Future trends in urban stormwater quality : effects of changes in climate, catchment characteristics and processes and socio-economic factors

Sammanfattning: Climate change and progressing urbanization cause numerous environmental concerns, including the impacts on urban drainage. Such impacts were addressed during the last two decades with focus on hydraulic overloading of drainage systems and the means of overload remediation by stormwater management. However, modern urban drainage also serves to provide and protect broad environmental services chiefly by controlling stormwater quality. During the past 40 years, a sizeable investment has been made in urban drainage systems to improve stormwater quality and protect receiving water ecosystems. Such investments are at risks, because of impaired performance of stormwater quality controls now and in the future for the following reasons: (i) Hydraulic and Pollution overloading (ii) the aging of stormwater management systems and (iii) insufficient attention paid to socio-economic issues. The primary objectives of the thesis that follows is to address the above issues by examining future trends in stormwater quality and the influential factors affecting these trends. Trends in urban stormwater quality, in response to projected changes in the climate, urban catchments and their drainage systems, and environmental practices and policies, were studied by systematically describing these changes by a set of scenarios, which were then applied to several test catchments in simulations with two well-established computer models of urban drainage (US EPA SWMM and WinSLAMM). In runoff simulations, stormwater quality was described Total suspended solids (TSS) and three heavy metals, namely Cu, Pb and Zn. The assessment of uncertainties in the simulation process and potential changes in sewer pipe materials further inspired two additional studies: Potential improvements in modelling trace metal transport and control by clarifying the role of coarse sediments on road surfaces, and water quality implications of using sewer pipes made from three different materials. Simulations with up-scaled rainfall data produced changes in stormwater quality, depending on the type of storm events. Generally pollutant loads increased due to climate changes characterized by higher depths and intensities of rainfall in future scenarios. Storms with low to intermediate depths and intensities showed the highest sensitivities to climatic changes, because runoff producing areas increased with higher storm intensities (i.e., leading to contributions of pervious areas), and sufficient pollutant supplies on catchment surfaces; for high intensity events, such supplies were quickly exhausted. TSS loads exported from catchments with low imperviousness were most sensitive to climatic changes, but the magnitudes of TSS loads were low compared to those from catchments with high imperviousness. Furthermore, potential changes in catchment characteristics and drainage systems were identified to be of importance. Future scenarios combining changes in climate and socio-economic factors showed that the impacts on stormwater quality caused by climatic changes were smaller than those caused by changes in socio-economic factors. However, future urbanization impacts on stormwater quality could be controlled by incorporating modern stormwater management measures in future catchments. Simulations of such controls indicated that they were highly effective in protecting the stormwater quality. Finally it was noted that the two applied computer models produced somewhat different results and high uncertainties when assessing the future stormwater quality. This was due to their different descriptions of the underlying processes. Hence, it was desirable to examine the feasibility of improving stormwater quality modelling, particularly with respect to heavy metals. During laboratory experiments coarse particles were identified to potentially release significant amounts of heavy metals (mostly in the particulate bound phase) during runoff events. Site/runoff event specific factors (e.g., traffic intensity and street sweeping routines) and characteristics of the particles (i.e. organic content) were identified as influential factors affecting the release of heavy metals. This finding may help improve the description of pollutant transport processes in stormwater quality models. Laboratory experiments showed that various pipe materials (PVC, concrete and corrugated steel) affected the stormwater quality differently, depending on the characteristics of the stormwater used in experiments. The concrete pipe contributed to increase pH of the transported stormwater. Metal concentrations were mostly unaffected in the PVC pipe, decreased in the concrete pipe (due to particle deposition and metal adsorption to the pipe surface), and while Zn concentrations increased in the corrugated steel pipe due to elution, Cu and Pb concentrations were reduced (due to particle deposition in the corrugations). Since the impact of climatic changes on stormwater quality was relatively small compared to changes in socio-economic factors, future efforts to maintain or improve stormwater quality should focus on implementing pollutant abatement strategies, including implementation of well-designed and maintained stormwater treatment measures.