Mine Tailings as Sources of Greenhouse Gas Emissions: A Multi-Site Investigation Incorporating Isotopic Signatures

Abstract

Tailings are the slurry waste product from mining operations deposited in impoundments in large quantities and are associated with challenging environmental issues such as acid mine drainage. Sulfide-rich tailings can be oxidized due to O2 ingress and water infiltration, producing H+ that dissolves surrounding carbonate minerals, leading to CO2 production. To characterize the seasonal CO2 emissions from tailings impoundments with various cover systems and explore the geochemical and physical controls on the emissions, field studies were conducted at five mine tailings sites in Canada. The sites included tailings that are uncovered (Giant Mine, NT), sand/gravel-covered (South Bay Mine, Long Lake Mine, and Nickel Rim North Tailings, ON), and multi-layer-covered with O2-consuming-organic material and desulfurized tailings (Strathcona Tailings Management Area, ON).

Physical properties were measured and analyzed including tailings water content, soil temperature, particle density, and porosity. Tailings solid and pore water samples were collected via manual coring and squeezing extraction from core samples. Gas depth profile sampling and gas flux chambers were used to quantify subsurface gas concentrations and CO2 fluxes in the subsurface and to the atmosphere. To characterize the stable carbon isotope signatures across phases and source-trace CO2 transport, the δ13C values of solid, aqueous, and gaseous phases in the investigated tailings systems were determined.

At uncovered and sand/gravel covered sites with high sulfide content, acidic conditions (pH <2), rapid O2 depletion, high subsurface CO2 concentrations (>20 vol.%), and substantial surface fluxes (up to 140 kg ha-1 day-1) to the atmosphere were measured. Effective covers (composite desulfurized tailings/organic materials) suppressed acid generation but still sustained considerable CO₂ fluxes to the atmosphere (~100-120 kg ha⁻¹ day⁻¹). Sites rehabilitated with different cover systems showed CO2 fluxes in June and July that were twice as high compared to September and October. δ13C values (-4‰ to +3‰) of pore-gas CO2 samples suggest that CO2 originated from geogenic carbonate mineral dissolution at the sand/gravel-covered site. At the multi-layer-covered site, the δ13C-CO2 values of <-20‰ in the organic material cover and -10‰ to -5‰ in the deeper desulfurized tailings, suggest mixed sources of CO2 production. This study demonstrates that tailings emit CO2 at rates exceeding or comparable to wetlands, forests, and farmland sites. A large portion of the CO2 is derived from primary carbonate minerals contained within the mine wastes. Integrating CO2 emissions into global C budgets is critical, and future cover designs must balance remediation control with C management to mitigate climate impacts.