• antimony
• arsenic
• cadmium
• chromium
• cobalt
• copper
• iron
• manganese
• mercury
• molybdenum
• nickel
• selenium
• sulphate
• uranium, and
• zinc.

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.