Formatted Title
Remediation of Chlorinated Solvents with Electrical Resistance Heating at an Active Industrial Site in Italy
Background/Objectives
Persistently high concentrations of chlorinated compounds were detected at an active facility in Italy where an active pump and treat (P&T) system had been in place since the early 2000s. The remedial plan for the Site was to leverage the significant distance between the primary impacts and the property boundary by coupling an application of electrical resistance heating (ERH), targeting the contaminant source zone, with natural attenuation processes to address residual groundwater impacts.
The application of ERH was technically challenging due to a variety of factors: the source zone was located under and adjacent to an active manufacturing facility with utilities and active municipal sewage lines dissecting the treatment volume; historic subsurface structures (concrete basins) were discovered during subsurface installation; building ambient air concentrations had to be maintained below occupational thresholds; and a multinational network of stakeholders and service providers had limited access to the Site due to the COVID-19 pandemic.
Approach/Activities
A pre-design investigation was performed which included high resolution site characterization using a membrane interface probe to delineate impacts; resistivity analyses to confirm ERH suitability; and oedometric tests to confirm heating would not result in significant subsidence.
A network of 38 co-located heating and vapor extraction points, 19 horizontal screened extraction wells (placed exclusively on the outside of the building), five multiphase extraction wells, six temperature monitoring points, and associated power delivery, extraction, and treatment infrastructure was installed in 2020, and operated for approximately six months. Additionally, a building monitoring plan was implemented above the treatment volume to confirm the absence of deformation in the structure during soil heating. This included the continuous measurement of the inclination of selected pillars and the relative displacement between pillars and external paneling, by means of six biaxial clinometers and five pairs of crack gauges.
During the application of ERH, several parameters were periodically measured to monitor remedial progress including temperature of the heated formation, liquid and vapor extraction rates (both at individual extraction points and cumulatively at the inlet of the treatment system), concentrations of contaminants in extracted liquid and vapor, and power and potable water quantities delivered to each electrode.
Results/Lessons Learned
The site was heated to an average temperature of 97°C (up to 105°C in the most impacted area). The site-wide target temperature of 88°C was achieved after 126 days of operations, after which diminishing returns in mass recovery were observed, and approximately 600 kg of contaminants were recovered. In unsaturated soils, the highest concentrations decreased by 97%, while those in saturated soils decreased by more than 99%. The contaminant removal efficiency was lower in samples collected beneath the operational building (16 to 49%) where no horizontal extraction wells were installed and lower temperatures were achieved in the soil. This suggests the importance of both localized extraction and temperature performance in the achievement of remedial outcomes.
Ultimately, the implementation of ERH at the site should result in an earlier cessation of the P&T system than could be expected without source zone treatment. Minimizing the operational period of the P&T system may result in an increase of the lifecycle sustainability of the remediation process, reducing the overall use of resources and costs.