This project was undertaken for Auckland Regional Council to identify significant sources of contaminants in the central Waitemata and southeastern Manukau Harbours. The efficacy of stormwater treatment and contaminant source control measures were also assessed.

The Issue

The iconic Waitemata and Manukau Harbours are a fundamental part of urban living in Auckland, New Zealand’s largest city. Aucklanders have strong economic, social, and cultural connections with these water bodies, making extensive use of them for recreation, industry, transport, fishing, trade, and tourism.

At the same time, the Harbours are sinks for the disposal of urban stormwater and associated contaminants. In places, the harbour sediments contain heavy metals at concentrations considered moderate or high risk to the harbour ecosystems. With a rapidly growing population and the continued expansion of the city, the potential exists for further accumulation of contaminants and resultant ecological degradation.

The Approach

Auckland Regional Council (ARC) contracted NIWA to undertake two studies to predict the accumulation of sediment, zinc, and copper in the Central Waitemata and Southeastern Manukau Harbours.

The study aims were to:

• identify significant sources of contaminants
• forecast their accumulation over a 100-year planning horizon
• assess the efficacy of stormwater treatment and contaminant source control measures.

Our approach involved the application of three linked mathematical models:

• the GLEAMS sediment-generation model
• the CLM contaminant load model
• the USC harbour sediment/contaminant accumulation model developed by NIWA.

To implement the USC model, we collected data describing physical and chemical properties of the harbour, including sediments and freshwater inputs. We also analysed spatial information describing catchment characteristics such as land use, topography, and soil type.

Steps in calibrating and validating the model:

• We performed a backwards-looking projection ('hindcast') of sediment accumulation rates by running the model for the past 50 years with inputs of known annual average sediment accumulation rates.
• We then calibrated the model by comparing this hindcasted sedimentation rate to actual sedimentation rates determined from sediment cores, and adjusting the model until it 'fitted' the actual data.
• Finally, we validated the calibrated model by comparing its hindcasted heavy-metal concentrations to present-day vertical profiles of zinc and copper concentrations in harbour sediments. These vertical profiles provide a record of heavy-metal accumulation in harbour sediments over the past 50 years.

We then used the calibrated, validated USC model to predict future rates of contaminant accumulation in each of several harbour subestuaries, given variations in sediment and heavy metal loads associated with different stormwater management scenarios. 

The Result

The results of the studies identify those parts of the harbour most at risk and those subcatchments where stormwater management interventions may be of greatest benefit.

Key predictions are:

• No immediate threat of elevated zinc and copper concentrations in many subestuaries, such as intertidal flats and tidal creeks draining predominantly rural subcatchments.
• High threat associated with elevated zinc and copper concentrations in tidal creeks draining heavily urbanised subcatchments. Sediment quality guidelines are already exceeded or are predicted to be exceeded in the next 20-30 years in these locations. 

Some of the results of the Waitemata and Manukau studies can be seen in the figures below. These respectively show:

1. Areas of the central Waitemata Harbour requiring attention based on predicted sediment, zinc, and copper loads under a baseline development scenario (the ARC's basic plan for development of the catchment).
2. How improved stormwater treatment will change outcomes in the southeastern Manukau Harbour compared to the baseline development scenario (Scenario 1). Outcomes are expressed in terms of exceeding sediment quality guidelines for heavy metal concentrations in the harbour bed sediments.

• TEL is Threshold Effects Level: the level where sediment quality goes from being acceptable to potentially degrading the environment.
• ERL is Effects Range – Low: above this level, you can expect to see biological effects.