Environmental Microbiology

Support or remediation of contaminated urban lakes: Advantages and risks of calcium nitrate and oxygenated water addition during summer stratification

In cooperation with Ingenieurbüro Wassmann / TU Berlin Soil Science / TU Berlin Environmental Chemistry and Air Research

This research project investigates the influence of a new process to support or remediate lakes with oxygen-poor water bodies and highly contaminated sediments.

In oxygen-deficient water bodies, compounds such as nitrate, sulfate, iron or carbon dioxide can serve as electron acceptors for microbial respiration. In addition, fermentation processes may take place. Degradation processes that require oxygen as a direct reaction partner are significantly slowed down or stopped under these conditions. In addition to the deposition of incompletely decomposed organic matter, the formation of hydrogen sulfide (toxic), ammonia (toxic to fish and oxygen depleting), and an increased release of phosphate may occur, further accelerating eutrophication. Urban lakes are additionally exposed to a variety of potentially toxic components such as aliphatic hydrocarbons, PAHs, halogenated organic compounds and metals that can be introduced via untreated rainwater from the urbanized environment.

Lake Schäfersee is a stratified lake in Reinickendorf, Berlin, originating from the last ice age. Over the last decades, an estimated amount of over 80 000 m3 of highly contaminated sediments built up in Lake Schäfer as a result of many years of untreated rainwater discharges from an urbanized storm sewer catchment area of over 250 ha. This resulted in an almost year-round oxygen deficient hypolimnion and fish die-offs occurred regularly.

To alleviate the lake, calcium-nitrate and oxygen-saturated water are added simultaneously to the hypolimnion of the lake, without disturbing its stratification. The nitrate shall serve as an electron acceptor for microbial respiration thereby potentially contributing to a reduced sulfide and methane formation and an enhanced organic matter removal via heterotrophic nitrate-respiring organisms. Oxygen shall stimulate oxygen-dependent enzymatic pollutant transformation processes. In addition, an increase of redox potential is predicted and shall help to prevent the re-dissolution of phosphate from the sediment, thus limiting the availability of phosphate. Positive effects such as the almost complete prevention of hydrogen sulfide formation and reduced phosphorus loading in the lake have already been observed as a result of the treatment.

Accompanying microbiological analyses will not only be used to understand the mode of action and efficiency of the process but also to develop control parameters and procedures based on rapidly detectable molecular biological parameters. Using 16S rRNA gene sequences and metagenome analyses to detect functional genes, qualitative and quantitative changes in microbial sediment populations will be analyzed.

Moreover, the influence of the addition of calcium nitrate and oxygen-enriched water on persistent organic pollutants present in the sediments will be analyzed as much as the reduction or formation of climate-relevant gases such as methane or nitrous oxides. At the same time, potential risks of the process are also being investigated, such as for instance the potential mobilization of (semi)metals from the sediment to the water body during the treatment.

The project is funded by the BMBF.

Funded by