Centre for Hydrogeology and Geothermics (CHYN), University of Neuchâtel , Rue Emile Argand 11, CH-2000 Neuchâtel, Switzerland.
Environ Sci Technol. 2011 Oct 1;45(19):8247-53. doi: 10.1021/es201415k. Epub 2011 Sep 15.
Compound-specific isotope analysis (CSIA) can potentially be used to relate vapor phase contamination by volatile organic compounds (VOCs) to their subsurface sources. This field and modeling study investigated how isotope ratios evolve during migration of gaseous chlorinated ethenes across a 18 m thick unsaturated zone of a sandy coastal plain aquifer. At the site, high concentrations of tetrachloroethene (PCE up to 380 μg/L), trichloroethene (TCE up to 31,600 μg/L), and cis-1,2-dichloroethene (cDCE up to 680 μg/L) were detected in groundwater. Chlorinated ethene concentrations were highest at the water table and steadily decreased upward toward the land surface and downward below the water table. Although isotopologues have different diffusion coefficients, constant carbon and chlorine isotope ratios were observed throughout the unsaturated zone, which corresponded to the isotope ratios measured at the water table. In the saturated zone, TCE became increasingly depleted along a concentration gradient, possibly due to isotope fractionation associated with aqueous phase diffusion. These results indicate that carbon and chlorine isotopes can be used to link vapor phase contamination to their source even if extensive migration of the vapors occurs. However, the numerical model revealed that constant isotope ratios are only expected for systems close to steady state.
化合物特定同位素分析(CSIA)可用于将挥发性有机化合物(VOC)气相污染与其地下源相关联。本野外和模拟研究调查了气态氯代乙烯在沙质滨海平原含水层 18 米厚非饱和带中迁移过程中同位素比值如何演变。在该地点,地下水中检测到高浓度的四氯乙烯(PCE 高达 380μg/L)、三氯乙烯(TCE 高达 31600μg/L)和顺式-1,2-二氯乙烯(cDCE 高达 680μg/L)。氯代乙烯浓度在地下水位处最高,并向上朝向地表和向下低于地下水位稳定降低。尽管同位素异构体具有不同的扩散系数,但在整个非饱和带中观察到恒定的碳和氯同位素比值,这与在地下水位处测量的同位素比值相对应。在饱和带中,TCE 沿着浓度梯度逐渐贫化,可能是由于与水相扩散相关的同位素分馏所致。这些结果表明,即使蒸气发生广泛迁移,碳和氯同位素也可用于将气相污染与其源联系起来。然而,数值模型表明,只有在接近稳态的系统中才有望出现恒定的同位素比值。