Concentration rebound following in situ chemical oxidation in fractured clay

A pseudo two-dimensional, transient flow and transport numerical model was developed to simulate in-situ chemical oxidation (ISCO) of trichloroethylene (TCE) and tetrachloroethylene (PCE) by potassium permanganate in fractured clay. This computer model incorporates Dense non aqueous phase liquid (DNAPL) dissolution, reactive aquifer material; multi-species matrix diffusion, and kinetic formulations for the oxidation reactions. A sensitivity analysis for two types of parameters, hydrogeological and engineering, including matrix porosity, matrix organic carbon, fracture aperture, potassium permanganate dosage, and hydraulic gradient was conducted. Remediation metrics investigated were the relative rebound concentrations and percent mass destroyed. A well-defined correlation was not found between the magnitude of rebound concentrations during post-remedy monitoring and the amount of contaminant, mass destroyed during the application. Results indicate that all investigated parameters affect ISCO remediation in some form. Results indicate that when advective transport through the fracture is dominant relative to diffusive transport into the clay matrix (large Peclet, number), permanganate is more likely to be flushed out of the system and treatment is not optimal. If the Peclet number is too small, indicating that diffusion into the matrix is dominant relative to advection through the fracture, permanganate does not traverse the entire fracture, leading to post-remediation concentration rebound. Optimal application of ISCO requires balancing advective transport, through the fracture with diffusive transport into the clay matrix.