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Electrical and electromagnetic geophysical techniques are increasingly used to capture the dynamics of solute transport in the shallow subsurface. Three-dimensional electrical imaging techniques were used to investigate solute transport, including possible activation of preferential flow paths, at an artificially-drained site located on a flat agricultural landscape where excess nutrients discharge into drainage ditches. A saline tracer was injected into the saturated zone at a rate previously determined to limit groundwater mounding. Subsequently, a sprinkler system was used to simulate a 25-year, 12-hour storm event (~120 mm). A fan array of 96 electrodes, designed based on solute transport simulations, was used to acquire 3D electrical imaging monitoring datasets during tracer injection and over the subsequent simulated storm event. The time lapse electrical geophysical datasets suggest the tracer transport was through storm-activated preferential pathways in the subsurface. Calibration of hydrological models using the electrical geophysical datasets as proxies of changes in solute concentration provides unique insights into the role of preferential flow in providing groundwater-sourced nutrient inputs to drainage ditches from agricultural fields., Open-Access Online Publication: May 29, 2023

Groundwater is considered as the primary source of drinking water in many countries around the world. For maintaining resource sustainability, clean and safe groundwater is a priority for water authorities worldwide. Unfortunately, groundwater resources are susceptible to various types of pollution, with arsenic (As) being a major pollutant in certain areas due to natural or anthropogenic activities. Due to its adverse health effects, As, a class-1 carcinogen, has been a topic of intense research. The technical difficulty and high cost incurred by conventional treatment systems for As removal have led many researchers to work on developing efficient, eco-friendly, and cost-effective technologies for the treatment of groundwater sources. Biochar has been widely used as an adsorbent for the decontamination of groundwater. High As removal rates have been achieved following metal (Fe, Zn, Mn) impregnation and magnetic modification to biochar. Despite numerous researches on biochar for As removal, very limited efforts have been made for upscaling to a convenient treatment system. Thus, this review highlights key findings of previous studies required for pragmatic applications. Following up with future trends, an overview is given on real-time monitoring techniques that can be combined with biochar for evaluating adsorption studies. Furthermore, research opportunities that exist in biochar adsorption studies have also been identified. This technical review is aimed for scientists, scholars and researchers as a supplementary guide for understanding As decontamination processes, highlighting the role of biochar as an adsorbent and conceptualizing the prospects of integration of real-time monitoring techniques.

Iron mineral transformations occurring in hydrocarbon-contaminated sites are linked to the biodegradation of the hydrocarbons. At a hydrocarbon-contaminated site near Bemidji, Minnesota, USA, measurements of magnetic susceptibility (MS) are useful for monitoring the natural attenuation of hydrocarbons related to iron cycling. However, a transient MS, previously observed at the site, remains poorly understood and the iron mineral phases acting as reactants and products associated with this MS perturbation remain largely unknown. To address these unknowns, we acquired mineral magnetism measurements, including hysteresis loops, backfield curves, and isothermal remanent magnetizations on sediment core samples retrieved from the site and magnetite-filled mineral packets installed within the aquifer. Our data show that the core samples and magnetite packs display decreasing magnetization with time and that this loss in magnetization is accompanied by increasing bulk coercivity consistent with decreased average grain size and/or partial oxidation. Low-temperature magnetometry on all samples displayed behavior consistent with magnetite, but samples within the plume also show evidence of maghemitization. This interpretation is supported by the occurrence of shrinkage cracks on the surface of the grains imaged via scanning electron microscopy. Magnetite transformation to maghemite typically occurs under oxic conditions, here, we propose that maghemitization occurs within the anoxic portions of the plume via microbially mediated anaerobic oxidation. Mineral dissolution also occurs within the plume. Microorganisms capable of such anaerobic oxidation have been identified within other areas at the Bemidji site, but additional microbiological studies are needed to link specific anaerobic iron oxidizers with this loss of magnetization. National Science FoundationNational Science Foundation (NSF) [1742938, 1742959]; U.S. Geological Survey Environmental Health Program Published version This material is based upon work supported by the National Science Foundation under Grant Nos. #1742938 and #1742959. We thank Dr. Nicholas Seaton from the Characterization Facility, College of Science and Engineering, the University of Minnesota for his help with the SEM imaging. We thank Selcen Yokus for her tremendous help with the sampling of the magnetite packs and assistance in helping to understand the mineral packets. We thank Doug Kent, U.S. Geological Survey, for his helpful review comments. We thank the three anonymous reviewers for their helpful comments, which significantly improved this manuscript. This project was supported by the U.S. Geological Survey Environmental Health Program. Public domain – authored by a U.S. government employee

SUMMARYSoil stabilization processes aim at enhancing soil's engineering properties. Although the concept is straightforward, it involves physical and chemical changes to the subsurface that could result in local environmental changes. Compared to conventional soil stabilization methods (such as cement grouting), bio-mediated soil stabilization, such as microbial-induced calcite precipitation (MICP), offers the opportunity to minimize environmental impact, but the underlying processes need to be well understood for proper applications. Accurate characterization and long-term monitoring are paramount for the success of soil improvement, especially MICP treatments. Spectral induced polarization (SIP), an established geophysical method, has shown to be sensitive to MICP processes and products (e.g. calcite). In this work, we performed a two-phase study to explore SIP's suitability as a monitoring tool. Phase 1 involved a laboratory scale MICP study under controlled conditions and phase 2 a pilot field scale study. In the laboratory, MICP was induced through the introduction of ureolytic microorganisms, while in the field, indigenous soil microbes were stimulated to promote ureolysis. In both cases, traditional geochemical monitoring, along with spatiotemporally dense SIP monitoring, were performed. Over the course of the laboratory study, SIP successfully tracked the MICP progress as well as the calcite precipitation behaviour. Similarly, the SIP results of the field scale study showed to be sensitive to the subsurface changes in response to MICP. SIP offered spatiotemporally rich information on the MICP progress and process status. The similarity between observed signal trends in the laboratory and field in this study clearly proved that SIP signals from MICP in controlled laboratory environments can be successfully used to study field MICP applications despite scale and complexity differences.

In this work, we explore the use of biochar as a remediation agent, and the sensitivity of the spectral‐induced polarization method as a remediation monitoring aid. Biochar amended columns were fully saturated with industrial wastewater (olive oil mill waste) with very high concentration of phenols (∼2485 mg/L) and other substances. The biochar‐amended columns achieved very high removal rates of phenols compared to the control (sand only). Geophysical monitoring over the duration of the experiment (10 days) showed changes in the spectral‐induced polarization signal (imaginary conductivity) consistent with phenol removal as confirmed by geochemical monitoring. This experiment confirmed the utility of biochar as a remediation agent. Furthermore, spectral‐induced polarization can serve as long‐term, high resolution, monitoring aid in organic contaminant degradation processes.

Microbial induced carbonate precipitation (MICP) is a promising soil stabilization method performed by stimulating soil microbes that are naturally occurring and ubiquitous in soil systems. The precipitated carbonate acts as a cementation agent to bind loose soil at grain to grain contacts. MICP has been extensively tested and proven in laboratory environments, as well as in limited field trials; however, long term field applications still remain challenging, partly due to quality control and monitoring issues. Induced polarization (IP), an established geophysical method in mineral exploration, is a prime candidate for MICP monitoring and characterization. This study presents the geophysical results of a 15-day field-scale MICP project. The MICP treatment involved the injection of molasses (carbon source for microbial proliferation) and urea in a Ca2+ rich aquifer. IP monitoring successfully delineated, spatially and temporally, the propagation of MICP in the treatment area, while common resistivity measurements failed to capture any MICP related changes. Reduced hydraulic conductivity in the treatment area versus untreated area, further supports that MICP has changed the physical properties of the subsurface. Furthermore, conventional geochemical monitoring as well as X-ray diffraction analysis confirmed carbonate precipitation in samples from discrete wells in the treatment area.

Geophysical investigations documenting enhanced magnetic susceptibility (MS) within the water table fluctuation zone at hydrocarbon contaminated sites suggest that MS can be used as a proxy for investigating microbial mediated iron reduction during intrinsic bioremediation. Here, we investigated the microbial community composition over a 5-year period at a hydrocarbon-contaminated site that exhibited transient elevated MS responses. Our objective was to determine the key microbial populations in zones of elevated MS. We retrieved sediment cores from the petroleum-contaminated site near Bemidji, MN, United States, and performed MS measurements on these cores. We also characterized the microbial community composition by high-throughput 16S rRNA gene amplicon sequencing from samples collected along the complete core length. Our spatial and temporal analysis revealed that the microbial community composition was generally stable throughout the period of investigation. In addition, we observed distinct vertical redox zonations extending from the upper vadose zone into the saturated zone. These distinct redox zonations were concomitant with the dominant microbial metabolic processes as follows: (1) the upper vadose zone was dominated by aerobic microbial populations; (2) the lower vadose zone was dominated by methanotrophic populations, iron reducers and iron oxidizers; (3) the smear zone was dominated by iron reducers; and (4) the free product zone was dominated by syntrophic and methanogenic populations. Although the common notion is that high MS values are caused by high magnetite concentrations that can be biotically formed through the activities of iron-reducing bacteria, here we show that the highest magnetic susceptibilities were measured in the free-phase petroleum zone, where a methanogenic community was predominant. This field study may contribute to the emerging knowledge that methanogens can switch their metabolism from methanogenesis to iron reduction with associated magnetite precipitation in hydrocarbon contaminated sediments. Thus, geophysical methods such as MS may help to identify zones where iron cycling/reduction by methanogens is occurring.

In a column set-up, Fe modified biochar produced from date palm leaves was used to remove As (1 mg L−1) from a laboratory-prepared wastewater. The wastewater treatment process was monitored in real-time by spectral induced polarization (SIP), over a wide range of frequencies (0.01–1000 Hz). Both 5 and 10% biochar-amended columns achieved As removal exceeding 98%. The SIP parameters appear to be sensitive on As removal processes, with the recorded trend following the conventional geochemical monitoring, while offering higher temporal resolution.

This review specifically deals with the latest advances in the application of nanotechnologies and nanocomposites for remediation of arsenic (As)-contaminated water and soil. Remediation mechanisms generally include physicochemical adsorption and (photo)chemical redox reactions and filtration. Recently, various types of engineered organic/inorganic nanocomposites have been designed in membrane forms, embedded structures, or composites with extraordinary physical-chemical properties, and outstanding capacity for removal or immobilization of As in contaminated sites. In the present article, we give an overview of engineered nanomaterials developed recently (2017–2021) and their interaction mechanisms with As in contaminated water and soil. Emerging approaches include the development of bio-nanocomposites and nanomaterials that show both oxidative and adsorptive capacities. For the first time, we set out to perform a comprehensive assessment of the advantages of nanomaterials in As-contaminated soils with the focus on the mechanisms of decreasing bioavailability and leaching of As. Although great researches have been developed, serious study gaps and a new direction to future researches have been identified.

We utilise a concept of specific polarizability (cs), represented as the ratio of mineral–fluid interface polarization per pore-normalised surface area Sp, to demonstrate the influence of clay-organic interaction on complex conductivity measurements. Complex conductivity measurements were performed on kaolinite– and illite–sand mixtures as a function of varying ethanol (EtOH) concentration (10% and 20% v/v). The specific surface area of each clay type and Ottawa sand was determined by nitrogen-gas-adsorption Brunauer–Emmett–Teller method. We also calculated the porosity and saturation of each mixture based on weight loss of dried samples. Debye decomposition, a phenomenological model, was applied to the complex conductivity data to determine normalised chargeability (mn). Specific polarizability estimates from previous complex conductivity measurements for bentonite–sand mixtures were compared with our dataset. The cs for all sand–clay mixtures decreased as the EtOH concentration increased from 0% to 10% to 20% v/v. We observe similar cs responses to EtOH concentration for all sand–clay mixtures. Analysis of variance with a level of significance α = 0.05 suggests that the suppression in cs responses with increasing EtOH concentration was statistically significant for all sand–clay mixtures. On the other hand, real conductivity showed only 10% to 20% v/v changes with increasing EtOH concentration. The cs estimates reflect the sensitivity of complex conductivity measurements to alteration in surface chemistry at available surface adsorption sites for different clay types, likely resulting from ion exchange at the clay surface and associated with kinetic reactions in the electrical double layer of the clay–water–EtOH media. Our results indicate a much larger influence of specific surface area and ethanol concentration on clay-driven polarization relative to changes in clay mineralogy.

We present a new Matlab-based11Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. frequency-domain electromagnetic (EM) inversion code (FEMIC) for analysis of datasets collected using multi-frequency EM-induction instruments. The code includes routines for data filtering and calibration, forward modeling, inverse modeling, image appraisal (i.e., calculation of the depth of investigation), and visualization. A one-dimensional forward model is assumed, but two or three dimensional lateral regularization constraints can be applied during the inversion. The code can take advantage of the parallel-processing capabilities of multi-processor computers, thus facilitating efficient inversion of large datasets. Synthetic and field examples demonstrate the operation of the FEMIC code and showcase its capabilities. Source code is provided as material supplementary to this paper to allow modifications and extensions by others.

Surface complexation models are widely used with batch adsorption experiments to characterize and predict surface geochemical processes in porous media. In contrast, the spectral induced polarization (SIP) method has recently been used to non-invasively monitor in situ subsurface chemical reactions in porous media, such as ion adsorption processes on mineral surfaces. Here we compare these tools for investigating surface site density changes during pH-dependent sodium adsorption on a silica gel. Continuous SIP measurements were conducted using a lab scale column packed with silica gel. A constant inflow of 0.05 M NaCl solution was introduced to the column while the influent pH was changed from 7.0 to 10.0 over the course of the experiment. The SIP measurements indicate that the pH change caused a 38.49 ± 0.30 μS cm− 1 increase in the imaginary conductivity of the silica gel. This increase is thought to result from deprotonation of silanol groups on the silica gel surface caused by the rise in pH, followed by sorption of Na+ cations. Fitting the SIP data using the mechanistic model of Leroy et al. (Leroyet al., 2008), which is based on the triple layer model of a mineral surface, we estimated an increase in the silica gel surface site density of 26.9 × 1016 sites m− 2. We independently used a potentiometric acid–base titration data for the silica gel to calibrate the triple layer model using the software FITEQL and observed a total increase in the surface site density for sodium sorption of 11.2 × 1016 sites m− 2, which is approximately 2.4 times smaller than the value estimated using the SIP model. By simulating the SIP response based on the calibrated surface complexation model, we found a moderate association between the measured and estimated imaginary conductivity (R2 = 0.65). These results suggest that the surface complexation model used here does not capture all mechanisms contributing to polarization of the silica gel captured by the SIP data.

We performed 2D resistivity and IP measurements over a known olive oil mill waste plume at a site in western Crete, Greece. The objectives of the survey were: (1) to determine whether IP is more diagnostic in delineating the spatial extent of the plume relative to resistivity measurements alone; (2) to evaluate whether the additional information content obtained from IP is worth the effort given longer data acquisition times and higher measurement errors that inevitably characterize field IP data acquisition. Complex conductivity inversion of the field IP dataset revealed that the organic plume is characterized as a region of high electrical conductivity (real part of complex conductivity) consistent with the conceptual model for the electrical structure of a biodegraded LNAPL contaminant plume. The plume is also characterized by a region of high polarizability (imaginary part of complex conductivity) that is more localized to the known plume location (based on conventional monitoring) relative to the high conductivity region in the electrical conductivity image. This observation is attributed to the fact that electrical conductivity is more strongly controlled by hydrogeological and geological characteristics of the site that mask the response from the biodegraded plume. This result encourages the use of field IP to improve the spatial delineation of organic contamination in the subsurface. However, more laborious field procedures are required to acquire reliable field IP data and the inversion of field IP data remains more challenging than resistivity data alone.

This paper provides an update on the fast‐evolving field of the induced polarization method applied to biogeophysics. It emphasizes recent advances in the understanding of the induced polarization signals stemming from biological materials and their activity, points out new developments and applications, and identifies existing knowledge gaps. The focus of this review is on the application of induced polarization to study living organisms: soil microorganisms and plants (both roots and stems). We first discuss observed links between the induced polarization signal and microbial cell structure, activity and biofilm formation. We provide an up‐to‐date conceptual model of the electrical behaviour of the microbial cells and biofilms under the influence of an external electrical field. We also review the latest biogeophysical studies, including work on hydrocarbon biodegradation, contaminant sequestration, soil strengthening and peatland characterization. We then elaborate on the induced polarization signature of the plant‐root zone, relying on a conceptual model for the generation of biogeophysical signals from a plant‐root cell. First laboratory experiments show that single roots and root system are highly polarizable. They also present encouraging results for imaging root systems embedded in a medium, and gaining information on the mass density distribution, the structure or the physiological characteristics of root systems. In addition, we highlight the application of induced polarization to characterize wood and tree structures through tomography of the stem. Finally, we discuss up‐ and down‐scaling between laboratory and field studies, as well as joint interpretation of induced polarization and other environmental data. We emphasize the need for intermediate‐scale studies and the benefits of using induced polarization as a time‐lapse monitoring method. We conclude with the promising integration of induced polarization in interdisciplinary mechanistic models to better understand and quantify subsurface biogeochemical processes.

The distribution of shallow frozen ground is paramount to research in cold regions, and is subject to temporal and spatial changes influenced by climate, landscape disturbance and ecosystem succession. Remote sensing from airborne and satellite platforms is increasing our understanding of landscape-scale permafrost distribution, but typically lacks the resolution to characterise finer-scale processes and phenomena, which are better captured by integrated surface geophysical methods. Here, we demonstrate the use of electrical resistivity imaging (ERI), electromagnetic induction (EMI), ground penetrating radar (GPR) and infrared imaging over multiple summer field seasons around the highly dynamic Twelvemile Lake, Yukon Flats, central Alaska, USA. Twelvemile Lake has generally receded in the past 30 yr, allowing permafrost aggradation in the receded margins, resulting in a mosaic of transient frozen ground adjacent to thick, older permafrost outside the original lakebed. ERI and EMI best evaluated the thickness of shallow, thin permafrost aggradation, which was not clear from frost probing or GPR surveys. GPR most precisely estimated the depth of the active layer, which forward electrical resistivity modelling indicated to be a difficult target for electrical methods, but could be more tractable in time-lapse mode. Infrared imaging of freshly dug soil pit walls captured active-layer thermal gradients at unprecedented resolution, which may be useful in calibrating emerging numerical models. GPR and EMI were able to cover landscape scales (several kilometres) efficiently, and new analysis software showcased here yields calibrated EMI data that reveal the complicated distribution of shallow permafrost in a transitional landscape. Copyright © 2016 John Wiley & Sons, Ltd.

Time-lapse electrical resistivity imaging (ERI) combined with salt tracers can enhance our understanding of subsurface flow and transport in ditch-drained agroecosystems. In a nearly level field adjacent to a drainage ditch, we installed 192 electrodes in a 72-m2 plot along with five hydrometric stations to enable time-lapse ERI monitoring during storms. Immediately prior to a wintertime rainstorm, we applied 5 kg of KBr to a shallow trench located roughly 11 m upgradient from the ditch. Antecedent conditions prior to the storm were wet, assuring a hydrologically connected state that favored tracer dissolution and movement in the subsurface. We monitored the evolution of the tracer plume with ERI over a seven-day period. Four-dimensional (4D) delineation of precipitation and tracer transport using time-lapse ERI showed that the tracer plume moved preferentially downward through a discontinuity in the argillic horizon within the first 57 hrs. Over the following 79 hrs, the tracer plume exhibited steady, lateral movement towards the drainage ditch within a more permeable sandy horizon situated just below the argillic. A spatial moment analysis of logarithmic changes in conductivity in the sand layer revealed seepage velocities ranging from 0.0027 to 0.0482 m d−1; these values were positively correlated with hydraulic gradients, suggesting laminar flow processes typical of sandy aquifers. Increasing groundwater velocities with hydraulic gradients suggest that solute transfers from groundwater to ditch are likely to be faster in storms, especially during wet periods. Even so, predominantly slow lateral transport within the sandy horizon, even during storms, indicates that critical source areas of pollutant loss are likely within a few meters of the ditch.

Soil stabilization involves methods used to turn unconsolidated and unstable soil into a stiffer, consolidated medium that could support engineered structures, alter permeability, change subsurface flow, or immobilize contamination through mineral precipitation. Among the variety of available methods carbonate precipitation is a very promising one, especially when it is being induced through common soil borne microbes (MICP - microbial induced carbonate precipitation). Such microbial mediated precipitation has the added benefit of not harming the environment as other methods can be environmentally detrimental. Carbonate precipitation, typically in the form of calcite, is a naturally occurring process that can be manipulated to deliver the expected soil strengthening results or permeability changes. This study investigates the ability of spectral induced polarization and shear-wave velocity for monitoring calcite driven soil strengthening processes. The results support the use of these geophysical methods as soil strengthening characterization and long term monitoring tools, which is a requirement for viable soil stabilization projects. Both tested methods are sensitive to calcite precipitation, with SIP offering additional information related to long term stability of precipitated carbonate. Carbonate precipitation has been confirmed with direct methods, such as direct sampling and scanning electron microscopy (SEM). This study advances our understanding of soil strengthening processes and permeability alterations, and is a crucial step for the use of geophysical methods as monitoring tools in microbial induced soil alterations through carbonate precipitation.

Conceptual models for the geophysical responses associated with hydrocarbon degradation suggest that the long-term evolution of an oil plume will result in a more conductive anomaly than the initial contamination. In response to the Deepwater Horizon (DH) oil spill into the Gulf of Mexico in 2010, an autonomous resistivity monitoring system was deployed on Grand Terre, Louisiana, in an attempt to monitor natural degradation processes in hydrocarbon-impacted beach sediments of this island. A 48-electrode surface array with a 0.5-m spacing was installed to obtain twice-daily images of the resistivity structure of the shallow subsurface impacted by oil. Over the course of approximately 18 months, we observed a progressive decrease in the resistivity of the DH spill-impacted region. Detailed analysis of pixel/point resistivity variation within the imaged area showed that long-term decreases in resistivity were largely associated with the DH-impacted sediments. A microbial diversity survey revealed the presence of hydrocarbon-degrading organisms throughout the test site. However, hydrocarbon degradation activity was much higher in the DH-impacted locations compared to nonimpacted locations, suggesting the presence of active hydrocarbon degraders, supporting biodegradation processes. The results of this long-term monitoring experiment suggested that resistivity might be used to noninvasively monitor the long-term degradation of crude oil spills.

Conventional characterization and monitoring of hydrocarbon (HC) pollution is often expensive and time-consuming. Magnetic susceptibility (MS) has been proposed as an inexpensive, long-term monitoring proxy of the degradation of HC. We acquired repeated down hole MS logging data in boreholes at a HC-contaminated field research site in Bemidji, MN, USA. The MS data were analyzed in conjunction with redox conditions and iron availability within the source zone to better assess whether MS can serve as a proxy for monitoring HC contamination in unconsolidated sediments. The MS response at the site diminished during the sampling period, which was found to coincide with depletion of solid phase iron in the source zone. Previous geochemical observations and modeling at the site suggest that the most likely cause of the decrease in MS is the transformation of magnetite to siderite, coupled with the exhaustion of ferrihydrite. Although the temporal MS response at this site gives valuable field-scale evidence for changing conditions of iron cycling and stability of iron minerals it does not provide a simple proxy for long-term monitoring of biodegradation of hydrocarbons in the smear zone.

Landfilling remains the most attractive waste management method for solid waste. Although not the most efficient and environmental-friendly option, landfills offer a cost-efficient solution compared to other alternatives. For any landfill to be successful site selection, construction, operation, and post-closure monitoring is critical. Synergistic use of geophysical methods and traditional point sampling (e.g., borehole sampling) allows for high resolution characterization and monitoring of landfills during all stages of operation; from guided site selection, to construction integrity and waste characterization, to leachate recirculation and leak monitoring. Geophysical methods offer advantages, such as high temporal and spatial resolution, non (or minimally) invasive and cost-efficient operation, rendering them a very powerful tool for characterization, and long-term monitoring of waste disposal sites. Since geophysical methods involve the indirect imaging of the subsurface cautious implementation, including direct sampling, is needed for successful application. Multiple geophysical methods have been shown to be suitable for landfill characterization and monitoring. Electrical (resistivity, induced polarization, and self potential) and electromagnetic (transient electromagnetic methods, ground penetrating) are the common geophysical methods employed in waste management operations due to the increased conductivity of waste and leachate. Seismic methodologies can also be used to describe subsurface geology and possible waste horizons. In certain cases, magnetic measurements can also be used for the monitoring and characterization of landfills. Typically, geophysical methods are used to: spatially delineate landfills and define landfill geometry, monitor and characterize the spatial distribution of moisture, gas content, and leachate inside landfills, identify classes of buried waste based on material composition, monitor the integrity of the liner, and identify and monitor leachate leaks, and the associated contamination plumes. With this chapter we aim to introduce common geophysical methods and provide examples for application in landfills. For the geophysical methods of interest the basics principles, along with up to date references are provided, and the advantages and limitations for waste management operations are discussed.

Numerous adverse effects are associated with the accidental release of ethanol (EtOH) and its persistence in the subsurface. Geophysical techniques may permit non-invasive, real time monitoring of microbial degradation of hydrocarbon. We performed complex resistivity (CR) measurements in conjunction with geochemical data analysis on three microbial-stimulated and two control columns to investigate changes in electrical properties during EtOH biodegradation processes in porous media. A Debye Decomposition approach was applied to determine the chargeability (m), normalized chargeability (mn) and time constant (τ) of the polarization magnitude and relaxation length scale as a function of time. The CR responses showed a clear distinction between the bioaugmented and control columns in terms of real (σ′) and imaginary (σ″) conductivity, phase (ϕ) and apparent formation factor (Fapp). Unlike the control columns, a substantial decrease in σ′ and increase in Fapp occurred at an early time (within 4 days) of the experiment for all three bioaugmented columns. The observed decrease in σ′ is opposite to previous studies on hydrocarbon biodegradation. These columns also exhibited increases in ϕ (up to ~ 9 mrad) and σ″ (up to two order of magnitude higher) 5 weeks after microbial inoculation. Variations in m and mn were consistent with temporal changes in ϕ and σ″ responses, respectively. Temporal geochemical changes and high resolution scanning electron microscopy imaging corroborated the CR findings, thus indicating the sensitivity of CR measurements to EtOH biodegradation processes. Our results offer insight into the potential application of CR measurements for long-term monitoring of biogeochemical and mineralogical changes during intrinsic and induced EtOH biodegradation in the subsurface.

The demand for more efficient and economic oil recovery techniques has driven research into novel extraction approaches, including microbial enhanced methods. Microbial enhanced oil recovery (MEOR) is an underutilized technology that could significantly enhance tertiary oil recovery. Previous research has shown the spectral induced polarization (SIP) method to be sensitive to microbial degradation of hydrocarbons, so the method should therefore be sensitive to MEOR treatments. To test this hypothesis, heavy-oil-containing sands were monitored for a period of approximately six months while undergoing MEOR treatment. SIP monitoring showed significant sensitivity to biodegradation induced changes. Increases in phase and imaginary conductivity, with a polarization peak centered on approximately 3–8 Hz, were observed for the two MEOR active columns. Similarly, the normalized chargeability, an integrated parameter of a Debye decomposition analysis of the spectra, showed a linear increase in time. Chromatographic methods confirmed oil biodegradation in the active columns. The SIP responses are likely the result of microbial processes and the changes they promote to oil properties, such as altering wettability, or possibly the effect of organic acid production. The results of this experiment indicate that SIP may be a viable method of monitoring MEOR processes.

We performed complex resistivity (CR) measurements on laboratory columns to investigate changes in electrical properties as a result of varying ethanol (EtOH) concentration (0% to 30% v/v) in a sand–clay (bentonite) matrix. We applied Debye decomposition, a phenomenological model commonly used to fit CR data, to determine model parameters (time constant: τ, chargeability: m, and normalized chargeability: mn). The CR data showed a significant (P ≤ 0.001) time-dependent variation in the clay driven polarization response (~ 12 mrad) for 0% EtOH concentration. This temporal variation probably results from the clay–water reaction kinetics trending towards equilibrium in the sand–clay–water system. The clay polarization is significantly suppressed (P ≤ 0.001) for both measured phase (ϕ) and imaginary conductivity (σ″) with increasing EtOH concentration. Normalized chargeability consistently decreases (by up to a factor of ~ 2) as EtOH concentration increases from 0% to 10% and 10 to 20%, respectively. We propose that such suppression effects are associated with alterations in the electrical double layer (EDL) at the clay–fluid interface due to (a) strong EtOH adsorption on clay, and (b) complex intermolecular EtOH–water interactions and subsequent changes in ionic mobility on the surface in the EDL. Changes in the CR data following a change of the saturating fluid from EtOH 20% to plain water indicate strong hysteresis effects in the electrical response, which we attribute to persistent EtOH adsorption on clay. Our results demonstrate high sensitivity of CR measurements to clay–EtOH interactions in porous media, indicating the potential application of this technique for characterization and monitoring of ethanol contamination in sediments containing clays.

Ethanol (EtOH), an emerging contaminant with potential direct and indirect environmental effects, poses threats to water supplies when spilled in large volumes. A series of experiments was directed at understanding the electrical geophysical signatures arising from groundwater contamination by ethanol. Conductivity measurements were performed at the laboratory scale on EtOH-water mixtures (0 to 0.97 v/v EtOH) and EtOH-salt solution mixtures (0 to 0.99 v/v EtOH) with and without a sand matrix using a conductivity probe and a four-electrode electrical measurement over the low frequency range (1-1000 Hz). A Lichtenecker-Rother (L-R) type mixing model was used to simulate electrical conductivity as a function of EtOH concentration in the mixture. For all three experimental treatments increasing EtOH concentration resulted in a decrease in measured conductivity magnitude (|σ|). The applied L-R model fitted the experimental data at concentration ≤0.4v/v EtOH, presumably due to predominant and symmetric intermolecular (EtOH-water) interaction in the mixture. The deviation of the experimental |σ| data from the model prediction at higher EtOH concentrations may be associated with hydrophobic effects of EtOH-EtOH interactions in the mixture. The |σ| data presumably reflected changes in relative strength of the three types of interactions (water-water, EtOH-water, and EtOH-EtOH) occurring simultaneously in EtOH-water mixtures as the ratio of EtOH to water changed. No evidence of measurable polarization effects at the EtOH-water and EtOH-water-mineral interfaces over the investigated frequency range was found. Our results indicate the potential for using electrical measurements to characterize and monitor EtOH spills in the subsurface.

We have developed an alternative formulation for Debye decomposition of complex electric conductivity spectra, by recasting it into a new set of parameters with a close relationship to the continuous formulation for the complex conductivity method. The procedure determines a relaxation time distribution (RTD) and two frequency-independent parameters that modulate the complex conductivity spectra. These two parameters represent (1) the direct current contribution and (2) the conductivity range spanned by the low- and high-frequency limits. The distribution of relaxation times quantifies the contribution of each distinct relaxation process. Assuming that characteristic times with insignificant contributions can be ignored, a minimum set of characteristic relaxation times is determined. Each contribution can then be associated with specific polarization processes that can be interpreted in terms of electrochemical or interfacial parameters of mechanistic models derived from inverted parameters obtained from the proposed approach. Synthetic tests show that the procedure can fit spectral induced polarization (SIP) data and successfully retrieve the RTD. We have applied the procedure to laboratory SIP data from experiments with sand and oil mixtures undergoing microbial degradation of hydrocarbons. The RTD reveals evidence of a length scale at which a new polarization process takes place as a result of the biodegradation process.

Field-scale lithologic applications of complex conductivity ([Formula: see text]) imaging have been hindered by the challenges of (1) acquiring reliable induced polarization (IP) measurements and (2) obtaining reliable [Formula: see text] images from the measurements. We performed a series of 2D time domain resistivity/IP surveys at the Hanford 300 Area (Richland, Washington) where the challenge was to image the spatial distribution of two lithologic units that control the exchange between groundwater and surface water of the Columbia River. Exploiting the equivalence between time domain and frequency domain measurements of polarization, a 2D [Formula: see text] inversion (real conductivity [Formula: see text], imaginary conductivity [Formula: see text], and phase angle [Formula: see text]) was used to image the spatial distribution of [Formula: see text] across the site. Synthetic studies were carried out to investigate the effects of noise on the resolution of [Formula: see text] images and to add confidence on the interpretation of possible paleochannels observed in the field data sets. The synthetic studies show that, with increasing representative noise levels, degradation of the resolution of lithologic structures in the parameters most controlled by the IP measurements ([Formula: see text] and [Formula: see text]) is significantly greater than degradation of resolution of [Formula: see text] images. However, the acquisition of IP measurements, and the analysis of changes in [Formula: see text] and [Formula: see text] constrains the lithological interpretation of the geoelectrical data set due to the strong dependency of [Formula: see text] on lithological properties. A threshold based on [Formula: see text] measurements from cores at the site was used to estimate the elevation of the contact between the two key units, which is consistent with boreholes at the site. Variation in the elevation of this contact provides evidence of a depression in the Hanford-Ringold contact connecting the aquifer and the Columbia River; this depression likely represents a paleochannel regulating flow and transport at the site.

Continuing advancements in subsurface electrical resistivity tomography (ERT) are increasing its capabilities for understanding shallow subsurface properties and processes. The inability of ERT imaging data to resolve unique subsurface structures and the corresponding need to include constraining information remains one of the greatest limitations, yet provides one of the greatest opportunities for further advancing the utility of the method. We propose a new method of incorporating constraining information into an ERT imaging algorithm in the form of discontinuous boundaries, known values, and spatial covariance information. We demonstrated the approach by imaging a uranium-contaminated wellfield at the Hanford Site in southeastern Washington State, USA. We incorporate into the algorithm known boundary information and spatial covariance structures derived from the highly resolved near-borehole regions of a regularized ERT inversion. The resulting inversion provides a solution which fits the ERT data (given the estimated noise level), honors the spatial covariance structure throughout the model, and is consistent with known bulk-conductivity discontinuities. The results are validated with core-scale measurements, indicating a significant improvement in accuracy over the standard regularized inversion and revealing important subsurface structure known to influence flow and transport at the site.

We performed laboratory experiments to investigate the sensitivity of the Spectral Induced Polarization (SIP) method to toluene contamination in clayey soils. We used mixtures of quartzitic sand and montmorillonite as soil samples, artificially contaminated with varying amounts of toluene. Care was taken to quantify the experimental uncertainty resulting from packing since such effects must be quantified if variations in SIP signatures between samples are to be reliably interpreted in terms of the effects of hydrocarbon concentration. The SIP response of all samples following addition of toluene was monitored for a period of 40 days following sample preparation. Stepwise regression was used to examine the statistical significance of correlations between (i) clay content and (ii) toluene concentration and SIP parameters. Both single-frequency real and imaginary conductivity measurements, along with the integral chargeability, normalized chargeability, DC conductivity and time constant obtained from a Debye decomposition fitting, were examined in this regression analysis. The SIP measurements show a clear time dependence following sample preparation, indicating that samples containing toluene may take significant time to reach an equilibrium electrical response. SIP measurements are significantly related to toluene content shortly after sample preparation, when the expected dependence of SIP on clay concentration is apparently suppressed. However, for the state of electrical equilibrium after 40 days (interpreted to indicate surface chemistry at equilibrium) there is no significant relation between SIP measurements and toluene content; instead SIP measurements are then significantly correlated with clay concentration. The total chargeability, normalized chargeability and relaxation time obtained from the Debye decomposition show no correlation with toluene content, indicating that this procedure, which likely integrates over multiple mechanisms, may not be suitable for understanding relationships between SIP and hydrocarbon contamination. We find only small low-frequency polarization signals observed in relation to toluene concentration (2 mrad at 0.01 Hz), which initially decreases the interfacial polarization. Unlike earlier works, our results do not support the use of the SIP method as a tool for monitoring toluene contamination in clay soils.

In recent years, geophysical methods have been shown to be sensitive to microbial-induced mineralization processes. The spectral induced-polarization (SIP) method appears to be very promising for monitoring mineralization and microbial processes. With this work, we study the links of mineralization and SIP signals, in the absence of microbial activity. We recorded the SIP response during abiotic FeS precipitation. We show that the SIP signals are diagnostic of FeS mineralization and can be differentiated from SIP signals from biomineralization processes. More specifically, the imaginary conductivity shows almost linear dependence on the amount of FeS precipitating out of solution, above the threshold value [Formula: see text] under our experimental conditions. This research has direct implications for the use of the SIP method as a monitoring and decision-making tool for sustainable remediation of metals in contaminated soils and groundwater.

We analyze the relationship between induced polarization (IP) parameters and the specific surface area normalized to the pore volume [Formula: see text] for an extensive sample database. We find that a single linear imaginary conductivity-[Formula: see text] relation holds across a range of single-frequency IP data sets composed of sandstones and unconsolidated sediments that lack an appreciable metallic mineral content. We also apply a recent approach defined as Debye decomposition (DD) to determine normalized chargeability [Formula: see text], a global estimate of polarization magnitude from available spectral IP (SIP) data sets. A strong linear relationship between [Formula: see text] and [Formula: see text] is also found across multiple data sets. However, SIP model parameters determined for samples containing metallic minerals are approximately two orders of magnitude greater than for the model parameters estimated for the nonmetallic sample database. We propose a concept of “polarizability of the mineral-fluid interface per unit [Formula: see text]” to explain this difference, which is supported by the observed dependence of IP parameters on fluid conductivity between sample types. We suggest that this linear IP-[Formula: see text] relation can be considered the IP equivalent of the classical Archie empirical relation. Whereas the Archie relation describes a power-law relation between electrical conductivity due to electrolytic conduction through the available interconnected pore volume, the IP-[Formula: see text] relation is an equivalent relation between mineral-fluid interfacial polarization and available pore surface area.