MICROBIAL INFLUENCE ON CARBONATE EQUILIBRIA IN A HYDROCARBON-CONTAMINATED AQUIFER

The introduction of hydrocarbon into an aquifer initiates a cascade of biogeochemical reactions as the native microbial consortia adjust to the new substrate and begin to degrade it. The microbial degradation results in changes in solution redox and pH that are coupled to the production of secondary metabolites and CO2. Carbonate minerals are particular sensitive to changes in pH and DIC that shift mineral equilibria.

The Bemidji aquifer was contaminated by crude petroleum in 1979, and the biogeochemical consequences of that event have been intensively studied as part of the US Geological Survey Toxic Hydrology Program. We directly examined the influence of microbial activity on the formation of authigenic carbonate minerals in this aquifer using both field and laboratory studies. Upgradient of the oil pool aerobic hydrocarbon oxidation produces acidity that is buffered by the dissolution of native calcite and dolomite. As the groundwater trends into iron reducing and methanogenic regions authigenic carbonate minerals are re-precipitated. We observed authigenic syntaxial calcite overgrowths, and ferroan calcite was also reported (Baedecker et al., 1993), likely driven by dissimilatory iron reduction. More recently a ferroan dolomite was identified in field experiments precipitating on microbial cell walls, with laboratory experiments finding ordered stoichiometric dolomite in a methanogenic system. These results demonstrate the microbial influence on carbonate equilibria in this system ranging from microscale nucleation and precipitation to bulk shifts in redox and geochemical equilibria resulting from their metabolic processes.