Assessing the potential impact of microbes in the Edwards and Trinity Aquifers of central Texas

The Edwards Aquifer in central Texas is one of the largest carbonate aquifers in the United States, supplying nearly two million people with water. The role of microorganisms in the development of deep carbonate aquifers has not been thoroughly investigated. The Edwards Aquifer is composed of a freshwater zone and a sulfidic, saline water zone. Down-well video surveys show an abundance of white filamentous and planktonic biomass floating in the water column, indicating that the saline water may serve as a habitat for microbes and provide a unique opportunity to investigate microbially enhanced deep subsurface karstification. A combination of geochemical analyses, modeling, and in situ¬ microcosms were used to study the effects of microbial colonization of aquifer rock on carbonate solubility. Ion speciation and mineral saturation states were calculated using the computer program Geochemist\u27s Workbench. The saline water modeled as being undersaturated to supersaturated with respect to calcite (SI = -0.3 to +1.5), but undersaturated with respect to gypsum (SI = -0.2 to -1.5) and supersaturated with respect to dolomite (SI = +0.5 to +3.9). In situ microcosms consisting of approximately 1 cm3 calcite and dolomite chips (totaling 10 g) were placed in freshwater and saline water wells from 150-220 meter water depths to test for rock dissolution due to microbial colonization. Examination of calcite surfaces for some of the wells, that modeled as being supersaturated with respect to calcite, showed dissolution pits, generally associated with microbes. Gypsum crystals were observed on some chips associated with cells, despite the waters being undersaturated with respect to gypsum. Dissolution of calcite and precipitation of gypsum may be due to local microbial metabolism. Results of this study provide insight into saline water carbonate dissolution, which may be enhanced by microbes and extend the depths to which karstification occurs