Geochemical and microbiological zonation of the Middendorf aquifer, South Carolina

We consider the nature and origin of the chemical and microbiological zonation of the Cretaceous Middendorf aquifer, in the Coastal Plain of South Carolina, in light of the chemical evolution of groundwater along a flow path there. Some types of microbial activity, most notably aerobic respiration and iron reduction, lead to a net generation of acid as CO2, but reaction of groundwater with minerals in the aquifer consumes the acid, driving up pH and controlling the evolution of the groundwater's major ion chemistry. The thermodynamic energy available to the various functional groups of microbes, except the autotrophic hydrogen-oxidizing methanogens, is everywhere along the flow path sufficient to drive microbial growth. The concentrations of acetate, formate, and lactate do not vary systematically from zone to zone along the flow path, as might be expected if each zone were dominated by a single functional group of microbes. The overall rate of microbial respiration in the aquifer appears to be limited by the rate of the initial fermentation of organic matter. We found no compelling evidence indicating that one functional group of microbes excludes others from any of the aquifer's zones. Sulfate reduction, for example, may be non-existent in the high-iron zone associated with iron reduction, or may account for up to 90% of the respiration occurring there. Zonation of the groundwater into high-iron and low-iron facies may result from a minor change along the aquifer in the balance between the activities of iron and sulfate reducers.