Thermodynamic theory predicts that microorganisms will utilize terminal electron accepting processes (TEAPs) sequentially, beginning with the process that yields the highest amount of energy, but field studies often document an apparent overlap of TEAP byproducts (e.g. Fe(II), CH4). There are several possible explanations for this observed overlap, including downgradient transport of Fe(II) and methane, use of vertically averaged data from wells screened over more than one TEAP, or simultaneous utilization of multiple electron acceptors occurring in microenvironments, or small pockets of microbial activity. In this study, we are testing the hypothesis that microenvironments of Fe(III) and CO2 utilization are created by heterogeneities in Fe(III) concentration and bioavailability. The heterogeneous nature of most soils and sediments allows for the possibility of small pockets of TEAPs that are different from the bulk TEAP.

Sediment cores, collected from a petroleum-contaminated site, were used to inoculate batch microcosms. Microenvironment formation is being controlled by the addition of sand-sized glass beads coated with Fe(III)-oxides. In one group of microcosms, the beads form a continuous layer on top of the sediment; the other group contains beads mixed throughout the sediment. In order to examine how microbes are utilizing solid phase iron coupled to BTEX oxidation, samples will be analyzed using scanning electron microscopy (SEM) and electron microprobe techniques in correlation with iron mineral extraction.

Results of this research will provide better understanding of whether microbes utilize TEAPs sequentially or simultaneously. This will allow for more accurate quantification of contaminant mass loss in the field, which will result in better numerical models for bioremediation designs, enabling more effective management of petroleum-contaminated sites.