Carpark pollutant yields from first flush stormwater runoff

Stormwater runoff from carparks should be treated to remove pollutants before they enter urban waterways; however, differences in traffic characteristics and surrounding land use activities can result in varying first flush pollutant types and concentrations requiring specific treatment approaches. An understanding of potential variations in first flush pollutant characteristics from carparks is necessary to design adequate treatment systems. Stormwater runoff from over 20 runoff events in three different carparks (university, hospital and industrial) in Christchurch, New Zealand were thus analyzed for Total Suspended Solids (TSS) and heavy metals (Zn, Cu and Pb) over a year. Pollutant concentrations were found to vary across the carparks, which were largely driven by land use activities such as traffic count, size of the vehicles, and surrounding topography. Mean concentrations of heavy metals (both dissolved and particulate) and TSS were significantly higher in the industrial carpark than in the other two urban carparks, which had statistically similar mean pollutant concentrations. Specific ratios of metal species (Zn:Cu, and Zn:Pb) were relatively high for the industrial carpark, indicating a greater contribution from the wear and tear of large commercial vehicles. TSS and total Zn wash-off concentrations from the hospital carpark when it was non-operational (passive) were found to be significantly lower than the hospital carpark when it was operational (active), confirming that pollutant concentrations increase with vehicular activity. Rainfall characteristics such as antecedent dry days and rain intensity and duration were found to only have a low positive correlation to pollutant concentrations for all carparks. The findings from this study highlight the importance of considering carpark characteristics in the implementation of on-site stormwater treatment systems.