The aquatic environment is continually exposed to a complex mixture of chemicals, with effluents of wastewater treatment plants (WWTPs) as one key source. The aim of the present study was to investigate whether environmental risk assessments addressing individual substances are sufficiently protective for such mixtures. Based on a literature review of chemicals reported to occur in municipal WWTP effluents and mode-of-action considerations, four different types of mixtures were composed containing human pharmaceuticals, pesticides, and chemicals regulated under REACH. The experimentally determined chronic aquatic toxicity of these mixtures towards primary producers and the invertebrate Daphnia magna could be adequately predicted by the concept of concentration addition, with up to 5-fold overestimation and less than 3-fold underestimation of mixture toxicity. Effluents of a municipal WWTP had no impact on the predictability of mixture toxicity and showed no adverse effects on the test organisms. Predictive environmental risk assessments for the individual mixture components based on the derived predicted no effect concentrations (PNECs) and median measured concentrations in WWTP effluents (MCeff ) indicated no unacceptable risk for any of the individual chemicals, while MCeff /PNEC summation indicated a possible risk for multi-component mixtures. However, a refined mixture assessment based on the sum of toxic units at species level indicated no unacceptable risks, and allowed for a safety margin of more than factor 10, not taking into account any dilution of WWTP effluents by surface waters. Individual substances, namely climbazole, fenofibric acid and fluoxetine, were dominating the risks of the investigated mixtures, while added risk due to the mixture was found to be low with the risk quotient being increased by less than factor 2. Yet, uncertainty remains regarding chronic mixture toxicity in fish, which was not included in the present study. The number and identity of substances composing environmental mixtures such as WWTP effluents is typically unknown. Therefore, a mixture assessment factor is discussed as an option for a prospective environmental risk assessment of mixtures of unknown composition.
For additional information, see the publication in Water Research:
Coors, A., Vollmar, P., Sacher, F., Polleichtner, C., Hassold, E., Gildemeister, D., Kühnen, U. (2018). Prospective environmental risk assessment of mixtures in wastewater treatment plant effluents – theoretical considerations and experimental verification. Water Res 140, 56-66.
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