BMR Journal of Australian Geology & Geophysics, 10, 391- 393 © Commonwealth of Australia 1988

Acid mine drainage at Captains Flat, New South Wales G. Jacobson1 & G.F. Sparksman 1

Acid drainage from the abandoned base-metals mine at Captains Flat, New South Wales, is the main contributor of zinc to the Molonglo River in low flow conditions. In 1982, zinc was detectable

Introduction The zinc-lead-copper mine at Captains Flat, New South Wales, was worked between 1882 and 1962, producing about 4 million tonnes of ore. The orebodies, of massive and disseminated sulphide, occurred in a north-south trending belt of Silurian shale and volcanics. Mining took place to depths of 600 m over a strike length of about one kilometre (Glasson & Paine, 1965; Oldershaw, 1965).

The closure of the mine antedated environmental legislation by several years. During the life of the mine, mill waste water and mine drainage water was discharged into the Molonglo River, and there were three serious pollution incidents when tailings dams collapsed during floods, resulting in damage to pastures downstream. After closure of the mine, the underground workings filled with water, which overflowed into the river. In the late 1960s public concern was aroused because of obvious instability of the tailings dumps and the well-documented pollution of the Molonglo River water and sediments upstream of Canberra, where the river had been dammed to form Lake Burley Griffin (Weatherley & others, 1967). Various possible remedial measures were canvassed by the Joint Government Technical Committee on Mine Waste Pollution of the Molonglo River (1974). Eventually, the tailings dumps were stabilised and vegetated at a cost of several million dollars (Ash, 1976; Craze, 1979).

The effectiveness of the remedial works on water quality in the Molonglo River was investigated in 1977-8 by the NSW State Pollution Control Commission, in the New South Wales section of the river, and by BMR in the ACT section of the river. It was concluded that the water quality of the NSW section had improved during median flow conditions and that zinc concentration was a small fraction of what it had been before rehabilitation (Brown & Train, 1983).

From an ACT point of view, the improvement was reflected by reduced salinity, pH, and zinc concentration (Haldane, 1977).

Water quality in the Molonglo River - the low•flow condition, 1982 The year 1982 was one of the driest years on record in this region, and flows in the Molonglo River were low throughout the year. During this year, water and sediment samples were taken by BMR at about 20 sites along the river (Figs. 1, 2), at approximately two-monthly intervals. The samples were analysed for zinc, lead and copper, as well as other elements (Sparksman, 1982). It was found that during low-flow conditions, the major source of zinc pollution was springs that discharge mine water directly into a 150 m section of the river, next to a rehabilitated tailings dump. The springs result from the high-pressure head of groundwater in the mine

1 Division of Continental Geology, Bureau of Mineral Resources, GPO Box 378, Canberra, ACT 2601

in the river for 50 km downstream of Captains Flat. Rehabilitation works at the mine site have effectively stabilised tailings dumps, and neutralised them as a source of major pollution.

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Figure 1. The Molonglo River, showing sample locations in New South Wales and the Australian Capital Territory.

(Fig. 2). The mine drainage water is acid, with a minimum recorded pH of 2.8; it has high salinity, with total dissolved solids up to 6000 mg/ L; and contains up to 220 mg/ L zinc and less than 2 mg/ L copper and lead.

In 1982, the North Spring (Fig. 2) was the main source of zinc pollution, supplying 25-100070 of the zinc load in the Molonglo River. In the period 1973 to 1982 the zinc level of the North Spring water decreased from a constant level of 230 mg/L to about 160 mg/ L, indicating a slowing down of the oxidation and leaching process in the mine.

Analyses of Molonglo River samples above the mine showed 'natural' zinc levels ranging from 1 to 3 mg/L. A sudden increase in zinc level and decrease in pH occurred in the river where the springs discharge, resulting in zinc levels of up to 30 mg/L and zinc loads of up to 60 kg/day. The low pH and high zinc levels persisted for about 0.5 km downstream to the Copper Creek junction. The addition of polluted waters from the Copper Creek catchment resulted in river waters containing 6-30 mg/ L zinc at pH 4.1-5.7, and from 4 to 62 kg/day. Water quality improved over the next 10 km downstream, with a decrease in zinc levels to about 4 mg/L and a rise in pH to near neutral (Table 1). Zinc levels gradually decreased further downstream, though zinc persisted in solution as far downstream as the NSW/ACT border (Fig. 1). Zinc loads at the border ranged from 0 to 5.3 kg/day, and increased with greater river flow.

Figure 3 compares the 1982 zinc and pH levels in the Molonglo River with those recorded in previous investigations. In the median-flow conditions of 1977-8 the

392 G. JACOBSON & G.P. SPARKSMAN

Table 1. Chemical analyses of water samples, July 1982

North Spring. Molonglo River Molonglo River Captains Flat 250 m downstream 10 km downstream

pH 2.9 4.9 6.5 Electrical

conductivity 3300 535 400 Calcium 412 46 30 Magnesium 720 32 21 Sodium 28 15 18 Potassium 3 1 1 Bicarbonate N.D. 7 35 Chloride 1 4 10 Sulphate 428 304 150 Dissolved zinc N.S. 15.7 5.9 Total zinc 160 16.7 5.9

Analyses in mg/ L; electrical conductivity in microsiemens/cm.

zinc levels were lower than those recorded before 1961 and in the period 1970-73, indicating an improvement in water quality since rehabilitation of the tailings dumps. However, in the low-flow conditions of 1982, polluted discharge from the mine springs had a considerable effect on pH and zinc levels in the upper Molonglo River. On the other hand, low river flow meant that river bed sediments were undisturbed, resulting in decreased zinc levels with increasing pH further downstream.

Figure 4 shows the content of copper, lead and zinc in sediments of the Molonglo River and Lake Burley Griffin in July, 1982. The highest content of metals in the Molonglo

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Figure 2. The abandoned mine at Captains Flat, New South Wales. The diagrammatic cross-section shows groundwater flow direction s.

River bed sediments was observed in a 2 km stretch of the river below the mine. The highest level of copper was 850 mg/ L at Station I, and copper levels decreased rapidly to 20-40 mg/L further downstream. The highest lead content was 5400 mg/L at Station 4, and lead levels decreased downstream from that point. Copper and lead levels were lowest at the head of Lake Burley Griffin, but then increased slightly within the lake. The highest concentration of zinc was 7000 mg/ L at Station 2; zinc levels decreased downstream, then increased again to about 4000 mg/ L at 40 km downstream. Continuing high levels of zinc in sediments of the Molonglo River remain a source of metal pollution to Lake Burley Griffin, especially during high river flow conditions when the sediments are mobilised.

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Figure 3. Variations in mean zinc and pH levels of the Molonglo River water (pre-1982 levels after National Capital Development Commission, 1981).

Conclusions The mine rehabilitation works of the mid-1970s have reduced the likelihood of a catastrophic pollution event such as the collapse of another tailings dump, and have reduced pollution levels in the Molonglo River in median-flow conditions. However, pollution of the Molonglo River is apparent in low•flow conditions from the discharge of acid mine drainage. Maintenance of the existing rehabilitation works at Captains Flat will be necessary for some time to come, and reduction of the pollution caused by acid mine drainage could require the construction of additional treatment works.

The Molonglo River is one of several Australian rivers that are polluted by heavy metals as a result of a century of unconstrained mining prior to the introduction of environmental controls in the 1970s. Even with large scale rehabilitation works, the effects of pollution persist for many decades.

ACID MINE DRAINAGE, CAPTAINS FLAT, NSW 393

Acknowledgements

This work was undertaken as a BMR contribution to the Joint Government Technical Committee on Mine Waste Pollution of the Molonglo River. We thank James Ferguson for comments on the draft manuscript.

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Figure 4. Copper, lead and zinc contents of Molonglo River bed sediments, July 1982.

References Ash, R.R., 1976 - Investigation for remedial works at the retention

dams and tailings dumps at Captains Flat near Canberra, Australia. Twelth International Congress on Large Dams, Mexico, 1976, 667-685 .

Brown, J.D., & Train, W., 1983 - The effects of rehabilitation of the mine dumps at Captains Flat on water quality of the Molonglo River - a summary of studies carried out during 1977179. State Pollution Control Commission, New South Wales - unpublished report.

Craze, B., 1979 - Mine waste pollution control at Captains Flat, New South Wales. Proceedings oj the Fourth International Symposium on Environmental Biogeochemistry, Canberra, 1979, 705-712.

Glasson, N.R. & Paine, V.R., 1965 - Lead-zinc-copper ore deposits of Lake George Mines, Captains Flat. In McAndrew, J. (editor) - Geology of Australian Ore Deposits. Eighth Commonwealth Mining and Metallurgical Congress, Melbourne, 1965, 1,423-431.

Haldane, A.D., 1977 - Effect of remedial treatment on water quality of the Molonglo River within ACT. Unpublished report in BMR files.

Joint Government Technical Committee on Mine Waste Pollution of the Molonglo River, 1974 - Mine Waste Pollution of the Molonglo River. Australian Government Publishing Service, Canberra.

National Capital Development Commission, 1982 - Waters of the Canberra region - metropolitan planning issues. National Capital Development Commission, Australia, Technical Paper 30.

Oldershaw, w., 1965 - Geological and geochemical survey of the Captains Flat area, New South Wales. Bureau oj Mineral Resources, Australia, Report 101.

Sparksman, G., 1983 - Heavy metal pollution in the Molonglo River, 1982. Bureau oj Mineral Resources, Australia, Record 1983 / 16.

Weatherley, A.H., Beevers, J.R., & Lake, P.S., 1967 - The ecology of a zinc-

polluted river. In Weatherley, A.H . (editor), Australian inland waters and their fauna - eleven studies. Australian National University Press, Canberra.