Cross‐borehole geoelectrical time‐lapse monitoring of in situ chemical oxidation and permeability estimation through induced polarization

Abstract

Worldwide, soil contamination due to industrial activities is a major issue. One method for remediation of contaminated sites is in situ chemical oxidation, where an oxidizing agent is injected into the contaminated soil. Normally, monitoring wells are established in the remediation area for tracking the oxidizing agent. However, wells only provide point information of the injectant spread. This issue can be addressed using cross‐borehole resistivity and induced polarization tomography, by mapping the electrical properties in the entire remediation volume and by deriving, through petrophysical relations, the hydraulic properties of the medium. Here we present a proof‐of‐concept study, performed over one year as part of a larger remediation project, where resistivity and time‐domain induced polarization data were acquired among 10 boreholes, before and after two rounds of injection of oxidizing agents. The time‐lapse resistivity models, obtained through a focusing inversion scheme that favours compact time‐lapse changes, clearly show the oxidizing agent spread as highly conductive anomalies and confirmed by water conductivity measurements in boreholes. The time‐lapse inversions also show spatial variability in the injectant spread, with some areas not reached. The induced polarization data quality decreased significantly just after the injection rounds, because of the decrease in resistivity and induced polarization signal level, so that induced polarization time‐lapse inversions were not feasible. However, the induced polarization data were used for background characterization and to estimate permeability. In particular, there is a good match between the imaged low‐permeability zones and the areas in which the injectant did not spread, identified by the time‐lapse resistivity inversions. Furthermore, geological samples confirm the presence of fine‐grained sediments in the estimated low‐permeability zones. While time‐lapse resistivity tomography may be used for documenting the injectant spread, induced polarization permeability estimates prior to injection can be used to better tailor the remediation in terms of dimension and location of injection filters.

High‐Resolution Geological Information from Crosshole Ground Penetrating Radar in Clayey Tills

“…Another cross‐borehole geophysical method, frequently used for environmental application, is electrical resistivity tomography (ERT), often used in combination with induced polarization (IP). Similarly to the crosshole GPR method, crosshole resistivity methods have been applied for various purposes such as monitoring of in‐situ remediation (e.g., Mao et al 2016; Bording et al 2021; Lévy et al 2022) and mapping of possible contaminant flow paths in clay till (Bording et al 2019). As mentioned previously, the resolution of resistivity tomography is, in contrast to GPR tomography, highest close to the boreholes (Day‐Lewis et al 2005).…”

Quantifying Reagent Spreading by Cross‐Borehole Electrical Tomography to Assess Performance of Groundwater Remediation

“…In practice, the K-distribution only weakly depends on salinity variations. Further details can be found in Bording et al (2021) and Fiandaca et al (2018b).…”

“…Cross‐borehole ERT has been successfully used to monitor the migration of saline tracers in well‐controlled tank experiments (Slater et al., 2002) and in the field (Singha & Gorelick, 2005; Wilkinson et al., 2010). It has also been installed at contaminated sites to monitor groundwater changes related to in situ remediation (Bording et al., 2021; Nivorlis et al., 2019). However, a largely missing step in surface and cross‐borehole ERT/IP investigations is to integrate these methods as impactful decision‐support tools for remediation (Singha et al., 2015).…”

“…In situ remediation methods often rely on the generation of a treatment zone (TZ), where a reactive agent is injected to intersect the contaminant plume. Examples of reactive components include strong chemical oxidants (Bording et al., 2021; Tsitonaki et al., 2010), microscale zero‐valent‐iron (mZVI; Xin et al., 2015) and electron donor and microbial enrichment cultures including specific degrading bacteria, e.g., Dehalococcoides (Scheutz et al., 2008).…”