Evaluation of the FOCUS-PRZM model for predicting acetochlor leaching and persistence in soil

Sorption of pesticides in soil is often time-dependent, with increasing adsorption with time. This time-dependent sorption kinetics can lead to groundwater concentrations that are significantly different from those predicted using an equilibrium adsorption model such as that employed in the U.S. EPA PRZM-GW model. Currently, the U.S. EPA is revising PRZM-GW to incorporate the time-dependent sorption kinetics that is used in FOCUS-PRZM3.5.2 model. While PRZM-GW with such time-dependent sorption kinetics is not yet available, the influence of the two sorption hypotheses on predicted acetochlor leaching and persistence in soil was evaluated using the FOCUS-PRZM3.5.2 model. This model is capable of simulating both equilibrium and time-dependent sorption processes, with the latter being modeled by a two-site, equilibrium and kinetic sorption model. The FOCUS-PRZM3.5.2 model was parameterized separately with the equilibrium adsorption model as well as the time-dependent sorption model. The equilibrium adsorption coefficient (Kd) was obtained from a laboratory batch equilibrium study, while the kinetic sorption parameters for the two-site, time-dependent sorption model were obtained by model calibration by minimizing the root mean square errors between measured and simulated acetochlor mass in the soil profile. The optimized kinetic sorption site is 4% of the total sorption site, indicating that acetochlor was primarily in equilibrium over the course of the study. Still, the calibrated time-dependent sorption model much better predicted the persistence and distributions of acetochlor in the soil profile than the equilibrium adsorption model did. This is especially obvious for the later periods after acetochlor application. Limited leaching of acetochlor in the soil makes it difficult to look further into the capability of the sorption models for leaching prediction. A sensitivity analysis indicates that the FOCUS-PRZM-predicted concentration distributions of acetochlor in the soil profile are sensitive to kinetic sorption parameters.