Review of hydrologic processes and the transport of pollutants operated by storm water runoff in the urban environment

Stormwater runoff discharges from the urban environment have been identified as one of the major causes of quality deterioration in receiving water bodies. The pollutant load associated with stormwater runoff from urbanised areas conveys a heterogeneous mixture of constituents including particulate matter (PM), organic and inorganic compounds, nutrients, oil and grease, and metals. The wash-off and transport of pollutants during a rainfall-runoff event is a complex process affected by several factors ranging from the specific hydrologic, land use and watershed characteristics, to the availability, transport, partitioning and dissolution mechanisms of particulate matter and pollutants. Since the seventies, experimental studies carried out to investigate non-point pollution source and the related impact on receiving water bodies pointed out the relevance of the pollutant load associated with stormwater runoff. Experimental water quality data collected during several monitoring campaigns are illustrated in this chapter in order to characterise storm-water runoff quality from impervious surfaces with different land uses. Although experimental results indicate a wide range of variation in terms of concentration values, particulate matter and heavy metals have been recognized as the major concern due to their impact onto the aquatic environment. Indeed, particulate matter is the carrier of hazardous pollutants, including metals, adsorbed on its surface thus representing a potential source of chronic toxicity while dissolved metals pose a threat to the aquatic ecosystems in term of acute toxicity, being readily bioavailable. The delivery behavior of pollutant mass across the rainfall-runoff process is presented and discussed here with a special focus on particulate matter and heavy metals. The knowledge of the interactions between hydrologic processes and the production of pollutants transported by stormwater aims at developing management and control strategies to effectively protect the receiving water bodies and their aquatic ecosystems. From data presented in this chapter, it emerges that distributed treatment systems can prove more effective if compared with end-of-pipe technical solutions since the first flush phenomenon is generally more pronounced in small catchments, thus optimizing the design of BMPs. In light of the finding presented in the this chapter, it is evident that the development of suitable models to predict the pollutant loads associated with stormwater runoff is still an open issue. Further research is expected to provide additional insights into the overall picture, with special needs emerging with reference to heavy metals partitioning and speciation, and to more closely linking the observed behaviour to hydrology and the specific land-use characteristics.