Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York
by Frederick Stumm, Andrew D. Lange, and Jennifer L. Candela
U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 00-4193
ABSTRACT
Manhasset Neck, a peninsula on the northern
shore of Long Island, N.Y., is underlain by
unconsolidated deposits that form a sequence of
aquifers and confning units. Ground water at
several public-supply wells has been affected by
the intrusion of saltwater from the surrounding
embayments (Manhasset Bay, Long Island Sound,
Hempstead Harbor). Twenty-two boreholes were
drilled during 1992-96 for the collection of
hydrogeologic, geochemical, and geophysical
data to delineate the subsurface geology and the
extent of saltwater intrusion within the peninsula.
A series of continuous high-resolution seismic-
reflection surveys was completed in 1993 and
1994 to delineate the character and extent of the
hydrogeologic deposits beneath the embayments
surrounding Manhasset Neck.
The new drill-core data indicate two
hydrogeologic units--the North Shore aquifer
and the North Shore confining unit--where the
Lloyd aquifer, Raritan confining unit, and the
Magothy aquifer have been completely removed
by glacial erosion.
Water levels at selected observation wells
were measured quarterly throughout the study.
These data, and continuous water-level records,
indicate that (1) the upper glacial (water-table)
and Magothy aquifers are hydraulically
connected and that their water levels do not
respond to tidal fluctuations, and (2) the Lloyd
and North Shore aquifers also are hydraulically
connected, but their water levels do respond to
pumping and tidal fluctuations.
Offshore seismic-reflection surveys in the
surrounding embayments, and drill-core samples,
indicate at least four 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 to 4 miles
long and about 1 mile wide, and (5) extend to
more than 400 feet below sea level.
Water from 12 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 two observation wells screened in the upper
glacial aquifer.
Five distinct areas of saltwater intrusion have
been delineated in Manhasset Neck; three extend
into the Lloyd and North Shore aquifers, and two
extend into the upper glacial and Magothy
aquifers. Borehole-geophysical-logging data
indicate that several of these saltwater wedges
range from a few feet to more than 125 feet in
thickness and have sharp freshwater-saltwater
interfaces, and that chloride concentrations
within these wedges in 1997 ranged from 102 to
9,750 milligrams per liter. Several public-supply
wells have either been shut down or are currently
being affected by these saltwater wedges. Data
show active saltwater intrusion in at least two of
the wedges.
Citation: Stumm, Frederick, Lange, Andrew D., and Candela, Jennifer L., 2002,
Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York:
U.S. Geological Survey Water-Resources Investigations Report 00-4193, 42 p.
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