Geophysical monitoring of ISRM remediation technique in unsaturated contaminated soils by Chromium VI: Spinetta Marengo Solvay pilot test site

The results of the geophysical monitoring of the in situ redox manipulation (ISRM) technique by using sodium dithionite injections are presented. The experiment was carried out in Spinetta Marengo industrial site where some near surface chromium VI contaminated areas, in unsaturated soils, are present, due to the activities of the chrome planting facility dismantled in the early 80s. Main goal of the pilot test is to evaluate the feasibility of geophysical monitoring in support the ISRM technique in order to: i) establish the effectiveness of the dithionite injection in terms of resistivity perturbations for different injection procedures (direct-push, injection wells, multistep direct push injection), ii) delineate the geometrical and physical features of the perturbed volume by the injections, and iii) help select the location of the control boreholes, in the treated contaminated area, where a decrease in chromium VI is expected. High-resolution electrical resistivity tomography and surface and borehole georadar techniques were used. In the experiment a time-lapse geophysical approach was adopted. The geophysical monitoring was planned to have the lesser logistic interference with ISRM technique. The results indicate the monitoring feasibility and its capacity to evidence clearly the perturbed volumes by the dithionite injection. The perturbed areas are characterized, in the depth range 1-6 m, by negative resistivity perturbations ranging between -10% and -70%, as well as clear perturbations in the phases of the georadar signals. The geophysical monitoring allows to perform a robust estimate of (i) the predicted action radius of the single injection, with a maximum of 1.50 m, (ii) the treated unsaturated soil volume which ranges from about 6 m3 to 9 m3. Furthermore the geophysical monitoring of the multi-step injection test allows us to infer some consideration about the kinematics and dynamics of the injection process. The geophysical results were verified with soil core samples which were collected in the location predicted by the geophysical monitoring. The laboratory analysis of the cores indicates, in the highly perturbed areas (resistivity anomalies lesser than -30%), a lowering the content of chromium VI, with respect the background condition, up to below the threshold of detectability. In general, the total Chromium remains at same background level indicating a chemical reduction of the Chromium VI in Chromium III, which is less toxic and mobile in the environment. The Spinetta Marengo tests demonstrate that geophysical monitoring constitutes an effectual support of the used ISRM remediation technique.