Evaluating four-dimensional time-lapse electrical resistivity tomography for monitoring DNAPL source zone remediation

The physicochemical interacting mechanisms and real-time spectral induced polarization monitoring of lead remediation by an aeolian soil

Compared to traditional lead-remediating materials, natural-occurring paleosol is ubiquitous and could be a promising alternative due to its rich content in calcite, a substance known for its lead-removal ability via carbonate dissolution-PbCO3 precipitation process. Yet, the capability of paleosol to remediate aqueous solutions polluted with heavy metals, lead included, has rarely been assessed. To fill this gap, a series of column permeation experiments with influent Pb2+ concentrations of 2000, 200, and 20 mg/L were conducted and monitored by the spectral induced polarization technique. Meanwhile, the SEM-EDS, XRD, XPS, FTIR and MIP tests were carried out to unveil the underlying remediation mechanisms. The Pb-retention capacity of paleosol was 1.03 mmol/g. The increasing abundance of Pb in the newly-formed crystals was confirmed to be PbCO3 by XRD, SEM-EDS and XPS. Concurrently, after Pb2+ permeation, the decreasing calcite content in paleosol sample from XRD test, and the appearance of Ca2+ in the effluent confirmed that the dissolution of CaCO3 followed by the precipitation of PbCO3 was the major mechanism. The accumulated Pb (i.e., the diminished Ca) in paleosol was inversely proportional (R2 >0.82) to the normalized chargeability (mn), an SIP parameter denoting the quantity of polarizable units (primarily calcite).

Characterization of solid waste deposit using electrical resistivity tomography and time domain induced polarization

The whopping increase in solid waste landfills poses serious threats to the environment. Compared to the drilling method, geophysical methods are effective, non-invasive techniques for delineating the contaminant distribution. In this study, electrical resistivity tomography (ERT) and induced polarization (IP) were used to investigate a solid waste deposit. The results of ERT/IP imaging illustrate the potential of the method in environmental studies. Based on the results of 21 survey lines, geo-electrical signals can be summarized as three types: with only high resistivity for construction & demolition wastes (CDWs) areas (RO type), contaminated soil for high chargeability (CO type), and contaminants under CDWs layer have both high resistivity and chargeability (RC type). Chargeability values over 10.2 mV/V correspond to contaminated soil with an overall concentration larger than 75 mg/kg. With the three-dimensional interpolation results and the determined chargeability criteria, the total volume of contaminated soil is 40,555 cubic meters. Finally, comparing the efficiency, cost and results of IP and drilling sampling methods shows that the IP is an efficient, low-cost and high-resolution contamination characterization. The results support that ERT/IP information can fulfill rapid and initial identification as a reliable tool in engineering and environmental investigations.

Multidisciplinary monitoring of an in-situ remediation test of chlorinated solvents

Pollutions on and within the underground poses risks for groundwater contamination and is a widespread global problem. Common remediation methods based on digging and removal can be expensive and have limitations, while in-situ remediation is an attractive alternative. However, there is a need to develop tools to monitor the effectiveness both in terms of the successful injection of remediation fluids but also the effectiveness of the treatment, i.e., degree of degradation/removal of the pollutants and possible metabolites. This paper presents a methodology for monitoring the changes following an in-situ remediation treatment of a site contaminated with chlorinated solvents. The methodology consists of two different methods, where Direct Current resistivity and time-domain Induced Polarization (DCIP) was used to acquire daily data and geochemical analyses on water samples were collected approximately every three months. The geophysical results provide insights on how the injected fluids are spreading and assist in acquiring a better understanding of the geological and hydrogeological system. On the other hand, the geochemical sampling enhances our knowledge about the hydrochemistry of the system and the concentration of the pollutants. Our research highlights the challenges of monitoring in-situ bioremediation experiments in complex environments and in cases where pollutants are situated in low hydraulic conductivity formations. The joint interpretation of the data shows the importance of an interdisciplinary approach to understand complex systems.

Research on the detection of leakage points in vertical barrier walls using a combined method of ERT and tracer methods

In order to improve the detection accuracy of vertical barrier leakage, three contamination leakage working conditions, including point leakage only, point and vertical leakage, and horizontal leakage, were simulated by small-scale soil tank tests. The dynamic evolution of soil resistivity over time was monitored by electrical resistivity tomography (ERT). The accuracy of the ERT detection results was validated through thermal tracer method, chloride tracer method, and soil true resistivity experiments. The results indicate that the resistivity profiles at different times can more accurately reflect information on the location of leakage points, the extent of contamination plumes, and the migration pathways of pollutants under different working conditions. The extent of anomalous areas in resistivity profiles is a crucial factor in representing the geometric shape of pollution leakage. However, the preferential seepage or lateral migration of contaminant in the soil significantly reduces the detection accuracy of ERT for identifying leakage points. The thermal tracer method and the chloride tracer method can produce better complementary interpretations of ERT monitoring results. The measurement points near the leakage point exhibit faster temperature response rates, which can serve as a characteristic for identifying the location of leakage points. Compared with the thermal tracer method, the chloride tracer method can monitor the migration of contaminants over a larger range. Therefore, the proposed combined diagnostic detection method in this paper presents a feasible solution with promising engineering applications in leakage detection for vertical barrier barriers.

DNAPL flow and complex electrical resistivity evolution in saturated porous media: A coupled numerical simulation

Induced Polarization (IP) is a non-intrusive geophysical method to monitor Dense Non-Aqueous Phase Liquid (DNAPL) contamination and remediation processes underground. In this study, an advanced numerical code simulating DNAPL flow and complex electrical resistivity is presented. The model was validated against existing IP results and image measurements that were carried out previously in a series of 2D tank experiment. Multiphase flow modeling in porous media is coupled with electrical current modeling to simulate the process of DNAPL migration and the associated IP response. This brings a broader view of the contamination in space and time compared to surface and borehole measurements, especially when the results are supported by field measurements or laboratory experiments. The simulations are developed in 3D and are performed in COMSOL Multiphysics®. The simulations using petrophysical relationships for in-phase and quadrature resistivity and the results of the experiments are in complete accordance with each other in the parts of the tank where the saturation of DNAPL is relatively low (i.e. especially in the cone of depression in the pumping scenario). However, the parts associated with high saturation of DNAPL show high errors between the in-phase resistivity simulations and the results from experiments. The present work can be regarded as a preliminary study toward further applications of coupled IP-multiphase flow for more accurate detection and monitoring of DNAPLs. It is suggested that the choice of tool/approach in this study be extended to larger-scale studies for further investigation.

Integrating hydraulic tomography, electrical resistivity tomography, and partitioning interwell tracer test datasets to improve identification of pool-dominated DNAPL source zone architecture

High-resolution characterization of complex dense non-aqueous phase liquid (DNAPL) contaminated sites is crucial for developing effective remediation strategies. The DNAPL source zone is usually characterized by hydraulic/partitioning tracer tomography (HPTT). However, the HPTT method may fail to capture the highly saturated pool-dominated DNAPL source zone architecture (SZA), because partitioning tracers tend to bypass the low-permeability zones where the pool DNAPL accumulates, resulting in a low-resolution DNAPL estimation. With a limited number of measurements, the estimation errors may be significant. To overcome these difficulties, time-lapse electrical resistivity tomography (ERT) was integrated with the partitioning interwell tracer test (PITT) and hydraulic tomography (HT) to characterize the pool-dominated DNAPL SZA. Herein, we proposed an iterative joint inversion framework coupling the multiphase flow model with the ERT forward model to estimate the heterogeneous permeability distribution and DNAPL SZA. Under this framework, permeability was estimated using the hydraulic head data from HT in stage 1, and the DNAPL SZA was subsequently estimated by assimilating both the PITT and ERT observations in stage 2. The permeability estimated from stage 1 was used as prior information for stage 2, and the DNAPL saturation estimated from stage 2 was served as prior information for stage 1 in the next loop to form an iterative loop to improve the estimation of both permeability and DNAPL SZA. The iterative joint inversion framework was evaluated in two numerical experiments with different heterogeneous structures by assimilating multi-source datasets, including hydraulic head, partitioning interwell tracer concentration, and electrical resistivity. Results show that with limited measurements of HPTT method, one can roughly capture the DNAPL distribution, missing the fine structure of the DNAPL SZA. In contrast, by incorporating multi-source datasets, the heterogeneous permeability and DNAPL SZA can be reconstructed with a higher resolution. Furthermore, the inversion accuracy of the DNAPL SZA improves progressively as the iteration proceeds, which demonstrates the advantage of utilizing complementary information from permeability and DNAPL distribution through the iteration framework. Comparing with the results without loop iteration, the estimation error is reduced by 17.3% for permeability and 8.6% for DNAPL saturation in Experiment 1; by 14.7% for permeability and 11.2% for DNAPL saturation in Experiment 2 through the iterative framework. To further evaluate our framework, we preformed the prediction of the depletion process of the DNAPL source zone and plume based on the estimated DNAPL SZA. Results show that using the iterative framework, the prediction of the SZA depletion is greatly improved, i.e., the estimation error of the dissolved downstream plume from the DNAPL source zone after 3 years is reduced by 20.9% in Experiment 1, and by 43.2% in Experiment 2, respectively, through the iterative framework. This significant improvement is because the iterative method can better capture the spread of DNAPL pool.

Characterizing natural degradation of tetrachloroethene (PCE) using a multidisciplinary approach

A site in mid-western Sweden contaminated with chlorinated solvents originating from a previous dry cleaning facility, was investigated using conventional groundwater analysis combined with compound-specific isotope data of carbon, microbial DNA analysis, and geoelectrical tomography techniques. We show the value of this multidisciplinary approach, as the different results supported each interpretation, and show where natural degradation occurs at the site. The zone where natural degradation occurred was identified in the transition between two geological units, where the change in hydraulic conductivity may have facilitated biofilm formation and microbial activity. This observation was confirmed by all methods and the examination of the impact of geological conditions on the biotransformation process was facilitated by the unique combination of the applied methods. There is thus significant benefit from deploying an extended array of methods for these investigations, with the potential to reduce costs involved in remediation of contaminated sediment and groundwater.

Integrated hydrogeophysical modelling and data assimilation for geoelectrical leak detection

Time-lapse electrical resistivity tomography (ERT) measurements provide indirectobservations of hydrological processes in the Earth's shallow subsurface at high spatial and temporal resolution. ERT has been used in the past decades to detect leaks and monitor the evolution of associated contaminant plumes. Specifically, inverted resistivity images allow visualization of the dynamic changes in the structure of the plume. However, existing methods do not allow the direct estimation of leak parameters (e.g. leak rate, location, etc.) and their uncertainties. We propose an ensemble-based data assimilation framework that evaluates proposed hydrological models against observed time-lapse ERT measurements without directly inverting for the resistivities. Each proposed hydrological model is run through the parallel coupled hydro-geophysical simulation code PFLOTRAN-E4D to obtain simulated ERT measurements. The ensemble of model proposals is then updated using an iterative ensemble smoother. We demonstrate the proposed framework on synthetic and field ERT data from controlled tracer injection experiments. Our results show that the approach allows joint identification of contaminant source location, initial release time, and solute loading from the cross-borehole time-lapse ERT data, alongside with an assessment of uncertainties in these estimates. We demonstrate a reduction in site-wide uncertainty by comparing the prior and posterior plume mass discharges at a selected image plane. This framework is particularly attractive to sites that have previously undergone extensive geological investigation (e.g., nuclear sites). It is well suited to complement ERT imaging and we discuss practical issues in its application to field problems.

Polyelectrolyte-grafted Ti3C2-MXenes stable in extreme salinity aquatic conditions for remediation of contaminated subsurface environments

Polyelectrolyte-grafted Ti₃C₂-MXenes display high colloidal stability and low adsorption to mineral substrates in extreme salinity aquatic media, while maintaining decent removal efficiency for aqueous organic dyes.

Multidisciplinary Characterization of Chlorinated Solvents Contamination and In-Situ Remediation with the Use of the Direct Current Resistivity and Time-Domain Induced Polarization Tomography

Soil contamination is a widespread problem and action needs to be taken in order to prevent damage to the groundwater and the life around the contaminated sites. In Sweden, it is estimated that more than 80,000 sites are potentially contaminated, and therefore, there is a demand for investigations and further treatment of the soil. In this paper, we present the results from a methodology applied in a site contaminated with chlorinated solvents, for characterization of the contamination in order to plan the remediation and to follow-up the initial step of in-situ remediation in an efficient way. We utilized the results from three different methods; membrane interface probe for direct measurement of the contaminant concentrations; seismic refraction tomography for investigating the depth to the bedrock interface; and direct current resistivity and time-domain induced polarization tomography to acquire a high-resolution imaging of the electrical properties of the subsurface. The results indicate that our methodology is very promising in terms of site characterization, and furthermore, has great potential for real-time geophysical monitoring of contaminated sites in the future.

The fate of DNAPL contaminants in non-consolidated subsurface systems - Discussion on the relevance of effective source zone geometries for plume propagation

Dense non-aqueous phase liquids, i.e., DNAPLs and the evolving contaminant plumes in aquifers provide significant potential to pose hazards affecting both environment and human health. Therefore, a proper assessment of contaminant spreading within the subsurface is critical. This includes a sufficient characterization of governing parameters describing both the subsurface and the contaminant itself. Thereby, knowledge on the contaminant source zone and especially the source zone geometry, i.e., SZG is critically required, yet very uncertain. This study identifies current limitations and open research questions in the formation and shape determination of source zone geometry, as well as its relevance for contaminant plumes. Our literature review reveals that existing characterization methods are subject to data interpretation uncertainties, while the application of these methods on field scale is limited by technical demands and accompanied efforts. In a next step, methods to implement increased source zone information into calculation methods are discussed. By means of an exemplary application of selected assessment tools, i.e., plume response models, results clearly proof the relevance of SZGs for site assessment. However, existing plume response models consider over-simplified geometries that may compromise their suitability. Our findings identify the demand for improved characterization of complex SZGs and the need to better evaluate the dependency of DNAPL migration on system properties and external influences. With emphasized knowledge on the most relevant SZG features, the delineation of "effective" SZGs allowing for straightforward implementation into plume response models and an adaption of the latter to incorporate more information on SZGs should be possible.

Time-Lapse 3D Electric Tomography for Short-time Monitoring of an Experimental Heat Storage System

A borehole thermal energy storage living lab was built nearby Torino (Northern Italy). The aim of this living lab is to test the ability of the alluvial deposits of the north-western Po Plain to store the thermal energy collected by solar panels. Monitoring the temperature distribution induced in the underground and the effectiveness of the heat storage in this climatic context is not an easy task. For this purpose, different temperature evolution strategies are compared in this paper: Local temperature measurements, numerical simulations and geophysical surveys. These different approaches were compared during a single day of operation of the living lab. The results of this comparison allowed to underline the effectiveness of time-lapse 3D electric resistivity tomography as a non-invasive and cost-effective qualitative heat monitoring tool. This was obtained even in a test site with unfavorable thermo-hydrogeological conditions and high-level anthropic noise. Moreover, the present study demonstrated that, if properly calibrated with local temperature values, time-lapse 3D electric resistivity tomography also provides a quantitative estimation of the underground temperature.

Numerical prediction of the long-term evolution of acid mine drainage at a waste rock pile site remediated with an HDPE-lined cover system

Remediation at former mining sites containing waste rock piles (WRPs) commonly involves the installation of a cover system over the waste rock to limit water and oxygen ingress and attenuate the impacts of acid mine drainage (AMD) to the environment. Cover systems containing high-density polyethylene (HDPE) liners have the attributes to be highly effective; however, their performance over the long-term is unknown. The objective of this study was to assess the long-term effectiveness of an 'in-service' HDPE-lined cover system for reducing AMD contamination at WRP sites. A numerical investigation of a former mining site containing a large WRP reclaimed with an HDPE cover is presented. A 3-D groundwater flow and contaminant transport model of the site was developed in FEFLOW to predict the spatial and temporal evolution of AMD over 100 years. Field parameters observed at 46 monitoring wells over a 5-year monitoring period (including hydraulic head, recharge, hydraulic conductivity and water quality) were used as key input and calibration parameters. The HDPE cover significantly reduced both water recharge to the waste rock (i.e., 512 to 50 mm/year) and AMD seepage to groundwater. Both the groundwater flow and contaminant transport (sulfate was used as an AMD tracer) components of the model were calibrated and verified to the observed field data, with strong correlations evident between observed and simulated hydraulic heads and sulfate concentrations, respectively. Long-term model predictions of AMD evolution indicated significant and continual reductions in sulfate concentrations over time at all well locations. Background concentration levels (25 mg/L) are expected to be reached within 40 years. This study has demonstrated that HDPE-lined cover systems can be highly effective in reducing AMD loading from WRPs and its impacts on the receiving environment.

Mechanism for detecting NAPL using electrical resistivity imaging

The detection of non-aqueous phase liquid (NAPL) related impacts in freshwater environments by electrical resistivity imaging (ERI) has been clearly demonstrated in field conditions, but the mechanism generating the resistive signature is poorly understood. An electrical barrier mechanism which allows for detecting NAPLs with ERI is tested by developing a theoretical basis for the mechanism, testing the mechanism in a two-dimensional sand tank with ERI, and performing forward modeling of the laboratory experiment. The NAPL barrier theory assumes at low bulk soil NAPL concentrations, thin saturated NAPL barriers can block pore throats and generate a detectable electrically resistive signal. The sand tank experiment utilized a photographic technique to quantify petroleum saturation, and to help determine whether ERI can detect and quantify NAPL across the water table. This experiment demonstrates electrical imaging methods can detect small quantities of NAPL of sufficient thickness in formations. The bulk volume of NAPL is not the controlling variable for the amount of resistivity signal generated. The resistivity signal is primarily due to a zone of high resistivity separate phase liquid blocking current flow through the fully NAPL saturated pores spaces. For the conditions in this tank experiment, NAPL thicknesses of 3.3cm and higher in the formation was the threshold for detectable changes in resistivity of 3% and greater. The maximum change in resistivity due to the presence of NAPL was an increase of 37%. Forward resistivity models of the experiment confirm the barrier mechanism theory for the tank experiment.

Tomographic inversion of time-domain resistivity and chargeability data for the investigation of landfills using a priori information

In this paper, we present a new code for the modelling and inversion of resistivity and chargeability data using a priori information to improve the accuracy of the reconstructed model for landfill. When a priori information is available in the study area, we can insert them by means of inequality constraints on the whole model or on a single layer or assigning weighting factors for enhancing anomalies elongated in the horizontal or vertical directions. However, when we have to face a multilayered scenario with numerous resistive to conductive transitions (the case of controlled landfills), the effective thickness of the layers can be biased. The presented code includes a model-tuning scheme, which is applied after the inversion of field data, where the inversion of the synthetic data is performed based on an initial guess, and the absolute difference between the field and synthetic inverted models is minimized. The reliability of the proposed approach has been supported in two real-world examples; we were able to identify an unauthorized landfill and to reconstruct the geometrical and physical layout of an old waste dump. The combined analysis of the resistivity and chargeability (normalised) models help us to remove ambiguity due to the presence of the waste mass. Nevertheless, the presence of certain layers can remain hidden without using a priori information, as demonstrated by a comparison of the constrained inversion with a standard inversion. The robustness of the above-cited method (using a priori information in combination with model tuning) has been validated with the cross-section from the construction plans, where the reconstructed model is in agreement with the original design.

Investigation of chlorinated solvent pollution with resistivity and induced polarization

Globally, an enormous number of polluted areas are in need of remediation to prevent adverse effects on health and environment. In situ remediation and especially the monitoring thereof needs further development to avoid costly and hazardous shipments associated with excavation. The monitoring of in situ remediation actions needs easier and cheaper nondestructive methods for evaluation and verification of remediation degree and degradation status of the contaminants. We investigate the Direct Current resistivity and time-domain Induced Polarization tomography (DCIP) method and its use within the context of a DNAPL (Dense Non-Aqueous Phase Liquids) contaminated site in Varberg, Sweden, where an in situ remediation pilot test has been performed by stimulated reductive dechlorination by push injection. Our results show that the DCIP technique is an emerging and promising technique for mapping of underground structures and possibly biogeochemical spatial and temporal changes. The methodology could in combination with drilling, sampling and other complementary methods give an almost continuous image of the underground structures and delineation of the pollutant situation. It can be expected to have a future in monitoring approaches measuring time lapse induced polarization (IP), if more research is performed on the parameters and processes affecting the IP-signals verifying the interpretations. The IP technique can possibly be used for verification of the effectiveness of in situ remediation actions, as the current sampling methodology is inadequate.