Determination of CO2 and DIC in high-CO2 waters: An application for the analysis of coal mine waters

Abstract

The impact of sulfuric-acid weathering on the global carbon cycle is increasingly being recognized as important in both recent and deep time estimates yet remains poorly constrained. Global-scale estimates of CO₂ fluxes are hampered by the lack of data, especially for acidic waters. Mining-impacted groundwaters and associated discharges, which can be acidic and contain CO₂ orders of magnitude greater than atmospheric equilibrium, are chemically unstable at the Earth surface and are often difficult to sample and accurately analyze. The direct CO₂ method used herein is accurate and precise with holding times up to 25 days. This modified method allows for the determination of CO₂ and DIC in samples lacking measurable alkalinity or having high CO₂ pressures; these samples may otherwise be excluded from estimates of regional carbon fluxes. To demonstrate the method, natural and CO₂-treated mine waters with pHs ranging from 3.1 to 7.1 were sampled and found to contain between 2 and 13 mM CO₂ (88–572 mg CO₂/L, 24–154 mg C/L) and 2–21 mM dissolved inorganic carbon, DIC (24–252 mg C/L). In comparison to standard DIC estimation methods, the direct method for analyzing CO₂ and determining corresponding concentrations of DIC is more accurate and reproducible over holding times of several weeks for water samples containing low-to-high CO₂ and with acidic-to-circumneutral pH.