Release of biocides in an exemplary urban river in Germany

Numerous chemicals enter surface waters via various emission pathways. One important group of those are biocides, which are used in a broad range of products, e.g. as disinfectants, agents against insects, for rodent control, or as material preservatives. Biocides are primarily used in urban areas, collected and transported in sewer systems either to sewage treatment plants (STP) or directly to the receiving water body (stormwater outfalls). During heavy rainfall events, however, sewage treatment plants cannot completely treat the increased sewage volume. Therefore, a part of the untreated sewage water, that contains also biocides, enters the surface waters as combined sewer overflow (CSO).

To assess the importance of different urban emission pathways of biocides, a study was conducted in Germany in the exemplary catchment of the river Alb flowing through the city of Karlsruhe. Water quality decreases by passing the city area from a good status to a bad status as defined in the Water Framework Directive (WFD). It is questioned if and which urban entry pathway contributes to which extent to the increasing pollution of the river Alb. Between 03/2021 and 12/2023 in total 130 samples were taken from the river Alb, the effluent of the municipal STP, combined sewer overflow and storm water outfalls and subsequently analysed for 42 biocidal substances, mainly disinfectants, material preservatives and pest control products. 26 out of 42 substances were detected: 13 in the river water while 26 substances were detectable in the urban emission pathways. These substances are mainly used as material preservatives, e.g. carbendazim, diuron, isoproturon and terbutryn, and were detected in approximately 90 % of all samples. For some substances the environmental quality standard (EQS) or the predicted no effect concentration (PNEC) are exceeded. For example, in CSO-samples the concentration of the insecticide permethrin exceeds 6 to 12-fold the PNEC for surface water. Even in consideration of a regulatory dilution factor of 10 for the discharge into surface waters, the PNEC is exceeded. This can be seen as an indication that pyrethroids used in urban areas are mainly transported into surface waters via CSOs, where they pose a risk to aquatic organisms. Due to low environmental concentrations and associated challenging analytical methods, other pyrethroids were not or only seldom detected.

Present results show that biocidal substances used in material preservation products are (continuously) released via numerous pathways into surface waters, and combined sewer overflows are important emission sources of biocides in the aquatic environment. However, the findings of a part of the substances investigated cannot be unambiguously attributed to a biocidal application, since the same substances can be used as, for example, human or veterinary pharmaceuticals. Nevertheless, the project provided important insights into the occurrence of biocides in urban runoff components and identified combined sewer overflows as a relevant emission pathway in urban areas which needs further to be investigated.