Mapping and Monitoring of DNAPL Source Zones With Combined Direct Current Resistivity and Induced Polarization: A Field‐Scale Numerical Investigation

Abstract

Direct current (DC) resistivity has been widely investigated for non‐invasive mapping of dense non‐aqueous phase liquids (DNAPLs); however, due to its difficulty in distinguishing DNAPLs from adjacent soils, the DC method is limited for static detection of DNAPLs and more often employed for monitoring DNAPL changes over time. Time‐domain induced polarization (IP) can provide complementary information to better discriminate between DNAPL, water and surrounding soils. Since highly resistive DNAPLs tend to laterally spread and pool on polarizable (chargeable) clay lenses, combined DC and IP (DCIP) has the potential to enhance static detection, and also monitoring of DNAPL source zones (SZs). The objective of this study is to assess DCIP for characterizing and monitoring DNAPL SZs at the field‐scale. A new DNAPL‐DCIP numerical model was first developed that couples a 3D multiphase flow model, which simulates DNAPL release and remediation scenarios, with a 3D DCIP model, which calculates the corresponding resistivity and chargeability response. The sensitivity of the DCIP response to key DNAPL and soil properties was then analyzed at a single subsurface location, closely matching previous theoretical and experimental observations. Finally, a field‐scale simulation of DNAPL release and remediation was conducted, with simultaneous mapping by DCIP surveys. Results demonstrate that chargeability can provide enhanced understanding of the lithological distribution that controls the variability in the DNAPL SZ, with time‐lapse resistivity being used to monitor DNAPL mass changes during SZ remediation. This numerical study suggests that combined DCIP can be valuable for site characterization and time‐lapse monitoring performance at DNAPL‐impacted sites.

Experimental study of the influence of saturation on the resistivity response of sandstone

“…With the saturation increases, the conductivity of sandstone transitions from electronic conduction in the solid state to ionic conduction in the pore water. As the saturation level increases, the ionic mobility also increases, resulting in enhanced conductivity of sandstone (Almpanis et al, 2021).…”

Assessing Contaminant Mass Discharge Uncertainty With Application of Hydraulic Conductivities Derived From Geoelectrical Cross‐Borehole Induced Polarization and Other Methods

“…The aqueous, uncharged chlorinated ethene contamination (TCE, DCE) and the uncharged degradation products (VC, ethene, ethane) are expected to have no influence on the DC‐signal or on the IP‐signal. The presence of separate phase non‐aqueous phase liquids (NAPL), shown to affect DC and IP‐signals (e.g., Almpanis et al., 2021), is very unlikely in the test area far from the source. This is confirmed by the maximum observed concentrations of TCE and PCE being less than 1% of the aqueous solubility, the US EPA indication threshold for NAPL presence (Kueper & Davies, 2008 and USEPA, 1992).…”

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

“…According to several studies, presence of DNAPL might affect the electrical conductivity. For example, simulations by Almpanis et al (2021) predict that 100% mass removal of a source zone containing 1-60% of DNAPL will lead to a conductivity increase by a factor 2 after 6 years. However, DNAPL is a source zone phenomenon: in contaminant plumes, where the contaminant concentrations are below the free-phase solubility limit, no DNAPL are expected.…”