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 covers about 150 acres and is located in a river-terrace deposit along the east bank of the Columbia River near Rocky Reach Dam, about 8 miles north of Wenatchee, Washington. It consists of the Upper and Lower Aquifers, which are highly permeable sand-and-gravel aquifers separated by the Clay Confining Unit. Where the Clay Confining Unit is absent in the northwestern part of the study area, the aquifers merge to form the Combined Aquifer. The primary use of the Eastbank Aquifer system is to supply water from the Lower and Combined Aquifers to the Eastbank Hatchery and the regional water system, which serves more than 65,000 people in and near the cities of Wenatchee and East Wenatchee. The hatchery is owned by the Public Utility District No. 1 of Chelan County (PUD) and compensates for fish losses resulting from the Rocky Reach and Rock Island Hydroelectric Projects. The Eastbank Hatchery needs relatively cool water for successful operations and, reportedly, temperatures of ground water pumped by the hatchery have been increasing. The PUD asked the U.S. Geological Survey to conduct a study of the Eastbank Aquifer system to help understand why the ground-water temperatures may have been increasing and to determine data needs for possible future evaluations of aquifer-system management options that maintain sufficiently cool ground water for hatchery operations.
The Upper, Lower, and Combined Aquifers are primarily recharged by water from the Columbia River. Ground-water discharge occurs as seepage around and through a subsurface cutoff wall and ground-water pumping. The main pumping centers are the RW well field in the central part of the study area, which supplies the regional water system, and the CT well field in the south-central part of the study area, which supplies the hatchery. The RW and CT well fields became operational in 1983 and 1989, respectively. From 1990 through 2000, annual mean pumpage from the RW well field was relatively constant, with a mean annual pumpage of 10.7 ft3/s. Pumpage increased by about 40 percent to a mean annual pumpage of 15.0 ft3/s from 2002 through 2006 due to an expansion of the service area to include the city of East Wenatchee. The mean annual pumpage from the CT well field probably has been relatively constant since 1994, although there is greater uncertainty in the historical pumpage estimate for the CT well field than the RW well field. In 2006, mean annual pumpage from the hatchery and regional water system was about 43 and 16 cubic feet per second (ft3/s), respectively.
Ground-water levels measured on July 18, 2007 indicate that there are two overlapping cones of depression in the Lower and Combined Aquifers with an approximately east-west trending ground-water divide between them. The cone of depression surrounding the RW well field draws water primarily from the west and secondarily from the north, while an additional, smaller amount of water is drawn in from the south, east, and probably from beneath the wells. The cone of depression surrounding the CT well field draws water primarily from the west and southwest, with an additional, smaller amount of water drawn in from the north and east. A spatial analysis of dissolved-constituent and bacterial concentrations in water sampled in nine wells and one river location in August 2007 was consistent with the ground-water flowpaths inferred from the July 18, 2007, water-level data.
The PUD has measured hourly water levels since 1990 in a monitoring network of 1 river site and 12 wells distributed throughout the Eastbank Aquifer system for the purpose of monitoring hydrologic and thermal conditions of the system. Because potentiometric gradients in the Lower Aquifer are small, the uncertainty in the historical water-level measurements was too large to use the data to analyze for possible trends in hydrologic conditions. The uncertainty in the historical water-temperature measurements was less, however, and these data were analyzed for trends in thermal conditions.
Analyses of interannual trends in time lags between changes in river temperatures and subsequent changes in ground-water temperatures showed that most of the Lower and Combined Aquifers have been in thermal equilibrium—defined by constant time lags, since 1999 and the equilibrium was reached during 1991–98. The only exceptions are the Combined Aquifer near well LR2-W, which had not reached thermal equilibrium by 2006, and the Lower Aquifer near wells TH1 and TH4, which reached thermal equilibrium prior to 1991.
Analyses of interannual trends in river temperatures showed increasing trends in annual mean and maximum river temperatures from 1999 through 2006. The mean annual increase was 0.07°C for the annual mean and 0.17°C for the annual maximum river temperature. There were no trends in the annual minimum temperatures from 1999 through 2006, and there were no trends in the annual minimum, mean, and maximum river temperatures from 1991 through 1998 and from 1991 through 2007. The increases in river temperatures from 1999 through 2006 resulted in corresponding increases in Lower and Combined Aquifer temperatures, except near well TH8. The increases in mean annual river temperatures and thus ground-water temperatures over a relatively short, multi-year period are within the natural variability of the river temperatures and decreases in mean annual river temperatures are likely during other multi-year periods.
Interannual trends in ground-water temperatures are controlled by interannual trends in river temperatures, interannual trends in seasonal pumpage patterns, and the extent of thermal equilibrium in the aquifer system. From 1999 through 2006, seasonal pumpage patterns were relatively stable and most of the aquifer system was in thermal equilibrium; thus reported trends of increasing temperatures of water pumped by the CT well field are most likely explained by increasing trends in river temperatures.
A numerical model could be used to evaluate if there may be pumping alternatives that can meet the water demand by the Eastbank Hatchery and the regional water system and also provide sufficiently cool water for the hatchery. Updating or constructing a numerical model would benefit from continued monitoring of the hydrologic and thermal conditions of the Lower Aquifer for at least 1 year. A numerical model would be more reliable if more detail were available about the source of the water flowing through the North and South Weirs and if the presence of a bedrock depression with possible cold-water storage near the RW well field could be confirmed.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubs.usgs.gov/sir/2008/5071
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