Remediation of deep soil contamination using thermal vacuum wells

A joint Shell/General Electric field experiment is described in which a new in-situ thermal desorption soil remediation process (ISTD-Thermal Wells) is shown to remove high-boiling-point contaminants from deep soils. For this pilot, a sand pit was prepared with surrogate soil contaminants placed in a cylindrical region 9 feet in diameter and 7 feet deep. Twelve heater/vacuum wells were completed in a triangular array with a 7.25-foot well spacing. During the remediation, electrical resistance heating and vacuum were applied to the wells for a period of 70 days. Soil temperatures were monitored throughout the experiment, and soil samples were taken with a Geoprobe coring unit to observe the removal of contaminants. Energy and material balance data were also collected to improve understanding of process mechanisms. Temperatures above 500 °F were achieved in the interwell regions, and contaminants were completely removed despite large inflows of ground water that resulted from heavy rains. A second test at a PCB-contaminated site avoided most of the water influx problems and demonstrated effective heating to over 1000 °F and complete removal of the PCBs.