St. Thomas, with an area of 32 square miles, is the second largest of the Virgin Islands of the United States. The island is mountainous, and slopes commonly exceed 35 degrees along a central ridge 800 to 1,200 feet high running the length of the island. The general appearance is a panorama of numerous steep interstream spurs and rounded peaks.
The island is made up of rocks of Cretaceous age, mostly volcanic flows and breccia s. A thin limestone and tuffaceous wacke complete the sequence of major rock types. All the rocks have been tilted and dip about 50 degrees north.
Water in Charlotte Amalie, the capital, is supplied by sea-water desalting and water barged from Puerto Rico and is augmented by hillside rain catchments and individual roof catchments. Rainwater augmented by water hauling and a few wells is the source of water for the rural areas.
Streamflow is meager--2 to 8 percent of the annual rainfall-and is predominantly storm runoff. Runoff after rainstorms seldom exceeds 5 percent of the rainfall. Runoff is rapid, however, and flash floods occasionally occur.
Test drilling has shown that water can be obtained from fractured volcanic rocks in nearly all parts of the island. Wells will yield, generally, less than 1,000 gpd (gallons per day). In the upper Turpentine Run Valley and the Lovenlund Valley, short-term yields of individual wells are as great as 100 gallons per minute.
Estimates of potential yield from these areas are 300,000 and 100,000 gpd, respectively. Two smaller areas--Long Bay and Lindberg Bay on the outskirts of Charlotte Amalie have estimated ground-water yields of 70,000 and 30,000 gpd, respectively. Fully developed, the surface- and ground-water resources of the island could yield 1.3 million gallons of water per day.
Ground water is slightly saline, commonly containing more than 1,000 milligrams per liter dissolved solids. The principal source of the minerals is bulk fallout of sea- and land-derived dust from the atmosphere. Solution of minerals from the rocks of the aquifers is the second largest contributor. Nitrate and some of the bicarbonate content of the water is probably derived from vegetation and animal and human wastes.
Surface water is similar in mineral content to ground water during base flow.
Additional publication details
USGS Numbered Series
A survey of the water resources of St. Thomas, Virgin Islands