At Lake Tegel in Berlin, Germany, drinking water is produced by induced bank filtration. Under such increased infiltration conditions, it is very important to understand the natural purification processes in the upper littoral zone (sediment depth of about 0–30 cm) in order to maintain a high drinking water quality. We therefore analyzed the pore water and measured the redox potential at Lake Tegel regularly to detect fluctuations in the concentrations of physicochemical parameters due to seasonal variations in the redox potential. The redox potential is not only influenced by the biological activity of the interstitial biocoenosis, but also by hydraulic conditions that occasionally produce unsaturated zones leading to an intrusion of gaseous oxygen. The result is an increase in the redox potential, which declines during the summer due to intensive dissimilatory reduction and microbial activity, thus leading to distinctly anaerobic conditions. When this is the case, the oxygen supplied by primary production and bioturbation cannot meet the oxygen demand needed for the mineralization of organic material. Negative redox values (minimal –200 mV) are accompanied by increases in nitrite (max. conc. 150 µg/L) and ammonium levels (max. conc. 0.45 mg/L), while the nitrate concentrations decreased (min. conc. 0.2 mg/L). This indicates that processes such as denitrification and ammonification occur, and that, after depletion of free oxygen reserves, other electron acceptors, such as nitrate and also heavy metal ions (Fe3+), are used.