Sorption of Pesticides Atrazine, Isoproturon, and Metamitron in the Vadose Zone

Knowledge of the sorption properties of the vadose zone is essential to provide sound risk assessments of groundwater contamination by pesticides. This study explored the sorption properties of the vadose zone of a 187‐ha agricultural catchment located 120 km northeast of Paris, France. Eocene geologic materials composing the vadose zone include limestones, marlstones, clays, and sands. Distribution coefficients ( K d ) of 14 C‐labeled atrazine (6‐chloro‐ N 2 ‐ethyl‐ N 4 ‐isopropyl‐1,3,5‐triazine‐2,4‐diamine), isoproturon [3‐(4‐isopropylphenyl)‐1,1‐dimethylurea], and metamitron [4‐amino‐4,5‐dihydro‐3‐methyl‐6‐phenyl‐1,2,4‐triazin‐5‐one] were measured in batch on 30 vadose zone samples, and their sorption isotherms measured on six representative samples. K d values for each pesticide were highly variable and ranged from 0.10 to 2.99 L kg −1 for atrazine, 0.06 to 1.27 L kg −1 for isoproturon, and 0.06 to 2.08 L kg −1 for metamitron. Two vadose zone materials, a lignitic clay and a clay with low organic C content, were found to have K d values higher than those of the topsoil. Most of the sorption isotherms in the vadose zone were found to be linear. K d s could be well predicted by the clay content (particle‐size fraction < 2 μm after organic matter and carbonates removal) of the vadose zone. This result suggests that simple but efficient pedotransfer functions may be derived to predict the sorption properties of the vadose zone at the catchment scale.