Presentation Description
Use of Thermal Conduction Heating for the Remediation of DNAPL in Fractured Bedrock
There still does not exist an effective technology for the remediation of DNAPL in fractured bedrock systems. This is because DNAPL in fractured bedrock presents several significant challenges; including: 1) defining the area to be treated; 2) potential impacts of matrix diffusion within and downgradient of the source zone; 3) discrete nature of fracture pathways and presence of dead-ends; and 4) accessing DNAPL within the fractures and the contaminant mass in the matrix. One technology however, that may be able to overcome many of these limitations is Thermal Conduction Heating (TCH) also known as In-Situ Thermal Desorption (ISTD). ISTD is the simultaneous application of heat, by TCH, and vacuum to the subsurface to remove organic chemicals. Heat is applied by installing electrically powered heaters at regular intervals throughout the zone to be treated. The heat moves out into the inter-well regions primarily by thermal conduction. Thermal conduction heating of fractured bedrock sites is capable of: 1) achieving thorough heating of the bedrock (matrix and fractures), 2) preventing unwanted condensation of steam and CVOC vapors, and 3) capture and removal of the CVOC mass liberated from the bedrock and unconsolidated deposits.

At a site located in the southeastern part of the U.S., TCH was used to remediate a TCE DNAPL source zone that extended 90 ft below the ground surface (bgs). The upper 75 ft of the treatment zone consisted of saprolite and weathered bedrock with the water table at 55 ft bgs. The bottom 15 feet of the treatment zone consisted of fractured gneiss. The primary objective of the remedial effort was to remove the TCE (DNAPL, sorbed, dissolved, and vapor) from the unconsolidated deposits (0 to 75 ft bgs) and to achieve a 95% UCL of the mean TCE soil concentration less than 60 μg/kg. In order to accomplish this and to minimize the threat of downward mobilization of DNAPL into the fractured gneiss, the heated/treatment interval extended to 90 ft to create a “hot floor” within the upper portion of the bedrock. Thus contaminants within this interval of the bedrock were also removed. In addition to the “hot floor”, the ISTD system also utilized ground water extraction to create an upward hydraulic gradient between the bedrock and unconsolidated soils to provide added security that any DNAPL potentially mobilized during heating was prevented from migrating vertically into the bedrock.

This paper describes the various design issues associated with implementation of ISTD in or above fractured bedrock and presents first-of-its-kind data showing the thorough and rapid heatup of the bedrock by TCH. Although, pre-and post-treatment samples of the underlying bedrock were not available for laboratory analysis, soil samples from within the treatment zone show a very thorough removal of contaminants. Measured starting concentrations of TCE were as high as 81,000,000 &#956;g/kg and 1,100,000 &#956;g/L in soil and water, respectively, and DNAPL was visually observed in soil and water samples. The post-remediation 95% UCL of the mean TCE soil concentration for the entire treatment zone, above and below the water table (based on 56 discrete soil samples), was 17 &#956;g/kg. The post-treatment concentration of TCE in groundwater samples from a monitoring well within the treatment zone that had starting TCE concentrations at saturation levels (1,100,000 &#956;g/L) was reduced to <5 &#956;g/L.