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Water-Table and Potentiometric-Surface Altitudes in the 

Upper Glacial, Magothy, and Lloyd Aquifers beneath 

Long Island, New York, April–May 2010 

By Jack Monti, Jr., Michael Como, and Ronald Busciolano

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 used by water managers and suppliers for 

aquifer management and planning purposes.

Water-level measurements made in 503 monitoring wells, a network of observation and supply 

wells, and 16 streamgage locations across Long Island during April–May 2010 were used to prepare the 

maps in this report. Measurements were made by the wetted-tape method to the nearest hundredth of a 

foot. Water-table and potentiometric-surface altitudes in these aquifers were contoured by using these 

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

16 streamgages, 349 observation wells, and 1 supply well screened in the upper glacial aquifer and (or) 

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

measurements at 67 observation wells and 27 supply wells screened in the middle to deep Magothy 

aquifer and (or) the contiguous and hydraulically connected Jameco aquifer. The Lloyd aquifer’s 

potentiometric-surface contours were interpreted from measurements at 55 observation wells and 

4 supply wells screened in the Lloyd aquifer or 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 so that the water levels in the wells could 

recover to the level of the potentiometric head in the surrounding aquifer. 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 

this report, all water-level altitudes are referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29).
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Hydrographs are included on these maps for selected wells that are instrumented with recording 

equipment. These hydrographs are representative of the 2010 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 Upper 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 glacial moraine along the center of the island with an isolated 

high in central Nassau County and another 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 highs 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 2010 ranged from 0 feet (ft) along the 

shore to more than 125 ft above NGVD 29 in northwestern Nassau County. In Kings County, 

26 measurements ranged from 1.39 to 18.36 ft; in Queens County, 42 measurements ranged from 

1.48 to 47.18 ft; in Nassau County, 72 measurements ranged from 5.36 to 125.41 ft; and in Suffolk 

County, 210 measurements ranged from 1.78 to 81.83 ft above NGVD 29.

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 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 has 

become the principal source of water supply in Nassau and Suffolk Counties during the past 50 years 

(1963–2013) as a result of contamination in the upper glacial aquifer.

This map depicts water levels during April–May 2010 in wells screened in the Magothy aquifer 

and the hydraulically connected Jameco aquifer (in parts of western Long Island). 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 measurements made in the 

upper part were used. Because vertical hydraulic gradients in the Magothy aquifer can be large, 

measured potentiometric heads at a given location can vary depending on the depth of the well screen.

 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. 
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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 has 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 2010 ranged from about 2 feet (ft) below NGVD 29 in extreme southwestern Nassau County 

to more than 83 ft above NGVD 29 in northeastern Nassau County. In Kings County, 2 measurements 

ranged from 1.33 to 5.56 ft; in Queens County, 9 measurements ranged from 4.68 to 35.52 ft; in Nassau 

County, 30 measurements ranged from -2.15 to 83.79 ft; and in Suffolk County, 53 measurements 

ranged from 4.85 to 78.70 ft above NGVD 29.

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 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). The Lloyd aquifer has become increasingly important as 

a source of water supply in some parts of Long Island in recent decades. 

This map depicts water levels during April–May 2010 in 60 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 2010 ranged from 

about 6 feet (ft) below NGVD 29 in northwestern Nassau County to more than 49 ft above NGVD 29 in 

northeastern Nassau County. In Kings County, 2 measurements ranged from 6.95 to 7.21 ft; in Queens 

County, 8 measurements ranged from 6.53 to 18.63 ft; in Nassau County, 44 measurements ranged from 

-6.34 to 49.21 ft; and in Suffolk County, 5 measurements ranged from 19.22 to 39.90 above NGVD 29.
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Depth to Water Table

This map depicts the depth to the water table beneath Long Island during April–May 2010. 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 349 observation wells, 

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 

an additional land surface terrain model of Suffolk County, which was 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 altitude was adjusted for the vertical datum differences 

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

results are shown as a depth to water table 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 ± 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 NGVD 29. Areas in which the depth to water table is shallow are shown in red and 

indicate areas where potential substructure flooding may occur.

Groundwater Level Data

Groundwater-level data were downloaded from the USGS National Water Information System 

(NWIS) database (http://waterdata.usgs.gov/nwis/si). This database consists of more than 

850,000 records of wells, springs, test holes, tunnels, drains, and excavations in the United States. 

Available site descriptive information includes well-location and completion information, such as 

latitude, longitude, well depth, and aquifer. NWIS water-level data 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 performed, and a resulting list of site numbers will appear. Click on the Site Number 

link, then click on the Field groundwater-level measurements link.
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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., and Swarzenski, W.V., 1966, Salt-water encroachment in southern Nassau and 

southeastern Queens Counties, Long Island, New York: U.S. Geological Survey Water-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 02–4284, 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.

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 Water-Resources 

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. 

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