Elucidating temperature effects on seasonal variations of biogeochemical turnover rates during riverbank filtration

Riverbank filtration (RBF) is a mechanism by which undesired substances contained in infiltrating surface waters are attenuated during their passage across the riverbed and its underlying aquifer towards production wells. In this study, multi-component reactive transport modeling was used to analyze the biogeochemical processes that occur during subsurface passage at an existing RBF system – the Flehe Waterworks located along the Rhine River in Düsseldorf, Germany. The reactive transport model was established on the base of a conservative solute transport model for which temperature and chloride data served as calibration constraints. The model results showed that seasonal temperature changes superimposed by changes in residence time strongly affected the extent of the redox reactions along the flow path. The observed temporal, especially seasonal, changes in the breakthrough of dissolved oxygen were found to be best reproduced by the model when the temperature dependency of the biogeochemical processes was explicitly considered. High floods in the Rhine drastically reduced the travel time to the RBF well from an average travel time of 25–40 days to less than 8 days. On the other hand, low flow conditions increased the subsurface residence times between the Rhine River and the RBF well to about 60 days. The model results revealed that short term changes in the terminal electron acceptor consumption (biodegrdation extent) were solely attributed to fluctuations in residence time, while more gradual changes in biodegradation extent were due to both seasonal variations of the river water temperature and gradual changes in residence time.