ASSESSING THE POTENTIAL IMPACT OF MICROBES IN THE EDWARDS AND TRINITY AQUIFERS OF CENTRAL TEXAS

The freshwater (FW) and saline-water (SW) interface marks the eastern boundaries of the karstified Edwards and Trinity aquifers in central Texas; this interface is known locally as the “bad-water line”. The bad-water line has been defined as the 1000 mg/L-line of total dissolved solids (TDS), compared to FW with 250-300 mg/L TDS. The SW also has high total dissolved sulfide (0.1 to >100 ppm). Video camera logs of open-hole SW wells reveal a complex network of fractures, cavities, and large caves, some having significant contributing flow into the wells. White filamentous and colloidal biomass in the water and thick biofilms covering carbonate rocks were also observed, suggesting that the SW is a unique habitat for microorganisms. Previous geochemical and hydrogeological work delineated some of the SW chemical attributes, but virtually nothing is known about SW microbiology and the possible role of microbes to SW geochemical and geological processes. SW reduced sulfur compounds (e.g., H₂S) could serve as rich energy sources for sulfur-oxidizing bacteria; sulfuric acid, produced as a metabolic product, would affect carbonate dissolution. Trinity SW wells from northern San Antonio and Edwards FW and SW wells from San Antonio and north along the bad-water line to San Marcos were evaluated for this investigation using unique geochemical and microbiological methods. Water samples were collected and analyzed for major and trace ions; field geochemistry was done for fugitive species, such as dissolved sulfide and oxygen. Ion speciation and mineral saturation states were calculated using the computer program Geochemist's Workbench.  Trinity SW is Ca-Na-SO₄-Cl water type, and Edwards SW shifts northward from Ca-HCO₃-SO₄ to Na-Ca-SO₄-Cl water-type. The SW is saturated with respect to calcite (SI = +0.04 to +0.89), but undersaturated with respect to gypsum (SI = -0.20 to -2.1) and slightly undersaturated to saturated with respect to dolomite (SI = -1.0 to +2.6). In situ microcosms consisting of approx. 1 cm³ calcite and dolomite chips (totaling approx. 10 g) were placed in FW and SW wells from 650-800 ft water depths to test for rock dissolution due to microbial colonization. Results provide insight into SW carbonate dissolution, which may be enhanced by microbes and extend the depths to which karstification occurs.