Bouchard Daniel, Höhener Patrick, Hunkeler Daniel
Center for Hydrogeology, University of Neuchâtel, Rue Emile Argand 11, CH-2009 Neuchâtel, Switzerland.
Environ Sci Technol. 2008 Nov 1;42(21):7801-6. doi: 10.1021/es800391s.
The study focuses on the effect of volatilization, diffusion, and biodegradation on the isotope evolution of volatile organic compounds (VOCs) in a 1.06 m long column filled with alluvial sand. A liquid mixture of 10 VOCs was placed at one end of the column, and measurements of VOC vapor concentrations and compound-specific isotope ratios (delta(13)C) were performed at the source and along the column. Initially, the compounds became depleted in 13C by up to -4.8% per hundred along the column axis, until at 26 h, uniform isotope profiles were observed for most compounds, which is expected for steady-state diffusion. Subsequently, several compounds (n-pentane, benzene, n-hexane) became enriched in 13C throughout the column. For the same compounds, a significant decrease in the source vapor concentration and a gradual enrichment of 13C by up to 5.3% per hundred at the source over a period of 336 h was observed. This trend can be explained by a larger diffusive mass flux for molecules with light isotopes compared to those with a heavy isotope, which leads to a depletion of light isotopes in the source. The isotope evolution of the source followed closely a Rayleigh trend and the obtained isotope enrichment factor corresponded well to the ratio between the diffusion coefficients for heavy and light molecules as expected based on theory. In contrastto diffusion, biodegradation had generally only a small effect on the isotope profiles, which is expected because in a diffusion-controlled system the isotope shift per decrease of mass flux is smaller than in an advection-controlled system. These findings open interesting perspectives for monitoring source depletion with isotope and have implications for assessing biodegradation and source variability in the unsaturated zone based on isotopes.
该研究聚焦于挥发、扩散和生物降解对填充冲积砂的1.06米长柱体中挥发性有机化合物（VOCs）同位素演化的影响。将10种VOCs的液体混合物置于柱体一端，并在源端和沿柱体进行VOC蒸汽浓度及化合物特异性同位素比率（δ¹³C）的测量。起初，沿柱体轴线，化合物的¹³C亏损高达每百分之 -4.8%，直到26小时时，大多数化合物观察到均匀的同位素分布，这对于稳态扩散而言是预期的。随后，几种化合物（正戊烷、苯、正己烷）在整个柱体中¹³C富集。对于相同的化合物，观察到源端蒸汽浓度显著降低，且在336小时内源端¹³C逐渐富集高达每百分之5.3%。这种趋势可以解释为，与重同位素分子相比，轻同位素分子具有更大的扩散质量通量，这导致源端轻同位素的亏损。源端的同位素演化紧密遵循瑞利趋势，所获得的同位素富集因子与基于理论预期的重、轻分子扩散系数之比吻合良好。与扩散相反，生物降解通常对同位素分布的影响较小，这是预期的，因为在扩散控制的系统中，每质量通量降低的同位素偏移比平流控制的系统中小。这些发现为利用同位素监测源端损耗开辟了有趣的前景，并对基于同位素评估非饱和带中的生物降解和源端变异性具有启示意义。
