Indicators for anthropogenic and natural water contributions to a small river

River quality is expected to change significantly as a consequence of climate change. Extended drought periods and decreasing groundwater levels will lower the contribution of groundwater to receiving streams and heavy rain will cause excessive surface runoff effects. Constant sources, like discharge from wastewater treatment plants or industrial installations, will be diluted in largely different ratios. The overall hydrochemical dynamics will, therefore, likely increase. This affects flood management schemes and other potential uses of river water. This study links trace substances (hydrochemistry, colloids) to the hydrological dynamics in the catchment. From this data the feasibility of infiltrating excess river water into a nearby aquifer (Flood-MAR) is assessed. The upper part of the study area is characterized by forests and meadows. There is a small village with one sewage treatment plant (1000 inhabitant equivalents) and a few hamlets. Additional emissions can be expected from surface runoff of one national road (deicing, tire abrasion, etc.). The hydrochemical characteristics show a decrease in the main cations and anions during a flooding event. Nitrate concentrations were low in both cases. Although particle concentrations were increasing during the flooding event, the overall concentrations were still below our expectations. This indicates that the meadows behind the retention dam, which were partly flooded, served as a settling ponds and filters. During low water conditions, organic material and algae were dominant. Few calcite particles and silicates are indicative of the composition of the quaternary gravels that make up the aquifer. The results confirm the risk assessment of the study area. The water quality during a flooding event met the legal thresholds set in the German Soil Protection Act, so infiltration into the downstream aquifer should be feasible.