Abstract

The Hanford Nuclear Reservation, located in south-central Washington, U.S.A., is the site of a large-scale and ongoing environmental cleanup effort, which includes the remediation of radionuclide- and chemical-contaminated groundwater. Identification of preferential pathways for groundwater contaminant flow near the top of the Columbia River Basalt Group is critical for the groundwater cleanup effort. High-resolution shallow seismic surveys were conducted using a 96 channel landstreamer with gimbaled geophones on the Hanford Central Plateau near the Gable Gap area of the Hanford Nuclear Site. A primary goal of the surveys was to demonstrate the feasibility of using a landstreamer to image the top of the basalt. We were able to collect an average of 606 m of profile line at 2 m station spacing per day, for a total of 12,722 m. The survey successfully imaged the top of the basalt and demonstrated that a landstreamer with gimbaled geophones can provide quality seismic data in this area. We found that the top of the basalt ranged in depth from 30 to 100 m deep, and it displays a rugose character caused by faults and erosion from turbulent flood waters of ice-age floods originating from Glacial Lake Missoula or from later ancestral Columbia River flooding. Erosional features present in the basalt might provide an avenue for mixing of water between the supra-basalt and the basalt-confined aquifers, and faults might provide pathways for groundwater flow.

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