USING STABLE ISOTOPES TO TRACK SOURCES AND FATE OF CARBON IN HIGH CO

Developing effective monitoring, verification, and accounting (MVA) tools for carbon sequestration operations based on laboratory or modeling experiments can be challenging due to the complexity, size, and long time scales involved. To test the applicability of a stable carbon isotope approach to understand CO₂ water-rock interactions and monitor CO₂ movement in complex geochemical environments and geological settings, samples were collected from two high-CO₂ coal mine discharges located at the base of the Pennsylvanian (≈290 Ma) Pittsburgh Formation of the Monongahela Group, in the eastern limb of the Irwin Syncline, in Allegheny County, Pennsylvania. Waters at both these sites have high dissolved CO₂ concentrations. Preliminary data indicates that carbon isotope signatures of dissolved inorganic carbon (δ¹³C_DIC) are sensitive to small shifts in dissolved CO₂ concentrations in the range of 0.01-0.20 g/L, demonstrating the power of this technique to monitor minor CO₂ variations. The carbon in these waters could originate from the oxidation of organic matter in the coal seams and/or soil organic matter or from dissolution of carbonate rocks. We used a Keeling plot approach to determine the major source of DIC in these waters. Using this approach, the δ¹³C_DIC values of samples are plotted against 1/DIC concentration and the regression of the data yields a straight line with y-intercept values, representing δ¹³C of added DIC, ranging from -5 to +2‰. The high δ¹³C of added DIC indicates that bicarbonate originating from carbonate rock dissolution is the major source of carbon in waters at these sites. This is consistent with strontium isotopic analysis and PHREEQc modeling that indicate that related net alkaline discharges develop as limestone dissolution is enhanced by cation exchange reactions of clay minerals with AMD mine pool waters, resulting in CO₂ degassing.