Abstract

Foam is a promising vehicle for delivering amendments into the vadose zone for in situ remediation; it is an approach being considered for in situ treatment and stabilization of metals and radionuclides located within the deep vadose zone of the Department of Energy’s Hanford Site in the state of Washington. A central aspect of evaluating the effectiveness of this approach is the ability to monitor foam distribution, its transformation, and the reactions that it induces in the subsurface, ideally in a noninvasive manner. In this study, we performed laboratory experiments to evaluate the potential of geophysical methods (complex resistivity and time-domain reflectometry [TDR]) as tools for monitoring foam-assisted amendment delivery in the deep vadose zone. Our results indicated great sensitivity of electrical methods to foam transportation and evolution in unsaturated porous media that were related to foam bubble coalescence and drainage processes. Specifically, we observed (i) a decrease in electrical resistivity (increase in electrical conductivity) by more than an order of magnitude in both silica sand and natural sediment matrices during foam transportation; (ii) an increase in resistivity (decrease in conductivity) of more than twofold during foam coalescence and drainage; and (iii) a distinct phase and imaginary conductivity signature related to the evolution of water films on sediment grains during foam injection and evolution processes. To assist with the interpretation of these data, TDR measurements were used to monitor moisture content, which provided complementary information about foam distribution and drainage. Our results clearly demonstrated the sensitivity of electrical and TDR signals to foam transportation and evolution in unsaturated porous media and suggest the potential of these methods for monitoring the response of a system to foam-based remediation treatments at field scales.

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