Infering the relevance of bank filtration processes for drinking water production sites on alluvial aquifers under drought conditions using residence time distributions and water quality parameters

During prolonged drought periods surface water contributions via bank filtration to drinking water production sites located on alluvial aquifers can become increasingly relevant due to associated changes in the hydraulic boundary conditions. Considering the predicted increase of occurance probabilities for hot and dry summers in the humid climate zone, these sites might be prone to an increased risk related to anthropogenic emissions into the connected surface water bodies in the near future.

We studied these processes at two well fields (8 active wells in total) located on the alluvial aquifer of the river Kyll, Germany, about 1 km downstream of the community of Kordel and the city´s waste water treatment plant. For six months we sampled bi-weekly water quality parameters in the wells (horizontal distances to the river varied between 30 and 420 m) and at three locations at the river Kyll. Samples were analyzed for stable water isotopes, EC, pH, DO, DOC, major ions, metals and selected pharmaceutical products. Based on a mixing analysis using major ion data we quantified the mean surface water contribution for each well, varying between 40 and 95 %. Using a darcian modelling approach based on continuous pumping rates, hydraulic gradients, existing information on hydraulic conductivities and the possible geometric connectivity of each of the wells to the river we were able to infer potential residence time distributions for the estimated surface water contributions for each of the wells. Comparing these distributions with nutrient gradients and oxic conditions we find significant correlations with the 0.05 quantile shortest residence time estimations, only. Resulting residence times of surface water contributions within the alluvial aquifer range from several days to weeks instead of previously estimated months to years. Dynamics in stable water isotope patterns in rainfall, surface water and groundwater show as well changes in surface water and groundwater composition within two weeks following the changes in the rainfall isotopic composition. Contrary to nutrient dynamics, the trace organic compounds, i. e. pharmaceutical products did not show spatial or temporal patterns, but a constant, substance specific degradation which leads to the conclusion that trace organic compound retention occurs in the nutrient rich near proximity of the river bed, i.e. the hyporheic zone.

These results demonstrate how quantitative and qualitative surface water contributions to groundwater / drinking water wells can have an increased relevance under drought conditions than previously anticipated.