Several physical, chemical and biological mechanisms play a role in the clogging of sediment interstices regularly observed in sand filter and infiltration basin systems. Whereas the hyporheic zone has been the focus of many investigations, little is known about the lenitic limnic zone, which is typical in lowland areas with lakes and low flow rivers. One must assume that clogging is regulated by both the build-up and the input of particulate organic matter (POM). In the present study, we collected samples from the littoral zone of Lake Tegel, Berlin, Germany, to analyze relevant carbon turnover processes. High concentrations of POM were detected in the upper sediment layer, with 3.4% ds down to 20 centimeters depth. A very high biomass of interstitial algae was found in the first 5 cm of sediment (25 µg Chl a per cm–3); this was 1000 times higher than in the lake water. The pore system of the sediment was filled to about 50% with POM, and the algae volume comprised about 25 % of POM. Only low amounts of POM were transported from the lake water downwards into the interstices, and the transport of FPOM (a few centimeters per day) was much lower than the water flow (32–260 cm d–1). The DOC concentrations in lake water (~8 mg L–1) and interstitial water (~6 mg L–1) were determined by the in situ bioactivity of interstitial organisms in addition to DOC input from lake water.

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.