Groundwater fate and transport modeling results demonstrate that matrix diffusion plays a role in attenuating the expansion of groundwater plumes of “non-degrading” or highly recalcitrant compounds. This is especially significant for systems where preferred destructive attenuation processes, such as biological and abiotic degradation, are weak or ineffective for plume control. Under these conditions, models of nondestructive physical attenuation processes, traditionally dispersion or sorption, do not demonstrate sufficient plume control unless matrix diffusion is considered. Matrix diffusion has been shown to be a notable emergent impact of geological heterogeneity, typically associated with back diffusion and extending remediation timeframes through concentration tailing of the trailing edge of a plume. However, less attention has been placed on evaluating how matrix diffusion can serve as an attenuation mechanism for the leading edge of a plume of non-degrading compounds like perfluoroalkyl acids (PFAAs), including perfluorooctane sulfonate (PFOS).

In this study, the REMChlor-MD model was parametrically applied to a generic unconsolidated and heterogeneous geologic site with a constant PFOS source and no degradation of PFOS in the downgradient edge of the plume. Low levels of mechanical dispersion and retardation were used in the model for three different geologic heterogeneity cases ranging from no matrix diffusion (e.g., sand only) to considerable matrix diffusion using low permeability (“low-k”) layers/lenses and/or aquitards. Our analysis shows that, in theory, many non-degrading plumes may expand for significant time periods before dispersion alone would eventually stabilize the plume; however, matrix diffusion can significantly slow the rate and degree of this migration.

For one 100-year travel time scenario, consideration of matrix diffusion results in a simulated PFOS plume length that is over 80% shorter than the plume length simulated without matrix diffusion. Although many non-degrading plumes may continue to slowly expand over time, matrix diffusion resulted in lower concentrations and smaller plume footprints. Modeling multiple hydrogeologic settings showed that the effect of matrix diffusion is more significant in transmissive zones containing multiple low-k lenses/layers than transmissive zones underlain and overlain by low-k aquitards. This study found that at sites with significant matrix diffusion, groundwater plumes will be shorter, will expand more slowly, and may be amenable to a physical, retention-based, Monitored Natural Attenuation (MNA) paradigm. In this case, a small “Plume Assimilative Capacity Zone” in front of the existing plume could be reserved for slow, de minimus, future expansion of a non-degrading plume. If potential receptors are protected in this scenario, then this approach is similar to allowances for expanding plumes under some existing environmental regulatory programs. Accounting for matrix diffusion may support new strategic approaches and alternative paradigms for remediation even for sites and conditions with “non-degrading” constituents such as PFAAs, metals/metalloids, and radionuclides.

地下水归宿和传输模拟结果表明，基质扩散在减弱“非降解”或高度顽固化合物的地下水羽流膨胀方面发挥作用。这对于首选破坏性衰减过程（例如生物和非生物降解）对羽流控制薄弱或无效的系统尤其重要。在这些条件下，非破坏性物理衰减过程模型，传统上是分散或吸附，并没有表现出足够的羽流控制，除非考虑基质扩散。基质扩散已被证明是地质异质性的显着紧急影响，通常与反扩散和通过尾流尾缘的浓度拖尾延长修复时间框架有关。然而，磺酸盐（PFOS）。

在这项研究中，REMChlor-MD 模型参数化地应用于具有恒定 PFOS 源且在羽流下坡边缘没有 PFOS 降解的一般松散和非均质地质场地。模型中使用了低水平的机械分散和延迟，用于三种不同的地质非均质情况，从无基质扩散（例如，仅沙子）到使用低渗透率（“低 k”）层/透镜和/或隔透层的相当大的基质扩散. 我们的分析表明，理论上，许多非降解羽流可能会膨胀很长一段时间，然后单独分散才能最终稳定羽流；然而，基质扩散可以显着减慢这种迁移的速度和程度。

对于一个 100 年旅行时间情景，考虑到基质扩散导致模拟的 PFOS 羽流长度比没有基质扩散时模拟的羽流长度短 80% 以上。尽管许多非降解羽流可能会随着时间的推移继续缓慢膨胀，但基质扩散导致浓度降低和羽流足迹更小。对多个水文地质环境进行建模表明，在包含多个低 k 透镜/层的透射区中，基质扩散的影响比低 k 隔水层下覆和上覆的透射区更显着。这项研究发现，在基质扩散明显的地点，地下水羽流会更短，膨胀更慢，并且可能适合物理的、基于保留的、受监测的自然衰减 (MNA) 范式。在这种情况下，de minimus ，非降解羽流的未来扩展。如果在这种情况下潜在的受体受到保护，那么这种方法类似于在某些现有环境监管计划下允许扩大羽流。考虑到基质扩散，即使对于具有“非降解”成分（如 PFAA、金属/准金属和放射性核素）的场地和条件，也可以支持新的战略方法和替代模式。