Occurrence, quality, and use of ground water in Orcas, San Juan, Lopez, and Shaw Islands, San Juan County, Washington

Ground water, which supplies most of San Juan County 's water needs, occurs in both bedrock and glacial drift. Water in the bedrock occurs in fractures in the otherwise dense, poorly permeable rock. Deposits of sand and gravel in the glacial drift provide the best yields to wells drilled into unconsolidated materials. Specific capacities of bedrock wells are typically low, and those of glacial-drift wells considerably higher. Ground water is high in dissolved solids and hardness; 29 of 56 wells sampled had water classified as very hard. Sixteen percent of the 171 ground-water sites tested for indicator bacteria had positive counts of one or more of these bacteria: total coliform , fecal coliform, fecal streptococcus. Nine percent of the 279 wells sampled for chloride in September 1981, appear to be affected by seawater intrusion. All of these wells are located within a mile of the coast; 60 percent of these wells are on Lopez Island. In 1980 an estimated total of 220 million gallons of ground water was withdrawn for all uses. Ninety percent of all ground-water use is for domestic and public supply purposes. Heavy pumpage on northern and southern Lopez Island correlates with areas having high chloride concentrations. (USGS)