Abstract
An estimation method for the Freundlich organic carbon normalized partitioning coefficient (Kfoc) is presented. It is based on regressions developed from batch equilibrium experiments submitted to U.S. EPA for pesticide registration. The regressions represent 18 specific agricultural soils from the U.S. and Europe, and are based on 41 pesticide active ingredients or metabolites. The predictive variable used is the subcooled liquid solubility (Sscl), whose calculation requires the compound's solubility, and if a solid (mp > 25 °C), its melting point. Sscl represents the compound's solubility as if it were a liquid at room temperature and does not assume that the compound is neutral. The estimation method was tested on 333 pesticides, and estimated means matched published Kfoc values to within 10× for 83%, 3× for 65%, and 2× for 43%. The estimation method (Kfoc Estimator v0.1) was further tested against data for 94 herbicides, 55 Superfund Priority Pollutants and 22 polychlorinated biphenyls. The estimated mean and median Kfoc values were highly correlated (R(2) > 0.63 to 0.87) with the measured values. Matching of reported to estimated values was generally good. The herbicides were 51%-54% within 2×, 74%-77% within 3×, and 93%-94% within 10×. The Priority Pollutants were 22%-29% within 2×, 38%-55% within 3×, and 73%-75% within 10×. The estimates for the PCBs were low, but the maximum estimated Kfoc was consistently within 10× of the reported Koc. This may be due to the PCB data representing a single soil, rather than the central-tendency estimates produced by Kfoc Estimator v0.1.