INTERACTING PROCESSES AFFECTING CONTAMINANT TRANSPORT IN UNSATURATED FRACTURED ROCK

In low-permeability unsaturated fractured rock, water flows predominantly through the interconnected fracture network, with some water imbibing into the neighboring matrix rock. Imbibition (driven by capillary pressure gradient) advectively transports contaminant from fractures into the matrix. Diffusion (driven by concentration gradient) can diffusively transport contaminant into the matrix (matrix diffusion), or from the matrix back into flowing fractures (back diffusion). Once in the matrix, sorbing contaminants can sorb onto matrix rock. All these interacting processes (imbibition, sorption, and diffusion) tend to retard breakthrough of a contaminant pulse released to the fracture network. A number of factors, such as the fracture flow rate, initial matrix saturation, matrix pore connectivity, and intermittent flow events, will influence these interacting processes.

This work presents new and innovative approaches [application of imbibition tests to probe pore connectivity, uniform initial saturation for cm-sized rock within relative humidity chambers controlled with saturated salts, utility of multiple tracers with different diffusivity and extent of sorption] and techniques [e.g., the unsaturated transport-sorption method, laser ablation coupled with inductively coupled plasma-mass spectrometry (LA*ICP/MS) for micro-scale solid sampling]. Coupled with pore-scale network modeling, these approaches and techniques are developed to understand and quantify the interacting unsaturated transport processes.

This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.