Evolution of in-situ thermal-enhanced oxidative remediation monitored by induced polarization tomography

Abstract

Traditional chemical analysis for monitoring the remediation process of contaminated soil and groundwater is limited in its spatiotemporal resolution and high cost. To overcome this shortcoming, we applied induced polarization (IP) tomograms to monitor the process of in-situ chemical oxidation coupled with thermal desorption in a field-scale NAPLs contaminated site. To compare the effectiveness of contaminant removal by different heating strategies, the contaminated site is divided into horizontal and vertical heating areas. The remediation lasted 25 days, including heating (days 1–14) and injection (days 15–25) stages. It is found that the variations in IP parameters shown in the tomograms correlate with temperature, groundwater level, oxidant transport and NAPLs removal. The resulting IP tomograms during heating reveal that continued heating of horizontal tubes and groundwater decline are dominant in IP variations within horizontal heating area, whereas temperature increase and NAPLs removal. The contaminant concentration during heating stage can be calculated based on variations in chargeability under stable groundwater level conditions, which facilitates the assessment of contaminant removal during heating. Furthermore, contaminant consumption with oxidant transport leads to a decrease in resistivity and chargeability for two heating areas during injection process. After stopping injection, there are large changes at shallow depths at 1–5 m bgs and modest changes at depths > 6 m bgs, indicating that the oxidant migrated downwards under density-driven transport. Our results demonstrate IP survey combined with hydrogeological parameters and geochemical measurement is suitable for quantifying contaminants removal during heating and identifying the migration pathway of the injected oxidant.