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-dimeth ylurea) 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 dep th of 0.3 m and analyzed for total chlortoluron concentrations at 0.03-m in tervals. A bromide (Br-) transport experiment was carried out at the same s ite. Thirty three, 61, and 136 d after application, corresponding to 54, 11 5, and 305 mm of precipitation, chlortoluron was found almost exclusively i n the upper two, four, and six layers, respectively. In these layers, coeff icients of variation (CVs) varied from about 50 to >200%. At the first samp ling, about 1% of applied chlortoluron was detected in the layer just above the plow-sole. Mean travel depths of chlortoluron were distinctly more var iable than those of Br-, showing no spatial correlation. The retardation fa ctor of chlortoluron, expressing its displacement relative to that of Br-, considerably increased during the experiment, on the average from 11.5 at t he first sampling to 26.3 at the third sampling. Only then, the local mean travel depths of chlortoluron showed a significant (positive) correlation w ith those of Br- and a significant (negative) correlation with organic C co ntents. Sorption kinetics of chlortoluron were investigated in laboratory s orption-desorption (SD) experiments at 3 and 20 degrees C. Sorption-desorpt ion isotherms showed pronounced hysteresis. The influence of experimental t emperature was small. The collected data are intended as a basis to evaluat e the ability of different modeling approaches to represent pesticide displ acement at the field scale.