Modeling improved ISCO treatment of low permeable zones via viscosity modification: Assessment of system variables

Abstract

A major challenge to successfully using in situ chemical oxidation (ISCO) for groundwater treatment is achieving uniform contact between the oxidant and contaminants in a heterogeneous aquifer. Viscosity modification technology, where a water-soluble polymer is mixed with remedial fluids, has been introduced in recent years to improve oxidant coverage of the target zone (i.e., sweep efficiency) and thus, treatment efficacy. In this work, we developed a numerical model to simulate the remedial fluid coverage from an ISCO injection with viscosity modification. Specifically, solution mixtures of xanthan and NaMnO4 were injected into a two-dimensional (2D) transport flow box that contained heterogeneous layers. Xanthan solutions were simulated as shear-thinning non-Newtonian fluids, where viscosity is a function of shear rate, polymer and NaMnO4 concentrations. Reactive transport of the polymer, NaMnO4, TCE, and reaction products were simultaneously modeled using advection dispersion reaction (ADR) equations coupled with the simulated flow field. The numerical model was validated using experimental data from the 2D cell experiments. Sensitivity analysis was conducted to investigate the relative contributions of system variables, such as polymer and permanganate concentrations, flow rate, permeability contrast, and different geometry settings. Results showed that higher concentration of permanganate and slower flow rate of the shear-thinning non-Newtonian fluids improved the oxidants ability to enter low permeable zones and react with the TCE. Higher permeability contrast decreased the velocity of the xanthan–MnO4− mixture inside the low permeable zone (LPZ), which increased TCE oxidation and product recovery. Changing the architecture of the LPZ from one zone to two smaller zones separated by a transmissive zone increased the overall product recovery. Thus, viscosity modification can improve both the sweeping efficiencies and TCE removal.