Wellfield installation and investigations, Creston Study Area, Eastern Washington

The report summarizes the design and implementation of field testing used to characterize the Roza Member basalt aquifer of the Wanapum Formation of the Columbia River Basalt Group. A research wellfield was implemented and tests conducted to lithologically classify subsurface conditions underlying an 800- by 1400-foot area located in the northwest quarter of Section 16, T.25N, R.34E, six miles south of the town of Creston in Lincoln County, Washington. The relatively high-yielding aquifers of the Roza Member basalt are separated by claystone aquitards and are hydrologically interrupted by at least two different subsurface hydrologic structures. Water-level data were used to determine the regional and local hydraulic gradient. Wellbore slug tests and passive (nonpumping) tracer tests permitted calculation of equivalent hydraulic apertures and effective porosities for the Roza basalt flow top and flow interiors. Analysis of data from pumping tests permitted calculation of transmissivities and storage coefficients for the Roza basalt. Measurements of water level are continuing, and additional tracer tests have been conducted both in the wellfield proper and in the larger study area. Applicable wellfield aquifer analyses along with model studies will provide guidelines for use in evaluating proposed sites for disposing of high-level nuclear waste in saturated fracturedmore » geologic media. 36 refs., 72 figs., 16 tabs.« less