Date of Award
Master of Science (MS)
Environmental Engineering and Earth Sciences
Ronald W Falta
Lawrence C Murdoch
James K Henderson
The back diffusion of dissolved chemicals from low permeability zones to aquifers can cause contaminant plumes to persist long after remediation (Chapman and Parker, 2005). Because of the complicated nature of some field sites, the effect of back diffusion on plume persistence can sometimes be ambiguous. A novel approach for simulating matrix diffusion effects was previously adapted from geothermal reservoir modeling, which combines numerical and analytical methods by discretizing only the high permeability parts of the aquifer and treating the matrix diffusion flux into the high permeability gridblocks as a concentration dependent source/sink term (Falta and Wang, 2017; Muskus and Falta, 2018). This semi-analytical/numerical method, as a result, is not as computationally intensive as conventional matrix diffusion modeling methods.The objective of this research is to better understand the parameterizations that affect the back diffusion signal in a chemical transport model, which is accomplished by applying the semi-analytical/numerical modeling method to theoretical scenarios that are representative of field conditions. This research aims to develop a better intuition for back diffusion effects that can be applied to future field studies. From this study, it was concluded that the observation of the most significant back diffusion effects in any aquifer system is dependent on monitoring well location relative to the highest concentrations within the aquifer and the low permeability/high permeability interfaces. The initial source concentration is critical for determining the magnitude at which back diffusion affects aquifer concentrations, which in some cases can be below the MCL. The low k zone degradation rate was found to be a key parameter for determining the magnitude of plume persistence caused by back diffusion. Diffusive mass flow was shown to be governed by porosity and the geometric parameterization for embedded low k material or fractures. Lastly, partial source zone remediation that results in a residual source mass can cause plume persistence that looks similar to the effect of back diffusion, and the relative contributions of a residual source mass and back diffusion to overall plume persistence are determined by the amount of source mass removed, the amount of low k material or fractures in the aquifer system, the location in the aquifer relative to the source zone, and the low k zone parameterization. Finally, a field site was assessed where the gained insights from this study were used to determine the potential risk for back diffusion at the site and to develop a model to evaluate any observed plume persistence for back diffusion.
Prater, Wesley, "Application and Parameterization of a Semi-Analytical/Numerical Method for Modeling Matrix Diffusion Effects in Groundwater Chemical Transport" (2019). All Theses. 3203.