The distribution and migration of sodium from a reclaimed upland to a constructed fen peatland in a post-mined oil sands landscape

Highlights: • Upland recharge was an important control of sodium migration. • The driest year had the highest solute flux from upland to fen. • Arrival times of sodium at the fen surface were estimated to be 4–11 years. • Sodium concentrations rose from 87 to 200 mg L⁻¹ in the fen rooting zone. Post-mine landscape reclamation of the Athabasca Oil Sands Region requires the use of tailings sand, an abundant mine-waste material that often contains large amounts of sodium (Na⁺). Due to the mobility of Na⁺ in groundwater and its effects on vegetation, water quality is a concern when incorporating mine waste materials, especially when attempting to construct groundwater-fed peatlands. This research is the first published account of Na⁺ redistribution in groundwater from a constructed tailings sand upland to an adjacent constructed fen peat deposit (Nikanotee Fen). A permeable petroleum coke layer underlying the fen, extending partway into the upland, was important in directing flow and Na⁺ beneath the peat, as designed. Initially, Na⁺ concentration was highest in the tailings sand (average of 232 mg L⁻¹) and lowest in fen peat (96 mg L⁻¹). Precipitation-driven recharge to the upland controlled the mass flux of Na from upland to fen, which ranged from 2 to 13 tons Na⁺ per year. The mass flux was highest in the driest summer, in part from dry-period flowpaths that direct groundwater with higher concentrations of Na⁺ into the coke layer, and in part because of the high evapotranspiration loss from the fen in dry periods, which induces upward water flow. With the estimated flux rates of 336 mm yr⁻¹, the Na⁺ arrival time to the fen surface was estimated to be between 4 and 11 years. Over the four-year study, average Na⁺ concentrations within the fen rooting zone increased from 87 to 200 mg L⁻¹, and in the tailings sand decreased to 196 mg L⁻¹. The planting of more salt-tolerant vegetation in the fen is recommended, given the potential for Na⁺ accumulation. This study shows reclamation designs can use layered flow system to control the rate, pattern, and timing of solute interactions with surface soil systems.