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

Abstract: 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 is designed to accomplish much more than just reduce runoff flows and volumes; it 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 by their modifications, or additions of new components, 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 overloading of stormwater quality control measures resulting from up-scaled precipitation in the changing climate and increasing areas contributing runoff due to expansion of urban areas and/or intensification of urban land use, (ii) Pollution overloading caused not only by the growing inputs of pollutants from progressing urbanization, but also by an increased intensity of pollutant wash-off and transport processes, (iii) the aging of stormwater management systems, which without effective maintenance suffer from deterioration of their performance, and (iv) insufficient attention paid to socio-economic issues concerning environmental policies and practices. 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. The scenarios were chosen to reflect both climate change (selected from those offered by the Swedish Meteorological and Hydrological Institute) and the socio-economic factors. The test catchments were selected somewhat opportunistically from those which represented typical Swedish urban catchments and were documented by the available data. Two simulation models were used: The US EPA SWMM representing one of the most widely applied physically-based models, and the WinSLAMM, which compared to SWMM, is a pollutant source based model. In runoff simulations, stormwater quality was described by several important parameters: Total suspended solids (TSS), which are arguably the most important descriptor of stormwater quality, and three ubiquitous heavy metals, namely Cu, Pb and Zn, which reflect the pollution generated by automobile traffic. 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 progress of urbanization will manifest itself in two ways: expansion of urban areas (urban sprawl) and intensification of land use in the existing areas. Urban sprawl leads to the growth of urban areas and generation of more runoff. Furthermore, it also leads to increased ‘kilometres travelled’, which contributes to an increased magnitude of traffic related pollution. 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, assuming the acceptability of the required investments. 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. For stormwater quality described by heavy metals, the wash-off/elution of metals from catchment surface sediment is important, so it was further examined in laboratory experiments. Coarse particles were identified to potentially release significant amounts of heavy metals during runoff events. The released metals were predominantly in the particulate bound phase, since coarse particles acted first as collectors of fine particles in dry weather, but released those particles during rainfall runoff. Site/runoff event specific factors (e.g., traffic intensity and street sweeping routines; energy input into sediment/water interaction) 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. Concerning the changes in sewer pipe materials (e.g., substituting PVC or corrugated steel pipes for concrete pipes), laboratory experiments showed that various pipe materials affected the stormwater quality differently, depending on the characteristics of the stormwater used in experiments. All three materials, and particularly concrete, contributed to increased pH of the transported stormwater. Even though metals could be potentially eluted from all the materials tested, 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. These reductions were explained by the fact that pipe corrugations acted as particle traps, and metal concentrations, except for Zn, were therefore significantly reduced. However, once these traps would fill up, such pollutant removals would cease. 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.