The impact of urbanisation on the water quality & hydrology of the Kaiwharawhara Stream, Wellington, New Zealand

Abstract

Studies have shown that urbanisation increases surface runoff and modifies water quality. The development of urban areas changes both the physical and chemical characteristics of water, however, these effects are often complex and tend to vary from place to place. The Kaiwharawhara Stream catchment provides a range of sub-catchments with land uses ranging from "natural" through to intensively urbanised. The impact of urbanisation has been evaluated by comparing the flow characteristics and water quality of these various sub-catchments. Water quality problems in the Kaiwharawhara Stream result from sewerage faults, such as leaks or cross-connections with the stormwater system, and leachate from closed landfill sites. Background levels, and concentrations of non-point source contaminants present in urban runoff are unknown, as is their potential impact. The temporal variation of over twenty different water quality parameters has been quantified at numerous sites within the catchment. The parameters examined include temperature, pH, suspended solids, dissolved oxygen, BOD5, nutrients, faecal coliforms, organics (FWAs, oil & grease), and various metals. The change in water quality during a storm event, and the sediment quality at various sites, has also been evaluated. The flow characteristics have been quantified at four locations within the catchment and contaminant yields calculated. Geographic Information Systems (GIS) have been used to combine, analyse, and display various geographic data layers. A comparison of the hydrological characteristics of the monitored sub-catchments has shown significant differences. However, while the urban sub-catchment showed many of the characteristics expected, including more small floods, a lower baseflow, and sharp 'peaked' hydrographs with a short lag time, it proved difficult to quantify the impact of urbanisation because of differences in the innate physical characteristics of the discrete catchments. The impact of land use change and the important role of interception by vegetation was clearly illustrated in the predominantly pastoral sub-catchment; an elevated baseflow and storm runoff volumes, leading to a relatively high specific yield was observed The Upper Karori Reservoir was also clearly shown to alter the natural flow regime, particularly with the attenuation of flood peaks. Large temporal and spatial variations in the water quality have been observed. The nature of the contamination, sources and main controls, have been identified. Many urban sites were found to have significantly reduced water quality when compared to non-urban sites. Leachate from the closed Curtis Street landfill site and urban runoff were recognised to have a significant effect on the water quality. As expected, a reduction in water quality during wet-weather flow was observed, resulting from elevations in suspended material and bacterial levels. The analysis of the water quality during a storm event has shown the complex interaction of various processes. A large proportion of the contaminants were found to be associated sediment, and a spatial variation in sediment quality was identified indicating various anthropogenic sources. This study has confirmed that urbanisation has a significant effect on the flow characteristics and water quality within the Kaiwharawhara Stream catchment. Leachate, sewerage faults and urban runoff may increase the amounts of suspended sediment, nutrients, oxygen demanding substances, heavy metals and oil & grease. Whether these loads are sufficient to cause 'significant' adverse effects to the environment is questionable. The better understanding of the processes that affect water quality gained, and an indication of areas that need improvement, will assist in the development of a range of resource management strategies to mitigate any negative effects of contamination.