Water Resources of the Caribbean
Island hydrology: Puerto Rico and the U.S. Virgin Islands
Allen Zack and Matthew C. Larsen
U.S. Geological Survey, GSA Center, Suite 400-15, 651 Federal Drive, Guaynabo, Puerto Rico, 00965-5703, USA
When Christopher Columbus sailed into the Caribbean Sea 500 years ago he initiated a profound transformation of the Caribbean islands. Within the next several centuries these forest-covered, humid-tropical islands that extend from Venezuela to Florida became important producers of sugar for the major European powers. This sugar production required importation of a massive labor force, the descendants of whom dominate the region to this day. Island ecology and hydrology prior to 1492 had been adequate to support small populations of mostly Arawak and Taino indians. However, by the 19th century, extensive deforestation began to increase soil erosion and the sediment transported by rivers. By the middle of the 20th century, deforestation in Puerto Rico reached its peak when more than 90 percent of the island forests had been cut.
The effects of high population density, and the conversion of tropical forest to agricultural, industrial and residential use has had significant effects on the quality and availability of water for island residents. These effects include the over-utilization of existing water supply, filling of public-supply reservoirs with sediment, and contamination of surface- and ground-water.
The hydrology of small tropical islands differs from that of temperate, continental areas. In the West Indies, precipitation, the origin of all freshwater resources, is controlled principally by the easterly tradewinds, the passage of tropical storms, and orographic effects in the islands with high relief. The geology, topography, and relative size of the islands determine the degree to which they collect and retain the rainfall that ultimately provides island water supplies.Sources of water: Puerto Rico
Puerto Rico has abundant ground-water and surface-water resources due to relatively heavy rainfall over the mountainous interior of the island and receptive, sedimentary rocks around the island's periphery. These alluvial and limestone formations form an extensive artesian aquifer system on the north coast. Water-table aquifers overly the north coast artesian aquifer and occur at shallow depths along most of Puerto Rico's coastline. Man-made reservoirs located on principal water courses collect runoff and are used for water supply, flood control, and limited hydroelectric power generation. Ground water accounts for about 30 percent of the total amount of water used in Puerto Rico, whereas surface water accounts for about 70 percent.Sources of water: U.S. Virgin Islands
In contrast, the three principal U. S. Virgin Islands of St. Thomas, St. Croix, and St. John are much smaller and of lower maximum elevation than Puerto Rico. The U.S. Virgin Islands therefore receive less rainfall and retain less freshwater for water supplies. The U.S. Virgin Islands have no perennial streams and only limited ground-water resources. Accordingly, 65 percent of freshwater supplies in the U.S. Virgin Islands are provided by energy-consumptive desalinated seawater, making it the most expensive publicly supplied water in the United States, at $4.20 per 1000 liters. The use of fossil fuels for the desalination of seawater is particularly expensive in the U.S. Virgin Islands because all fuel is imported. At present, 22 percent of the freshwater supply originates as ground water and 13 percent is provided from rainfall by rooftop catchments. Recently, the scant ground-water resources that occur in coastal embayment areas have been selectively developed, particularly in St. Croix, and are providing less expensive drinking water to island residents. Where well fields are recharged by downward leakage of the occasional runoff in intermittent streambeds, additional ground water can be recharged to underlying aquifers by constructing retention dams in the streambeds to form small ponds. The additional recharge enabled by retention dams is not without its price, however, as the dams also trap sediment. The sediment must be periodically evacuated or the dams become useless.Pollution problems
Contamination in both Puerto Rico and the U.S. Virgin Islands from such sources as accidental spills and leaking septic tanks has limited the amount of ground-water and surface-water supplies that can be developed without treatment. In Puerto Rico, only the deep artesian aquifer on the north coast is free of downward percolating contamination. The continued use of the artesian aquifer, however, is threatened by poorly designed and constructed industrial artesian wells. Ruptured or corroded well casings provides hydraulic connection to the overlying water-table aquifer, diminishing artesian pressure and limiting the amount of water available to wells screened in the artesian aquifer. The overlying water-table aquifer has been contaminated in some areas by irresponsible or accidental releases of volatile organic compounds (VOC's) or other toxic wastes. These chemicals have infiltrated soils and entered the shallow ground-water regime along the north coast, as well as many other areas of Puerto Rico.
In coastal locations of Puerto Rico and the U.S. Virgin Islands where fresh ground water is withdrawn for water supplies, saline-water encroachment threatens the continued use of fresh ground- water and limits ground-water withdrawals. In the U.S. Virgin Islands, and to a lesser extent in Puerto Rico, leaky septic tanks and inadequate sewage treatment facilities have degraded the quality of near-surface ground-water supplies.Reservoir sedimentation
Reservoirs are used throughout most of Puerto Rico for water supply; including the San Juan metropolitan area. Many of the principal reservoirs have become partially filled with sediment since their construction, reducing their effectiveness in providing water supplies and mitigating flood peaks (table 1). Hundreds of tons of suspended sediment are transported into reservoirs during heavy rainstorms that occur several times per year on average. Reduced velocity gradients permit this suspended sediment to settle and accumulate in the reservoirs. The majority of this sediment originates from poorly controlled agricultural practices and urban development within the watersheds. Annual suspended sediment loads in some watersheds are as high as 13,000 metric tons/km2/year. In addition, landslides triggered by heavy rainfall have transported an average of 300 metric tons/km2/year into river channels in some watersheds. This sediment is then carried into downstream reservoirs. The principal water supply for the San Juan metropolitan area is Carraizo Reservoir which has lost 58 percent of its initial capacity since it was constructed almost 40 years ago. In addition to the loss of potable water storage capacity of the reservoir (particularly during droughts), Carraizo Reservoir becomes less efficient each year at reducing flood peaks.
Hydrologic hazards caused by insufficient or excessive rainfall have caused considerable human suffering in both Puerto Rico and the U.S. Virgin Islands. Droughts are rare in Puerto Rico, but are frequent and severe in the U.S. Virgin Islands due to limited fresh ground-water supplies and the lack of perennial streams and reservoirs. Any minor depletions in rainfall dramatically affect agriculture and require water rationing throughout the U.S. Virgin Islands.
Flash flooding and landsliding, the most dangerous of hydrologic hazards in both Puerto Rico and the U.S. Virgin Islands, can be attributed to the physiographic features of the islands combined with periodic heavy rainfall typical of the humid tropics. Flash flooding, in which peak discharge occurs during and shortly after intense rainfall, is common where stream courses are short, characterized by steep gradients, and channels are narrow and shallow. Poor drainage on the flood plain has increased the vulnerability of these areas to flooding, threatening the dense populations residing in the lower valleys.
Landsliding of steeply sloping, unstable terrain has caused many deaths and extensive property damage in Puerto Rico, since the 1950's (Larsen and Torres Sanchez, 1993). Triggered by intense and/or prolonged rainfall landslides have caused most damage near population centers where low-cost urban development has spread to steeply sloping unstable areas. In October, 1985, a 600 mm rainfall occurring in a 24 hour period triggered a landslide near Ponce, Puerto Rico that claimed the lives of more than 130 people residing in crowded, substandard housing (Jibson, 1989).Solutions
Much of the ground-water contamination in Puerto Rico, although regrettable, can be accommodated by treating the withdrawn ground water before allowing the water to re-enter the potable water-supply network. Scavenger wells can be used to extract freshwater near the coast while avoiding the upconing of saline-water through hydrodynamic stabilization of the saline-freshwater interface. Brackish ground-water can be treated by reverse osmosis using a membrane that filters the sodium and chloride ions from the water. Air strippers can be used for detoxifying ground water contaminated VOC's.
Sedimentation of reservoirs in the humid tropics is difficult to control because of frequent high intensity rainfall. Comprehensive programs controlling land use practices are essential to the mitigation of soil erosion. In addition periodic flushing of sediment from reservoirs, in combination with regular dredging, will prolong reservoir life. Some dredging costs can be offset by the sale of the sand fraction of dredged spoils for construction use.
Natural hazard mitigation requires early warning by civil defense agencies charged with evacuation of people living in hazardous areas. The U.S. Geological Survey maintains a satellite transmitting, real-time hydrologic data-collection network that provides early warning of these hydrologic hazards. The network consists of meteorologic stations, streamflow stations, and raingages that continuously monitor threatening hydrologic conditions throughout Puerto Rico and the U.S. Virgin Islands. The system is operated cooperatively with local government agencies in Puerto Rico and the U.S. Virgin Islands.Long-term changes in Caribbean climate
Caribbean islands may be undergoing gradual, but significant, hydrologic depletions that began several centuries ago, with islands of low elevation exhibiting the most obvious depletions. The reduced streamflow and diminished ground-water supplies may be attributed to decreasing rainfall over the tropical oceans, islands, and coastal areas since the industrial revolution. Oral history reveals that perennial streams occurred in the islands of Vieques (Puerto Rico) and St. Croix several centuries ago. Fresh ground-water supplies in the Esperanza Valley, Vieques, have gradually diminished during the past 200 years even though ground-water withdrawals have remained constant. For approximately 250 years after its discovery, Isla de Mona (Puerto Rico) was shown on nautical charts as a watering port. Reports of the abundance of good water continued into the 1790's. Today, available water supplies, other than rainfall, are meager and brackish (Jordan, 1972). Rainfall in San Juan and San German, Puerto Rico appears to have decreased during the last 100 years (Colon, 1987). Similar decreases have been observed in the Panama Canal basin (Windsor and others, 1990) and at lowland sites in Costa Rica (Windsor and Rand, 1985).Summary
Insular societies are forced to carefully allocate their limited natural resources, the most important of which is water. Only by good planning, prudent use of water resources, and employment of the most developed technology for exploitation of these water resources can a modern industrial society with a high population sustain itself under the constraints of limited geography and finite water resources. The water resource problems encountered in this type of setting vary somewhat from those water resource problems experienced in continental areas. However, island hydrology can be seen as a microcosm of continental-scale hydrology, and a predictor of water-resource management problems that may be encountered on a continental scale.
Zack, Allen, and Larsen, M.C., 1994, Island hydrology: Puerto Rico and the U.S. Virgin Islands: National Geographic Research & Exploration: Water Issue, p. 126-134.