Fate of leaf litter deposits and impacts on oxygen availability in bank filtration column studies.

Degradation of particulate organic carbon (POC) such as leaf litter might deplete dissolved oxygen within the upper layers of bank filtration, an efficient and robust barrier for pathogens and for various organic micro-pollutants (OMP) in water supply systems worldwide. The degradation of OMP during bank filtration depends on the redox conditions. The present study aimed at identifying the impacts and fates of different local leaves on the oxygen consumption and the possible biological degradation of indicator OMP. Oxygen concentrations initially decreased within the columns from around 8 mg/L in the influent to low concentrations indicating extensive consumption within a short travel distance. Still a substantial oxygen consumption was observed after 250 days. OMP concentrations were not significantly affected by the microbial processes. A layer of calcium carbonate crystallites was observed on the POC layer. Some leaf fragments appeared to be persistant towards degradation and the carbon content relative to nitrogen and sulfur contents decreased within 250 days. The results demonstrate that trees at bank filtration sites might have a strong long-term impact on the subsurface redox conditions.

Characterization and quantification of dissolved organic carbon releases from suspended and sedimented leaf fragments and of residual particulate organic matter.

Bank filtration is a powerful and established barrier for pathogens and organic pollutants. The aerobic degradation of the pollutants competes with the microbial respiration of dissolved and particulate organic matter (DOC and POC). The fate of terrestrial POC (tPOC) in bank filtration is currently not fully understood. In the present study, fallen leaves of different local trees were milled, characterized and investigated as tPOC in both batch and column experiments. The respective contents of carbon and nitrogen differed slightly, but the different leaves released significantly different DOC fractions as determined by size-exclusion chromatography. While high molecular weight biopolymers were degraded by indigenous microorganisms, humic substances were not degraded within 96days in batch experiments. DOC release and POC wash-out in column experiments led to a slight decrease of tPOC depositions, but more than 80% of the initial tPOC remained after 54days thus representing a long-term organics reservoir for microbial respiration. The release of humic substances from autumnal leaf litter inputs is a plausible explanation for comparably high DOC concentrations (approximately 4.5mg/L) in Berlin drinking water that mainly originates from lake bank filtration.