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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5045

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Summary and Conclusions

The Yakima River Basin aquifer system in south-central Washington encompasses an area of about 6,900 mi² including the entire Yakima River Basin and lands to the east extending to the Columbia River. The five hydrogeologic units delineated, from top to bottom, are the Saddle Mountains, Mabton, Wanapum, Vantage, and Grande Ronde. Information for these units was compiled and combined from several data types and sources, which include a simplified geology map, hydrogeologic contour maps from previous studies, and interpretations of about 3,000 well records from previous studies and investigators. This information was used to delineate the extent of and the depth to the top of three basalt and two interbed hydrogeologic units within the study area.

All data and information were converted to digital data layers and entered into a GIS software program in order to construct a 3D digital framework of the unit extents and layers using a gridded 30-meter cell size. Data layers for each basalt and interbed unit include a DEM, simplified surficial-geology map, previously constructed hydrogeologic unit contour maps (where available), mapped extent of the hydrogeologic unit, and well-log point values of the depth to the top of the individual unit. The original data interpretations were honored as much as possible in developing the 3D framework. The calculated depth to the top of the unit-cell values and (or) mapped contours for the hydrogeologic units were compared to the original well and (or) mapped contour data then adjusted to reflect more accurately the original interpretations.

The hydrogeologic framework defines the physical, lithologic, and hydrologic characteristics of the basalt and interbed hydrogeologic units that compose part of the ground-water system in the Yakima River Basin. The hydrogeologic characteristics vary from unit to unit, and the extent and depth-to-top of each hydrogeologic unit is described.

The Saddle Mountains unit is the youngest and least extensive of the basalt units encompassing an area of about 2,290 mi². The Saddle Mountains unit ranges in altitude from 4,290 ft where it is exposed at land surface to a depth of 1,840 ft below land surface with most of the unit lying beneath the basin-fill deposits. The hydrogeologic unit consists predominantly of basalt flows and interbed members of the Saddle Mountains Basalt whose flow texture and composition differ greatly throughout its extent.

The Mabton unit is the sedimentary interbed between the Saddle Mountains Basalt and Wanapum Basalt. It encompasses an area of about 2,210 mi², and generally is present beneath the Saddle Mountains unit. The depth to the top of this unit ranges from 80 to 2,000 ft below land surface. The hydrogeologic unit generally consists of clay, shale, claystone, clay with basalt, clay with sand, and sandstone, but also may contain small amounts of sand and sand-and-gravel.

The Wanapum unit extends to the north and west beyond the Saddle Mountains unit and encompasses an area of about 3,450 mi². The Wanapum unit ranges in altitude from 5,680 ft where it is exposed at land surface to a depth of 2,050 ft below land surface with most of the unit lying beneath the basin-fill deposits, Saddle Mountains unit or Mabton unit. The hydrogeologic unit consists predominantly of basalt flows and interbed members of the Wanapum Basalt. The basalt flows generally are medium-grained to moderately plagioclase-phyric, olivine bearing and relatively high in iron and titanium oxides.

The Vantage unit is the sedimentary interbed between the overlying Wanapum Basalt and Grande Ronde Basalt. It encompasses an area of about 3,090 mi² with most of the unit present beneath the Wanapum unit. The depth to the top of this unit ranges from 40 to 2,790 ft below land surface. The hydrogeologic unit generally consists of clay, shale, sandstone, tuff with claystone, and clay with basalt, but also may contain small amounts of sand and sand-and-gravel.

The Grande Ronde unit is the oldest and most extensive of the basalt units encompassing an area of about 5,390 mi². The Grande Ronde unit ranges in altitude from 6,900 ft where it is exposed at land surface to a depth of 2,800 ft below land surface with most of the unit lying beneath the basin-fill deposits. The hydrogeologic unit consists predominantly of basalt flows and interbed members of the Grande Ronde Basalt. The basalt flows are aphyric with microphenocrysts of plagioclase and clinopyroxene.

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