Department of Water Management, Delft University of Technology, Stevinweg 1, 2628 CN Delft, the Netherlands.
School of Geology and Geophysics, University of Oklahoma, 100 E. Boyd Street, SEC 710, Norman, OK 73019, United States of America.
J Contam Hydrol. 2019 Oct;226:103520. doi: 10.1016/j.jconhyd.2019.103520. Epub 2019 Jul 8.
Back-diffusion of chlorinated ethenes (CEs) from low-permeability layers (LPLs) causes contaminant persistence long after the primary spill zones have disappeared. Naturally occurring degradation in LPLs lowers remediation time frames, but its assessment through sediment sampling is prohibitive in conventional remediation projects. Scenario simulations were performed with a reactive transport model (PHT3D in FloPy) accounting for isotope effects associated with degradation, sorption, and diffusion, to evaluate the potential of CSIA data from aquifers in assessing degradation in aquitards. The model simulated a trichloroethylene (TCE) DNAPL and its pollution plume within an aquifer-aquitard-aquifer system. Sequential reductive dechlorination to ethene and sorption were uniform in the aquitard and did not occur in the aquifer. After 10 years of loading the aquitard through diffusion from the plume, subsequent source removal triggered release of TCE by back-diffusion. In the upper aquifer, during the loading phase, δC-TCE was slightly enriched (up to 2‰) due to diffusion effects stimulated by degradation in the aquitard. In the upper aquifer, during the release phase, (i) source removal triggered a huge δC increase especially for higher CEs, (ii) moreover, downstream decreasing isotope ratios (caused by downgradient later onset of the release phase) with temporal increasing isotope ratios reflect aquitard degradation (as opposed to downstream increasing and temporally constant isotope ratios in reactive aquifers), and (iii) the carbon isotope mass balance (CIMB) enriched up to 4‰ as lower CEs (more depleted, less sorbing) have been transported deeper into the aquitard. Thus, enriched CIMB does not indicate oxidative transformation in this system. The CIMB enrichment enhanced with more sorption and lower aquitard thickness. Thin aquitards are quicker flushed from lower CEs leading to faster CIMB enrichment over time. CIMB enrichment is smaller or nearly absent when daughter products accumulate. Aquifer CSIA patterns indicative of aquitard degradation were similar in case of linear decreasing rate constants but contrasted with previous simulations assuming a thin bioactive zone. The Rayleigh equation systematically underestimates the extent of TCE degradation in aquifer samples especially during the loading phase and for conditions leading to long remediation time frames (low groundwater flow velocity, thicker aquitards, strong sorption in the aquitard). The Rayleigh equation provides a good and useful picture on aquitard degradation during the release phase throughout the sensitivity analysis. This modelling study provides a framework on how aquifer CSIA data can inform on the occurrence of aquitard degradation and its pitfalls.
底水扩散会导致地下水中氯代乙烯（CEs）从低渗透性层（LPL）中反向扩散，即使主要污染带已经消失，污染物也会持续存在很长时间。LPL 中自然发生的降解会降低修复时间框架，但在传统修复项目中，通过沉积物采样进行评估是不可行的。通过使用考虑与降解、吸附和扩散相关的同位素效应的反应迁移模型（PHT3D 在 FloPy 中）进行情景模拟，评估了含水层中 CSIA 数据在评估含水层隔水层降解中的潜在应用。该模型模拟了三氯乙烯（TCE）DNAPL 及其在含水层-含水层隔水层-含水层系统中的污染羽流。在含水层隔水层中，连续的还原脱氯生成乙烯和吸附作用是均匀的，而在含水层中则没有发生。通过羽流从含水层隔水层中扩散，经过 10 年的加载后，随后的源去除会引发 TCE 通过反向扩散的释放。在上层含水层中，在加载阶段，由于含水层隔水层中降解刺激的扩散效应，δC-TCE 略有富集（最多 2‰）。在上层含水层中，在释放阶段，（i）源去除触发了 δC 的巨大增加，特别是对于更高的 CEs；（ii）此外，下游同位素比值的降低（由于释放阶段下游的开始时间较晚），伴随着时间的增加同位素比值反映了含水层隔水层的降解（与反应性含水层中下游增加和时间常数的同位素比值相反）；（iii）碳同位素质量平衡（CIMB）增加了 4‰，因为较低的 CEs（更贫化、吸附性更低）已被输送到含水层隔水层更深的地方。因此，在该系统中，富集的 CIMB 并不表示氧化转化。CIMB 富集随着吸附作用的增加和含水层隔水层厚度的减小而增强。薄含水层隔水层中低 CEs 更快地被冲洗掉，导致 CIMB 随时间的增加而更快地富集。当女儿产物积累时，CIMB 富集较小或几乎不存在。在假设存在薄的生物活性带的情况下，指示含水层隔水层降解的含水层 CSIA 模式在线性递减速率常数的情况下是相似的，但与以前的模拟相比则相反。瑞利方程系统地低估了含水层样品中 TCE 降解的程度，特别是在加载阶段和导致修复时间框架较长的条件下（地下水流速低、含水层隔水层较厚、含水层隔水层中吸附作用较强）。瑞利方程在整个敏感性分析中为含水层隔水层降解提供了一个良好且有用的图像。这项建模研究提供了一个框架，说明如何利用含水层 CSIA 数据了解含水层隔水层降解的发生及其缺陷。
