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.