Soil and herbicide properties influenced mobility of atrazine, metolachlor, and primisulfuron-methyl in field lysimeters

Understanding herbicide mobility in soils is necessary to prevent groundwater contamination. We studied the mass balance distribution of three C-14-labeled herbicides (atrazine, metolachlor, and primisulfuron-methyl) in four soils (Dothan, Portsmouth, Rion, and Wagrarn) 128 d after application to soil column field lysimeters. Analyses were made of surface soil, subsoil, and leachate samples, and metabolites were identified in surface soil and leachate. Our objectives were to examine, measure, and correlate the leaching patterns of the chemicals and correlate their leaching characteristics with the physicochemical properties of the soils. Metolachlor was the most mobile herbicide as indicated by the retardation factor (R-f) (R-f = 0.35 in 1992 and 0.17 in 1993), followed by atrazine (R-f = 0.119 in 1992 and 0.09 in 1993) and primisulfuron-methyl (R-f = 0.15 in 1992 and 0.12 in 1993). Herbicide mobility (R-f) was related to leachate volume collected from the four soils, herbicide aqueous solubility, and longevity of the chemicals. The herbicides were of greatest mobility in Rion and Wagrarn soils and of lowest mobility in Portsmouth and Dothan soils. Soil factors affected the weakly basic atrazine differently than the nonionizable metolachlor or the weakly acidic primisulfuron-methyl. Volatility losses of the herbicides were inversely related to longevity (disappearance time in the field (DT50) of the compounds and to humic matter contents of the soils. Carbon-14 herbicide in the subsoil and in the leachate was correlated with herbicide mobility (R-f) total leachate volume, and 50% disappearance time values. Herbicide mobility was in agreement with predictability using a simple decision-aid model.