The Geochemistry and Carbon Dioxide Flux of Two Mine Discharges, Allegheny County Pennsylvania

Concerns about the rise in earth surface temperatures potentially linked to increases in atmospheric CO2 concentration have led to proposals to sequester anthropogenic CO2 emissions in subsurface geologic formations. Potential for leakage from these sites is a significant obstacle to this approach. Abandoned coal mines often exhibit measurable CO2 flux and may be useful analogs for CO2 sequestration sites. Discharge from two abandoned coal mines in Allegheny County, PA were studied to determine the presence of CO2 flux, to quantify the flux and to determine the geochemical controls on the system. The two sites demonstrated considerable variability in chemistry and quantity of flux. One site had relatively high concentration of Fe and moderate alkalinity in the discharge water. The other site had discharge water with comparatively low concentration of Fe, comparatively high concentration of Al and low alkalinity. Both sites had relatively high concentration of sulfate compared to most natural waters and the data at both sites are consistent with the dissolution of pyrite generating sulfuric acid and the generation of CO2 by the dissolution of carbonate minerals. Isotope data and mass balance calculations support the conclusion that the majority of carbon in these systems is "old" carbon -- either from the dissolution of carbonate rocks or the oxidation of the remaining coal. By comparing the modeled and empirically measured CO2 concentration it was determined that conventional methods based on alkalinity for determining PCO2 and/or H2CO3* may have limitations in this geochemical system that can lead to inaccurate estimations of the CO2 flux. The results also demonstrated that inorganic carbon assumes a variety of forms in these systems (H2CO 3 *, HCO3-, and CO3 2-). In a CO2 sequestration scenario, depending on the chemistry, it may be more useful to determine total dissolved inorganic carbon flux instead of CO2 flux. The CO2 flux at these two sites closely correlates to mine discharge and SO42- concentrations; if other sites demonstrate similar chemical characteristics these two parameters may be a useful screening tool for use on a regional scale to evaluate potential point sources of significant carbon flux.