Multi-scale Characterization and Prediction of Coupled Subsurface Biogeochemical-Hydrological Processes Page: 2 of 5
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provide raw material for more detailed mineralogical characterization, including electron
microscopy, x-ray diffraction, and analysis of acid-extractable metals. For the first test, the clay-
sized fraction (< 2-pm) was isolated from the bulk sediments and reduced enzymatically using a
site-isolated strain of Geobacter. The complex resistivity data revealed a slight decrease in both
the phase response and the imaginary component of resistivity following bioreduction,
suggesting that the alteration of clay minerals by microorganisms is capable of decreasing the
polarization response of sediments containing iron-bearing phyllosilicates. While the primary
mechanism responsible for the polarization decrease is believed to be the decrease in clay
mineral surface area exposed to the pore fluid following reduction, additional factors, such as
carbonate precipitation, may also be contributing to the observed decrease under field conditions.
The second test utilized FeS precipitates obtained from enrichments of acetate- and ferrous iron-
amended site groundwater. The precipitates were first rinsed before being added to a column
filled with silica sand and saturated with oxygenated groundwater. As oxidation of the FeS
proceeded, complex resistivity data were collected. As indicated by a gradual change in color
from black to orange, FeS oxidation proceeded over the course of several days. In contrast, the
change in the complex resistivity response was rapid, with the polarization response
characteristic of the FeS precipitates decreasing to that of the background silica sand within 24-
hours. The final test was designed to test the hypothesis that abiotic processes are capable of
significantly altering the real component of resistivity through the precipitation of low
conductivity phases, such as elemental sulfur created through the oxidation of bisulfide by ferric
iron present in crystalline iron (oxy)hydroxides. Oxidized aquifer sediments were added to
sulfide-rich groundwater lacking a readily available source of organic carbon (e.g. acetate) and
the complex resistivity response was measured over the course of 2 weeks. During this time, the
real component of conductivity (i.e. bulk conductivity) decreased by up to 16%, presumably as a
result of the precipitation of SO and/or disseminated, disordered FeS. Over the same time period,
the polarization response increased significantly (44% at 10-Hz), most likely as a result of the
precipitation of FeS and in agreement with our previous observations monitoring metal sulfide
2b. Field Scale Characterization and Monitoring. Crosshole seismic, radar and complex resistivity,
and SP data were collected this year between several well pairs and within all three flow cells at
the Rifle Site. These data are being compared with electrical, geological, and flowmeter logs
available from boreholes to assess hydrogeological heterogeneity at the site and to monitor
responses associated with biostimulation experiments. Preliminary analysis suggests that an
interpretation of heterogeneity across datasets is consistent across the datasets, and that the
geophysical information should be useful for understanding hydrogeological variations across
and within the sites.
Ideally, geophysical field-scale monitoring would be performed after all key features of
the project (laboratory, reactive flow modeling, field characterization, and estimation
framework) have been developed. However, during this year, we took advantage of ongoing
field-scale biostimulation experiments to monitor the responses using time-lapse geophysical
datasets. Most interesting are the responses observed in the time lapse, surface electrical
resistivity tomographic (ERT) datasets and responses associated with self-potential (SP)
measurements. The surface ERT datasets were collected perpendicular to experiments conducted
during 2006 in what are called the '2004' and '2005' Rifle flow cells. In agreement with
laboratory analysis (see Section 2a), these datasets displayed signatures associated with iron and
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Hubbard, Susan; Williams, Ken; Steefel, Carl; Banfield, Jill; Long, Phil; Slater, Lee et al. Multi-scale Characterization and Prediction of Coupled Subsurface Biogeochemical-Hydrological Processes, report, June 1, 2006; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc883197/m1/2/: accessed November 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.