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Hydrogeology and Extent of Saltwater 

Intrusion in the Northern Part of the Town 

of Oyster Bay, Nassau County, New York: 

1995-98

By Frederick Stumm, Andrew D. Lange, and Jennifer L. Candela

In cooperation with 

NASSAU COUNTY DEPARTMENT OF PUBLIC WORKS

Water-Resources Investigations Report 03-4288

U.S. Department of the Interior

U.S. Geological Survey
This page has been left blank intentionally.
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Coram, New York: 2004

For additional information write to:

U.S. Geological Survey

2045 Route 112, Bldg. 4

Coram, NY 11725

This report is available on-line at: http://ny.usgs.gov

For sale by U.S. Geological Survey, Information Services 

Box 25286, Denver Federal Center 

Dnever, Co 802225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.
iii

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Previous investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Purpose and scope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Methods of study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Drilling logs and core samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Marine-seismic-reflection surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Borehole geophysical logs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Water-level measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Water-quality data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Hydrogeology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Bedrock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Aquifers and Confining Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Lloyd aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Raritan clay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Magothy aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

North Shore aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

North Shore confining unit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Upper glacial aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Interpretation of Geophysical Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Marine-seismic-reflection surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Borehole geophysical logs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Precipitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Ground water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Pumpage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Water levels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Upper glacial and Magothy aquifers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Lloyd and North Shore aquifers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Water quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Chloride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Volatile organic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Inorganic constituents, nitrate, and metals . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Extent of saltwater intrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Characteristics of saltwater intrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Saltwater wedges A, B, C, and D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Saltwater wedge A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Saltwater wedge B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Saltwater wedge C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Saltwater wedge D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Contents
iv

Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

References cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

FIGURES

1. Map showing location of the Oyster Bay study area and seismic-reflection profiles, 

Nassau County, N.Y. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

 2. Map showing locations of production, observation, industrial and golf-course wells 

in the Oyster Bay study area, Nassau County, N.Y. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

 3. Map showing surface altitude of bedrock underlying Oyster Bay study area, Nassau 

County, N.Y. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

 4. Hydrogeologic sections in Oyster Bay study area, Nassau County, N.Y.:

 A. Section A-A ́ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

 B. Section B-B ́ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

 C. Section C-C ́ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

 D. Section D-D ́ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

 5-10. Maps of Oyster Bay study area, N.Y., showing extent and upper surface altitude 

of selected hydrogeologic units:

 5. Lloyd aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

 6. Raritan clay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

 7. Magothy aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

 8. North shore aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

 9. North shore confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

 10. Upper glacial aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

 11. Vertical sections showing interpreted stratigraphy along seismic-reflection profiles, 

E-E ́, F-F ́, G-G ́, and H-H ́ off Oyster Bay study area, 1996: 

 A. Section E-E ́ in Long Island Sound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

 B. Section F-F ́ in Long Island Sound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

 C. Section G-G ́ in Long Island Sound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

 D. Section H-H ́ in Cold Spring Harbor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

 12. Geophysical logs of well N12774, Bayville, N.Y. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

 13-14. Maps of Oyster Bay study area, Nassau County, N.Y., showing water levels in 

July 1997:

13. Water-table altitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

14. Potentiometric-surface altitude in the Lloyd and North Shore aquifers . . . . . . . . . . . 40

 15. Graphs showing water levels from Lloyd and North Shore aquifer wells in 

Oyster Bay study area, Nassau County, N.Y.: A. Well N12697, B. Well N11798, 

C. Well N11280, D. Well N110, E. Well N12667, F. Well N12774 . . . . . . . . . . . . . . . . . . . . . . 42

 16. Maps showing chloride concentrations in the upper glacial and Magothy aquifers in 

Oyster Bay study area, Nassau County, N.Y., 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

 17. Maps showing chloride concentrations in the Lloyd and North Shore aquifers in 

Oyster Bay study area, Nassau County, N.Y., 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

 18. Geophysical logs of two observation wells in northern Oyster Bay study area, 

Nassau County, N.Y.: A. Well N12790, B. Well N12747 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

 19. Graphs showing chloride concentration and pumpage at selected production wells 

in northern Oyster Bay study area, Nassau County, N.Y.: A. Well N7643, 

B. Well N10144, C. Well N8776. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
v

TABLES

1. Generalized description of hydrogeologic units underlying the Oyster Bay study area, 

Nassau County, N.Y. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2. Total annual pumpage from the Oyster Bay Study area, Nassau County, N.Y., by aquifer, 

1993-97 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 

CONVERSION FACTORS, AND DATUM

 Multiply By To Obtain

 Length

 inch (in) 25.40 millimeter

 foot (ft) 0.3048 meter

 mile (mi) 1.609 kilometer

 Area

 square mile (mi 2) 2.590 square kilometer

 Flow

 million gallons per day (Mgal/d) 0.0438 cubic meters per second

 Hydraulic conductivity

 foot per day (ft/d) 0.3048 meter per day

 Volume

 ounce, fluid (fl oz) 29.57 milliliter

 Acoustic velocity

 foot per second (ft/s) 0.3048 meter per second

 Other abbreviations used

 hour (h)

 milligrams per liter (mg/L)

 million gallons (Mgal)

 millisiemens per meter (mS/m)

 minute (min)

 gallons per minute per foot (gal/min)/ft of drawdown

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29).

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).

Altitude, as used in this report, refers to distance above the vertical datum.
vi
HYDROGEOLOGY AND EXTENT OF SALTWATER 

INTRUSION IN THE NORTHERN PART OF THE TOWN OF 

OYSTER BAY, NASSAU COUNTY, NEW YORK: 1995-98

By Frederick Stumm, Andrew D. Lange, and Jennifer L. Candela

Abstract

The Oyster Bay study area, in the northern part of Nassau 

County, N.Y., is underlain by unconsolidated deposits that 

form a sequence of aquifers and confining units. At least one 

production well has been affected by the intrusion of saltwater 

from Hempstead Harbor, Long Island Sound, and Cold Spring 

Harbor. Nineteen boreholes were drilled during 1995-98 for 

the collection of hydrogeologic, geochemical, and geophysical 

data to delineate the subsurface geology and the extent of 

saltwater intrusion. Continuous high-resolution marineseismic-reflection surveys in the surrounding embayments of 

the Oyster Bay study area were conducted in 1996.

New drill-core data indicate two hydrogeologic units—

the North Shore aquifer and the North Shore confining 

unit—where the Lloyd aquifer, the Raritan confining unit, 

and the Magothy aquifer have been completely removed by 

glacial erosion.

Water levels at 95 observation wells were measured 

quarterly during 1995-98. These data and continuous waterlevel records indicated that (1) the upper glacial (water-table) 

and Magothy aquifers are hydraulically connected and that 

their water levels did not respond to tidal fluctuations, and 

(2) the Lloyd and North Shore aquifers are hydraulically 

connected and their water levels responded to pumping and to 

tidal fluctuations.

Marine seismic-reflection surveys in the surrounding 

embayments 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 coarsegrained deposits, (2) are asymmetrical and steep sided, (3) 

trend northwest-southeast, (4) are several miles long and 

about 1 mile wide, and (5) extend to more than 500 feet below 

sea level.

Water samples taken during 1995-98 from three 

production wells and six observation wells screened in 

the upper glacial and Magothy aquifers contained volatile 

organic compounds in concentrations that exceeded the New 

York State Department of Health Drinking Water Maximum 

Contaminant Levels. High iron or nitrate concentrations 

were detected in water samples taken in 1997-98 from 39 

observation wells. Previous high concentrations resulted in the 

shutdown of two production wells.

Four distinct areas of saltwater intrusion in the Oyster 

Bay study area were delineated— three were in the upper 

glacial aquifer, and the fourth was in the Lloyd aquifer. 

Borehole-geophysical-logging data indicated that three of 

these saltwater “wedges” ranged from a few feet thick to 

more than 100 feet thick and had sharp freshwater-saltwater 

interfaces. Chloride concentrations in water from eight 

observation wells within these wedges in 1997 ranged from 

125 to 13,750 milligrams per liter. One production well in 

Bayville has been shut down as of 1996 and others in the area 

may be affected by these saltwater wedges.

Introduction

The ground-water system within the coastal areas of 

northern Nassau County is under increasing stress from 

pumping by production, industrial, and golf-course-irrigation 

wells. Increasing chloride concentrations in ground water 

pumped from production wells have caused a need for detailed 

information on the area’s hydrogeologic system. Groundwater-resources managers in Nassau County’s northern 

coastal area are concerned about increasing concentrations of 

chloride in ground water pumped from production wells and 

will require information on the hydrogeologic framework, the 

effect of pumping on ground-water levels, and the extent of 

saltwater intrusion into the ground-water system. 

In 1991, the U.S. Geological Survey (USGS), in 

cooperation with the Nassau County Department of Public 

Works (NCDPW), began a long-term study to delineate 

the hydrogeologic framework and the extent of saltwater 

intrusion in northern Nassau County. Results of the study 

will be used by water-resources managers for the protection 

and conservation of the local ground-water supply. 

The study entailed an evaluation of the current (1998) 

observation-well network; drilling of new observation wells 

in areas of probable saltwater intrusion and in areas where 

hydrogeologic data were scarce; collection of geologic, 
2 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

hydrologic, geophysical (borehole and marine-seismicreflection) and water-quality data; and compilation of data to 

characterize the hydrogeologic framework and the extent of 

saltwater intrusion.

The study area and its surrounding embayments 

encompass about 64 mi2 in the northeastern part of Nassau 

County (fig. 1). The Oyster Bay study area is bounded on 

the west by Hempstead Harbor, on the north by Long Island 

Sound, and on the east by Cold Spring Harbor (fig. 1). The 

southern boundary of the study area is between Northern 

Boulevard (Route 25A) and the Long Island Expressway (fig. 

1). In 1996, the population of the study area was estimated 

to be 160,750, or about 11 percent of Nassau County’s 

population (Nassau County Department of Health, 1997).

Previous Investigations

The ground-water resources of Long Island were first 

studied by Veatch and others (1906), who described the 

geologic deposits and occurrence of ground water throughout 

Long Island. Fuller (1914) revised and supplemented much 

of the earlier geologic work. The geologic correlations used 

herein are revised from those of Fuller (1914) as well as 

those of Isbister (1966), and Kilburn and Krulikas (1987). 

Suter and others (1949) contoured the upper surface of the 

major geologic units underlying Long Island. Swarzenski 

(1963) described the hydrogeology and hydrologic conditions 

in northwestern Nassau County during 1955-57; many of 

his interpretations and general hydrogeologic framework 

were verified by the drill-core and marine-seismic-reflection 

data. Isbister (1963) discussed the relation of freshwater 

to saltwater in Centre Island, and Isbister (1966) described 

the hydrogeology and hydrologic conditions in the northern 

part of Oyster Bay in 1958-60. Kilburn and Krulikas (1987) 

attempted to restructure and rename parts of the hydrogeologic 

framework in the northern section of the Town of Oyster Bay, 

largely from driller’s logs (some of which were questionable). 

The hydrogeologic framework was remapped by recent 

USGS work that resulted in the identification of the North 

Shore aquifer and North Shore confining unit in the Great 

Neck peninsula by Stumm (2001) and in the Manhasset Neck 

peninsula by Stumm and others (2002). The most recent 

water-table reports that include the study area are Kimmel 

(1971) for 1970, Donaldson and Koszalka (1983) for 1979, 

Doriski (1987) for 1984, and Busciolano and others (1998) 

for 1997.

Smith (1958) described the first use of high-power, lowfrequency marine-seismic-reflection equipment to measure the 

depth to bedrock beneath selected areas of Long Island Sound. 

Oliver and Drake (1951) used marine-seismic-reflection 

techniques to measure the depth to bedrock in Long Island 

Sound, and Williams (1981) used high-resolution marineseismic-reflection surveys and core samples to identify the 

sediments beneath Long Island Sound. Grim and others (1970) 

described the geology and subsurface morphology of Long 

Island and the Manhasset Bay embayment. Lewis and Stone 

(1991) conducted a systematic marine-seismic-reflection 

survey to map the deposits beneath most of Long Island 

Sound, except the shallow coastal areas and embayments 

along the northern shore of western Long Island. Tagg and 

Uchupi (1967) used continuous marine-seismic-reflection 

profiles to define the subsurface morphology of Long 

Island Sound. Stumm (2001) and Stumm and others (2002) 

completed marine-seismic-reflection surveys along the north 

shore of Nassau County and correlated these units with 

observation-well core samples.

Purpose and Scope

This report (1) characterizes the hydrogeology underlying 

the Oyster Bay study area, (2) describes the ground-water flow 

system, and (3) delineates the extent of saltwater intrusion 

within the major aquifers as of 1998. It also presents maps 

and hydrogeologic sections that summarize the geologic 

and hydrologic data in the area of investigation. Included 

are water-table and potentiometric-surface maps of major 

aquifers, surface-altitude maps of hydrogeologic units, and 

hydrogeologic sections that show the extent and thickness of 

the units and chloride concentration within these units during 

1992-98. 

Acknowledgments

The authors extend thanks to James Mulligan, Brian 

Schneider, Raymond Mazza, Kenneth Fischgrund, and 

others of NCDPW for their assistance and technical 

support throughout this study. Thanks are also given to the 

Incorporated village of Bayville, city of Glen Cove Water 

Department, Jericho, Locust Valley, Old Westbury, Oyster 

Bay, and Roslyn Water Districts, and Sea Cliff Water 

Company for providing pumpage information and access to 

their production well sites. The Seawanaka Yacht Club in 

Centre Island is thanked for providing dock space during the 

marine-seismic-reflection survey. The authors thank the USGS 

Branch of Geophysics for their technical support and use of 

the marine seismic-reflection equipment and the R/V F. Peter 

Haeni. The authors express appreciation to Delta Well and 

Pump Co., Inc., Hydro Group, Inc., and R and L well drillers 

for providing lithologic samples and access to boreholes for 

geophysical logging at various sites in the study area. 

Methods of Study

Data collected during this study included (1) drilling 

logs and core samples from observation-wells, (2) boreholegeophysical logs, (3) marine-seismic-reflection surveys, (4) 

water-level measurements, and (5) water-quality analyses. 

Additional information was obtained from the NCDPW, the 
 3

FIGURE 1. Location of Oyster Bay study area and marine-seismic-reflection profiles, Nassau County, N.Y.

0 5 10 MILES

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MAP

NEW YORK

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F

F'G G'

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H'

NORTHERN

BOULEVARD

( ROUTE 25A )

LONG

ISLAND EXPRESSWAY

74o 73o 72o

40o

45'

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40o

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 Methods of Study 
4 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

Nassau County Department of Health (NCDH), the New 

York State Department of Environmental Conservation, and 

from previous studies. The maps and sections are based on all 

available geologic data and well-driller’s logs, as of 1998.

Drilling Logs and Core Samples

Nineteen boreholes were drilled during 1995-98 to collect 

hydrogeologic, water-quality, and borehole-geophysical 

data. The drilling program consisted of mud-rotary drilling 

and split-spoon core sampling. Most boreholes were drilled 

through the unconsolidated sediments into bedrock, and 

core samples were obtained at regular intervals for geologic 

analysis. The thickness of each hydrogeologic unit was 

estimated primarily from core data obtained during the drilling 

of observation wells, from borehole-geophysical logs, driller’s 

logs, and through interpolation. Color descriptions of the 

core samples were based on the standard Munsell color chart 

(Kollmorgan Instruments Corporation, 1994). 

Marine-Seismic-Reflection Surveys

Several continuous high-resolution marine-seismicreflection profiles were completed in 1996 along the coast 

of the study area to identify the unconsolidated deposits 

from Hempstead Harbor to Cold Spring Harbor (fig. 1) 

and to correlate these deposits with hydrogeologic and 

borehole-geophysical data obtained during the drilling of 

the observation wells. Locations of four of the survey lines 

are shown in figure 1. The continuous seismic-reflection 

system consisted of a graphic recorder, an amplifier and 

filter, a power generator, a catamaran-mounted sound source, 

a hydrophone array, and a digital tape recorder, all of which 

were installed in a shallow-draft 22-ft boat. The sound 

source and hydrophone array were towed behind the slowly 

moving boat, where the marine-seismic-reflection data were 

digitally recorded and filtered for noise and displayed in 

real-time graphics. 

The continuous high-resolution marine-seismic-reflection 

surveys were used to interpret the depth and continuity 

of seismic reflectors and lithology (Haeni, 1986, 1988). 

Applications of this technique for hydrogeologic and waterresource studies have been described by Haeni (1986, 1988), 

Reynolds and Williams (1988), Stumm (2001), Stumm and 

Lange (1994, 1996), and Stumm and others (2002).

The sound source generates seismic signals that 

travel through the water column and penetrate the deposits 

underlying the sea floor. Some of the seismic signal is 

reflected back to the water surface from the sea floor. 

Stratigraphic interfaces will show a change in the acoustic 

impedance (the product of the density and acoustic velocity 

of each medium) (Haeni, 1986; Robinson and Çoruh, 1988). 

The reflected signals received by the hydrophone array 

produce an electrical signal that is amplified, recorded on 

digital tape, filtered, and then plotted. The resulting seismic 

sections, which resemble geologic sections, were correlated 

with geologic logs of nearshore observation wells. In this 

situation, the vertical axis is a function of the time required 

for the seismic signal to travel from the source to the reflector 

and back (Haeni, 1986). Several seismic-reflection and 

refraction studies indicate that the average acoustic velocity 

of unconsolidated saturated glacial deposits is about 5,000 ft/s 

(Haeni, 1988; Reynolds and Williams, 1988); this was used as 

an average velocity in this study. 

Borehole Geophysical Logs

Borehole geophysical logs were collected from 

observation wells to provide information that could not be 

obtained by drilling and sampling. The geophysical-logging 

systems used in this study provided continuous digital 

records that were dependent upon the physical properties of 

the sediment, the rock matrix, and the interstitial fluids. At 

several sites, natural-gamma radiation (gamma), spontaneous 

potential (SP), single-point-resistance (SPR), and short-and 

long-normal resistivity (R) logs were collected in mud-filled 

open boreholes before the casing was installed, then focused 

electromagnetic-induction (EM) logs were obtained after 

installation of the polyvinyl chloride (PVC) casing. The types 

of geophysical logs are described below.

Natural-gamma radiation (gamma) logs—provide a 

record of the total gamma radiation detected in a borehole 

(Keys, 1990). Clays and fine-grained sediments tend to be 

more radioactive than the quartz sand that forms the bulk of 

the deposits on Long Island. Gamma logs commonly are used 

for lithologic and stratigraphic correlation.

Spontaneous-potential (SP) logs—provide a record 

of the electrical potential, or voltage, that develops at the 

contact between clay beds and sand aquifers within a borehole 

(Keys, 1990). SP differs from one formation to the next and 

is measured in millivolts (Serra, 1984). SP is a function of 

the chemical activity of the borehole fluid, the water in the 

adjacent sediments, the water temperature, and the type and 

quantity of clay. SP logs are used to determine lithology, bed 

thickness, and salinity of formation water (Keys, 1990).

Single-point-resistance logs (SPR)—provide a measure 

of the resistance, in ohms, between an electrode in the 

borehole and an electrode at land surface. The volume of 

surrounding material to which the SPR probe is sensitive is 

spherical and only 5 to 10 times the electrode diameter, and is 

affected by the borehole fluid (Keys, 1990). SPR logs are used 

to obtain high-resolution lithologic information.

Normal-resistivity (R) logs—measure apparent resistivity 

in ohm-meters and are used to interpret lithology and water 

salinity (Keys, 1990). This technique uses two electrodes 

typically spaced 16 to 64 in. apart in the borehole, called 

short and long normal logs, respectively. The volume of 

surrounding material to which normal-resistivity probes 
 5

are sensitive is spherical, with a diameter about twice the 

electrode spacing (Serra, 1984; Keys, 1990). Only short 

normal resistivity logs were used in this study.

Focused electromagnetic-induction (EM) logs—measure 

formation conductivity and are used in conjunction with 

gamma logs for distinguishing between conductive fluids 

and conductive clays. This technique uses an electromagnetic 

emitter coil that induces current loops within the surrounding 

formation to generate a secondary electromagnetic field. 

The intensity of the secondary field received by the receiver 

coil is proportional to the formation conductivity (Keys, 

1990; Serra, 1984; Keys and MacCary, 1971). EM logs are 

measured in units of millisiemens per meter (mS/m) and are 

inversely related to the ohm-meter value of normal-resistivity 

logs (Keys, 1990; Serra, 1984; Keys and MacCary, 1971). 

The normally low conductivity of geologic deposits on Long 

Island is favorable for induction logging to delineate highly 

conductive fluids such as saltwater or leachate. For this 

reason, EM logging has been used on Long Island to delineate 

the freshwater-saltwater interface (Chu and Stumm, 1995; 

Stumm, 1993, 1994, 2001; Stumm and Lange, 1994, 1996; 

Stumm and others, 2002).

Water-Level Measurements

Water levels were measured quarterly at selected 

observation wells throughout the study area from 1992 to 

1998. All wells were measured on the same day to minimize 

the effect of nearby sporadic pumping. All water-level data 

were checked and entered into the USGS data base in Coram, 

N.Y. A total of 95 observation wells were measured within the 

Oyster Bay study area and surrounding areas (fig. 2).

Continuous water-level recorders were installed at 

seven wells, each screened in a specific aquifer. Five of these 

recorders were digital pressure-transducer data loggers that 

recorded at 1-hour intervals; the other two were continuous 

analog recorders. The continuous analog recorder at well 

N7152 (near Oak Neck, figs. 1, 2), was replaced with a 

digital pressure-transducer data logger in 1997. Historical 

(pre-1992) continuous water-level recorder information from 

12 additional wells was used to supplement the data. The 

resulting hydrographs were used to identify tidal effects on 

water levels, possible aquifer interconnections, and the effect 

of pumping from nearby production wells.

Water-Quality Data

Water samples were collected from 42 observation wells 

during 1994-97 by the NCDPW and analyzed by the NCDPW 

Cedar Creek Special Projects Laboratory. Additional data on 

ground-water quality in the study area are available from the 

NCDH and the NCDPW. The results were compared with the 

New York State Department of Health (NYSDH) maximum 

contaminant levels (MCL’s) for drinking water for 1996 

to summarize the general water quality of the study area. 

These wells were also sampled for chloride concentration 

and specific conductance for delineation of the freshwatersaltwater interface.

Filter-press samples were obtained from selected core 

samples for chloride and specific-conductance measurement at 

the USGS office in Coram. Filter-press samples were collected 

using nitrogen-gas pressure to force interstitial water from 

uninvaded (by drilling mud) parts of a split-spoon core sample 

obtained during drilling (Lusczynski, 1961). The sample 

volume typically was about 0.07 fl oz. (2 mL); chloride 

concentration and specific conductance were measured by 

hand-held probes. The resulting values were used only as a 

general indicator of brackish or salty water within a formation 

and were correlated with borehole geophysical logs and the 

values from samples collected from screened wells.

Hydrogeology

The Oyster Bay study area is underlain by unconsolidated 

Holocene deposits, glacial deposits of Pleistocene age, 

and Coastal-Plain deposits of Late Cretaceous age. These 

sediments consist of gravel, sand, silt, and clay underlain 

by crystalline bedrock of early Paleozoic age. The bedrock 

is relatively impermeable and forms the base of the groundwater-flow system on Long Island.

The relations between the hydrogeologic and geologic 

units, and the hydraulic characteristics and description of 

the aquifers and confining units underlying the study area 

are shown in table 1. The upper and lower boundaries of the 

glacially derived hydrogeologic units are mainly delineated 

from lithologic differences between units rather than the age 

of the deposits. 

The geologic and hydrologic units underlying Long 

Island are described by Suter and others (1949); Perlmutter 

and Geraghty (1963); Swarzenski (1963); Kilburn (1979); 

Kilburn and Krulikas (1987); and Smolensky and others 

(1989). The drill-core data obtained in this study and previous 

studies in Great Neck and Manhasset Neck provide a basis 

for the naming of two hydrogeologic units—the North Shore 

confining unit and the North Shore aquifer (Stumm, 2001; 

Stumm and Lange, 1994, 1996; Stumm and others, 2002) 

for the first time. All other geologic and hydrogeologic unit 

names used in this report are those currently (1998) used by 

the USGS. 

Stratigraphy

The unconsolidated deposits beneath the study area 

are of Cretaceous and Pleistocene age and are underlain by 

crystalline (metamorphic) bedrock. Microscopic mineralogic 

analysis of drill-core samples and borehole geophysical logs 

were used to delineate the stratigraphic sequence.

Hydrogeology
6 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

NORTHERN

BOULEVARD

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124

202

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GOLF-COURSE WELL

PRODUCTION WELL

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0 1 2 KILOMETERS

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1149

3892

5071

4432

Numbers are assigned by New York State

(Prefix N for Nassau County is omitted;

Department of Environmental Conservation.

suffix T represents test boring.)

prefix S represents Suffolk County;

SUFFOLK CO.

NASSAU CO.

S31039T

FIGURE 2. Locations of production, observation, 

industrial, and golf-course-irrigation wells within 

the Oyster Bay study area, Nassau County, N.Y. 

(Location is shown in fig. 1.)

Hydrogeology
8 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

Bedrock

The bedrock underlying the study area has been mapped 

as the Hartland Formation of Middle Ordovician to Lower 

Cambrian Age (Baskerville, 1992). The bedrock surface 

generally slopes southeastward from about 350 to 800 ft below 

sea level (fig. 3), except in the northernmost parts of the study 

area where glacial scouring has created north-northwestward 

dipping valleys. Drill-core data indicate the bedrock is a 

biotite-garnet schist and gneiss. A 50-to 100-ft-thick zone of 

saprolite (highly weathered bedrock), is present, except in the 

northwestern part of the study area, where some or all of it 

was removed by glacial erosion. The bedrock surface forms 

a virtually impermeable boundary for Long Island’s groundwater system (McClymonds and Franke, 1972).

The saprolitic zone consists of mostly yellow, gray, 

and white clay with quartz fragments. This zone has been 

previously interpreted by drillers to be a clay or clayey gravel 

layer above bedrock. All observation wells drilled during 

this study penetrated weathered bedrock except N12855 and 

N12921 (fig. 2), where this layer has been removed through 

glacial erosion.

Glacial erosion or scouring of the overlying Cretaceous 

and possibly early Pleistocene deposits and weathered 

bedrock left at least four valleys that truncate the surrounding 

material. The four buried valleys are in (1) Hempstead 

Harbor, (2) the area along the northwestern part of the study 

area, (3) in Bayville, and (4) in Cold Spring Harbor (fig. 1). 

The weathered bedrock at the base of these valleys has been 

partly or totally removed. Drill-core data and driller’s logs 

from wells N835 and N12921, along the northern shore of 

the study area (fig. 2), indicate that the penetrated bedrock 

is devoid of a saprolitic zone. The time during which the 

weathered bedrock was removed is unknown, but it probably 

was during successive glacial advances during the Pleistocene. 

In general, bedrock beneath Cretaceous sediments has a 

saprolitic zone, and the absence of Cretaceous deposits 

typically is accompanied by a lack of saprolite. The marineseismic-reflection data collected in this study in 1996 

Table 1. Generalized description of hydrogeologic units underlying the Oyster Bay study area, Nassau County, N.Y. 

[Modified from Smolensky and others, 1989, sheet 1; McClymonds and Franke, 1972]

Hydrogeologic unit Geologic unit Description and hydraulic characteristics

Upper glacial aquifer Upper Pleistocene deposits

Till and outwash deposits of sand, silt, and clay and boulders. 

Varied permeability with an average horizontal hydraulic 

conductivity of 270 feet per day and an anisotropy of 10:1. 

Outwash has the highest hydraulic conductivity. 

North Shore confining 

unit Pleistocene deposits

Marine and postglacial lake deposits. Clay and silt deposits 

with minor lenses containing shells. The clay is olive brown 

and olive gray and poorly permeable. Unit contains a minor 

sand unit that is moderately permeable.

North Shore aquifer Pleistocene deposits Sand, silt, and gravel; brown and olive gray, poor to moderate 

sorting. Moderately permeable.

Magothy aquifer Matawan Group-Magothy Formation, 

undifferentiated

Fine sand with silt and interbedded clay. Gray and pale yellow 

quartz sand. Lignite and iron-oxide concretions common. 

Moderately permeable with an average horizontal hydraulic 

conductivity of 50 feet per day and an anisotropy of 100:1.

Raritan confining unit

(Raritan clay)

Unnamed clay member of the 

Raritan Formation

Clay; solid with multicolors such as gray, white, red, or tan. 

Very poorly permeable. Confines water in underlying unit. 

Average vertical hydraulic conductivity of 0.001 foot per day. 

Lloyd aquifer Lloyd Sand Member of the 

Raritan Formation

Fine to coarse sand and gravel with clay lenses. White and 

pale-yellow sand is well sorted. Moderately permeable with an 

average horizontal hydraulic conductivity of 40 feet per day, 

and an anisotropy of 10:1.

Bedrock Hartland Formation; crystalline 

bedrock

Highly weathered biotite-garnet-schist with low hydraulic 

conductivity. A thick saprolitic zone 50 to 100 feet thick, 

consisting of white, yellow, and gray clay, underlies most of 

the peninsula except in the northernmost part. Impermeable to 

poorly permeable.
 9

indicate that the bedrock surface beneath the embayments 

of Long Island Sound is undulating; this is consistent with 

observations of Tagg and Uchupi (1967), who found the 

bedrock surface beneath most of Long Island Sound to have 

been severely eroded during the Pleistocene. The buried 

valley in the northwestern part of the study area appears to 

extend southeastward through the central part, thus creating 

a northeastward dip in the bedrock surface due to extensive 

erosion as far south as well N12855 (fig. 2, 3). Bedrock is 

anomalously deeper (711 ft below sea level) at this well site.

Aquifers and Confining Units

Lloyd aquifer—The Lloyd Sand Member of the Raritan 

Formation of Late Cretaceous age (Suter and others, 1949; 

Cohen and others, 1968) consists of an upward fining 

sequence of white and pale-yellow (Munsell descriptive color 

value 2.5Y7/3) sand and gravel with white clay lenses. Some 

white and grayish silt is in the upper sections of the unit.

The Lloyd Sand overlies bedrock and generally is 

overlain by Raritan clay. The upper surface of the Lloyd Sand 

generally dips to the southeast and ranges from 150 ft to more 

than 400 ft below sea level (fig. 5). The Lloyd Sand, where 

present, ranges from 150 to 300 ft. in thickness. The Lloyd 

Sand has been removed from the northwestern, central, and 

northeastern parts of the study area by glacial erosion; this 

erosional pattern is also seen in the Great Neck peninsula 

(Stumm, 2001) and the Manhasset Neck peninsula (Stumm 

and others, 2002). The Lloyd Sand was not encountered within 

the buried valleys onshore or offshore (fig. 5), nor at wells 

N835, N12921, N11280 and N12855 (fig. 2, 4). A deep glacial 

valley, more than 6 mi long and more than 1 mi wide, was 

delineated on the basis of marine-seismic-reflection-survey 

data, geophysical logs, and drill cores from observations wells 

N12921, N11280 and N12855. No Cretaceous sediments were 

encountered within this valley.

The Lloyd Sand forms the Lloyd aquifer, which is a 

major source of water supply along the northern parts of the 

study area. In northern Nassau County, the Lloyd aquifer has 

an average horizontal hydraulic conductivity of about 60 ft/d 

(McClymonds and Franke, 1972) and a horizontal to vertical 

anisotropy of 10:1 (Smolensky and others, 1989).

Raritan clay—The unnamed clay member of the Raritan 

Formation, referred to as the Raritan clay in this report, 

overlies and confines the Lloyd aquifer (Suter and others, 

1949; Stumm, 2001; Stumm and others, 2002). It is a major 

confining unit throughout Long Island (Smolensky and others, 

1989) and is present throughout the study area except along 

the northwestern, central, and northeastern parts. It is a solid, 

compact clay that includes red (10R4/8), gray (2.5Y4/1), white 

(2.5Y9/1), tan (2.5Y4/2), and variegated colors.

The upper-surface altitude of the Raritan clay in the study 

area ranges from 75 ft to more than 350 ft below sea level; 

thickness ranges from less than 26 ft to more than 125 ft. The 

Raritan clay was removed by extensive glacial erosion in 

the northern, central, and northeastern parts of the study area 

(fig. 6). Evidence of similar glacial erosion is present in the 

Great Neck peninsula (Stumm, 2001) and the Manhasset Neck 

peninsula (Stumm and others, 2002). The Raritan clay was not 

penetrated at wells N835, N12921, N12855, and N11280 (figs. 

2, 4, 6). Marine-seismic-reflection profiles indicate that the 

Raritan clay underlies the southernmost parts of Hempstead 

Harbor and Cold Spring Harbor and in two isolated offshore 

ridges north of Oak Neck and Centre Island (fig. 1, 6). The 

Raritan clay is poorly permeable, with an average vertical 

hydraulic conductivity of about 0.001 ft/d (Smolensky and 

others, 1989).

Magothy aquifer—The Magothy aquifer consists of 

fine micaceous sand, silt, and interbedded clay sediments 

of Cretaceous age (Suter and others, 1949; Isbister, 1966; 

Kilburn and Krulikas, 1987). It has an upward fining 

sequence of gray, white, or pinkish sands and clays of the 

undifferentiated Matawan Group and Magothy Formation 

(Isbister, 1966; Kilburn and Krulikas, 1987; Smolensky and 

others, 1989; Stumm and others, 2002). Drill-core samples 

indicate that the Magothy aquifer consists of subrounded, 

silty, gray quartz sand (5Y6/1) and pale-yellow quartz sand 

(2.5Y7/2) with associated lignite and chemically stable, heavy, 

opaque minerals.

The Magothy aquifer has been removed by glacial 

erosion from the northern and central parts of the study area 

and is present mainly in the southern part, where it overlies 

the Raritan clay (figs. 4,7). Its upper surface ranges from more 

than 60 ft above sea level to more than 170 ft below sea level 

(fig. 7). Glacial erosion has produced a steep, undulating, 

northeast-trending scarp in the Magothy aquifer along the 

southern part of the study area (fig. 7). Similar features are 

present beneath the Great Neck (Stumm, 2001) and Manhasset 

Neck (Stumm and others, 2002) peninsulas. The Magothy 

aquifer has an average horizontal hydraulic conductivity of 50 

ft/d and an anisotropy of 100:1 (Smolensky and others, 1989).

North Shore aquifer—The North Shore aquifer consists 

of a sequence of Pleistocene-age sediments found only 

in the northwestern, central, and northeastern parts of the 

study area, and is described in detail by Stumm (2001) and 

Stumm and others (2002). These types of deposits are found 

in similar hydrogeologic settings elsewhere on the northern 

shore of Long Island; including to the west on the Great Neck 

peninsula (Stumm, 1994, 2001) Manhasset Neck peninsula 

(Stumm and Lange, 1994, 1996; Stumm and others, 2002), 

and in northeastern Queens County (Chu and Stumm, 1995). 

The North Shore aquifer deposits were called the Jameco 

Gravel by Swarzenski (1963) and Isbister (1966), and the 

Port Washington aquifer by Kilburn and Krulikas (1987). The 

name “North Shore aquifer” is used in this report in preference 

to Jameco Gravel or Jameco aquifer because these imply 

correlation with Jameco deposits in southern Queens County; 

such a correlation is questionable. The Port Washington 

aquifer, as described by Kilburn (1979) and Kilburn and 

Krulikas (1987), contains deposits of Cretaceous age (Lloyd 

aquifer and Raritan clay)(Stumm, 2001; Stumm and others, 

2002). The existence of the Port Washington aquifer or 

Hydrogeology
10 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

NORTHERN

BOULEVARD

-467

-597

-511

-490

-437

-431

-546

-458

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-424

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-401
 11

ROUTE

25A

-551

-688

-595

-642

-711

-451

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3o32' 73o28'

OBSERVATION WELL

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GOLF-COURSE WELL

PRODUCTION WELL

INDUSTRIAL WELL

0 1 2 KILOMETERS

0 1 2 MILES

-511

-297

BEDROCK CONTOUR – Shows altitude

of bedrock surface. Dashed where

approximately located. Contour interval is

50 feet. Datum is sea level.

����

WELL TYPES

Number, where present, indicates depth

above or below sea level, in feet.

FIGURE 3. Surface altitude of bedrock underlying 

Oyster Bay study area, Nassau County, N.Y. 

(Location is shown in fig. 1.)

Hydrogeology
12 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

Jameco Gravel above the Raritan clay in the study area is not 

supported by the drill-core samples, marine-seismic-reflection 

data, and hydrologic data (Stumm and Lange, 1996; Stumm 

and others, 2002).

The North Shore aquifer is a distinct hydrogeologic 

unit that either rests upon bedrock or the eroded surface 

of the Lloyd aquifer and is overlain by a thick sequence of 

clay and silt (the North Shore confining unit) (figs. 4, 8). 

It is a sequence of poorly to moderately sorted, dark-olivebrown (2.5Y3/3) and olive-gray (5Y6/2) gravel, sand, and 

silt. It contains subangular to subrounded quartz grains, rock 

fragments, unstable opaque minerals, and a large percentage 

of biotite and muscovite. The aquifer consists of moderately 

sorted stratified drift and outwash deposits that infilled the 

low-lying areas after the partial removal of the Cretaceous 

deposits and parts of the bedrock (saprolitic zone) by glacial 

erosion. It was then capped by clay and silt of the North Shore 

confining unit. Subsequent glacial advances here and in nearby 

areas carved northwest-southeastward trending valleys that 

reached the bedrock surface; they also removed previously 

deposited sediments that may have included parts of the North 

Shore aquifer. These valleys later became filled with hundreds 

of feet of clay and silt (figs. 4, 8, 11). The buried-valley 

deposits probably represent the most recent glacial advance 

in the area and may be younger than the North Shore aquifer 

material in other parts of the study area; therefore, they are 

grouped with undifferentiated Pleistocene-age deposits of the 

North Shore confining unit. The North Shore aquifer is about 

100 ft thick in the northern part of the study area and about 

230 ft thick in the central part (fig. 4A, 4B, 4D). The top of 

the North Shore aquifer ranges from 150 ft to almost 500 ft 

below sea level (fig. 8). The rapid response of water levels to 

tides and (or) pumping indicates that this aquifer is confined 

and moderately permeable. No hydraulic conductivity values 

for this unit were calculated; however, golf-course irrigation 

well N5152, which is screened in the North Shore aquifer, 

was calculated to have a specific capacity of 11.2 (gal/min)/ft. 

Similar hydraulic responses observed in the surrounding Lloyd 

aquifer indicate the North Shore aquifer is confined and in 

hydraulic connection with the Lloyd aquifer. The North Shore 

aquifer infills the lower part of the buried valley in the central 

part of the study area (fig. 8).

North Shore confining unit—The name “North Shore 

confining unit” is given to a sequence of Pleistocene-aged 

clay and silt deposits that are locally present along the 

northern shore of Queens County (Chu and Stumm, 1995), 

Nassau County (Stumm and Lange, 1994, 1996; Stumm, 

2001; Stumm and others, 2002), and Suffolk County 

(Soren, 1971). The North Shore confining unit underlies 

the northernmost and central parts of the study area and 

extends offshore. The term “North Shore confining unit” is 

extensively described in Stumm (2001) and is used here for 

the first time in reference to the northern parts of the Town of 

Oyster Bay. These deposits have been described by Grim and 

others (1970), Lewis and Stone (1991), and Williams (1981) 

as part of the stratified glacial-lake clay and silt that infill 

Long Island Sound and Manhasset Bay. Previous studies have 

used marine-seismic-reflection surveys, borehole geophysical 

logs, and drill-core samples to correlate these deposits 

with buried valleys that extend across the northernmost 

part of Nassau County (Stumm, 2001; Stumm and Lange 

1994, 1996; Stumm and others, 2002). The stratigraphy and 

geologic age of the North Shore confining unit were not 

investigated in this project.

Swarzenski (1963) and Isbister (1966) called these 

deposits the “Gardiners Clay”; Kilburn (1979) referred to them 

as the “Port Washington confining unit”. The name “North 

Shore confining unit” is used here in preference to the name 

“Gardiners Clay” because correlation with the Gardiners Clay 

elsewhere on Long Island is questionable. 

The Gardiners Clay is along the southern shore of Long 

Island and was probably deposited during an interglacial 

period. It is a greenish-brown or gray marine clay containing 

diatoms, foraminifera, and shell fragments (Suter and others, 

1949). The current widely accepted interpretation is that the 

term “Gardiners Clay” should be applied only to the shallow 

marine clay unit along the south shore of Long Island. 

The term “Port Washington confining unit” is similarly 

questionable. This unit, described by Kilburn (1979) and 

Kilburn and Krulikas (1987), was delineated from drillers’ 

logs and includes parts of the Raritan clay of Cretaceous 

age. Drill-core data obtained in this study are inconsistent 

with many of the previous interpretations and delineations; 

therefore, the term Port Washington confining unit is no 

longer considered valid in this area.

Tagg and Uchupi (1967) conducted seismic profiles in 

Long Island Sound and describe the bedrock surface as deeply 

cut by flat-bottomed, U-shaped troughs that extend to more 

than 600 ft below sea level. They interpreted these troughs or 

valleys to be glacially eroded but were unable to determine 

whether any of them extend beneath Long Island because 

nearshore seismic data were lacking. Reeds (1927) described 

varved clay deposits in the New York City area that imply 

clay deposition in postglacial freshwater lakes occupying lowlying basins in southern New York. He also proposed several 

interconnected lakes (Glacial Lake Flushing) within parts of 

Long Island Sound and northern Queens County (fig. 2) and 

describes these glacial lakes as containing extensive layers of 

varved clays and silt.

The North Shore confining unit consists of olive-brown 

(2.5Y4/2) varved clay and minor amounts of silt and dark 

olive-gray clay (5Y4/2). Its upper surface ranges from 13 ft 

to 140 ft below sea level (figs. 4, 9) and its thickness ranges 

from 0 to less than 130 ft, except where it fills the buried 

valleys beneath Hempstead Harbor, Oyster Bay, and Cold 

Spring Harbor. In contrast, these embayments are described 

by Swarzenski (1963), Isbister (1966), Kilburn (1979), and 

Kilburn and Krulikas (1987) as infilled with mostly sand and 

gravel. Marine-seismic-reflection and drill-core data obtained 

during this study indicate that most of these buried valleys are 

filled with more than 300 ft of clay and silt. In general, the 

North Shore confining unit is a relatively flat-lying deposit.
 13

FIGURE 4. Hydrogeologic sections in Oyster Bay study area, Nassau County, N.Y. A. Section A-A ́. B. Section B-B ́. C. Section C-C ́. D. Section D-D ́. (Locations are shown in fig. 2.)

-100-200-300-400-500-600

0

100.0

FEET

-700

0.5 MILE 1

0.5 KILOMETER 1

0

0

Sea

Level

Hempstead

Harbor

Mill Neck

Bay

Long Island

Sound

Bedrock Bedrock

Bedrock

Upper glacial aquifer

EXPLANATION

North Shore confining unit

UG

NScu

GEOLOGICAL CONTACT– dashed where approximately locatedWELL BORING AND TRACE OFGEOPHYSICAL (GAMMA RAY) LOG – well locations are shown in figure 2

Upper glacial

aquifer

Upper glacial

aquifer UG

Lloyd

aquifer

Lloyd

aquifer

Lloyd

aquifer

North Shore

confining unit

Vertical exaggeration x 21

Raritan

clay

Raritan

clay Raritan

clay

(NW) A' A

N12646

N12667

N12790

North Shore

confining unit North Shore confining unit

NorthShoreaquifer

N12790

����

NScu

UG

Hydrogeology
14 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

FIGURE 4. (continued) Hydrogeologic sections in Oyster Bay study area, Nassau County, N.Y. A. Section A-A ́. B. Section B-B ́. C. Section C-C ́. D. Section D-D ́. (Locations are shown in fig. 2.)

Mag

-100

200100

-200-300-400-500-600-700-800

0 Sea

Level

0.5 MILE 1

0.5 KILOMETER 1

0

0

N11280

Manhasset

Bay

Bedrock

Bedrock

Bedrock

EXPLANATION

Magothy aquifer MagGEOLOGICAL CONTACT– dashed where approximately locatedWELL BORING AND TRACE OFGEOPHYSICAL (GAMMA RAY) LOG – well locations are shown in figure 2

Upper glacial

aquifer

Magothy

aquifer

Magothy

aquifer

Magothy

aquifer

Upper glacial

aquifer

Upper glacial

aquifer

Lloyd

aquifer

Lloyd

aquifer

North

Shore

confining

unit

North

Shore

aquifer

Lloyd

aquifer

Vertical exaggeration x 21

Raritan

clay

Raritan

clay

Raritan

clay

(NNW) B'

B

N11798

N12506

N11394

N11280

FEET
 15

FIGURE 4. (continued) Hydrogeologic sections in Oyster Bay study area, Nassau County, N.Y. A. Section A-A ́. B. Section B-B ́. C. Section C-C ́. D. Section D-D ́. (Locations are shown in fig. 2.)

Hydrogeology

EXPLANATIONUpper glacial aquiferNorth Shore confining unit

UG

NScu

GEOLOGICAL CONTACT– dashed where approximately locatedWELL BORING AND TRACE OFGEOPHYSICAL (GAMMA RAY) LOG – well locations are shown in figure 2

0.5 MILE 1

0.5 KILOMETER 1

0

0

Vertical exaggeration x 22

N12697

N12697

Sea

Level

Oyster Bay

Harbor

Oyster Bay

Harbor

Centre

Island Cold Spring

Harbor

Long IslandSound

Bedrock

Bedrock

200100

0

-100-200-300-400-500-600-700

Upper glacial

aquifer Upper glacial

aquifer

Upper glacial

aquifer

North

Shore

aquifer

Lloyd

aquifer

North Shore

confining unit

North

Shore

confining

unit

North

Shore

confining

unit

North

Shore

confining

unit

Lloyd

aquifer

Lloyd

aquifer

Lloyd

aquifer

Magothy

aquifer

Raritan

clay

Raritan

clay

Raritan

clay

Raritan clay

(N) C' C

N12646

N12635

N12733 Cove

Neck

FEET

UG

NS

cu

UG

Upper glacial

aquifer

Upper

glacial

aquifer UG UG UG
16 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98 Sea Level

D

(ENE)

N12774

N12697

N11280

Raritan clay

Raritan clay

Raritan clay

Bedrock

Bedrock

North

Shore

Aquifer

NScu

Magothy

aquifer

Magothy

aquifer

Long Island

Sound

Bayville Mill Neck

Bedrock

Upper glacial aquifer

Upper glacial

aquifer

Upper glacial

aquifer

North Shore

confining unit

Lloyd aquifer

Lloyd aquifer

Lloyd aquifer

D'

NScu

Magothy

aquifer

RC

N11280

0

-100

200100

-200-300-400-500-600-700-800

EXPLANATIONNorth Shore confining unitRaritan Clay

NScu

RC

GEOLOGICAL CONTACT– dashed

 where approximately locatedWELL BORING AND TRACE OFGEOPHYSICAL (GAMMA RAY) LOG –

 well locations are shown in figure 2

0.5 1 MILE

0.5 1 KILOMETER

0

0

Vertical Exaggeration x 22

FEET

FIGURE 4. (continued) Hydrogeologic sections in Oyster Bay study area, Nassau County, N.Y. A. Section A-A ́. B. Section B-B ́. C. Section C-C ́. D. Section D-D ́. (Locations are shown in fig. 2.)
 17

Upper glacial aquifer—The upper glacial aquifer 

includes the saturated parts of the upper Pleistocene deposits. 

It consists of beds of stratified fine- to coarse-grained sand 

and gravel, boulders, clay, till, and some small, shallow pond 

deposits (Suter and others, 1949) and, in the northern part 

of the town of Oyster Bay, is underlain in some areas by 

the North Shore confining unit. In some areas, Cretaceous 

deposits have been thrust upward by ice shove and are 

incorporated into the upper glacial aquifer. An example of this 

can be observed in outcrops along the northwestern coast at 

the Nassau County Department of Parks’ Garvies Point (fig. 

1) Museum and Preserve; where several blocks of Raritan 

clay have been ice-shoved and crop out 75 ft above their 

original stratigraphic locations. The upper glacial aquifer 

is poorly to moderately sorted and contains mostly quartz 

minerals, rock fragments, and biotite and muscovite; it also 

contains brown sand (7.5Y4/6), dark yellowish-brown sand 

(10YR4/6), and varying amounts of reworked Cretaceous 

deposits. Marine-seismic-reflection surveys and drill-core data 

indicate that this aquifer does not extend very far offshore, 

nor does it infill the buried valleys beneath Hempstead Harbor 

and Cold Spring Harbor, as interpreted by Kilburn (1979) 

and Kilburn and Krulikas (1987). The upper glacial aquifer 

overlies the Magothy aquifer in the southernmost part of the 

study area, and overlies the Raritan clay or the North Shore 

confining unit in the rest of the study area (figs. 4, 10). The 

present land surface consists of recent Holocene deposits and 

the unsaturated upper part of the upper glacial aquifer. The 

upper glacial aquifer has an average horizontal hydraulic 

conductivity of 270 ft/d (Smolensky and others, 1989).

Interpretation of Geophysical Data

Marine-seismic-reflection surveys — The seismicreflection survey results were correlated with geologic logs of 

nearshore observation wells. Four seismic-reflection profiles 

(E-E ́, F-F ́, G-G ́ and H-H ́, figs. 11A-D) show interpretations 

of the major subsurface features observed during the marineseismic-reflection surveys.

Seismic-reflection profile E-E ́ (fig. 11A) is about 8,600 

ft long and was surveyed 1.5 mi off the northern coast of 

the study area (fig. 1). The sea floor in this section is about 

40 ft below sea level. The seismic-reflection survey shows 

subhorizontal parallel reflections, which are interpreted to be 

silt and clay deposits that fill two buried valleys that extend to 

more than 250 ft below sea level. These valleys are bounded 

by coarse-grained deposits and are apparently underlain by 

bedrock. Borehole geophysical logs and drill cores from wells 

N12921, N12855, and N11280 indicate that these buried 

valleys may merge to form the 6 mi-long valley in the central 

part of the study area.

Seismic-reflection profile F-F ́ (fig. 11B) is about 9,600 

ft long and was surveyed 1 mi north of the eastern shore 

of the Bayville area (fig. 1). The profile indicates a buried 

valley that extends to the bedrock surface, about 360 ft 

below sea level. The profile of the valley is dominated by 

subhorizontal parallel reflections, which are interpreted to be 

silt and clay. Borehole geophysical logs and drill cores from 

observation well N12790, near Bayville, indicate that this 

valley becomes increasingly shallower as it approaches the 

Bayville shore. The valley is infilled with more than 300 ft 

of silt and clay. A strong reflector along the eastern boundary 

of the buried valley correlates with the Raritan Clay and 

is interpreted as the offshore extension of the Cretaceous 

deposits on Centre Island.

Seismic-reflection profile G-G ́ (fig. 11C) is about 11,500 

ft long and is roughly perpendicular to the strike or trend 

of a buried valley beneath Cold Spring Harbor (fig. 1). The 

Cold Spring Harbor valley is about 1 mi wide and extends to 

bedrock, which is about 420 ft below sea level. The sea floor 

here ranges from 10 to 60 ft below sea level. The buried valley 

truncates the surrounding coarse-grained deposits (chaotic 

reflectors) and is infilled with more than 400 ft of silt and 

clay deposits, as interpreted from the subparallel horizontal 

reflectors. Draped bedding is indicated along either edge of 

the valley.

Seismic-reflection profile H-H ́ (fig. 11D) trends along 

the apex or strike of the buried valley within Cold Spring 

Harbor (fig. 1). The profile is about 13,500 ft long. The sea 

floor in this section ranges from 20 to 60 ft below sea level. 

The seismic-reflection profile shows subparallel horizontal 

reflectors that are interpreted as silt and clay infilling the 

buried valley. The valley appears to extend to more than 540 

ft below sea level at its deepest part and is infilled with more 

than 500 ft of silt and clay. The valley’s decreasing depth 

southward is consistent with ice-contact erosion documented 

in other areas of northern Nassau County (Stumm, 2001; 

Stumm and others, 2002). A strong horizontal reflector at 

about 300 ft below sea level in the southern part of the seismic 

section is interpreted as the Raritan Clay.

Borehole geophysical logs —The borehole geophysical 

logs (gamma, SP, SPR, R, and EM) for well N12774, near 

Bayville (location shown on fig. 2), are shown in figure 12. 

The gamma log indicates (1) a zone of clay and silty clay at 

150 ft below land surface (130 ft below sea level) and (2) 

a clay unit in the Lloyd aquifer at about 240 ft below land 

surface (220 ft below sea level). This well was drilled into 

bedrock, and the base of the well was set at 392 ft below land 

surface (372 ft below sea level) within the Lloyd aquifer. 

The SP log shows a steady deflection with decreasing values 

from the top of the clay zone at 150 ft below land surface 

(130 ft below sea level). The SPR log shows a zone of lowest 

resistance at about 135 ft below land surface (115 ft below 

sea level). This zone of high conductivity also appears on the 

R and EM logs. The R log shows this zone to be in the areas 

with the lowest resistivity values. The EM log indicates a 

sharp increase in conductivity above the clay unit at 150 ft 

below land surface (130 ft below sea level). The filter-press 

sample obtained from 147 ft below land surface (127 ft below 

sea level) had a chloride concentration of 19,800 mg/L. This 

is interpreted as a wedge of saltwater 15-ft thick at the base 

of the upper glacial aquifer. The conductivity of ground water 

Hydrogeology
18 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

NORTHERN

BOULEVARD

ROUTE

��

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NP

-271

NP

-198

-245

-311

-318

-316

-361

-297

-377

-260

-287

-377

-278

-140

NP

-176

NP

-360 -387

-231

-317

-401

-263

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73o40' 73o36' 73o32' 

40o54'

40o52'

40o50'

40o48'

Base from New York State Department of Transportation, 1981, 1:24,000

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 19

ROUTE

2 5 A

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-311

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73o28'

-443

SUFFOLK CO.

NASSAU CO.

����

����

����

����

��

0 1 2 KILOMETERS

0 1 2 MILES

OBSERVATION WELL

�����������

STRUCTURE CONTOUR – Shows altitude

of Lloyd aquifer surface. Dashed where

approximately located. Contour interval is

50 feet. Datum is sea level.

GOLF-COURSE WELL

PRODUCTION WELL

INDUSTRIAL WELL

��

377

360

EXTENT OF LLOYD AQUIFER

NP LLOYD AQUIFER NOT PRESENT

WELL TYPES

Number, where present, indicates depth above

or below sea level, in feet.

FIGURE 5. Extent and upper surface altitude of the Lloyd 

aquifer in Oyster Bay study area, Nassau County, N.Y. 

(Location is shown in fig. 1.)

Hydrogeology
20 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

NORTHERN

BOULEVARD

ROUTE 

-312

-301

-128

��

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�

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-116

NP

-138

-75

-280 -151

-157

-230

-206

-150

-127

-275

-168

-107

NP

-150 -236

-281

NP

-29

-270

-261

-113

-237

-188

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73o40' 73o36' 73o32' 

40o54'

40o52'

40o50'

40o48'

Base from New York State Department of Transportation, 1981, 1:24,000

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 21

ROUTE 25A

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73o28'

����

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0 1 2 KILOMETERS

0 1 2 MILES

OBSERVATION WELL

�����������

STRUCTURE CONTOUR – Shows altitude

of Raritan clay surface. Dashed where

approximately located. Contour interval is

50 feet. Datum is sea level.

GOLF-COURSE WELL

PRODUCTION WELL

INDUSTRIAL WELL

����

157

281

EXTENT OF RARITAN CLAY

NP RARITAN CLAY NOT PRESENT

WELL TYPES

Number, where present, indicates depth above

or below sea level, in feet.

FIGURE 6. Extent and upper surface altitude of the Raritan 

clay in Oyster Bay study area, Nassau County, N.Y. 

(Location is shown in fig. 1.)

Hydrogeology
22 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

NORTHERN

BOULEVARD

-87

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-177

+9

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-158

NP

NP

NP

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NP

NP

NP

NP

NP

+67

NP

-115

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73o40' 73o36' 73o32' 

40o54'

40o52'

40o50'

40o48'

Base from New York State Department of Transportation, 1981, 1:24,000

-150

0

����

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-100

-50

+50

0

-50

-100

-150
 23

ROUTE 25A

NP

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+30

NP

NP

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NASSAU CO.

SUFFOLK CO.

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' 73o28'

EXTENT OF MAGOTHY AQUIFER

NP MAGOTHY AQUIFER UNIT NOT PRESENT

����

����

���

�

0 1 2 KILOMETERS

0 1 2 MILES

OBSERVATION WELL

�����������

STRUCTURE CONTOUR – Shows altitude

of Magothy aquifer surface. Dashed where

approximately located. Contour interval is

50 feet. Datum is sea level.

GOLF-COURSE WELL

PRODUCTION WELL

INDUSTRIAL WELL

-100

-401

-424

WELL TYPES

Number, where present, indicates depth above

or below sea level, in feet.

FIGURE 7. Extent and upper surface altitude of Magothy 

aquifer in Oyster Bay study area, Nassau County, N.Y. 

(Location is shown in fig. 1.)

Hydrogeology
24 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

NORTHERN

BOULEVARD

R

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-166

-398

NP

NP

NP

NP

NP

NP

NP

-203

NP

NP

-229

NP

NP

-481

NP N

NP

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73o40' 73o36' 73o32' 

40o54'

40o52'

40o50'

40o48'

Base from New York State Department of Transportation, 1981, 1:24,000
 25

ROUTE

25A

NP

�

WELL TYPES

Number, where present, indicates depth above

or below sea level, in feet.

NP

����

����

NP

NP

NP

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73o28'

0 1 2 KILOMETERS

0 1 2 MILES

OBSERVATION WELL

�����������

STRUCTURE CONTOUR – Shows altitude

of North Shore aquifer surface. Dashed where

approximately located. Contour interval is

50 feet. Datum is sea level.

GOLF-COURSE WELL

PRODUCTION WELL

INDUSTRIAL WELL

����

176

140

SUFFOLK CO.

NASSAU CO.

EXTENT OF NORTH SHORE AQUIFER

NORTH SHORE AQUIFER NOT PRESENT

FIGURE 8. Extent and upper surface altitude of the North 

Shore aquifer in Oyster Bay study area, Nassau County, N.Y. 

(Location is shown in fig. 1.)

Hydrogeology
26 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

NORTHERN

BOULEVARD

��

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-56

-46

-13

-68

NP

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NP

NP

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NP

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NP

-140

-41

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NP 

-84

NP

-68

NP

-15

Base from New York State Department of Transportation, 1981, 1:24,000

-100-130

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73o40' 73o36' 73o32' 

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STRUCTURE CONTOUR – Shows altitude

of North Shore confining unit surface. Dashed

where approximately located. Contour interval is

50 feet. Datum is sea level.

GOLF-COURSE WELL

PRODUCTION WELL

INDUSTRIAL WELL

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56

71

EXTENT OF NORTH SHORE CONFINING UNIT

NP NORTH SHORE CONFINING UNIT NOT PRESENT

SUFFOLK CO.

NASSAU CO.

WELL TYPES

Number, where present, indicates depth above

or below sea level, in feet.

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FIGURE 9. Extent and upper surface altitude of the North 

Shore confining unit in Oyster Bay study area, Nassau 

County, N.Y. (Location is shown in fig. 1.)

Hydrogeology
28 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

NORTHERN

BOULEVARD

170

60

44

45

56

20

18

209

261

194

154

122

96

112

141

157

128

229

221

218

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190

143

145

131 140

95 34

54

123

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9

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Base from New York State Department of Transportation, 1981, 1:24,000
 29

ROUTE

25A

218

212

203

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228

249

198

172

90

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STRUCTURE CONTOUR – Shows altitude

of upper glacial aquifer surface. Dashed where

approximately located. Contour interval is

50 feet. Datum is sea level.

GOLF-COURSE WELL

PRODUCTION WELL

0 1 2 KILOMETERS

0 1 2 MILES

��

140

198

INDUSTRIAL WELL

WELL TYPES

Number, where present, indicates depth above

or below sea level, in feet.

FIGURE 10. Extent and upper surface altitude of the upper 

glacial aquifer (land surface) in Oyster Bay study area, 

Nassau County, N.Y. (Location is shown in fig. 1.)

Hydrogeology
30 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

FIGURE 11. Interpreted stratigraphy along seismic-reflection profiles: A. E-E ́ in Long Island Sound. B. F-F ́ in Long Island Sound. C. G-G ́ in Long Island Sound. D. H-H ́ in Cold Spring Harbor. (Trace of profile is shown in fig. 1.)

Vertical Exaggeration x 9.9

DEPTH BELOW SEA LEVEL, IN FEET

(Assumed velocity of sound 5,000 feet per second)

SEA FLOOR

SILT AND

CLAY

SILT AND

CLAY

BEDROCK

E

120180240300360420480540600

60

0

(S) E'

250 METERS

0

0

1000 FEET 500 250

COARSE

GRAINED

DEPOSITS

COARSE

GRAINED

DEPOSITS

? ?

?

BEDROCK

BEDROCK
 31

Vertical Exaggeration x 10.8

DEPTH BELOW SEA LEVEL, IN FEET

(Assumed velocity of sound 5,000 feet per second)

SEA FLOOR

SILT AND

CLAY RARITAN CLAY

COARSE

GRAINED

DEPOSITS

BEDROCK

F

120180240300360420480540600

60

0

(NW) F'

250 METERS

0

0

1000 FEET 500 250

BEDROCK

BEDROCK

? ?

FIGURE 11. (continued) Interpreted stratigraphy along seismic-reflection profiles: A. E-E ́ in Long Island Sound. B. F-F ́ in Long Island Sound. C. G-G ́ in Long Island Sound. D. H-H ́ in Cold Spring Harbor. (Trace of profile is shown in fig. 1.)

Hydrogeology
32 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

FIGURE 11. (continued) Interpreted stratigraphy along seismic-reflection profiles: A. E-E ́ in Long Island Sound. B. F-F ́ in Long Island Sound. C. G-G ́ in Long Island Sound. D. H-H ́ in Cold Spring Harbor. (Trace of profile is shown in fig. 1.)

Vertical Exaggeration x 11.6

DEPTH BELOW SEA LEVEL, IN FEET

(Assumed velocity of sound 5,000 feet per second)

SEA FLOOR

SILT AND

CLAY

GAS

DEPOSITS

BEDROCK

G

120180240300360420480540600

60

0

(NW) G'

250 METERS

0

0

1000 FEET 500 250

BEDROCK

BEDROCK

COARSE

GRAINED

DEPOSITS

COARSE GRAINED

DEPOSITS

?

?

?

RARITAN CLAY
 33

FIGURE 11. (continued) Interpreted stratigraphy along seismic-reflection profiles: A. E-E ́ in Long Island Sound. B. F-F ́ in Long Island Sound. C. G-G ́ in Long Island Sound. D. H-H ́ in Cold Spring Harbor. (Trace of profile is shown in fig. 1.)

Vertical Exaggeration x 9.5

DEPTH BELOW SEA LEVEL, IN FEET

(Assumed velocity of sound 5,000 feet per second)

SEA FLOOR

SILT AND

CLAY

RARITAN

CLAY

GAS

DEPOSITS

BEDROCK

H

120180240300360420480540600

60

0

(E)

H'

250 METERS

0

0

1000 FEET 500 250

?

COARSE

GRAINED

DEPOSITS

Hydrogeology
34 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

G 0 200 SP -140 -40 SPR 575 1025 SNR 100 1900 EM 0 600

50

100 upper glacial

aquiferNorth Shoreconfining unitRaritan clayLloyd aquiferBedrock(weathered)

150200250300350400

4,250

5

5

EXPLANATION

DEPTH BELOW LAND SURFACE, IN FEET 

Gamma log (counts per second)

G Well screen-zone sample location, number

 is chloride concentration (milligrams per liter) Spontaneous Potential log (millivolts) SP

Single-Point-Resistance log (ohms) SPRShort-Normal-Resistivity log (ohm-meters) SNRElectromagnetic induction log (millisiemens per meter) EM

51

732

19,800

Filter-press sample location, number is chloride concentration (milligrams per liter)

51

0

FIGURE 12. Geophysical logs of well N12774, Bayville, N.Y. (Location is shown in fig. 2.)
 35

tends to increase with depth in saltwater wedges and typically 

rests upon impermeable clay horizons.

Precipitation

All freshwater in the Oyster Bay study area originates 

as precipitation. Some of the precipitation that falls infiltrates 

the land surface, and some runs off to storm sewers and 

to tidewater. Of the water that infiltrates the soil, some 

is returned to the atmosphere through evaporation and 

transpiration; the rest percolates to the water table and 

becomes shallow ground water. Some of this shallow ground 

water moves laterally and discharges to stream channels as 

base flow; the remainder moves downward into the deeper 

hydrogeologic units and eventually discharges upward to the 

surrounding saltwater bodies.

The mean annual precipitation recorded at Mineola, in 

central Nassau County (fig. 1) by NCDPW for 1937-97 is 

44.42 in; the mean annual precipitation for the Oyster Bay 

study area calculated by Miller and Frederick (1969) is about 

43.5 in. The precipitation regime of Long Island during 1951-

65 was studied by Miller and Frederick who found the average 

warm-season precipitation to be almost equal to cold-season 

precipitation, and that precipitation is greatest in the spring 

and least in the fall. The relation between precipitation and 

recharge on Long Island is further described in Busciolano and 

others (1998).

Ground Water

Ground water in the Oyster Bay study area is used 

primarily for residences, small industry, businesses, and golfcourse irrigation. Water levels in all major aquifers within the 

Oyster Bay study area were measured at observation wells on 

a quarterly basis during 1995-98.

Pumpage

Production wells in the Oyster Bay study area are 

owned and operated by eight companies—the Incorporated 

village of Bayville Water Department, the city of Glen Cove 

Water Department, the Jericho Water District, the Locust 

Valley Water District, the Old Westbury Water District, the 

Oyster Bay Water District, the Roslyn Water District, and 

the Sea Cliff Water Company. All of these companies pump 

water from the upper glacial, Magothy, and Lloyd aquifers. 

Pumpage from the Magothy aquifer is much larger than that 

from the Lloyd aquifer. Mean annual pumpage for the upper 

glacial, Magothy, and Lloyd aquifers during 1993-97 is listed 

in table 2A; mean annual pumpage values for total public 

supply, industry, and golf-course irrigation are summarized in 

table 2B. 

Twelve industrial wells in the Oyster Bay study 

area report pumpage to New York State Department of 

Environmental Conservation, which categorizes them as 

Table 2. Total annual pumpage in Oyster Bay study area, Nassau County, N.Y., by aquifer, 1993-97.

[Values are in million gallons; Mgal/d, million gallons per day. Data from the Incorporated Village of Bayville Water 

Department, the City of Glen Cove Water Department, the Jericho, Locust Valley, Old Westbury, Oyster Bay, and 

Roslyn Water Districts; the Sea Cliff Water Company, and the New York 

State Department of Environmental Conservation]

Year Total pumpage, in millions of gallons

Upper glacial

aquifer

Magothy

aquifer

Lloyd

aquifer

A. Total pumpage, by year

1993 900.4 8,481.3 567.0

1994 860.8 8,580.2 678.4

1995 874.7 8,856.2 812.7

1996 558.7 7,889.0 976.4

1997 825.8 8,669.6 623.7

Mean 804.1 8495.3 731.6

(2.2Mgal/d) (23.3Mgal/d) (2.0Mgal/d)

B. Total pumpage, by well category

Public Supply 434.0 8,312.9 664.1

 (1.2Mgal/d) (22.8Mgal/d) (1.8Mgal/d)

Industrial 261.6 40.6 33.7

 (0.7Mgal/d) (0.1Mgal/d) (0.1Mgal/d)

Golf-course irrigation (values are pumping-season 108.4 141.8 33.9

means for the 34 wells; March-October) (0.4Mgal/d) (0.6Mgal/d) (0.1Mgal/d)

Hydrogeology
36 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

industrial use or unspecified withdrawal. Six of these wells 

(N2316, N5672, N5994, N8937, N11090, and N11906) (fig. 

2) are screened within the upper glacial aquifer, four (N3752, 

N5708, N8432, N8601) are screened within the Magothy 

aquifer, and two (N660, N7614) (fig. 2) are screened within 

the Lloyd aquifer. 

The study area contains 34 golf-course irrigation wells, 

which supply 23 golf courses. Of these, 16 are screened within 

the upper glacial aquifer, 14 in the Magothy aquifer, and 4 in 

the Lloyd aquifer. The irrigation season extends from March 

through October (table 2B).

Water Levels

Pumping lowers water levels in aquifers and thereby 

induces a flow gradient toward the pumping wells. A major 

concern for water suppliers is that contaminated water from 

adjacent or overlying aquifers may be induced to flow toward 

the pumping wells.

Wells near the coast of Long Island generally are affected 

by tides; thus, the USGS Coram office protocol for water-level 

measurements is that these wells be measured during the 2-

hour (h) period of high-tide in the nearest embayment. Water 

levels in observation wells were measured quarterly during 

1995-98; those in July 1997 are shown depicted in figures 

13 (water-table aquifers) and 14 (confined aquifers). The 

following sections describe water levels in the four aquifers of 

the Oyster Bay study area during 1995-98.

Upper glacial and Magothy aquifers (fig. 13)—The water 

table is within the upper glacial aquifer in most of the study 

area, except in the southernmost part where it is within the 

Magothy aquifer. Water levels measured during July 1997 

in wells screened in the water table (fig. 13) ranged from 3 

ft to 79 ft above sea level. The highest water levels were at 

well N6670 in the north-central part of the study area and at 

well N10605 in the south-central part. Typically, the watertable tends to parallel the land-surface topography. The 

Magothy aquifer becomes increasingly confined with depth. 

Water-level data obtained during quarterly measurements 

in the Magothy aquifer, where it is present, generally are 

representative of the water table because the Magothy in this 

area is hydraulically connected with the upper glacial aquifer; 

thus, heads in the two aquifers differ by only a few feet 

(Busciolano and others, 1998).

Water-level data from selected nearshore wells indicate 

that the upper glacial aquifer near the shore of the study 

area is affected by tidal fluctuations only in areas where it is 

confined by localized silt and clay layers. Two tidal studies 

and analyses of digital water-level records of the upper glacial, 

North Shore, and Lloyd aquifers on the north shore of Nassau 

County indicate that the upper glacial aquifer is only slightly 

affected by tidal fluctuations (Stumm, 2001; Stumm and 

others, 2002). High concentrations of saltwater in the upper 

glacial aquifer in parts of Centre Island and Bayville (wells 

N12755 and N12791) caused water levels to be nonequivalent 

to freshwater heads; thus, these values were not used for 

water-level contouring.

Lloyd and North Shore aquifers (fig. 14)—Analysis of 

water-level data from quarterly measurements and continuous 

water-level recorders indicate the Lloyd and North Shore 

aquifers are affected by tidal fluctuations along the coast and 

are hydraulically connected to each other (Stumm, 2001; 

Stumm and others, 2002). Both aquifers respond to local 

production pumping and show a large cone of depression 

near Bayville (fig. 14). Predevelopment (1900) water levels 

within the Lloyd aquifer are inferred to have been above 

sea level throughout the northern part of Nassau County 

(Kimmel, 1973) and were found to be above sea level in 

1947 (Lusczynski, 1952), 1971 (Kimmel, 1973), 1975 (Rich 

and others, 1975), 1979 (Donaldson and Koszalka, 1983), 

1980 (Kilburn and Krulikas, 1987) and in March-April 1997 

(Busciolano and others, 1998). Water-level data from quarterly 

synoptic measurements made during 1994-98 indicate that 

water in the Lloyd and North Shore aquifers flows northward 

from a local ground-water high in the south-central part of the 

study area (fig. 14). A large cone of depression in the Bayville 

coastal area is attributed to local production pumping of the 

Lloyd aquifer. Two wells along the coast of the study area 

(N12667 and N12774) show tidal fluctuations of as much 

as 3 ft (figs. 2, 15). Lloyd and North Shore aquifer wells in 

the central part of the study area, such as wells N11798 and 

N11280, do not respond to tidal fluctuations (fig. 15). Water 

levels in the Lloyd and North Shore aquifers increase in the 

fall and winter in response to a seasonal decrease in pumping.

The potentiometric surface of the Lloyd and North Shore 

aquifers in July 1997 (fig. 14) indicates that water levels 

ranged from 58 ft above mean sea level at the local groundwater high in the south-central part of the study area to about 

2 ft below mean sea level in the Bayville coastal area. The 

large cone of depression extended offshore into Long Island 

Sound. The potentiometric surface at several wells in western 

Bayville was below sea level in July 1997. Summer pumping 

causes ground-water flow in the Lloyd and North Shore 

aquifers to reverse direction from seaward to landward; that is, 

from the surrounding saltwater embayments inland toward the 

pumping wells (areas of lowest hydraulic head). This reversal 

is the primary reason for the past and present (1998) saltwater 

intrusion in the study area.

The hydrograph of observation well N12697 (fig. 15), 

screened in the Lloyd aquifer near the intersection of C-C ́ 

and D-D ́ (fig. 2), shows the smallest (1.0 ft) tidal fluctuation 

of any recorder well near the coast because it is the farthest 

from the coast. The two hydrographs with the least amount 

of fluctuations are from wells in the central part of the study 

area, well N11798 screened in the Lloyd aquifer, and well 

N11280 screened in the North Shore aquifer. The aquifers in 

this part of the study area are not affected by tidal fluctuations 

but are affected by local pumping and natural factors such 

as barometric and climatic changes. The hydrograph of 

observation well N110 near Hempstead Harbor (fig. 15), 
 37

screened in the Lloyd aquifer, shows sharp declines in water 

levels in response to localized production pumping. The 

water level at this well was 14 ft above sea level during the 

quarterly synoptic measurement of July 1997, but this was 

lower than normal as a result of summer pumping; it also was 

below sea level during October 1997 (fig. 15) (Spinello and 

others, 1999). In general, all hydrographs from the Lloyd and 

North Shore aquifers for July-August 1997 indicate similar 

trends—the lowest hydraulic heads in mid-July, an increase or 

peak in late July, and a slow increase during August (fig. 15). 

The similarity between the hydrograph for well N11280, (fig. 

15), which is screened in buried valley deposits of the North 

Shore aquifer, and those of the other observation wells, also 

screened in the Lloyd aquifer (N12697, N11798, N12667 and 

N12774) (fig. 15), suggests hydraulic connection. A similar 

hydraulic connection between the North Shore aquifer and the 

surrounding Lloyd aquifer in the Great Neck and Manhasset 

Neck peninsulas has been documented by Stumm (2001), 

and Stumm and others (2002). The narrower range in water 

levels at observation well N11280 (North Shore aquifer) than 

at other Lloyd observation wells probably indicates that the 

North Shore aquifer within that area is less permeable, or that 

the hydraulic connection is less efficient than in other buried 

valleys in northern Nassau County. The installation of new 

observation wells within this buried valley was completed 

too late in the study to identify the cause of these measured 

fluctuations.

Water Quality 

Data on ground-water quality in the Oyster Bay study 

area were obtained from analyses made by the NCDH and the 

NCDPW. A brief summary of the current (1998) water-quality 

and review of the historical data were made to obtain chloride 

concentrations and specific-conductance values for delineation 

of the freshwater-saltwater interface. These data were 

compared with the New York State Department of Health’s 

MCL for Drinking Water (Nassau County Department of 

Health, 1997), but a detailed analysis of water quality and 

review of historical data were beyond the scope of this study.

Chloride—The chloride concentrations at production 

wells screened in the upper glacial and Magothy aquifers 

in the Oyster Bay study area generally range from 5 to 75 

mg/L. One Lloyd production well (N10144, near Bayville) 

had a peak chloride concentration of 101 mg/L in 1994 

and a concentration of 5 mg/L in 1995. As of 1998, no 

production wells screened in the Lloyd aquifer had chloride 

concentrations that exceeded the State’s MCL (250 mg/L).

Water from seven observation wells screened in the 

upper glacial aquifer (N12791, N12856, N12755, N12946, 

N12775, N12945, N12754, fig. 2) exceeded the State’s MCL 

for chloride; the values ranged from 1,025 mg/L to 13,750 

mg/L. Drever (1988) states that seawater has an average 

chloride concentration of 19,350 mg/L. Water from one Lloyd 

observation well (N12790, near Bayville) had a chloride 

concentration of 700 mg/L in 1997, and 1,800 mg/L in 1998, 

and water from one observation well screened in the Magothy 

aquifer (N9059, in the southern part of the study area) had a 

chloride concentration of 170 mg/L in 1994.

Volatile organic compounds—The three most commonly 

detected volatile organic compounds (VOC’s) in raw-water 

samples from Nassau County production wells in 1996 were 

tetrachloroethylene (PCE), trichloroethylene (TCE), and 

1,1,1-trichloroethane (TCA) (Nassau County Department of 

Health, 1997). No high concentrations of VOC’s have been 

detected (as of 1998) at production wells or observation wells 

screened in the Lloyd aquifer; the Lloyd generally is protected 

from the downward movement of contaminants by the Raritan 

clay. Water from one production Magothy well (N9334, near 

Garvies Point, fig. 2) is treated with an airstripper to remove 

VOC’s from the raw water. Two production upper glacial 

wells (N8327 and N3466, both near Garvies Point, fig. 2) have 

been shut down (as of 1989) because the VOC concentrations 

exceeded the State’s MCL (0.005 mg/L). 

Water from six observation wells—four screened in the 

upper glacial aquifer (N12856, N12791, N9100, N12775) and 

two in the Magothy aquifer (N10609, N11280)—have VOC 

concentrations above the State’s MCL’s.

Inorganic constituents, nitrate, and metals—As of 1998, 

no inorganic constituents have been detected in production 

wells screened in the Lloyd aquifer at concentrations above 

the State’s MCL’s. Nitrate contamination (MCL-10 mg/L) 

has led to restricted use of one upper glacial production well 

(N7643, near Bayville). Iron concentrations exceeded the 

State’s MCL (0.3 mg/L) at one unused production Magothy 

well (N5261, near Garvies Point). 

Water from three observation wells screened in the upper 

glacial aquifer (N12775, N10199, N12754) exceeded the 

State’s MCL for iron and manganese combined (0.5 mg/L) 

and water from 10 others (N12945, N12711, N1152, N9670, 

N9478, N12256, N9189, N12856, N9100, N9353) exceeded 

the State’s MCL for iron. 

Water from 11 Magothy observation wells (N11824, 

N9059, N1176, N11823, N11281, N 9115, N 10606, N10609, 

N 9117, N11732, N 7478) had iron concentrations above the 

State’s MCL for drinking water and water from three Magothy 

observation wells contained several other constituents 

in concentrations that exceed the State’s MCL. Nitrate 

concentrations at well N10246 (near Hempstead Harbor) 

exceeded the State’s MCL. The State’s MCL for zinc and iron 

concentrations (5 and 0.3 mg/L, respectively) were exceeded 

at well N1243 (near the Suffolk County border) and the State’s 

MCL for lead, iron and manganese concentrations (0.015 and 

0.5 mg/L, respectively) were exceeded at well N2528 (near the 

center of the study area, fig. 2). 

Iron concentrations in water from 13 observation wells 

screened within the Lloyd aquifer exceeded the State’s MCL 

(wells N12075, N11574, N11798, N11279, N 12697, N12733, 

N12853, N12855, N12880, N12929, N12747, N12790, 

N12870).

Extent of Saltwater Intrusion
38 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

NORTHERN

BOULEVARD

ROUT

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9117 11304 8430

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9190

10732

11732

9100

10246

11964

124

202

486

511

835

7152

7190

10145

11280 

11281

11279

11798

11574

12618

12635 12646

12667

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12747

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9478

9127

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10199

1149

5250

9670

1152

10245

1

10606

9087

6294

6668

6879

1190

1194

12754

12755

8716 9189

9334

9066

6289

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8995

5188

9259

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7486

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8183

8962

9480

4432

1771

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8713

7570

7665

9607

5994

9708

7546

842

7620

7643

8776

3892

5261

3466 8326

8327

5762

9210

9211

110 109

3474

3475

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4246

5201 

6651

7446

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8355

11076

11295

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116 117

118 119

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1651

5152

107

7549

8658

11909

585

3486

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2400

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1327

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8681

2616

7834

8642 4633

7830

7115

8542

5071

4639

114

4462

9800

638

8885

4760

4891

9023

7858

12711

1870

1871-7

12579

12921

12775

12774

12785 12870

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12790

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12855

12871

12929 

12853

1212

7781

12939

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1207

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901-9

1037

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73o40' 73o36' 73o32' 

40o54'

40o52'

40o50'

40o48'

Base from New York State Department of Transportation, 1981, 1:24,000
 39

ROUTE

25A

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NASSAU CO.

SUFFOLK CO.

23

4

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49

7

�

11454

10609

1243

9317

1227

11823

11394 

430

10607

11822

9154

486

1486

11453

12075

12733

12747

9316

9152

10605

10608

1228

1477

1225

12754

12755

12743

9076

8995

5188

9259

175

7486

8183

2560

6675

9708

198

199 570

4133

245

4246

6092

6093

6651

7593

7772

7773

8043

8355

10149

6

95

4400

9520

8021

7419

4760

4891

9023

7034

9806

2855

2871

9 12768

11645

1212

12939

7525

7644

7

S21 119T

11090 

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2' 73o28'

OBSERVATION WELL

�����������

WATER-TABLE CONTOUR – Shows

altitude of water table. Contour interval

10 feet. Datum is sea level.

GOLF-COURSE WELL

PRODUCTION WELL

INDUSTRIAL WELL

0 1 2 KILOMETERS

0 1 2 MILES

��

62

WELL TYPES

Number, where present, indicates depth

above or below sea level, in feet.

FIGURE 13. Water-table altitude in the upper glacial 

and Magothy aquifers in Oyster Bay study area, 

Nassau County, N.Y., July 1997.

Extent of Saltwater Intrusion
40 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

NORTHERN

BOULEVARD

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58

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40

11

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73o40' 73o36' 73o32' 

40o54'

40o52'

40o50'

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Base from New York State Department of Transportation, 1981, 1:24,000

BAYVILLE
 41

ROUTE

25A

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INDUSTRIAL WELL

73o28'

OBSERVATION WELL

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GOLF-COURSE WELL

PRODUCTION WELL

0 1 2 KILOMETERS

0 1 2 MILES

25

POTENTIOMETRIC SURFACE CONTOUR –

Shows altitude of potentiometric surface.

Dashed where approximately located. Hachures

indicate depression. Contour interval 10 feet.

Datum is sea level.

SUFFOLK CO.

NASSAU CO.

EXTENT OF THE LLOYD AND

NORTH SHORE AQUIFERS

20

WELL TYPES

Number, where present, indicates depth

above or below sea level, in feet.

FIGURE 14. Potentiometric-surface altitude in the Lloyd 

and North Shore aquifers in Oyster Bay study area, 

Nassau County, N.Y., July 1997.

Extent of Saltwater Intrusion
42 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

FIGURE 15. Graphs of water level data from Lloyd and North Shore aquifer wells in Oyster Bay study area, Nassau County, N.Y.: A. Well 

N12697, July 13-August 28, 1997., B. Well N11798, July 13-August 29, 1997., C. Well N11280, July 13-August 28, 1997., D. Well N110, October 

1988-September 1998., E. Well N12667, July 13-August 28, 1997., F. Well N12774, July 13-August 28, 1997. (Locations are shown in fig. 2)

1997

15 20 25 31 5 10 15 20 25

July August

-9

6

-8

-6

-4

-2

0

2

4

F. N 12774 (Lloyd)

15 20 25 31 5 10 15 20 25

July August 1997

3

14

4

6

8

10

12

WATER LEVEL, IN FEET ABOVE MEAN SEA LEVEL

E. N 12667 (Lloyd)

D. N110 (Lloyd)

WATER YEARS

1989

25

20

15

10

5

-5

0

1990 1991 1992 1993 1994 1995 1996 1997 1998

WATER LEVEL, IN FEET ABOVE MEAN SEA LEVEL -10

15 20 25 31 5 10 15 20 25

July August 1997

58.4

57.0

57.2

57.4

57.6

57.8

58.0

58.2

C. N 11280 (N. Shore)

15 20 25 31 5 10 15 20 25

July August 1997

26.0

27.4

26.2

26.4

26.6

26.8

27.0

27.2

B. N 11798 (Lloyd)

15 20 25 31 5 10 15 20 25

July August 1997

14

22

15

16

17

18

19

20

21

A. N 12697 (Lloyd)
 43

EXTENT OF SALTWATER INTRUSION

Saltwater intrusion is the most common type of waterquality degradation in coastal-plain aquifers (Fetter, 1994). 

In coastal areas, the hydraulic head under predevelopment 

(nonpumping) conditions is higher on land than in the 

surrounding saltwater embayments; thus, fresh ground 

water flows seaward (from areas of high potential to areas 

of lower potential) and meets saltwater at an equilibrium 

point (interface) offshore. If the natural hydraulic gradient 

is reversed by pumping, however, fresh ground water 

flows toward the pumping well instead of seaward toward 

the interface, the interface moves landward, and saltwater 

intrusion occurs. 

Characteristics of Saltwater Intrusion

Seawater has an average chloride concentration of 19,350 

mg/L (Drever, 1988). A grab sample taken in 1996 from the 

nearshore area of Long Island Sound, near Great Neck, had 

a chloride concentration of 13,995 mg/L; this indicated some 

freshwater inflow into Long Island Sound (Stumm, 2001). 

The predevelopment concentration of chloride in fresh ground 

water on Long Island was 10 mg/L or less (Lusczynski and 

Swarzenski, 1966), and shallow (upper glacial) ground water 

in urbanized areas of Long Island generally has an ambient 

chloride concentration of less than 40 mg/L (Buxton and 

others, 1981; Heisig and Prince, 1993). The increase since 

the predevelopment period is associated with contamination 

from land-surface sources. In this report, “ambient” water 

is defined as ground water with a chloride concentration of 

less than 40 mg/L; “brackish” water as ground water with a 

chloride concentration of 40 to 250 mg/L; and “saltwater” as 

ground water with a chloride concentration greater than 250 

mg/L (Lusczynski and Swarzenski, 1966; Chu and Stumm, 

1995; Stumm, 2001; Stumm and others, 2002). The following 

section describes the extent of saltwater intrusion into the 

Oyster Bay study area.

Historic chloride concentrations at production wells and 

in samples from inland observation wells within the study 

area indicate that the background chloride concentration in 

the Lloyd aquifer is 5 to 10 mg/L. Any increase in chloride 

concentrations above predevelopment levels in the Lloyd and 

North Shore aquifers within the study area would be indicative 

of saltwater intrusion from the surrounding saltwater 

embayments in response to pumping. Ground water has been 

affected by saltwater intrusion in at least one production well 

(N10144, near Bayville. 

The 50 years of chloride-concentration and pumpage 

data from production wells on Great Neck (to the west of 

the study area) indicate that once a concentration of 50 

mg/L is exceeded at a production well screened in the Lloyd 

aquifer, the concentrations will remain above 50 mg/L even 

if pumping is decreased (Stumm, 2001). The technique of 

pumping only in alternate years at wells with elevated chloride 

concentrations only slightly delays an inevitable rapid increase 

in chloride concentrations. This is because once the toe, or 

leading edge, of the saltwater wedge reaches the production 

well, it is “upconed” into the screen zone of the well and 

responds to decreased pumping for only a short time before 

stabilizing into a rapid, continuous increase.

Filter-press samples were obtained from 16 boreholes 

installed for this study and were analyzed for chloride 

concentrations. If multiple boreholes were in the same 

vicinity, only one was sampled. The values were then 

correlated with those in water samples from the well-screen 

zone and with data from geologic and geophysical logs to 

delineate the extent of the saltwater intrusion. Water samples 

from production wells were also analyzed for chloride 

concentrations.

Saltwater Wedges A,B,C, and D

Four wedge-shaped zones of saltwater intrusion were 

identified within the Oyster Bay study area (figs. 16, 17)—

three (A,B,C; fig. 16B) in the upper glacial aquifer, and one 

(D; fig. 17B) in the Lloyd aquifer. Typically, the saltwater 

wedges form at the base of an aquifer above the surface of an 

impermeable or poorly permeable layer because the density 

of saltwater is greater than that of freshwater. The underlying 

impermeable or poorly permeable layer can be bedrock, as 

with the Lloyd or North Shore aquifer, or a confining unit, as 

with the upper glacial aquifer. Saltwater wedges decrease in 

thickness landward and have relatively thin (about 10-ft thick) 

saltwater-freshwater interfaces in northern Nassau County, 

N.Y. (Stumm, 1993).

Saltwater wedge A (fig. 16) was at the base of the upper 

glacial aquifer in the easternmost part of Bayville and most 

of Centre Island. Water from well N12791, screened at 52 

to 72 ft below sea level, had a chloride concentration of 

13,750 mg/L in 1997. Geophysical logs obtained at the site 

in 1997 indicate that saltwater had intruded the entire upper 

glacial aquifer in the vicinity of this well (fig. 18A). Similar 

conditions were detected at observation well N12946 near 

Bayville, screened 80 to 100 ft below sea level, where the 

chloride concentration was 8,500 mg/L. Saltwater wedge 

A extended into Centre Island. In addition, Isbister (1966) 

described high chloride concentrations at two wells in the 

upper glacial aquifer in Centre Island, N175 (85 to 155 ft 

below land surface, BLS), and N6578 (61 ft BLS)(fig. 2). 

Water from a new observation well near Bayville, N12755, 

which is screened from 89 to 109 ft below sea level in the 

upper glacial aquifer, had a chloride concentration of 10,250 

mg/L in 1997. Geophysical logs of well N12747, from this site 

indicate that all of the upper glacial aquifer had been intruded 

by saltwater (fig. 18B). Although parts of eastern Bayville are 

low lying, much of Centre Island is not. It is unknown whether 

overpumping of private wells in the past or insufficient 

freshwater heads were the cause of saltwater in wedge A. 

No production wells were pumping from the upper glacial 

Extent of Saltwater Intrusion
44 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

NORTHERN

BOULEVARD

R

9117(M)

7450(M)

9115(M)

9100(M)

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2528(M)

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7152(UG)

9478(UG)

12256(UG)

10199(UG)

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73°40' 73°36' 73°32' 

40°54'

40°52'

40°50'

40°48'

Base from New York State Department of Transportation, 1981, 1:24,000

00

0

0

0

0

0
 45

12743(UG)

��

ROUTE 25A

1243(M)

11823(M)

��

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G)

12755 (UG)

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871(UG)

G)

Area shown in Figure 16B

������������������

������

2' 73°28'

0 1 2 KILOMETERS

0 1 2 MILES

OBSERVATION WELL – Top number is well

number; letter in parentheses is aquifer designation,

(M=Magothy, UG=upper glacial), bottom number

(����������) indicates chloride concentration in

milligrams per liter.

EXPLANATION

���

SUFFOLK CO.

NASSAU CO.

���������

0

GOLF-COURSE WELL

PRODUCTION WELL

INDUSTRIAL WELL

FIGURE 16. Chloride concentrations in the upper glacial 

and Magothy aquifers, Oyster Bay study area, Nassau 

County, N.Y., 1997. (Location is shown in fig. 1.)

Extent of Saltwater Intrusion
46 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

7152(UG)

9478(UG)

12256(UG)

12755 (UG)

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12711(UG)

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��

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12856(UG)

12946(UG) ����

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73o34'

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A

C

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100

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100

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73o30'

40o53'

40o54'

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LINE OF EQUAL CHLORIDE CONCENTRATION -- 

Dashed where inferred. Concentration is in milligrams per liter.

SALTWATER WEDGE

OBSERVATION WELL -- Top number is well number; letter

in parentheses is aquifer designation (UG=upper glacial).

Bottom number (����������) indicates chloride concentration, in milligrams

per liter.

1000

A

12856(UG)

����

0

0

1

1 2

MILES

KILOMETERS

�����

PRODUCTION WELL

GOLF-COURSE WELL

BAYVILLE

CENTRE

ISLAND

COVE

NECK

FIGURE 16B. Chloride concentrations in the upper glacial and Magothy aquifers, Oyster Bay study area, Nassau County, N.Y., 

1997. (Location is shown in fig. 16A.)
 47

aquifer in this area as of 1998. Private wells on Centre Island 

are screened in the Lloyd aquifer, probably because the upper 

glacial aquifer in this area contains saltwater.

Saltwater wedge B (fig. 16) was about 0.4 mi northwest 

of wedge A and is in the upper glacial aquifer. Pumping of 

production well N7643, in western Bayville and screened 

in the upper glacial aquifer, appears to have resulted in an 

increase in chloride concentration during 1965-84. Chloridebreakthrough curves for this well indicate rapid increases in 

chloride concentration during the 1960’s and 70’s (fig. 19A); 

chloride concentrations were less than 20 mg/L in 1965 and 

reached a peak of about 57 mg/L in 1974 (fig. 19A). This well 

was shut down in 1984 and no chloride data is available since 

that time. Water from observation well N12775, screened 

106 to 126 ft below sea level, in the upper glacial aquifer 

near Bayville, had a chloride concentration of 4,250 mg/L 

in 1997. Geophysical logs of the adjacent deep observation 

well N12774 from 1997 indicated a saltwater wedge about 

25-ft thick with a sharp interface at the base of the upper 

glacial aquifer (fig. 12). The sharp interface indicated active 

saltwater intrusion in response to overpumping of the upper 

glacial aquifer in this area. A wedge of saltwater appeared 

to be migrating landward toward the production pumping 

center south of observation well N12774. Saltwater wedge 

B appeared to extend landward as of 1997 past observation 

well N7152, at which the chloride concentration at 35 to 

70 ft below land surface was 5,400 mg/L in 1961 (Isbister, 

1966). The chloride concentration was only 25 mg/L in 1996 

in well N9478, which is in the upper glacial aquifer, and is 

approximately 1300 ft west of N7152.

Saltwater wedge C (fig. 16), along the northwest coast 

of Cove Neck, appeared to be within the upper glacial 

aquifer. Drillers’ logs on file at the USGS Coram office 

indicate that water at wells screened in the upper glacial 

aquifer had elevated chloride concentrations during the late 

1950’s to the early 1960’s. The extent and thickness of this 

wedge was undefined because this area lacks observation 

wells. Well N6675, in Cove Neck (86 ft below land surface), 

was described by Isbister (1966) as a well in the upper 

glacial aquifer with high chloride concentrations. Borehole 

geophysical logs and filter-press samples taken during this 

study in 1996 at observation well N12743, in the north-central 

part of Cove Neck, did not indicate brackish or salty water.

Saltwater wedge D (fig. 17B), in eastern Bayville, 

caused the shutdown and abandonment of production well 

N10144, screened in the Lloyd aquifer, after only 10 years 

of operation. Chloride-breakthrough curves (1984-95) for 

this well indicated rapid intrusion of saltwater into the 

Lloyd aquifer because of overpumping (fig. 19B). Chloride 

concentrations at this well were less than 5 mg/L in 1984 

and rose to more than 50 mg/L within 6 years of operation. 

The maximum chloride concentration was 101 mg/L in 

1994. Despite a drastic reduction in pumpage, chloride 

concentrations generally continued to increase until the well 

was subsequently taken out of service. Water from observation 

well N12790, approximately 1300 ft northeast of N10144 on 

the coast of eastern Bayville and screened at the base of the 

Lloyd aquifer, had a chloride concentration of 1,800 mg/L 

in 1998. Geophysical logs and water samples from this well 

indicated that a thin wedge of saltwater had intruded the Lloyd 

aquifer in the eastern part of Bayville because of overpumping 

(fig. 18A). Observation well N12870 (fig. 2), approximately 

2000 ft west of N10144 and screened at the base of the Lloyd 

aquifer, had a peak chloride concentration of 125 mg/L in 

1998. This well is near the western boundary of saltwater 

wedge D. 

Production pumping of the Lloyd aquifer by wells 

N7620 and N8776 (fig. 14), in western Bayville, appeared to 

have lowered the water levels to below sea level. Chloride 

concentrations at observation well N12774 in 1997 did not 

indicate that the saltwater-freshwater interface had moved 

inland; however, the rapid saltwater intrusion into the Lloyd 

aquifer in eastern Bayville indicates that western Bayville may 

be prone to similar saltwater intrusion in the future. Chloridebreakthrough curves from 1972-95 for production well N8776, 

screened in the Lloyd aquifer near Bayville, indicated low 

chloride concentrations (fig. 19C) except for one outlier point 

at which the chloride concentration peaked at 38 mg/L in 

1990. The data seem to indicate that the Lloyd aquifer in the 

western part of Bayville, as of 1998, had not been affected by 

saltwater intrusion .

SUMMARY AND CONCLUSIONS

The intrusion of saltwater toward several production 

wells within the northern part of the town of Oyster Bay 

prompted a study by the USGS in cooperation with the 

NCDPW from 1995 through 1998 to collect hydrogeologic, 

geophysical, and water-quality data, define the subsurface 

geology, and delineate the extent of saltwater intrusion. 

Nineteen boreholes were drilled during 1995-98, and 

continuous high-resolution marine-seismic-reflection surveys 

were completed to define the hydrogeologic framework 

from Hempstead Harbor to Cold Spring Harbor. Borehole 

geophysical logs (gamma, electric, and EM induction) were 

used to delineate hydrogeologic units and the extent of 

saltwater intrusion at selected observation wells.

New drill-core data led to the identification and naming 

of two hydrogeologic units—the North Shore aquifer and 

the North Shore confining unit. The North Shore aquifer is a 

sequence of Pleistocene-age sediments in the northwestern, 

central, and northeastern parts of the study area. The North 

Shore confining unit is a sequence of Pleistocene-age clay 

and silt deposits that occur locally along the northern parts 

of Queens, Nassau, and Suffolk Counties where the Lloyd 

aquifer, Raritan clay, and Magothy aquifer were severely 

eroded or removed by glacial scouring.

The Raritan clay overlies and confines water in the Lloyd 

aquifer. A glacially eroded buried valley appears to incise the 

Raritan clay from the northwest to the central part of the study 

Summary and Conclusions
48 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

NORTHERN

BOULEVARD

ROU

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73o40' 73o36' 73o32' 

40o54'

40o52'

40o50'

40o48'

Base from New York State Department of Transportation, 1981, 1:24,000

11280(L)

12618(L)

12635(L) 12646(L)

12667(L)

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12697(L)

12747(

5201(L)

12921(NS)

12774(L)

12870(L)

12790(L)

12880(L)

12855(L)

12929(L)

12853(L)
 49

ROUTE 25A

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73o28'

12733(L)

0(L)

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55(L)

12768(L)

2929(L)

Area shown in Figure 17B

SUFFOLK CO.

NASSAU CO.

0 1 2 KILOMETERS

0 1 2 MILES

OBSERVATION WELL - Top number is well 

number; letter in parentheses is aquifer 

designation (L=Lloyd, NS=North Shore.). 

Bottom number (����������) indicates chloride 

concentration in milligrams per liter.

�����������

PRODUCTION WELL

5201(L)

12921(NS)

��

�

GOLF-COURSE WELL

INDUSTRIAL WELL

FIGURE 17. Chloride concentrations in the Lloyd and 

North Shore aquifers, Oyster Bay study area, Nassau 

County, N.Y., 1997. (Location is shown in fig. 1.)

Summary and Conclusions
50 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

FIGURE 17B. Chloride concentrations in the Lloyd and North Shore aquifers, Oyster Bay study area, Nassau County, N.Y., 1997. 

(Location is shown in fig. 17A.)

������������������

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12733(L)

12618(L)

��

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�� ��

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12697(L)

12747(L)

12921(NS)

12774(L)

12870(L)

12790(L)

D �����

���

10144(L) ���

73o34' 73o30'

40o53'

40o54'

�����������

LINE OF EQUAL CHLORIDE CONCENTRATION -- 

Dashed where inferred. Concentration is in milligrams per liter.

SALTWATER WEDGE

OBSERVATION WELL -- Top number is well number; letter

in parentheses is aquifer designation (L = Lloyd, NS = North

Shore). Bottom number (����������) indicates chloride concentration in

milligrams per liter.

PRODUCTION WELL

GOLF-COURSE WELL

1000

D

12921(NS)

�

10144(L)

���

BAYVILLE

COVE

NECK

CENTRE

ISLAND
 51

FIGURE 18. Geophysical logs from two observation wells in northern Oyster Bay study area, Nassau 

County, N.Y.: A. Well N12790. B. Well N12747. (Locations are shown in figure 2.)

North Shore

confining unit

Upper glacial aquifer

5

Well screen-zone sample depth. Number

 is chloride concentration in milligrams per liter.

Filter-press sample depth. Number

 is chloride concentration in milligrams per liter.

EXPLANATION

G Gamma log (counts per second)

SP Spontaneous potential log (millivolts)

SPR Single-Point-Resistance log (ohms)

SNR Short-Normal-Resistivity log (ohm-meters)

Electromagnetic induction log (milliSiemens

 per meter)

EM

DEPTH, IN FEET BELOW LAND SURFACE DEPTH, IN FEET BELOW LAND SURFACE

0 200 -800 SP -500 300 SPR 800 0 EM 600

50

100

150

200

250

Upper glacial aquifer

Upper glacial

aquifer

North Shore

confining unit

Raritan clay

Raritan clay

Lloyd aquifer

Lloyd aquifer

Bedrock

300

350

400

B. N12747

A. N12790

0 200 300 SP 400 0 SPR 50 0 SNR 700 0 EM 700

50

100

150

200

250

300

350

400

450

500 38

1,025

1,800

13,750

1,200

4,080

4,080

16,400

10,250

4,460

G

G

50

Summary and Conclusions
52 Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County,

New York: 1995-98

FIGURE 19. Chloride concentration and pumpage at selected 

production wells in northern Oyster Bay study area, Nassau 

County, N.Y.: A. Well N7643. B. Well N10144. C. Well N8776. 

(Locations are shown in figure 2; incorporated village of Bayville.)

area, where the Raritan has been completely removed. The 

Magothy aquifer has been removed by glacial erosion in the 

northern and central parts of the study area, except just west 

and south of Oyster Bay Harbor. 

Marine-seismic-reflection surveys, borehole geophysical 

logs, and drill-core samples were used to correlate offshore 

glacial-lake deposits with buried valleys that extend across the 

study area. Marine-seismic-reflection profiles in Long Island 

Sound, Hempstead Harbor, Oyster Bay Harbor, and Cold 

Spring Harbor indicate silt and clay deposits that infill four 

sharply defined buried valleys that truncate the surrounding 

coarse-grained deposits. The valleys are asymmetrical, steep 

sided, northwest-southeast-trending, are several miles long, 

and about 1 mi wide. The floors of these valleys extend to 

bedrock; their depths range from 250 ft to more than 500 ft 

below sea level.

The upper glacial aquifer consists of till, sand, gravel, 

silt, and clay deposits underlain in some areas by the North 

Shore confining unit. The marine-seismic-reflection surveys 

and drill-core data indicate that the upper glacial aquifer does 

not extend far offshore and does not infill the valleys beneath 

Hempstead Harbor and Cold Spring Harbor, as proposed in 

previous studies. The water table is in the upper glacial aquifer 

in all but the southern part of the town of Oyster Bay, where it 

is in the Magothy aquifer.

Water-level data from quarterly measurements and 

continuous water-level recorders during 1995-98 indicate 

that the upper glacial aquifer is hydraulically connected to 

the Magothy aquifer in the study area and that water levels in 

these aquifers did not respond to tidal fluctuations. However, 

water levels in the Lloyd and North Shore aquifers, which are 

hydraulically connected, were affected by production pumping 

and by tides.

The potentiometric surface of the Lloyd and North Shore 

aquifers in July 1997 indicates a large cone of depression that 

results from production pumping. Water levels ranged from 58 

ft above to about 2 ft below mean sea level. However, water 

levels were measured more than 10 ft below mean sea level. 

Increased pumping in summer causes ground-water flow in 

the Lloyd and North Shore aquifers to reverse direction; that 

is, to flow inland from the surrounding saltwater embayments 

toward the pumping wells. This reversal of the normal 

seaward flow is the primary cause of saltwater intrusion in the 

study area.

Water from three production wells and six observation 

wells screened in the upper glacial and Magothy aquifers 

contained several VOC’s in concentrations greater than the 

New York State Department of Health maximum contaminant 

level (MCL) of 0.005 mg/L. Two production wells contained 

high iron or nitrate concentrations, and were shut down as a 

result; water from 39 observation wells screened in the upper 

glacial, Magothy and Lloyd aquifers had iron concentrations 

in excess of the State’s MCL of 0.3 mg/L.

1960 1965 1970 1975 1980 1985 1990 1995

YEAR

0

10

20

30

40

50

60

ANNUAL PUMPAGE, IN MILLIONS OF GALLONS

0

10

20

30

40

50

60

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

PUMPAGE

CHLORIDE

A. ������, 1965-1994 (Upper glacial)

1980 1985 1990 1995 2000

YEAR

0

50

100

150

200

250

300

0

30

60

90

120

B. ������, 1984-1995 (Lloyd)

1970 1975 1980 1985 1990 1995 2000

YEAR

0

85

170

255

340

0

10

20

30

40

C. �����, 1972-1995 (Lloyd)

PUMPAGE

CHLORIDE

PUMPAGE

CHLORIDE
 53

The chloride concentration at production wells screened 

in the upper glacial and Magothy aquifers ranged from 5 to 

75 mg/L. Production well N10144, which is near Bayville 

and is screened in the Lloyd aquifer, had a maximum chloride 

concentration in 1994 of 101 mg/L and was subsequently shut 

down. No production wells screened in the Lloyd aquifer had 

chloride concentrations that exceeded the State’s MCL of 250 

mg/L in 1998. Chloride concentrations at seven observation 

wells screened in the upper glacial aquifer and one well 

screened in the Lloyd aquifer, exceeded the State MCL; 

one observation well screened in the Magothy aquifer had a 

chloride concentration of 170 mg/L. 

Four areas of saltwater intrusion were delineated—three 

in the upper glacial aquifer and one in the Lloyd aquifer. The 

saltwater wedges decreased in thickness landward and had 

relatively sharp saltwater-freshwater interfaces.

Saltwater wedge A was at the base of the upper glacial 

aquifer in the easternmost part of Bayville and most of Centre 

Island. Water from observation wells N12791, N12946, and 

N12755 had chloride concentrations of 13,750, 8,500, and 

10,250, respectively. Geophysical logs of wells N12791 and 

N12747 from 1997 indicated that all of the upper glacial 

aquifer at these sites contained saltwater.

Saltwater wedge B was in the upper glacial aquifer about 

0.4 mi northwest of wedge A. Observation well N12775 had 

a chloride concentration of 4,250 mg/L. In 1997, the wedge 

at well N12774 was about 25 ft thick; its sharp interface 

indicated active saltwater intrusion from overpumping. In 

1997, wedge B appeared to be migrating landward toward the 

production pumping center south of well N12774. The wedge 

extended past well N7152, at which the chloride concentration 

in 1966 was 5,400 mg/L.

Saltwater wedge C, within the upper glacial aquifer, was 

on the northwest coast of Cove Neck. Drillers’ logs of several 

upper glacial wells indicated elevated chloride concentrations; 

but the wedge’s extent or thickness was unknown because 

observation wells were lacking.

Saltwater wedge D caused the shutdown and 

abandonment of production well N10144, which is near 

Bayville and is screened in the Lloyd aquifer. Chloride 

concentrations at this well increased so rapidly in the 10 years 

after the well’s installation in 1984 that this well had to be shut 

down. Water from observation well N12790, along the eastern 

coast of Bayville, had a chloride concentration of 1,800 

mg/L in 1998. The wedge was thin and extended westward 

to the vicinity of well N12870, which had a peak chloride 

concentration of 125 mg/L in 1997.

Production pumping of Lloyd aquifer wells in western 

Bayville appears to have lowered Lloyd aquifer water levels 

to below sea level as of 1997. The Lloyd aquifer in western 

Bayville was not affected by saltwater intrusion as of 1998, 

but the rapid saltwater intrusion in eastern Bayville indicates 

that it may be prone to saltwater intrusion.

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