Hyrogeology Of The West Plains Area Of Spokane County Washington

Author : William B. Deobald
Publisher :
ISBN 13 :
Total Pages : 404 pages
Book Rating : 4.:/5 (33 download)

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Download or read book Hyrogeology of the West Plains Area of Spokane County, Washington written by William B. Deobald and published by . This book was released on 1995 with total page 404 pages. Available in PDF, EPUB and Kindle. Book excerpt: "The West Plains Study Area comprises about 130 square miles (337 square kilometers) in western Spokane County, Washington and is part of the northeast corner of the Columbia Plateau. The area is underlain by a basement rock complex of Precambrian age metasedimentary rock and Cretaceous-Tertiary to Eocene age intrusives. The basement rock is overlain by Miocene age flood basalts of the Columbia River Basalt Group (CRBG), except where the basement crops out as a cluster of steptoes along the southern margin of the study area near Medical Lake. Flows of two CRBG formations, the magnetostratigraphic unit N2 of the Grande Ronde Formation, and the overlying younger Priest Rapids Member of the Wanapum Formation, comprise the majority of the local bedrock. Lacustrine and fluvial sedimentary deposits are interbedded with the basalt flows throughout the region, where sediments accumulated in drainages behind dams of the advancing basalt flows. The Wanapum Formation has a maximum thickness of 292 ft (89 m), with an average of 125ft (38m). The maximum thickness ofthe Grande Ronde Formation is 514ft (!57 [sic] m). The contact of the two basalt formations generally occurs at the elevation of 2,100-2,200 ft (640-670 m) and is typically marked by an interbed that varies in thickness from absent to 120ft (37m), with an average of 37 ft (11). A surficial cover consisting mostly of Quaternary age glacial Lake Missoula flood deposited material blankets the basalt surface in deposits ranging from absent to about 400ft (122m) in thickness. For the purpose of delineating the occurrence of ground water on the West Plains, the local geology was divided into five hydrostratigraphic units; the overburden unit, the Grande Ronde aquifer, the Wanapum aquifer, the Grande Ronde-Wanapum interbed confining unit, and the basement complex unit. The two basalt formations, and their interbedded sediments, are the most important hydrostratigraphic units of the aquifer system. The basement complex, because of its relatively low permeability, forms the lower boundary of the basalt aquifers Ground water in both the Wanapum and Grande Ronde aquifers generally flows from topographic highs to topographic lows. Thus, ground water moves generally from the steptoes in the southwest near Medical Lake, north and east toward the major surface water bodies of Deep Creek and the Spokane River. The system is recharged primarily by precipitation. The Wanapum aquifer discharges to the underlying Grande Ronde aquifer. The Grande Ronde aquifer discharges to the basement rock below and possibly to the Spokane Valley-Rathdrum Prairie aquifer on the west side of the Spokane River. The steptoes form a ground water divide that isolates the West Plains area from the eastern Columbia Plateau aquifer system where ground water movement is generally to the southwest. Additionally, a subsurface basement rock ridge may form an east-west ground water divide under Fairchild Air Force Base. However, the east-west ground water divide in the Wannum aquifer may be the result of artificial mounding caused by Fairchild's leaky wastewater system. The Wanapum and Grande Ronde aquifers are semiconfined to confined systems. Hydraulic gradients range from 0.006 to 0.121 and 0.011 to 0.182 respectively in the Wanapum and Grande Ronde aquifers. The vertical hydraulic gradient is downward from the Wanapum aquifer to the Grande Ronde aquifer. The Wanapum-Grande Ronde interbed forms an aquiclude and causes a difference in head between the Wanapum and Grande Ronde aquifers that ranges from approximately 20-210 ft (6-64 m), and tends to increase north and east away from the steptoes in the southwest. From May to October, 1994, hydraulic heads were observed to decline about five feet in both aquifers. Wanapum aquifer parameters estimated for the West Plains include hydraulic conductivities from 0.18 to 12. 1 ft/d (0.06 to 3.69 m/d), transmissivities from 4.8 to 20. 9 ft2/d (0.4 to 1.9 m2/d), and a range of storativity from 0.0000177 to 0.0005500. The thickest surficial deposits of unconsolidated sands, gravels, and boulders reside in what appear to be paleochannels cut into basalt, the most prominent one extending from the Riddle Hill area northward under Airway Heights to Deep Creek. A second channel runs northward, but farther to the east. Ground water is unconfined in the paleochannels, and the unconsolidated sediments may serve as a conduit for the migration of water to the lower aquifer. A background ground water quality characterization of the West Plains revealed that ground water quality in the area is generally good. Forty-six wells were sampled in the month of May, 1994 and analyzed for pH, specific conductance, total hardness, chloride, iron and nitrogen as nitrate plus nitrite. With the exception of pH and iron, water quality parameters were typically found at higher concentrations in the Wanapum aquifer with respect to the underlying Grande Ronde aquifer. In the Wanapum, the mean values for pH, specific conductance, total hardness as CaC03, chloride, nitrogen as nitrate plus nitrite and iron were 7.42, 271 J.lmhos/cm, 115 mg/L, 7. 15 mg/L, 5.38 mg/L and 0.024 mg/L respectively. In the Grande Ronde for comparison, these are 7.47, 254 J.lmhos/cm, 101 mg/L, 3.98 mg/L, 2.00 mg/L and 0.201 mg/L. The highest mean values overall for pH, specific conductance, total hardness, chloride and nitrogen as nitrate plus nitrite were observed in overburden wells. The highest values for specific conductance, total hardness as CaC03 and chloride were observed to be 1,485 J.lmhos/cm, 588 mg/L and 370.3 mg/L, respectively, in a shallow overburden well only 11 ft (3.4 m) deep. Aside from relatively poor water quality in this 11 ft (3.4 m) deep well and five basalt wells that exceeded the Maximum Contaminant Level for iron, all other water quality parameter values fell within acceptable limits."--Document.