Time-Continuous ANALYSIS of Mass Flux Reduction as a Function of Source ZONE Mass Removal at Two Field Sites

The degree of mass flux reduction associated with a given amount of contaminant mass removed is a key determinant 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.  There are two general approaches to evaluating the mass flux reduction/mass removal relationship, end-point analysis and time-continuous analysis.  Time-continuous analysis, based on continuous monitoring of mass removal and mass flux under aqueous flushing conditions, was performed for two sites: the Tucson International Airport Area (TIAA) Superfund Site and the Canadian Forces Base Borden Site.  The reduction in mass flux was significantly different for the two sites (90% vs. ~8%) for similar mass removals (< 40%).  These results illustrate the dependence of the mass flux reduction/mass removal relationship on source-zone architecture and mass-transfer processes.  The minimal mass flux reduction (or non-favorable) behavior was observed for the Borden site with relatively ideal mass transfer processes (equilibrium dissolution).  Conversely, favorable mass flux reduction/mass removal behavior was observed for the TIAA site with significantly non-ideal mass removal (pseudo-rate-limited dissolution).  The mass flux reduction/mass removal relationship for the TIAA site exhibited a multi-step behavior, which cannot be predicted using some of the available simple estimation functions.