Scientific Basis for Establishing Specific Numerical Values

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The Pesticide Contamination Prevention Act (PCPA) requires DPR to establish thresholds known as SNVs for six physical-chemical parameters: water solubility, Koc, hydrolysis half-life, aerobic soil metabolism half-life, anaerobic soil metabolism half-life, and field dissipation half-life. These parameters are presumed by the PCPA to correlate with the potential of a pesticide to leach to ground water. Water solubility and Koc are considered indicators of a pesticide's mobility within the soil, while the half-lives for hydrolysis, aerobic and anaerobic soil metabolism and field dissipation are considered indicators of the persistence of the pesticide in the soil. SNVs are required to be at least equal to those established by the U.S. EPA; presently there is no federal SNV for field dissipation. To view the current SNVs in regulation, please see 3CCR Section 6804.

SNV development

Statistical comparison procedures were used to calculate the SNVs. Based on nationwide ground water studies, a list of pesticides was created and separated into two groups: pesticides that had been detected in ground water as a result of legal agricultural use (leachers) and (2) chemicals that had been sampled for and not found in ground water as a result of legal agricultural use (nonleachers). Values for the physical-chemical parameters of chemicals in each group were determined from the open literature and DPR-approved studies submitted by pesticide registrants in fulfillment of the data call-in requirements in FAC section 13143. The data for each parameter was tested for their usefulness in discriminating between leachers and nonleachers by determining whether the means of the two groups were significantly different. The tests showed that the means of the data for water solubility, hydrolysis half-life, Koc, and the anaerobic soil metabolism half-life for chemicals identified as leachers were significantly different from the means of chemicals identified as nonleachers. The SNVs for these properties were established as those values that would identify as leachers 90 percent of the chemicals found in ground water due to agricultural use (Wilkerson and Kim, 1986). The means of the two groups for aerobic soil metabolism, however, were not significantly different. Because the PCPA requires DPR to establish an SNV for each physical-chemical parameter, the SNV for the aerobic soil metabolism half-life was set at a value that minimized its importance in the discrimination procedure.

The SNVs established by DPR are reevaluated periodically to incorporate new data into the calculations. Details on the creation and revision of the SNVs are at the following links:

  • Setting Specific Numerical Values April 1991, PDF. (1 mb), Report EH91-06, Johnson, B. 1991.
  • Setting Specific Numerical Values October 1989, PDF. (920 kb), Report EH89-13, Johnson, B. 1989.
  • Setting Specific Numerical Values November 1988, PDF. (1.4 kb), Report EH88-12, Johnson, B. 1988.
  • The Pesticide Contamination Prevention Act: Setting Specific Numerical Values, PDF. (2.2 mb), Report EH86-02, Wilkerson, M.R., K.D. Kim. 1986.

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