2.4 Potential Contaminants of Concern
Potential contaminants of concern in mining and mineral processing include: (i) solutes used in metal recovery such as cyanide at some gold mines; (ii) blasting residuals such as nitrate that enter the mine water circuit or are incorporated as soluble residues within a waste rock facility; (iii) compounds such as arsenic and selenium that can be leached and mobilized from tailings and waste rock under near-neutral pH conditions; (iv) metals released from tailings and waste rock if these wastes contain sulfide minerals that oxidize without subsequent neutralization, producing acidic drainage; and (v) high-salinity pore water released from some types of ores and concentrated during water – recycling through the mill. The geochemical characteristics of impacted mine contact waters can generally be described as acid mine drainage, non-saline neutral mine drainage, and saline neutral mine drainage. Acid mine drainage can occur as a result of the oxidation of the mineral pyrite, if it is present within the mine waste. If there is insufficient chemical buffering capacity within the waste then the effluent becomes acidic which in turns enhances metal mobility resulting in the release of metals such as zinc, copper and nickel. Pyrite oxidation can also occur in the case of non-saline drainage; however, in this case there is sufficient buffering capacity to keep the effluent non-acidic. However, the oxidation process results in elevated concentrations of sulphate, sometimes with concentrations up to several thousand mg/L. In some jurisdictions sulphate is regulated as a potential contaminant of concern in surface water.
Other potential water quality concerns rise in specific geologic settings. For example, in karstic terrain, it is possible that while there may be no specific solute with concentrations in excess of environmental standards, because of the very open nature of the flow pathways in karst, the possibility exists for exceedance of limits on the total suspended solids at emergence points in groundwater springs. In oil sands mining, there is a focus on the persistence in seepage water of naturally-occurring organic compounds such as naphthenic acids present in the supernatant pond water.
Not all mining wastes pose a risk of significant impact to groundwater resources. For example, at some iron ore mines where itabirite (metamorphosed iron-oxide bearing rock) is processed, the mine waste is not chemically reactive and seepage water has essentially the same chemical composition as the background water quality in the watershed.