Low permeability zones (LPZs) are major sources of groundwater contamination after active remediation to remove pollutants in adjacent high permeability zones (HPZs). Slow back diffusion from LPZs to HPZs can extend management of polluted sites by decades. Numerical models are often used to simulate back diffusion, estimate cleanup times, and develop site management strategies. Sharp concentration gradients of pollutants are present at the interface between HPZs and LPZs, and hence accurate simulation requires fine grid sizes resulting in high computational burden. Since the MODFLOW family of codes is widely used in practice, we develop a new approach for modeling pollutant back diffusion using MODFLOW/RT3D that eliminates the need for fine discretization of the LPZ. Instead, the LPZ is treated as an impermeable region in MODFLOW, while in RT3D the LPZ is conceptualized as a series of immobile zones coupled with a mobile zone at the HPZ/LPZ interface. Finite volume discretization of diffusion and reaction within the LPZ is then modeled as mass transfer and reaction among several immobile species. This results in a simulation domain with significantly fewer grid cells compared to that required if all LPZs are discretized, providing potential for improved computational efficiency. Cases, including a layer of HPZ over an LPZ, a thin/thick lens of LPZ embedded in HPZ, and multiple lens of LPZs embedded in HPZ are tested by the new approach for tracer and reactive scenarios.

低渗透区（LPZs）是积极修复去除相邻高渗透区（HPZs）中污染物后的主要地下水污染源。从LPZ向HPZ的缓慢向后扩散可以将污染场地的管理延长数十年。数值模型通常用于模拟反向扩散，估计清理时间并制定站点管理策略。HPZ和LPZ之间的界面处存在污染物的急剧浓度梯度，因此准确的模拟需要精细的网格大小，从而导致较高的计算负担。由于MODFLOW系列代码在实践中得到了广泛使用，我们开发了一种使用MODFLOW / RT3D建模污染物反向扩散的新方法，从而无需对LPZ进行精细离散。而是将LPZ视为MODFLOW中的不可渗透区域，而在RT3D中，LPZ被概念化为一系列固定区域，并在HPZ / LPZ接口处与移动区域耦合。然后将LPZ内扩散和反应的有限体积离散化建模为几种不动物种之间的传质和反应。与所有LPZ离散化相比，这导致仿真域的网格单元数明显减少，从而为提高计算效率提供了可能。通过示踪剂和反应性场景的新方法测试了包括在LPZ上的HPZ层，嵌入在HPZ中的LPZ的薄/厚透镜以及嵌入在HPZ中的LPZ的多透镜在内的情况。然后将LPZ内扩散和反应的有限体积离散化建模为几种不动物种之间的传质和反应。与所有LPZ离散化相比，这导致仿真域的网格单元数明显减少，从而为提高计算效率提供了可能。通过示踪剂和反应性场景的新方法测试了包括在LPZ上的HPZ层，嵌入在HPZ中的LPZ的薄/厚透镜以及嵌入在HPZ中的LPZ的多透镜在内的情况。然后将LPZ内扩散和反应的有限体积离散化建模为几种不动物种之间的传质和反应。与所有LPZ离散化相比，这导致仿真域的网格单元数明显减少，从而为提高计算效率提供了可能。通过示踪剂和反应性场景的新方法测试了包括在LPZ上的HPZ层，嵌入在HPZ中的LPZ的薄/厚透镜以及嵌入在HPZ中的LPZ的多透镜在内的情况。