In groundwater, pesticide metabolites tend to occur more frequently and at higher concentrations than their parent pesticides, due to their higher mobility and persistence. These properties might also favor their transfer across surface water - groundwater interfaces. However, the effect of surface water - groundwater interactions on the metabolite occurrence in groundwater and pumping wells has so far received little attention. We investigated the spatial distribution of metabolites in an unconsolidated aquifer, which interacts with two surface water bodies originating from catchments with contrasting land use. We focused on metabolites of the herbicide chloridazon, namely desphenyl-chloridazon (DPC) and methyl-desphenyl-chloridazon (MDPC) and characterized surface water - groundwater interactions with various environmental tracers (e.g. electrical conductivity, stable water isotopes, wastewater tracers). In zones influenced by a river from a mountainous area, metabolite concentrations were low (median values ≤0.50 μg L-1 for DPC, ≤0.19 μg L-1 for MDPC). In contrast, high concentrations occurred in areas dominated by recharge from agricultural fields and/or influenced by a stream from an adjacent intensely farmed catchment (median values up to 1.9 μg L-1 for DPC and up to 0.75 μg L-1 for MDPC). An endmember analysis using hydro-chemical data suggested that about 20% of the DPC mass in a pumping well originated from the neighboring catchment and on its own would cause a concentration above 0.1 μg L-1 for DPC. Our findings highlight that the mobile metabolites can be imported from zones with intense agriculture outside of the exploited aquifer via surface-water groundwater interactions influencing the metabolite concentration level and long-term dynamics in the aquifer.
Keywords: Agriculture; Chloridazon; Groundwater; Metabolite propagation; Pesticide metabolites; Surface water – groundwater interactions.
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