INTRODUCTION
The Kingshill aquifer is located in the central to southwestern parts of the island of St. Croix in the Virgin Islands of the United States (fig. 113). The aquifer consists of limestone and marl that has a maximum saturated thickness of about 200 feet. Although the aquifer produces only small volumes of water of marginal chemical quality, it is the only significant aquifer in the U.S. Virgin Islands and supplies a large proportion of the water needed for public supply and industry on St. Croix. Water is scarce on St. Croix. Streamflow is meager and not reliable; aquifers are small and yield mineralized water, much of which is unfit for human use.
St. Croix has a total area of about 80 square miles and is dominated
in the northwestern (Northside Range) and eastern (East End Range)
parts by highland areas that rise to altitudes of 1,088 feet and
866 feet, respectively, and that are underlain chiefly by poorly
permeable intrusive, volcanic, volcaniclastic, and sedimentary
rocks. The Kingshill aquifer, which lies between these two ranges,
is in a lowland that slopes from the south flank of the Northside
Range southward to the sea. The surface of the limestone and marl
that compose the aquifer has been deeply eroded, but about one-fourth
of it is covered by a blanket of alluvium, alluvial fan, debris
flow, and slope wash deposits as much as 80 feet thick, which
moderates the dissected topography and forms a broad rolling plain
through which low, rounded limestone hills are exposed.
St. Croix has four major streams that flow intermittently. All four rise in the Northside Range, and River Gut, the largest, flows southward across the Kingshill aquifer to the sea. The other three streams flow mostly across volcanic rocks to the west coast. The remainder of the streams on the island are very short and flow only after heavy rains.
Although average annual rainfall in St. Croix is abundant, ranging from 30 inches on the east to 50 inches in the mountains of the Northside Range, most of the rainfall is returned to the atmosphere by evapotranspiration, and not more than about 3 percent is available for recharge to aquifers.
HYDROGEOLOGIC UNITS
The island of St. Croix is roughly divided into thirds, both geologically
and topographically. The mountainous northwestern and eastern
parts are underlain by faulted and deformed volcaniclastic and
sedimentary rocks of the Mount Eagle Group of Late Cretaceous
age (fig. 73; table 7) that have been
intruded by the Fountain Gabbro in the Northside Range and the
Southgate Diorite in the East End Range. These two ranges are
separated by a sediment-filled graben structure that has surface
expression as a broad plain that occupies the central and southwestern
one-third of the island. The northeastern part of the central
plain is characterized by a hilly and dissected carbonate highland
area. The plain slopes southeastward from about 400 feet above
sea level where it adjoins the Northside Range to sea level on
the southern coast.
The lower part of the graben is filled to an estimated depth of 6,000 feet with the Jealousy Formation of Miocene to Oligocene (?) age (table 7). The top of the Jealousy Formation is buried throughout its extent; its upper surface generally slopes southeastward from an elevation of about 100 feet above sea level in the north-central part of the island (fig. 114) to about 200 feet below sea level on the south coast. The formation, which is described as a blue clay by well drillers, is made up primarily of deep-water calcareous mud, marl, or limestone. The Jealousy Formation represents hydrologic basement due to the progressively poorer quality water with increasing depth.
Conformably overlying the Jealousy Formation is the Kingshill Limestone (fig. 115; table 7), which consists mostly of deep-water limestone and marl, calcareous clay, and some conglomerate. Recent work on St. Croix indicates that the contact of the Kingshill Limestone and the Jealousy Formation may represent only a color change due to oxidation of the Kingshill sediments by circulating ground water. Thus, the two formations may represent continuous deposition. It does appear, however, that the contact represents a hydrologic boundary and, therefore, the Jealousy Formation forms the bottom of the Kingshill aquifer.
Unconformably overlying the Kingshill Limestone in the southern and western parts of the central plain are unnamed post-Kingshill carbonate strata (fig. 115; table 7) that consist of shallow-water calcareous sediments of limestones and minor dolomite of Pliocene age with some silt, sand, and gravel. The Kingshill Limestone and unnamed post-Kingshill carbonate rocks compose the Kingshill aquifer; the rocks dip and thicken southeastward.
Alluvial material, including sandy to clayey river valley deposits, alluvial fan, debris flow, colluvial deposits, and beach terraces, overlies the Kingshill Limestone and unnamed post-Kingshill carbonate rocks. The alluvial material fills bedrock valleys and subdues the topography of the dissected surface of the Kingshill Limestone. The alluvial material thickens southward to a maximum of about 80 feet, whereas silt- and clay-rich alluvial fan and debris flow deposits are usually less than 30 feet in thickness.
AQUIFERS AND CONFINING UNITS
The post-Kingshill carbonate rocks are far more permeable than
the carbonate rocks of the Kingshill aquifer. However, the two
units act together as a single unconfined aquifer. Alluvial deposits,
which are made up of clay, silt, sand, and gravel, mostly fill
erosional valleys in the Kingshill Limestone and the post-Kingshill
carbonate rocks and generally underlie existing drainage. The
alluvial deposits serve principally as zones in which recharge
from rainfall and streamflow is stored temporarily and then percolates
downward to recharge the underlying Kingshill aquifer. Upland
areas between streams have only a shallow, unsaturated soil that
overlies the aquifer. The saturated thickness of the Kingshill
aquifer ranges from less than 50 feet near the Northside Range
to more than 200 feet near the coast in the south-central part
of the island (fig. 116).
A band of clayey alluvial fan and debris flow deposits is present along the southern flank of the Northside Range (fig. 117). The clayey deposits are poorly permeable and generally yield less than 5 gallons per minute to wells. Elsewhere, the alluvium tends to be coarser grained and yields from 5 to 100 gallons per minute to wells.
HYDRAULIC PROPERTIES
The marl of the Kingshill Limestone generally has microscopic
porosity and is permeable only where it is fractured. However,
the fractures might be partially closed and poorly connected.
Wells that encounter connected fractures in the marl yield about
5 to 20 gallons per minute. Wells completed in limestone beds
of the Kingshill or in minor terrigenous deposits of the formation
might yield larger amounts of water. The post-Kingshill carbonate
rocks are more permeable because they contain well-defined fractures
and solution cavities, as well as intergranular porosity. Yields
to wells completed in the post-Kingshill carbonate rocks range
from 20 to 100 gallons per minute, and the wells have specific
capacities from about 0.5 to 50 gallons per minute per foot of
drawdown.
The estimated transmissivity of the Kingshill aquifer (fig. 118) reflects the increased saturated thickness, as well as the higher permeability, where the post-Kingshill carbonate rocks occur along the southern part of the aquifer. Transmissivities are least in a wide band parallel to the south flank of the Northside Range. The aquifer dips and thickens southeastward toward the ocean and is thinnest in this wide band.
GROUND-WATER FLOW
Regional ground-water flow in the Kingshill aquifer is southeastward,
approximately parallel to the dip of strata that compose the aquifer
(fig. 119). Locally, movement is north-eastward
toward Salt River Bay in the north-central part of the island.
The principal recharge to the Kingshill aquifer is from infiltration of an estimated 3 percent of the precipitation that falls on the aquifer and is not lost to evapotranspiration. Some recharge also is from streams, especially during periods of storm runoff, when the altitude of the water in the stream is above that of the water table. Some water from runoff of the Northside Range may enter the aquifer at its contact with the volcaniclastic and sedimentary rocks. Water readily moves into the permeable alluvium and into fractures in the marl and limestone.
Ground water moves down the hydraulic gradient defined by contours on the water table (fig. 119). The broad spacing of the contours in the north-central part of the aquifer indicates a shallow gradient that changes to a steeper gradient (indicated by closer spacing) in the central part of the aquifer. A shallow gradient also is shown by the broad contour spacing along the south coast. The gradient changes probably reflect permeability changes in the aquifer related to fracturing, thickness of alluvium, and the presence of the more permeable post-Kingshill carbonate rocks along the south coast.
Discharge from the Kingshill aquifer is largely to wells and to the ocean, but some water also probably discharges to streams during high stages of the water table. Evapotranspiration also accounts for some discharge of water where the water table is less than 20 feet below land surface.
A freshwater-saltwater interface occurs where the aquifer meets the sea. The interface is in the form of a transition zone from freshwater to saltwater and slopes downward and landward from the coastline. Freshwater from the aquifer moves up this interface to discharge at the sea floor.
GROUND-WATER QUALITY
The chemical quality of water in the Kingshill aquifer is marginal
for human consumption and most other uses. A criterion on the
island for use of water from the aquifer for drinking purposes
is that the water contain chloride concentrations of less than
500 milligrams per liter; this is nearly twice the concentration
of chloride recommended for drinking water by the U.S. Environmental
Protection Agency. Dissolved-solids concentrations generally exceed
1,000 milligrams per liter, which is the definition of saline
water used in this report. These values are for the part of the
aquifer that contains the least mineralized water. Water quality
deteriorates with depth and is highly saline in some areas.
The island environment of the Kingshill aquifer has an influence on ground-water quality. The source of ions in the water is not only from partial dissolution of aquifer minerals, but includes ions from seawater that is transported to the land surface by precipitation, waves, and sea spray and percolates downward to recharge the aquifer. Chloride in the ground water is largely from sea spray or residual salts in the aquifer matrix. Other ions are probably derived from dissolution of aquifer minerals, which include carbonate minerals as well as siliceous volcaniclastic material incorporated in the aquifer.
Analyses of water from the western and southern parts of the aquifer indicate that the water typically is a sodium chloride type and that average dissolved-solids concentrations exceed 1,600 milligrams per liter (fig. 120). Although water that contains dissolved-solids concentrations of 1,000 milligrams per liter is considered to be saline water in this report, on St. Croix water with concentrations of dissolved solids in the order of 2,000 to 5,000 milligrams per liter is used for many purposes because no less-mineralized water is available in most places.
Water in the Kingshill aquifer becomes highly mineralized with depth. The chloride concentration of the water increases rapidly with increasing depth below sea level (fig. 121). The distribution of dissolved-solids concentrations in water from the shallow part of the aquifer (fig. 122) indicates a general increase downgradient and toward the sea.
GROUND-WATER WITHDRAWALS
Total ground-water withdrawals from the Kingshill aquifer during
1985 were about 0.96 million gallons per day (fig.
123). The aquifer produced only small quantities of water,
most of which was highly mineralized. About 81 percent of the
water withdrawn, or about 0.78 million gallons per day, was used
for domestic and commercial purposes. About 15 percent of the
withdrawals, or about 0.14 million gallons per day, was used for
public supply; only about 3 percent, or about 0.04 million gallons
per day, was withdrawn for industrial, mining, and agricultural
uses. No ground water was used for thermoelectric power production.
Most of the water withdrawn is mixed with seawater and used to
feed desalination plants for public supply. In many households
not served by public supply systems, water from the Kingshill
aquifer is supplemented with freshwater obtained from roof-top
collection systems.
