Numerical simulation of isotope fractionation in steady-state bioreactive transport controlled by transverse mixing
Numerical simulation of isotope fractionation in steady-state bioreactive transport controlled by transverse mixing
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DOI：
10.1016/j.jconhyd.2012.08.010
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发表时间：
2012-10-01
影响因子：
3.6
通讯作者：
Cirpka, Olaf A.
中科院分区：
文献类型：
--
作者：
Eckert, Dominik;Rolle, Massimo;Cirpka, Olaf A.
化合物特定稳定同位素分析(CSIA)越来越多地被用作评估受污染现场的固有生物降解的工具。通常，瑞利方程被用来根据测量的污染物的同位素比率来估计生物降解的程度。然而，如果整个降解的限速步骤不是微生物反应本身，瑞利方程可能不再适用。在这项研究中，我们模拟了地下水系统中连续排放的石油碳氢化合物的生物降解。这些污染物在羽流边缘被有效降解，在那里横向扩散使它们与周围地下水中存在的溶解电子受体混合。我们模拟反应输运，以研究横向混合和生物降解对碳同位素特征空间模式的耦合影响，以及基于深度整合采样的碳同位素特征解释，这代表了污染场地评估中最常见的设置。我们提出了模拟水力均匀的实验室实验和考虑非均匀导电场的现场规模应用的场景。我们将考虑同位素特定的横向弥散的情况与忽略这种额外的分馏过程的情况进行比较。我们发现，这些效应会导致同位素信号的显著变化，并可能导致对生物降解的高估。此外，我们的结果提供了证据，横向混合对整体降解的限速效应在空间上沿稳态污染物羽流的长度变化。横向分散对生物降解的控制和分散的分馏效应单独调节了微生物反应引起的分馏。因此，在瑞利图中观察到了显著的非线性同位素模式。非均质流模拟表明，这些影响在较大的场尺度上持续存在，并对由导水场的非均质性决定的混合增强程度和地下水流速敏感。(C)2012爱思唯尔B.V.保留所有权利。
Compound-specific stable isotope analysis (CSIA) has increasingly been used as a tool to assess intrinsic biodegradation at contaminated field sites. Typically, the Rayleigh equation is used to estimate the extent of biodegradation from measured isotope ratios of the contaminant. However, if the rate-limiting step in overall degradation is not the microbial reaction itself, the Rayleigh equation may no more be applicable. In this study we simulate biodegradation of continuously emitted petroleum hydrocarbons in groundwater systems. These contaminants are effectively degraded at the plume fringe where transverse dispersion makes them mix with dissolved electron acceptors present in the ambient groundwater. We simulate reactive transport to study the coupled effects of transverse mixing and biodegradation on the spatial patterns of carbon isotope signatures and their interpretation based on depth-integrated sampling which represents the most common setup in the assessment of contaminated sites. We present scenarios mimicking a hydraulically uniform laboratory experiment and a field-scale application considering heterogeneous conductivity fields. We compare cases in which isotopologue-specific transverse dispersion is considered to those where this additional fractionation process is neglected. We show that these effects cause significant shifts in the isotopic signals and may lead to overestimation of biodegradation. Moreover, our results provide evidence that the rate-limiting effect of transverse mixing on the overall degradation spatially varies along the length of a steady-state contaminant plume. The control of biodegradation by transverse dispersion and the fractionating effect of dispersion modulate the fractionation caused by the microbial reaction alone. As a consequence, significantly nonlinear isotopic patterns are observed in a Rayleigh plot. Simulations in heterogeneous flow domains show that these effects persist at larger field scales and are sensitive to the degree of mixing enhancement, determined by the heterogeneity of the hydraulic conductivity fields, and to the groundwater flow velocity. (C) 2012 Elsevier B.V. All rights reserved.