Carbon dioxide content and isotopic composition of gas in the unsaturated zone as an index of geochemical activity in reclaimed mineral lands, Alberta, Canada

Full text: The results of a study of gases in the unsaturated zone suggest that the partial pressures of CO2 (PCO2) in the natural, undisturbed settings are relatively stable or increasing slightly over time, and that the values range between 0.03 and 3.5 mol%. The PCO2 levels in one mine (Vesta) are similar to the natural levels (0.1-4.0 mol%) while the CO2 levels in the other mine (Diplomat) reached a maximum of 23 mol% and varied significantly during the course of the one-year monitoring period. Values of PCO2 generally increased with respect to depth in the unsaturated zone. Higher-than-normal levels of CO2 in the unsaturated zone were attributed to the land disturbance and were believed to be indicative of nonequilibrium geochemical conditions. The values of δ13C/12C of CO2 from the Vesta and Diplomat mines (-17 to -19 per mille and -11 to -12 per mille, respectively) were sufficiently different that a dual source of CO2 was deduced. The first source was organic carbon, which was reflected by the values of δ13C/12C of CO2 from the natural, unmined settings (-21 to -17 per mille). The second source consisted of marine carbonates, hydrolysed by sulphuric acid produced during pyrite oxidation (∼=0 per mille). The samples from the two mines represent mixtures of CO2 from both sources. There was an excellent correlation between the values of PCO2 actually measured and those calculated on the basis of an equilibrium chemical model (WATEQF) from the chemical composition of groundwater. The maximum value of PCO2 for the Diplomat mine samples was about two times that of the Vesta mine (6.0 versus 3.0 mol%), in keeping with the range of PCO2 actually measured through gas sampling. (author)