Scientific Investigations Report 2006–5116
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5116
Three classes of well information were used to delineate the hydrogeologic framework for the sedimentary deposits in the six basins. The classes include the wells inventoried during this study, wells that were previously inventoried by USGS and WaDOE personnel, and wells that were not inventoried. The latter class of wells were selected to help refine the hydrogeologic framework in strategic areas where available inventoried well data were limited.
The inventory of wells for this study began with the compilation of existing well, piezometer, geophysical, test hole, and geochemical records obtained from files of the USGS, WaDOE, consulting firms, and well owners. Beginning in the spring of 2000, more than 12,000 well records were reviewed; from these records, about 4,700 sites were selected for field inventory. Selections were based on the well owner’s or tenant’s permission to visit the well, the location and depth of the well, the availability of a driller’s log or equivalent, and the ease of access to the well. Priority also was given to wells that were previously inventoried and had water levels measured by the USGS, wells in the WaDOE observation network, and wells that were open to only one hydrogeologic unit.
During the field inventory and water-level measurement period (autumn of 2000, 2001 and spring of 2001, 2002), 2,017 sites were inventoried by USGS, Reclamation, WaDOE, and the YN. Information gathered at all inventoried sites included site location, land-surface altitude, primary use of water, owner’s or tenant’s comments on water quality, yield, surrounding land-use practices, and any changes in construction details from the original well log. In addition, the depth to water in the wells was measured when possible and, where available, the WaDOE identification tag numbers were recorded on field sheets.
Site locations for inventoried wells were plotted on USGS 1:24,000 scale topographic maps. Latitude and longitude locations were determined using a satellite-based global positioning system (GPS) with a horizontal accuracy of ±10 ft, or from field maps compiled during the inventory. Altitudes of the land surface at each well were interpolated from the topographic maps based either on the GPS or the mapped location of the well. Altitude accuracy generally was between 5 and 20 ft. Information collected during the inventory was entered into the USGS National Water Information System (NWIS) database.
The goal of the inventory was to obtain an even areal distribution of the wells in the study area. However, this was not possible for the entire study area because of time constraints and a lack of wells in less populated areas. Therefore, wells selected in this study also include wells that had been previously inventoried and wells that were not inventoried.
Wells selected to help define the hydrogeologic framework that had been previously inventoried by USGS and WaDOE, but were not re-inventoried during this study, had been field inventoried in a similar manner as the field inventory described above. The major difference between the field inventory in this study and in the previous field inventories was that fewer wells were inventoried using a GPS and the well locations were plotted on topographic maps compiled in the field. The resulting latitude and longitude values for the wells were calculated from these topographic field maps. Locations of these field checked wells generally are accurate within a radius of several hundred feet of their actual location.
Wells selected to help define the hydrogeologic framework that were not inventoried were plotted based on an available address that matched the driller’s reported township, range, section, and quarter-quarter section. If the address did not match, the well was plotted in the center of the 40-acre driller’s reported quarter-quarter section. Locations of these non-field-checked wells are generally accurate within a radius of 0.25 mi of their actual location.
The surficial geology for the Yakima River Basin was simplified from twelve 1:100,000 digital quadrangle maps available from Washington State Department of Natural Resources (DNR) Division of Geology and Earth Resources (DGER). These included the quadrangles for: Skykomish, Chelan, Snoqulamie, Banks Lake, Wenatchee, Mount Rainier, Yakima, Priest Rapids, Mount Adams, Toppenish, Richland, and Goldendale. In addition, published paper maps and detailed descriptions of the geologic units were acquired from DGER and the USGS to help in simplification and grouping of the geologic units.
The 12 maps were merged into 1 digital surficial geologic map; no attempt was made to reconcile matching of the surficial geologic units across mapped quadrangle boundaries. The surficial geologic units generally were then grouped by age (Quaternary, Tertiary, Mesozoic, Paleozoic, and preCambrian), and by type of deposit. Some deposits were grouped by formation, others were grouped by igneous phase. The CRBG was already mapped into the Grande Ronde, Wanapum, and Saddle Mountain Formations for most of the study area. Where it was mapped as a formation member, it was grouped into the associated CRBG formation. This process resulted in the original 512 surficial units being grouped into 64 units.
The resulting simplified surficial geology was used to assist in delineating the extent of six structural sedimentary basins and to delineate the hydrogeologic framework of each of these basins. The delineated basins are from north to south, the Roslyn, Kittitas, Selah (also referred to as the Selah-Wenas), Yakima (also referred to as the Ahtanum-Moxee), Toppenish, and Benton Basins (fig. 6). The geologic structure in the Yakima River Basin clearly defines most of the sedimentary basins (fig. 7).
Thickness and extent maps of the hydrogeologic units in the basin-fill deposits were constructed for the six basins based on: (1) the simplified surficial geology, (2) well interpretations from previous investigations and maps, (3) unit interpretations from sections constructed in each basin, and (4) unit interpretations made from well information collected from about 4,700 wells. Available data for the six basins were first integrated into a three-dimensional (3D) representation of the hydrogeologic framework for each basin. The deposits in the basins were then divided and mapped into hydrogeologic units and a total thickness of the basin-fill deposits. Except for the Roslyn Basin, the total thickness for each basin was based on the thickness of the deposits to the first encounter of the CBRG. The total thickness of the basin-fill deposits for the Roslyn Basin was based on the first encounter of the consolidated continental sedimentary bedrock deposits. The accuracy of the delineations of the hydrogeologic units are primarily dependent on the methods of identification. The delineations made from a geologists’ lithologic log or geophysical log are relatively accurate, but vary according to the complexity of the geology. The delineations made from drillers’ logs are the least accurate, particularly in structurally complex areas.
In addition, previously delineated and mapped hydrogeologic units were incorporated as part of this study, in particular for the Toppenish and Benton Basins. Mapping of hydrogeologic units in parts of the Yakima River Basin was previously completed by Drost and Whiteman (1986, and 1990), Drost and others (1997), Newell Campbell, (unpub. maps produced for Yakama Nation, 2001), and Reidel and Thorn, Battelle Corp. (written commun., 2003 and 2005). How this previous work was incorporated is described in more detail in the discussions of the hydrogeologic framework for the applicable basins.
All data types were entered into a Geographic Information System (GIS) software program in order to construct a gridded, digital hydrogeologic framework for each basin using a 10-meter (m) cell size. Data types for each basin included a digital elevation model (DEM), the simplified surficial geology, previously constructed hydrogeologic unit contour maps (where available), mapped extents of hydrogeologic units, well-log point values of the tops and thicknesses of a hydrogeologic unit, and the interpretations from sections that were constructed for each basin.
In developing the 3D framework, the original data interpretations were honored as much as possible. Thus, the calculated top and thickness cell values and/or mapped contours for the hydrogeologic units were compared to the original well, section interpretations, and/or mapped contour data and adjusted to more accurately reflect the original interpretations. The areas where the calculated units are less accurate are in areas of (1) data gaps, (2) where the surficial geology changes abruptly, (3) structural complexity over short distances, and (4) where the well locations are less accurate and where unit contour intervals were more generalized. However, most discrepancies were reconciled through the use of a 10-m scale during the 3D framework construction.
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U.S.
Department of the Interior | U.S. Geological
Survey
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