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Comparison of Instantaneous Equilibrium and Equilibrium-Kinetic Sorption Models for Simulating Simultaneous Leaching and Runoff of PESTICIDES1

Two chemical sorption models, a two-site equilibrium-kinetic sorption model (E-K) and an instantaneous equilibrium sorption model (I-E), were used to describe pesticide leaching and runoff. Concentrations of bromide, atrazine, and fenamiphos in leachate and runoff water from 1.0 m X 0.5 m X 0.1-m boxes of packed air-dry Tifton loamy sand (fine-loamy, siliceous, thermic Plinthic Kandiudults) were compared with simulated concentrations predicted by the Root Zone Water Quality Model (RZWQM). Atrazine and bromide (as KBr) were applied to the soil surface and fenamiphos was incorporated into the top 7 cm.Water (100 mm in 2 h) was applied using sprinkler rainfall simulator 24 h later. The E-K model and the I-E model, which are the two submodels for chemical sorption in RZWQM, were used for pesticide sorption simulations, and a nonuniform mixing model was used for pesticide runoff simulations. Model parameter values were obtained by measurements, from the literature, and by calibration with measured atrazine and fenamiphos breakthrough curves (BTCs) and a runoff chemograph from an independent experiment. Results showed that the E-K model simulated atrazine and fenamiphos leaching and atrazine runoff better than the I-E model. The I-E sorption model underestimated pesticide leaching and overestimated atrazine runoff. Compared with previous studies showing that the E-K model simulated pesticide sorption and leaching better than the I-E sorption model with initially saturated soils and under steady state water flow conditions, this study showed that the E-K model also worked better than the I-E sorption model for initially air-dry soil and under fast and transient water flow conditions for leaching as well as runoff of a pesticide.