Electrical Resistance HeatingICN Pharmaceutical Portland, Oregon Case Study
Chlorinated solvent contamination in groundwater at concentrations indicative of a dense non-aqueous phase (DNAPL) was discovered at property owned by ICN Pharmaceuticals in 1995. The contamination was associated with a dry well that consisted of a 20-foot deep hole backfilled with gravel. TCE, cis-1,2 DCE, and vinyl chloride were detected at concentrations up to 10% of their aqueous solubility in groundwater. Contamination was present primarily in the Overbank Aquifer which lies between 20 and 60 feet below ground surface. The underlying Troutdale Gravel Aquifer (TGA) is tapped by the City of Portland for back-up water supply and is hydraulically connected to the Overbank Aquifer.
Electrical resistive (six-phase) heating was selected to remediate the DNAPL. A system consisting of electrodes, temperature monitoring strings, vapor extraction points, pressure monitoring points, and a 950 kW transformer was installed at the site and began operating May 2000. The system was expanded twice during operation to address lateral steam and contaminant movement outside the treatment area. This lateral movement was caused by steam being trapped beneath cooler, finer-grained soils present within the Overbank deposits. Deep vents were installed within and along the southern periphery of the treatment area to remove the trapped steam. Groundwater sampling techniques were modified to protect field personnel from pressurized steam and collect representative samples of very hot water. Many of the monitoring wells at the site were damaged as a result of heat-induced deformation of PVC and CPVC casings. In November 2001, heating was terminated based on declining contaminant concentrations in the groundwater and reduced rates of contaminant removal in the extracted vapor. Subsequent groundwater sampling has revealed significantly reduced contaminant concentrations in groundwater. No DNAPL remains; concentrations over much of the site are below drinking water standards.