Department of Geology
Eastern Washington University
Recent Masters theses in Geology
HYDROGEOLOGY OF THE WEST PLAINS AREA OF SPOKANE COUNTY, WASHINGTON
William B. Deobald
Abstract
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 125 ft (38 m). The maximum thickness of
the Grande Ronde Formation is 514 ft (157 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 120 ft (37 m), 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 400 ft (122 m) 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
Grade Ronde-Wanapum interbed confining unit, and the basement complex unit.
The two basalt formations, and their interbedded sediments, are the most
important hydrostatigraphic 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 in the
Wanapum 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 (6-64m), 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 nitrate. 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 CaCO3, chloride, nitrogen as nitrate plus nitrate and iron were
7.42, 271 µmhos/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
µmhos/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 nitrate were observed in overburden wells. The
highest values for specific conductance, total hardness as CaCO3 and
chloride were observed to be 1,485 µmhos/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.