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water on the islands is desalination of ocean water, which is very expensive. Ground <br />water is the primary source of drinking water on most islands, yet the amount of ground- <br />water recharge and the flow properties of many aquifers are poorly characterized. <br />Saltwater intrusion from pumping is degrading the major sources of domestic water <br />supply on several islands. Localized contamination of some aquifers also reduces the <br />availability of ground water. Surface water, which has typically been used for agricultural <br />irrigation, is not a reliable source of drinking water because stream flow is variable and <br />poorly characterized, plus storage capacity is limited. Environmental and cultural factors <br />are reducing the amount of surface water that can be diverted for off - stream uses, and the <br />in- stream flow requirements of streams have not been defined. Thus, there is abundant <br />surface water on many of the islands, but a lack of information about streamflow <br />characteristics and ecosystem requirements prevents surface water from being used as a <br />reliable source of water supply. <br />21 Caribbean Region – The Caribbean Region includes Puerto Rico and the U.S. Virgin <br />Islands; the majority of the population in the region resides in the San Juan metropolitan <br />area. Puerto Rico has abundant ground- and surface -water resources as a result of heavy <br />rainfall over the mountainous central part of the island and permeable sedimentary rock <br />aquifers along the coasts. As a result of orographic effects and the prevailing wind <br />direction, the north coast of Puerto Rico receives about 30% more rainfall than the south <br />coast. Because of the terrain and the absence of suitable sites for storage reservoirs, most <br />of the surface runoff rapidly flows out to sea. Heavy rainfall on the mountains results in <br />large sediment yields, which greatly reduce storage in the few water - supply reservoirs. <br />About 30% of the water supply in Puerto Rico is from ground water. As a result of <br />ground -water withdrawals, seawater intrusion into freshwater aquifers has been a <br />problem along the south coast of the island. The U.S. Virgin Islands —St. Thomas, St. <br />Croix, and St, John —have scarce freshwater - supply resources as a result of their fairly <br />small area on low -lying topography. There are no perennial streams and about 70% of the <br />water supply is from desalination, 20% is from ground -water withdrawals, and 10% is <br />from rooftop rainfall catchments, making water supplies in the Virgin Islands one of the <br />most expensive in the U.S. <br />24 <br />