Hydrogeologic assessment of the Sequim-Dungeness area, Clallam County, Washington

The Sequim-Dungeness area covers
116 square miles (mi²) on the northern part of the
Olympic Peninsula in northwestern Washington.
The central part of this area (74 mi²) was designated
as a primary study area. During the past two
decades, the population has rapidly increased, land
use has changed from mostly agricultural to residential,
and salmon populations in the Dungeness River
have appreciably declined. The increasing competition
for water combined with a close relation
between ground water, the Dungeness River, and an
extensive irrigation system has created a need for a
better understanding of ground water and the relation
between ground water and surface water in the study
area.

The Sequim-Dungeness area is underlain
by as much as 2,000 feet of unconsolidated
Quaternary deposits that are mostly of glacial origin.
Interpretation of 10 hydrogeologic cross sections
and lithologic logs of about 600 wells led to the
delineation of three aquifers, two confining beds, and
a lower unit of undifferentiated deposits. A bedrock
unit at the bottom is considered the base of the
ground-water system.

Ground water in the study area is recharged
from infiltration and percolation of precipitaton, percolation
of unconsumed irrigation water, leakage
from irrigation ditches, subsurface inflow through
the southern study-area boundary, and leakage from
streams. Average annual recharge for the study
period (December 1995 to September 1997) was
estimated to be 17.7 inches (in.) ( 151 cubic feet per
second (ft³/s)). The distribution of recharge was
8.6 in. (74 ft3fs) from precipitation, 2.7 in. (23 ft³/s)
from subsurface inflow, 3.1 in. (26 ft³/s) from irrigation,
and 3.3 in. (28 ft³/s) from leakage from the
Dungeness River. The 8.6 in. of recharge from precipitation
is much higher than would be expected in
an average year because average annual precipitation
during the study period was about 28 in., which
is 1.35 times higher than long-term average annual precipitation.
The long-term average annual
recharge from precipitation was estimated to be
5.4 in. (48 ft³/s).

Ground water discharges as subsurface flow
to saltwater bodies, flow to streams, flow to springs,
and as withdrawals from wells. Subsurface flow to
saltwater bodies and flow to springs were not estimated
in this study. Estimated average annual discharge
was 3.2 inches (in.) (27 ft³/s)) to the
Dungeness River and 4.6 in. (39 ft³/s) to other
streams in the study area. Gross withdrawals from
wells in 1996 were estimated to be 1.0 in. (8.4 ft³/s).

There was a small but statistically significant
increase in nitrate concentrations in ground water
from 1980 to 1996. Median concentrations in the
primary study area were 0.37 milligrams per liter
(mg/L) in 1980 and 0.46 mg/L in 1996. The areal
pattern of elevated nitrate concentrations has not
changed appreciably during the past 15 years. Elevated
concentrations were found in a large area east
of the Dungeness River and at scattered locations
west of the Dungeness River.

About 543,200 pounds of nitrogen are estimated
to enter the ground-water system in the primary
study area each year. Four sources account for
about 85 percent of the nitrogen; residential fertilizers,
septic systems, mineralization of soil organic
matter, and agricultural fertilizers .. It appears that the
four major sources are approximately equivalent in
amounts of nitrogen.

Concentrations of nitrate in the shallow aquifer
were significantly higher under residential areas
than under natural grasslands or forests. Median
nitrate concentrations were 1.3 mg/L under residential
areas, 0.55 mg/L under agricultural areas, and
0.12 mg/L under natural grasslands or forests.