Emission standards versus immission standards for assessing the impact of urban drainage on ephemeral receiving water bodies

摘要

In the past, emission standard indicators have been adopted by environmental regulationnauthorities in order to preserve the quality of a receiving water body. Such indicators are basednon the frequency or magnitude of a polluted discharge that may be continuous or intermittent.nIn order to properly maintain the quality of receiving waters, the Water Framework Directive,nfollowing the basic ideas of British Urban Pollution Manual, has been established. The Directivenhas overtaken the emission-standard concept, substituting it with the stream-standard conceptnthat fixes discharge limits for each polluting substance depending on the self-depurativencharacteristics of receiving waters. Stream-standard assessment requires the deployment ofnmeasurement campaigns that can be very expensive; furthermore, the measurement campaignsnare usually not able to provide a link between the receiving water quality and the pollutingnsources. Therefore, it would be very useful to find a correlation between the quality status of thennatural waters and the emission-based indicators. Thus, this study is aimed to finding a possiblenconnection between the receiving water quality indicators drawn by environmental regulationnauthorities and emission-based indicators while considering both continuous (i.e. from thenwastewater treatment plants) and intermittent pollution discharges (mainly from combined sewernoverflows). Such research has been carried out by means of long-term analysis adopting a holisticnmodelling approach. The different parts of the integrated urban drainage system were modellednby a parsimonious integrated model. The analysis was applied to an ephemeral river boundingnBologna (Italy). The study concluded that the correlation between receiving water quality andnpolluting emissions cannot be generally stated. Nevertheless, specific analyses on pollutingnemissions were pointed out in the study highlighting cause—effect link between polluting sourcesnand receiving water quality.