Dissolved inorganic carbon and stable carbon isotopic evolution of neutral mine drainage interacting with atmospheric CO2(g)

Abstract

We investigated the spatial variations in the concentrations of dissolved inorganic carbon (DIC), the stable carbon isotopic composition (δ13C) of DIC and the δ13C of carbonate precipitated from neutral mine drainage interacting with the atmospheric CO2(g). We assessed the chemical, DIC and δ13CDIC evolution of the mine drainage and the δ13C evolution of carbonate precipitates for a distance of 562m from the end of an 8km tunnel that drains a mine. Our results show that as the mine drainage interacts with atmospheric CO2(g) the outgassing of CO2 due to the high initial partial pressure of CO2 (pCO2) causes the DIC to evolve under kinetic conditions followed by equilibration and then under equilibrium conditions. The carbonate evolution was characterized by spatial increases in pH, decreasing concentrations of Ca2+ and DIC and by the precipitation of carbonate. The δ13CDIC showed a larger enrichment from the tunnel exit to 38m, moderate continuous enrichment to 318m and almost no enrichment to 562m. On the other hand, the δ13C of the carbonate precipitates also showed large enrichment from the tunnel exit to 38m, moderate enrichment to 318m after which the δ13C remained nearly constant. The enrichment in the δ13C of the DIC and the carbonate precipitates from 0 to 38m from kinetic fractionation caused by CO2(g) outgassing was followed by a mix of kinetic fractionation and equilibrium fractionation controlled by carbon exchange between DIC and atmospheric CO2(g) to 318m and then by equilibrium fractionation from 318 to 562m. From the carbonate evolution in this neutral mine drainage, we estimated that 20% of the carbon was lost via CO2 outgassing, 12% was sequestered in sediments in the drainage ponds from calcite precipitation and the remainder 68% was exported to the local stream.