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Application of Two-Site/Two-Region Models for Studying Simultaneous Nonequilibrium Transport and Degradation of Pesticides

Dep. of Soil Science, Univ. of Florida, Gainesville, FL 32611

R. J. Wagenet^*

Dep. of Soil, Crop, and Atmospheric Sciences, Cornell Univ., Ithaca, NY 14853

M. Th. van Genuchten

USDA-ARS, U.S. Salinity Lab., 4500 Glenwood Drive, Riverside, CA 92501

*Corresponding author.

ABSTRACT

Quantitative laboratory study of pesticide sorption and degradation during transport can provide insight into the basic processes affecting pesticide fate in field soils. Accordingly, we demonstrate the application of analytical solutions of two-site/two-region transport models useful in studying simultaneous pesticide sorption and degradation. Soil-column displacement experiments involving ³H₂O, Cl, and atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) were conducted during steady-state water flow at two pore-water velocities and two pesticide concentrations. The soil used is a Valois silty loam (coarse-loamy, mixed, mesic Typic Dystrochrept). Effluent data from these experiments were used to demonstrate the application of these analytical solutions, as well as a parameter-estimation computer program based on these solutions. The ability to use laboratory-derived estimates of equilibrium sorption parameters to describe sorption under flowing conditions was evaluated at each flow velocity. Significant correlation between soil sorption partitioning and degradation prevents the simultaneous determination of both processes using these solutions. However, estimates of degradation obtained from mass balances of the column data were useful in identifying the equilibrium sorption parameters. Data collected elsewhere for 2,4,5-T (2,4,5-trichlorophenoxyacetic acid) herbicide transport were used as an additional example of the application of the model with degradation. Strengths and weaknesses of the models, and suggestions for further study, are presented.

Joint contribution from the Dep. of Soil, Crop, and Atmospheric Sciences, Cornell Univ., and the U.S. Salinity Lab.

Received for publication June 29, 1988.

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