TRACING GEOCHEMICAL PROCESSES IN COAL MINE WATERS USING CO2

The partial pressure of CO₂ has long been used as an indicator of water source and reactions in karst systems but it can also be used to trace reactions in coal mine drainage waters. In coal mine waters the carbonate dissolution is primarily driven by sulfuric acid rather than carbonic acid, similar to what is found in karst systems formed by sulfuric acid such as Carlsbad Caverns in the USA and Frasassi Cave in Italy. Depending on the pH of the system, the CO₂ generated by mine waters can be exported downstream as HCO₃ or lost by degassing as CO₂. In this study we tracked CO₂ fluxes and geochemistry at several mines for approximately one year to evaluate temporal change. The sites are located in southwest Pennsylvania and northern West Virginia. The three sites had average pHs that ranged from 2.9 to 5.3 and average CO₂ concentrations that ranged from 2.9 to 4.8 mM. The site with the highest pH had the highest CO₂ concentration as well as the greatest variability (~23%). The site with the lowest average CO₂ concentration was the least variable (~1% variability). There was no clear relationship between CO₂ concentrations and discharge at the sites suggesting that changes in the CO₂ concentrations cannot be attributed solely to storm-related dilution. Although CO₂ may be a useful tracer of geochemical processes in coal mine waters, changes in concentration over time may be more difficult to interpret than in karst waters.