Investigation of Mass Flux Reduction as a Function of Source-Zone Mass Removal for Immiscible-Liquid Contaminated Aquifers

Abstract

The magnitude of contaminant mass flux reduction associated with a specific amount of contaminant mass removed is a key consideration for evaluating the effectiveness of a source-zone remediation effort. Thus, there is great interest in characterizing, estimating and predicting relationships between mass flux reduction and mass removal. Intermediate-scale flow- cell experiments and published data for several field studies were examined to evaluate factors controlling the mass-flux-reduction/mass-removal relationship. Flow-cell experiments evaluated the impact of source-zone architecture and flow-field heterogeneity on mass-flux-reduction/mass-removal behavior. Significant reductions in mass flux occurred for systems wherein immiscible-liquid mass was present at both residual saturation and in high saturation pools. For a system with immiscible liquid present in multiple zones of different permeability, an increase in mass flux was observed for late stages of mass removal. Image analysis confirmed that the late stage increase in mass flux was attributed to changes in relative permeability. Early reductions in mass flux were also observed for systems wherein immiscible-liquid mass was poorly accessible to flowing water. End-point analysis, based on comparing masses and mass fluxes measured before and after a source-zone remediation effort, conducted for 21 field remediation projects ranged from slightly less than to slightly greater than one-to-one. Time-continuous analysis, based on continuous monitoring of mass removal and mass flux, performed for two sites illustrated the dependence of the mass-flux-reduction/mass-removal relationship on source-zone architecture and mass-transfer processes. Minimal mass flux reduction was observed for a system wherein mass removal was relatively efficient. Conversely, a significant degree of mass flux reduction was observed for a site wherein mass removal was inefficient. A simple mass-removal function was used to evaluate the measured data at both the intermediate and field scales. This function was unable to capture the complex behavior observed for some of the systems unless specific measurable system parameters were incorporated into the function. Finally, mathematical models of varying complexity used to simulate immiscible liquid dissolution illustrated the dependence of the calibrated dissolution rate coefficient on implicit and explicit consideration of larger-scale factors influencing immiscible liquid dissolution.