Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York
by Frederick Stumm
U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 99-4280
ABSTRACT
Great Neck, a peninsula, in the
northwestern part of Nassau County, N.Y., is
underlain by unconsolidated deposits that form a
sequence of aquifers and confining units. Seven
public-supply wells have been affected by the
intrusion of saltwater from the surrounding
embayments (Little Neck Bay, Long Island Sound,
Manhasset Bay). Fifteen observation wells were
drilled in 1991-96 for the collection of
hydrogeologic, geochemical, and geophysical data
to delineate the subsurface geology and extent of
saltwater intrusion within the peninsula.
Continuous high-resolution seismic-reflection
surveys in the embayments surrounding the Great
Neck peninsula and the Manhasset Neck peninsula
to the east were completed in 1993 and 1994.
Two hydrogeologic units are newly
proposed herein.the North Shore aquifer and the
North Shore confining unit. The new drill-core
data collected in 1991-96 indicate that the Lloyd
aquifer, the Raritan confining unit, and the
Magothy aquifer have been completely removed
from the northern part of the peninsula by
extensive glacial erosion.
Water levels at selected observation wells
were measured quarterly throughout the study.
The results from two studies of the effects of tides
on ground-water levels in 1992 and 1993 indicate
that water levels at wells screened within the
North Shore and Lloyd aquifers respond to tides
and pumping effects, but those in the overlying
upper glacial aquifer (where the water table is
located) do not. Data from quarterly water-level
measurements and the tidal-effect studies indicate
the North Shore and Lloyd aquifers to be
hydraulically connected.
Offshore seismic-reflection surveys in the
surrounding embayments indicate at least two
glacially eroded buried valleys with
subhorizontal, parallel reflectors indicative of
draped bedding that is interpreted as infilling by
silt and clay. The buried valleys (1) truncate the
surrounding coarse-grained deposits, (2) are
asymmetrical and steep sided, (3) trend
northwest-southeast, (4) are 2-4 miles long and
about 1 mile wide, and (5) extend to more than
200 feet below sea level.
Water from six public-supply wells
screened in the Magothy and upper glacial
aquifers contained volatile organic compounds in
concentrations above the New York State
Department of Health Drinking Water Maximum
Contaminant Levels, as did water from one
public-supply well screened in the Lloyd aquifer,
and from three observation wells screened in the
upper glacial and Magothy aquifers.
Four distinct wedge-shaped areas of
saltwater intrusion have been delineated within
the aquifers in Great Neck; three areas extend into
the Lloyd and North Shore aquifers, and the
fourth area extends into the upper glacial aquifer.
Three other areas of saltwater intrusion also have
been detected. Borehole-geophysical-logging data
indicate that four of these saltwater wedges range
from 20 to 125 feet in thickness and have sharp
freshwater-saltwater interfaces, and that
maximum chloride concentrations in 1996 ranged
from 141 to 13,750 milligrams per liter. Seven
public-supply wells have either been shut down or
are currently being affected by saltwater intrusion.
Citation: Stumm, Frederick, 2001, Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York: U.S. Geological Survey Water-Resources Investigations Report 99-4280, 41 p.
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