Development of a Borehole Electrical Technology for Assessing Diffusion and Dual Domain Mass Transfer

New technologies are needed to better understand the field-scale variations in effective diffusion coefficients, as well as the mass transfer parameters controlling contaminant transport, for low permeability zones (LPZs). Electrical geophysical measurements are theoretically related to the pore geometrical properties that control effective diffusion coefficients. Time lapse electrical measurements are sensitive to the transport of mass between LPZs and mobile zones, and can also sense sorption and desorption of mass from strongly sorbing matrix constituents. We have developed a borehole technology to interrogate diffusive mass transfer properties and processes on isolated sections of a borehole. The technology measures the complex (capturing both conduction and interfacial polarization mechanisms) electrical resistivity of a volume of sediment or rock mass adjacent to the borehole wall. Time-lapse measurements of the complex resistivity are recorded during injection of a tracer, from which diffusive mass transfer rates can be inferred. Preliminary measurements have been acquired at two Navy sites where back diffusion of contaminant mass from fractures into the rock matrix is documented. Laboratory tests on cores from the test sites validate the methodology for gaining insight into effective diffusion coefficients and mass transfer parameters in situ from a borehole.