Occurrence of ground water and potential for seawater intrusion, Island County, Washington

The data from a study of groundwater availability and quality in Island County, Washington, are presented. Increased groundwater withdrawals associated with the population increase in Island County have caused concern about groundwater availability and potential seawater intrusion. The most widely used aquifer lies near sea level. Locally, available data indicate that one or more water-bearing zones lie above the sea-level aquifer. Pumpage in 1979 was about 1.67 billion gallons, about 90% of which was pumped from the sea-level aquifer. Most large producing wells in the county have pumping water levels near or below sea level, so that if pumping continues for a long enough time, seawater intrusion would result. Chloride concentrations in water samples taken in July 1978, April 1980 and August 1980 indicate that seawater intrusion is occurring in northeastern and southern Camano Island and in central Whidbey island. (USGS)