Department of Water Management, Delft University of Technology, Stevinweg 1, Delft 2628 CN, the Netherlands.
Department of Water Management, Delft University of Technology, Stevinweg 1, Delft 2628 CN, the Netherlands.
J Contam Hydrol. 2020 May;231:103638. doi: 10.1016/j.jconhyd.2020.103638. Epub 2020 Mar 20.
Degradation of chlorinated ethenes (CEs) in low conductivity layers of aquifers reduces pollution plume tailing and accelerates natural attenuation timeframes. The degradation pathways involved are often different from those in the higher conductive layers and might go undetected when only highly conductive layers are targeted in site assessments. Reactive transport model simulations (PHT3D in FloPy) were executed to assess the performance of dual carbon and chlorine compound specific stable isotope analysis (CSIA) in degradation pathway identification and quantification in a coupled physical-chemical heterogeneous virtual aquifer. Degradation rate constants were assumed correlated to the hydraulic conductivity: positively for oxidative transformation (higher oxygen availability in coarser sands) and negatively for chemical reduction (higher content of reducing solids in finer sediments). Predicted carbon isotope ratios were highly heterogeneous. They generally increased downgradient of the pollution source but the large variation across depth illustrates that monotonously increasing isotope ratios downgradient, as were associated with the oxidative component, are not necessarily a common situation when degradation is favorable in low conductivity layers. Dual carbon-chlorine CSIA performed well in assessing the occurrence of the spatially separated degradation pathways and the overall degradation, provided appropriate enrichment factors were known and sufficiently different. However, pumping to obtain groundwater samples especially from longer well screens causes a bias towards overestimation of the contribution of oxidative transformation associated with the higher conductive zones. As degradation was less intense in these highly conductive zones under the simulated conditions, overall degradation was underestimated. In contrast, in the usual case of limited CSIA data, dual CSIA plots may rather indicate dominance of chemical reduction, while oxidative transformation could go unnoticed, despite being an equally important degradation pathway.
含水层低电导率层中氯代乙烷（CEs）的降解减少了污染羽流的拖尾，并加速了自然衰减的时间框架。所涉及的降解途径通常与高电导率层中的途径不同，如果仅在现场评估中针对高电导率层进行评估，可能会检测不到这些途径。执行了反应迁移模型模拟（FloPy 中的 PHT3D），以评估双碳和氯化合物特定稳定同位素分析（CSIA）在耦合物理化学非均质虚拟含水层中降解途径识别和定量中的性能。降解速率常数假定与水力传导率相关：对于氧化转化呈正相关（较粗砂中氧气供应更充足），对于化学还原呈负相关（较细沉积物中还原固体含量更高）。预测的碳同位素比值高度不均匀。它们通常在污染源的下游增加，但沿深度的大变化表明，与氧化成分相关的单调增加的同位素比值沿下游不一定是在低电导率层中有利于降解的常见情况。双碳-氯 CSIA 在评估空间分离的降解途径和整体降解的发生方面表现良好，前提是已知并具有足够差异的适当富集因子。然而，抽取地下水样品，特别是从较长的井筛中抽取地下水样品，会导致对与高电导率区相关的氧化转化贡献的高估。由于在模拟条件下这些高电导率区的降解程度较弱，因此整体降解被低估。相比之下，在通常情况下，有限的 CSIA 数据可能表明化学还原的主导地位，而氧化转化可能会被忽略，尽管它是同样重要的降解途径。
