St. Croix consists of two mountainous volcanic rock cores separated by a graben containing clays with minor limestone and conglomerate that is overlain by about 300 feet of marl and limestone. Predominantly fine-grained alluvium mantles much of the limestone and marl area and fills overdeepened south-trending valleys to depths of as much as 100 feet. Rainfall follows an orographic pattern ranging from about 30 inches in eastern St. Croix to 55 inches in the northwestern mountains.
Four streams, all heading in the Northside Range, have intermittent reaches. All other streams in St. Croix are ephemeral, flowing only for a few hours or days following major rainstorms. Flow in the intermittent streams ranges from about 1 to 9 percent of the total rainfall and usually half or more of the flow is storm runoff resulting from two or three major storms. Storm runoff from individual storms seldom exceeds 5 percent of the rainfall. Only from 2 storms, one of 5 inches and the other of 7 inches in less than 48 hours, did runoff reach 20 percent, both times on River Gut at Golden Grove. The lack of storm runoff is attributed to the capability of the soil zone to accept large volumes of water and deficient soil moisture most of the year.
The dissolved-solids content of the water of St. Croix. reflects the influence of the sea and land. Bulk precipitation is believed to be the source of the initial mineral content of the island water. Additional mineralization, particularly of ground water, results from the solution of soluble salts, mixing with residual sea water, and concentration by evapotranspiration. Water in the volcanic rocks is basically a calcium bicarbonate sodium chloride type with dissolved solids ranging from 500 to 1,000 mg/l (milligrams per liter), and chloride concentration of 100 to 300 mg/1. By contrast, water in the limestone is a sodium bicarbonate sodium chloride type with dissolved-solids content ranging from less than 1,000 to more than 20,000 mg/1 and chloride concentration from less than 100 to more than 10,000 mg/l. The mineral content of water in the limestone invariably increases with depth. Water in the alluvial deposits and in the streams usually reflects the characteristics of water from the adjacent bedrock.
The retention of large volumes of rainfall in the soil zone from which it is evaporated and transpired by plants greatly reduces the water available for recharge to the aquifers of the island. Estimates of effective recharge to the aquifers range from less than 0.5 inch in some volcanic and marl rocks to 5 inches annually in more porous limestone and alluvium. Long-term yield from the aquifers is also affected by their storage capacity which may range from less than 1 percent in volcanic rocks and marl to 10 to 15 percent in limestone and alluvium. The ground-water potential (equivalent to the quantity of recharge) of St. Croix is estimated at 3.9 mgd (million gallons per day)--0.9 mgd from the Northside Range (Area 1) ; 0.4 mgd from the East End Range (Area 2), and 2.6 mgd from the central lowlands (Area 3). Most areas where major ground-water supplies are .available, principally in Central St. Croix, have already been developed. The Castle Coakley area, with a potential yield of 400,000 gpd, is the only major ground-water area still undeveloped.
The ground-water potential could be increased by reducing the brush and forest cover thus reducing water losses from transpiration; artificial recharge of alluvial aquifer by water spreading; utilizing streamflow (including storm runoff) or treated sewage effluent; or by lowering the ground-water level adjacent to streams to induce infiltration.
Advancements in desalination have made the brackish ground water in the Kingshill Marl (estimated at 35 billion recoverable gallons by Robison, 1972) a potential source of water. Recovery of this water would partly de-water the aquifer, which would cause water of better quality in the overlying rocks to