Ranking of Potentially Toxic Pollutants and Their Impact on Microbial Communities in Water Bodies Receiving Wastewater Discharges

With the increasing level of chemical pollution, it becomes impossible to identify and control all pollutants in the aquatic environment. At the same time, the high toxicity, difficult biodegradation, and persistence of some hazardous pollutants require long-term continuous monitoring for the assessment of associated risks and environmental impacts. It is urgent to develop new strategies for screening target compounds, especially in water bodies receiving wastewater discharges with unknown residual pollutants and the approach of ranking/prioritization has the potential to contribute to this. The purpose of our work was to rank potentially toxic elements in the model sub-catchment according to the risk they pose to the environment and assess the impact of the pollution on microbial communities. The study is based on field samplings from the discharge zone of a municipal wastewater treatment plant in the upper part of Iskar River, Bulgaria, and subsequent laboratory analyses. A ranking system based on criteria, such as occurrence (measured concentrations in water and sediments by ICP-MS) and ecotoxicological data was applied for scoring potentially toxic elements. The effect of pollution on microbial communities in the discharge zone was assessed by the determination of the share of living cells and inhibition of their metabolic activity (CTC/DAPI staining method with digital image analysis). The data showed an increased level of risk, associated with seasonal dynamics of Cu. The Cd, Ni, and Pb being priority pollutants must be taken under consideration, too. The effect on microbial communities was assessed by indicators “share of the live bacteria”, and “mean fluorescence intensity”. The microbial communities in the wastewater discharge area were affected by pollutants and by high nutrients and organic content in effluent. The combination of ranking with the assessment of metabolic activity of sediment microbial communities allows us to identify the most harmful contaminants in discharge zones and to analyze the possible impact on biota. The applied approach has the potential to help us focus the future site-specific monitoring for pollution control in water bodies, receiving wastewater discharges, and contributes to a better understanding of the potential ecological risks from hazardous pollutants.