Geochemistry of the Yegua Aquifer system and its relation to microbial processes

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Schlichenmeyer, Jeannette Leone
Texas A&M University
geology.; Major geology.
thesis / dissertation description
Two sediment cores (NP-I and NP-3) were taken from the Yegua formation in East-central Texas and analyzed for sedimentology, geochemistry, and microbiology to assess the relationship between geochemistry and microbial processes. Both cows suggest a deltaic origin for this region. NP-1 is likely an overbank or floodplain deposit. NP-3 is likely a bar or channel deposit. Groundwater wells were installed in the boreholes and screened in the watersaturated sands (37-42 feet and 9 1-I 01 feet below the surface, respectively). Another well (NP-2), 50 feet to the south of NP-3, was screened at 37-47 feet below the surface. The wells were sampled routinely for geochemistry and microbiology. Total organic carbon (TOC) contents range from 0.01 to over 60 weight percent, and, except in soil intervals, have the 813C signature Of C3 plants. Soil intervals have a noticeable 13C contribution from C4 plants. Dissolved inorganic carbon (DIC) contents and 5 13C values suggest that NP-I water has been influenced by dissolution of a carbonate with a 813C of less than 09o'o. NP-2 and NP-3 waters have been influenced by carbonate dissolution and organic matter oxidation. Microbial and geochemical data suggest that sulfide oxidation is occurring in NP-I and NP-2 waters. These waters have moderate to high sulfate and Fe2+ concentrations, low pH (-6 or less), and are supersaturated with respect to jarosite, a weathering product of pyrite, according to the speciation model WATEQF. Both sulfur oxidizing and sulfate reducing bacteria (SRB) are present in these waters. NP-3 sediments from the screened interval contain SRB. NP-3 waters are low in sulfate and Fe2+, and have a pH above 7. This suggests that sulfate reduction is more dominant at deeper depths. Sulfur oxidizing bacteria and SRB can be cultured from these waters as well. A syntrophic relationship may exist in these sediments and waters between sulfur oxidizers and sulfate reducers. Iron cycling in these sediments appears to be dependent on sulfur cycling, rather than an independent process. SRB numbers correlate with sand content, and bacterial numbers are not limited by carbon source or electron acceptors.