Azimuthal Resistivity Investigation of an Unconfined Aquifer at the Hanford Integrated Field Research Challenge Site

Developing a robust large-scale groundwater contaminate transport model requires quantifying the effect of heterogeneity and anisotropy on solute transport. Here we investigated the feasibility of using surface azimuthal resistivity methods to characterize near-surface anisotropy and heterogeneity in order to improve the conceptual model for uranium transport through unconsolidated sediment at the Integrated Field Research Challenge Site (IFRC) which borders the Columbia River. A generalized azimuthal resistivity array was constructed with seven telescoping radii and 15° rotations between each electrode. Azimuthal array data were acquired by multiplexing with the MPT-DAS1 system connected to 172 surface electrodes. Array geometries included the square array, arrow array, offset wenner and equatorial dipole-dipole. Effective depths of exploration ranged between 5 and 57 m. Results from the upper 5m of exploration depth exhibit an isotropic resistivity which is consistent with the excavation and homogonous fill depth of the waste ponds at the IFRC. Exploration depths beyond 5 m are influenced by the Hanford and Ringold Formations. These formations exhibit a strong anisotropic resistivity which increases with depth. Assuming that the response is entirely controlled by hydrologic anisotropy, these azimuthal resistivity data suggest a preferential path with a mean azimuth between 150° and 170°. This azimuthal resistivity trend coincides with an incision feature in the Ringold formation measured in a suite of core logs and is consistent with the trajectory of a tracer plume from an injection test conducted in March 2009. Surface azimuthal resistivity methods may also have application in characterizing localized anisotropy and heterogeneity within shallow alluvial deposits at Hanford allowing for the optimal placement of tracer injections and borehole electrodes.