Abstract
The hydrogeology, hydrology, and geochemistry
of groundwater and surface water in the upper (western)
860 square miles of the Yakima River Basin in Kittitas County,
Washington, were studied to evaluate the groundwater-flow
system, occurrence and availability of groundwater, and the
extent of groundwater/surface-water interactions. The study
area ranged in altitude from 7,960 feet in its headwaters in the
Cascade Range to 1,730 feet at the confluence of the Yakima
River with Swauk Creek. A west-to-east precipitation gradient
exists in the basin with the western, high-altitude headwaters
of the basin receiving more than 100 inches of precipitation
per year and the eastern, low-altitude part of the basin
receiving about 20 inches of precipitation per year. From the
early 20th century onward, reservoirs in the upper part of the
basin (for example, Keechelus, Kachess, and Cle Elum Lakes)
have been managed to store snowmelt for irrigation in the
greater Yakima River Basin. Canals transport water from these
reservoirs for irrigation in the study area; additional water
use is met through groundwater withdrawals from wells and
surface-water withdrawals from streams and rivers. Estimated
groundwater use for domestic, commercial, and irrigation
purposes is reported for the study area.
A complex assemblage of sedimentary, metamorphic,
and igneous bedrock underlies the study area. In a structural
basin in the southeastern part of the study area, the bedrock
is overlain by unconsolidated sediments of glacial and
alluvial origin. Rocks and sediments were grouped into six
hydrogeologic units based on their lithologic and hydraulic
characteristics. A map of their extent was developed from
previous geologic mapping and lithostratigraphic information
from drillers’ logs. Water flows through interstitial space
in unconsolidated sediments, but largely flows through
fractures and other sources of secondary porosity in bedrock.
Generalized groundwater-flow directions within the
unconfined part of the aquifers in unconsolidated sediments
indicate generalized groundwater movement toward the
Yakima River and its tributaries and the outlet of the
study area.
Groundwater movement through fractures within the
bedrock aquifers is complex and varies over spatial scales
depending on the architecture of the fracture-flow system
and its hydraulic properties. The complexity of the fracturedbedrock
groundwater-flow system is supported by a wide
range of groundwater ages determined from geochemical
analyses of carbon-14, sulfur hexafluoride, and tritium in
groundwater. These geochemical data also indicate that
the shallow groundwater system is actively flushing with
young, isotopically heavy groundwater, but isotopicallylight,
Pleistocene-age groundwater with a geochemicallyevolved
composition occurs at depth within the fracturedbedrock
aquifers of upper Kittitas County. An eastward
depletion of stable isotopes in groundwater is consistent
with hydrologically separate subbasins. This suggests that
groundwater that recharges in one subbasin is not generally
available for withdrawal or discharge into surface-water
features within other subbasins. Water budget components
were calculated for 11 subbasins using a watershed model
and varied based on the climate, land uses, and geology of
the subbasin.
Synoptic streamflow measurements made in August 2011
indicate that groundwater discharges into several tributaries
of the Yakima River with several losses of streamflow
measured where the streams exit bedrock uplands and flow
over unconsolidated sediments. Profiles of stream temperature
during late summer suggest cool groundwater inflow over
discrete sections of streams. This groundwater/surfacewater
connection is further supported by the stable-isotope
composition of stream water, which reflects the local stableisotope
composition of groundwater measured at some wells
and springs.
Collectively, these hydrogeologic, hydrologic, and
geochemical data support a framework for evaluating the
potential effects of future groundwater appropriations on
senior surface-water and groundwater rights and streamflows.
Although total pumping rates in upper Kittitas County of
about 3.5 cubic feet per second are small relative to other
components of the water budget, the magnitude, timing, and
location of withdrawals may have important effects on the hydrologic system. The heterogeneous and variably fractured
bedrock in the study area precluded a detailed evaluation of
localized effects of pumping, but several generalizations about
the groundwater and surface-water systems can be made.
These generalizations include evidence for the continuity
between the groundwater and surface-water system apparent
from synoptic streamflow measurements, stream-temperature
profiles, and stable-isotope data of groundwater and
surface waters.