Evidence that bio-metallic mineral precipitation enhances the complex conductivity response at a hydrocarbon contaminated site
Introduction
Previous studies have suggested the use of geophysical methods as complementary, rapid, cost-effective, and minimally-invasive tools for detecting and monitoring the extent and fate of oil spills in the subsurface (Atekwana and Atekwana, 2010, Atekwana et al., 2004a, Che-Alota et al., 2009, Werkema et al., 2003). In our arsenal of geophysical tools, complex conductivity has been suggested as a technique with high sensitivity to the presence of contaminants (Börner et al., 1993, Flores Orozco et al., 2012, Kemna et al., 2004, Olhoeft, 1985, Revil et al., 2011, Schmutz et al., 2010, Schmutz et al., 2012, Vanhala et al., 1992) as well as accompanying bio-physicochemical processes associated with hydrocarbon biodegradation (Abdel Aal et al., 2004, Abdel Aal et al., 2006). Nonetheless, the interpretation of complex conductivity data from field sites undergoing active microbial degradation remains challenging partly because several factors may contribute to the observed complex conductivity response including for instance the effect of the pore water composition and conductivity, temperature, the cation exchange capacity of the minerals, the presence of bacteria, non-wetting phases, and metallic particles.
The presence of oil stimulates microbial activity, resulting in bio-physicochemical changes induced in the subsurface during the biodegradation of hydrocarbons by microbes (Cassidy et al., 2002, Cozzarelli et al., 1994, Cozzarelli et al., 2001). Microbes provided with an organic carbon source and nutrients can result in (1) an increase in cell density and the formation of biofilms on the grain surfaces and between pore openings in rocks and sediments that induce physical changes in the porous material itself (Abdel Aal et al., 2010, Atekwana and Atekwana, 2010, Atekwana and Slater, 2009, Clement et al., 1996, Thullner et al., 2002) and (2) the production of organic and carbonic acids that enhance the weathering of aquifer solids causing an increase in the surface roughness of the mineral grains and the ionic strength of the pore water (Abdel Aal et al., 2006, Atekwana et al., 2004b).
The presence of terminal electron acceptors (TEA's) governs nutrient utilization by microbes during the breakdown of organic carbon (Bekins et al., 2001, Cozzarelli et al., 2001). TEA's are sequentially utilized from O2, NO3−, Fe(III), Mn (IV), SO42 −, and CO2. Some iron(III)-reducing microorganisms can use hydrocarbons as a carbon source to reduce iron(III) to Fe(II) (Lovley et al., 1989). When hydrocarbon biodegradation is coupled to iron reduction it can lead to the formation of ferrous bio-minerals such as magnetite (Lovley et al., 1987, Rijal et al., 2010), pyrite (Prommer et al., 1999), ferroan calcite (Baedecker et al., 1992) and siderite (Tuccillo et al., 1999). For example, magnetite can be formed through either biologically induced (abiotic) mineralization or biologically controlled (biotic) mineralization. In the biologically induced mineralization magnetite can be nucleated and grow abiotically by chemical reactions involving byproducts as a result of biotic metabolic activity, while during biologically controlled mineralization, magnetite can be synthesized at a specific location within or on the cell (Bazylinski and Frankel, 2003). Following these two pathways, iron(III) can be converted to iron(II), then magnetite can be precipitated either by nucleation or growth that is controlled by ferrous iron concentration and/or pH (Hansel et al., 2005).
In several recent studies, magnetic susceptibility data from hydrocarbon contaminated sites have documented zones of enhanced magnetic susceptibility within the smear zone coincident with the free phase hydrocarbon plume (Mewafy et al., 2011, Rijal et al., 2010, Rijal et al., 2012). In these studies, magnetite was documented as the dominant mineral within the enhanced zone of magnetic susceptibility, suggesting biomineralization due to iron reduction. This raises an important question regarding the contribution of bio-metallic mineral phases such as magnetite to the complex conductivity response at hydrocarbon contaminated sites.
In this study, we acquired laboratory complex conductivity measurements on core sediments retrieved from a hydrocarbon contaminated site where hydrocarbon biodegradation has been extensively documented. Additionally, we conducted several controlled experiments simulating the field conditions to aid interpretation of our results. Our specific objective was to elucidate the major factors controlling the complex conductivity response at hydrocarbon contaminated sites undergoing biodegradation.
Section snippets
Site history
The National Crude Oil Spill Fate and Natural Attenuation Research Site at Bemidji, MN (Fig. 1) is a natural laboratory available for investigating bio-physicochemical processes associated with the intrinsic bioremediation of a crude oil spill (Cozzarelli et al., 2001, Eganhouse et al., 1993). In August 1979, a high pressure crude oil pipeline ruptured, releasing 1,700,000 L of crude oil. Oil pooled in low-lying areas (~ 2000 m2) over a total area of 6500 m2 (Fig. 1). According to Essaid et al.
Complex conductivity
Silver-chloride non-polarizable electrodes were used to measure the resulting difference of the electrical potential across a sample in response to current flow. The impedance magnitude (|σ|) and phase (φ) of the sample were recorded relative to a precision reference resistor upon stimulus with a sine-wave current. The measured frequency dependent complex conductivity σ* (ω) behavior of a porous medium can be generally represented with an in-phase (real) conductivity component (σ′) or
Retrieval of cores
A total of four ~ 5-cm diameter cores were retrieved from both uncontaminated and contaminated locations at the study site (Fig. 1). Two cores were collected in July 2010; one from the contaminated location (core C1010) within the free phase plume, and one from the uncontaminated location (core C1006). Two additional cores were retrieved in July 2011; core C1110 from within the free phase plume and core 1101D from the edge of the dissolved phase plume (Fig. 1). All the cores extended from the
Complex conductivity measurements for Bemidji cores
Our results are separated according to field conditions into “saturated” (for samples obtained from core segments at or below the highest water table mark) and vadose zone measurements (for samples obtained from core segments above the highest water table mark). Fig. 3 shows the complex conductivity sampling locations and hydrocarbon distribution along the 2010 and 2011 cores. The saturated zone as defined here for the contaminated free product locations (cores C1010, C1110) comprises the
Variability in the complex conductivity response of oil contaminated sediments
The variability in the complex conductivity of oil contaminated sediments is driven by several factors including soil physical properties, wettability, hydrocarbon saturation, salinity and saturation effects (Revil et al., 2012a, Revil et al., 2012b; Schmutz et al., 2011), and hydrocarbon phase (Flores Orozco et al., 2012). However, at sites where extensive biodegradation has taken place, the variability of the complex conductivity results from biophysicochemical changes imparted on the
Conclusions
We investigated the complex conductivity response of oil contamination at a site undergoing active bioremediation where iron reduction is the dominant terminal electron process. The magnitude of the real and imaginary components of the complex conductivity is higher for most of the samples from the smear zone (where the oil saturation is greatest) compared to the dissolved phase saturated zone where the oil saturation is lower. Saturated zones of enhanced complex conductivity responses are
Acknowledgments
This material is partially based on work supported by the Enbridge Energy (Ltd.), the Minnesota Pollution Control Agency, and the U.S.G.S. Toxic Waste Substances Program. We thank W. Herkalrath, F. Day-Lewis, J. Lane, M. Erickson, and J. Trost (U.S.G.S.), J. Heenan and C. Zhang (Rutgers-Newark) for valuable field support. The U.S. EPA Office of Research and Development funded and collaborated in the research described here under EP-10-D-000488. It has been subjected to Agency review and
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