一维对流-分散输运与扩散有关的同位素分馏效应。,Science of the Total Environment

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一维对流-分散输运与扩散有关的同位素分馏效应。
Science of the Total Environment ( IF 8.2 ) Pub Date : 2016-01-28 , DOI: 10.1016/j.scitotenv.2016.01.114
Bruce S Xu ₁ , Barbara Sherwood Lollar ₂ , Elodie Passeport ₃ , Brent E Sleep ₁

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研究了碳同位素的水相扩散相关同位素分馏（DRIF），以研究其中运输被认为是一维的常见地下水污染物。本文不仅着重于这些系统中理论上可观察到的DRIF效应，而且还介绍了基于现场测量规模所施加的约束以及检测和分析不确定度的标准极限来约束“可观察” DRIF的重要概念。具体来说，根据扩散分馏因子，污染物的初始浓度（C0），同位素分析的方法检测限（MDL），运输时间，以及纵向机械分散系数与有效分子扩散系数的比值（Dmech / Deff）。结果可以确定田间条件，在这种条件下，DRIF可能是使用稳定碳同位素测量评估一维对流-弥散运移的污染物运移和转化的重要因素。这项研究表明，对于BTEX，MTBE和氯化乙烯的扩散占主导地位的运输，只有当C0 / MDL比率为50或更高时，对于摩尔质量较小的化合物（如氯乙烯），才可以检测到DRIF效应。为了使摩尔质量较高的化合物能够检测到DRIF，需要更大的C0 / MDL比（对应于更高的源浓度或更低的检测限）。对于相对年轻的扩散羽（<100年），对于VC而言，DRIF的可观察距离很小（小于1m），并且通过使用“典型”井距（至少几个井距）的常规采样方法将不容易检测到DRIF。米）。通过平流，机械分散和分子扩散进行污染物运输，这项研究表明，在Dmech / Deff大于10的野外场所，普通地下水污染物可能不会观察到DRIF效应。重要的是，在大多数现场条件下，通常在纵向上满足Dmech /Deff≥10，这表明在大多数污染物运移主要是一维的地下水系统中，DRIF不太可能被观察到。

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Diffusion related isotopic fractionation effects with one-dimensional advective-dispersive transport.

Aqueous phase diffusion-related isotope fractionation (DRIF) for carbon isotopes was investigated for common groundwater contaminants in systems in which transport could be considered to be one-dimensional. This paper focuses not only on theoretically observable DRIF effects in these systems but introduces the important concept of constraining "observable" DRIF based on constraints imposed by the scale of measurements in the field, and on standard limits of detection and analytical uncertainty. Specifically, constraints for the detection of DRIF were determined in terms of the diffusive fractionation factor, the initial concentration of contaminants (C0), the method detection limit (MDL) for isotopic analysis, the transport time, and the ratio of the longitudinal mechanical dispersion coefficient to effective molecular diffusion coefficient (Dmech/Deff). The results allow a determination of field conditions under which DRIF may be an important factor in the use of stable carbon isotope measurements for evaluation of contaminant transport and transformation for one-dimensional advective-dispersive transport. This study demonstrates that for diffusion-dominated transport of BTEX, MTBE, and chlorinated ethenes, DRIF effects are only detectable for the smaller molar mass compounds such as vinyl chloride for C0/MDL ratios of 50 or higher. Much larger C0/MDL ratios, corresponding to higher source concentrations or lower detection limits, are necessary for DRIF to be detectable for the higher molar mass compounds. The distance over which DRIF is observable for VC is small (less than 1m) for a relatively young diffusive plume (<100years), and DRIF will not easily be detected by using the conventional sampling approach with "typical" well spacing (at least several meters). With contaminant transport by advection, mechanical dispersion, and molecular diffusion this study suggests that in field sites where Dmech/Deff is larger than 10, DRIF effects will likely not be observable for common groundwater contaminants. Importantly, under most field conditions, Dmech/Deff≥10 is usually satisfied in the longitudinal direction, suggesting that DRIF is not likely to be observable in most groundwater systems in which contaminant transport is predominantly one-dimensional. Given the importance in the MDL it is recommended that MDL should always be explicitly reported in both modeling and field studies.

更新日期：2019-11-01

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