Full-scale removal of DNAPL constituents using steam-enhanced extraction and electrical resistance heating

In 2003, the United States Department of Energy completed a full-scale non-aqueous phase liquid (NAPL) remediation of Area A of the Northeast Site at the Young-Rainey STAR Center, Largo, Florida. Area A covered an area of 930 m 2 (10,000 square feet) and extended to a depth of 10.7 m (35 feet), representing a total cleanup volume of 9930 m 3 (12,960 cubic yards). The site was contaminated with similar to 2500 kg (5500 lb) of NAPL constituents such as trichloroethylene, cis-1,2-dichloroethylene, methylene chloride, toluene, and petroleum hydrocarbons. The site consists of a fine-grained sand aquifer underlain by a Hawthorn clay at 9 m (30 feet) depth. The upper 1.5 m (5 feet) of this clay formed part of the remediation volume, as dense non - aqueous phase liquid was present in this layer. The site was remediated using a combination of steam-enhanced extraction and electrical resistance heating. Operations lasted 4.5 months. The site was heated to the target temperatures within 6 weeks, at which time the mass removal rate increased more than 1000-fold. After the target volume had been heated to or near boiling temperatures, pressure cycles were used to increase the mass removal rates, until a final phase of diminishing returns was reached. Postoperational sampling of soil and ground water at randomly selected locations showed the concentrations of all contaminants of concern (COC) to be well below the remedial goals. The majority of the ground water samples were below maximum contaminant level (MCL) for all the COC. The estimate of volatile organic contaminant (VOC) mass removed from the site (1130 kg 2500 lb) agreed well with the estimate of VOC present before operation (1170 kg 2580 lb). The postoperational sampling showed that similar to 0.5 kg (1 pound) of VOCs remained in the remedial volume, and showed remedial efficiencies of between 99.85% and 99.99% for the four chemicals of concern. Since the postoperational sampling shows all concentrations to be below or close to ground water MCLs, the thermal remedy may be satisfactory for site closure without a polishing phase.