Environmental management at Canadian mines has primarily focused, to date, on the issue of acid drainage related to tailings and mine rock. The large potential liability to the industry represented by acid drainage has, to some degree, overshadowed other water quality issues related to mine waste. There are, however, other chemical elements of interest (EOIs) related to non-acid generating mine waste that can result in unacceptably elevated concentrations in drainage that can enter the environment. Several chemicals do not require acidic conditions to maintain elevated concentrations above environmental guideline levels and can therefore be important in neutral pH drainage, including:
• antimony,
• arsenic,
• cadmium,
• chromium,
• cobalt,
• copper,
• iron,
• manganese,
• mercury,
• molybdenum,
• nickel,
• selenium,
• sulphate,
• uranium, and
• zinc.
There were 57 operating metal mines in Canada in 2003 with a contribution to the GDP in excess of $9 billion. By value, the top metals mined in Canada include gold, nickel, copper, iron and zinc. Coal and diamonds are among the top ten “minerals” if sand, gravel and stone are not considered. With few exceptions, the elements of interest for the potential contamination of water and air also represent economic products from mines across Canada. The 2002 annual production rates (tonnes) were 140 for antimony, 900 for cadmium, 2,000 for cobalt, 7,500 for molybdenum, 180,000 for nickel, 226 for selenium and 890,000 for zinc. Arsenic and mercury are no longer produced as economic commodities in Canada, but are associated with deposits that have been exploited for other purposes.
In general, these chemicals are a concern because there is some environmental or regulatory driver (or potential environmental impact) associated with each, they are not removed from solution at neutral pH and for many there are specific challenges for cost-effective removal from effluent and waste-waters.
Most of the regulatory drivers for these chemicals are related to the protection of aquatic organisms. Exceptions to this include antimony and chromium with low drinking water standards, manganese that has aesthetic issues and can form unwanted precipitates in the receiving water, and sulphate that can be toxic to some forms of moss that inhabit streams. The sources of these chemicals vary and many can originate from different types of mining, including base metal, gold and uranium mines.
There are, however, other considerations for potential toxicity for at least two of these chemicals that should also be considered. Both molybdenum and selenium can also play a role in toxicity to terrestrial animals at relatively low concentrations. Molybdenum can be toxic to mammals, and a great deal of attention has been focused on the assessment of the effects on ruminants such as deer and moose around mines with elevated molybdenum in British Columbia and Saskatchewan. Selenium can bioaccumulate in waterfowl, as well as exhibit toxic effects in other wildlife such as ungulates and is therefore also a concern to more than aquatic organisms.
Waste and mine water are commonly treated at mines to remove chemical constituents and to comply with regulatory requirements for discharged effluent. Although no reliable values exist, it is likely that the present value of water and effluent treatment (capital plus operating) at Canadian mines exceeds $1 billion. The estimate of the incremental cost for upgrades and additions to treatment plants outside of Ontario to comply with the 2002 MMER amendments exceeded $600M alone to address nine parameters (arsenic, copper, lead, nickel, zinc, radium-226, total cyanide, total suspended solids and pH). One of the difficulties for treatment of EOIs in neutral drainage is that several require special or non-traditional treatment systems. For example, arsenic and molybdenum are generally treated with ferric sulphate, but may require more than one pH adjustment step. Although promising technologies have been identified for several EOIs, these will generally depend on site-specific conditions. In addition, even metals like nickel that can be treated by lime addition may require other approaches with neutral drainage at some mines where removal efficiency is poor or where traditional treatment (addition of lime) creates other concerns such as elevated dissolved solids concentrations.
The costs of non-lime treatment (or pH control) systems can only be guestimated at this time. However, it may be fair to assume that an arsenic, molybdenum or selenium circuit or stand-alone treatment plant would have a similar capital cost to a lime treatment system. Operating costs may also be similar because, while the reagent use may be lower per unit volume of effluent, the unit cost of chemicals (i.e., ferric sulphate) will be higher than that for lime. There may be some uncertainty associated with long-term sludge stability and management. This has been recognized for arsenic treatment for which long-term stability of sludge remains an open question.
If it is assumed that non-traditional treatment systems represent only one-half of the existing treatment systems for operating mines, the net present value for treating neutral drainage may represent a collective liability to the Canadian mining industry of more than $500M.
The priority EOIs identified in this review include arsenic, molybdenum and selenium. The main drivers for priority assessment are the low regulatory limits and potential terrestrial effects (for Mo and Se) that are outside of the classical scope of environmental effects monitoring (EEM) programs that focus on the aquatic environment only. In addition, the issue of elevated sulphate may represent a significant liability to the mining industry if the British Columbia guideline of 100 mg/L is applied nation-wide. This issue should be examined carefully by the mining industry.
The following recommendations are proposed:
• Develop technology-transfer and collaborative initiatives and events (e.g., workshops) to focus on selected EOIs, including arsenic, molybdenum and selenium.
• Develop or expand guidelines for the assessment of metal leaching at neutral pH for environment-specific disposal strategies such as flooding, in-pit, etc.
• Evaluate the long-term stability of sludge produced by the addition of ferric sulphate in the treatment of antimony, arsenic, molybdenum and selenium.
• Review terrestrial toxicity related to molybdenum and selenium, and evaluate potential liabilities for application of toxicity benchmarks or regulatory limits.
• Assess the implications of the British Columbia sulphate guideline to protect aquatic mosses.
