RESUMEN
In situ thermal recovery is utilized extensively for unconventional bitumen extraction in the Cold Lake-Beaver River (CLBR) basin in Alberta, Canada. Public health concerns have been raised over potable groundwater contamination and arsenic release adjacent to these operations within the CLBR basin, which have been linked to subsurface heating of aquifer sediments. Under localized heated conditions, As-bearing aquifer sediments have been shown to undergo water-rock interactions and release constituents at near neutral pH conditions; however, release mechanisms have yet to be conclusively reported. To investigate the hydrogeochemical processes of aquifer heating and solute transport in detail, this study applies a novel heated column design to mimic saturated aquifer materials in contact with a thermal recovery well while constraining flow and geochemical conditions. Two column experiment scenarios were considered using: 1) quartz [SiO2] sand with 0.6 wt% pyrite [FeS2]; and 2) aquifer sediments collected from the CLBR region. Heated temperature gradients between 50 °C and 90 °C were maintained within a 0.6 m section of the 3 m column with a flow rate of one pore volume per week. During heated low oxygen (<3 mg L-1) conditions, results generally show increases in pH, Al, As, B, Mn, Mo, Si and Zn concentrations within and downgradient of the column heating section. Constituent release is primarily attributed to thermal desorption from Fe oxides, clay and silicate mineral dissolution, competitive anion exchange, and increased mixing. Overall results suggest that these mechanisms are responsible for increasing constituent concentrations in groundwater adjacent to in situ thermal recovery operations.