Advances in time-domain induced polarization monitoring with application on chlorinated solvents contamination : Towards scalable real-time geophysical monitoring

Environmental pollution is a significant concern for scientists, practitioners, authorities and the society. Among the various pollutants, chlorinated solvents pose a considerable risk to our groundwater resources. These hazardous chemicals, often used in industrial processes, can contaminate soil and water, posing a threat to both human health and ecosystems. Detecting and tracking the spread of these contaminants is crucial to prevent further damage and facilitate remediation efforts.The research focuses on developing and refining a technique called direct current resistivity and time-domain induced polarization (DCIP) monitoring, which is a geophysical method to measure the electrical properties of subsurface materials. The method can provide images of the subsurface, like medical imaging, showing the change of the electrical properties over time. By tracking those changes researchers can monitor dynamic processes in the ground. The focus of the study is to use the methodology to follow changes that happen in the ground following an in-situ bioremediation treatment of a site contaminated with chlorinated solvents.The research shows that the joint use of geophysical and hydrochemical data enhances the overall understanding of in-situ remediation processes and indicates that the ongoing remediation is successfully reducing the concentration of contaminants in the ground. While geophysical imaging can potentially provide qualitative answers in areas where it is challenging to collect water samples, follow-up mostly relies on groundwater sampling to delineate information regarding the concentration of contaminants. Furthermore, the study highlights the importance of considering seasonal variations in data interpretation, as well as the need for consistent water sampling during the same period. Geophysical imaging offers insights into the spreading of injected fluids, while groundwater chemistry data is crucial for a qualitative analysis of contaminants in the water. Together, these methods complement each other to better understand the changes occurring during in-situ remediation experiments. Also, the study demonstrates the importance of using a multimethod geophysical approach together with auxiliary data to update existing geological models and improve the understanding regarding the subsurface conditions prior to a monitoring experiment.In the rapidly evolving field of geoelectrical monitoring, managing and interpreting large volumes of data is a constant challenge. The research study presents an efficient methodology that simplifies the process of collecting, processing, and displaying geoelectrical monitoring data, making it more accessible and user-friendly for experts and stakeholders alike. One of the most interesting aspects of this research is its scalability. The newly developed methods can be readily applied to small- and large-scale monitoring projects, making it a cost-effective and practical solution for environmental protection agencies and industries alike. With the ability to track in-situ bioremediation experiments in real-time, authorities can respond more quickly to mitigate the spread of pollutants, saving precious time and resources in the process. Furthermore, the research shows great potential in other geophysical monitoring applications

Temporal filtering and time-lapse inversion of geoelectrical data for long-term monitoring with application to a chlorinated hydrocarbon contaminated site

We present a solution for long-term direct current resistivity and time-domain induced polarization (DCIP) monitoring, which consists of a monitoring system and the associated software that automates the data collection and processing. This paper describes the acquisition system that is used for remote data collection and then introduces the routines that have been developed for pre-processing of the monitoring data set. The collected data set is pre-processed using digital signal processing algorithms for outlier detection and removal; the resulting data set is then used for the inversion procedure. The suggested processing workflow is tested against a simulated time-lapse experiment and then applied to field data. The results from the simulation show that the suggested approach is very efficient for detecting changes in the subsurface; however, there are some limitations when no a priori information is used. Furthermore, the mean weekly data sets that are generated from the daily collected data can resolve low-frequency changes, making the approach a good option for monitoring experiments where slow changes occur (i.e. leachates in landfills, internal erosion in dams, bioremediation). The workflow is then used to process a large data set containing 20 months of daily monitoring data from a field site where a pilot test of in situ bioremediation is taking place. Based on the time-series analysis of the inverted data sets, we can detect two portions of the ground that show different geophysical properties and that coincide with the locations where the different fluids were injected. The approach that we used in this paper provides consistency in the data processing and has the possibility to be applied to further real-time geophysical monitoring in the future

Multimethod characterization of a chlorinated solvents contaminated site and geoelectrical monitoring of in-situ bioremediation

Soil contamination is a widespread problem and actions need to be taken in order toprevent damage to the groundwater and the life around the contaminated sites. InSweden more than 80.000 sites are potentially contaminated, therefore there is ademand for accurate and efficient methods for site characterization and soilremediation. In the past, the preferred methodology for soil remediation involvedthe excavation of the contaminated mass which was either deposited in landfills (digand dump) or treated elsewhere (dig and treat). However, these techniques areassociated with significant high risk (secondary exposure) and long-term costs. Onthe other hand, in-situ bioremediation has the potential to address these issuesoffering a safer, more sustainable and cost-efficient alternative for soil remediation.Unfortunately, monitoring the progress of in-situ treatments requires soil/watersampling and laboratory analysis, which, if done frequently, can increase the costdramatically. For this reason, there is a demand for new methodologies that can beused to follow the progress of in-situ bioremediation.The work presented in this thesis involves a former dry-cleaning facility located inAlingsås (Sweden). The site is contaminated with chlorinated solvents and a pilotin-situ bioremediation plan was launched in November 2017. First, we adapted amultimethod approach for site characterization using several methods: DirectCurrent resistivity and time-domain Induced Polarization (DCIP), SeismicRefraction Tomography (SRT) and the Membrane Interface Probe (MIP). The aimwas to build a refined geological conceptual model. Second, we developed anautonomous and fully automated system for geophysical monitoring with the DCIPmethod that aims to follow the daily changes in the subsurface. We present acomplete workflow that includes data acquisition, pre-processing, inversion andvisualization of the daily DCIP monitoring data. The proposed scheme is robust andshows that DCIP monitoring has great potential to record the changes due to thebioremediation; however, it needs to be paired with more information (temperature,geochemistry, contaminant concentrations) to better understand the changes thattake place in the subsurface

Combined DC resistivity and induced polarization (DC-IP) for mapping the internal composition of a mine waste rock pile in Nova Scotia, Canada

Mine waste rock piles (WRPs) can contain sulfidic minerals whose interaction with oxygen and water can generate acid mine drainage (AMD). Thus, WRPs can be a long-term source of environmental pollution. Since the generation of AMD and its release into the environment is dependent on the net volume and bulk composition of waste rock, effective characterization of WRPs is necessary for successful remedial design and monitoring. In this study, a combined DC resistivity and induced polarization (DC-IP) approach was employed to characterize an AMD-generating WRP in the Sydney Coalfield, Nova Scotia, Canada. Two-dimensional (2D) DC-IP imaging with 6 survey lines was performed to capture the full WRP landform. 2D DC results indicated a highly heterogeneous and moderately conductive waste rock underlain by a resistive bedrock containing numerous fractures. 2D IP (chargeability) results identified several highly-chargeable regions within the waste, with normalized chargeability delineating regions specific to waste mineralogy only. Three-dimensional (3D) DC-IP imaging, using 17 parallel lines on the plateau of the pile, was then used to focus on the composition of the waste rock. The full 3D inverted DC-IP distributions were used to identify coincident and continuous zones (isosurfaces) of low resistivity (0.4 mS/m) that were inferred as generated AMD (leachate) and stored AMD (sulfides), respectively. Integrated geological, hydrogeological and geochemical data increased confidence in the geoelectrical interpretations. Knowledge on the location of potentially more reactive waste material is extremely valuable for improved long-term AMD monitoring at the WRP

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

Monitoring internal erosion in embankment dams using 3D Electrical Resistivity Tomography: Älvkarleby test embankment dam

One major risk threatening embankment dam integrity is internal erosion of the core. Internal erosion progresses inside the dam but it is difficult to detect with conventional methods. Electrical Resistivity Tomography (ERT) is a potential-based method that can sense the interior of the dam, and this study aims to evaluate the capability of ERT as a complementary monitoring technique for discovering internal erosion. A test embankment dam with some defects incorporated inside the core and fine filter in Älvkareby, Sweden, has been constructed with the purpose of assessing different monitoring techniques including ERT. Buried electrodes and sensors for other monitoring instruments were placed inside the dam. The collected ERT data were inverted using a 3D time-lapse inversion model implemented in pyGIMLI/pyBERT package. The results revealed a layered resistivity structure in the core and several unintentional anomalous zones. Furthermore 2 out of 5 defects in the core were located, namely horizontal and vertical crushed rock zones, with a slight location shift for the horizontal zone. A concrete block defect was indicated, although not as distinctly and with a lateral shift. The two remaining defects, a crushed rock zone at the abutment and a wooden block were not discovered

Pre-study for geoelectrical monitoring for detection of internal defects and anomalous seepage in the Älvkarleby test embankment dam

Electrical resistivity tomography (ERT) can be used to monitor the interior of hydropower embankments dams, and thereby detect zones with anomalous material properties and flow induced variation in the resistivity caused by changes in total dissolved solids (TDS) and temperature. Furthermore, monitoring of embankment dams in connection with a substantial change in the reservoir water level can detect anomalous leakage paths via differential wetting of zones with different hydraulic properties. In Sweden, where the available hydropower energy capacity is utilised, installation of electrodes must be done post-construction of embankment dams, which for practical reasons generally means installed along its crest, in the top of the core, using a 2D ERT approach. This has the advantage of focusing the sensitivity to the core itself, which is the part of the dam that shall stay impervious over time. However, the orientation of the electrode layout in combination with the 2D approximation leads to severe 3D effects, which distorts the inverted model resistivities and geometry. Furthermore, the resolution decreases with depth, which is a major limitation for high dams. A way ahead would be if electrodes could be installed on deeper levels inside the dam close to the core, which might be possible using modern drilling technology. This electrode lay-out concept was investigated with numerical modelling using extended gradient, cross-line bipole-bipole and corner arrays between horizontal-horizontal, vertical-vertical and vertical-horizontal lines respectively. To interpret the data 3D inversion is required to handle the structure’s geometry due to the zoned construction with materials that have large contrasts in resistivity. A test embankment dam installation was built during the autumn of 2019, with electrodes and various sensors installed inside the dam to evaluate the applicability of the suggested approach. We present results of numerical modelling simulating potential defect scenarios where several measurement sequences of close to 8,000 data points using the abovementioned arrays are inverted all at once. In order to resolve subtle variations inside the core, the finite element grid design is based on prior knowledge about the internal material distribution, with broken smoothness constrains at known material boundaries. In combination with region-based control of the resistivities of the different material zones, the inversion in combination with a time-lapse approach it shows promising results