Contributions of iron (III) and sulfate-reducing bacteria to attenuation of an Acid Mine Drainage site: Linking microcosm studies and geochemistry

Acid Mine Drainage (AMD) is a consequence of mining activity; it results from bacterial and chemical oxidation of pyrite and other sulfide minerals in waste rock and tailings. AMD is characterized by low pH, and elevated sulfate, iron and often heavy metal concentrations that cause severe damage to the environment. Microorganisms indigenous to highly acidic environments are very diverse and include microorganisms capable of generating alkalinity. The objective of this research was to investigate the biological attenuation of AMD in Davis Mine, an abandoned pyrite mine in Western Massachusetts. The main focus was to evaluate the effects of dissimilatory iron reducing bacteria (DIRB) and sulfate-reducing bacteria (SRB) on the geochemistry of Davis Mine using in situ (ISM) and laboratory (LBM) microcosms. Evidence of ongoing microbial activity was indicated by geochemical changes observed only in live LBMs: increase in concentration of ferrous iron over time, the development of reducing conditions, increase in pH and the development of black precipitates. Data showed that indigenous bacteria change the water chemistry and mineral composition favoring the natural attenuation of the site. However, the results from this study were not sufficient to confirm whether or not SRB or DIRB were metabolically active. The LBM experiments showed that in a closed system and under favorable environmental conditions (temperature, Eh, pH) biological reduction was one of the main mechanisms for the remediation of AMD. Indigenous bacteria were capable of remediating AMD without the addition of an external carbon source, and attenuation rates increased with the addition of glycerol, nitrogen and phosphorous. A program for aqueous geochemical calculations, PHREEQC, confirmed the precipitation of some minerals from the water geochemistry resulting from the microbial activity. Due to hydrological problems in the ISM, it was not possible to obtain in situ rates of degradation. The ISM was affected by the introduction of water with different chemical characteristics. The in situ experiments showed that groundwater transport and diffusion played an important role in the groundwater chemistry. Further in situ experiments are needed to account for environmental factors such as soil pore diameter, temperature, groundwater flow and geochemical processes, availability of electron donor and nutrients in order to determine the rate of microbial activities