedure based on Principal Components Analysis (PCA). Grape, citrus, and olive growing areas of Fresno and Tulare Counties were targeted, areas where pre-emergence herbicide residues had been detected in well water. Overall, herbicide residues were detected in 75 of 176 sampled wells, a high frequency of detection in relation to results from previous targeted well sampling studies. Since residues were also detected in the not-classified group, the classification procedure was modified using an approach based on Canonical Variates Analysis (CVA). More sections were classified into vulnerable soil clusters with the CVA approach than with the PCA method. Data from two other explanatory variables, depth to ground water and amount of pesticide used per section, were included to illustrate how additional information can be incorporated into this approach of identifying vulnerable areas.