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

摘要

A fractured sandstone site located in the U.S. Midwest was impacted beginningbefore 1970 by a multicomponent DNAPL mainly composed of 1,1,1-TCA, PCE,TCE and BTEX. A groundwater VOC plume containing the above-mentioned compoundsand degradation products extends for more than 3 km from the DNALP input area. InSeptember 2007, groundwater samples were collected for evaluation of redox parametersand VOC and carbon isotope analyses as part of a study aiming to evaluate the fate of theVOC in the plume. The VOC data indicated almost complete degradation of PCE/TCE tocis-DCE and around 80% transformation of 1,1,1-TCA to 1,1-DCA in the area from thesource to the middle of the plume which is consistent with anaerobic condition reportedfor 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 areabetween 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 and1,1,1-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 biodegradationis much less than in the middle of plume, which is consistent with the more aerobic conditionmeasured at the fringes of the plume. Degradation of 1,1,1-TCA to 1,1-DCAshowed much less degradation of TCE to cis-DCE and a clear trend of decreasing extentalong 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 isotopesanalyses significantly improved the understanding of the fate of chlorinated ethenes andethanes in a regional VOC plume developed in a fractured rock aquifer.