Sulfate is becoming more of a water quality problem for mining companies. Regulations governing sulfate concentrations have been flexible, and the unstated mitigation measure is often dilution of sulfate downstream. Federal water quality guidance in the United States characterises sulfate as a secondary contaminant with a maximum concentration of 250mg/l for drinking water. At the state level, sulfate concentrations may come into play when limiting total dissolved solids for water used for irrigation or livestock watering. In Canada, there is no federal but often provincial guidance for protecting aquatic life. British Columbia recently proposed a 30-day average sulfate concentration guideline of 65mg/l, and other provinces are also considering lower limits.
Regulatory scrutiny of sulfate is increasing and may affect mining operations and projects, as follows:
• In the southwest US, a sulfate groundwater plume impacting a drinking water resource is now actively managed by intercepting groundwater and incorporating its reuse into the mine plan
• In British Columbia, developing and implementing a sulfate management plan is now required for restarting a copper mine
• Sulfate concentrations in neutralised nickel laterite and copper leach solutions do not meet World Bank freshwater discharge standards
How can sulfate be managed to prevent impacts on water quality? The easiest solution is to limit the amount of water contacting sulfur-bearing material or process solutions. The most expensive solution is treatment. Treatment options include sulfate precipitation, ion exchange or membrane processes (e.g. reverse osmosis or nano-filtration) and biologically mediated sulfate reduction. All of these processes produce a residual sludge or brine that must be managed. Treatment costs for sulfate depend on site conditions, method, and sludge or brine disposal. Benchmarking studies suggest an operations and maintenance cost of $5/1000 gallons ($1.3/m3), not including sludge or brine disposal. Treating for sulfate is expensive and results from not planning for or managing sulfate impacts during operations.
The regulation of sulfate will only become more stringent. Sulfate loading will be questioned more frequently during permitting and become a greater environmental liability during operations and at closure. The time to begin managing sulfate is during mine planning. At a minimum, sulfate management should be incorporated into the operation’s water management plan. Working with mine planners and operators to develop sulfate load balances is a good first step for predicting impacts, and these balances can be used to assess the benefit of mitigation alternatives. Comparing the cost of sulfate mitigation through improved water management to the cost of sulfate treatment can help justify proactive management to reduce sulfate treatment liability.
