Flood event impact on pesticide transfer in a small agricultural catchment (Montoussé at Auradé, south west France)

In this paper, pesticide transfer dynamic is studied during two flood events in a small experimental catchment close to Toulouse (south west France). Thirteen pesticide molecules (herbicides, fungicides) have been analysed by multi-residue technique on filtered and unfiltered waters. The results show very high pesticide concentrations in the different fractions compared to low flow periods and to the data collected by the French institutional networks in charge of the pesticide river water pollution survey. Several molecules present concentration higher than 0.1 µg L-1 and even higher than 1 µg L-1 in the unfiltered waters. In the suspended matters the concentrations vary respectively between 0.1 and 30 µg g-1 according to the molecules and can represent 40 to 90% of the total concentration for low soluble molecules. All the molecule concentrations and fluxes increase during the flood flows and have positive relationships with the stream discharge, but hysteresis between rising and falling periods can be observed for some molecules. Pesticide concentrations in unfiltered waters and partitioning between dissolved and particulate fractions (Kd = [diss]/[part]) are controlled by dissolved organic carbon and total suspended matter. A good negative relationship can be established between log Kd and log Kow for 6 molecules