Department of Geology
Eastern Washington University
Recent Masters theses in Geology
AQUIFER DELINEATION AND BASELINE GROUNDWATER QUALITY INVESTIGATION OF A
PORTION OF NORTH SPOKANE COUNTY, WASHINGTON
Reanette M. Boese
Abstract
The purpose of this study is to delineate and describe the aquifers that
exist in the southern half of the Little Spokane River drainage basin that
lies within Spokane County. This comprises the area from a line about two
miles north of Chattaroy to the Little Spokane River's confluence with the
Spokane River.
The Little Spokane River basin is surrounded by crystalline basement rock
highlands to the east and west, and similar rocks also underlie the whole
study area. These crystalline rocks consist of two units of Mt. Spokane
granite, granodiorite, and some pre-Cambrian metamorphic rocks. The basin
formed in the crystalline basement rocks is filled with rocks and sediments
of younger ages. During the Miocene Epoch basalt flows from the Grande
Ronde Formation and the Priest Rapids Member of the Wanapum Basalt
Formation were emplaced in the study area. Latah Formation lacustrine
clays and fluvial sands were deposited concurrent to the basalts when the
Columbia River Basalt blocked regional drainage systems. During the
Pleistocene Epoch catastrophic flood sands and gravels and glaciolacustrine
clays, as well as Palouse loess, were deposited in the study area.
Mass-wasting deposits were also probably generated during the Pleistocene
Epoch. Holocene deposits include eolian sand dunes and modern alluvium.
Seven important hydrogeologic units are recognized within the geologic
framework in the study area. They are (1) the crystalline basement
aquifer, (2) the lower sand and gravel aquifers in the Latah and
Pleistocene deposits, (3) aquitards composed of the clays of the Latah and
Pleistocene deposits, (4) upper (surficial) sand and gravel aquifers, (5)
landslide aquitards, (6) the Grande Ronde Basalt aquifers, and (7) the
Wanapum Basalt Priest Rapids Member aquifers.
Groundwater occurs in the weathered and fractured portions of the
crystalline basement rock. Recharge to this unit is by direct infiltration
of precipitation where it is exposed and from overlying units where it is
buried. Crystalline basement rocks with little weathering or fracturing
act as the lower hydrogeologic boundary throughout the whole study area.
Grande Ronde Basalt makes up the basalt aquifer west of the Little Spokane
River under Wild Rose and Half Moon Prairies. This aquifer is unconfined
where it is in direct contact with unconsolidated sediments above. Lower
sections of the basalt, where vertical water flow is restricted by clay
interbeds and massive basalt, exhibit confined aquifer characteristics.
Recharge to the Grande Ronde Basalt aquifer is from direct infiltration of
precipitation where the units are exposed, from leakage through overlying
units, and possibly from underlying units. These lower basalt units mainly
discharge to the surface streams. There is also some leakage to lower
units.
Wanapum Basalt aquifers are found on Green Bluff and Orchard Bluff.
Estimates of transmissivity on Five Mile Prairie range from 134.8 to 192.5
ft2/day (12.5 to 17.9 m2/day). Estimates of the lateral hydraulic
conductivity of Wanapum Basalts in the West Plains area of Spokane County
range from 0.18 to 12.1 ft/day (0.05 to 3.69 m/day). Recharge to the
Wanapum Basalt aquifers is mainly through precipitation with some
irrigation return. Discharge is to springs along the edges of the mesas
and to lower units.
Lower sand and gravel aquifers exist as discontinuous units within the
clays of the Pleistocene lake deposits and in the Latah Formation. The
lower sand and gravel aquifers consist of both Latah sand and Pleistocene
flood deposits. Transmissivity values from a few wells tested in this unit
ranged from 10,000 to 40,000 ft2/day (929 to 3720 m2/day). Hydraulic
conductivities at the same wells ranged from 100 to 230 ft/day (13.4 to
30.8 m/day). The lower sand and gravel aquifers are recharged by overlying
units and discharge to lower units and ultimately to the streams of the
area. Becausethese lower aquifers are surrounded by clay, recharge to them
is slowed and not noticeably affected by seasonal or yearly changes in
precipitation.
The upper sand and gravel aquifer units in the area are discontinuous.
They consist mainly of sand dunes in the south and Pleistocene catastrophic
flood deposits found mainly along the Little Spokane River and in the
Hillyard trough. These discontinuous unconsolidated sediment bodies
represent unconfined, perched aquifers. Transmissivities in similar
materials near the study area range from 722 to 518,400 ft2/day (67 to
48,159 m2/day). The upper sand and gravel aquifers are recharged by direct
infiltration of precipitation and by irrigation to the land surface.
Discharge from these aquifers is through springs to the streams of the area
and to lower units.
Several sites in or near the study area have had contaminated groundwater
in the past. These are the Colbert Landfill, the Heglar-Kronquist
Landfill, the North Market Street site, Kaiser Mead pot-lining disposal
sites, and an egg ranch near Deer Park. Remediation has occurred or is
under study at all these sites.
Limited reconnaissance level sampling of 44 water wells in the study area
was conducted from April 23 to June 4, 1996. The static water level was
measured in 37 of these water wells. The water samples were analyzed for
nitrogen as nitrate+nitrite, iron, pH, chloride, calcium and magnesium
hardness, and specific conductance. Based on the data gathered in this
study, the water quality of the study area is good except for high
concentrations of iron. The major source of iron in groundwater in the
study area is basalt. Uranium is also naturally enriched in some
groundwater in the study area.
Iron concentration values ranged from less than 0.010 to 14.9 mg/L with a mean
concentration of 2.368 mg/L. Nitrogen as nitrate+nitrite concentration
values ranged from less than 0.01 to 9.86 mg/L with a mean concentration of 1.20
mg/L. The pH values ranged from 6.51 to 8.18 with a mean value of 7.35.
The total hardness as CaCO3 concentration values ranged from less than 1 to 497.5
mg/L with a mean of 188.8 mg/L. The chloride concentration values ranged
from 1.06 to 234.68 mg/L with a mean value of 9.50 mg/L. The specific
conductance values ranged from 196 to 1163 mmhos/cm with a mean value of
373 mmhos/cm. A comparison of water quality parameters by aquifer type
found some minor differences.