East King County is a 250-square-mile area east of Seattle underlain by as much as 1,200 feet of unconsolidated deposits of glacial and nonglacial origin. A surficial geology map and 12 geohydrologic sections were constructed and used to delineate 10 geohydrologic units, 4 of which are major aquifers. Annual precipitation over the study area averages 57 inches, of which 31 inches, or 413,000 acre-feet, enter the ground-water system as recharge. Some 98,500 acre-feet of ground water is estimated to discharge to surface water bodies each year, 9,540 acre-feet is discharged through springs, and 4,270 acre-feet is withdrawn from wells. The chemical quality of the ground water in east King County is typical of that in other areas of western Washington. The median dissolved-solids concen- tration of 124 samples analyzed was 115 milligrams per liter, and 95 percent of the water samples were classified as soft or moderately hard. The median nitrate concentration was 0.07 milligrams per liter, and no widespread nitrate contamination was apparent.