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.