Quick Dips

Metallic Rivers

Are our estuaries conduits or sinks for metal-bearing wastes? This is the question being asked by researchers at the Institute of Environmental Science Research and the Institute of Geological and Nuclear Research, who are studying the fate of the metals released into three North Island estuaries -- the Hatea estuary in Whangarei Harbour, the Waihou estuary at Coromandel and the Mahurangi estuary at Warkworth.

While each estuary has the two features which typify Northland estuarine systems -- mud and mangroves -- each also drains a different type of metal-bearing catchment. Dr Jenny Webster and Dr Cathy Butler of ESR and Dr Kevin Brown of IGNS are focusing on on the processes which affect metals transported in fresh water as it mixes with sea water.

At the heart of the research is an attempt to discover what really happens to the metals flushed "out to sea" each year. The amount of metal carried into estuaries can be significant, particularly in urban and mineralised catchments. Although the use of metal criteria to control metal concentrations in fresh water near effluent discharge sites is now widespread, contaminant metal impacts downstream can not always be controlled in this way. Not only is metal accumulation a concern for estuaries, but the distribution of metals between the water and the sediment can change. Dissolved metals are more toxic than those bound in a solid phase and, for some metals, an increase in salinity can result in an increase in a toxic form of the metal.

Evidence of estuarine metal accumulation has already been identified in the Waihou estuary, where the concentrations of heavy metals, such as copper, lead, zinc and arsenic, have been measured in the sediments and waters of the Waihou River and its major tributaries. The Waihou River runs from its headwaters near Paeroa into the Firth of Thames, where the final 20 kilometres is affected by tides which alter the flow and chemistry of the freshwater river. Sediment metal concentrations are raised in the input from Tui Stream, which drains the base-metal sulphide tailings dam of the former Tui mine, and from the Ohinemuri River which drains a mineralised catchment which has been extensively mined. Metal concentrations in the Waihou sediments continue to rise downstream from these tributaries, becoming erratic in the estuary. The average increase over normal, background concentrations in the estuary ranges from 1.8 times in the case of iron to 20.8 times for lead.

The Hatea River, on the other hand, drains a suburban catchment in Whangarei, mixing with the harbour water in a marina very near to the business/industrial centre. Stormwater drains discharge directly into suburban streams and the marina, raising the lead and zinc concentrations of many sediments to levels similar to those of Auckland's Manukau and Waitemata Harbours, and exceeding those of the most contaminated Waihou River sediments.

The Mahurangi River in Warkworth drains into an estuary formed by the Snell's beach peninsula, and then into Mahurangi Harbour. Metal accumulation in the southern region of this estuary is of particular concern, as it is a popular site for oyster farms. As oysters are filter feeders, their flesh can accumulate metals. Research carried out on the fate of cadmium applied to the surrounding farm land uses the metal content of the oysters to trace cadmium movement through the estuary.

At ESR, laboratory experiments duplicating estuarine chemistry are being undertaken to attempt to identify the most important factor contributing to the accumulation, release or removal of each metal under artificial estuarine conditions. The results of these experiments and the field studies will be used, together with two computer programmes which can model estuarine and freshwater chemistry, to help predict the future of metals in our New Zealand estuaries.

Dr Jenny Webster, ESR