Scientific Investigations Report 2008–5071
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5071
Conceptual Model of Hydrologic and Thermal Conditions
The Eastbank Aquifer system is located in sedimentary deposits east of and adjacent to Rocky Reach Dam, which is a run-of-the-river hydroelectric dam on the Columbia River north of Wenatchee and East Wenatchee, Washington (fig. 1). Construction of the dam began in 1956 and the dam is owned and operated by Public Utility District No. 1 of Chelan County (PUD). By the time the dam was put into commercial operation in 1961, the water level of the Columbia River was raised from a natural low water level of about 610 ft (Stone and Webster Engineering Corporation, 1959) to a full-pool level of 707 ft. This rise formed a lake called Lake Entiat, which extends from Rocky Reach Dam upstream to Wells Dam (fig. 1). The water-level rise of the Columbia River similarly increased the saturated thickness of the Eastbank Aquifer system. As part of dam construction, a subsurface cutoff wall was constructed in the aquifer system (fig. 2) to minimize seepage around the eastern extent of Rocky Reach Dam and prevent destabilization of the east bank. This cutoff wall in effect acts as a “subsurface dam” and helps maintain elevated ground-water levels in the Eastbank Aquifer system to the north of the cutoff wall. The result is that the drop in ground-water levels from north to south of the cutoff wall is similar to the drop in river water levels from upstream to downstream of Rocky Reach Dam.
The primary use of the Eastbank Aquifer system is to supply water for an on-site fish hatchery called the Eastbank Hatchery (fig. 2), and the regional water system serving the cities of Wenatchee, East Wenatchee, and parts of unincorporated Chelan and Douglas Counties. In 2006, mean annual pumpage from the regional water system was about 16 ft3/s and mean annual pumpage from the Eastbank Hatchery was about 43 ft3/s. The regional water system pumps water from the RW well field—wells RW1, RW2, RW3, and RW4 (fig. 2) and serves more than 65,000 people through about 26,000 connections (M. Cockrum, City of Wenatchee, written commun., 2008). As the population of the service area continues to grow, the regional water system may need to pump more water in the future to serve additional customers. The hatchery pumps water from the CT well field—wells CT1, CT2, CT3, and CT4 (fig. 2). The secondary use of the Eastbank Aquifer system is to supply irrigation water for Lincoln Rock State Park (LR well field—wells LR1, LR2-E, and LR2-W; fig. 2), a small quantity of industrial water to lubricate turbines of the Rocky Reach Hydroelectric Project (wells SW13 and SW14 of the SW well field; fig. 2), and a small quantity of irrigation water for miscellaneous sites outside Lincoln Rock State Park (well SW11 of the SW well field; fig. 2).
The Eastbank Hatchery is owned by the PUD and operated by the Washington Department of Fish and Wildlife. The hatchery forms part of the Anadromous Fish Agreement and Habitat Conservation Plans that allow the PUD to operate the Rocky Reach and Rock Island Hydroelectric Projects under license agreements with the U.S. Federal Energy Regulatory Commission (FERC). The hatchery helps compensate for losses of sockeye, spring and summer Chinook salmon, and summer steelhead (Public Utility District No. 1 of Chelan County, 2007a). Successful operation of the hatchery relies on access to relatively cool ground water, preferably not exceeding 13°C (Ian Adams, Public Utility District No. 1 of Chelan County, written commun., 2008). Ground-water temperatures are reported to have increased in recent years. If these increases continue, the PUD would either need a different approach for supplying appropriate water to the hatchery or alternative solutions for meeting its hatchery obligations.
To help understand why the ground-water temperatures may have been increasing and to determine the data needs for possible future evaluations of aquifer-system management alternatives that maintain sufficiently cool ground water for the successful production of fish in the Eastbank Hatchery, the PUD requested that the U.S. Geological Survey (USGS) conduct a study of the Eastbank Aquifer system. The objective of this study is to improve the understanding of the hydrologic and thermal conditions of the Eastbank Aquifer system and the processes that affect those conditions. The objective was met by evaluating available hydrologic, water-temperature, and related information, identifying data gaps, collecting new data, and developing an updated data-collection program.
This report documents the development of a conceptual model of hydrologic and thermal conditions of the Eastbank Aquifer system near Rocky Reach Dam, Douglas County, Washington, and the need for additional data and analyses to improve the understanding of the Eastbank Aquifer system. Information used to develop the conceptual model includes reports that document the design and construction of the subsurface cutoff wall east of Rocky Reach Dam (Stone and Webster Engineering Corporation, 1959), analyses of the hydrology and hydrogeology of the Eastbank Aquifer system (CH2M Hill, 1977 and 1988; Water & Environmental Systems Technology, Inc., 1990), vertical temperature profiles of the ground-water system (Water & Environmental Systems Technology, Inc., 1990), and numerical models of the Eastbank Aquifer system that simulate hydrologic and thermal conditions starting in 1989–90 (Water & Environmental Systems Technology, Inc., 1990 and 1998). Additional information evaluated include hourly river and aquifer water levels and water temperatures collected by the PUD in a monitoring network since 1990, miscellaneous data from PUD files, historical ground-water pumpage from wells supplying the regional water system and Eastbank Hatchery, and water-level, water-temperature, and other water-quality data collected during 2007–08 as part of this study.
The study area is located in Douglas County, Washington, in a river-terrace deposit along the east side of the Columbia River about 8 mi north of Wenatchee (fig. 1). The study area, which covers about 150 acres, includes Lincoln Rock State Park and the area to the south, including a subsurface cutoff wall constructed as part of Rocky Reach Dam. The dam is located at river mile (RM) 473.7 and creates Lake Entiat (fig. 1), which ranges in altitude from a normal low pool of 703 ft to a normal full pool of 707 ft (Public Utility District No. 1 of Chelan County, 2007b). The Columbia River drainage area upstream of Rocky Reach Dam is about 88,000 mi2 and drains parts of Washington, Idaho, and Montana; and British Columbia, Canada. Mean discharge at Rocky Reach Dam for the period of record (October 1961 through September 2006) is 113,900 ft3/s with a maximum discharge of about 535,000 ft3/s on June 10, 1961, and a minimum daily discharge of 25,100 ft3/s on November 11, 1973 (U.S. Geological Survey, 2007).
The subsurface of the study area consists of coarse- and fine-grained sediments deposited along the east side of the Columbia River Valley on top of metamorphic bedrock. This bedrock forms the base beneath Rocky Reach Dam and crops out along the west bank of the river. East of the study area, metamorphic bedrock is overlain by Columbia River flood basalts that form the Columbia Plateau. The topography of the study area has low relief, with an average altitude of about 740 ft (fig. 2). Bedrock on the west and east sides of the river steeply rises to altitudes exceeding 2,400 and 2,000 ft, respectively, within 1 mi of the study area.
The climate of the study area exhibits characteristics of both maritime and continental climates (National Oceanic and Atmospheric Administration Western Regional Climate Center, 2007a). The prevailing wind direction is from the southwest or west, which brings in the remnants of humid air masses generated over the Pacific Ocean after their flow has been impeded by the Cascade Range. Extreme summer and winter temperatures occur when the wind direction shifts to the north and east and continental air flows into the area (National Oceanic and Atmospheric Administration Western Regional Climate Center, 2007a). Generally, summers in the study area are warm and dry and winters are cold and also relatively dry. During the most recent climate-normal period (1971–2000) at the National Weather Service climate station in nearby Wenatchee (site 459074), the mean monthly precipitation ranged from 0.3 in. in July to 1.5 in. in December and the mean monthly minimum and maximum air temperatures ranged from -4.9 and 1.7°C in January, respectively, to 16.1 and 31.0°C in July, respectively (National Oceanic and Atmospheric Administration Western Regional Climate Center, 2007b; fig. 3). Mean annual precipitation was 9.1 in. and mean annual minimum and maximum air temperatures were 5.4 and 16.9°C, respectively.
Except for irrigated lawns, shrubs, and trees in Lincoln Rock State Park and limited additional sites, the study area is sparsely vegetated with primarily grasses and shrubs that have adapted to local conditions.
As part of the design process for Rocky Reach Dam in the 1950s, geotechnical engineering surveys and hydrologic assessments were conducted in and near the study area. Results of this work near the subsurface cutoff wall (fig. 2) were summarized by Stone and Webster Engineering Corporation (1959) when they described the design and construction of the subsurface cutoff wall. The feasibility of using the Eastbank Aquifer system as a public water supply for the Wenatchee urban area was investigated by R.W. Beck and Associates (1973) and included an aquifer assessment by Robinson and Noble, Inc. Subsequently, Robinson and Noble, Inc. conducted a more detailed aquifer assessment that was reported by CH2M Hill (1977) as part of a predesign study of the regional water system. The study included descriptions of the installation and testing of well RW1, the first well of the regional water system. CH2M Hill documented the installation and testing of wells RW2, RW3, and RW4 (1979; as reported by Water & Environmental Systems Technology, Inc., 1990) and wells CT1, CT2, CT3, and CT4 (1988). CH2M Hill (1988) also presented results of seismic-refraction and electrical-resistivity surveys of the Eastbank aquifer system and parts of a draft report dated 1987 that included detailed hydrogeologic cross sections of the system (appendix 1).
Water & Environmental Systems Technology, Inc. (1990) analyzed the feasibility of using the Eastbank Aquifer system as a source of water for the Eastbank Hatchery over the long term by considering both the availability and temperature of ground water. As part of this analysis, they developed numerical ground-water models to help evaluate the hydrologic and thermal conditions of the ground-water system in 1989–90 and possible future conditions. The first of these models was a finite-difference ground-water flow model, MODFLOW (McDonald and Harbaugh, 1988), that was used to verify the conceptual model of the flow system and refine aquifer properties. The second model was a finite-element model, CFEST (Gupta and others, 1987), that was used to assess hydrologic and thermal conditions in 1989–90 and evaluate possible future conditions for different combinations of river temperature and seasonal pumping. Water & Environmental Systems Technology, Inc. (1990) also designed a network for monitoring river and aquifer water levels and water temperatures. Data from this network collected by the PUD since 1990 were used by Water & Environmental Systems Technology, Inc. to verify the previously developed CFEST model (Water & Environmental Systems Technology, Inc., 1998). The model was subsequently used to assess possible impacts on the hydrologic and thermal conditions of the Eastbank Aquifer system, assuming increased pumping by the regional water system to accommodate a possible expansion of the system’s service area to include East Wenatchee (Water & Environmental Systems Technology, Inc., 1998). The expansion of the service area and the increase in pumping took place in 2001.
In Washington, the USGS assigns wells identifiers that describe their locations with respect to township, range, section, and tract. For example, number 24N/20E-35G01 (fig. 4) indicates that the well is in township 24 North (N) and Range 20 East (E) of the Willamette base line and meridian. The number immediately following the hyphen indicates the section (35) within the township; the letter following the section gives the tract within the section, as shown in figure 4. The two-digit sequence number (01) following the letter indicates that the well was the first inventoried by USGS personnel in that tract. The nominal size of a section is 1 mi2 and the nominal size of a tract is 40 acres. In the study area, sections and tracts may deviate from their nominal sizes because they do not have standard rectangular shapes.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubs.usgs.gov/sir/2008/5071
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