Water for domestic and municipal supply on St. John, in the past, has been obtained from rain catchments, dug wells, and barge shipments from St. Thomas and Puerto Rico. As a result of this study, small ground-water supplies have been developed for the Virgin Islands National Park.
Ground water occurs in significant but limited quantities in the fractured volcanic rock throughout most of the Island. Yield of wells in this aquifer ranges from less than 100 to about 2,000 gpd (gallons per day). The average long-term yield of the three drilled wells in use by the National Park Service in 1967 was about 1,000 gpd. Yield ofl,000 to 5,000 gpd may be expected in the Coral Bay and the Reef Bay areas.
Estimated total recharge of the fractured volcanic rock on St. John, based on a recharge of 1 to 3 inches per year, is 1,000,000 to 3,000,000 gpd. Perhaps as much as a quarter to a third of this water could be developed practically, depending on the rainfall in a given year. The chemical quality of the ground water in the fractured-rock aquifer in areas uncontaminated by sea water ranges from 600 to 2,000 mg/l (milligrams per liter) or more dissolved solids. Water from-formations in the higher altitudes is of better quality than that in the lower formations.
Small quantities of ground water are available from beach sand, alluvium, and fractured rock near the sea. However, these sources tend to be brackish and are subject to salt-water encroachment.
There are no perennial streams on St. John. There are a few spring-fed pools in stream channels, however, that are sustained, except in severe drought. Storm runoff is estimated to average 1 inch over the island annually, and evaporation from open water surfaces is about 70 inches per year. Ponds can be developed, but because of the high .evaporation they may be unreliable during droughts.
Rain water collected in cisterns from roofs and catchments yield about 50 gpd per 1,000 square feet of catch area during an average year of rainfall. This is the main method of Water supply for domestic use on the Island; it will probably be continued even if a public distribution system is made available, because of the limited quantity of other natural water.