USGS Washington Water Science Center
|U.S. Department of the Interior
U.S. Geological Survey
Release: May 8, 2000
253-428-3600, ext. 2618
Has seawater entered the only major source of fresh water on Washington's Lopez Island--its ground water? Chloride concentrations in the island's ground water indicate that seawater intrusion has likely occurred in some wells, according to a new report published by the U.S. Geological Survey, Department of the Interior.
Lopez Island, one of the San Juan Islands, does not have the lakes and perennial streams to supply more fresh surface water, and it gets only 20 to 30 inches of precipitation a year to recharge its ground water. But the island's growing population and continued development are increasing demands for fresh water.
"Pumping more fresh water from wells would at some point lower the water table. And that could raise the transition zone between the fresh water and seawater enough to bring seawater into the freshwater aquifer," said the USGS's Laura Orr, author of the report.
Because of such concern, the USGS, in cooperation with the San Juan Conservation District, launched a 1997 study of the potential of seawater intrusion on Lopez Island. In 1997 the USGS collected samples from 185 selected well or spring sites on the island and analyzed the samples for chloride concentrations.
Of the 185 samples, 46 percent had chloride concentrations in excess of 100 milligrams per liter, indicating possible seawater intrusion. The concentrations ranged from 12 mg/L to 420 mg/L, with a median of 92 mg/L. Moreover, the concentrations had increased over time since a previous USGS study in 1981.
Although the study did not find two other indications of seawater intrusion--chloride concentrations consistently increasing in wells closer to the shoreline or at greater depths--Orr cautioned that not finding these other indications does not prove anything either way.
"One indicator tells us that seawater intrusion has apparently occurred in some places. But there are other possible causes for high chloride that should be ruled out to confirm that the increased chloride has come from intruding seawater, especially in individual areas. We also need to study the extent of the intrusion," Orr said.
The fact sheet report, "Is Seawater Intrusion Affecting Ground Water on Lopez Island, Washington?," by Laura Orr, is published as U.S. Geological Survey Fact Sheet 057-00. It is available for reading at the U.S. Geological Survey office at 1201 Pacific Avenue, Suite 600, Tacoma, Washington 98402. It can be purchased from the U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225-0286, telephone (303) 202-4200.
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