Journal
JOURNAL OF CONTAMINANT HYDROLOGY
Volume 136, Issue -, Pages 106-116Publisher
ELSEVIER
DOI: 10.1016/j.jconhyd.2012.04.006
Keywords
Back diffusion; Remediation; Modeling; Grid discretization; Groundwater plumes; Chlorinated solvents
Funding
- Strategic Environmental Research and Development Program (SERDP) [ER-1740]
- University Consortium for Field-Focused Groundwater Contamination Research
- DuPont
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It is now widely recognized that contaminant release from low permeability zones can sustain plumes long after primary sources are depleted, particularly for chlorinated solvents where regulatory limits are orders of magnitude below source concentrations. This has led to efforts to appropriately characterize sites and apply models for prediction incorporating these effects. A primary challenge is that diffusion processes are controlled by small-scale concentration gradients and capturing mass distribution in low permeability zones requires much higher resolution than commonly practiced. This paper explores validity of using numerical models (HydroGeoSphere, FEFLOW, MODFLOW/MT3DMS) in high resolution mode to simulate scenarios involving diffusion into and out of low permeability zones: 1) a laboratory tank study involving a continuous sand body with suspended clay layers which was 'loaded' with bromide and fluorescein (for visualization) tracers followed by clean water flushing, and 2) the two-layer analytical solution of Sale et al. (2008) involving a relatively simple scenario with an aquifer and underlying low permeability layer. All three models are shown to provide close agreement when adequate spatial and temporal discretization are applied to represent problem geometry, resolve flow fields and capture advective transport in the sands and diffusive transfer with low permeability layers and minimize numerical dispersion. The challenge for application at field sites then becomes appropriate site characterization to inform the models, capturing the style of the low permeability zone geometry and incorporating reasonable hydrogeologic parameters and estimates of source history, for scenario testing and more accurate prediction of plume response, leading to better site decision making. (C) 2012 Elsevier B.V. All rights reserved.
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