Microbially mediated reactions facilitate the in situ degradation of many contaminants in groundwater thus contributing substantially to the water-purification capabilities of the subsurface (Griebler & Avramov, 2015). The ability to understand and predict contaminant removal through microbially mediated reactions in aquifers hinges on our ability to monitor and quantify reaction rates (Pinay et al., 2015) and microbial activity in the subsurface (Leckie et al., 2004). Microbially mediated reactions are subject to spatial variability and temporal dynamics (Li et al., 2021). In general, that spatial variability depends on the co-occurrence of reactants and Abstract Spectral Induced Polarization (SIP) has been suggested as a non-invasive monitoring proxy for microbial processes. Under natural conditions, however, multiple and often coupled polarization processes co-occur, impeding the interpretation of SIP signals. In this study, we analyze the sensitivity of SIP to microbially-driven reactions under quasi-natural conditions. We conducted flow-through experiments in columns equipped with SIP electrodes and filled with natural calcareous, organic-carbon-rich aquifer sediment, in which heterotrophic denitrification was bio-stimulated. Our results show that, even in the presence of parallel polarization processes in a natural sediment under field-relevant geochemical conditions, SIP is sufficiently sensitive to microbially-driven changes in electrical charge storage. Denitrification yielded an increase in imaginary conductivity of up to 3.1 𝐴𝐴 μS cm −1 (+140%) and the formation of a distinct peak between 1 and 10 Hz, that matched the timing of expected microbial activity predicted by a reactive transport model fitted to solute concentrations. A Cole-Cole decomposition allowed separating the polarization contribution of microbial activity from that of cation exchange, thereby helping to locate microbial hotspots without the need for (bio) geochemical data to constrain the Cole-Cole parameters. Our approach opens new avenues for the application of SIP as a rapid method to monitor a system's reactivity in situ. While in preceding studies the SIP signals of microbial activity in natural sediments were influenced by mineral precipitation/dissolution reactions, the imaginary conductivity changes measured in the biostimulation experiments presented here were dominated by changes in the polarization of the bacterial cells rather than a reaction-induced alteration of the abiotic matrix. Plain Language SummaryTo better predict the contribution of microbes to groundwater clean-up it is important to locate microbes in the ground that are actively removing contaminants and measure how fast they are doing so. Our ability to do so, however, is limited by the difficulty in visualizing underground processes. Electrical methods such as spectral induced polarization (SIP) have been applied to monitor microbes and provide an alternative to visualize them underground. SIP, however, has so far only been shown to work in controlled environm...
<p>Solid organic matter (SOM) is an important component of natural sediments and plays a crucial role in providing substrate for microbial reactions and the degradation of contaminants in soil and groundwater. Knowledge about its distribution in the subsurface is crucial for the delineation of potential hotspots of microbial activity. The subsurface is, however, difficult to access, limiting our ability to reliably delineate the spatially heterogeneous distribution of SOM. Recently, the geophysical method induced polarization (IP) has been shown to be a potentially promising mapping tool, able to detect the presence of SOM. However, the mechanisms controlling IP signals in the presence of SOM are not (yet) well understood, with a handful of studies highlighting inconclusive results (Katona et al., 2021; Mellage et al., 2022; Ponziani et al., 2012; Schwartz & Furman, 2014). Moreover, a non-negligible contribution of polarization from the organic matrix can yield signals that may cause misinterpretation of other petro-physical relationships in unconsolidated sediments.</p> <p>In this study, we measured the spectral IP (SIP) response of aquifer sediment cores (2 &#8211; 8 m depth) collected from an alluvial floodplain aquifer in southwest Germany. The total organic carbon (TOC) content in the cores and the cation exchange capacity (CEC) exhibit a positive correlation with the magnitude of polarization (i.e. imaginary conductivity). In addition, strong differences in the frequency dependence of the IP measurements as a function of TOC fraction were observed for the otherwise calcareous matrix devoid of other strongly polarizing mineral phases (e.g. pyrite or clay minerals). While the CEC at the site is strongly dominated by the amount of SOM, polarization is more strongly linked to SOM than CEC. We hypothesize that the weaker correlation between SOM and CEC highlights the contribution of poorly understood charge storage mechanisms within the polydisperse organic matrix that differ from polarization at mineral surfaces. Ongoing experiments with artificial soil mixtures of calcitic sand and varying fractions of peat, under controlled conditions (i.e. constant electrical conductivity of the pore fluid), will help to shed light on the controls behind our field-derived relationships. We expect that our combined field and laboratory investigations will provide insights into the petro-, or rather, <em>organo-</em>physical relationship between SOM and the imaginary conductivity, and thus contribute to a conceptualization of the underlying polarization mechanisms in organic matrices.</p> <p>&#160;</p> <p><em>References</em></p> <p>Katona,&#160;T., Gilfedder,&#160;B.&#160;S., Frei,&#160;S., B&#252;cker,&#160;M., & Flores Orozco,&#160;A. (2021). High-resolution induced polarization imaging of biogeochemical carbon-turnover hot spots in a peatland. <em>Biogeosciences</em>, <em>18</em>(13), 4039&#8211;4058.</p> <p>Mellage,&#160;A., Zakai,&#160;G., Efrati,&#160;B., Pagel,&#160;H., & Schwartz,&#160;N. (2022). Paraquat sorption- and organic matter-induced modifications of soil spectral induced polarization (SIP) signals. <em>Geophysical Journal International</em>, <em>229</em>(2), 1422&#8211;1433. https://doi.org/10.1093/gji/ggab531</p> <p>Ponziani,&#160;M., Slob,&#160;E.&#160;C., Vanhala,&#160;H., & Ngan-Tillard,&#160;D. (2012). Influence of physical and chemical properties on the low-frequency complex conductivity of peat. <em>Near Surface Geophysics</em>, <em>10</em>(6), 491&#8211;501. https://doi.org/10.3997/1873-0604.2011037</p> <p>Schwartz,&#160;N., & Furman,&#160;A. (2014). On the spectral induced polarization signature of soil organic matter. <em>Geophysical Journal International</em>, <em>200</em>(1), 589&#8211;595. https://doi.org/10.1093/gji/ggu410</p> <p>&#160;</p>
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