Emission standards versus immission standardsfor assessing the impact of urban drainage onephemeral receiving water bodies

In the past, emission standard indicators have been adopted by environmental regulationauthorities in order to preserve the quality of a receiving water body. Such indicators are basedon the frequency or magnitude of a polluted discharge that may be continuous or intermittent.In order to properly maintain the quality of receiving waters, the Water Framework Directive,following the basic ideas of British Urban Pollution Manual, has been established. The Directivehas overtaken the emission-standard concept, substituting it with the stream-standard conceptthat fixes discharge limits for each polluting substance depending on the self-depurativecharacteristics of receiving waters. Stream-standard assessment requires the deployment ofmeasurement campaigns that can be very expensive; furthermore, the measurement campaignsare usually not able to provide a link between the receiving water quality and the pollutingsources. Therefore, it would be very useful to find a correlation between the quality status of thenatural waters and the emission-based indicators. Thus, this study is aimed to finding a possibleconnection between the receiving water quality indicators drawn by environmental regulationauthorities and emission-based indicators while considering both continuous (i.e. from thewastewater treatment plants) and intermittent pollution discharges (mainly from combined seweroverflows). Such research has been carried out by means of long-term analysis adopting a holisticmodelling approach. The different parts of the integrated urban drainage system were modelledby a parsimonious integrated model. The analysis was applied to an ephemeral river boundingBologna (Italy). The study concluded that the correlation between receiving water quality andpolluting emissions cannot be generally stated. Nevertheless, specific analyses on pollutingemissions were pointed out in the study highlighting cause—effect link between polluting sourcesand receiving water quality.