Variation of pesticide sorption isotherm in soil at the catchment scale

The variation of the sorption isotherm of pesticides has seldom been explored at the catchment scale. Such a study was conducted at the scale of a 187‐ha agricultural catchment for three herbicides: atrazine, isoproturon and metamitron. Partition coefficient ( K d ) values were measured in batch experiments on 51 topsoil samples, and showed moderate variability at the catchment scale (coefficient of variation CV ∼ 30%). Values of K d ranged from 0.47 to 1.70 litre kg −1 for atrazine, 0.47 to 1.81 for isoproturon, and 0.55 to 2.21 for metamitron. A clustering method was used to reduce the number of samples on which to measure sorption isotherms to 14. Sorption isotherms agreed with the Freundlich rather than the linear model. K f parameters had CV values similar to those for K d , with values ranging from 0.78 to 2.13 mg   1− N   flitre  N   fkg −1 for atrazine, 0.61 to 1.82 for isoproturon, and 0.69 and 2.58 for metamitron. N f exponents showed little variation (CV < 5%). N f values were between 0.86 and 0.98 for atrazine, 0.85 and 0.90 for isoproturon, and 0.82 and 0.87 for metamitron. More than 97% of the K f catchment‐scale variations could be explained by the variations of the soil organic carbon content. © 2002 Society of Chemical Industry