Do pesticides degrade in surface water receiving runoff from agricultural catchments? Combining passive samplers (POCIS) and compound-specific isotope analysis.

Pesticides lead to surface water pollution and ecotoxicological effects on aquatic biota. Novel strategies are required to evaluate the contribution of degradation to the overall pesticide dissipation in surface waters. Here, we combined polar organic chemical integrative samplers (POCIS) with compound-specific isotope analysis (CSIA) to trace in situ pesticide degradation in artificial ponds and agricultural streams. The application of pesticide CSIA to surface waters is currently restricted due to environmental concentrations in the low μg.L range, requiring processing of large water volumes. A series of laboratory experiments showed that POCIS enables preconcentration and accurate recording of the carbon isotope signatures (δC) of common pesticides under simulated surface water conditions and for various scenarios. Commercial and in-house POCIS did not significantly (ΔδC < 1 %) change the δC of pesticides during uptake, extraction, and δC measurements of pesticides, independently of the pesticide concentrations (1-10 μg.L) or the flow speeds (6 or 14 cm.s). However, simulated rainfall events of pesticide runoff affected the δC of pesticides in POCIS. In-house POCIS coupled with CSIA of pesticides were also tested under different field conditions, including three flow-through and off-stream ponds and one stream receiving pesticides from agricultural catchments. The POCIS-CSIA method enabled to determine whether degradation of S-metolachlor and dimethomorph mainly occurred in agricultural soil or surface waters. Comparison of δC of S-metolachlor in POCIS deployed in a stream with δC of S-metolachlor in commercial formulations suggested runoff of fresh S-metolachlor in the midstream sampling site, which was not recorded in grab samples. Altogether, our study highlights that the POCIS-CSIA approach represents a unique opportunity to evaluate the contribution of degradation to the overall dissipation of pesticides in surface waters.