Field-scale study of chlortoluron movement in a sandy soil over winter: I. Experiments

The transport of the herbicide chlortoluron (3-(3-chlor-p-tolyl)-1,1-dimethylurea) was monitored in Northern Germany in a sandy soil between November 1994 and March 1995. On a 0.86-ha grid, soil cores were taken down to a depth of 0.3 m and analyzed for total chlortoluron concentrations at 0.03-m intervals. A bromide (Br-) transport experiment was carried out at the same site. Thirty three, 61, and 136 d after application, corresponding to 54, 115, and 305 mm of precipitation, chlortoluron was found almost exclusively in the upper two, four, and six layers, respectively. In these layers, coefficients of variation (CVs) varied from about 50 to >200%. At the first sampling, about 1% of applied chlortoluron was detected in the layer just above the plow-sole. Mean travel depths of chlortoluron were distinctly more variable than those of Br-, showing no spatial correlation. The retardation factor of chlortoluron, expressing its displacement relative to that of Br-, considerably increased during the experiment, on the average from 11.5 at the first sampling to 26.3 at the third sampling. Only then, the local mean travel depths of chlortoluron showed a significant (positive) correlation with those of Br- and a significant (negative) correlation with organic C contents. Sorption kinetics of chlortoluron were investigated in laboratory sorption-desorption (SD) experiments at 3 and 20 degrees C. Sorption-desorption isotherms showed pronounced hysteresis. The influence of experimental temperature was small. The collected data are intended as a basis to evaluate the ability of different modeling approaches to represent pesticide displacement at the field scale.