New insight into pesticide partition coefficient Kd for modelling pesticide fluvial transport: application to an agricultural catchment in south-western France.

Pesticides applied on crops are leached with rainfall to groundwater and surface water. They threat the aquatic environment and may render water unfit for human consumption. Pesticide partitioning is one of the pesticide fate processes in the environment that should be properly formalised in pesticide fate models. Based on the analysis of 7 pesticide molecules (alachlor, atrazine, atrazine's transformation product deethylatrazine or DEA, isoproturon, tebuconazole and trifluralin) sampled from July 2009 to October 2010 at the outlet of the river Save (south-western France), the objectives of this study were (1) to check which of the environmental factors (discharge, pH, concentrations of total suspended matter (TSM), dissolved organic carbon (DOC) and particulate organic carbon (POC) could control the pesticide sorption dynamic, and (2) to establish a relationship between environmental factors, the partition coefficient Kd and the octanol/water distribution coefficient Kow. The comparison of physico-chemical parameters values during low flow and high flow shows that discharge, TSM and POC are the factors most likely controlling the pesticide sorption processes in the Save river network, especially for lower values of TSM (below 13mgL(-1)). We therefore express Kd depending on the widely literature-related variable Kow and on the commonly simulated variable TSM concentration. The equation can be implemented in any model describing the fluvial transport and fate of pesticides in both dissolved and sorbed phases, thus, Kd becomes a variable in time and space. The Kd calculation method can be applied to a wide range of catchments and organic contaminants.