Scientific Basis for Identifying Ground Water Protection Areas

Back to Identifying and Protecting Ground Water Protection Areas

How DPR identified areas vulnerable to pesticide movement to ground water

Pesticides have been found in a variety of climatic, soil, and depth to ground water conditions. In the 1990s DPR scientists wondered whether there was a way to characterize these various conditions so that pesticide movement to ground water could be predicted, and thus prevented before contamination occurs. The following sections describe the source of pesticide detection data, and the various statistical techniques, the mathematical model, and depth-to-ground-water data used by DPR to identify vulnerable areas. These vulnerable areas where unmitigated use of certain pesticides is likely to contaminate ground water were adopted in regulation as "ground water protection areas."

Source of pesticide detection data: Well inventory database

The Pesticide Contamination Prevention Act (PCPA) requires DPR to sample wells for pesticides with the potential to pollute ground water. The PCPA further requires all local, county and state agencies to submit all results of well sampling for pesticides to DPR. In addition, DPR encourages other entities, such as the United States Geological Service, to submit results. DPR is also required to maintain a database of these well sampling results. As a result, DPR has a large data set of wells containing pesticide residues from legal agricultural use. DPR used this data set to determine if there was a relationship between detections and local climate, soil, and depth to ground water characteristics. DPR´s well inventory database and well sampling strategies are summarized in:

Statistical methods and modeling

Because of the wide range of climatic and soil conditions associated with pesticide contamination of ground water, an empirical statistical approach was devised to determine whether vulnerable areas could be described. The empirical analysis was conducted in two steps. In the first step, a cluster statistical procedure was used to identify groups of contaminated wells with similar soil properties. Initially, principal component analysis was used to associate sections of land with those clusters. These procedures are described in the following two publications - a DPR report available on the website, and a journal article available from John Troiano at

Later, an alternative procedure (canonical variates analysis) was used to improve characterization of vulnerable areas. The following two publications describe how DPR tested that procedure by sampling wells inside and outside of vulnerable areas, considering depth to ground water as an additional factor. The statistical model developed to identify vulnerable areas is called the CALVUL model. Copies are available from John Troiano at

  • Profiling areas of Ground Water Contamination by Pesticides in California: Phase II - Evaluation and Modification of a Statistical Model. Troiano, J., C. Nordmark, T. Barry, and B. Johnson. Environ. Monitor. Assess. 45:301-318. 1997.
  • Pesticide Movement to Ground water: Application of Areal Vulnerability Assessments and Well Monitoring to Mitigation Measures. Troiano, J., C. Nordmark, T. Barry, B. Johnson, and F. Spurlock. In Ballatine et al. (ed.) Triazine Herbicides Risk Assessment. ACS Symposium Series 683. p.239-251. 1998.

Application of the CALVUL model

Previously, DPR sampled for pesticides in ground water based on pesticide use data only. However, in determining where to sample ground water for norflurazon residues, DPR used the CALVUL model and pesticide use data. High norflurazon detection rates indicated that the CALVUL model is an effective tool to identify areas where contamination is likely. The norflurazon study also indicated that depth-to-ground water was another important variable in determining vulnerable areas. These results are found in the following report:

The current method used to group and profile sections of land based on soil data is explained in report EH 00-05 and the method to determine depth-to-ground water estimates for sections of land in report EH 00-02. Both reports are listed below and are available on the website.

Using this vulnerability analysis, studies have been conducted to identify the pathways of contamination and appropriate management practices.

For content questions, contact:
John Troiano
1001 I Street, P.O. Box 4015
Sacramento, CA 95812-4015
Phone: (916) 324-4115