TRANSPORT AND ATTENUATION OF ARSENIC IN AQUIFER SEDIMENT AT THE CAMPBELL MINE, BALMERTOWN, ONTARIO

Inherent to the extraction of some gold ores is the mobilization of naturally occurring arsenic. In addition to concentrating gold, the ore enrichment process accumulates arsenic in the mill tailings, resulting in an increased potential for leaching into groundwater systems. This release and potential transport of arsenic is a principal environmental concern for the gold mining industry. At the Campbell Gold Mine (Balmertown, Ontario), high concentrations of dissolved arsenic derived from the reductive dissolution of arsenic bearing iron oxides are observed near the base of the tailings impoundment. Concentrations decrease, however, along the groundwater flow path in the underlying aquifer, suggesting that arsenic is partially attenuated. Groundwater measurements indicate the presence of significant concentrations of both iron and hydrogen sulfide (HS^-). Calculations using the geochemical speciation program MINTEQA2 indicate super-saturation with respect to the solid phase As₂S₃ (orpiment), the phase most readily formed by arsenic in the presence of aqueous sulfide. Arsenic adsorption onto secondary iron phases and precipitation as orpiment were investigated in laboratory experiments.

In batch experiments conducted with simulated plume water, 15% of arsenic was adsorbed. Column experiments were conducted to quantify the mass of arsenic which can be attenuated by sediment from the mine site under continuous flow conditions. Arsenic breakthrough was observed after only 7 pore volumes, but after 60 pore volumes 90% of the total arsenic mass introduced to the system had been retained on the column sediment. Hydrogen sulfide was detected in the column effluent at concentrations of up to 0.060 mg/L. Calculations using the geochemical speciation program MINTEQA2 indicate super-saturation with respect to orpiment, however, further sediment analyses are required to confirm its presence or absence. Leaching column results suggest that adsorbed arsenic may represent an additional concern at mine closure. Aquifer material directly underlying the tailings has the potential to release arsenic at concentrations of up to 5 mg/L for many pore volumes under different geochemical conditions.