Fate of Chlorinated Compounds in a Fractured Sandstone Aquifer in the U.S. Midwest

摘要

A fractured sandstone site located in the U.S. Midwest was impacted beginning before 1970 by a multicomponent DNAPL mainly composed of 1,1,1-TCA, PCE, TCE and BTEX. A groundwater VOC plume containing the above-mentioned compounds and degradation products extends for more than 3 km from the DNALP input area. In September 2007, groundwater samples were collected for evaluation of redox parameters and VOC and carbon isotope analyses as part of a study aiming to evaluate the fate of the VOC in the plume. The VOC data indicated almost complete degradation of PCE/TCE to cis-DCE and around 80% transformation of 1,1,1-TCA to 1,1-DCA in the area from the source to the middle of the plume which is consistent with anaerobic condition reported for this area. The leading edge of the plume was characterized by much less degradation. The VOC degradation pattern was confirmed by the carbon isotopes data. In the area between the source and the middle of the plume, the δ13C values of the daughter product, cis-DCE and 1,1-DCA, tend to the isotope composition of the precursors, PCE/TCE and 1,1,1-TCA, which indicates that cis-DCE or 1,1-DCA are not been affected by biodegradation. The δ13C values at the fringes of the plume indicated the degree of biodegradation is much less than in the middle of plume, which is consistent with the more aerobic condition measured at the fringes of the plume. Degradation of 1,1,1-TCA to 1,1-DCA showed much less degradation of TCE to cis-DCE and a clear trend of decreasing extent along the flow path. This is also confirmed by the δ13C values of 1,1,1-TCA and 1,1-DCA. This study showed that a combined approach of VOC concentration and carbon isotopes analyses significantly improved the understanding of the fate of chlorinated ethenes and ethanes in a regional VOC plume developed in a fractured rock aquifer.