Contaminant Transport with Time-Dependent DNAPL Source Dissolution Kinetics and Biodecay: Effects of Partial Mass Removal, Flow Reduction and Enhanced Biodecay

A simple parametric model is described for field-scale DNAPL dissolution kinetics as a function of mean groundwater velocity, initial DNAPL mass, mass remaining, and a mass depletion exponent, field-scale mass transfer coefficient, and source zone biodecay coefficient. Model validation was performed by assessing model fit to synthetic data sets generated from high-resolution numerical simulations. Results indicate that field- scale dissolution rates vary in direct proportion to relative DNAPL mass remaining raised to an empirical depletion exponent, which is <1 for source zones dominated by DNAPL pools or lenses and is >1 for finger- like residual DNAPL regions. The parametric model is capable of accurately describing mass flux versus time from finger-dominated or pool-dominated source regions individually. When both types of geometries occur within a given source zone, a dual continuum approach provides better accuracy than a lumped model if sufficient data is available for calibration. The source model, coupled with a dissolved phase transport model, is applied to evaluate the effects of three source zone remediation strategies: 1) partial mass removal, 2) partial source zone flow reduction, and 3) enhanced source zone biodecay for both a finger-dominated and pool-dominated DNAPL sources. Each of the strategies is compared to a corresponding "no action" scenario. For each scenario, source mass flux and concentrations at downgradient receptor locations are evaluated versus time. Mass removal and enhanced biodecay reduce source mass flux, downgradent concentrations and source duration. Flow reductions reduce source mass flux and downgradient concentrations in the short term at the expense of increasing source duration. Depending on compliance point location, maximum allowable concentration, and other remediation criteria, any of these strategies may be reasonable and applicable under certain site conditions. Combining two or more of the methods and integrating dissolved plume management strategies may further improve remediation performance.