Contaminant Sources, transport and fate in stormwater runoff in Christchurch, New Zealand
Stormwater quality is receiving increased scrutiny to reduce ecological degradation of urban waterways. In order to predict the fate of key contaminants in stormwater runoff, a model applicable to the local conditions in Christchurch is necessary. We are developing a model to estimate contaminant sources, transport and fate, which will help decision makers ascertain best structural and management practices to reduce contaminant loading to urban waterways. Necessary input parameters include coefficients for contaminant build-up and wash-off functions that describe the deposition during antecedent dry days and dislodgement during a rain event. To derive these parameters, we constructed thin boards of different street materials (e.g. asphalt, concrete), which were exposed at various locations within a University of Canterbury carpark over a nine day period before being placed under a rainfall simulator to collect surface runoff. Our experimental results showed that asphalt retains more contaminant particles compared to concrete surfaces. Spatial variability of contaminant distribution within the carpark was high. First flush TSS wash-off concentrations from concrete boards, for example, ranged from 63 to 164 mg/L. Similar variability was also observed for copper and lead concentrations from both concrete and asphalt boards. Variation was attributed to factors such as the number and type of vehicles parking over the boards. The application of this experimental data to parameterize our initial model resulted in predictions that seemed realistic and are comparable to measured TSS and copper concentrations in stormwater.