Formulation of a Finite-difference Groundwater Flow Model for the Spokane Valley Aquifer, Washington PDF Download

Author: Iain A. Olness
Publisher:
ISBN:
Category : Aquifers
Languages : en
Pages : 202

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Book Description
"The Spokane Valley - Rathdrum Prairie Aquifer system is unique in that it is one of the most productive aquifers in the United States, and is the only source of good quality water for the Spokane Valley. Recharge to the aquifer comes mainly from Lake Coeur d'Alene, Lake Pend Oreille, the Spokane River, precipitation, and runoff. The aquifer extends from Lake Pend Oreille, Idaho to Long Lake, Washington and covers approximately 425 square miles. The aquifer framework is comprised predominantly of Quaternary flood deposits which originated from Glacial Lake Missoula. Precambrian-age through Tertiary-age consolidated rocks make up the bedrock beneath the Spokane Valley, along the valley margins, and in the highlands in the surrounding area. A modular three-dimensional finite-difference flow model was developed to simulate the flow of groundwater through the Spokane aquifer using MODFLOW/EM[trademark]. The modeled area extended two (2) miles east of the Washington- Idaho state line to two (2) miles west of Havana Street, the eastern corporate city limit of Spokane, Washington. The model domain consists of 6,300 cells in seventy (70) rows and ninety (90) columns, with approximately 2,900 active cells. The cells comprising the northern and southern boundaries of the aquifer, and all cells that lay outside these boundaries, were designated as no-flow cells. Cells within the aquifer boundaries were modeled as variable head and cells simulating the lakes were modeled as constant head. The eastern boundary of the modeled area was designed as a general- head boundary. Input for the model consisted of inputting data into the following seven (7) packages: (1) basic, (2) block centered flow, (3) river, (4) recharge, (5) well, (6) evapotranspiration, and (7) general-head boundary. All of the data was obtained from previously published and unpublished reports, with the majority of the data obtained from Boike and Vaccaro (1980). Little or no change in the water levels was found in the Spokane aquifer for the 1920-1991 period, indicating the system is more or less in equilibrium. Calibration of the model was accomplished by primarily adjusting hydraulic conductivity values in the variable head cells and the river bottom conductance values for the river nodes until the least differences were observed between the heads calculated by this model and the 1980 water-table map constructed by Boike and Vaccaro (1980). The groundwater flow across the state line calculated using this model was 305 cubic feet per second (cfs) and the overall groundwater flow was 752 cfs. The sensitivity of the model was tested by routinely adjusting the various physical factors (saturated thickness, hydraulic conductivity, well pump rates) uniformly and noticing the effect these changes had on the leakage to/from the Spokane River and the heads of the Spokane aquifer. The most important physical factors ascertained from the sensitivity analysis were the saturated thickness values, the hydraulic conductivity values, and the well pump rates. Groundwater flow at the state line increased 84%, flow throughout the entire model increased 34%, and an average difference in water levels of -1.45 feet resulted from using saturated thickness values determined by the U.S. Army Corps of Engineers (1951) as opposed to those determined by Newcomb and others (1953). Groundwater flow at the state line decreased by 58%, flow throughout the entire model decreased by 32%, and groundwater levels increased by an average of 2.8 feet when the hydraulic conductivity values of the separate zones was decreased by 50%. Increasing the hydraulic conductivity values of the separate zones by 100% resulted in an increase of groundwater flow at the state line by 112%, flow through the entire model increased by 63%, and water levels dropped by an average of 1.9 feet"--Document.