Transport of Chlorsulfuron through Soil Columns

Chlorsulfuron [2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl) amino]carbonyl-benzenesulfonamide is an anionic sulfonylurea herbicide with high soil persistence; consequently, there is interest in predicting its mobility in agricultural soils. The objectives of this study were to determine the transport characteristics of chlorsulfuron in disturbed and undisturbed soil columns and evaluate the capabiiities of LEACHM (Leaching Estimation and Chemistry Model) for predicting chlorsulfuron transport. Soil column experiments were conducted with two Montana soils (Amsterdam silt loam, file-silty, mixed Typic Haploboroll, and Haverson silty clay loam, fine-loamy, mixed (calcareous), mesic Ustic Torrifluvent) under unsaturated flow conditions. Unit gradient was established in all columns by balancing surface water input to outflow at the bottom of each column through a stainless steel porous plate connected to a vacuum chamber containing a fraction collector. A nonlinear least squares approach (CXTFIT) was used to fit breakthrough curves (BTCs) for Br- and C-14-labeled chlorsulfuron using the linear equilibrium adsorption model (i.e., nonequilibrium assumption or LEA model) and the bicontinuum model (i.e., nonequilibrium assumption). Observed Br- BTCs were best described by the bicontinuum model indicating physical nonequilibrium due to immobile water regions. Observed chlorsulfuron BTCs demonstrated both chemical and physical nonequilibrium during transport. The best fit to observed chlorsulfuron BTCs was obtained with the bicontinuum model using the dispersion coefficient optimized (i.e., fixed) from the Br- BTCs. LEACHM was used to generate predicted BTCs for chlorsulfuron utilizing independently measured or estimated soil physical parameters as input data. Predicted BTCs utilizing the Br- -derived dispersion coefficient (D) and the bicontinuum model-derived partition coefficient (K(oc)) did not adequately reflect observed BTCs primarily because the current version of LEACHM has no capability for accepting input parameters relating to nonequilibrium conditions. The D and K(oc) values for chlorsulfuron BTCs derived from the LEA model improved predictions; however, it is important to note that LEA-derived D and K(oc) values for chlorsulfuron BTCs essentially compensate for nonequilibrium behavior. Finally, the experimentally determined chlorsulfuron BTCs confirmed the high mobility of this chemical at neutral soil pH values.