The study provides recommendations to prevent exceedances of health related guide values but also concludes that public health is not generally at threat when elevated contributions of wastewater effluents occur.
Around 17 % of Germany’s drinking water that is supplied by the public water networks is derived via bank filtrate or artificial groundwater recharge.
Rivers and lakes play a significant role in that. However, precipitation, agricultural drainage water, industrial discharges and effluents from wastewater treatment plants may introduce chemical pollutants and pathogens into these surface water bodies.
The study “Dynamic of wastewater effluent contributions to surface waters and potential challenges for drinking water abstraction in Germany” allows to determine for the first time the relative contribution of wastewater effluents in all German surface water bodies during various flow conditions, and to assess their relevance for drinking water abstraction via bank filtration or artificial groundwater recharge.
Based on the results of this study, the contributions from wastewater effluents during average flow conditions vary in most upper river basins in Germany between 0 and 5 %. Contributions from >5-10 % and >10-20 % are dominant in river basins up- and downstream of urban centers (e.g. rivers Havel, Neckar, lower Rhine, Maas, middle Rhine) as well as river stretches characterized by generally low-flow conditions (i.e., rivers Main, Ems, Neckar). During low-flow conditions this picture changes dramatically. Effluent contributions of >10-20 % are dominating under low-flow conditions nationwide in a large number of river basins. Several watersheds exhibit wastewater effluent contributions of >20-30 % (e.g., rivers Elbe/Saale, Weser, middle Rhine). Portions of the river Main, Ems, Weser and Havel as well as tributaries of the Rhine river are characterized by effluent contributions of >30-50 %. In portions of the river Neckar, Ems, the Baltic Sea estuary and the lower and middle Rhine watersheds effluent contributions are above 50 % during low-flow conditions.
The presence of elevated contributions of wastewater effluents is not per se a potential risk for human health where drinking water is abstracted via bank filtration or groundwater recharge. Essential for the risk assessment are site specific characteristics as hydrobiological conditions of the soil passage. Case studies presented in the report show which conditions may result in an exceedance of health related guide values for chemical pollutants.
The results of the study emphasise that an overall understanding of the dynamics in the whole watershed are needed when water is abstracted via bank filtration or groundwater recharge – especially where bank filtrate constitutes a significant share of the raw water while the contribution of wastewater effluents is high in the respective waterbody. In many cases, the needed risk assessment has not yet been done by the respective authorities or water providers.
Considering impacts from climate change, the relative contribution of wastewater effluents in receiving streams is likely going to increase and thereby not only affecting the ecological and chemical status of surface water but also drinking water abstraction.
Potential measures to reduce micropollutants in waters have been outlined in a recent UBA Background paper.
The findings from the study should also be taken into consideration regarding other water uses from surface waters, e.g. agricultural irrigation. Especially under low-flow conditions the presence of wastewater born pollutants and pathogens can be high, potentially requiring additional treatment processes to reduce risk for the health and environment. This should also be taken into account in the current discussions on water reuse – without risk assessment one cannot assume that surface waters are generally safe or more suitable for irrigation than reclaimed water that has undergone advanced treatment processes.
Therefore a holistic view is needed to ensure that surface waters can fulfill different uses and meet the requirements that are needed for a healthy ecosystem, safe drinking water abstraction or irrigation.