A test of soil heating for cold climates was conducted at the West Quartermaster's Fueling System in Operable Unit 5 of Fort Wainwright near Fairbanks, Alaska. This site had experienced a release of gasoline and diesel fuel into a gravelly sand aquifer that flows into the Chena River. Three independent study sites were established: an unheated soil vapor extraction and air sparging (SVE/AS) area, an SVE/AS area with radio frequency heating, and an SVE/AS area with electrical resistance heating (ERH). In situ sensors were used to measure oxygen, pressure, soil moisture, and temperature at each site, and data were recorded twice daily by data loggers. Soil samples were collected before heating, at the end of moderate heating, and at the end of high-temperature heating. Both radio frequency heating and ERH systems were evaluated on the basis of their ability to heat a column of soil 40 feet (12 m) in diameter and 6-18 feet (2-6 m) below grade. Soil heating was tested because the low native subsurface temperature (1-4°C) strongly inhibited both biodegradation and volatile removal. Two ranges of temperatures were tested: moderate-temperature operation that was intended to stimulate aerobic bioremediation and high-temperature operation that was intended to maximize removal as vapor.
Before the field study began, laboratory column studies were conducted on site soil to predict the effects of heat on biodegradation rates. Laboratory studies showed that although biodegradation rates were low at temperatures less than 5°C, rates more than doubled when the soil was heated to 10 to 15°C. The studies also indicated that the optimum temperature for the microbial consortia existing in the soil was about 20°C and biodegradation rates declined at temperatures above 30°C. For this reason, the goal of the moderate-temperature ERH system was to heat the site to 15 to 30°C.
Fort Wainwright - Fairbanks, Alaska - Case Study