Evaluation of the efficacy of stormwater treatment devices for reducing water borne ecological and human health risks

Stormwater runoff is widely recognised as a primary source of pollution and cause of negative ecological effects in urban river networks. In an effort to mitigate the degradation of receiving waters, tertiary stormwater treatment devices, such as constructed wetlands and bioretention systems, are increasingly being retrofitted to urban catchments at a considerable cost as part of the Water Sensitive Urban Design (WSUD) strategy. These devices reduce the direct connection of impervious areas to receiving waterways and have the potential to combine natural biological, chemical and physical processes to treat urban stormwater runoff. However, there has been relatively limited field validation of their benefits, particularly under Australian conditions and in terms of improving ecological health, despite their increasing popularity as part of urban water policies and strategies. The overarching aim of this research was to assess the efficacy of tertiary stormwater treatment devices retrofitted to urban catchments in Sydney, Australia,to improve water quality, and reduce potential risk of harm to ecological and human health. This was achieved by: 1) assessing the water quality improvement capacity of tertiary stormwater treatment devices; 2) assessing the toxicity of untreated and treated stormwater to freshwater biota using single-species toxicity tests in the laboratory with algae (Pseudokirchneriella subcapitata), a crustacean [Ceriodaphnia dubia] and fish embryos [Melanotaenia duboulayi]), and in situ with shrimp (Paratya australiensis); 3) assessing the influence of untreated and treated stormwater upon higher-levels of biological organisation (i.e. community and ecosystem responses) and 4) assessing the potential of constructed stormwater wetlands to provide a breeding ground for mosquitoes. Chapter 2 demonstrated the ability of a constructed wetland to reduce the majority of stormwater pollutants tested across three storm events and the toxicity of stormwater to freshwater biota across two storm events. However, the potential risk of stormwater to stimulate primary production remained following treatment. In Chapter 3, evaluation of a retrofitted bioretention basin across six storm events revealed that it had little impact on the majority of influent pollutant concentrations. Leaching of several analytes from the system occurred consistently and the toxicity of stormwater to freshwater biota was not always reduced. Chapter 4 highlighted that a larger-sized (with respect to the contributing catchment area) constructed wetland treated stormwater to a standard that enabled good improvements in ecological health at the wetland outlet, evidenced by its ability to support consistently high survival of P. australiensis, the presence of sensitive macroinvertebrate taxa, and reduced rates of organic decomposition. By contrast, two other wetlands that were smaller with respect to catchment area and had design constraints had a reduced capacity to achieve the same ecological improvements. Chapter 5 indicated that stormwater Zn concentrations reflective of those found in treated stormwater from a constructed wetland reinstated normal foraging behaviour in shrimp, indicating that the treatment of metal contaminants in stormwater can have discernible ecological benefits. Chapter 6 highlighted the variability of tertiary stormwater treatment systems (one bioretention basin and three constructed wetlands) to reduce indicator bacteria, particularly enterococci. Median outflow concentrations generally exceeded public health criteria for primary and secondary contact. Public health risks were further substantiated by the occurrence of immature mosquito larvae at all surface flow systems. Correlation analysis indicated that the higher nutrient concentrations in the inlet zone in comparison to the outlet zone provided conditions that were more conducive to mosquito larvae production. The empirical evidence generated from this study has important implications for the assessment and design of future stormwater treatment systems as well as the pursuit of WSUD strategies where the primary goal is to protect and rehabilitate urban waterways. Despite some beneficial outcomes evident in the chemical and ecological analyses, the overall results presented in this thesis suggest that stormwater treatment devices are far from a panacea for the adverse impacts of urbanisation on river networks. Indeed, the data suggest that it may not be possible to treat stormwater to prescribed levels (such as those set out in national water quality guidelines). It is clear that appropriate design of treatment systems is paramount and further research and subsequent design modifications is needed if the goal of improving the condition of urban waterways using stormwater treatment devices is to be achieved. This study has also demonstrated the importance of using a combination of chemical analysis and biological measures to provide an integrative assessment of any subsequent water quality improvements, and how this approach is necessary to quantify the benefits of tertiary stormwater treatment devices and to confirm if they function as planned.