Ground water is present on Block Island as a lens of freshwater that overlies saltwater. Yields of 2 to 5 gallons per minute are obtainable throughout the island, and yields of 25 gallons per minute are possible at many wells. Annual water use during 1990 is estimated to have been 53 million gallons, of which approximately 17 million gallons was delivered from a water company at Sands Pond. Demand by water company customers from May through October averages 74,000 gallons per day. The sustainable yield of Sands Pond during the drought years estimated to be only 45,000 gallons per day. Withdrawal of the remaining 29,000 gallons per day from Fresh Pond, proposed as an alternative source, would produce an estimated water-level decline of less than 1 foot. Block Island consists of a Pleistocene moraine deposit that includes meltwater deposits, till, sediment-flow deposits, and glacially transported blocks of Cretaceous strata and pre-Late Wisconsinan glacial deposits. The water table is a subdued reflection of the land-surface topography and flow is generally from the central, topographic highs toward the coast. Layers of low hydraulic- conductivity material impede vertical flow, creating steep vertical gradients. No evidence of widespread ground-water contamination was found during this study. Nitrate concentrations were below Federal Maximum Contaminant Levels at each of the 83 sites sampled. No evidence of dissolved organic constituents was found in groundwater at the 10 sites sampled, and ground-water samples collected near the landfill showed no evidence of contamination from landfill leachate. Dissolved-iron concentrations exceeded the Federal Secondary Maximum Contaminant Level in groundwater at 26 of 76 wells sampled. High iron concentrations were found predominantly in the eastern and northern parts of the island and are attributed to the presence of iron-bearing minerals and organic matter in the aquifer.