Department of Geotechnical Engineering, Tongji University, Si Ping Road 1239, Shanghai 200092, China.
Department of Geotechnical Engineering, Tongji University, Si Ping Road 1239, Shanghai 200092, China; Key Laboratory of Geotechnical and Underground Engineering of the Ministry of Education, Tongji University, Shanghai 200092, China.
Water Res. 2022 Aug 15;222:118925. doi: 10.1016/j.watres.2022.118925. Epub 2022 Jul 29.
Contaminants stored in the low permeability sediments will continue to threaten the adjacent shallow groundwater system after the aquifer is remediated. Understanding the storage and discharge behavior of contaminants in the aquitards is essential for the efficient remediation of contaminated sites, but most of the previous analytical studies focused on nonreactive solutes in a single homogenous aquitard. This study presents novel analytical solutions for the forward and back diffusion of contaminants through multi-layer low permeability sediments considering abiotic and biotic environmental degradation. Three representative source depletion patterns (i.e., instantaneous, linear, and exponential patterns) were selected to describe the dissolution of dense non-aqueous phase liquids (DNAPL) in the aquifer more realistically. At the forward diffusion stage, the mass storage of contaminants in the aquitards with the instantaneous pattern is the largest, nearly twice that with the exponential pattern. A simple equivalent homogeneous model is generally adopted in the risk assessment. However, relative to the proposed multi-layer model, it will significantly underestimate the onset of the back-diffusion of heterogeneous aquitards and overestimate the persistence of aquifer plumes. The previously-reported semi-infinite boundary assumption is also not applicable, with a maximum error of over 200% in the long-term prediction of back diffusion behavior of a thin aquitard. Moreover, when the degradation half-life is less than 16 years, less than 10% of the contaminants stored in the aquitards will diffuse into the overlying aquifer, suggesting that biostimulation or bioaugmentation can effectively mitigate back-diffusion risk. Overall, the proposed diffusion-reaction coupled model with multi-layer media is of great value and high demand in predicting the back-diffusion behavior of heterogeneous aquitards and guiding the soil bioremediation.
污染物在含水层得到修复后，仍会继续储存在低渗透性沉积物中，威胁到相邻的浅层地下水系统。了解含水层隔水层中污染物的储存和排放行为，对于受污染场地的有效修复至关重要，但之前的大部分分析研究都集中在单一均质含水层中无反应溶质上。本研究提出了一种新的分析解，用于考虑非生物和生物环境降解作用下，污染物通过多层低渗透性沉积物的正向和反向扩散。选择了三种有代表性的源耗竭模式（即瞬时、线性和指数模式）来更真实地描述致密非水相液体（DNAPL）在含水层中的溶解。在正向扩散阶段，瞬时模式下污染物在隔水层中的质量储存最大，几乎是指数模式的两倍。在风险评估中，通常采用简单的等效均质模型。然而，与提出的多层模型相比，它会显著低估非均质隔水层反向扩散的起始时间，并高估含水层羽流的持久性。之前报道的半无限边界假设也不适用，对于薄隔水层反向扩散行为的长期预测，最大误差超过 200%。此外，当降解半衰期小于 16 年时，储存在隔水层中的污染物中只有不到 10%会扩散到上覆含水层中，这表明生物刺激或生物增强可以有效降低反向扩散风险。总的来说，提出的具有多层介质的扩散-反应耦合模型对于预测非均质含水层的反向扩散行为和指导土壤生物修复具有重要价值和高度需求。
