Mining operations frequently encounter saline groundwater below permafrost on the Canadian Shield. Saline groundwater on the shield is primarily a calcium chloride brine. Management options need to meet compliance limits for total dissolved solids (TDS) and chloride in receiving waters.
TDS concentrations in groundwater increase with depth below the permafrost and can exceed 50,000 mg/L. The Canadian Council of Ministers of the Environment long term chloride limit for protection of aquatic life is 120mg/L. A TDS limit of 1,000mg/L has also been set for some mines in the north. Treatment options are limited, expensive, and energy intensive. Conventional treatment involves a concentration step (typically a membrane process). At TDS concentrations over 30,000mg/L, most membrane processes are not feasible. Evaporation and crystallisation is cost prohibitive for mine operations. This leaves two feasible options for managing saline groundwater: discharge to the environment or storage in completed pits or underground workings.
Discharge to the environment requires enough assimilative capacity (dilution) in the receiving water to meet compliance limits. The assimilative capacity varies seasonally in the north, which may require mine water storage during low flow periods of the year. Discharge to a lake may initially meet compliance limits but if the loading to the lake exceeds the rate at which TDS is flushed from the lake, concentrations may eventually exceed the compliance limit. Discharge to the marine environment is a viable option but depends on proximity.
Saline water can also be disposed of in completed underground workings and pits. This requires integrating water management and mine planning. Disposal in underground workings isolates the saline water from the environment but sufficient volume needs to be available when saline inflows begin. Saline water can be stored in a pit by forming density stratified lakes where the saline water is deposited first and overlain with less dense freshwater. Creating this type of lake in a pit is challenging, but has the potential for the long-term storage of saline water without impacting the environment.
