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.