Understanding Water Quality in Raglan Harbour

Raglan Harbour is located on the west coast of New Zealand’s North Island and is bordered by the Raglan township. Seven major river sub-catchments and a number of smaller streams drain to the harbour. Land use in the... [ view full abstract ]

Raglan Harbour is located on the west coast of New Zealand’s North Island and is bordered by the Raglan township. Seven major river sub-catchments and a number of smaller streams drain to the harbour. Land use in the watershed is dominated by dairy farming and forestry, which impact harbour water quality.

A waste water outfall is located at the harbour mouth and is licensed to release from 30 minutes before high tide until 1 hour before low tide. Raglan is a popular holiday destination. During summer, the population nearly triples, adding significant pressure to the waste water system. Over the years, there have been a number of reported spills and unlicensed releases from the treatment facility into the harbour leading to temporary health warnings, and impacted recreation and food gathering in the harbour. However, there is little context of the scale of the operation, and of the spills, against contaminant levels from inflowing rivers which are strongly affected by land use practices.

We present a numerical model to assess water quality in Raglan Harbour which includes inflow from 13 rivers, the waste water outfall and waste water spills. The model has two parts: a calibrated hydrodynamic model to simulate the water movement within the model domain and a water quality model which uses the results from the hydrodynamic model to track the release, movement and decay of contaminants. The water quality model uses releases based on relationships developed from water quality monitoring.

Results from year long modelling indicate strong seasonality in pollutant concentrations and are used to map the spatial variability in concentration. The model has helped to establish the relative impacts of the different sources of pollution, specifically the waste water outfall and riverine inputs. It is also being used to identify high impact areas, and the sources responsible for each area of increased concentration. The project is supported by the local council, and the model is currently being extended so that it can be used as a management tool to better understand and predict water quality in recreational areas.