Geology and ground-water conditions in southern Nassau and southeastern Queens Counties, Long Island, N.Y.

Test drilling, electrical logging, and water sampling of 'outpost' and other
wells have revealed the existence of a deep confined body of salt water in the
Magothy(?) formation beneath southwestern Nassau and southeastern Queens
Counties, Long Island, N.Y. In connection with a test-drilling program, cooperatively sponsored by the U.S. Geological Survey, the Nassau County Department
of Public Works, and the New York State Water Resources Commission (formerly
Water Power and Control Commission), 13 wells ranging in depth from about
130 to 800 feet were drilled during 1952 and 1953 and screened at various depths
in the Magothy(?) formation and Jameco gravel. On the basis of the preliminary
geologic, hydrologic, and chemical data from these wells, a detailed investigation
of ground-water conditions from the water table to the bedrock was begun in a
200-square-mile area in southern Nassau and southeastern Queens Counties.
The Inain purposes of the investigation were to delineate the bodies of fresh and
salty ground water in the project area, to relate their occurrence and movement
to geologic and hydrologic conditions, to estimate the rate of encroachment, if
any, of the salty water, and to evaluate the effectiveness of the existing network
of outpost wells as detectors of salt-water encroachment.
About a million people in the report area, residing mainly in southern Nassau
County, are completely dependent on ground water as a source of supply. Fortunately, precipitation averages about 44 inches per year, of which approximately
half is estimated to percolate into the ground-water reservoir.
The ground water is contained in and moves through eight differentiated geologic units composed of unconsolidated gravel, sand, and clay, of Late Cretaceous,
Pleistocene, and Recent age, having a maximum total thickness of about 1,700
feet. The underlying metamorphic and igneous crystalline basement rocks are
of Precambrian age and are not water bearing.
The water-yielding units from the surface down are (1) the upper Pleistocene
deposits, (2) the principal artesian aquifer, composed of the Jameco gravel and
Magothy(?) formation, and (3) the Lloyd sand member of the Raritar formation.
The confining units are the '20-foot' clay, the Gardiners clay, and the clay member of the Raritan formation. The upper Pleistocene deposits contain an extensive unconfined body of fresh water. Fresh water under artesian conditions is
contained in the principal artesian aquifer and the Lloyd sand member. The
piezometric surface of the principal artesian aquifer is similar in shape to the south-ward-sloping water table; it ranges in altitude from about sea level to 55 feet
above.
The chemical quality of the fresh ground water in most of the area in all aquifers
is good to excellent, and concentrations of dissolved solids and of chloride generally
are below 100 ppm (parts per million) and 10 ppm, respectively. Analyses of
water samples from selected wells show no progressive increase in concentration of
chloride in most of the area. The data on quality of water have been used to
delineate one major and several minor bodies of salty ground water. The wedgeshaped main confined salt-water body, in which the concentration of chloride
reaches about 17,000 ppm, is in the Magothy(?) formation and Jameco gravel in
extreme southwestern Nassau County and southeastern Queens County. The
base of the salt-water wedge is about at the top of the clay member of the Raritan
formation. Beneath the barrier beach in south-central and southeastern Nassau County a shallow extension of the main confined salt-water body contains as much as 4,000 ppm of chloride and is separated from the lower main salt-water body by fresh ground water. Shallow, thin bodies of unconfined salty ground water are common in the upper Pleistocene and Recent deposits adjacent to salty surface water in tidal creeks, bays, and the Atlantic