COMPARISON OF INSTANTANEOUS EQUILIBRIUM AND EQUILIBRIUM-KINETIC SORPTION MODELS FOR SIMULATING SIMULTANEOUS LEACHING AND RUNOFF OF PESTICIDES

Two chemical sorption models, a two-site equilibrium-kinetic sorption model (E-K) and an instantaneous equilibrium sorption model (I-E), wer e used to describe pesticide leaching and runoff, Concentrations of br omide, 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-loam y, siliceous, thermic Plinthic Kandiudults) were compared with simulat ed concentrations predicted by the Root Zone Water Quality Model (RZWQ M), 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 mod el and the I-E model, which are the two submodels for chemical sorptio n in RZWQM, were used for pesticide sorption simulations, and a nonuni form mixing model was used for pesticide runoff simulations, Model par ameter values were obtained by measurements, from the literature, and by calibration with measured atrazine and fenamiphos breakthrough curv es (BTCs) and a runoff chemograph from an independent experiment, Resu lts showed that the E-K model simulated atrazine and fenamiphos leachi ng and atrazine runoff better than the I-E model, The I-E sorption mod el 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 wit h initially saturated soils and under steady state water flow conditio ns, 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 transi ent water flow conditions for leaching as well as runoff of a pesticid e.