Numerical Simulation of DNAPL Emissions and Remediation in a Fractured Dolomitic Aquifer

The knowledge of Dense Nonaqueous Phase Liquid (DNAPL) source zone architecture and corresponding emissions is necessary in quantifying the efficacy of partial source zone removal on site closure and corresponding cleanup time estimation. Research on movement and dissolution mechanisms of pure-phase DNAPL and the migration of the aqueous-phase plumes is limited in fractured rocks. This study focusses on the numerical modeling of various chlorinated solvents that have penetrated the fractured dolomitic bedrock at the Smithville site in Ontario, Canada, where a large aqueous phase plume has developed over the last several decades and since 1989 has been hydraulically controlled by a pump-and-treat remediation system. We adopt a two-step procedure in which a multiphase compositional model CompFlow is first applied to simulate the migration of DNAPLs in a discretely fractured porous medium with hydrostratigraphy representing the Smithville site. In the second step, the CompFlow results are employed to define the source term for a regional-scale transport simulation using HydroGeoSphere by treating the layered, fractured dolomitic rocks as an equivalent porous continuum. Transport simulations are conducted both prior to and after the operation of the pump-and-treat system. Results reveal that good agreement with the observed mass removal data and TCE plume can be achieved by modifying the composition of the DNAPL source and by reducing the hydraulic conductivity in the source zone region to account for preferential flow around it. Our transport model results support the conceptual model of TCE contamination which posits a mixed source (2 to 4%) of pure DNAPL with limited contact with actively flowing groundwater that is undergoing equilibrium dissolution. Model results also reveal that the pump-and-treat system has neither been effective in stabilizing the plume nor removing a significant amount of contaminant mass. Simulation results indicate that the stability of the plume is likely due to biodegradation.