PNNL

Abstract

Migration of groundwater contaminants in the Gable Gap area of the Hanford Site, in southeastern Washington State, is strongly influenced by the distribution and permeability of basalts that lie beneath an unconfined aquifer. Locally, folding and faulting of the Columbia River Basalt associated with the Yakima fold and thrust belt followed by erosion due to the Lake Missoula floods resulted in a complex basalt surface that represents either an impermeable lower boundary to the confined aquifer system or localized regions of increased permeability that potentially promote communication between the unconfined aquifer system and deeper confined aquifer systems. In 2011, a seismic landstreamer campaign was carried out to image the basalt surface and produced pre-stack depth migrated reflection images. The reflection images identified two large troughs that may represent paleo-channels and several areas of possible faulting. Here, the streamer data are re-analyzed using refraction travel-time and Rayleigh wave dispersion analyses to obtain images of compressional and shear wave velocities within the suprabasalt sediment sections and the upper basalt surface. The combined interpretation of reflection and seismic velocity images shows complexity in the basalt velocity and surface elevation, and, could potentially guide future boreholes needed to quantify vertical flow between the confined and unconfined aquifers.