Source Control or Traditional BMPs? An Assessment of Stormwater Quality Benefits and Costs in Auckland City (New Zealand)

Sediment quality monitoring identified zinc as a contaminant of concern for the Auckland Region's receiving, environments. Within Auckland City receiving waters, Tamaki Estuary, Whau Estuary and Waterview Inlet are identified as key receiving environments at threat. This study compared the total reductions in zinc and total suspended solids (TSS) loads in stormwater runoff achievable by implementing a range of stormwater quality options, including: traditional stormwater quality ponds; on-site treatment of industrial roofs; and source control methods such as galvanised roof replacement, high efficiency street sweeping and increased cesspit cleaning; and estimated the cost of implementing these options. Pond locations were selected for high contaminant load areas and where they could be feasibly sited. An overall treatment efficiency rate per receiving environment (for zinc and TSS) was determined for each of the individual and combined suites of ponds and source control methods. The likely practical combinations of methods were then assessed and the combined effectiveness and cost calculated to compare the cost-benefit ratio of each option (in terms of annual zinc load reduction achieved per dollar spent). Results show that while stormwater quality ponds are generally the most cost-effective options within the catchments studied, the relatively small contributing sub-catchment for each pond and upper limits on physical contaminant removal means that significant load reductions can only be achieved through implementing source control.