There is a large demand for water in Puerto Rico and the Virgin Islands; thus, even minor sources of ground water that can be developed for domestic, commercial, or other purposes are important locally. An atlas of the ground-water resources of these islands, therefore, would be incomplete without at least a synopsis of these minor aquifers.
Puerto Rico and the Virgin Islands each have a central core of faulted and folded volcaniclastic, igneous, and sedimentary rocks (fig. 124). Although these rocks have little or no intergranular porosity, some secondary permeability has developed as a result of weathering, faulting, and fracturing. These secondary openings transmit and store small volumes of water that can be recovered through wells.
Freshwater also is present in small alluvial deposits in coastal embayments of the Virgin Islands (fig. 124). These deposits are commonly exploited for small supplies of water.
VOLCANICLASTIC-, IGNEOUS-, AND SEDIMENTARY-ROCK AQUIFERS
The volcaniclastic-, igneous-, and sedimentary-rock aquifers consist of the upper 50 feet to a maximum of 300 feet of fractured and weathered volcaniclastic, plutonic, and sedimentary rocks that make up the core of each of the islands (fig. 124). Where exposed, these rocks store and transmit small quantities of water in fractures and overlying saprolite. The water can be recovered through domestic and small-diameter commercial wells that generally yield 5 to 10 gallons per minute or less. Although yields to individual wells are small, the area of the volcaniclastic-, igneous-, and sedimentary-rock aquifers comprises a large part of each of the islands, and the aquifers generally are the only source of ground water in those areas
Fresh ground-water withdrawals from these aquifers during 1985 amounted to about 11 million gallons per day in Puerto Rico and about 0.36 million gallons per day in the Virgin Islands. This constituted about 6 percent of the total ground-water withdrawals in Puerto Rico and about 25 percent of the total ground water withdrawn on the Virgin Islands.
Water in the volcaniclastic-, igneous-, and sedimentary- rock aquifers generally is very hard and locally contains large concentrations of sodium, chloride, bicarbonate, iron, and manganese.
COASTAL EMBAYMENT AQUIFERS
Coastal embayment aquifers in the Virgin Islands consist of alluvial valley-fill deposits that grade into beach sands as the bedrock valleys open onto coastal embayments (fig. 124). The alluvium, which commonly ranges in thickness from 30 to 50 feet, generally is fine grained and consists of clay, silt, and fine sand eroded primarily from volcanic rocks. Where they contain mostly fine-grained sediments, the aquifers yield only small amounts of water and are semiconfined. Locally, the alluvium is coarse sand and gravel, and the aquifer is unconfined. The alluvial deposits interfinger and grade into beach deposits that consist primarily of coarse coral sand. These deposits are permeable and yield only a few gallons per minute to wells. However, water in the coastal embayment aquifers is generally brackish to saline. Approximately 0.11 million gallons per day was withdrawn from the coastal embayment aquifers during 1985. The water is used primarily for domestic nondrinking purposes and as feed water to supply reverse osmosis desalination units.