Recent mining of beach sand has resulted in significant erosion and loss of beaches and dunes.
Many of Puerto Rico's beaches are eroding, and though rates of erosion vary, it is a major concern for the tourism and residential development industries. More than 85 percent of the population lives within 7 kilometers of the coast and they are heavily dependent on tourists that are attracted by the island's beaches and coral reefs. High-quality scientific data are needed to help formulate public policy regarding residential and commercial construction along the coast, beach replenishment, and future use of marine resources. Scientists have long recognized that the causes of coastal land loss are not limited to a relative rise in sea level, but can be manmade as well. For example, sediment supply to beaches especially along the north shore of Puerto Rico has been strongly affected by upstream river channeling, dam construction, various agricultural practices, paving and urbanization, as well as shallow-water oceanographic processes. The response to coastal erosion in Puerto Rico has been mostly crisis based leading to engineered solutions that have a negative effect on the coastal environment.
Puerto Rico is exhausting its sand and gravel for construction and recreation.
USGS scientists conducted reconnaissance geologic mapping for new sand and gravel resources.
Geologists have mapped the shelf of Puerto Rico using seismic-reflection profiling augmented by bottom sampling. Three offshore sand and gravel deposits have been identified which may be economically important. The largest of these, known as Escollo de Arenas, extends off the northwest corner of the island of Vieques, and is estimated to contain 90 million cubic meters of sand and gravel. A second deposit, Cabo Rojo, lies in a shallow trough 1 kilometer offshore near the southwest corner of the main island and is estimated to be nearly as large. The third deposit, Isabela, off the northwest corner of Puerto Rico, may not be economically viable. The geometry and nature of each deposit was determined by seismic surveys, diver-operated Vibracores, surface sampling, and size analysis to determine their suitability for construction use.
USGS studies of the Escollo de Arenas sand and gravel deposit suggest that it is a significant resource.
Escollo de Arenas extends 6 kilometers northwest from Punta Arenas on the island of Vieques. Its surface shape can be seen clearly from aerial photographs as it varies between 100 and 1,000 meters in width. Sand waves move back and forth as tidal currents sweep over the shoal four times a day at speeds up to 150 centimeters per second. It is these strong tidal currents that anchor the shoal in one place. Geological evidence suggests that the shoal is a "sink" for sand moving west along the north and north along the west coasts of the island. The composition of the sediment is primarily terrigenous nearshore, but becomes increasingly biogenic at the distal end. This evidence suggests that removal of the sand from this shoal should have little effect on the beaches because they are naturally nourished by sand derived from erosion. Other geological evidence suggests that an ancient sand body underlies the shoal, and that the Punta Arenas is actually eroding back from a point about 6 kilometers northwest of its present location.
USGS studies of other sand and gravel deposits suggest that they may be viewed as future resources.
Unlike the Escollo de Arenas, the Cabo Rojo deposit is composed of calcium carbonate materials of biogenic origin, such as shell and coral debris, with very little terrigenous sediment. It is probably a modern isolated deposit in an energy-low trough that collects sediment from longshore transport of materials along Puerto Rico's south coast. Recent DNR studies of cores taken from this deposit indicate that it rests on a layer of fine sediment, such as silts and clays, that would be unsuitable for construction or recreation. The risk to the environment and coral ecosystems by disturbing these fine-grained sediments could make this deposit an expensive resource. The smallest of the three deposits, Isabela, has been discounted as a viable resource. It is not a single, continuous deposit, but is made up of relatively thin lenses of sand spread over a large area in deep water. In addition, the northwest coast of Puerto Rico is a high-energy environment that would hamper recovery of sand and gravel.
USGS researchers continue reconnaissance mapping on the shelf to identify other potential sand and gravel resources.
Preliminary investigations suggest that other sites have potential to be significant economic resources of sand and gravel. Details of their composition and geometry are not available without careful study. The urgency of the depleting sand and gravel resources of Puerto Rico is underscored by the very high costs associated with building on other nearby islands, such as St. Thomas, where sand and gravel are imported in order to preserve materials for recreational purposes.
U.S. Department of the Interior, U.S. Geological Survey
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Last modified: 19:38:24 Wed 09 Jan 2013
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