Soil vacuum extraction of perchloroethylene from the borden aquifer

An assessment of a conventional soil vacuum extraction (SVE) system to remo ve perchloroethylene (PCE), a dense nonaqueous phase liquid (DNAPL), was co nducted in a dewatered 9 X 9 m block of the surficial sand aquifer located at the Canadian Forces Base (CFB), Borden, Ontario. Approximately two years prior to the installation and operation of the SVE system, 1250 kg of PCE were released under saturated conditions into this experimental cell, formi ng a heterogeneous DNAPL distribution. Using ground penetrating radar and n eutron probe data in conjunction with detailed soil core data, it was deter mined that approximately 600 kg of PCE were available for removal by the SV E system after dewatering. Performance assessment of this SVE system was ba sed on soil core data and monitoring of in situ gas concentration and press ure, soil temperature, and moisture content. After the first 25 days of operation, the SVE system had removed 230 kg of PCE, during which time the off-gas concentration decreased by 70% from an i nitial concentration of close to 3500 ppm, After 432 days of operation, 63% (376 kg) of the estimated available mass was removed from the SVE target z one. In situ pressure measurements and sulphur hexafluoride tracer tests in dicated that advective air movement occurred throughout the SVE target zone . Changes to the extraction well configuration and the use of short-screene d extraction drive points did not increase the mass removal performance. After 250 days of operation, eight soil cores were removed from the test ce ll, and bulk soil concentration and soil moisture content measurements,were performed at 0.05 m increments along the length of each core. These data r evealed zones where significant quantities of PCE remained, even though the cumulative mass removal profile for the SVE system was nonzero asymptotic. In general, these zones coincided with higher moisture content zones refle ctive of the heterogeneous nature of this porous medium. This investigation provides insight into some of the limitations of convent ional SVE technology to clean up a sand aquifer contaminated with a DNAPL, given a reasonably good estimate of the before-extraction mass conditions. In particular, the findings clearly show that the heterogeneous nature of t he porous medium gives rise to a nonuniform moisture content, a heterogeneo us pure phase distribution, and the development of preferential gas phase f low pathways, As a consequence, this will in general limit this technology to remove a considerable fraction of the DNAPL mass present. The results al so shed light on the use of soil gas and bulk soil concentration data to pr ovide a representative picture of the in situ mass distribution. And finall y, an analysis of the system off-gas data showed that continuous rather tha n pulsed operation of the extraction system increased mass removal.