Abstract
This report presents information used to characterize
the groundwater-flow system on the Kitsap Peninsula, and
includes descriptions of the geology and hydrogeologic
framework, groundwater recharge and discharge, groundwater
levels and flow directions, seasonal groundwater-level
fluctuations, interactions between aquifers and the
surface‑water system, and a water budget. The Kitsap
Peninsula is in the Puget Sound lowland of west-central
Washington, is bounded by Puget Sound on the east and
by Hood Canal on the west, and covers an area of about
575 square miles. The peninsula encompasses all of Kitsap
County, the part of Mason County north of Hood Canal, and
part of Pierce County west of Puget Sound. The peninsula
is surrounded by saltwater and the hydrologic setting is
similar to that of an island. The study area is underlain by
a thick sequence of unconsolidated glacial and interglacial
deposits that overlie sedimentary and volcanic bedrock units
that crop out in the central part of the study area. Geologic
units were grouped into 12 hydrogeologic units consisting
of aquifers, confining units, and an underlying bedrock unit.
A surficial hydrogeologic unit map was developed and used
with well information from 2,116 drillers’ logs to construct
6 hydrogeologic sections and unit extent and thickness maps.
Unconsolidated aquifers typically consist of moderately
to well-sorted alluvial and glacial outwash deposits of sand,
gravel, and cobbles, with minor lenses of silt and clay. These
units often are discontinuous or isolated bodies and are of
highly variable thickness. Unconfined conditions occur
in areas where aquifer units are at land surface; however,
much of the study area is mantled by glacial till, and
confined aquifer conditions are common. Groundwater in
the unconsolidated aquifers generally flows radially off the
peninsula in the direction of Puget Sound and Hood Canal.
These generalized flow patterns likely are complicated by the
presence of low-permeability confining units that separate
discontinuous bodies of aquifer material and act as local
groundwater-flow barriers.
Groundwater-level fluctuations observed during
the monitoring period (2011–12) in wells completed in
unconsolidated hydrogeologic units indicated seasonal
variations ranging from 1 to about 20 feet. The largest
fluctuation of 33 feet occurred in a well that was completed in
the bedrock unit. Streamgage discharge measurements made
during 2012 indicate that groundwater discharge to creeks in
the area ranged from about 0.41 to 33.3 cubic feet per second.
During 2012, which was an above-average year of
precipitation, the groundwater system received an average of
about 664,610 acre-feet of recharge from precipitation and
22,122 acre-feet of recharge from return flows. Most of this
annual recharge (66 percent) discharged to streams, and only
about 4 percent was withdrawn from wells. The remaining
groundwater recharge (30 percent) left the groundwater system
as discharge to Hood Canal and Puget Sound.
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First posted June 10, 2014
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