Sulfate Influence on Methylmercury Generation and Wild Rice Harvesting – Hammond Reef Project (Technical Memorandum)

The Government Review Team (GRT) expressed concerns regarding the Hammond Reef Project's (HRP) discharge of sulphate to Marmion Reservoir and its potential effects on methylmercury (MeHg) generation and downstream wild rice harvesting. Baseline data showed some fish already exceed mercury consumption guidelines. GRT worries sulphate could increase MeHg production and negatively impact wild rice near Raft Lake Dam.

Sulphate Release and Dilution

The HRP's geology is non-acid generating, meaning sulphate release is primarily from cyanide treatment, not natural rock. Predicted sulphate release (150–250 mg/L) is much lower than sulfide-rich sites. An effluent diffuser ensures significant mixing and dilution; sulphate concentrations within 100m of the diffuser are expected to increase by only about 0.3 mg/L above background. Downstream at Raft Lake Dam, sulphate is projected to be around 1.8 mg/L, close to the 1.6 mg/L baseline. These minor expected increases are considered within regional variability.

Wild Rice Protection

The MPCA (Minnesota Pollution Control Agency) framework links wild rice health to sediment porewater sulfide, which is influenced by sulphate, iron, and organic carbon. High levels of sulfide can reduce wild rice viability. Based on HRP area sediment data (average iron ~23,000 µg/g), the calculated wild rice sulphate protection threshold is approximately 12.0 mg/L. The HRP’s projected sulphate concentrations at Raft Lake Dam (1.8 mg/L) are well below this threshold, suggesting low risk to wild rice.

Methylmercury Dynamics

MeHg generation in freshwater reservoirs is a complex balance of production (often by sulfate-reducing bacteria, stimulated by moderate sulphate) and inhibition (by high sulfide and photo-demethylation). While some studies show sulphate can enhance methylation, high sulphide concentrations typically inhibit it by reducing mercury bioavailability. Photo-demethylation in surface waters also reduces MeHg. Marmion Reservoir has multiple natural mercury inputs and processes like photo-demethylation that influence overall MeHg levels, making precise predictions for small, incremental sulphate changes difficult.

Responses and Monitoring

Due to the many competing processes and the small predicted sulphate additions, precisely foreseeing MeHg uptake by large fish from HRP is not scientifically feasible. The HRP will participate in the Metal Mining Effluent Regulations (MMER) Environmental Effects Monitoring (EEM) program, involving spatial and temporal monitoring of fish tissue, habitat, and water quality. Mitigation is challenging given existing mercury impacts and the minor predicted project-related changes. CMC commits to ongoing collaboration with regulators and First Nations, providing data to update fish consumption guidelines if fish tissue mercury levels rise, and performing ongoing water quality and periodic fish tissue mercury monitoring. If higher-than-predicted sulphate levels occur, fish tissue monitoring frequency will increase.