MEND Project 2.11.1a-a
June 1989

EXECUTIVE SUMMARY

Finding an environmentally safe, yet economical, method of disposing of reactive mine wastes is a challenge facing both the mining industry and government. When such materials contain sulphides, the conventional practice of land-based disposal has often resulted in the generation of acidic water and the concomitant leaching of trace metals from the mine wastes. Acid production in tailings and waste rock is a result of the oxidation of sulphide minerals (principally iron pyrite). Acid generation results in various, often severe, impacts on water chemistry and biological resources. The environmental implications are considerable, particularly since the problem can persist after active mining has ceased.

One method for controlling acid generation which is receiving increasing attention is the practice of depositing reactive mine wastes underwater. While the Metal Mining liquid Effluent Regulations, authorized under the federal Fisheries Act, currently prohibit lake disposal of mine tailings, an exemption can be issued through a federal cabinet Order-in-Council. An argument for subaqueous disposal is based on the premise that acid generation is suppressed in submerged mine wastes that are essentially unexposed to oxygen and bacterial action. This suggestion is predicated on knowledge of the biogeochemical nature of lacustrine sediments, and appears to be supported by observations made in specific field studies. Hence, it is consistent to suggest that sulphide-rich mine wastes may be disposed of underwater without significant release of metals to the overlying waters. In some circumstances, however, a number of factors can act individually or in concert to promulgate release of metals, with the associated potential for environmental degradation. Nevertheless, with sufficient knowledge of post-depositional chemical reactivity of the specific tailings and adherence to disposal criteria, these factors can be mitigated to various extents and impacts on water quality and indigenous biota minimized.

While this conventional wisdom supporting subaqueous disposal may be correct, only a limited number of reviews and even fewer field studies have been undertaken that add significant insight Consequently, the efficacy of underwater disposal remains largely unproven.

Responding to the clear need for establishing effective methods to mitigate acid mine drainage, and recognizing the considerable promise subaqueous disposal holds in this regard, the British Columbia Acid Mine Drainage Task Force issued a call for proposals to complete a literature review on underwater disposal of reactive mine wastes. The study was to focus on the state-of-the-art in theoretical knowledge and case studies of British Columbia mines disposing of wastes in a freshwater environment. Consideration was to be given to potential impacts on freshwater biological systems, including both physical and chemical effects of mine waste disposal. As a consequence, Rescan Environmental Services Ltd. was retained to undertake the literature review on the Subaqueous Disposal of Reactive Mine Wastes.

This study was completed based on a comprehensive review of all aspects of subaqueous disposal of reactive mine wastes in a freshwater environment, both theoretical and applied. While emphasis was placed on the British Columbia experience with underwater disposal practices, as mandated in the RFP, attention was also given to other parts of Canada and the United States. During the review, numerous and varied sources of literature were consulted through both desk and computer-aided search methods. Sources of information included, among others, literature on acid mine drainage mechanisms and chemistry, aquatic chemistry, geochemistry, hydrogeochemistry, biochemistry, microbiology, limnology and aquatic biology/ecology. Whenever possible, relevant case study data were obtained which often addressed one or more of the above areas to various degrees of detail. Although the scope of this report is confined to freshwater (i.e. lake disposal), the literature review includes documentation on land-based tailings and leach dumps or heaps insofar as they contained data relevant to subaqueous disposal. Similarly, results from marine disposal operations are only included where such work illustrates principles which are applicable universally to aquatic systems. The results of this literature review are summarized below. Section 7.0 provides a comprehensive bibliography of information on both the theory and practice of subaqueous disposal.

Of all the conclusions drawn from this study, perhaps the most salient is that AMD poses serious disadvantages for land-based disposal of reactive mine wastes and that the underwater disposal of such wastes holds considerable promise for suppressing acid generation. Nevertheless, the potential long-term impacts associated with subaqueous disposal remain poorly understood.

Various factors, sometimes acting synergistically, determine the potential for mine wastes deposited underwater to generate acid and, consequently, the potential for biological impacts. These factors include, among others, the natural -chemistry of the receiving environment, physicochemical conditions which may help limit concentrations of dissolved metals, hydrochemical conditions that may increase heavy metal solubility and the composition of the mine wastes being deposited. Of the range of predictive tests available to evaluate potential for acid generation (Section 2.3), the kinetic shake flask test appears somewhat suitable for subaqueous storage of reactive mine wastes.

The complex processes of bioavailability of metals in lake-bottom sediments and bioaccumulation in the freshwater food chain are not well understood, particularly with regard to reactive mine waste disposal. To help improve the level of understanding, lake studies should be conducted whereby post-depositional reactivity of submerged wastes is evaluated to determine if benthic effluxes of selected metals, i.e. Cu, Pb, Zn, Cd, Mn, Fe, As, and Hg are present and to what extent they are obviated by the gradual deposition of a veneer of natural sediments. Apart from potential impact, other biological effects of underwater disposal include turbidity, sedimentation on lake bottoms and toxicity to aquatic organisms.

Following a review of the literature relating to acid mine drainage, subaqueous disposal, and its potential biological implications, numerous case studies documenting existing occurrences of subaqueous disposal in a freshwater environment were reviewed. The cases analyzed within British Columbia include Buttle Lake, Benson Lake, Babine Lake, Bearskin Lake (proposed), Brucejack Lake (proposed), Kootenay Lake, Pinchi Lake, Summit Lake,. Equity Silver Mines Ltd. (flooded open-pit), Endako Mine (flooded open-pit), Cinola Gold Project (proposed) and Phoenix Mine. Other Canadian and U.S. cases examined include Garrow Lake, Northwest Territories; Mandy Lake, Anderson Lake, and Fox Lake, Manitoba; and Reserve Mining Co. Ltd., Silver Bay, Minnesota. Generally it was concluded that although the case studies reviewed represented a diversity of environments, the results yielded were somewhat inconclusive. Data were generally superficial, only remotely relevant, reflect questionable sampling practices, and were not gathered with a view toward better understanding the long-term impacts associated with the subaqueous disposal of reactive mine wastes.

Based on the results of the literature review, it is recommended that field studies be undertaken to evaluate the post-depositional reactivity of sulphide-bearing mine wastes and to conduct more detailed, site-specific investigations of potential biological impacts in the freshwater receiving environment Studies should include a detailed evaluation of the ore and tailings mineralogy, particle size distributions, predicted settling behaviour of mine wastes, and leaching behaviour or reactivity of the wastes once exposed to freshwater. It is considered critical that geochemical and limnological field investigations be completed in concert to both increase our knowledge of the factors which control metal release or uptake by tailings and the potential associated, direct or indirect, impacts that might accrue to the biological community.

The geochemical studies recommended require analyzing interstitial waters collected from suites of cores raised from submerged tailings deposits in a number of lakes including Buttle Lake, Benson Lake and/or Kootenay Lake, British Columbia; Mandy Lake, Fox Lake, and/or Anderson Lake, Manitoba. Such studies should embrace a variety of deposits including unperturbed sediments and tailing.,, with contrasting mineralogies, and should include assessment of alteration effects and connate water and/or groundwater chemistry in contrast to tailings disposed on-land. The studies should include locations no longer receiving mine wastes and active depositional regimes. It is highly recommended that the geochemical investigations include chemical analyses of selected major and minor element concentrations in the solid phases from which the pore waters are extracted, mineralogical characterization, and measurements of organic carbon concentrations.

Comparative mineralogical studies of both fades should be undertaken to contrast the extent and nature of mineral alteration where tailings of the same composition have been discharged both underwater and on-land (e.g., Buttle and Benson Lakes). Such comparisons have the potential to provide a particularly enlightening suite of examples of the relative disgenetic behaviour of tailings exposed to the atmosphere versus those submerged in a freshwater environment.

In association with the geochemical analyses described above, limnological and biological investigations must be completed to link the complex process of metals release from submerged wastes to their potential uptake by aquatic organisms and bioaccumulation in the food chain. The purpose of these studies will be to describe the lake(s) considered in terms of features that can assist in predicting the impacts of mine wastes deposited in similar lakes. Lake morphology and hydrology, physical and chemical limnology and biological characteristics should all be measured to allow investigators to calculate lake turnover and residence time, determine circulation and mixing features, and evaluate the potential for wastes to be mobilized and the rate at which contaminants would be dispersed from the mine waste deposit. A better understanding of metal transfer between sediments and the aquatic food chain would also be achieved.

Site-specific experiments on lacustrine biota should be designed to establish the impacts of heavy metals on both infauna and epifauna. Metal levels within the tissues of these organisms may reflect metal uptake rates and the potential for bioaccumulation in the food chain. It is also advised that one or two suitable fish species (i.e. those characterized by low mobility, long life-spans, and/or higher trophic level feeding) be chosen for tissue metals analysis due to the high interest in fish by both regulatory agencies and the general public.

Finally, based on a combination of theory as documented in this literature review, and empirical field study data, it is recommended that a decision model be developed to evaluate the suitability-of future underwater waste disposal strategies. Initial attempts at this have been confounded by the insufficient and/or unreliable data describing conditions at existing subaqueous disposal sites.

The type of decision model proposed would incorporate physical, chemical, geochemical, biochemical, limnological and biological conditions, identified in theory and refined through field investigation, in a critical path framework to evaluate the environmental implications associated with strategies for the subaqueous disposal of mine wastes. It would provide a pragmatic method for screening disposal alternatives by both industry and government regulators, based on a fatal flaw approach that would identify key potential problem areas for given proposed discharge strategies. The decision model would be developed based on theoretical and case study information collected through this review, coupled with empirical data gathered through field studies such as those outlined above, and would assist both industry in effectively choosing methods of reactive mine waste disposal and government charged with the responsibility for ensuring wastes are disposed of in an environmentally acceptable manner.

Last Modified: 2003-11-26		Important Notices

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