Influence of surface water – groundwater interactions on the spatial distribution of pesticide metabolites in groundwater

In groundwater, pesticidemetabolites tend to occurmore frequently and at higher concentrations than their parent
pesticides, due to their highermobility 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 pumpingwells has so far received little attention.Weinvestigated 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 onmetabolites 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 fromamountainous area,metabolite concentrations
were low(median values ≤0.50 μg L−1 for DPC, ≤0.19 μg L−1 forMDPC). In contrast, high concentrations occurred
in areas dominated by recharge fromagricultural fields and/or influenced by a streamfroman adjacent intensely
farmed catchment (median values up to 1.9 μg L−1 for DPC and up to 0.75 μg L−1 forMDPC). 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 longterm
dynamics in the aquifer.