Permeable reactive barriers (PRBs), such as mulch biowalls, have been installed at numerous groundwater cleanup sites, and laboratory and field studies have demonstrated biotic and abiotic processes that degrade chlorinated volatile organic compounds (CVOCs) in groundwater passing through these engineered remedies. However, the longevity of mulch biowalls remains a fundamental research question. Soil and groundwater sampling at seven mulch biowalls at Altus Air Force Base (AFB) approximately 10 years after installation demonstrated the ongoing degradation of CVOCs. Trichloroethene was not detected in five of seven groundwater samples collected from the biowall despite upgradient detections above federal drinking water standards. Microbial sampling established the presence of key dechlorinating bacteria and the abundance of genes encoding specific enzymes for degradation, high methane concentrations, low sulfate concentrations, and negative oxidation‐reduction potential, all indicative of highly reducing conditions within the biowalls and favorable conditions for CVOC destruction via microbial reductive dechlorination. High cellulose content (>79%) of the mulch, elevated total organic carbon (TOC) content in groundwater, and elevated potentially bioavailable organic carbon (PBOC) measurements in soil samples further supports an ongoing, long‐lived source of carbon. These results demonstrate the ongoing and long‐term efficacy of the mulch biowalls at Altus AFB. In addition, concentrations of bacteria, TOC, PBOC, and other geochemical parameters suggest a modest impact of the biowalls downgradient. The continued presence of CVOCs downgradient may be attributable to back diffusion from low‐permeability shale. However, the biowalls continue to provide benefits by removing CVOCs in groundwater, thus reducing further CVOC loading to the downgradient, low‐permeability strata.

中文翻译：

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覆盖膜生物墙处理氯化溶剂性能的长期评估

渗透性反应屏障（PRB），例如覆盖生物墙，已安装在许多地下水净化场所，实验室和现场研究表明，通过这些工程方法，生物和非生物过程可降解地下水中的氯代挥发性有机化合物（CVOC）。但是，覆盖生物墙的寿命仍然是一个基本的研究问题。安装后约10年，在Altus空军基地（AFB）的七个覆盖生物墙处对土壤和地下水进行了采样，证明了CVOC的持续降解。尽管从联邦政府的饮用水标准中检出的污染物含量上升，但在从生物墙收集的七个地下水样品中有五个未检出三氯乙烯。微生物采样确定了关键的脱氯细菌的存在，以及编码降解的特定酶的基因的丰富，高甲烷浓度，低硫酸盐浓度和负氧化还原潜力，所有这些都表明生物壁内的高度还原条件和CVOC破坏的有利条件通过微生物还原脱氯。覆盖物中较高的纤维素含量（> 79％），地下水中总有机碳（TOC）含量升高以及土壤样品中潜在的可生物利用的有机碳（PBOC）含量升高，进一步支持了持续存在的长寿命碳源。这些结果证明了Altus空军基地覆盖生物墙的长期有效性。此外，细菌，TOC，PBOC，其他地球化学参数表明，生物墙下降的影响较小。CVOCs降级的持续存在可能归因于低渗透性页岩的向后扩散。然而，生物隔离墙通过去除地下水中的CVOC继续提供好处，从而进一步降低了CVOC对低梯度，低渗透率地层的负荷。