Subsurface pyrite oxidation has the potential to lower groundwater pH and to release constituent trace elements to solution. Although the central role of microbial activity in pyrite oxidation is well understood in acid mine drainage and other low-pH (e.g. pH < 2) environments, the potential for microorganisms to mediate pyrite oxidation under circumneutral pH conditions is not well understood. This study examined the potential for aerobic microbial metabolism to promote circumneutral pH oxidation of pyrite in Cambrian-age sandstones from Trempealeau County, WI. Results from a 580-day incubation experiment showed that microbial activity greatly accelerated (ca. 5-fold) the rate and extent of sulfate generation (a direct measure of pyrite oxidation) in microcosms containing reduced pyrite-bearing sediments. Microbial growth coupled to pyrite oxidation was indicated by a detectable increase in ATP content. pH values dropped to 3 in the microcosms which contained limited carbonate (dolomite) buffering capacity. The decline in pH could be directly attributed to microbial acceleration of pyrite oxidation. Ca²⁺ and Mg²⁺ were proportionally released to solution with sulfate as a result of carbonate and/or Ca-aluminosilicate dissolution by the acid generated from pyrite oxidation. When the amount of acid from pyrite oxidation exceeded the system buffering capacity, metals were selectively released from the geological material. No significant release of trace metals took place in abiotic reactors, which showed much lower rates of pyrite oxidation. These finding suggest that groundwaters in contact with pyrite-containing geological formations contain microorganisms capable of accelerating the oxidation of native pyrite in those formations. The results of this study have key implications for controls on the onset of low-pH conditions and associated changes in groundwater quality in private and municipal drinking water wells located within pyrite-bearing geological formations.

Left: Magnified image of the Tunnel City Group rock core sample with pyrite lining the inside of a vugg, collected at about 186 feet depth. Right: Magnified image of Wonewoc Formation rock core sample with abundant well-rounded quartz sand. Sample was collected from about 360 feet depth.

Lisa Haas

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