Coupled Hydrologic and Geophysical Inversion for Characterization of Nonaqueous Phase Source Zones

Concentration plume evolution and chemical persistence at chlorinated solvent contaminated sites are typically controlled by the presence of dense nonaqueous phase liquid (DNAPL) mass within a highly contaminated region or DNAPL source zone. Characterization of both the extent and structure of the source zone DNAPL mass distribution is, thus, of great importance to site assessment and successful remedial design. This paper provides an overview of ongoing research in our group designed to develop new approaches to DNAPL source zone characterization based on the joint inversion of hydrological and geophysical data. Hydrological data are obtained from three-dimensional (3D) simulations of DNAPL infiltration, entrapment, and subsequent rate-limited dissolution for a representative contaminant (PCE) under a range of geologic conditions. Electrical resistance tomography (ERT) is employed as the geophysical modality, where electrical resistivity is mapped to local saturations through an Archie's law expression. Simulated down gradient transect concentrations and cross-hole ERT measurements are then used, in conjunction with a shape-based inversion technique, to infer the source zone envelope (i.e, the region of space over which DNAPL saturation is nonzero), as well as the average saturation in this region. The potential performance of this approach for DNAPL source zone characterization is illustrated and discussed.