Mathematical Modelling of DNAPL Source Zone Remediation

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West, Michael
DNAPL , Remediation , ISCO , SEAR , bioremediation , modelling
Mathematical modelling was utilized to evaluate trichloroethylene (TCE) and tetrachloroethylene (PCE) dense non-aqueous phase liquid (DNAPL) source zone remediation in the subsurface environment. Semi-analytical solutions were derived, tested, and employed to evaluate the benefits of source zone concentration reduction and solute degradation mechanisms on the evolution of plumes in porous media and fractured rock domains. Simulations of treatment in complex DNAPL source zones using different remedial technologies were completed with a numerical model that was developed, tested, calibrated, and applied to nine idealized heterogeneous porous media sites. Analytical modelling revealed that, in domains dominated by matrix diffusion, aggressive and moderate source zone concentration reduction may have similar effects on the leading edge of the plume. The tailing (near source) edge of the plume may be more responsive to aggressive concentration reduction, particularly when diffusion processes are negligible. Both the near-field (near-source) and far-field plume responses were strongly influenced by the matrix decay half-life for both transient and steady-state conditions. The degradative capacity of the matrix largely dictated plume extent and life-span for the fractured bedrock site considered here. Numerical simulations of in situ source zone treatment with chemical oxidation (ISCO), enhanced bioremediation (ISEB), and surfactants (SEAR) were compared and contrasted. Treatment efficacy was site specific, with benefits observed at some sites, and detrimental impacts observed at others. Each technology demonstrated some degree of performance enhancement relative to dissolution only (no treatment). The maximum DNAPL mass depletion enhancement factors for ISCO, ISEB and SEAR, were 1.44, 2.91, and 2.70 after 10 years, respectively. Similarly, the maximum boundary mass flux enhancement factors for ISCO, ISEB and SEAR were 9.78, 3.32, and 3.97, respectively. While notable enhancements were observed for many sites during active treatment, the long-term performance of pre-maturely terminated ISCO and ISEB, and to a lesser degree SEAR, was similar to dissolution. Overall, the partial depletion of DNAPL mass from source zones produced on-going persistent boundary mass flux signatures. Only the complete removal of DNAPL mass, which was attained for one site with SEAR, successfully eliminated downgradient boundary mass flux.
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