Evaluating health hazard of bathing waters affected by combined sewer overflows

Combined Sewer Overflows (CSO) affect bathing water quality of receiving water bodies by bacterial contamination. The aim of this is study is to assess the health hazard of bathing waters affected by CSOs. This is useful for bathing water managers, for risk assessment purposes and for further impact and economical assessments. Contaminant hazard was evaluated based on two novel indicators proposed in this study: the mean duration of insufficient bathing water quality (1) over a period of time (i.e. several years) and (2) after single CSO/rain events. Particularly, a novel correlation between the duration of sea water contamination and the event rainfall volume was developed. Contaminant hazard was assessed through a state-of-the-art coupled urban drainage and sea water quality model that was developed, calibrated and validated based on local observations. Furthermore, hazard assessment was based on a novel statistical analysis of continuous simulations over a 9 year period using the coupled model. Finally, a validation of the estimated hazard is also shown. The health hazard was evaluated for the case study of Badalona (Spain) even though the methodology presented can be considered generally applicable to other urban areas and related receiving bathing water bodies.