Prepared in cooperation with

Long Island Water Conference

Nassau County Department of Public Works

New York City Department of Environmental Protection

Port Washington Water District

Suffolk County Department of Health Services

Towns of North Hempstead and Shelter Island

Manhasset-Lakeville Water District

Nassau Suffolk Water Commissioners Association

New York State Department of Environmental Conservation

Sands Point Water Department

Suffolk County Water Authority

Water Authority of Great Neck North

Water-Table and Potentiometric-Surface Altitudes in the 

Upper Glacial, Magothy, and Lloyd Aquifers of Long 

Island, New York, April–May 2013 

By Michael D. Como, Michael L. Noll, Jason S. Finkelstein, Jack Monti, Jr., and Ronald Busciolano

Pamphlet to accompany

Scientific Investigations Map 3326

U.S. Department of the Interior 

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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must be secured from the copyright owner.

Suggested citation:

Como, M.D., Noll, M.L., Finkelstein, J.S., Monti, Jack, Jr., and Busciolano, Ronald, 2015, Water-table and 

potentiometric-surface altitudes in the Upper Glacial, Magothy, and Lloyd aquifers of Long Island, New York, April–

May 2013: U.S. Geological Survey Scientific Investigations Map 3326, 4 sheets, scale 1:125,000, 6-p. text, 

http://dx.doi.org/10.3133/sim3326.

ISSN 2329-132X (online)

2
Water-Table and Potentiometric-Surface Altitudes in the 

Upper Glacial, Magothy, and Lloyd Aquifers of Long 

Island, New York, April–May 2013 

Sheet 1–Water table

Introduction

The U.S. Geological Survey (USGS), in cooperation with State and local agencies,

systematically collects groundwater data at varying measurement frequencies to monitor the hydrologic 

conditions on Long Island, New York. Each year during April and May, the USGS conducts a synoptic 

survey of water levels to define the spatial distribution of the water table and potentiometric surfaces 

within the three main water-bearing units underlying Long Island—the upper glacial, Magothy, and 

Lloyd aquifers (Smolensky and others, 1989)—and the hydraulically connected Jameco (Soren, 1971) 

and North Shore aquifers (Stumm, 2001). These data and the maps constructed from them are 

commonly used in studies of Long Island’s hydrology and are utilized by water managers and suppliers 

for aquifer management and planning purposes.

Water-level measurements made in 502 monitoring wells (observation and supply wells) and 16 

streamgage locations across Long Island during April–May 2013 were used to prepare the maps in this 

report. Groundwater measurements were made by the wetted-tape method to the nearest hundredth of a 

foot. Contours of water-table and potentiometric-surface altitudes were created by using the 

groundwater measurements. The water-table contours were interpreted by using water-level data 

collected from 16 streamgages, 334 observation wells, and 1 supply well screened in the upper glacial 

aquifer or the shallow Magothy aquifer; the Magothy aquifer’s potentiometric-surface contours were 

interpreted from measurements at 70 observation wells and 31 supply wells screened in the middle to 

deep Magothy aquifer and the contiguous and hydraulically connected Jameco aquifer. The Lloyd 

aquifer’s potentiometric-surface contours were interpreted from measurements at 58 observation wells 

and 8 supply wells screened in the Lloyd aquifer and the contiguous and hydraulically connected North 

Shore aquifer. Many of the supply wells are in continuous operation and therefore, were turned off for a 

minimum of 24 hours before measurements were made to allow the water levels in the wells to recover 

to ambient (non-pumping) conditions. Full recovery time at some of these supply wells can exceed 24 

hours; therefore, water levels measured at these wells are assumed to be less accurate than those 

measured at observation wells, which are not pumped (Busciolano, 2002). In addition to pumping 

stresses, elevated chloride concentrations (saline water) also lower the water levels measured in certain 

wells. This reduction in water level is the result of saline water being denser than freshwater 

(Lusczynski, 1961). In this report, all water-level altitudes are referenced to the National Geodetic 

Vertical Datum of 1929 (NGVD 29).

The land surface or topography was downloaded from the National Map portal 

(http://nationalmap.gov), which represents the most currently available terrain representation as a 10-

meter digital elevation model (DEM). The National Map terrain representation was combined with 

additional land surface terrain models of Suffolk County and New York City, which were collected 

using lidar to produce a high accuracy three-dimensional land surface altitude model based on the 

3
geospatial product for coastal flood mapping. The datum for land surface altitude is North American 

Vertical Datum of 1988 (NAVD 88). NAVD 88 is approximately 1 – foot lower than NGVD 29 on 

Long Island.

Hydrographs are included on these maps for selected wells that have digital recording 

equipment. These hydrographs are representative of the 2013 water year1 to show the changes that have 

occurred throughout that period. The synoptic survey water level measured at the well is included on 

each hydrograph.

Upper Glacial and Shallow Magothy Aquifers (Water Table)

The upper glacial aquifer of Pleistocene age is the uppermost unit in Long Island’s groundwater 

system and contains the water table throughout the island, except in parts of central and eastern Nassau 

County and western Suffolk County, where the entire upper glacial aquifer is unsaturated and the water 

table is in the shallow Magothy aquifer. The upper glacial aquifer was the principal source of water 

supply throughout Long Island for several decades, but contamination in many areas resulted in the 

widespread curtailment of its use for public supply. The general configuration of the water table is an 

east-west mound that coincides with the topographic high (region of higher land surface altitude) and the 

glacial moraine along the center of the island with isolated areas of higher water levels in central Nassau 

County and in central Suffolk County. Local highs also are present in northwestern Nassau County 

(Stumm and others, 2002) and the central part of the southern peninsula of eastern Suffolk County 

(Schubert and others, 2004). These areas of higher water levels are a result of the low hydraulic 

conductivity of the geologic units underlying these wells.

The water-table altitude on Long Island during April–May 2013 ranged from 0 feet (ft) along the 

shore to more than 121 ft above NGVD 29 in northwestern Nassau County. In Kings County, 20 

measurements ranged from 1.19 to 16.55 ft; in Queens County, 34 measurements ranged from 0.74 to 

46.39 ft; in Nassau County, 71 measurements ranged from 1.84 to 121.71 ft; and in Suffolk County, 210 

measurements ranged from 1.29 to 78.67 ft above NGVD 29.

References Cited 

Busciolano, Ronald, 2002, Water-table and potentiometric-surface altitudes of the Upper Glacial, 

Magothy, and Lloyd Aquifers on Long Island, New York, in March–April 2000, with a summary of 

hydrogeologic conditions: U.S. Geological Survey Water-Resources Investigations Report 01–4165, 17 

p., 6 pl.

Lusczynski, N.J., 1961, Head and flow of ground water of variable density: Journal of 

Geophysical Research, v. 66, no. 12, p. 4247−4256.

Schubert, C.E., Bova, R.G., and Misut, P.E., 2004, Hydrogeologic framework of the North Fork 

and surrounding areas, Long Island, New York: U.S. Geological Survey Water-Resources Investigations 

Report 2002−4284, 23 p., 4 pl., scale 1:120,000.

Smolensky, D.A., Buxton, H.T., and Shernoff, P.K., 1989, Hydrologic framework of Long 

Island, New York: U.S. Geological Survey Hydrologic Investigations Atlas HA–709, 3 sheets, scale 

1:250,000.

Soren, Julian, 1971, Results of subsurface exploration in the mid-island area of western Suffolk 

County, Long Island, New York: Oakdale, N.Y., Suffolk County Water Authority, Long Island 

Resources Bulletin 1, 60 p.

4
Stumm, Frederick, Lange, A.D., and Candela, J.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.

1

A water year is the 12-month period beginning October 1 and ending September 30. It is 

designated by the year in which it ends.

Sheet 2–Potentiometric surface in the Magothy and Jameco aquifers

Magothy and Jameco Aquifers

The Magothy aquifer of upper Cretaceous age is the most extensive unit in Long Island’s 

groundwater system. This aquifer underlies most of Long Island and the offshore waters, except in parts 

of western and northern Kings and Queens Counties, northern Nassau County, and northwestern and 

northeastern Suffolk County, where it has been removed by erosion and glacial scour. The altitude of its 

upper surface ranges from more than 200 feet (ft) above the National Geodetic Vertical Datum of 1929 

(NGVD 29) in parts of north-central Nassau County and west-central Suffolk County to more than 600 

ft below NGVD 29 in north-central Suffolk County (Smolensky and others, 1989). Aquifer thickness 

ranges from zero in northern and western Kings and Queens Counties, northern Nassau County, and 

parts of northern Suffolk County to more than 1,000 ft in south-central Suffolk County (Soren and 

Simmons, 1987). The Magothy aquifer is the principal source of water supply in Nassau and Suffolk 

Counties.

This map depicts water levels during April–May 2013 in wells screened in the Magothy aquifer 

and the hydraulically connected Jameco aquifer. Most water-level measurements used to prepare this 

map were made in wells screened near the middle of the aquifer and were supplemented by some 

measurements from the basal zone. Only a few water-level measurements made in the upper part of the 

aquifer were used. Measured potentiometric heads at a given location can vary depending on the depth 

of the well screen, because vertical hydraulic gradients in the Magothy Aquifer can be large.

The general configuration of the potentiometric surface in the Magothy aquifer is similar to that 

of the water table, rising gradually from the western part of Long Island to form an east-west-trending 

mound in Nassau and western Suffolk Counties. The surface of the mound then gradually declines 

toward the eastern end of Long Island. In areas where deep channels have been eroded into the Magothy 

aquifer and filled with glacial deposits, the potentiometric-surface contours were drawn from water 

levels measured in wells screened in these upper glacial aquifer deposits, which are laterally contiguous 

and hydraulically connected to the Magothy aquifer. The inferred northern limit of the Magothy aquifer, 

the inferred extent of the Jameco aquifer, and the aquifer in which each well is screened are indicated on 

this map. Public-supply pumping and elevated chloride levels have lowered the potentiometric surface 

below NGVD 29 in southern Nassau County (Lusczynski and Swarzenski, 1966).

The altitude of the potentiometric surface in the Magothy aquifer on Long Island during April–

May 2013 ranged from about 5 ft below NGVD 29 in extreme southwestern Nassau County to more 

than 80 ft above NGVD 29 in northeastern Nassau County. In Kings County, 2 measurements ranged 

from 2.77 to 6.58 ft; in Queens County, 8 measurements ranged from 4.26 to 32.05 ft; in Nassau County, 

29 measurements ranged from -4.90 to 80.00 ft; and in Suffolk County, 55 measurements ranged from 

3.61 to 75.90 ft above NGVD 29.

5
References Cited

Lusczynski, N.J., and Swarzenski, W.V., 1966, Salt-water encroachment in southern Nassau and 

southeastern Queens Counties, Long Island, New York: U.S. Geological SurveyWater-Supply Paper 

1613−F, 76 p.

Schubert, C.E., Bova, R.G., and Misut, P.E., 2004, Hydrogeologic framework of the North Fork 

and surrounding areas, Long Island, New York: U.S. Geological Survey Water-Resources Investigations 

Report 2002−4284, 23 p., 4 pl., scale 1:120,000.

Smolensky, D.A., Buxton, H.T., and Shernoff, P.K., 1989, Hydrologic framework of Long 

Island, New York: U.S. Geological Survey Hydrologic Investigations Atlas HA–709, 3 sheets, scale 

1:250,000.

Soren, Julian, and Simmons, D.L., 1987, Thickness and hydrogeology of aquifers and confining 

units below the upper glacial aquifer on Long Island, New York: U.S. Geological Survey WaterResources Investigations Report 86–4175, 3 sheets, scale 1:125,000.

Sheet 3–Potentiometric surface in the Lloyd and North Shore aquifers.

Lloyd and North Shore Aquifers

The Lloyd aquifer of upper Cretaceous age is the basal unit of Long Island’s groundwater 

system. This aquifer continuously underlies most of Long Island and the offshore waters, except in parts 

of western and northern Kings, Queens, and Nassau Counties, and extreme northeastern Suffolk County, 

where it has been removed by erosion. The upper surface of the aquifer ranges in altitude from less than 

100 feet (ft) below the National Geodetic Vertical Datum of 1929 (NGVD 29) in extreme northern parts 

of Queens County to more than 1,500 ft below NGVD 29 in south-central Suffolk County (Smolensky 

and others, 1989). Aquifer thickness ranges from zero in northern and western parts of Kings, Queens, 

and Nassau Counties and extreme northeastern Suffolk County to more than 500 ft in extreme 

southeastern Nassau and southwestern Suffolk Counties (Soren and Simmons, 1987).

This map depicts water levels during April–May 2013 in 66 wells screened in the Lloyd and 

North Shore aquifers. Wells screened in the Pleistocene deposits of the North Shore aquifer in northern 

Nassau County are included because in these areas the North Shore aquifer is hydraulically connected to 

the Lloyd aquifer (Stumm, 2001; Stumm and others, 2002 and 2004). The inferred northern extent of the 

Lloyd aquifer, the inferred extent of the North Shore aquifer, and the aquifer in which each well is 

screened are indicated on this map.

The general configuration of the potentiometric-surface altitude in the Lloyd aquifer is similar to 

those in the overlying water-table and Magothy aquifers. The potentiometric surface gradually rises 

from the western part of Long Island to form an east-west-trending mound in eastern Nassau County that 

continues east into central Suffolk County. The mound then gradually declines toward the eastern end of 

Long Island. Public-supply and golf-course pumping has lowered the potentiometric surface in the 

Lloyd aquifer below NGVD 29 in northwestern Nassau County (Stumm and others, 2002).

The potentiometric-surface altitude in the Lloyd aquifer on Long Island in 2013 ranged from 

about 14 ft below NGVD 29 in northwestern Nassau County to more than 49 ft above NGVD 29 in 

northeastern Nassau County. In Kings County, 3 measurements ranged from 5.87 to 8.80 ft; in Queens 

County, 7 measurements ranged from 6.59 to 17.38 ft; in Nassau County, 49 measurements ranged from 

-14.25 to 49.48 ft; and in Suffolk County, 7 measurements ranged from 14.47 to 37.25 above NGVD 29.

6
References Cited

Smolensky, D.A., Buxton, H.T., and Shernoff, P.K., 1989, Hydrologic framework of Long 

Island, New York: U.S. Geological Survey Hydrologic Investigations Atlas HA–709, 3 sheets, scale 

1:250,000.

Soren, Julian, and Simmons, D.L., 1987, Thickness and hydrogeology of aquifers and confining 

units below the upper glacial aquifer on Long Island, New York: U.S. Geological Survey WaterResources Investigations Report 86–4175, 3 sheets, scale 1:125,000.

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.

Stumm, Frederick, Lange, A.D., and Candela, J.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.

Stumm, Frederick, Lange, A.D., and Candela, J.L., 2004, Hydrogeology and extent of saltwater 

intrusion in the northern part of the town of Oyster Bay, Nassau County, New York: U.S. Geological 

Survey Water-Resources Investigations Report 03–4288, 55 p.

Plate 4–Depth to water table

Depth to Water Table

This map depicts the depth to the water table beneath Long Island during April–May 2013. Areas 

in which the depth to water table is shallow are shown in red and indicate areas where potential 

substructure flooding may occur.

A geographic information system was used to create a continuous surface of the water table 

using an iterative finite difference interpolation technique with measurements from 334 observation 

wells, 1 supply well, 16 streamgages, interpreted 10-foot (ft) contour intervals, and the coastline. 

The land surface or topography was downloaded from the National Map portal 

(http://nationalmap.gov), which represents the most currently available terrain representation as a 10-

meter digital elevation model (DEM). The National Map terrain representation was combined with 

additional land surface terrain models of Suffolk County and New York City, which were collected 

using lidar to produce a high accuracy three-dimensional land surface altitude model based on the 

geospatial product for coastal flood mapping.

The continuous surface of the water-table was adjusted for the vertical datum differences across 

Long Island. This interpolated surface was then subtracted from the topography at the same location. 

The results are shown as a continuous depth to water- table map. Field measurements of depth to water 

(accurate to a hundredth of a foot) are also plotted on the map. 

The general configuration of the depth to the water table reflects the topography data; however, 

because the map scales of the topography (1:24,000) and water-table altitude (1:125,000) differ, the 

horizontal accuracy of the depth to water table is 1:125,000 with a vertical error of plus or minus 5 feet. 

Areas in which no water-level data were available for comparison are shown in gray; however, in areas 

along the south shore of Long Island, including marshes and the barrier island, the water-table altitude 

was assumed to be the National Geodetic Vertical Datum of 1929 (NGVD 29). 

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Groundwater-level Data Retrieval

Groundwater-level data used to create this map product were downloaded from the USGS 

National Water Information System (NWIS) database (http://waterdata.usgs.gov/nwis/si). The NWIS 

database consists of more than 850,000 records of wells, springs, test holes, tunnels, drains, and 

excavations in the United States. Available site information includes well-location and completion 

information, such as latitude, longitude, well depth, and aquifer. Water-level data from the NWIS 

database can be queried and used to construct historical time-series hydrographs for wells. As an 

example, for Suffolk County mapped well "S16783," search NWIS on Site Name “16783" and choose 

the option button “match any part”; then click Submit. A search will be completed, and a resulting list of 

site numbers will appear. Click on the Site Number link, and then click on the Field groundwater-level 

measurements link.

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