Effluents from combined and separated sewer systems cause discharges of water and pollutants to receiving waters, that can be critical for the integrity of these ecosystems. The nature of factors affecting aquatic life in running waters can be either physical (i.e. shear stress), chemical (i.e. oxygen deplition, non-ionized ammonia) or combined, dependent on local conditions. Over the last years, extensive studies signifiantly improved the knowledge on receiving water impact, whereas little effort was introduced to methods for the specific assessment of effects caused by urban stormwater discharges from particular catchments. Therefore, a working group of the German Association of Water Pollution Control (ATV) developed a systematic approach in order to identify critical combinations of urban stormwater catchments and receiving water properties. Based on meso-scale laboratory experiments, model calculations and analysis of field studies, a data base was generated in order to quantify the potential of both single and combined physical and chemical stress to aquatic life. The data base was proved to be sufficient to derive a scheme describing the ecological impact of urban stormwater discharges. For selected parameters, ‘critical thresholds’ were used to assess the relative importance of hydraulic and chemical factors being harmful for the ecological properties of receiving waters. These were linked with the effects of meaures and summarized in decision schemes.

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