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Modeling the Probabilities of Groundwater Contamination by Pesticides

Soil Science and Plant Nutrition, Univ. of Western Australia, Nedlands, WA 6907, Australia

*Corresponding author (aylmore{at}uniwa.uwa.edu.au).

ABSTRACT

Field soils show significant spatial variations in properties, such as organic matter content, bulk density, and moisture content, that can affect the mobility and persistence and thus fate of organic pesticides in the soil environment. A simple model incorporating the variations in soil and pesticide parameters has been developed to assess the groundwater contamination potential of pesticides. The model is based on linear, equilibrium, and reversible sorption, first-order degradation, and steady piston flow, and allows the unsaturated soil zone to be divided into a number of layers of different thickness and properties. For each input parameter, 500 random data were generated from normal distributions that characterize the variability of the parameters. The fate of 29 pesticides were assessed using soil and environmental conditions of the Swan Coastal Plains of Western Australia and pesticide properties reported in the literature. The predicted pesticide residue fractions remaining at 150-, 300-, and 500-cm depths were described by beta distributions, and the corresponding travel times by normal distributions. Fourteen out of the 29 pesticides are predicted to reach 150 and 300 cm and 13 are predicted to reach the 500-cm depth with mean residue fractions 0.01%. The predicted mean travel times for pesticides that may reach 300 cm vary from about 2 mo to about 18 yr. Significant standard deviations are associated with these mean residue fractions and travel times. Six pesticides, fenamiphos, simazine, metribuzin, linuron, fenarimol, and metalaxyl, have been identified as having high cumulative probabilities of 0.82 to 1.00 for residue concentrations to be >0.01% at the 300-cm depth.

Received for publication November 14, 1995.

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