TOWARD IMAGING DISSIMILATORY IRON REDUCTION RATES IN THE SUBSURFACE WITH HYDROGEOPHYSICAL TOOLS

As organic contaminants are introduced into aquifers as pollution, biologic reactions create zones of anoxia in which dissimilatory iron reduction (DIR) is possible. The region of DIR often roughly defines the boundary between contaminated and uncontaminated waters. While there are numerous geochemical and hydrological techniques available for monitoring contaminant plumes, many of these techniques are costly, provide a limited amount of data, and cannot yield time course information. Geophysical techniques may provide a cost effective way to expand these small data sets. Recently, electrical resistivity, induced polarization, and self potential have been used to map the spatial extent of contaminant plumes containing anything from landfill leachate to petroleum products. We demonstrate that geochemical and biogeochemical effects like redox changes, variations in total dissolved solids, and bacterial activity can be quantifiably linked to an electrical geophysical response. For example, abiotic iron reduction with ascorbic acid produces a quantifiable and theoretically predictable change in electrical resistivity at the bench scale. The resistivity of a solution of ferrihydrite and water at pH 5.8 remains unchanged unless mineral acid or ascorbic acid is added, and each effect is theoretically predictable. In batch experiments, during abiotic iron reduction, cumulative 20-40% increases in measured resistivity (~300 uS/cm) can be attributed to a decrease in conductivity from increasing pH (DpH = 3.25 -> 5.07, -201 uS/cm) and an increase in conductivity from increasing dissolved Fe(II) (D[Fe] = 2.2 - 3.3 mM, 400 -700 uS/cm). We are also quantifying the resistivity response associated with abiotic iron reduction in the presence of iron-reducing enzymes from Shewanella oneidensis MR-1 (in vitro DIR) and in the presence of metabolizing cells (in vivo DIR) with both batch and column experiments. Changes in electrical resistivity for in vivo or in vitro DIR will be modeled based on observed changes in bio-film formation, pH, Fe(II) and other solutes. Induced polarization is also expected to change where iron precipitates form. The change in these measured geophysical properties with time will enable us to image biologically mediated iron reduction rates and plume evolution in the laboratory and the field.