The research team, led by the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), discovered that the extent of chemical pollution in the ponds is determined less by the degree of surrounding built-up areas than by the type of water flowing into them. Chemicals from tire wear enter the ponds primarily via stormwater discharge; while pharmaceutical residues enter via streams fed by treated wastewater. The study was conducted as part of the research project POUNDER in collaboration with the Leibniz Institute for Zoo and Wildlife Research (IZW) and the University of Iowa and published in the journal “Water Research.”
In all 43 urban ponds examined, the researchers found dissolved trace organic contaminants, including tire-associated chemicals as well as corrosion inhibitors and pharmaceuticals. The median sum concentration of trace organic contaminants in the pond water was 1.7 micrograms per liter. Synthetic chemicals — especially in complex mixtures — can pose a threat to ecosystems and drinking water resources even at these low concentrations.
Street runoff also affects distant ponds
Tire-associated chemicals were found particularly frequently and in high concentrations. Their total concentration reached up to 9.3 micrograms per liter in a single pond. The most frequently detected compound was 1,3-diphenylguanidine (DPG), a vulcanization accelerator used in tires, which was found in 42 of the 43 ponds. The average concentration of DPG was 0.4 micrograms per liter. DPG is used in tire manufacturing and is therefore an indicator for runoff from streets.
The study showed that the type of water inflow had a greater impact on high trace organic contaminant concentrations than the surrounding land use. In particular, ponds receiving stormwater runoff through storm drains can be affected by pollutants transported from more distant, connected areas. In ponds with no stormwater discharge via storm drains, tire-associated chemicals can enter small water bodies through diffuse runoff from adjacent impervious surfaces, such as parking lots and streets.
Pharmaceuticals indicate the input of wastewater
Pharmaceuticals, their metabolites, and transformation products were detected in 74 percent of the ponds studied. One pharmaceutical the researchers found was carbamazepine, a drug used to treat epilepsy, which is not fully removed in conventional wastewater treatment plants and therefore occurs in treated wastewater and receiving rivers and streams. The corrosion inhibitor benzotriazole, which is used in dishwashing detergents, was also commonly found in urban ponds. Elevated concentrations of pharmaceuticals in urban ponds were related to streams located within a 50 meter radius around the pond shore.
Several streams and rivers in Berlin, including the Panke, Nordgraben, and Erpe receive substantial inputs from wastewater treatment plants, which can account for up to 80 percent of the water volume of the streams and rivers. Pharmaceuticals might also enter ponds through raw sewage, for example, due to leakages in the sewer system. However, potential input of raw sewage into Berlin‘s ponds was not quantified in this study.
Stable water isotopes help to investigate pond water inflows
To determine the origin and input pathways of the water and pollution in the ponds, the researchers combined a variety of measurements: They analyzed 37 trace organic contaminants, metals such as zinc, stable water isotopes, and general water quality parameters such as nutrients, while also taking characteristics of the urban environment into account. “This multi-tracer approach enabled us not only to identify which pollutants were present in urban ponds, but also where the pond water stems from and what potential entry routes for pollutants exist,” said Elise Malsch-Fröhlich, who is pursuing her Ph.D. on this topic at IGB. For example, the researchers were able to distinguish between inputs from stormwater and street runoff and those from wastewater-impacted streams. “Whilst analyses of the pollutants alone show the current contamination level in the pond water, they provide only limited insights into the different input pathways,” explained the doctoral candidate.
This approach can also be used to investigate potential connections of ponds to groundwater. For ten of the sampled ponds, the pond water depth and groundwater levels in the surrounding area indicated a possible inflow of groundwater. For seven of these ponds, the stable water isotope data confirmed potential groundwater-pond water interactions. In case of falling groundwater levels, organic contaminants could also migrate from the pond into the groundwater; however, this cannot be determined based on the available data.
Type of water inflow plays a crucial role for urban water management
Urban ponds are not isolated water bodies but part of the urban water cycle. “Our results show that the chemical water quality of an urban pond cannot be determined solely by the degree of surrounding urbanization. What matters are the water sources — and thus which pollutants enter the pond via stormwater runoff, surface waters, or potentially groundwater,” explained IGB researcher Dr. Stephanie Spahr, who led the study. For ponds connected to storm drains, also more distant areas can contribute to the pollutant load. In small watersheds, however, the surrounding degree of impervious surfaces can play a greater role for water quality. “For the management of small urban water bodies, this means looking beyond the individual pond’s edge to its tributaries and potential pollution sources.”
How such contaminants can be reduced more effectively in the future was demonstrated by another study led by Stephanie Spahr: The team investigated whether biochar is suitable for treating urban stormwater. They were able to demonstrate that biochar can activate the chemical oxidant peroxydisulfate (persulfate) and thus promote the transformation of trace organic contaminants. A combination of biochar and persulfate could, in the future, complement conventional stormwater treatment and, in particular, improve the removal of persistent and mobile compounds.
