Remediation of Deep Soil Contamination Using Thermal Vacuum Wells (1997)

Abstract 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. Introduction The difficulty in remediating the large number of sites contaminated by toxic, carcinogenic, or radioactive chemicals has generated interest in developing improved processes for cleaning these sites. At present, the typical clean-up response is either (1) capping with an impermeable surface seal to reduce direct exposure of contaminants to human contact and leakage to aquifers, or (2) excavation of the contaminated soil, followed by ex-situ treatment or disposal at another site. In-situ processes, which either destroy contaminants in place or remove them without disturbing the soil, offer advantages over those requiring excavation, by minimizing disturbance at the surface and by reducing costs of full remediation. One of the most versatile and effective of these in-situ processes is In-Situ Thermal Desorption (ISTD), in which heat and vacuum are applied simultaneously to subsurface soils. For shallow soil contamination, an ISTD method using surface heater blankets has been developed. Recently, ISTD-Thermal Blankets have been demonstrated to he highly effective in removing PCBs from soils, and commercial remediation services are now available. For deep soil contamination, a similar thermal vacuum process using heater wells (ISTD-Thermal Wells) has been proposed. As with the thermal blanket, this process is a clean, closed system that is simple and fast. It destroys pollutants in place without having to move the soil. It can be used under roads, foundations and other fixed structures. If required, the thermal wells can be slanted or drilled horizontally. The operations are low profile, quiet, and cause little disruption of adjoining neighborhoods. The process possesses a high removal efficiency because the narrow range of soil thermal conductivities provides excellent sweep efficiency and because its high operating temperature assures complete displacement efficiency of contaminants in the gas phase. Unlike fluid injection processes, ISTD is applicable to tight soils, clay layers, or in soils with wide variations in permeability and water content. In homogeneous soils, fluid injection can be used along with heat injection to speed up the process, provided a vacuum is maintained throughout the heated region. The ISTD-Thermal Wells process is an adaptation of two oil field production methods: vacuum pumping wells to enhance light oil recovery, and well heating for increasing heavy oil production. In one application of technology, an array of heater/vacuum wells is placed vertically in the ground in triangular patterns. The wells are equipped with high-temperature electric heaters and connected to a vacuum blower. P. 905^