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An Evaluation of Kinetic and Equilibrium Equations for the Prediction of Pesticide Movement Through Porous Media¹

ABSTRACT

Miscible displacement techniques were used to study the movement of picloram (4-amino-3,5,6-trichloropicolinic acid) through a water-saturated Norge loam soil. The equilibrium adsorption and desorption isotherms for picloram and Norge loam soil were not single-valued relations. Picloram mobility was reduced significantly when the average pore-water velocity was decreased from 145 to 14.2 cm/day. Observed and predicted effluent concentration distributions were compared. Predictions were made with a S/360 CSMP simulation model, using two kinetic rate equations and an equilibrium Freundlich equation. The two kinetic models and the equilibrium model each satisfactorily described the observed effluent concentration distributions at low pore-water velocities provided the nonsingle-valued character of the adsorption-desorption process was included in the calculations. At high pore-water velocities, the kinetic adsorption models were found inadequate to predict the picloram movement. An empirical model was then developed, based on the assumption that equilibrium existed during displacement and that only a fraction of the soil participated in the adsorption process. This fraction was found to be a function of the average pore-water velocity. With the empirical model, a reasonable fit between data and calculated effluent curves was obtained for all pore-water velocities.

¹ Contribution from the Departments of Agronomy at New Mexico State University, Las Cruces and Oklahoma State University, Stillwater. The work upon which this report was based was supported in part by funds obtained from the U. S. Department of the Interior, Office of Water Resources Research, as authorized under the Water Resources Research Act of 1964, and by E. P. A. Research Grant no. R-800364 at Oklahoma State University. Journal Article no. 463, Agr. Exp. Sta., New Mexico State Univ., Las Cruces, New Mexico, 88003.

² Graduate Research Assistant, New Mexico State Univ., Associate Professor, Oklahoma State Univ., and Associate Professor, New Mexico State Univ., respectively.

Received for publication April 3, 1973. Accepted for publication September 5, 1973.