Using in situ remediation (ISR-MT3DMS) model to estimate back-diffusion timeframe for thin silts and clays

Low-permeability soils like silts and clays may act as storage reservoirs for dissolved contaminant mass. As the original source is depleted, back-diffusion may sustain groundwater plumes for decades to centuries or longer. There are some tools available for evaluating back-diffusion from thick clay/silt layers (e.g. Matrix Diffusion ToolKit); however, there are significant limitations for estimating timeframes for back-diffusion from thin layers of clay and silt, and for evaluating the influence of contaminant degradation on back-diffusion timeframe. Use of numerical models may be prohibitive in some cases, particularly for three-dimensional (3-D) models. For example, representing one thin silt or clay unit in a numerical model may require incorporation of dozens to a hundred or more model layers. Representing multiple thin silt/clay units distributed vertically is even more prohibitive. This study presents an innovative approach for simulating diffusion using a local domain approach, which has been incorporated into a new public domain model called In Situ Remediation MT3DMS (ISR-MT3DMS). ISR-MT3DMS was developed based on the MT3DMS framework, and utilizes an array of one-dimensional local domain models for simulating diffusion-dominated processes at different spatial and temporal resolution than the global domain. ISR-MT3DMS also includes the flexible biogeochemical reaction framework from BioRedox (Carey et al., 1998) for simulating natural and enhanced degradation. (author)