Inorganic carbon dynamics and CO₂ flux associated with coal-mine drainage sites in Blythedale PA and Lambert WV, USA

Abstract

Drainage from coal mines, where carbonate dissolution is driven by sulfuric acid, can result in a net transfer of geologically-bound carbon to the atmosphere. The flux and downstream evolution of dissolved inorganic carbon (DIC) is presented for two coal mine sites that discharge high concentrations of DIC (3.7–4.5 mM C) producing a total flux of DIC from the mine from 13 to 249 kg-C/year (18–364 metric tons of CO₂/year). More than 65 % of the total DIC is lost via CO₂ evasion with the remaining DIC is exported downstream as dissolved species. The fate of the DIC depends upon the pH of the water which is controlled by evasion of CO₂, the concentration of pre-existing alkalinity, carbonate precipitation and dissolution, and metal hydrolysis reactions. The CO₂ concentrations and fluxes from the study sites are comparable to those estimated from literature data for other coal mine sites in the Appalachian region. The total flux estimated from a dataset of 140 coal mines was comparable in magnitude to the CO₂ emissions from a small coal-fired power plant. The extent of CO₂ degassing from mine waters is poorly constrained because (1) flux estimates can be biased low when acid waters are excluded in alkalinity-based estimates; (2) flux estimates can be biased high if non-carbonate alkalinity is present in the mine waters; and (3) mine waters react rapidly following discharge hampering the measurement process. The study sites presented illustrate the impact of coal mining as an anthropogenic influence on carbon cycling; however, more data are necessary to fully estimate the importance of this impact on regional scales.