The University of Texas at Austin, Department of Civil, Architectural, and Environmental Engineering, 301 E. Dean Keeton St., Stop C1786, Austin, TX 78712, USA.
University of Illinois at Urbana-Champaign, Department of Geology, 1301 West Green St., Urbana, IL 61801, USA.
J Contam Hydrol. 2019 Jul;224:103480. doi: 10.1016/j.jconhyd.2019.04.003. Epub 2019 Apr 11.
Low permeability source zones sustain long-term trichloroethene (TCE) groundwater contamination. In anaerobic environments, TCE is transformed by both biological reductive dechlorination and abiotic reactions with reactive minerals. Little is known about the relative contribution of these two pathways as TCE diffuses from low permeability zones (LPZs) into high permeability zones (HPZs). This study combines a flow cell experiment, batch experiments, and a diffusion-reaction model to evaluate the contributions of biotic and abiotic TCE transformation in LPZs. Natural clay (LPZ) and sand (HPZ) from a former Air Force base were used in all experiments. In batch, the LPZ material transformed TCE and cis-1,2-dichloroethene (cis-DCE) to acetylene with pseudo first-order rate constants of 8.57 × 10 day and 1.02 × 10 day, respectively. Biotic and abiotic pathways were then evaluated together in a bench-scale flow cell (16.5 cm × 2 cm × 16.5 cm) that contained a LPZ layer, with a source of TCE at the base, overlain by a HPZ continuously purged with lactate-amended groundwater. Diffusion controlled mass transfer in the LPZ, while advection controlled migration in the HPZ. The mass discharge rate of TCE and its biotic (cis-DCE and vinyl chloride) and abiotic (acetylene) transformation products were measured over 180 days in the flow cell effluent. Depth profiles of these compounds through the LPZ were determined after terminating the experiment. A one-dimensional diffusion-reaction model was used to interpret the effluent and depth profile data and constrain reaction parameters. Abiotic transformation rate constants for TCE to acetylene, normalized to in situ solids loading, were approximately 13 times greater in batch than in the flow cell. Slower transformation rates in the flow cell indicate elevated TCE concentration and/or further degradation of acetylene to other reduced gas compounds in the flow cell LPZ (thereby partially masking TCE abiotic transformation). Biotic and abiotic parameters used to interpret the flow cell data were then used to simulate a field site with a 300 cm thick LPZ. Abiotic processes contributed to a 2% reduction in TCE flux after 730 days. When abiotic rate constants were changed to that observed in batch, or to rate constants previously reported for a pyrite rich mudstone, the TCE flux reduction was 21% and 53%, respectively, after 730 days. Though biotic processes dominated TCE transformation in the flow cell experiment, the simulations indicate that abiotic processes have potential to significantly contribute to TCE attenuation in electron donor limited environments provided suitable reactive minerals are present.
低渗透性源区会持续造成三氯乙烯（TCE）地下水污染。在厌氧环境中，TCE 会通过生物还原脱氯和与反应性矿物的非生物反应而转化。由于 TCE 从低渗透性区（LPZ）扩散到高渗透性区（HPZ），因此对于这两种途径的相对贡献知之甚少。本研究结合流动池实验、批量实验和扩散-反应模型，评估了 LPZ 中生物和非生物 TCE 转化的贡献。所有实验均使用来自前空军基地的天然粘土（LPZ）和沙子（HPZ）。在批量实验中，LPZ 材料将 TCE 和顺-1,2-二氯乙烯（cis-DCE）转化为乙炔，其假一级速率常数分别为 8.57×10-1 天-1和 1.02×10-1 天-1。然后，在一个包含 LPZ 层的台式流动池（16.5cm×2cm×16.5cm）中同时评估了生物和非生物途径，该流动池的底部有 TCE 源，顶部由连续用添加了乳酸的地下水冲洗的 HPZ 覆盖。LPZ 中扩散控制质量转移，而 HPZ 中则是对流控制迁移。在流动池流出物中测量了 180 天内 TCE 及其生物（cis-DCE 和氯乙烯）和非生物（乙炔）转化产物的质量排放速率。在实验结束后，通过 LPZ 测定了这些化合物的深度分布。使用一维扩散-反应模型来解释流出物和深度分布数据，并约束反应参数。与流动池相比，TCE 转化为乙炔的非生物转化速率常数在批量实验中约高 13 倍，在流动池中则较慢。这表明在流动池 LPZ 中，TCE 浓度升高和/或乙炔进一步降解为其他还原气体化合物，从而部分掩盖了 TCE 的非生物转化（LPZ）。用于解释流动池数据的生物和非生物参数随后用于模拟一个具有 300cm 厚 LPZ 的现场。非生物过程导致 730 天后 TCE 通量减少 2%。当非生物速率常数更改为批处理中观察到的速率常数，或更改为先前报道的富含黄铁矿泥岩中的速率常数时，730 天后 TCE 通量减少分别为 21%和 53%。尽管生物过程在流动池实验中主导了 TCE 转化，但模拟表明，在电子供体有限的环境中，只要存在合适的反应性矿物，非生物过程就有可能显著有助于 TCE 的衰减。
