The tailings at Kristineberg, northern Sweden, have a very low content of organic carbon, a feature common with many sulfidic tailing impoundments. Three different experiments were set-up to assess the role of carbon dioxide in a depth profile. Firstly, pore gas was collected in vials from ground water pipes at various points in the profile of a dry covered tailings impoundment and analyzed in the laboratory for CO2, O2, N2, H2, and CH4 contents. Secondly, pore water was extracted from tension lysimeters at various depths. This water was analyzed for numbers of bacteria (iron-oxidizing and sulfur-oxidizing, both by MPN), and low molecular weight organic acids. Thirdly, Acidithiobacillus ferrooxidans (Strain DSMZ No 1927) was grown on a mixture of irradiated tailings and sterile water. The amount of organic acids produced was monitored.The largest bacterial count of iron-oxidizing bacteria, 4.7 × 105/g tailings, was at the oxidation front, while the heterotrophic sulfur-oxidizing bacteria were mainly found in the unsaturated, oxidized zone, 2.65 × 104/g tailings. The oxidation front was also the location where the largest amount of organic acids was found in the field study (formate 0.83 mg/l and acetate 0.51 mg/l). The acetic acid found coincides with the highest count of iron-oxidizing bacteria.The intrusion of O2 and CO2 at the studied location is enough for microbiological activity, although the overall effect on AMD production is not addressed.The results from laboratory incubations indicate that the microbial community produces organic carbon with CO2 as the sole carbon source, up to 1.35 mg/l after 16 weeks measured as TOC.To conclude, we suggest that knowledge of the intrusion of both CO2 and O2 is vital for a full understanding of the microbial ecology, and thus the weathering processes, in a dry covered tailings impoundment. Hence, the CO2 produced in the till cover and entering the tailings ecosystem is crucial to the function of the ecosystem.