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Increased ground-water use associated with population growth in recent years has caused concern about deterioration of the fresh ground-water supply by intrusion of Puget Sound seawater into aquifers underlying Island County, Washington. Whidbey and Camano. Islands, the two largest islands in the county, are the subject of this study and are located about 25 miles northwest of Seattle. Ground water occurs in glacial deposits having a maximum thickness of about 3,000 feet; these deposits were divided into five aquifers (designated as "A" through "E") and five confining units (designated as "A" through "E") extending to 900 feet below sea level. Fresh ground water moves from recharge areas on land surface, downward through aquifers and confining units, to discharge as springs above and below sea level and to discharge from pumped wells. Most wells pump from aquifers C and D. A chloride concentration of 100 mg/L (milligrams per liter) was used to distinguish fresh ground water from seawater in the mixing zone. In August 1981, chloride concentrations exceeded 100 mg/L in 24 percent of the sampled wells that were drilled below sea level. The largest chloride concentration was 14,000 mg/L, but chloride concentrations did not exceed 1,000 mg/L in most of the sampled wells. Chloride concentrations were usually large in wells drilled in low-altitude areas along the coastline, and chlorides generally increased with depth below sea level. Four models were constructed to simulate steady flow of fresh ground water in multiple aquifers and confining units, and to locate a sharp interface between moving freshwater and stationary seawater. Three of the models were constructed for Whidbey Island and one for Camano Island. Each model was calibrated using time-averaged data. Model computations show that most of the recharge is discharged from aquifers C and D as springs below sea level, and only a small fraction of the recharge moves downward below aquifer C. Also, model computations of interface depths and hydraulic head indicate that aquifers in all areas except northeast Camano Island are not recharged by ground water moving from the mainland through aquifers beneath Puget Sound. Beneath northeast Camano Island, gradients of observed hydraulic heads indicate that aquifers may be recharged by ground water from the mainland. The computed interface was deepest beneath the central parts of the islands and sloped upward near the coastlines to intersect the bottom of Puget Sound. The maximum computed depth of the interface exceeded 900 feet below sea level, beneath southern Whidbey Island. In most of the areas separating Whidbey and Camano Islands the interface intersects the bottom of Puget Sound, indicating that there is no movement of fresh ground water between the two islands.

Ground-water resources and simulation of flow in aquifers containing freshwater and seawater, Island County, Washington