Supervising Scientist Division

The behaviour of copper in sediments and waters of Macquarie Harbour, western Tasmania

Supervising Scientist Report 111
Teasdale P, Apte S, Batley G and Ford P
Supervising Scientist, 1996
ISSN 1325-1554
ISBN 0 642 24310 7

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The Mount Lyell copper mine in Queenstown, western Tasmania, had discharged tailings and waste water into the King River from 1916 to 1994. It has been estimated that, over this period, in excess of 100 million cubic metres of tailings, slag and topsoil was deposited in the King River and Macquarie Harbour. As a consequence of this activity, high copper concentrations have been measured in the King River and Macquarie Harbour. This waste material has accumulated along the banks of the King River and has formed a large, unstable delta where the river enters the harbour. Exposed tailings material both on the delta and on the river banks are highly acidic as a result of pyrite oxidation and this contributes to the dissolution of both iron and copper from these sediments.

The Mount Lyell Remediation Research and Demonstration Program is undertaking a comprehensive study of the impacts of the Mount Lyell mine operation on the King River and Macquarie Harbour as part of the development of a remediation strategy. This report describes a comprehensive study of the cycling of copper in the sediments and waters of Macquarie Harbour, undertaken as project 12 within this program. The specific objectives of this project were to characterise the potential for sediment in Macquarie Harbour to release metals to the water column, to determine the conditions under which this will occur and to estimate the magnitude of the impact on the level of metals in the water column. Any options for reducing the potential impact on the release of metals from the sediment were also to be canvassed.

Field trips to Macquarie Harbour were in June and July 1995. Ten study sites of primary interest were selected, including three sites in the vicinity of the King River delta, one site in the entrance to Long Bay, five sites spaced approximately equidistantly along the east coast of the harbour, between the mouths to the King and Gordon Rivers, and a western harbour site. Water and sediment core samples were taken at each site and analysed for a range of chemical parameters. Pore water samplers (peepers) were also deployed in order to obtain information on the pore water copper concentrations. Sediment traps were also deployed at selected sites in the harbour in order to determine the sedimentation fluxes of particulate material.

Elevated concentrations of dissolved copper were measured throughout Macquarie Harbour. The highest dissolved copper concentrations were measured in the delta waters (up to 560 µg/L). Elsewhere, surface water copper concentrations ranged from about 100 µg/L in the north to 12 µg/L in the south. Bottom water concentrations ranged from about 30 µg/L on the delta to 4 µg/L in the south. Dissolved copper concentrations were determined largely by dilution of King River water with seawater low in copper and precipitation of river-borne copper in the low salinity zone of the estuary. About 65-85% of the copper at all sites was in the filterable fraction(<0.45 µm) and is probably present in the form of small particles (colloids) in association with iron and manganese. The copper complexation capacity of dissolved organic matter was exceeded in most harbour waters and a significant proportion of the dissolved copper may therefore be bioavailable.

Particulate copper concentrations in sediment from all sites, except site 7, exceeded sediment quality guideline criteria formulated overseas, reaching over 1300 mg/kg at some sites. High particulate copper concentrations result from the deposition of copper-laden tailings material in the northern harbour and the precipitation of colloidally-associated copper in the southern harbour. Copper was effectively immobilised in the southern harbour sediments by the presence of high levels of reactive sulfide which forms insoluble precipitates with copper. Acid volatile sulfide measurements predicted low sediment toxicity at the southern harbour sites, whereas the toxicity of sediments from the northern harbour sites were predicted to be potentially high as a consequence of the low sulfide levels.

Pore water copper concentrations measured on the delta were as high as 4000 µg/L in locations that experienced recent sediment deposition or disturbance. This has serious implications for proposed dredging activities on the tailings material. At the more stable regions of the delta, and for the rest of the northern harbour region where tailings material had been deposited, pore water copper concentrations were in the order of 50-500 µg/L. Based on the pore water copper profiles, a flux of about 35 700 kg/y of dissolved copper from the sediment to the overlying water was calculated for the delta region. The pore water copper concentrations in the south were less than 8 µg/L at the surface sediments and decreased to less than 2 µg/L at depth. The low copper concentrations are a result of high sulfide concentrations which control the solubility of copper in the sediments and maintains a flux of dissolved copper from the water column into the sediment.

Fluxes of copper in sedimenting particles, which also act to remove copper from the water column, were calculated to be 46 000 kg/y on the delta. This is slightly greater than the flux of dissolved copper out of the sediments, thereby making the delta a net sink of copper under the conditions experienced during this study. The flux of particulate copper from the water column into the sediment for the rest of the harbour was about 85 000 kg/y. A mass balance calculation indicated that copper inputs for the King River were at least 95 000 kg/y, making it the source of 75% of the copper into the harbour waters. It must be stressed that the mass balance calculations presented in this work represent only a snapshot of the system and do not take into account temporal variations in copper transport or cycling.

A preliminary conceptual model of copper cycling in Macquarie Harbour has been proposed. Based on the nature of the sediment and the measured copper fluxes, there are three distinct zones. In the King River delta zone there are significant fluxes of dissolved copper from the sediments to the water column. The 'northern' harbour zone is characterised by a lower flux of copper but is a geographically larger area. The 'southern' harbour zone however, acts as a sink for dissolved copper as a consequence of the sulfide-rich surface sediments.

Based on this study, it is clear that there is a substantial input of dissolved copper to the Harbour from the delta sediments. This will remain a potential problem even after the major source of copper to the system, the King River, has been remediated. The pore water chemistry studies suggest that there is likely to be remobilisation of dissolved copper during dredging operations. Further studies are required to assess the magnitude of this problem. Increasing the sulfide and organic matter concentrations of the sediments, possibly through revegetation, is likely to reduce the flux of copper from the Delta.

Cover - The behaviour of copper in sediments and waters of Macquarie Harbour, western Tasmania

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