As summarized in recent reports released by the EPA, ITRC, NRC, and SERDP, the presence of denser-than-water immiscible liquids (DNAPLs) in the subsurface remains as one of the most critical issues associated with hazardous waste sites. Immiscible liquids serve as long-term sources of contamination, and their presence can greatly impact the costs and time required for site remediation. To accurately assess the human health risks associated with DNAPLs in the subsurface, and to design effective remediation systems for such contamination, it is essential to understand contaminant mass-transfer and massflux behavior associated with DNAPL source zones. The contaminant mass flux or mass discharge emanating from a source zone, also referred to as the source strength or source function, is a primary determinant of the risk associated with a contaminated site. Concomitantly, the reduction in mass flux achieved with a specific level of source-zone mass removal (or mass depletion) is a key metric 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. The nature of the relationship between mass flux reduction and mass removal will be mediated by the properties and distribution of the porous medium and of the DNAPL (source-zone architecture), and their resultant impacts on the pore-water velocity field and mass-transfer dynamics. In addition, the relationship between mass flux reduction and mass removal may change with time due to temporal changes in source-zone architecture and mass-transfer dynamics (i.e., source-zone aging). Furthermore, the mass-flux-reduction/mass removal relationship may be affected by source-zone remediation efforts. These issues will be illustrated using the results of studies spanning a range of spatial and temporal scales.
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