ESTIMATION OF DUAL-DOMAIN MASS-TRANSFER COEFFICIENT USING MULTIPLE APPROACHES FOR GROUNDWATER REMEDIATION

In situ groundwater remediation can be technologically challenging due to the heterogeneous nature of the subsurface environment. The dual-domain conceptual model can partially account for these heterogeneities, and has been widely validated in both porous and fractured media to represent rapidly and slowly conducting pore sequences. In remediation modeling, diffusive mass transfer between the mobile and immobile domains is generally considered to be non-equilibrium and may be approximated using a first-order mass transfer rate coefficient. Such dual-domain models have been increasingly used to help manage and remediate sites contaminated with organic chemicals such as trichloroethylene (TCE). However, the dual-domain mass-transfer coefficient, a key model parameter required in the dual-domain approach, is one of the most difficult parameters to characterize through site investigation efforts.

This study provides estimates of the dual-domain mass transfer coefficient determined using multiple quantitative approaches in alluvial soils at a site contaminated with TCE. The first approach was based on results of a fluorescent-dye tracer test conducted during remedial design activities. This mass transfer coefficient was estimated during calibration of a MODFLOW/MT3D numerical model to tracer breakthrough curves. Alternative approaches included derivations based on: (1) the geometrical structure of soils; (2) permeability contrasts between coarser-grained and finer-grained soils; and (3) the effective surface area between mobile and immobile domains. These various estimates of the mass transfer coefficient were compared and evaluated. Results demonstrate that the mass transfer coefficient could be reasonably estimated using physically-based methods, but the tracer test provided the most definitive answer. Results also provided insights that may allow mass transfer coefficients to be more accurately estimated using conventional site characterization data such as geologic logging and permeability testing. Uncertainties remain, however, due to the scale-dependent nature of dual-domain mass transfer.