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Possible Impact Of Ok Tedi Mining Wastes On The Waters Of Torres Strait

by J. Dent

The Ok Tedi mine is located in the Star Mountains in the western part of Papua New Guinea. Mining is carried on for gold at present, but after these deposits are exhausted the main mineral to be extracted will be copper. Processing of ore began in May 1984, but site clearance, preparation, construction, etc., had begun some two to three years beforehand. The planned throughput of the mine in the current phase is 17,000 tonnes per day (t.p.d.). Extraction of gold from the milled ore is by cyanidisation, and the principal waste products are cyanide (CN-), copper (Cu), lead (Pb), zinc (Zn), and cadmium (Cd), as well as rock and soil waste.

A detailed Environmental Impact Study (Anon. 1982) was carried out well in advance of the mine's development, which considered, inter alia, the effects of chemical effluents on water quality and aquatic life. A principal component of the means of environmental control was a permanent tailings dam on the Ok Ma (a tributary of the Ok Tedi) which in addition to trapping a proportion of solid tailings, would by reason of detention time, allow for natural breakdown of cyanide in tailings. However, in January 1984, failure of an abutment of this dam during its construction led to its abandonment. In an endeavour to keep the production start date up to schedule, the government of Papua New Guinea agreed to the mine consortium's proposal of an interim tailings scheme. Whilst the developers were investigating possibilities for alternative sites and eventual construction of another tailings dam, permission was granted to dispose of some treated tailings direct into the river system. Treatment of the tailings entailed:

1. separating coarse and fine sediments by cycloning, the coarse fraction being deposited as fill in the Ok Ninqi Valley, the finer fraction, or slimes, being transferred by pipeline to the Ok Mani;
2. neutralisation of excess cyanide by dosing with hydrogen peroxide.

The expected division of coarse to fine sediments was 50:50 though operating results now show a ratio close to 30:70. The initial agreed throughput or the mine was 17,000 t.p.d., and although this has not been consistently achieved since production start-up in May 1984, it has sometimes been exceeded, and requests have been made by the consortium to have the agreed throughput raised to 22,000 t.p.d. or higher. The intention was that the interim tailings scheme should operate up until the end of 1985. The subsequent history of the mine's development has negated this simple concept, and although the detailed future is unclear, should the mine continue, the operation or the interim tailings disposal scheme is certain to be one or two years more than originally intended, given that the government's insistence an the construction of a permanent tailings dam is maintained.

There is therefore a considerable quantity of pollutants in soluble or solid form being introduced into the headwaters of the Fly River. A recent study by the Delft Hydraulics Laboratory (Anon. 1984a), commissioned by the developers, has highlighted the possible impacts on the lower reaches of the river and the immediate offshore area, points which were given no consideration by the Environmental Impact Study. With little or no reported data to work on, a theoretical model study has produced the following predictions, assuming the non-existence of a permanent tailings dam:

1. Annual sediment production will be 17.5 million tonnes per year (mt/yr) of suspended solids and 2.5 mt/yr bedload, of which 25% will reach the sea sooner or later. Estimates of the natural sediment transport capacity of the Fly are sketchy, but range between 27 mt/yr and 60 mt/yr;
2. Sediments reaching the sea will contain 320 ppm copper on average;
3. Over a mine life of 30 years, the result of sediment disposal into the river would cause the deposition of 1 m depth of polluted sediments over an area of 1000 km2 the offshore delta.

Presence of a permanent tailings dam could reduce concentration of pollutants by 50 per cent. The most significant unknown factors in these assessments is the degree of which particulate cooper may be re-mobilised into soluble form, especially the most toxic ionic species. This remobilisation would result from changes in the pH of the ambient water. Release of copper into solution happens in acidic environments and would entail a significant drop in the river pH which is generally steady around 7. However in the backswamps where decaying vegetation produces an excess of humic acid, pH can be as low as 4.5 which would cause release of copper into solution. Changes in the water chemistry in estuarine and delta areas due to salt water can also lead to mobilisation of heavy metals into solution.

Once the Interim tailings Disposal Scheme had been approved by the State, the Department of Minerals and Energy engaged consuItants, Australian Mineral Development Laboratories (A.M.D.E.L.) to report on likely impacts and recommend monitoring requirements and quality criteria to afford some environmental protection (Anon. 1984b, 1984c). The basis of their recommended criteria were from the U.S. Environmental Protection Agency (U.S.E.P.A.), for total recoveable metal. In order to relate the total recoverable figure more directly to toxic levels, the criteria for soluble forms of metals were also defined. Recommended criteria, related by dilution at mean flow conditions were given at Tabubil (Lower Ok Tedi Bridge) and Ningerum and are shown In Table 1.

Table 1. Recommended Water Quality Criteria (ppb)

	Soluble	Total Recoverable Metal
Lower Ok Tedi Bridge
Cu	125	288
Pb	50	550
Zn	125	1000
Cd	12.5	25
CN-	5	-
Ningerum
Cu	50	115
Pb	20	220
Zn	50	400
Cd	5	10
CN-	3.5	-

River conditions were monitored in several ways:

1. Daily measurements by Ok Tedi Mining Ltd (O.T.M.L.) at Ok Tedi Bridge;
2. Periodical sampling visits by Bureau or Water Resources (B.W.R.), analyses by A.M.D.E.L.;
3. Half-hourly monitoring of cyanide and tailings discharge in ore processing cycle;
4. Toxicity tests on tailings samples carried out by the Biology Department, University of Papua New Guinea.

Any intention to use results from this monitoring as a basis for control should effluent levels exceed criteria quickly proved impossible. Apart from the time lag of results being made available to B.W.R., legal complications arose in that OK Tedi Mining Limited disputed the applicability of the Water Resources Act, 1982 to their activities. Their claim was that all environmental provisions are dealt with under the Mining (Ok Tedi Agreement) Act, 1976, which is a specific Act having precedence over a later general Act. Thus any attempt by the State, under the provisions of the Water Resources Act, to fine, to limit activities or to close down the mine as a result of O.T.M.L's failure to meet the criteria would be contested.

During the period June to November 1984 there have been numerous occurrences where the copper criteria has been exceeded at both Ok Tedi Bridge and Ningerum recorded by both the O.T.M.L. and A.M.D.E.L. analyses. Levels of copper have fluctuated considerably and this has been attributable to the changes in the ore body, which is highly variable. Peaks reaching twice the level or criteria have been recorded. Failure to meet water quality criteria have also been recorded at Ok Tedi Bridge in levels of lead, cadmium, and most frequently, cyanide. The latter has been suggested to have some link with the level of copper in the ore being processed, as excess cyanide forms weak linked complexes with copper which prevents complete neutralisation in the plant, but can still be recorded as free cyanide in the monitoring tests.

There are no complementary data available at present to indicate the biological impact of chemical levels. However, toxicity tests on tailings has indicated that tailings are becoming progressively more toxic. Tests were carried out on shrimps and mosquito fish on four sets of tailings, collected in early and late June, October and November 1984. As the percentage of solids in the tailings sample has risen, from 14% to 51%, so the concentration of tailings reaching a 96 hour LC50 for both animals has fallen, and is shown in Taole 2 (Mowbray in prep.).

Table 2. Result of Toxicity Tests on Ok Tedi Tailings (Provisional)

		Concentration of Tailings %
Sample Date	Ave % solids	LC50 fish	LC5O shrimps
June 19	13.9	16.7	4.6
June 30	42.1	8.7	0.5
October 4	45.8	1.5	0.3
November 28	50.9	1.2	0.2

The above chemical and biological measurements only indicate the likelihood of impacts on the river environments. As yet no drastic impacts on fish or other aquatic life have been observed. Even the two significant incidents involving spillages of cyanide at the mine site and loss of containers of cyanide at the mouth of the Fly delta do not appear to have caused major environmental damage. This may be due to the rapid decay characteristics or cyanide or to the remoteness of the locations where incidents occurred which enable effects to go unnoticed. The main concern, however, must remain with the long-term effects of heavy metal pollution, which are cumulative within the ecosystem. While a suitable monitoring system program can be developed to include the vast areas of the Fly delta and offshore, the question as to its purpose must be addressed. By the time the impacts of deteriorating conditions have been identified, the effectiveness of any control or remedial measures would be minimal.

Anon. (1982), Ok Tedi Environmental Study Vol 1. Main Report Vol 4. Sedimentation Analysis and Results: Chemical Effects. Maunsell and Partners Pty Ltd, June 1982.

Anon. (1984a), Review of the Environmental Impact of Mining Operations of Ok Tedi Mining Ltd on the Ok Tedi/Fly River System. Delft Hydraulic Laboratory, September 1984.

Anon. (1984b), Interim Tailings Disposal Scheme (O.T.M.L.) Proposed monitoring of Mining Effluents A.M.D.E.L. Report No. 1534, March 1984.

Anon. (1984c) Interim Tailings Disposal Scheme (O.T.M.L.) Impacts and Legislation. A.M.D.E.L. Report No. 1535, March 1984.

Mowbray, D. (in prep.), Pollution Problems In Papua New Guinea: Ok Tedi. D. Mowbray, University of Papua New Guinea (in preparation).

WOLANSKI: Concerning delta sediment transport, what will cause the increase in sediment compared with total discharge?

DENT: The river is already turbid. The sediment load will increase by a quarter to a third.

POLLUNIN: We will need base line data to ascertain any changes in heavy metal levels in the future. What is happening about collecting base-line data?

HAINES: Nothing an the Australian side at Present. I have informally discussed the requirements for a program with Great Barrier Reef Marine Park Authority, Australian Institute of Marine Science and James Cook University of North Queensland staff. It seems there would be need for a 12 months pilot study to collect base-line data and establish a monitoring Program. We very roughly costed the pilot program at $200,000: $100,000 for staff and the rest for collecting and analysing samples. This would be the total resource commitment - it may be that a program could utilise existing staff and therefore be a lot cheaper. It really needs somebody to find out what can be done and then co-ordinate a program.