The significance of microbial processes in hydrogeology and geochemistry

Microbial processes affect the chemical composition of groundwater and the hydraulic properties of aquifers in both contaminated and pristine groundwa ter systems. The patterns of water-chemistry changes that occur depend upon the relative abundance of electron donors and electron accepters. In many pristine aquifers, where microbial metabolism is limited by the availabilit y of electron donors (usually organic matter), dissolved inorganic carbon ( DIC) accumulates slowly along aquifer flow paths and available electron acc epters are consumed sequentially in the order dissolved oxygen > nitrate > Fe(III) > sulfate > CO2 (methanogenesis). In aquifers contaminated by anthr opogenic contaminants, an excess of available organic carbon often exists, and microbial metabolism is limited by the availability of electron accepte rs. In addition to changes in groundwater chemistry, the solid matrix of th e aquifer is affected by microbial processes. The production of carbon diox ide and organic acids can lead to increased mineral solubility, which can l ead to the development of secondary porosity and permeability. Conversely, microbial production of carbonate, ferrous iron, and sulfide can result in the precipitation of secondary calcite or pyrite cements that reduce primar y porosity and permeability in groundwater systems.