The island of Key West lies at the end of the Florida Keys, about 150 miles southwest of Miami. The public-water supply for the island is provided by the Florida Keys Aqueduct Authority Well Field near Miami. However, there are many privately owned wells on the island that tap the local fresh ground-water lens for potable and nonpotable water supply. The number of people who use water from the wells for drinking purposes is unknown. From 1985 to 1988, the U.S. Geological Survey, in cooperation with the South Florida Water Management District, conducted an investigation to characterize the Key West freshwater lens. Observation wells were drilled to determine the extent of the lens and to characterize the water quality. Previous well logs and well-core data collected during the investigation showed the aquifer to be a highly permeable, porous, solution-riddled, oolitic limestone that allows rainfall recharge to quickly seep into the ocean and saltwater to easily intrude the aquifer. The small freshwater lens (250 milligrams per liter of chloride concentration, or less) averages 5 feet in thickness below the center of the western half (Old Town) of the island. The lens contains about 20 million gallons of fresh-water during the dry season and about 30 million gallons during the wet season. Underlying the freshwater lens is a transition zone of freshwater-saltwater mix that extends to the saltwater interface (19,000 milligrams per liter of chloride concentration), which is about 40-feet deep at the center of the lens. The water table fluctuates and the configuration of the lens constantly changes, largely as a result of tidal effects. Other events, such as rainfall, pumping, and evapotranspiration, are masked by the tidal effects. The freshwater lens is a calcium bicarbonate water that grades to a sodium chloride type near the saltwater interface. Elevated concentrations of nitrate nitrogen were found in water samples from wells in the Old Town district. However, concentrations generally were not above the maximum contaminant level of 10 milligrams per liter for drinking water established by the Florida Department of Environmental Regulation. Water samples near an old land-fill in the eastern half of the island had concentrations of iron (600-1,900 micrograms per liter) and lead (40-800 micrograms per liter) that extended maximum contaminant levels of 300 and 50 micrograms per liter. These trace-element concentrations generally decreased with distance from the landfill. Although the freshwater lens is a potential source of water for additional nonpotable water needs in Key West, any large-scale pumping could quickly exhaust the freshwater lens, and saline water could be rapidly drawn though the porous limestone aquifer. Water-quality data indicate that the lens is an unlikely source of potable water.