Projects & Programs
Spatial Heterogeneity of Microbial Iron Reduction Potential in Subsurface Sediments
Objectives: The goal of this research was to determine the spatial distribution of microbial iron reducers in the subsurface, and the relationship of the bacterial distribution to the geological and geochemical properties of the sediments. Integrated models for the distribution of microbial iron reduction, physical properties and geochemical properties are needed as input to reactive flow and transport models that can be used to predict the potential effectiveness of microbial iron reduction for bioremediation of metals and radionuclides in groundwater.
Approach: Several boreholes were cored and sampled at the South Oyster Focus Area. The site is near Oyster, Virginia and serves as an analog to contaminated the U.S. Department of Energy sites in sandy sediments. The sediments at the site are coastal plain sediments that include fine-grained lagoonal and back-bay sediments as well as sand-rich sediment layers, that are similar to those found at the DOE Savannah River Site. The samples from the boreholes were analyzed for microbial iron reduction potential using a low-cost batch measurement method developed for the project. Researchers on the project also measured the extractable iron oxyhydroxide content, hydraulic conductivity, bulk density, porosity, organic matter concentration and grain size of the samples. High-resolution cross-hole radar and seismic tomographic data were recorded between each pair of boreholes. Geostatistical methods were used to integrate the data and estimate the microbial iron reduction potential between the boreholes.
Results: Results indicate that iron-reducing bacteria are present in all sediment types at the South Oyster site but have a very patchy distribution. Almost 45% of the samples did not show any microbial iron reduction potential. The geological and geochemical properties of samples where microbes reduced ferric iron were significantly different from samples where no reduction occurred. In addition, the geophysical properties of samples with activity were significantly different, especially the radar attenuation. The observed relationships allowed the use of geostatistical methods to integrate the tomographic radar data and the core data to predict where microbiological activity would be present between the boreholes. The project generated a large number of geostatistical simulations that made it possible to estimate the probability that activity would be found at any location between the existing boreholes. The results of this research project provide a model for the distribution of iron reducing bacteria and microbial iron reduction potential on the Atlantic Coastal Plain and a methodology that can be applied to develop similar models for other locations where metal-reducing bacteria may be used for contaminant remediation.