Prepared in cooperation with the New York State Department of Environmental Conservation

Use of Time Domain Electromagnetic Soundings and 

Borehole Electromagnetic Induction Logs To Delineate 

the Freshwater/Saltwater Interface on Southwestern 

Long Island, New York, 2015–17

Open-File Report 2020–1093

U.S. Department of the Interior

U.S. Geological Survey
Cover. Front: U.S. Geological Survey scientist collecting a 100-square-meter time domain 

electromagnetic survey in Juniper Park, Middle Village, Queens County, New York. Photograph by 

the U.S. Geological Survey. Back: U.S. Geological Survey scientists collecting 100-square-meter time 

domain electromagnetic surveys in (upper photograph) Roy Wilkins Park, Jamaica, Queens County, 

and (lower photograph) Phil “Scooter” Rizzuto Park, South Richmond Hill, Queens County, New York. 

Photographs by the U.S. Geological Survey.
Use of Time Domain Electromagnetic 

Soundings and Borehole Electromagnetic 

Induction Logs To Delineate the 

Freshwater/Saltwater Interface 

on Southwestern Long Island, 

New York, 2015–17

By Frederick Stumm, Michael D. Como, and Marie A. Zuck

Prepared in cooperation with the 

New York State Department of Environmental Conservation

Open-File Report 2020–1093

U.S. Department of the Interior

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

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2020

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Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Stumm, F., Como, M.D., and Zuck, M.A., 2020, Use of time domain electromagnetic soundings and borehole 

electromagnetic induction logs to delineate the freshwater/saltwater interface on southwestern Long Island, New 

York, 2015–17: U.S. Geological Survey Open-File Report 2020–1093, 27 p., https://doi.org/10.3133/ofr20201093.

Associated data for this publication:

Como, M.D., Zuck, M.A., and Stumm, F., 2020, Time domain electromagnetic surveys collected to estimate the extent 

of saltwater intrusion in Nassau and Queens County, New York, October–November 2017: U.S. Geological Survey 

data release, https://doi.org/10.5066/P90B6OTX.

ISSN 2331-1258 (online)
iii

Acknowledgments

The authors thank the staff at the New York State Department of Environmental Conservation 

for their assistance and technical support during the study.

The authors thank the Nassau County Department of Public Works for granting access to their 

observation wells for borehole-geophysical measurements.

The authors also thank Jason Finkelstein of the U.S. Geological Survey for his assistance with 

several of the figures.

v

Contents

Acknowledgments........................................................................................................................................iii

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

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

Hydrogeologic Setting .........................................................................................................................3

Historic Pumpage and Saltwater Intrusion ......................................................................................3

Methods...........................................................................................................................................................4

Time Domain Electromagnetic Soundings ........................................................................................4

Borehole Electromagnetic Induction Logs .......................................................................................6

Location of the Freshwater/Saltwater Interface on Southwestern Long Island ..............................16

Upper Glacial-Jameco-Magothy Aquifer Complex Isochlor Map ..............................................16

Lloyd Aquifer Isochlor Map ...............................................................................................................20

Summary........................................................................................................................................................24

References Cited..........................................................................................................................................24

Figures

1. Map showing location of the study area, surface geophysical soundings, and 

wells on southwestern Long Island, New York ........................................................................2

2. Cross section showing generalized hydrogeology of western Long Island, 

New York ........................................................................................................................................3

3. Graph showing the time domain electromagnetic smooth one-dimensional 

model at site QTDEM2 in Queens, New York ...........................................................................6

4. Map and graphs showing locations and individual soundings of time domain 

electromagnetic surveys collected in Queens and Nassau Counties, New York ..............7

5. Graph showing generalized geology, natural gamma log, electromagnetic 

induction conductivity log, and chloride concentrations in observation well 

N 12506, Nassau County, New York .........................................................................................17

6. Map showing the location of hydrogeologic cross section A to A′ in Kings, 

Queens, and Nassau Counties, New York ..............................................................................18

7. Hydrogeologic cross section A to A′ with gamma and electromagnetic 

induction conductivity logs for wells in Kings, Queens, and Nassau Counties, 

New York ......................................................................................................................................19

8. Isochlor map showing the 5,000 milligram per liter chloride concentration in 

the upper glacial, Jameco, and Magothy aquifer complex and delineated in the 

shallow, intermediate, and deep parts of the aquifer complex in Kings, Queens, 

and Nassau Counties, New York ..............................................................................................21

9. Isochlor map showing the 5,000 milligram per liter chloride concentration in the 

Lloyd aquifer in Kings, Queens, and Nassau Counties, New York ......................................22

10. Graph showing gamma, induction resistivity, and calculated electromagnetic 

conductivity logs at test well Q 3655 showing the extent of saltwater intrusion 

in the Magothy and Lloyd aquifers, Queens County, New York ..........................................23
vi

Tables

1. Site identification of time domain electromagnetic soundings collected on 

southwestern, Long Island, New York, 2017 .............................................................................5

2. Well location and borehole electromagnetic induction log information ...........................20

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Flow rate

million gallons per day (Mgal/d) 0.04381 cubic meter per second (L/s)

Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83) 

or the North American Datum of 1927 (NAD 27).

Elevation refers to the distance above or below the vertical datum.

Supplemental Information

Chloride concentrations are given in milligrams per liter.

Conductivity is given in millisiemens per meter, and resistivity is given in ohm-meters.

Transmitter loop size is given in square meters, and intercoil spacing is given in centimeters.

Natural gamma is given in counts per second.

Base frequencies are given in hertz.

Abbreviations

EM electromagnetic induction

TDEM time domain electromagnetic

USGS U.S. Geological Survey
Use of Time Domain Electromagnetic Soundings and 

Borehole Electromagnetic Induction Logs To Delineate 

the Freshwater/Saltwater Interface on Southwestern 

Long Island, New York, 2015–17

By Frederick Stumm, Michael D. Como, and Marie A. Zuck

Abstract

The U.S. Geological Survey, in cooperation with the New 

York State Department of Environmental Conservation, used 

surface and borehole geophysical methods to delineate the 

freshwater/saltwater interface in coastal plain aquifers along 

the southwestern part of Long Island, New York. Over pumping of groundwater in the early 20th century combined with 

freshwater/saltwater interfaces at the coastline created saltwater intrusion in the upper glacial, Jameco, Magothy, and Lloyd 

aquifers. This study documents, for the first time, extensive 

saltwater intrusion of the Lloyd aquifer along the southwestern 

coast of Long Island, N.Y. Several public-supply wells in the 

southern parts of Nassau, Queens, and Kings Counties have 

been adversely affected by saltwater intrusion causing supply wells to be shutdown and abandoned. Due to the ongoing 

groundwater pumping in southern Nassau County, the freshwater/saltwater interface requires delineation and monitoring 

for any inland movement.

In 2015–17, the U.S. Geological Survey collected time 

domain electromagnetic soundings at 12 locations and borehole electromagnetic induction conductivity logs at 9 wells 

within the study area to delineate several saltwater intrusion 

wedges. The upper glacial, Jameco, and Magothy aquifers 

were grouped into one aquifer complex within the study 

area to simplify interpretations. The coastal plain sediments 

increase in thickness from west to east and north to south 

because of their regional dip toward the southeast. Three 

separate wedges, shallow, intermediate, and deep, of saltwater 

intrusion were delineated in the upper glacial, Jameco, and 

Magothy aquifer complex. In addition, analysis of geophysical 

logs collected in an open borehole of a test well in southern 

Queens County in 1989 revealed the Lloyd aquifer was nearly 

completely intruded by saltwater with an estimated chloride 

concentration of 15,000 milligrams per liter. The geophysical 

logs from this well provides, for the first time, definitive proof 

of saltwater intrusion of the Lloyd aquifer on Long Island’s 

south shore, suggesting the freshwater/saltwater interface was 

at the coastline and not miles offshore as theorized by previous studies.

Introduction

The population of southwestern Long Island, New 

York (fig. 1), grew rapidly during the 20th century, and the 

demand for fresh drinking-water supplies increased proportionately. During the first half of the 20th century, most of 

the public-supply water serving this area was supplied from 

wells screened in the underlying unconsolidated deposits of 

Pleistocene and Cretaceous age including the upper glacial, 

Jameco, Magothy, and Lloyd aquifers (fig. 2). Overpumping 

in some areas caused saltwater intrusion resulting in the 

shutdown of multiple supply wells in Queens and Kings 

Counties and affected several supply wells in Nassau County 

(Lusczynski, 1952; Buxton and others, 1981). Since the 1990s, 

the drinking water for Queens and Kings Counties has been 

supplied from a network of surface-water reservoirs in the 

Delaware, Catskills, and Croton watersheds (not shown) of 

New York. In Nassau County, all drinking water continues to 

be supplied from wells distributed throughout the county. The 

extent of saltwater intrusion was mapped in the southern part 

of Queens and Nassau Counties using water-quality samples 

collected from public-supply and observation wells in 1938, 

1952, 1963, 1966, and 1997 (Sandford, 1938; Lusczynski, 

1952; Perlmutter and Geraghty, 1963; Lusczynski and 

Swarzenski, 1966; and Terraciano, 1997). However, the last 

time the Magothy aquifer was mapped with respect to saltwater intrusion was more than 30 years ago, and none of these 

previous studies included information on saltwater intrusion in 

the Lloyd aquifer. Surface and borehole geophysical methods 

have not been widely used to map the extent of saltwater intrusion in these areas.

The U.S. Geological Survey (USGS), in cooperation 

with the New York State Department of Environmental 

Conservation, used time domain electromagnetic (TDEM) 

soundings and borehole electromagnetic induction (EM) 

conductivity logs to estimate the location of the freshwater/

saltwater interface underlying southwestern Long Island, N.Y. 

(fig. 1). The purpose of this report is to describe the application of TDEM sounding and borehole EM conductivity logging methods and to present an integrated analysis of results 
!( NTDEM6 NTDEM4 NTDEM3 QTDEM4 QTDEM1 QTDEM6 QTDEM2 QTDEM3 QTDEM5 NTDEM2 NTDEM1 NTDEM5 Q 3655 K 3414 

N 10620 N 13804 N 13682 N 13879 N 13700 N 13559 K 3613 74°00’ 73°40' 40°43’ 40°33’ 0 5 

0 5 10 KILOME

KINGS COUNTY QUEENS COUNTY BRONX COUNTY NASSAU COUNTY NEW YORK COUNTY Long Beach NEW JERSEY NEW YORK LONG ISLAND Jones Be

Time domain electromagnetic survey  and identifier Observation well and identifier EXPLANATION Q 3655 NTDEM6 Jones Inlet Base from National Oceanic and Atmospheric Administration digital data, 1:70,000, digital vector shoreline Lambert Conformal Conic Projection North American Datum of 1983 ATLANTIC OCEAN Jamaica Bay East River Hudson River Little Neck Bay 

Figure 1. Location of the study area, surface geophysical soundings, and wells on southwestern Long Island, New York.
Introduction 3

Bedrock

Lloyd aquifer

Raritan clay

Magothy aquifer

Jameco aquifer

Gardiners clay

Upper glacial aquifer

Unsaturated zone

Long Island Sound ATLANTIC OCEAN

SCHEMATIC ONLY, NOT TO SCALE

NORTH SOUTH

LONG ISLAND

Figure 2. Generalized hydrogeology of western Long Island, New York.

from those methods to delineate zones of electrically conductive groundwater caused by saltwater intrusion in the major 

aquifers of southwestern Long Island in 2015–17. Data used 

in this report are available in a USGS data release (Como and 

others, 2020) and the USGS GeoLog Locator (USGS, 2020b).

Hydrogeologic Setting

The study area is underlain by unconsolidated deposits 

that constitute three major aquifers separated by two confining units. The unconsolidated deposits of Late Cretaceous, 

Pleistocene, and Recent age thicken southeastward and 

overlie southeast dipping crystalline bedrock of Paleozoic and 

Precambrian age (Smolensky and others, 1990; Lusczynski, 

and Swarzenski, 1966) (fig. 2). The areal extent and lithology 

of the unconsolidated deposits are important factors in the 

distribution and movement of fresh and salty groundwater in 

the report area (Lusczynski, and Swarzenski, 1966; Masterson 

and others, 2015).

The water table (upper glacial aquifer of Pleistocene 

age) in the study area consists of unconsolidated sand, 

gravel, and clay, and is underlain throughout part of the 

area by the Gardiners clay of Pleistocene age (Soren, 1978). 

The Gardiners clay is a gray clay that overlies and confines 

the Jameco aquifer throughout the southern part of Queens 

County. The Jameco aquifer is a sand and gravel deposit 

of Pleistocene age (Soren, 1978) that overlies the Magothy 

aquifer of Cretaceous age. The Magothy aquifer consists 

of sand, silt, and clay (Franke and McClymonds, 1972). 

Underlying the Magothy aquifer is the multicolored Raritan 

clay of Cretaceous age, which overlies and confines water in 

the Lloyd aquifer of Cretaceous age. The Lloyd aquifer is the 

deepest unconsolidated unit and overlies weathered bedrock 

below and consists of fine to coarse quartz-rich beds of sand 

and gravel and interbedded sandy clay and clay (Lusczynski, 

and Swarzenski, 1966; Franke and McClymonds, 1972). The 

Lloyd aquifer is an artesian aquifer and contains freshwater in 

shoreline and inland areas. The Lloyd aquifer underlies most 

of Long Island and consists of an upward fining sequence of 

gravels, sand, silt, and clay lenses (Suter and others, 1949). 

Due to extensive intrusion of saltwater in the aquifers above 

the Raritan clay in the coastal parts of the study area, the 

Lloyd aquifer is the only aquifer that supplies freshwater to 

the barrier island of Long Beach in Nassau County in the 

southern part of the study area (Perlmutter and Crandell, 1959; 

Garber, 1986).

Historic Pumpage and Saltwater Intrusion

During 1904–16, most of the public-supply water in 

Kings and Queens Counties was pumped from the upper 

glacial aquifer at average rates of 21 and 37 million gallons 

per day, respectively (Johnson and Waterman, 1952). Total 

public- and industrial-supply pumpage from all aquifers in 

Kings County during that period averaged 60 million gallons 

per day, most of which was from the upper glacial aquifer 

(Lusczynski, 1952). The predevelopment chloride concentration of freshwater on Long Island was 10 milligrams per liter 

(mg/L) (Lusczynski and Swarzenski, 1966). The ambient chloride concentration of shallow groundwater in urbanized areas 
4 Use of TDEM Soundings and Borehole EM Induction Logs To Delineate the Freshwater/Saltwater Interface

of Long Island is about 40 mg/L (Buxton and others, 1981; 

Heisig and Prince, 1993); this increase is attributed to contamination from land surface sources. In this report, freshwater is 

defined as groundwater with a chloride concentration less than 

250 mg/L, “brackish” water has a chloride concentration of 

250 to 1,000 mg/L, and saltwater has a chloride concentration 

greater than 1,000 mg/L.

By 1936, overpumping in central Kings County had 

created a major cone of depression in the water table that 

extended to the south shore of much of Kings County and 

into southwestern Queens County (Chu and Stumm, 1995). 

Maximum drawdowns in the area exceeded 35 feet below sea 

level. By the 1930s, chloride concentrations in Kings County 

had increased to more than 100 mg/L (Lusczynski, 1952; 

Buxton and others, 1981), and, by the 1940s, chloride concentrations at inland wells exceeded 250 mg/L. The elevated 

chloride concentrations resulted in the shutdown of publicsupply wells in Kings County in 1947 (Lusczynski, 1952). 

After the shutdown, industrial pumping continued at rates of 

about 23 Mgal/d in northern Kings County, where groundwater levels remained below sea level into the 1960s (Buxton and 

others, 1981). The cessation of pumping in Kings County was 

balanced by increased public-supply pumping in southwestern 

Queens County. By 1961, however, a major cone of depression developed in that area due to excessive pumping, and, 

in 1974, saltwater intrusion resulted in the shutdown of those 

wells (Buxton and others, 1981). Public-supply pumping then 

shifted eastward to southeastern Queens County. Soren (1971)

documented saltwater intrusion in three areas of Queens 

County: the northwestern section, the north-central section, 

and the Woodhaven (southwestern) section. Lusczynski and 

Swarzenski (1966) delineated intermediate and deep saltwater 

wedges in southern Nassau and southeastern Queens Counties 

from analyses of water from monitoring wells, filter-press core 

samples, and geophysical logs.

Methods

The USGS applied TDEM sounding and borehole EM 

conductivity logging methods to map the extent of electrically conductive groundwater on southwestern Long Island 

(Nassau, Queens, and Kings Counties).

Time Domain Electromagnetic Soundings

The TDEM sounding method uses a transmitter to drive 

an electrical current through a square loop of insulated cable 

on the ground and a receiver to measure the current induced in 

the subsurface. The current has equal on-and-off periods and 

base frequencies that generally range from 30 to 300 hertz. 

Termination of the current flow is not instantaneous, but 

occurs over a few microseconds, during which the magnetic 

field varies. The time-variant nature of the primary electromagnetic field creates a secondary electromagnetic field in the 

ground beneath the loop that, in accordance with Faraday’s 

law, generally mirrors the primary field of the transmitter loop 

(Christiansen and others, 2006; North Carolina Division of 

Water Resources, 2006). This secondary field immediately 

begins to decay, generating additional eddy currents that propagate downward and outward into the subsurface similar to a 

series of smoke rings (U.S. Army Corps of Engineers, 1995).

Measurements of the secondary currents are made during the time-off period by one or more receivers in the center 

of the transmitter loop. The signal strength of the decaying 

currents at specific times and depths is controlled by the bulk 

conductivity of the subsurface, which includes the conductivity of subsurface rock and sediment units and their contained 

fluids (Fitterman and Stewart, 1986; Stewart and Gay, 1986; 

McNeill, 1994; Auken and others, 2008). The subsurface conductivity is estimated from the voltage-decay curve collected 

at the receiver through an inversion process described below. 

The depth of investigation depends upon the equipment’s ability to monitor the early and late times of the transient signal, 

the initial magnitude of the magnetic moment (proportional 

to the size of the loop and the current), the attenuation of 

the signal by the resistivity of the subsurface, and the background noise.

TDEM sounding locations were selected on the basis 

of several factors including the location where saltwater was 

suspected to be in the subsurface, site access, amount of 

open space to accommodate wire loop sizes, and the need to 

minimize electromagnetic interference. Soundings made at 

sites near power lines and buried utilities have a higher likelihood of electromagnetic interference (anthropogenic sources 

of noise). TDEM surface geophysical data were collected at 

12 sites using 20-, 40-, or 100-meter, square transmitter loops 

(fig. 1; table 1) and analyzed to develop layered and smoothed 

earth resistivity models for each location. Data from each 

sounding were used to calculate apparent resistivity to depths 

of about 375–1,100 feet below land surface (Como and others, 

2020). Generally, the largest possible loop size was used at 

any location to increase the likelihood of obtaining the deepest aquifer measurements (Payne and Teeple, 2011). A larger 

transmitter loop size increases the depth of measurement and 

increases resistance of the loop wire, which decreases the current. Insulated wire loops (antennas) were used to transmit and 

receive the data. The apparent resistivity values were examined for data quality at depth.

At each sounding, an integration time of several seconds 

was used to measure 10 different datasets (the compilation of 

these datasets is referred to as a stack). The integration time is 

the length of time the system transmits and receives continuously (every few milliseconds) to determine one voltage 

value. The mean value of all the soundings collected over the 

integration time is stored. The values stored in the stack are 

averaged, before the inversion process, to ensure data quality 

and repeatability. Longer integration time increases late-time 

data quality (late time is the length of time after the electrical 

current is shut off), which generally are data collected from 

greater depths. However, the increase in data quality resulting 
Methods 5

Table 1. Site identification of time domain electromagnetic soundings collected on southwestern, Long Island, New York, 2017.

[Data are from Como and others (2020). Collection dates are given in month/day/year. Latitude and longitude are given in decimal degrees. NAD 83, North 

American Datum of 1983; NAVD 88, North American Vertical Datum of 1988]

Site identifier 

(fig. 1) Date collected Transmitter loop size 

(square meter)

Latitude 

(NAD 83)

Longitude 

(NAD 83)

Elevation 

(foot NAVD 88)

NTDEM1 10/26/2017 40 40.600486 −73.646063 6

NTDEM2 11/02/2017 100 40.626277 −73.603247 8

NTDEM3 11/03/2017 100 40.657701 −73.60186 15

NTDEM4 11/03/2017 100 40.651436 −73.534794 9

NTDEM5 11/07/2017 100 40.681053 −73.691764 31

NTDEM6 11/07/2017 40 40.680695 −73.691876 31

QTDEM1 10/27/2017 100 40.720539 −73.879669 96

QTDEM2 10/27/2017 40 40.69507 −73.821718 52

QTDEM3 10/27/2017 40 40.68183 −73.786856 17

QTDEM4 11/01/2017 100 40.686716 −73.772327 24

QTDEM5 11/01/2017 100 40.68093 −73.803097 33

QTDEM6 11/08/2017 100 40.706693 −73.839967 127

from a longer integration time is less than the increase in data 

quality resulting from a larger magnetic moment created by 

a larger loop size (Payne and others, 2007). For each TDEM 

sounding, a global positioning system unit, which is built 

into the WalkTEM acquisition system, was used to obtain the 

precise location of one corner of the loop.

The relation between the TDEM voltage-decay curve 

recorded at the receivers, and the subsurface conductivity, 

was determined using Aarhus HydroGeophysics Group SPIA 

software and methods described by Auken and others (2015). 

The data at each site were filtered to identify and remove 

background noise caused by the measurement electronics, the 

anthropogenic sources, or both. For all TDEM surveys, the 

following steps were taken to mitigate noise: (1) removal of 

early-time noise (essentially, signal not related to the geohydrology); (2) application of a standard-deviation threshold 

to data at each time gate (to remove extraneous outliers) 

and removal of data points that exceed that threshold; and 

(3) identification of the background noise and removal of all 

late-time data encountering the noise floor (conditions at each 

site where no current is applied to the transmitter). For each 

sounding, data from both magnetic moments (caused by low 

current and high current) and both receivers were combined 

and inverted together.

The TDEM decay data were inverted by using smoothand layered-model approaches to generate resistivity models 

of the subsurface. The smooth models have multiple (about 

20) layers with fixed thicknesses that change gradually, 

whereas the layered models have fewer layers with blocky or 

stepped transitions between the layers. Resistivity units were 

converted to conductivity units using equation 1:

 σ = 1_

ρ × 1,000 (1)

where

σ is conductivity, in millisiemens per meter; and

ρ is resistivity, in ohm-meters.

The conductivity of a sand-and-water mixture as measured by the TDEM sounding method is related to the conductivity of the water by the empirical relation described by 

Archie (1942) and presented by McNeill (1980) as follows:

_σχ

σω = nm (2)

where

σχ is the conductivity of the 

sand-and-water mixture,

σω is the conductivity of water,

n is the porosity of the sand-and-water 

mixture, and

m is a constant (values listed in McNeill, 1980).

The depth of investigation of the TDEM soundings were 

determined by using methods described in Christiansen and 

Auken (2012). The TDEM-sounding locational information, 

raw data, and inversions are available in Como and others 

(2020). An example of a processed TDEM sounding is shown 

in figure 3. The smooth one-dimensional model indicates 

conductive groundwater in the deep part of the aquifer resting 

upon the Raritan clay. The Raritan clay shows a reduction in 

conductivity below the saltwater wedge.

Twelve TDEM soundings were collected in Queens and 

Nassau Counties (figs. 1 and 4). The soundings were used 

to delineate the freshwater/saltwater interface in the upper 

glacial, Jameco, and Magothy aquifers. The sounding at site 

NTDEM3 was not used in figure 4 due to excessive anthropogenic interference. The sounding at site QTDEM1 was not 

used in the mapping because the saltwater encountered at that 
6 Use of TDEM Soundings and Borehole EM Induction Logs To Delineate the Freshwater/Saltwater Interface

0 1,200

50

100

150

200

250

300

350

400

450

millisiemens per meter

Depth, in feet below land surface

0

Upper glacial

 aquifer

Raritan

 clay

 Freshwater

 

 Saltwater

 

 Magothy

 aquifer

 Shallow

 

 Intermediate

 

 Deep 

EXPLANATION 

Freshwater zone

Saltwater zone

Clay zone

Zone demarcation (shallow, intermediate, deep) 

Contact

Time domain electromagnetic conductivity

Figure 3. Time domain electromagnetic smooth one-dimensional model at site QTDEM2 in Queens, New York. Survey location shown 

on figure 1.

location may be indicative of another wedge of saltwater in the 

northwestern part of Queens County first indicated by Soren 

(1971, 1978). The TDEM sounding at site QTDEM1 seems to 

indicate a southern extent of that wedge, which is beyond the 

scope of this report.

Borehole Electromagnetic Induction Logs

Borehole geophysical logs used for this study included 

natural gamma and focused EM conductivity. Several publications describe the logging methods (Archie, 1942, Keys 

and MacCary, 1971; Serra, 1984; Keys, 1990; McNeill, 

1986; Williams and Lane, 1998). Gamma logs were used for 

lithologic and stratigraphic correlation. Gamma log response 

is generally low in the quartz-rich sand aquifers of Pleistocene 

and Cretaceous age found within Long Island’s coastal plain 

deposits. The exception was the Raritan clay that exhibits substantial gamma responses (Suter and others, 1949; Buxton and 

others, 1981). EM conductivity logs provided an electricalconductivity profile of the formations being measured, from 

which groundwater conductivity and chloride concentrations 

can be inferred (Metzger and Izbicki, 2013; Stumm, 2001; 

Stumm and others, 2002, 2004; Stumm and Como, 2017).

EM conductivity logs were collected using a Geonics 

model EM–39 borehole tool that uses coaxial coil geometry 

with an intercoil spacing of 50 centimeters to allow a radius of 

measurement into the formation with centimeter-scale vertical resolution (Taylor and others, 1989; McNeill, 1986). EM 

conductivity logs are unaffected by conductive borehole fluid 

or the presence of plastic casing. The combination of a large 

conductivity range, high sensitivity, and very low noise and 

drift allows accurate measurement of subsurface conditions 

(Taylor and others, 1989).
! ! ! !

!

!

!

!

!

NEW JERSEY NEW YORK

Time domain electromagnetic surve and identifier

Outpost and historical well and iden

Q 3655

NTDEM6

?

0 1,200

50

100150200250300350400450500550600650700750800850900950

TDEM Conductivity

millisiemens per meter

Depth, in feet below land surface

0

Magothy aquiferUpper Glacial aquifer

Raritan clay

 Freshwater

0 

50

100150200250300350400450500550600650700750800850900950

1,0001,0501,100

0

Depth, in feet below land surface

TDEM Conductivity

millisiemens per meter

Raritan clay

 Lloydaquifer

Magothy aquifer

Upper Glacial aquifer Saltwater (overwash)

 Freshwater

0 1,200

50

100150200250300350400

0

Depth, in feet below land surface

TDEM Conductivity

millisiemens per meter

Magothy aquifer

Upper Glacial aquifer

 Saltwater

 Saltwater

0 1,200

50

100150200250300350400450500550600650700750800850

0

Depth,in feet below land surface

TDEM Conductivity

millisiemens per meter

Upper Glacial aquifer

Magothyaquifer Lloydaquifer

Bedrock

 Saltwater?

Raritan clay

 Freshwater

0 1,200

50

100150200250300350400

0

Depth, in feet below land surface

TDEM Conductivity

millisiemens per meter

Clay?

Raritan clay

 Freshwater

 Saltwater

Upper Glacial aquifer Shallow

0 1,200

50

100150200250300350400450

0

Depth, in feet below land surface

TDEM Conductivity

millisiemens per meter

Raritan clay

Upper Glacial aquifer

 Freshwater

 Saltwater

0 1,200

50

100150200250300350400450

TDEM Conductivity

millisiemens per meter

Depth, in feet below land surface

0

Upper Glacial aquiferRaritan clay

 Freshwater

 Saltwater

 Magothy aquifer

 Shallow

 Intermediate

 Deep 

0 1,200

50

100150200250300350400450500550600

millisiemens per meter

0

Depth, in feet below land surface

TDEM Conductivity

 Lloyd aquifer

Raritan clay

Upper Glacial aquifer

 Freshwater

Shallow

Intermediate

Deep

Upper glacial, Jameco, and Magotht

ShallowIntermediateDeep 

EXPLANATION

NTDEM6

NTDEM4

NTDEM3

QTDEM4

QTDEM1

QTDEM6

QTDEM2

QTDEM3 QTDEM5

NTDEM2

NTDEM1

NTDEM5

Q 3655

K 3414

N 10620

N 13804

N 13682

N 13879

N 13700

N 13559

K 3613

74°00’ 73°40’

40°43’

40°33’

0 5 

0 5 10 KILOME

KINGS COUNTY

QUEENS COUNTY NASSAU COUNTY

A

B

C

D

E

F

G

H

I

Figure 4. Locations and individual soundings of time domain electromagnetic surveys collected in Queens and Nassau Counties, New York. A, locati(larger version available for download at https://doi.org/10.3133/ofr20201093); B, QTDEM2 sounding; C, QTDEM4 sounding; D, NTDEM5 sounding; E, NTNTDEM1 sounding; G, QTDEM3 sounding; H, QTDEM5 sounding; and I, QTDEM6 sounding.
8 Use of TDEM Soundings and Borehole EM Induction Logs To Delineate the Freshwater/Saltwater Interface

0 600

50

100

150

200

250

300

350

400

450

Time domain electromagnetic conductivity

millisiemens per meter

Depth, in feet below land surface

0

Upper Glacial

 aquifer

Raritan

 clay

 Freshwater

 

 Saltwater

 

 Magothy

 aquifer

Shallow

 

Intermediate

 

Deep

 

EXPLANATION 

Freshwater zone

Saltwater zone

Clay zone

Zone demarcation (shallow, intermediate, deep) 

Contact

B. QTDEM2

Figure 4. —Continued
Methods 9

0 1,200

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

850

0

Depth, in feet below land surface

millisiemens per meter

Upper glacial

 aquifer

Magothy

 aquifer

 Lloyd

 aquifer

Bedrock

 Saltwater?

 

Raritan

 clay

 Freshwater

 

Shallow

Intermediate

Deep

EXPLANATION 

Freshwater zone

Saltwater zone

Clay or bedrock zone

Zone demarcation (shallow, intermediate, deep) 

Contact

C. Time domain electromagnetic conductivity QTDEM4

Figure 4. —Continued
10 Use of TDEM Soundings and Borehole EM Induction Logs To Delineate the Freshwater/Saltwater Interface

0 1,200

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

850

900

950

millisiemens per meter

Depth, in feet below land surface

0

Magothy

 aquifer

Upper Glacial

 aquifer

Raritan

 clay

 Freshwater

 

Shallow

Intermediate

Deep

EXPLANATION 

Freshwater zone

Saltwater zone (none)

Clay zone

Zone demarcation (shallow, intermediate, deep) 

Contact

Time domain electromagnetic conductivity D. NTDEM5

Figure 4. —Continued
Methods 11

0 1,200

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

850

900

950

1,000

1,050

1,100

0

Depth, in feet below land surface

millisiemens per meter

Raritan

 clay

 Lloyd

 aquifer

Magothy

 aquifer

Upper glacial

 aquifer Saltwater (overwash)

 

 Freshwater

 

Shallow

Intermediate

Deep

EXPLANATION 

Freshwater zone

Saltwater zone

Clay zone

Zone demarcation (shallow, 

 intermediate, deep) 

Contact

E. Time domain electromagnetic conductivity NTDEM4

Figure 4. —Continued
12 Use of TDEM Soundings and Borehole EM Induction Logs To Delineate the Freshwater/Saltwater Interface

0 1,200

50

100

150

200

250

300

350

400

0

Depth, in feet below land surface

millisiemens per meter

Magothy

aquifer

Upper Glacial

 aquifer

 Saltwater

 

 Saltwater

 

Shallow

Intermediate

EXPLANATION 

Freshwater zone

Saltwater zone

Clay zone (none)

Zone demarcation (shallow, intermediate, deep) 

Contact

F. NTDEM1 Time domain electromagnetic conductivity

Figure 4. —Continued
Methods 13

0 1,200

50

100

150

200

250

300

350

400

0

Depth, in feet below land surface

millisiemens per meter

Clay?

Upper glacial

 aquifer

Raritan

 clay

 Freshwater

 

 Saltwater

 

EXPLANATION 

Freshwater zone

Saltwater zone

Clay zone

Zone demarcation (shallow, intermediate, deep) 

Contact

Shallow

Intermediate

Deep

Time domain electromagnetic conductivity

G. QTDEM3

Figure 4. —Continued
14 Use of TDEM Soundings and Borehole EM Induction Logs To Delineate the Freshwater/Saltwater Interface

EXPLANATION 

Freshwater zone

Saltwater zone

Clay zone

Zone demarcation (shallow, intermediate, deep) 

Contact

Time domain electromagnetic conductivity

0 1,200

50

100

150

200

250

300

350

400

450

0

Depth, in feet below land surface

milliseimens per meter

Raritan

 clay

Upper glacial

 aquifer

 Freshwater

 

 Saltwater

 

Shallow

Intermediate

Deep

H. QTDEM5

Figure 4. —Continued
Methods 15

EXPLANATION 

Freshwater zone

Saltwater zone (none)

Clay zone

Zone demarcation (shallow, intermediate, deep) 

Contact

Time domain electromagnetic conductivity

0 1,200

50

100

150

200

250

300

350

400

450

500

550

600

millisiemens per meter

0

Depth, in feet below land surface

 Lloyd

aquifer

Raritan

 clay

Upper Glacial

 aquifer

 Freshwater

 

Shallow

Intermediate

Deep

I. QTDEM6

Figure 4. —Continued
16 Use of TDEM Soundings and Borehole EM Induction Logs To Delineate the Freshwater/Saltwater Interface

On Long Island, the EM conductivity log responses for 

brackish- to saltwater-saturated materials are tens to hundreds 

of times greater than the responses associated with lithologic 

changes in the regional sediments (fig. 5) (Stumm, 1993; 

Stumm and Como, 2017). EM conductivity logs were used 

to delineate saltwater intrusion in other studies in Florida and 

California (Paillet and others, 1999; Hanson, 2003). In the 

California study, a nonlinear relation between bulk EM resistivity and pore-fluid conductance was determined (Land and 

others, 2004). On Long Island, the aquifers and groundwater 

are highly resistive; therefore, EM conductivity log response 

is very sensitive to slight increases in groundwater conductivity caused by increased dissolved solids (Mack, 1993; Stumm, 

1993; Stumm and Como, 2017). Chu and Stumm (1995) used 

EM conductivity logs to delineate the freshwater/saltwater 

interface in selected areas of Kings and Queens Counties. The 

relation between EM conductivity log response and chloride concentration were used to allow the conversion of EM 

conductivity log response in wells where direct chloride water 

sampling to determine chloride concentrations in the aquifer 

was not possible (Stumm and Como, 2017).

A linear relation was observed in the data from Long 

Island wells between EM conductivity log response and 

chloride concentration collected from screen zones in the well 

and pore fluid samples obtained from cores of aquifers during 

drilling (filter press; Lusczynski, 1961; Stumm and Como, 

2017). Stumm and Como (2017) developed a least-squares 

regression to relate changes in EM conductivity to changes in 

chloride concentration in groundwater for the coastal plain on 

Long Island, N.Y:

Cl−=25.26(σ)+10.1 (3)

where

Cl− is the chloride concentration, in milligrams 

per liter, in groundwater from screen zones 

of wells and filter press samples from 

cores; and

σ is the peak electromagnetic conductivity, 

millisiemens per meter, from the EM 

conductivity log.

Paine (2003) used borehole EM conductivity logs and 

surface TDEM soundings at a monitoring well and determined 

TDEM soundings produce a good general fit to measured 

borehole conductivities, although their vertical resolution is 

poor in comparison.

Location of the Freshwater/Saltwater 

Interface on Southwestern Long Island

Groundwater on Long Island originates as precipitation 

onto the land surface and recharges the groundwater-flow 

system. Groundwater migrates downward in the central parts 

of Long Island and flows northward and southward toward the 

embayments surrounding Long Island as underflow to downgradient areas. Groundwater flow discharges to streams along 

the north and south shores of Long Island near the coast.

Using open borehole-geophysical logs collected over 

the past 20 years, a cross section was constructed from well 

K 3414 (Kings County) in the west to well N 12894 (Nassau 

County) in the east (figs. 6 and 7, table 2). The gamma logs 

indicate the presence of clay lenses in the Magothy aquifer, 

variations in the amount of fines in the Raritan clay, and the 

weathered bedrock below the Lloyd aquifer (fig. 7). EM conductivity logs at the wells indicate the extent of the saltwater 

wedges along the coast (fig. 7). The EM logs indicate the 

Magothy aquifer is completely intruded with saltwater in the 

western part of the study area, and the intrusion separates into 

two wedges toward the east (fig. 7).

In 2015–17, the USGS collected TDEM soundings and 

EM conductivity logs on southwestern Long Island, N.Y. 

(fig. 1). Using equation 3, chloride concentrations were 

estimated for the upper glacial, Jameco, and Magothy aquifer 

complex and the Lloyd aquifer. These data were integrated 

into two isochlor maps, one for the aquifers above the Raritan 

clay (upper glacial, Jameco, and Magothy) and another for 

the Lloyd aquifer (figs. 8 and 9). An isochlor is a line of equal 

chloride concentration in groundwater. The isochlors represent 

the extent of intruded saltwater at a given concentration. A 

chloride concentration of 5,000 mg/L was considered a definitive indication of saltwater.

Upper Glacial-Jameco-Magothy Aquifer 

Complex Isochlor Map

An isochlor map for the combined upper glacial, Jameco, 

and Magothy aquifers as an undifferentiated sequence (aquifer 

complex) was produced using the processed TDEM soundings 

from 12 sites and EM conductivity logs from 9 wells (fig. 8). 

The thickness of the aquifer complex decreases from east to 

west and from south to north due to the regional dip to the 

southeast in the coastal plain sediments. In addition, the aquifer complex contains substantial and extensive clay lenses that 

result in compartmentalization of saltwater intrusion. Due to 

these features, the aquifer complex above the Raritan clay was 

divided into thirds, a shallow, intermediate, and deep subunit. 

Within each of these subunits, the peak conductivity value was 

converted to a chloride concentration, in milligrams per liter, 

using equation 3.

Three separate wedges of saltwater intrusion were delineated in the aquifer complex. The isochlors suggest the extent 

of the saltwater intrusion in the aquifer complex is variable 

with depth (fig. 8). This study indicated that the variable depth 

of the public-supply pumpage in the Magothy aquifer and 

aquifer heterogeneity produces compartmentalized stresses. 

These stresses create differential rates of saltwater intrusion, 

with saltwater of variable concentrations present at variable 

depths in the aquifer. In the southeastern part of the isochlor 

map (southeastern Nassau County), the deep and intermediate 

parts of the aquifer complex contain freshwater, and the shallow part contains saltwater farther inland (fig. 8). In contrast, 
Location of the Freshwater/Saltwater Interface on Southwestern Long Island 17

50

100

150

200

250

300

350

400

450

500

Depth, in feet below land surface 

Natural gamma

0 200

Electromagnetic induction conductivity

0 600

counts per second

Generalized geology millisiemens per meter

Gray clay and silt

Well sorted sand

and clay lenses

Gray clay and silt

Gray clay

Well sorted sand

and clay lenses

Weathered bedrock

12,700 mg/L

11,200 mg/L

8,730 mg/L

7,630 mg/L

10,100 mg/L

9,750* mg/L

9,230 mg/L

533 mg/L

1,250 mg/L

Well screen

0

All concentrations are chloride, in milligrams per liter (mg/L), from

filter press samples. Asterisk * indicates well screen sample.

Figure 5. Generalized geology, natural gamma log, electromagnetic induction conductivity log, and chloride concentrations in 

observation well N 12506 (U.S. Geological Survey station 404944073392601), Nassau County, New York (modified from Stumm and Como, 

2017). Well location shown on figure 1.
!( !( !( !( !( !( !( 

!( !( G G G G G G G G G G GGNTDEM6 NTDEM4 NTDEM3 QTDEM4 QTDEM1 QTDEM6 QTDEM2 QTDEM3 QTDEM5 NTDEM2 NTDEM1 NTDEM5 Q 3655 K 3414 

N 10620N 13804 N 13682 N 13879 N 13700 N 13559 74°00’ 73°40’ 40°43’ 40°33’ 0 5 

0 5 10 KILOM

KINGS COUNTY QUEENS COUNTY NASSAU COUNTY Long Beach Jones 

K 3613 NEW JERSEY NEW YORK LONG ISLAND Cross section A to A’ EXPLANATION 

A ! Time domain electromagnetic surve

 and identifier Outpost and historical well and iden

Q 3655 NTDEM6 Jones Inlet G( Figure 6. Location of hydrogeologic cross section A to A′ in Kings, Queens, and Nassau Counties, New York.
VERTICAL EXAGGERATION 42X 00 

100200300400500600700800900

1,0001,1001,2001,3001,4001,5001,6001,700

1,800

1,9002,000

0 K 3613 Land surface

N 13879

N 13700

N 13559

Lloyd aquifer

Bedrock

Lloyd aquifer

Raritan clay

Magothy aquifer

Magothy aquifer

A 

Bedrock

EXPLANATION

Gamma log

EM log

Q 3655

Magothy aquifer

Lloy

Raritan clay

K 3414

N 13804

N 13682

Raritan clay

Lloyd aquifer

Bedrock

Bedrock

Lloyd aquifer

Raritan clay

Magot

Natural gamma

250

1,000

00

Electromagnetic induction (EM) conductivity

counts per second

millisiemens per meter

Saltwater

Freshwater

Saltwater

Saltwater

Saltwater

Saltwater

?

?

Freshwater

Freshwater

Freshwater

Long Beach Jones Beach

FEET

NAVD 88

BEND IN SECTION

BEND IN SECTION

BEND IN SECTION

BEND IN SECTION

BEND IN SECTION

Jones Inlet QUEENS COUNTY NASSAU COUNTY KINGS COUNTY 

Freshwater zoneSaltwater zoneClay zone

Figure 7. Hydrogeologic cross section A to A′ with gamma and electromagnetic induction conductivity logs for wells in Kings, Queens, and Nassau Cross-section location shown on figure 6.
20 Use of TDEM Soundings and Borehole EM Induction Logs To Delineate the Freshwater/Saltwater Interface

Table 2. Well location and borehole electromagnetic induction log information.

[Site number refers to U.S. Geological Survey (USGS) site identifier in the National Water Information System (USGS, 2020a). Log dates are given in month/

day/year. Latitude and longitude are given in degrees (°), minutes (′), and seconds (′′). NAD 83, North American Datum of 1983; NAVD 88, North American 

Vertical Datum of 1988; N/A, not applicable; SP, Spontaneous potential; SPR, Single Point Resistance (USGS, 2020b)]

Site number

Well 

identifier 

(fig. 1)

Log date

Latitude 

(NAD 83, unless 

otherwise noted)

Longitude 

(NAD 83, unless 

otherwise noted)

Elevation 

(feet 

NAVD 88)

Link to 

induction 

log data

Link to additional log data

403900073541301 K 3613 06/07/2013 40°38′59′′ 73°54′12′′ 12 N/A Gamma, SP, SPR, and resistivity

403431073581101 K 3414 04/19/1995 40°34′31′′ 

(NAD 27)

73°58′11′′ 

(NAD 27)

7.0 Induction Gamma

403918073494001 Q 3655 04/20/1988 40°39′18′′ 

(NAD 27)

73°49′40′′ 

(NAD 27)

11 Induction Gamma

403649073453901 N 13804 10/26/2015 40°36′49′′ 73°45′39′′ 10 Induction Gamma, SPR, resistivity, and 

conductivity

403517073445601 N 13682 10/26/2015 40°35′17′′ 73°44′56′′ 11 Induction Gamma, SP, SPR, resistivity, 

and conductivity

403505073402201 N 13879 10/26/2015 40°35′05′′ 73°40′22′′ 8.0 Induction Gamma, SPR, resistivity, and 

conductivity

403511073375001 N 13700 01/10/2008 40°35′11′′ 73°37′50′′ 7.0 Induction Gamma, SPR, and resistivity

403507073333801 N 13559 5/7/2006 40°35′07′′ 73°33′38′′ 13 Induction Gamma, SP, SPR, and resistivity

403637073254401 N 12894 10/27/2005 40°36′37′′ 73°25′44′′ 8.0 Induction Gamma, SP, and SPR

in the central part of the saltwater wedge in southwestern 

Nassau and southeastern Queens Counties, all three parts of 

the complex are intruded (fig. 8). In southwestern Queens 

County and parts of Kings County, saltwater seems to intrude 

the deep and intermediate parts farther inland (fig. 8).

Currently (2020), public-supply wells screened in the 

upper glacial and Magothy aquifers in southwestern Nassau 

County are pumping groundwater which previous studies have 

indicated produce cones of depression that have resulted in a 

state of active saltwater intrusion in this part of the study area 

(Perlmutter and Geraghty, 1963; Lusczynski and Swarzenski, 

1966). Ongoing monitoring of the observation network using 

EM conductivity logs indicates active intrusion. Continuation 

of long-term monitoring would be needed to determine the 

rate of saltwater intrusion.

Lloyd Aquifer Isochlor Map

Under ambient conditions, the Lloyd aquifer throughout 

Long Island typically has a chloride concentration of less 

than 10 mg/L (Stumm, 2001). Chloride data in the southwestern part of Long Island from the early 20th century suggests 

elevated chloride concentrations were observed soon after 

pumping of Lloyd aquifer supply wells along the barrier 

islands (Leggette, 1937). The rate of saltwater intrusion and 

concentrations of chloride indicate the freshwater/saltwater 

interface in the Lloyd aquifer was close to the coastline and 

not miles offshore as theorized by previous investigations, 

which questioned observations of elevated (above 250 mg/L) 

chloride concentrations observation wells screened in the 

Lloyd aquifer on southwestern Long Island and attributed 

these data to leaking casings (Buxton and others, 1981; 

Terraciano, 1997). Recent reanalysis of open borehole geophysical logs from test well Q 3655, drilled in 1989 along the 

coast in southern Queens County, indicates complete saltwater 

intrusion in the Magothy aquifer and nearly complete intrusion of the Lloyd aquifer (figs. 1 and 10). Using equation 3, 

the estimated chloride concentration in the Lloyd aquifer at 

this site was about 15,000 mg/L. Charles (2016) used available 

chloride water-quality data to delineate the chloride concentrations in the Lloyd aquifer in the Long Island area. Using 

the interpreted chloride concentration for the Lloyd aquifer 

at Q 3655, it appears the 5,000 mg/L isochlor is much farther 

inland than had previously been mapped (Charles, 2016) 

(fig. 9). The freshwater/saltwater interface in the Lloyd aquifer 

had to have been much closer to the coast of Long Island during predevelopment than previously theorized by other studies 

due to the rapid rates of saltwater intrusion and high concentrations of chloride.

Currently (2020), several new deep observation wells are 

being drilled on southwestern Long Island. These new wells 

will provide additional information on the extent of saltwater intrusion in this part of Long Island and provide salinity 

ground truth in areas where only TDEM soundings were 

measured. Long-term monitoring of the deep observation-well 

network would provide valuable information on the potential 

for saltwater intrusion in the Lloyd aquifer on a barrier island 

(Long Beach) along the southern part of Nassau County that 

relies upon the Lloyd aquifer for their sole source of potable water.
74°00’ 73°40’

40°43’

40°33’

0 5 

0 5 10 KILOME

KINGS COUNTY

QUEENS COUNTY

NASSAU COUNTY

Long Beach

NEW JERSEY NEW YORK

LONG ISLAND 

Time domain electromagnetic survey locat and identifier

Outpost and historical well and identifier Q 3655

NTDEM6

?

NTDEM4

NTDEM2

NTDEM6 NTDEM3

NTDEM5

QTDEM4 QTDEM2

QTDEM3

QTDEM1

QTDEM6

QTDEM5

NTDEM1

N 13879 N 13559

N 13700

N 10620

N 13682

N 13804

Q 3655

K 3414

K 3613

Upper glacial, Jameco, and Magothy aquif

ShallowIntermediateDeep 

EXPLANATION

Figure 8. The 5,000 milligram per liter chloride concentration in the upper glacial, Jameco, and Magothy aquifer complex and delineated in the shalldeep parts of the aquifer complex in Kings, Queens, and Nassau Counties, New York.
QUEENS COUNTY NASSAU COUNTY KINGS COUNTY 74°00’ 73°45' 73°30' 40°50' 40°45' 40°40' 40°35' 

0 5 

0 5 1

ATLANTIC OC

Base digital data from The National Atlas 1:100,000 series Projection: State Plane New York, Long Island North American Datum of 1927 EXPLANATION 5,000 milligram per liter 

 concentration isochlo

Lloyd aquifer. Dashed 

Lloyd outpost wells Q 3655 Long Beach Jones Beach NEW JERSEY NEW YORK BRONX COUNTY NEW YORK COUNTY LONG ISLAND 

Figure 9. The 5,000 milligram per liter chloride concentration in the Lloyd aquifer in Kings, Queens, and Nassau Counties, New York.
Location of the Freshwater/Saltwater Interface on Southwestern Long Island 23

Gamma

0 150

Induction resistivity

0 200

Electromagnetic induction conductivity

0 millisiemens per meter 1,000

50

100

150

200

250

300

350

400

450

500

550

600

650

Lloyd 

aquifer Screen

zone

0

Raritan clay

Magothy

aquifer

Upper glacial

aquifer Freshwater

Saltwater

Saltwater

Freshwater

Bedrock

counts per second

Depth, in feet below land surface

700

* Estimated chloride concentration, in milligrams 

per liter, from the electromagnetic induction log Saltwater-freshwater interface

Hydrogeologic contact

20,000*

Q 3655

Screen

zone

15,000*

Jameco?

aquifer

Gardiners clay

Saltwater

Gardiners clay

Ohm-meters

EXPLANATION

Freshwater zone

Saltwater zone

Clay zone

Figure 10. Gamma, induction resistivity, and calculated electromagnetic conductivity logs at test well Q 3655 showing the extent of 

saltwater intrusion in the Magothy and Lloyd aquifers, Queens County, New York. Well location shown on figure 1.
24 Use of TDEM Soundings and Borehole EM Induction Logs To Delineate the Freshwater/Saltwater Interface

Summary

The U.S. Geological Survey (USGS), in cooperation 

with the New York State Department of Environmental 

Conservation, used surface and borehole-geophysical methods 

to estimate the location of the freshwater/saltwater interface on 

southwestern Long Island, New York. These electromagnetic 

methods provided conductivity values of the aquifers underlying the study area. These conductivity values were used in an 

equation that relates conductivity collected by electromagnetic 

induction (EM) methods to estimated chloride concentrations 

in aquifers on Long Island.

Early to late 20th century public-supply pumpage from 

the upper glacial, Magothy, and Lloyd aquifers in Nassau, 

Queens, and Kings Counties within the study area, produced 

large cones of depression and extensive saltwater intrusion 

soon after large-scale pumping began. This indicates that the 

freshwater/saltwater interface was likely at the coastline at that 

time and not miles offshore as previous research theorized.

In 2015–17, the USGS collected time domain electromagnetic (TDEM) soundings at 12 locations and analyzed 

EM conductivity logs from 9 observation wells on southwestern Long Island (Nassau, Queens, and Kings Counties) to 

delineate the freshwater/saltwater interface on southwestern 

Long Island, N.Y.

An analysis of the TDEM soundings, combined with 

the EM conductivity logs, indicate saltwater has intruded 

a large part of the study area in the upper glacial, Jameco, 

and Magothy aquifers. The aquifers contain substantial and 

extensive clay lenses, which create compartmentalized saltwater intrusions within the aquifer complex. To better map 

the intrusions the aquifer complex above the Raritan clay was 

divided into thirds containing a shallow, intermediate, and 

deep saltwater wedge.

Public-supply pumpage in the Magothy aquifer tends to 

produce compartmentalized stresses that translate to differential rates of saltwater intrusion resulting in saltwater being 

found at variable depths in the aquifer. Three separate wedges, 

shallow, intermediate, and deep, of saltwater intrusion were 

delineated in the upper glacial, Jameco, and Magothy aquifer 

complex. In southeastern Nassau County, the deep and intermediate parts of the aquifer complex contain freshwater, and 

the shallow part contains saltwater farther inland. In contrast, 

in the central part of southwestern Nassau and southeastern 

Queens Counties, all three parts of the aquifer complex are 

intruded. In southwestern Queens County and parts of Kings 

County, saltwater seems to intrude farther into the deep and 

intermediate parts of the aquifer complex. Monitoring of the 

observation network using EM conductivity logs is critical in determining the status and rate of change of saltwater 

intrusion.

Under ambient conditions, the Lloyd aquifer throughout 

Long Island typically has a chloride concentration less than 

10 milligrams per liter. Chloride data in the southwestern part 

of Long Island from the early 20th century suggest elevated 

chloride concentrations were observed soon after pumping 

of supply wells along the barrier islands. The rapid change in 

chloride concentration in response to pumping indicates that 

the freshwater/saltwater interface in the Lloyd aquifer was 

close to the coastline and not miles offshore, as theorized by 

previous investigations. Recent reanalysis of open boreholegeophysical logs from test well Q 3655 drilled in 1989 along 

the coast in southern Queens County indicates complete intrusion of saltwater in the Magothy aquifer and nearly complete 

intrusion of the Lloyd aquifer. Using a linear least squares 

equation, the estimated chloride concentration in the Lloyd 

aquifer at this site was about 15,000 milligrams per liter. The 

borehole geophysical logs from this well provides definitive 

proof of extensive saltwater intrusion of the Lloyd aquifer 

along the southwestern shore of Long Island indicating that 

the freshwater/saltwater interface in the Lloyd aquifer was at 

the coastline under predevelopment conditions in this part of 

Long Island.

References Cited

Archie, G.E., 1942, The electrical resistivity log as an aid 

in determining some reservoir characteristics: Petroleum 

Transactions of the AIME, v. 146, no. 1, p. 54–62. [Also 

available at https://doi.org/10.2118/942054-G.]

Auken, E., Christiansen, A.V., Jacobsen, L.H., and Sørensen, 

K.I., 2008, A resolution study of buried valleys using laterally constrained inversion of TEM data: Journal of Applied 

Geophysics, v. 65, no. 1, p. 10–20, accessed July 2018 at 

https://doi.org/10.1016/j.jappgeo.2008.03.003.

Auken, E., Christiansen, A.V., Kirkegaard, C., Fiandaca, G., 

Schamper, C., Behroozmand, A.A., Binley, A., Nielsen, 

E., Effersø, F., Christensen, N.B., Sørensen, K., Foged, N., 

and Vignoli, G., 2015, An overview of a highly versatile 

forward and stable inverse algorithm for airborne, groundbased and borehole electromagnetic and electric data: 

Exploration Geophysics, v. 46, no. 3, p. 223–235. [Also 

available at https://doi.org/10.1071/EG13097.]

Buxton, H.T., Soren, J., Posner, A., and Shernoff, P.K., 1981, 

Reconnaissance of the ground-water resources of Kings and 

Queens Counties, New York: U.S. Geological Survey OpenFile Report 81–1186, 64 p. [Also available at https://doi.org/

10.3133/ofr811186.]

Charles, E.G., 2016, Regional chloride distribution in the 

Northern Atlantic Coastal Plain aquifer system from Long 

Island, New York, to North Carolina: U.S. Geological 

Survey Scientific Investigations Report 2016–5034, 

37 p., accessed April 13, 2020, at https://doi.org/10.3133/

sir20165034.
References Cited 25

Christiansen, A.V., and Auken, E., 2012, A global measure for 

depth of investigation: Geophysics, v. 77, no. 4, p. WB171–

WB177, accessed July 2018 at https://doi.org/10.1190/

geo2011-0393.1.

Christiansen, A.V., Auken, E., and Sørensen, K., 2006, The 

transient electromagnetic method, in Kirsch, R., ed., 

Groundwater geophysics: Berlin, Heidelberg, Springer, 

p. 179–225, accessed April 12, 2020, at https://doi.org/

10.1007/3-540-29387-6_.

Chu, A., and Stumm, F., 1995, Delineation of the saltwaterfreshwater interface at selected locations in Kings and 

Queens Counties, Long Island, New York, through use of 

borehole geophysical techniques, in Geology of Long Island 

and metropolitan New York: Stony Brook, N.Y., Programs 

with Abstracts, Apri1 22, 1995, p. 21–30.

Como, M.D., Zuck, M.A., and Stumm, F., 2020, Time domain 

electromagnetic surveys collected to estimate the extent 

of saltwater intrusion in Nassau and Queens County, New 

York, October–November 2017: U.S. Geological Survey 

data release, https://doi.org/10.5066/P90B6OTX.

Fitterman, D.V., and Stewart, M.T., 1986, Transient electromagnetic sounding for groundwater: University of South 

Florida Scholar Commons, Geology Faculty Publications 

1, 12 p., accessed April 14, 2020, at https://doi.org/10.1190/

1.1442158.

Franke, O.L., and McClymonds, N.E., 1972, Summary of 

the hydrologic situation on Long Island, New York, as a 

guide to water-management alternatives: U.S. Geological 

Survey Professional Paper 627–F, 59 p. [Also available at 

https://doi.org/10.3133/pp627F.]

Garber, M., 1986, Geohydrology of the Lloyd aquifer, Long 

Island, New York: U.S. Geological Survey Water-Resources 

Investigations Report 85–4159, 40 p. [Also available at 

https://pubs.usgs.gov/wri/1985/4159/report.pdf.]

Hanson, R.T., 2003, Geohydrologic framework of recharge 

and seawater intrusion in the Pajaro Valley, Santa Cruz and 

Monterey Counties, California: U.S. Geological Survey 

Water-Resources Investigations Report 03–4096, 100 p., 

accessed April 15, 2020, at https://pubs.usgs.gov/wri/

wri034096.

Heisig, P.M., and Prince, K.R., 1993, Characteristics of 

a ground-water plume derived from artificial recharge 

with reclaimed wastewater at East Meadow, Long 

Island, New York: U.S. Geological Survey WaterResources Investigations Report 91–4118, 58 p. [Also 

available at https://pdfs.semanticscholar.org/abae/

41c1770dfc01a2cbb514bb97268426e7e2e2.pdf.]

Johnson, A.H., and Waterman, W.G., 1952, Withdrawal 

of ground water on Long Island, New York: New York 

State Water Power and Control Commission Bulletin 

GW–28, 13 p.

Keys, W.S., 1990, Borehole geophysics applied to waterresources investigations: U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 2, 

chap. E2, 165 p. [Also available at https://pubs.usgs.gov/

twri/twri2-e2/.]

Keys, W.S., and MacCary, L.M., 1971, Application of 

borehole geophysics to water-resources investigations: 

U.S. Geological Survey Techniques of Water-Resources 

Investigations, book 2, chap. E1, 134 p. [Also available at 

https://pubs.usgs.gov/twri/twri2-e1/.]

Land, M., Reichard, E.G., Crawford, S.M., Everett, R.R., 

Newhouse, M.W., and Williams, C.F., 2004, Groundwater quality of coastal aquifer systems in the West Coast 

Basin, Los Angeles County, California, 1999–2002. 

U.S. Geological Survey Scientific Investigations 

Report 2004–5067, accessed August 21, 2017, at 

https://pubs.usgs.gov/sir/2004/5067.

Leggette, R.M., 1937, Record of wells in Kings County, 

N.Y.: Albany, New York State Water Power and Control 

Commission, Bulletin GW–3, p. 175, 1 pl.

Lusczynski, N.J., 1952, The recovery of ground-water 

levels in Brooklyn, New York, from 1947 to 1950: 

U.S. Geological Survey Circular 167, 29 p. [Also available 

at https://doi.org/10.3133/cir167.]

Lusczynski, N.J., 1961, Filter-press method of extracting 

sample for chloride analysis: U.S. Geological Survey WaterSupply Paper 1544–A, 8 p.

Lusczynski, N.J., and Swarzenski, W.V., 1966, Salt-water 

encroachment m southern Nassau and southeastern Queens 

Counties, Long Island, New York: U.S. Geological Survey 

Water-Supply Paper 1613–F, 76 p., 5 pls. [Also available at 

https://pubs.usgs.gov/wsp/1613f/report.pdf.]

Masterson, J.P., and Pope, J.P., Monti, J., Jr., Nardi, M.R., 

Finkelstein, J.S., and McCoy, K.J., 2015, Hydrogeology 

and hydrologic conditions of the Northern Atlantic Coastal 

Plain aquifer system from Long Island, New York, to North 

Carolina (ver. 1.1, September 2015): U.S. Geological 

Survey Scientific Investigations Report 2013–5133, 76 p. 

[Also available at https://doi.org/10.3133/sir20135133.]

McNeill, J.D., 1980, Electrical conductivity of soil 

and rocks: Mississauga, Ontario, Canada, Geonics 

Limited, Technical Note TN–5, 22 p. [Also available at 

http://www.geonics.com/pdfs/technicalnotes/tn5.pdf.]
26 Use of TDEM Soundings and Borehole EM Induction Logs To Delineate the Freshwater/Saltwater Interface

McNeill, J.D., 1986, Geonics EM39 borehole conductivity 

meter theory of operation: Mississauga, Ontario, Canada, 

Geonics Limited, Technical Note TN–20, 18 p. [Also 

available at http://www.geonics.com/pdfs/technicalnotes/

tn20.pdf.]

McNeill, J.D., 1994, Principles and application of time 

domain electromagnetic techniques for resistivity sounding: Mississauga, Ontario, Canada, Geonics 

Limited Technical Note TN–27, 16 p. [Also available at 

http://www.geonics.com/pdfs/technicalnotes/tn27.pdf.]

Mack, T.J., 1993, Detection of contaminant plumes by borehole geophysical logging: Ground Water Monitoring and 

Remediation, v. 13, no. 1, p. 107–114. [Also available at 

https://doi.org/10.1111/j.1745-6592.1993.tb00427.x.]

Metzger, L.F., and Izbicki, J.A., 2013, Electromagneticinduction logging to monitor changing chloride concentrations: Ground Water, v. 51, no. 1, p. 108–121. [Also available at https://doi.org/10.1111/j.1745-6584.2012.00944.x.]

North Carolina Division of Water Resources, 2006, Time 

domain electromagnetic geophysics: North Carolina 

Department of Environmental Quality web page, accessed 

April 19, 2020, at https://www.ncwater.org/Education_and_

Technical_Assistance/Ground_Water/TDEM/.

Paillet, F., Hite, L., and Carlson, M., 1999, Integrating surface 

and borehole geophysics in ground water studies—An 

example using electromagnetic soundings in South Florida: 

Journal of Environmental & Engineering Geophysics, v. 4, 

no. 1, p. 1–79. [Also available at https://doi.org/10.4133/

JEEG4.1.45.]

Paine, J.G., 2003, Determining salinization extent, identifying 

salinity sources, and estimating chloride mass using surface, 

borehole, and airborne electromagnetic induction methods: 

Water Resources Research, v. 39, no. 3, 10 p. [Also available at https://doi.org/10.1029/2001WR000710.]

Payne, J.D., Kress, W.H., Shah, S.D., Stefanov, J.E., Smith, 

B.A., and Hunt, B.B., 2007, Geophysical delineation of 

the freshwater/saline-water transition zone in the Barton 

Springs segment of the Edwards aquifer, Travis and Hays 

Counties, Texas, September 2006: U.S. Geological Survey 

Scientific Investigations Report 2007–5244, 21 p., 6 apps. 

[Also available at https://pubs.usgs.gov/sir/2007/5244/.]

Payne, J.D., and Teeple, A.P., 2011, Time-domain electromagnetic soundings collected in Dawson County, Nebraska, 

2007–09: U.S. Geological Survey Data Series 581, 46 p. 

[Also available at https://doi.org/10.3133/ds581.]

Perlmutter, N.M., and Crandell, H.C., 1959, Geology and 

ground-water supplies of the south-shore beaches of Long 

Island, N.Y: Annals of the New York Academy of Sciences, 

v. 80, no. 4, p. 1060–1076. [Also available at https://doi.org/

10.1111/j.1749-6632.1959.tb49280.x.]

Perlmutter, N.M., and Geraghty, J.J., 1963, Geology and 

ground-water conditions in southern Nassau and southeastern Queens Counties, Long Island, N.Y: U.S. Geological 

Survey Water-Supply Paper 1613–A, 212 p. [Also available 

at https://pubs.usgs.gov/wsp/1613a/report.pdf.]

Sandford, J.H., 1938, Report on the geology and hydrology 

of Kings and Queens Counties, Long Island: New York 

State Water Power and Control Commission, Bulletin 

GW–7, 68 p. [Also available at https://archive.org/details/

usgswaterresourcesnewyork-bull_gw_7.]

Serra, O., 1984, Fundamentals of well-log interpretation: New York, N.Y., Elsevier, 423 p. [Also available at 

https://www.academia.edu/10053890/Fundamentals_of_

Well_Log_Interpretation_OSerra.]

Smolensky, D.A., Buxton, H.T., and Shernoff, P.K., 1990, 

Hydrologic framework of Long Island, New York: 

U.S. Geological Survey Hydrologic Investigations Atlas 

HA–709, 3 sheets, scale 1:250,000, accessed April 19, 2020, 

at https://doi.org/10.3133/ha709.

Soren, J., 1971, Ground-water and geohydrologic conditions 

in Queens County, Long Island, New York: U.S. Geological 

Survey Water-Supply Paper 2001–A, 39 p. [Also available 

at https://doi.org/10.3133/wsp01A.]

Soren, J., 1978, Subsurface geology and paleogeography of 

Queens County, Long Island, New York: U.S. Geological 

Survey Water-Resources Investigations Report 77–34, 17 p. 

[Also available at https://doi.org/10.3133/wri7734.]

Stewart, M., and Gay, M.C., 1986, Evaluation of transient 

electromagnetic soundings for deep detection of conductive 

fluids: Ground Water, v. 24, no. 3, p. 351–356. [Also available at https://doi.org/10.1111/j.1745-6584.1986.tb01011.x.]

Stumm, F., 1993, Use of focused electromagnetic induction borehole geophysics to delineate the saltwaterfreshwater interface in Great Neck, Long Island, New 

York: Symposium on the Application of Geophysics to 

Engineering and Environmental Problems, v. 2, p. 513–525. 

[Also available at https://doi.org/10.4133/1.2922029.]

Stumm, F., 2001, Hydrogeology and extent of saltwater 

intrusion of the Great Neck Peninsula, Great Neck, Long 

Island, New York: U.S. Geological Survey Water-Resources 

Investigations Report 99–4280, 41 p. [Also available at 

https://pubs.usgs.gov/wri/1999/4280/wri19994280.pdf.]

Stumm, F., and Como, M.D., 2017, Delineation of saltwater intrusion through use of electromagnetic-induction 

logging—A case study in southern Manhattan Island, New 

York: Water (Basel), v. 9, no. 9, p. 631. [Also available at 

https://doi.org/10.3390/w9090631.]
References Cited 27

Stumm, F., Lange, A.D., and Candela, J.L., 2002, 

Hydrogeology and extent of saltwater intrusion 

on Manhasset Neck, Nassau County, New York: 

U.S. Geological Survey Water-Resources Investigations 

Report 2000–4193, 42 p., accessed April 19, 2020, at 

https://doi.org/10.3133/wri004193.

Stumm, F., Lange, A.D., and Candela, J.L., 2004, 

Hydrogeology and extent of saltwater intrusion in the north-

ern part of the town of Oyster Bay, Nassau County, New 

York—1995–98: U.S. Geological Survey Water-Resources 

Investigations Report 2003–4288, 55 p., accessed April 19, 

2020, at https://doi.org/10.3133/wri034288.

Suter, R., de Laguna, W., and Perlmutter, N.M., 1949, 

Mapping of geologic formations and aquifers of Long 

Island, New York: Albany, N.Y., New York State Water 

Power and Control Commission, Bulletin GW–18, 

212 p. [Also available at https://archive.org/details/

usgswaterresourcesnewyork-bull_gw_18/mode/2up.]

Taylor, K.C., Hess, J.W., and Mazzela, A., 1989, Field evalu-

ation of a slim-hole borehole induction tool: Ground Water 

Monitoring and Remediation, v. 9, no. 1, p. 100–104. 

[Also available at https://doi.org/10.1111/j.1745-

6592.1989.tb01125.x.]

Terraciano, S.A., 1997, Position of the freshwater/saltwa-

ter interface in southeastern Queens and southwestern 

Nassau Counties, Long Island, New York, 1987–88: 

U.S. Geological Survey Open-File Report 96–456, 17 p. 

[Also available at https://doi.org/10.3133/ofr96456.]

U.S. Army Corps of Engineers, 1995, Geophysical explora-

tion for engineering and environmental investigations: 

Engineer Manual 1110–1–1802, chap. 4, 57 p. [Also avail-

able at https://www.publications.usace.army.mil/Portals/76/

Publications/EngineerManuals/EM_1110-1-1802.pdf.]

U.S. Geological Survey [USGS], 2020a, USGS water data 

for the Nation: U.S. Geological Survey National Water 

Information System database, accessed April 2020 at 

https://doi.org/10.5066/F7P55KJN.

U.S. Geological Survey [USGS], 2020b, USGS GeoLog 

Locator: U.S. Geological Survey database, accessed 

June 28, 2020, at https://doi.org/10.5066/F7X63KT0.

Williams, J.H., and Lane, J.W., 1998, Advances in bore-

hole geophysics for ground-water investigations: 

U.S. Geological Survey Fact Sheet 002–98, 4 p. [Also avail-

able at https://doi.org/10.3133/fs00298.]

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