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Environmental concerns about the effects of effluents from wastewater treatment plants in tourist areas of the Alps: toxicity in aquatic microorganisms

Are the effluents of wastewater treatment plants in high mountains of concern for aquatic biodiversity? To answer this question, we carried out an experimental study testing the short-term toxicity of some Pharmaceutical Active Compounds (PhACs) in the effluents of a plant in a mountain valley of the Italian Alps sampled during the high tourist season (i.e., the ski season) when PhACs contamination is higher. We used different tools, taking as a model the bacterium Aliivibrio fischeri: the “whole-mixture approach” (Microtox test), “component-based approach”, predictive models “Concentration Addition (CA)”, “Independent Action (IA)”, and Combination Index (CI)”. We investigated the nature of interactions potentially occurring among seven selected PhACs (clarithromycin, naproxen, acetaminophen (paracetamol), ibuprofen, diclofenac, carbamazepine, and amoxicillin). This study showed that anti-inflammatory ibuprofen and diclofenac have higher short-term toxicity (IC50 <100 mg L-1) for A. fischeri compared with antibiotics, whose toxic effects are expected to become visible in the long term. Furthermore, based on the CI method, the seven PhACs seem not to interact in a synergistic or antagonistic way, but the final effect of their mixture seems to be equal to the sum of their individual effects. Notwithstanding the high tourist pressure, the Microtox test reported an overall toxicity of only 21%, which drops to 7% in the receiving water body, the Vermigliana stream. These values, besides the predictions by CA and IA, are not alarming per se, i.e., the treated effluent of the plant in the period of maximum tourist pressure can be considered no harmful to aquatic microorganisms. However, based on other studies highlighting negative effects of the diluted treated effluent of the same plant on macroinvertebrate community structure, we suggest that other model organisms be considered, including algae, insects, and fish, to assess the real ecological risk to wildlife of an effluent. The experimental tests on A. fischeri are useful for fast, preliminary information on the level of risk for freshwater ecosystems, but they should be combined with field studies and experiments on the wild populations to increase the ecological realism.