The Integrated Catchment Study of Auckland City (New Zealand): Contaminant Load Discharge to Coastal Receiving Environments

Contaminant loads were computed as part of the Integrated Catchment Study (ICS) of Auckland City (New Zealand). Overflows and stormwater from 29,000 hectares discharge through 3,000 outfalls to the Waitemata and Manukau Harbours. Enterococci, total suspended sediment and total zinc pollutographs were produced at 81 consolidated outfalls by combining outputs from (1) combined and wastewater overflows from network modelling and (2), consolidated catchment stormwater quality modelling. The extent of network models was based on addressing key issues, including designed overflow structure locations and principal sources of stormwater contamination. 35 to 80% of the public pipe network was modelled in each catchment, totalling 43,000 nodes. Network models were developed in MOUSE and calibrated and validated to nearly 350 flow and rain gauges. Rainfall-runoff was simulated with the Model B + RDI module, reflecting a combination of kinematic wave plus rainfall dependent inflow methods. Pipe hydraulic routing was calculated solving the St. Venant equations, with the energy equation applied at manholes. Overflow contaminant loads were generated by factoring combined and wastewater overflow volumes by a Volume Weighted Wastewater Event Mean Concentration method. Stormwater loads were produced by combining consolidated catchment MOUSE hydrographs with STORMQUAL, an urban stormwater quality model. STORMQUAL is based on contaminant build-up and wash-off. Model parameters were calibrated to commercial, industrial, and residential land uses from monitored subcatchments. Parameter values estimated from monitored land uses were regionalised based on the known mix of land use in unmonitored consolidated catchments. The largest enterococci discharges occur from outfalls in the older parts of the city that contain substantial portions of combined networks and large number of designed overflow structures. Industrial areas containing zinc coated roofs discharging to the Whau Estuary, Waterview Inlet, Tamaki Estuary, and One Tree Hill contribute the largest zinc contaminant loads in the city.