Mines paper # 6,
September 1996
In 1955 Mary Kathleen Uranium Ltd (MKU) was formed, with a majority of the shares held by the Rio Tinto Mining Company of Australia Limited. A sales contract with the United Kingdom Atomic Energy Authority was signed in 1956. Mining commenced at the end of 1956 and the treatment plant was commissioned in mid 1958. In this first phase of operation to 1963, Mary Kathleen treated 2.9 million tonnes of ore. Tailings were emplaced in a 12 hectare tailings dam in a small valley west of the plant. This overflowed into an evaporation pond of some 60 hectares.
The mine then remained closed for more than a decade. New sales contracts with overseas utilities were negotiated and recommissioning began in 1974. The Company made a share issue to raise capital, and the Commonwealth Government, through the Australian Atomic Energy Commission underwrote this, thereby obtaining a 42% holding in the Company. CRA, a successor of Rio Tinto Mining, held 51%, and the public 7%.
The mine and much upgraded mill reopened early in 1976. At the end of 1982 the mine finally closed down after some 6.3 million tonnes of ore had been treated in its second phase of operation.
Rehabilitation Plan
Notwithstanding the minimal conditions imposed in relation to the 1954 leases, from 1974 the Company took the view that it should conform to relevant current environmental and occupational health standards. Consequently, before the recommissioning in 1976 a full environmental impact study was undertaken and this incorporated a rehabilitation plan for the whole site.
Operations conformed to the national Code of Practice on Radiation Protection in the Mining and Milling of Radioactive Ores (1975 and 1980), and in its rehabilitation procedures the Company effectively complied with the Code of Practice on the Management of Radioactive Wastes from the Mining and Milling of Radioactive Ores (1982).
The Company's detailed rehabilitation plan was developed with the assistance of local and international consultants and in consultation with the Queensland Department of Mines. This plan predates the 1982 code mentioned above and is based on three principles:
In developing the Rehabilitation Plan, a detailed study of shut-down procedures and regulatory systems for uranium mines world wide was carried out by company staff. However, it became obvious that because of the variables of climate and land use, a site-specific rehabilitation plan was needed. The rehabilitation work was largely carried out by Minenco Pty Ltd.
Mine Pit
Mining operations ceased in September 1982, with the open cut approximately 230 metres deep, measured from the highest part of the rim. After ensuring that the rock face of the pit wall was stable, benches and the main decline were blocked off to vehicular and cattle access by barriers of waste rock. The haul road was ripped and seeded.
Rain water and some seepage water from the evaporation ponds area remains in the pit and it is expected that the depth of this will finally stabilise at around 40 to 50 metres.
Waste Dumps
The waste rock dumps contained some 24 million tonnes and were physically stable. Leaching of salts from them was not expected to be a problem due to the large excess of acid neutralising potential in the waste rock. Monitoring has confirmed this.
In 1981-82 the top surfaces of most waste dumps were levelled and covered with a layer of waste rock containing suitable fine material to promote plant growth and attenuate radiation levels. The surfaces were subsequently ripped and seeded with a mixture of seeds from local grasses, trees and shrubs. A good vegetative cover has become established on these surfaces. The low rainfall during the 1984-85 wet season showed that this was in fact securely established.
The two dumps which were not treated initially provided material to cover the surfaces of the tailings and evaporation ponds areas. These were then levelled, contoured and seeded immediately prior to the 1985-86 wet season.
Tailings Area
The tailings disposal area covers about 28.5 hectares and contains more than seven million tonnes of tailings material deposited behind the main dam wall. During the clean up operations, all contaminated equipment, soil from the treatment plant area and the material from the base of the evaporation area was buried here. The final quantity of liquor from the evaporation ponds was also treated in this area.
The surface of the tailings was contoured to slopes of about 1 in 200 leading to the spillways located in the perimeter of the area.
Initial test work and overseas experience suggested that a one metre layer of waste rock over the surface of the tailings would provide substantial erosion cover, prevent leaching of radioactive materials from the tailings, provide sufficient radiation shielding and restrict the rate of release of radon gas satisfactorily. In the event, it became difficult to demonstrate the last, due to the extremely low concentrations involved.
In 1983 about 60% of the upper south section of the tailings dam was covered with one metre of waste rock as originally intended. However, it was then found that a half metre layer of soil-clay mixture would give greatly improved attenuation of radon release from the tailings. Therefore, the rock cover already in place was removed and a half metre layer of roller-compacted soil-clay mixture from the upper, uncontaminated sections of the evaporation pond walls was placed on the upper south section of the tailings area. The one metre cover of waste rock was then replaced and completed.
On the upper north section of the tailings dam a half metre layer of contaminated clay, soil and evaporite mixture removed from the bases of the evaporation ponds was first placed on the tailings surface, followed by a further half metre layer of roller-compacted uncontaminated soil and clay. Over this, a one metre layer of waste rock was placed. Covering both upper sections of the tailings area was completed in January 1985.
The lower north section of the tailings dam was used as an evaporation area until late in 1985. The final quantity of evaporation pond liquid was transferred here in March 1985 and as it evaporated, slimes, evaporite and contaminated ground from the remaining section of the evaporation ponds area were tipped around the edge of it. The remaining liquid was then treated with finely crushed limestone to precipitate dissolved salts and radionuclides. Following a drying period, soil and waste rock material was added to the resulting precipitate to assist solidification and provide a suitable stable base for the cover material. The half metre of soil/clay cover on this section was completed in October and the one metre layer of waste rock cover was in place in November 1985.
In 1983 the coarse tailings, which had been deposited on the downstream side of the main wall during operations, were levelled and waste rock material was placed adjacent to the wall and on top of these tailings to produce a batter slope of 2.5 to 1, from the top of the wall. Finally, in 1984 a filter zone was constructed beyond the level portion of the toe of the wall to trap any fine particles carried from the tailings material. A two metre thick zone of compacted waste material topped by large boulders covered the filter zone to protect it.
Evaporation Ponds
The system was designed to contain all liquid wastes and to enable the evaporation of water from them. Seepage control was therefore important during both operation and rehabilitation phases. The principal seepage path under the evaporation pond wall was along the original stream bed. This was impounded in a collection pond and pumped back to the evaporation pond or to the treatment plant for re-use.
At the close of operations in October 1982, approximately 1.6 million cubic metres of liquor were contained in the evaporation ponds. Two thirds of this evaporated in the first twelve months. Since it was not expected that the evaporation ponds would ever completely dry out naturally, the closure plan proposed a means of treatment and disposal of the remaining liquor, utilising 755 metres of infiltration trenches in alkaline clays near the main evaporation ponds. Here neutralisation and cation exchange rapidly improved the water quality of seepage, which was then picked up in the borehole system and pumped back to the mine pit.
Heavy wet seasons in 1982-83 and 1983-84 set back the liquor disposal programme. In particular, the volume of liquor remaining increased from 400,000 cubic metres in December 1983 to almost 700,000 cubic metres in March 1984.
The material on the base of the evaporation ponds area consisted of evaporite, slimes and contaminated ground. As the liquor level receded in the ponds, the dried exposed surface was cleared of precipitate and the top layer of surface soil. This material was buried in the tailings area as already described.
Cover material, consisting of 0.6 metres of waste rock on the pond 1 area and 0.4 metres of soil and clay followed by 0.6 metres of waste rock in the pond 2 area, was placed on the cleared surface.
The evaporation pond walls, with the exception of the eastern wall which diverts the Eastern Branch of the Cameron Creek, were removed. The material from the walls was placed in the tailings disposal area. Diversion of the creek was maintained to minimise erosion of the rehabilitated evaporation area surface. The east wall was armoured with large rocks and the diversion channel was enlarged to reduce the impact of flood waters.
Treatment Plant Area
After operations ceased in 1982, the plant was decommissioned and prepared for auction in April 1983. Alpha and gamma radiation surveys were taken over all equipment, structures and areas to provide a basis for the cleaning and decontamination program.
Several items which were contaminated and would have required a considerable amount of work to decontaminate were demolished and buried in the tailings dam. These included the product dryer, with associated equipment and building, solvent extraction tanks and pipework, product thickeners and pipework, clarification equipment, tailings pipelines and leaching tanks.
The remainder of the equipment was cleaned and the contaminated solids (including soil) and liquids were removed to the tailings dam.
Final clean up of the area commenced following demolition and removal of purchased items. Uncontaminated discarded material was dumped at the bottom of the mine pit, while minor concrete foundations and contaminated soil were buried in the tailings dam. Some major concrete foundations were left in position, and those of the power station were covered with a layer of waste rock material and soil. The area was then ripped and seeded, with most of this work being completed in August 1984.
The final average radiation levels, as elsewhere on the site, are well below maximum levels universally accepted for members of the general public.
Town and Support Facilities
The Mary Kathleen community and township formed an important social and economic unit in the area and it was desirable that it be maintained if at all possible. The Company decided that it would support any proposal offering an economically viable future for the town. However, no viable alternative use was proposed and therefore in October 1982 the decision was made to offer the town facilities for sale and removal. Final clean-up was undertaken early in 1984, leaving only bitumen roads, concrete slabs and vegetation .
The two company-built water storages, Lake Corella and Lake Mary Kathleen were transferred to the state Water Resources Commission.
Conclusion
In the light of the normally very arid environment at Mary Kathleen all rehabilitation work was designed to minimise the long term potential for dust generation while nevertheless avoiding the creation of a rockscape with low potential for establishment of vegetation. In the event, vegetation has become well established across the whole site, and increasingly it resembles that on the dry rocky hillsides nearby.
After detailed evaluation it was concluded that apart from the mine pit, the only significant residual impact from the operation is the water quality at one waterhole just downstream of the minesite. This did not affect stock, as an alternative and more reliable bore water supply had been provided.
Thus Mary Kathleen was the site of Australia's first major rehabilitation project of a uranium mine, which was completed at the end of 1985 at a cost of some A $19 million. This work won an award for environmental excellence from the Institution of Engineers Australia.
In the late 1980s, following relinquishment of the leases, MKU was liquidated.
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URANIUM INFORMATION CENTRE Ltd.
A.C.N. 005 503 828
GPO Box 1649N, Melbourne 3001, Australia
phone (03) 9629 7744
fax (03) 9629 7207