GEOCHEMICAL AND ISOTOPIC INVESTIGATION OF CARBON EVOLUTION IN MINE DISCHARGE WATERS: IMPLICATIONS FOR CO2 SEQUESTRATION

We investigated the evolution of carbon in a series of high partial pressures of CO₂ (pCO₂)-water in sequentially connected settling ponds that impound drainage that discharges from a tunnel which extends about 8 km to a precious metal mine in Colorado. We determined the principal controls on carbon behavior and identified the isotopic fractionation associated with both CO₂ exsolution and carbonate mineral precipitation in alkaline mine discharge waters. We conducted standard field measurements and laboratory chemical and carbon isotopic measurements on the aqueous samples and chemical and isotopic measurements on the mineral precipitates. There was a general increase in pH and decrease in dissolved inorganic carbon (DIC) concentration from the near-tunnel to the distal ponds. Generally, the metal concentrations of the aqueous samples decreased from the near to the distal ponds and stable carbon isotopes of the DIC (δ¹³C_DIC) of both the aqueous and mineral precipitates were continuously enriched from the near to the distal ponds. X-ray diffraction (XRD) analyses revealed that calcite was the principal mineral in the precipitates and the relatively higher Fe and Mn concentrations in the mineral precipitates must have co-precipitated with calcite or adsorbed on its surfaces. The δ¹³C_DIC values of both the aqueous and mineral precipitate samples were in agreement with those predicted from kinetic and equilibrium exchange reactions between aqueous solutions and atmospheric CO_2(g). The measured δ¹³C_DIC of the aqueous samples are consistent with dissolved carbonates in the mine pits and along the tunnel. The measured δ¹³C_DIC of the mineral precipitates results from both the kinetic and carbon isotopic exchange associated with both CO₂ outgassing and calcite precipitation. Isotopic mass-balance calculations revealed that the principal control on CO₂ loss in these aqueous samples is through calcite precipitation rather than CO₂ outgassing, implying that mineral trappings of CO₂ is the rate-limiting step in carbon fixation in these mine discharge ponds. The sequestration of carbon is enhanced by high pCO₂ and the trapping of CO₂ by calcite precipitation is the rate limiting step during CO₂ equilibration between mine discharge waters and atmospheric CO_2(g).