Field-scale study of chlortoluron movement in a sandy soil over winter: II. Modeling

This study was conducted to evaluate different modeling approaches for thei r ability to predict chlortoluron movement observed in a 4-mo field study. The data base includes the results of a Br- transport experiment performed at the same site and of laboratory sorption-desorption (SD) experiments. Th e approaches tested are based on the one dimensional convection dispersion equation (CDE) with two-stage or equilibrium sorption. The CDE was applied either at the field scale or, to account for parameter variability, within a parallel soil columns (PSC) approach. Parallel soil column simulations we re realized by the Monte Carlo (MC) method. Transport and sorption paramete rs and their distributions were estimated from Br- concentration profiles a nd SD isotherms, respectively. Chlortoluron degradation had been investigat ed in a joint project. The CDE with two-stage sorption represented the move ment of the chlortoluron peak well, but not ifs spreading. Similar results were obtained assuming equilibrium sorption, provided the same sorption par ameters were used. If the sorption parameters were derived from the 1-d sor ption isotherm instead, displacement was overpredicted Accounting far the v ariability of sorption and transport parameters through the PSC approach af fected the simulation results only slightly. All simulations substantially underpredicted chlortoluron spreading, probably because the time scales of the laboratory and the field experiments were too different. To convert the estimated rate parameter to the appropriate time scale, we derived a simpl e formula that is based on the hypothesis of intrasorbent diffusion. Applic ation of the CDE with the rescaled rate parameter distinctly improved the a greement between modeled and measured chlortoluron concentration profiles.