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Scientific Investigations Map 3326

Prepared in cooperation with the
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

Sheet 1 - Water table
     Sheet 1 - Water table
Sheet 2 - Potentiometric surface in the Magothy and Jameco aquifers
     Sheet 2 - Potentiometric surface in
     the Magothy and Jameco aquifers
Sheet 3 - Potentiometric surface in the Lloyd and North Shore aquifers
     Sheet 3 - Potentiometric surface in
     the Lloyd and North Shore aquifers
Sheet 4 - Depth to water table
     Sheet 4 - Depth to water table


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 (, 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 datum for land surface altitude is North American Vertical Datum of 1988 (NAVD 88). On Long Island NAVD 88 is approximately 1-foot lower than NGVD 29.

Hydrographs are included on these maps for selected wells that have digital recording equipment. These hydrographs are representative of the 2013 water year to show the changes that have occurred throughout that period. The synoptic survey water level measured at the well is included on each hydrograph.

First posted May 19, 2015

For additional information, contact:
Director, New York Water Science Center
U.S. Geological Survey
425 Jordan Road
Troy, NY 12180
(518) 285-5600

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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. pamphlet,

ISSN 2329-132X (online)

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