U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 03-4315

Occurrence of Arsenic in Ground Water of 

Suffolk County, New York, 1997-2002

In cooperation with

Suffolk County Water Authority
Cover. Brushs Creek, Laurel, New York, © Joseph R. Melanson of skypic.com
Occurrence of Arsenic in Ground Water of 

Suffolk County, New York, 1997-2002 

By Richard A. Cartwright

In cooperation with

Suffolk County Water Authority

Water-Resources Investigations Report 03-4315

U.S. Department of the Interior

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

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For additional information about this report write to:

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reproduce any copyrighted materials contained within this report.
iii

Contents

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

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

Approach ............................................................................................................................................... 2

Purpose and Scope ............................................................................................................................. 3

Acknowledgments .............................................................................................................................. 3

Description of Study Area .................................................................................................................. 3

Data and Sampling ....................................................................................................................................... 3

Data from 1997 through 2001 .............................................................................................................. 3

SCWA Data .................................................................................................................................. 3

SCDHS Data ................................................................................................................................. 4

Sampling in 2002 .................................................................................................................................. 4

Occurrence of Arsenic in Ground Water .................................................................................................. 4

Potential Sources of Arsenic in Suffolk County .............................................................................. 8

Concentrations and Distribution ....................................................................................................... 8

Summary and Conclusions ........................................................................................................................ 10

References Cited ....................................................................................................................................... 10

Figures

1. Map of Suffolk County, N.Y., showing locations of wells for which arsenic-concentration 

data from 1997 to 2002 are presented. .................................................................................................. 2

2. Map of Suffolk County, N.Y., showing land-use categories, locations of wells with reported 

arsenic concentrations exceeding 10 micrograms per liter, and locations of wells sampled 

during June through August 2002 .......................................................................................................... 7

Tables

1. Suffolk County Water Authority (SCWA) wells with detectable arsenic concentrations, 

January 2000 through May 2002 ............................................................................................................ 4

2. Arsenic concentrations in ground-water samples collected by Suffolk County Department 

of Health Services, April 1997 through July 2001 ................................................................................ 5

3. Arsenic concentrations at selected wells in Suffolk County, N.Y., 1997 through 2002 .................. 9
iv

CONVERSION FACTORS, ABBREVIATIONS, AND VERTICAL DATUM

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

 Other Abbreviations Used in this Report

 micrograms per liter (μg/L)

Vertical coordinate information is referenced to the National Geodetic Vertical 

Datum of 1929 (NGVD of 1929).
Occurrence of Arsenic in Ground Water of Suffolk County, 

New York, 1997-2002 

By Richard A. Cartwright

Abstract

 Water-quality data from public and private drinkingwater supply wells that were sampled from October 1997 

through March 2001 in Suffolk County, New York were 

evaluated to define the occurrence and concentrations of 

arsenic throughout the county. The data bases of the Suffolk 

County Water Authority (SCWA) and the Suffolk County 

Department of Health Services (SCDHS) included 14 wells 

at which arsenic concentrations approached or exceeded 

the 2002 U.S. Environmental Protection Agency (USEPA) 

drinking-water guideline of 10 micrograms per liter (μg/L). 

As a followup, 19 wells were sampled from June through 

August 2002; 7 were wells previously reported to have had 

high arsenic concentrations; 7 were near other wells reported 

to have high concentrations, and the remaining 5 were in areas 

where detectable concentrations of arsenic were suspected. 

Arsenic concentrations near 10 μg/L were detected at only 2 of 

the 19 wells sampled; arsenic concentrations in samples from 

the remaining 17 wells were reported as less than the USGS 

Central Laboratory reporting limits of 2 μg/L or 4 μg/L.

 The elevated concentrations previously reported 

(1997 through 2001) for at least three of the wells were due 

to incompletely flushed carbon filters in the supply-well 

system or were typographical errors. A decrease in arsenic 

concentration was indicated at six of the seven resampled 

wells; no reasons are apparent. Arsenic concentrations in 

ground water that exceed drinking-water guidelines were 

found only at one site. On the basis of limited sampling data 

available for this study, the concentrations of arsenic above 

drinking-water standards (10 μg/L) do not appear to indicate a 

countywide problem with regards to arsenic concentrations in 

ground water.

Introduction

Long-term exposure to arsenic in drinking water has 

been linked to cancer of the bladder, lungs, skin, kidney, 

nasal passages, liver and prostate; other effects of arsenic 

ingestion include cardiovascular, pulmonary, immunological, 

neurological, and endocrine disorders (National 

Research Council, 1999). On February 22, 2002, the U.S. 

Environmental Protection Agency (USEPA) set a new 

standard for arsenic in drinking water to reduce public health 

risks; this new standard lowered the maximum concentration 

limit in drinking water from 50 μg/L to 10 μg/L. Community 

water systems are required to meet this new standard by 

January 23, 2006 (U.S. Environmental Protection Agency, 

2001b). The imposition of the new standard, and concern 

over potential health risks related to arsenic in drinking 

water, prompted reevaluations of public-water supplies across 

the nation. 

An evaluation was conducted by the U.S. Geological 

Survey (USGS) to provide an overview of arsenic 

concentrations in ground water throughout the United States 

(Welch and others, 2000). Results indicated that 24 percent of 

the counties from which data were available reported arsenic 

concentrations exceeding 10 μg/L in at least 10 percent of 

the samples collected. The new USEPA standard for arsenic 

in drinking water, and results from Welch and others (2000), 

prompted an investigation by the USGS on Long Island, N.Y., 

in cooperation with Suffolk County’s major water supplier, 

the Suffolk County Water Authority (SCWA), to document 

the occurrence and concentrations of arsenic in ground water 

throughout the county (fig. 1).

Long Island’s ground-water system has been designated 

by the USEPA as a sole-source aquifer (SSA), which is 

defined as an aquifer that supplies at least 50 percent of the 

drinking water consumed in the overlying area and that, if 

contaminated, would create a significant hazard to public 

health (http://www.epa.gov/safewater/swp/ssa.html; accessed 

5/21/03). Designation of Long Island’s ground-water system 

as a sole-source aquifer requires that the USEPA review all 

proposed projects within the county that are receiving federal 

financial assistance; this review is intended to ensure that the 

projects do not endanger the water source. The designation 

of Long Island’s ground-water system as a SSA is further 

justification for determining arsenic concentrations in ground 

water, in addition to the new USEPA arsenic standard, and 

results from Welch and others (2000).
Approach

An inventory of data on arsenic concentrations in ground 

water from 1997 through 2001 was made from data bases of 

the USGS, SCWA, SCDHS, the New York State Department 

of Environmental Conservation (NYSDEC), and the Cornell 

Cooperative Extension of Suffolk County to (1) define the 

occurrence of arsenic detections, and (2) determine whether 

further study was warranted. Most of the arsenic-concentration 

data were found in the SCDHS and SCWA data bases; 

together these data bases provided a thorough areal coverage 

of Suffolk County. 

Nineteen public and private drinking water-supply wells 

were selected for sampling and analysis. Fourteen of the wells 

had a reported history of arsenic concentrations close to, or 

exceeding, 10 μg/L. Seven of these wells were sampled; the 

other seven were no longer available for resampling; therefore, 

seven nearby wells were sampled. The remaining five wells 

were in areas suspected to have potential for elevated arsenic 

concentrations on the basis of their down-gradient position 

in relation to other arsenic-contaminated wells or agricultural 

areas where past arsenic use was likely.

Figure 1. Locations of wells for which 1997-2002 arsenic-concentration data are presented.

New York

Location map

Suffolk

County

Long Island

0 10 20 MILES

0 10 20 KILOMETERS

Nassau County

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404549072583401 404812072541301

404923073122401

405359073111201

410049072300801

41004972291401

410009072175001

405652072142501

405235072254001

410406071523001

410406071523101 LONG ISLAND SOUND

ATLANTIC OCEAN

410811072181701

�����������

SAMPLED WELLS

SAMPLES COLLECTED AND ANALYZED BY SCWA FROM

JANUARY 2000 through MAY 2002 at which arsenic

concentration was equal ro or greater than 1 �g/L (514 wells)

SAMPLES COLLECTED AND ANALYZED BY SDCHS

FROM OCOTBER 1997 through APRIL 2001 at which arsenic

concentration exceeded 2 �g/L

WELL AT WHICH SCWA OR SCDHS ARSENIC

CONCENTRATION DATA EXCEEDED 10 �g/L

ABBREVIATIONS

SCDHS - Suffolk County Department of Health Services

SCWA - Suffolk County Water Authority

�g/L - micrograms per liter

Base from U.S. Geological Survey digital data, 1:24,000,

Universal Transverse Mercator projection, NAD27, Zone 18

2 Occurrence of Arsenic in Ground Water of Suffolk County, New York, 1997-2002
Purpose and Scope

This report summarizes arsenic-concentration data 

collected from October 1997 through March 2001 from public 

and private supply wells in Suffolk County. These data were 

compiled from several data bases and then compared with data 

from 19 public and private drinking-water supply wells that 

were sampled from June through August 2002. On the basis 

of the information compiled and the sampling performed in 

2002, an assessment was made of the occurrence of arsenic 

in ground water in Suffolk County from 1997 through 2002. 

Analytical data are presented in three tables. Well locations 

and associated land uses are shown in maps.

Acknowledgments 

 Thanks are extended to Bob Murray, Karen Randazzo 

and Kevin Durk of the SCWA for cooperation during the 

sampling of public-supply wells and for providing historical 

water-quality data. The author also thanks Paul Ponturo of the 

SCDHS for providing historical water-quality data for private, 

community and non-community well samples.

Description of Study Area

Suffolk County occupies the eastern three-quarters of 

Long Island, N.Y. (fig.1) and extends about 85 miles eastward 

from the eastern boundary of Nassau County. The aquifer 

system beneath Suffolk County consists of a sequence of 

unconsolidated deposits comprising three aquifers that overlie 

a southeastward-dipping bedrock surface. Sediment thickness 

ranges from about 500 feet in the northwest to almost 2,000 

feet beneath the barrier beaches in southwestern Suffolk 

County (Smolensky and others, 1989). The uppermost aquifer 

is the Pleistocene-aged upper glacial aquifer, which is about 

700 feet thick in the northern half of the island and generally 

thins to about 100 feet thick in the south. It is found within 

the upper Pleistocene deposits, which consist primarily of 

two terminal moraines, outwash, glaciolacustrine deposits, 

and marine clay on the extreme south shore (Pluhowski and 

Kantrowitz, 1964). Till, an unsorted mixture of clay, sand, 

gravel and boulders, is found mostly along the north shore and 

in the moraines. Outwash deposits that consist of fine to very 

coarse quartzose sand and gravel are found between and south 

of the moraines. Glaciolacustrine deposits that consist of silt, 

clay, and some sand and gravel are found mostly in central 

and eastern Suffolk County. Most private wells in the county 

are completed in the upper glacial aquifer. 

Below the upper glacial aquifer is the Cretaceous-aged 

Magothy aquifer, which ranges in thickness from 0 in extreme 

northwestern Suffolk County to more than 1,000 feet in the 

south and extends as deep as 1,200 feet below land surface 

(Smolensky and others, 1989). The Magothy aquifer is within 

the Magothy Formation and Matawan Group, undifferentiated, 

and it consists of fluviodeltaic sediments deposited in a coastal 

plain environment. Sediments in the Magothy aquifer include 

fine to medium sand, clayey in part, with interbedded lenses 

and layers of coarse sand and sandy and solid clay. Gravel 

is common in the basal 50 to 200 feet (McClymonds and 

Franke, 1972). The Magothy aquifer is generally less affected 

by surface contamination than the upper glacial aquifer and is 

more commonly tapped by public-water suppliers.

The deepest aquifer (Lloyd aquifer) is part of the 

Cretaceous-aged Raritan Formation and is separated from the 

Magothy aquifer by the Raritan confining unit. There are very 

few public-supply wells that penetrate the Lloyd aquifer. It is 

not discussed further in this report.

Data and Sampling

Data bases of the USGS, SCWA, SCDHS, New York 

State Department of Environmental Conservation (NYSDEC), 

and the Cornell Cooperative Extension of Suffolk County 

were reviewed for arsenic records from 1997-2001; the 

results prompted a reconnaissance and resampling in 2002. 

A discussion of SCWA and SCDHS sampling and analyzing 

techniques is not included in this report because these data are 

not being used to compare with USGS-collected data; they are 

only used to aid in selecting sampling sites.

Data from 1997 through 2001

Most of the reported detections of arsenic in ground 

water were found in the SCDHS data base, probably because 

SCDHS often collects water-quality data from areas with 

a history or a potential of contamination, whereas SCWA 

installs production wells in areas that are likely to yield 

uncontaminated water. Any SCWA production well that 

contains unacceptable concentrations of arsenic (or any other 

contaminant) is taken out of service until the situation is 

remedied; this, in effect, minimizes the number of arsenic 

detections in the SCWA data base.

Locations of wells from which arsenic data from 1997 

through 2002 were available are shown in figure 1. Only wells 

at which concentrations exceeded 2 μg/L (from SCDHS’s 

supply-well data) are shown because inclusion of all 12,743 

wells would make the map unreadable. 

SCWA Data

A review of SCWA’s arsenic-concentration data from 

2000 through 2002 for 514 untreated supply-well samples 

identified only 24 wells with arsenic concentrations equal 

to or greater than the detection limit of 1 μg/L, and only two 

that had concentrations near the new USEPA Drinking Water 

Guideline of 10 μg/L (table 1). Locations of SCWA wells with 

arsenic-concentration data are included in figure 1.

Data and Sampling 3
SCDHS Data

A review of data collected from 1997 through 2001 

from the 12,743 supply-well samples in the SCDHS data 

base indicated only 227 samples with arsenic concentrations 

greater than the detection limit of 1 μg/L. Most samples are 

from untreated ground water, but some were collected after 

the water had passed through a carbon filtration system. Many 

samples had arsenic concentrations between 1 μg/L and 2 

μg/L, therefore, only samples with concentrations exceeding 

2 μg/L, and the well locations, are listed in table 2. Arsenicconcentration data were also reviewed from test-well samples 

collected and analyzed by the SCDHS as part of a 1997-

2002 pesticide-occurrence study done in cooperation with 

the NYSDEC. That study also included the public-supplywell data mentioned above. The data are reported in three 

documents resulting from three consecutive investigations 

beginning in 1997; these are available on the NYSDEC web 

page—http://www.dec.state.ny.us/website/dshm/prl/scdhs1c.

htm (accessed 05/21/03), in Bradley and others (2000), and 

in Bradley and others (2002). The following detections (at 

the minimum reporting level of 2 μg/L) were reported for the 

following periods of the pesticide-occurrence study: 

 Most arsenic concentrations were below 10 μg/L. Data 

from these three sources that were not in the SCDHS data base 

had little site information and are not included in this report.

Sampling in 2002

Initially, 14 wells with reported arsenic concentrations 

ranging from 8 μg/L to 93.5 μg/L during 1997 through 2001 

were selected from the SCWA and SCDHS data sets possible 

sample collection in 2002. Some of the homes served by these 

wells had been recently connected to a public water supply, 

and their wells were no longer accessible for sampling. Where 

this occurred, a sample was collected from a neighboring 

site if possible. Additional wells were selected for sampling 

on the basis of their proximity to sites with previously 

reported arsenic concentrations greater than 10 μg/L. These 

included wells downgradient from agricultural land and 

wells downgradient of till deposits in which elevated arsenic 

concentrations were reported.

In all, 19 wells (3 public supply, 12 private, 3 community 

supply, and 1 noncommunity supply) were sampled from 

June through August 2002 and analyzed for arsenic and 

other metals at the USGS National Water Quality Laboratory 

(NWQL) in Denver, Colo., for comparison with the 1997 

through 2001 data. Locations of these 19 wells are plotted 

on the land-use map shown in figure 2. Water samples were 

collected from spigots as close to the wellhead as possible 

and ahead of any filtering systems. Samples were collected 

only after enough water had been run to allow stabilization of 

temperature, pH, and specific conductance, so that the sample 

would be as representative of ground-water quality as possible. 

Samples were collected, treated, and filtered in accordance 

with NWQL protocols, then packed in ice and shipped 

overnight to NWQL for analysis. Arsenic concentration was 

measured by graphite furnace-atomic absorption spectrometry 

(Jones and Garbarino, 1999). Duplicate samples were 

collected and analyzed at two sites by SCWA (termed split 

samples in table 3).

Occurrence of Arsenic in Ground Water

Arsenic is a naturally occurring element in certain 

types of bedrock. Geochemical reactions within fractured 

bedrock can release arsenic into ground water. Arsenic can be 

Table 1. Untreated supply-well samples collected and analyzed by 

Suffolk County Water Authority (SCWA), with detectable arsenic 

concentrations (equal to or greater than 1 μg/L), January 2000 

through May 2002, Suffolk County, New York.

[Sampling was done at 514 supply wells; (mo/d/yr), month/day/year; μg/L, 

micrograms per liter. Boldface indicates arsenic concentration near or above 

10 μg/L.]

Hamlet

Well 

No.

Sample date

(mo/d/yr)

Arsenic 

concentration

(μg/L)

Middle Island S-49606 04/09/02 1

Wading River S-100608 02/07/02 1

West Babylon S-51457 01/28/02 1

Southampton SCUMW1 04/12/00 1.1

Mastic S-71881 02/18/02 1.2

Mastic S-71882 04/19/02 1.2

Lakeland S-77010 07/19/00 1.2

Sag Harbor DIVWM 04/12/00 1.3

Sag Harbor DIVWM4 01/12/00 1.3

West Islip S-101321 02/02/02 *1.4

Bohemia S-67074 05/04/02 1.5

Central Islip S-67197 02/27/02 1.6

Smithtown S-106565 04/15/02 *1.6

Bohemia S-68690 01/21/02 1.6

Oakville S-108161 01/20/02 *1.6

Yaphank S-47438 05/07/02 1.6

Oakville S-94286 01/21/02 *1.8

Southampton S-79293 04/12/00 1.9

Southold S-106416 04/19/00 *3.3

Central Islip S-74505 01/21/02 3.8

Southampton S-106976 04/12/00 *4.2

West Islip S-22389 02/03/02 4.4

Montauk S-76305 02/25/02 8

Montauk S-76304 04/23/02 10.2

*Well not plotted in figure 1 because coordinates are not available

October 1997 through March 1999: 42 detections in 1,867 wells

April 1999 through March 2000: 5 detections in 743 wells

April 2000 through March 2001: 11 detections in 1,099 wells

4 Occurrence of Arsenic in Ground Water of Suffolk County, New York, 1997-2002
Table 2. Arsenic concentrations greater than 2 μg/L in ground-water samples collected from April 

1997 through July 2001 and analyzed by Suffolk County Department of Health Services (SCDHS), 

Suffolk County, New York.

[NCOM, noncommunity supply well (serves schools, hotels, restaurants, etc.); PRIV, private supply well; COMM, 

community supply well (serves small community); (mo/d/ yr), month/day/year; μg/L, micrograms 

per liter. Boldface indicates arsenic concentration near to or exceeding 10 μg/L.] 

Hamlet Well No.

Type of

site 

Sample date

(mo/d/yr)

Arsenic

concentration 

(μg/L)

Amagansett NCOM 8/25/99 3.03

Amagansett PRIV 4/12/99 2.83

Amagansett PRIV 8/5/98 2.39

Amagansett PRIV 1/11/99 2.38

Amagansett PRIV 6/5/00 2.53

Bellport S-14710 COMM 11/15/99 2.07

Bohemia S-67074 COMM 4/21/99 2.4

Brentwood S-30234 COMM 5/7/01 8.8

Bridgehampton PRIV 9/14/98 4.02

Brookhaven PRIV 11/22/99 6.64

Brookhaven PRIV 4/17/00 82

Brookhaven PRIV 5/9/00 3.77

Calverton PRIV 11/12/98 3.72

Calverton PRIV 10/14/98 2.72

Calverton PRIV 4/27/00 2.09

Calverton X-00050 COMM 7/11/00 3.25

Center Moriches PRIV 3/17/99 2.94

Central Islip S-67197 COMM 6/14/01 2.3

Central Islip S-74505 COMM 3/17/99 3.91

Copiague S-113006 COMM 8/23/99 2.08

Cutchogue NCOM 8/14/00 2.39

Cutchogue NCOM 5/19/98 2.95

Cutchogue PRIV 3/13/98 43.5

Cutchogue PRIV 7/28/97 2.6

Cutchogue PRIV 11/5/98 43.7

Cutchogue PRIV 3/27/00 2.66

East Hampton PRIV 5/7/98 2.2

East Hampton PRIV 8/23/99 2.03

East Hampton PRIV 8/23/99 2.12

East Patchogue PRIV 6/8/00 18.3

Fishers Island S-92463 COMM 6/19/01 2.3

Hampton Bays NCOM 8/5/98 3.97

Hampton Bays PRIV 3/21/00 3.83

Hauppauge S-15746 COMM 5/7/01 28

Laurel PRIV 6/19/00 5.83

Laurel PRIV 6/26/00 2.65

Lloyd Harbor PRIV 4/10/01 4.1

Lloyd Neck PRIV 4/10/01 5.8

Manorville NCOM 3/14/00 4.47

Manorville PRIV 4/25/97 2.9

Manorville PRIV 5/21/97 2.4

Manorville PRIV 8/11/97 2.5

Manorville PRIV 8/11/97 3

Manorville PRIV 8/12/97 4.8

Manorville PRIV 8/12/97 5.6

Mattituck PRIV 7/14/98 4.3

Montauk NCOM 8/23/99 2.07

Montauk NCOM 8/23/99 2.23

Occurrence of Arsenic in Ground Water 5
Hamlet Well No.

Type of

site 

Sample date

(mo/d/yr)

Arsenic

concentration 

(μg/L)

Montauk NCOM 8/23/99 2.17

Montauk NCOM 8/23/99 2.06

Montauk NCOM 8/23/99 2.22

Mount Sinai PRIV 5/11/00 3.28

Nesconset PRIV 6/23/97 5.7

Nesconset PRIV 3/10/98 7.29

Nesconset PRIV 5/20/98 7.84

Nesconset PRIV 5/20/98 6.19

Nesconset PRIV 5/21/98 7.75

Nesconset PRIV 6/25/98 8.16

Nesconset PRIV 4/17/00 3.75

New Suffolk NCOM 8/18/99 2.1

Nissequogue PRIV 10/1/97 20.5

Nissequogue PRIV 2/19/98 2.25

Nissequogue PRIV 3/3/98 5.87

Nissequogue PRIV 6/22/98 3.53

Nissequogue PRIV 8/6/98 6.64

Nissequogue PRIV 8/6/98 4.48

Oakville S-94286 COMM 4/18/01 2.4

Old Field PRIV 8/6/98 3.03

Orient NCOM 6/8/00 93.5

Peconic NCOM 5/14/01 2.4

Peconic PRIV 7/29/97 3.9

Peconic PRIV 2/6/01 3.5

Riverhead COMM 8/31/98 3.02

Riverhead PRIV 8/4/97 2.9

Riverhead PRIV 3/13/00 6.45

Riverhead S-01322 COMM 4/3/00 3.66

Riverhead S-07261 COMM 7/25/01 4.3

Riverhead S-111777 COMM 7/25/01 2.9

Riverhead S-15117 COMM 9/15/99 2.65

Riverhead S-30271 COMM 7/25/01 2

Riverhead S-66685 COMM 7/19/00 3.04

Riverhead S-88466 COMM 7/25/01 4.2

Riverhead X-00041 COMM 7/11/00 2.98

Sag Harbor NCOM 6/2/99 55.4

Sag Harbor PRIV 6/11/97 2.5

Sag Harbor S-36860 COMM 9/14/98 4.17

Sagaponack PRIV 5/5/98 2.17

Sagaponack PRIV 6/25/98 4.04

Saint James PRIV 8/20/97 2.4

Saint James PRIV 3/30/98 3.33

Saint James PRIV 11/30/99 3.98

Setauket PRIV 12/18/97 2.86

Shirley PRIV 8/6/98 3.66

Smithtown NCOM 8/8/00 6.26

Smithtown PRIV 9/16/97 2.2

Smithtown PRIV 5/20/98 2.71

Smithtown PRIV 8/6/98 3.18

Smithtown PRIV 11/13/00 2.71

Southampton PRIV 4/7/98 34.3

Table 2. (Continued) Arsenic concentrations greater than 2 μg/L in ground-water samples collected 

from April 1997 through July 2001 and analyzed by Suffolk County Department of Health Services 

(SCDHS), Suffolk County, New York.

6 Occurrence of Arsenic in Ground Water of Suffolk County, New York, 1997-2002
Hamlet Well No.

Type of

site 

Sample date

(mo/d/yr)

Arsenic

concentration 

(μg/L)

Southold PRIV 10/2/99 2.11

Southold PRIV 7/7/99 5.61

Southold PRIV 4/25/00 2.18

Southold PRIV 2/29/00 7.54

Southold PRIV 7/5/01 7.2

Springs NCOM 8/7/00 2.3

Upton S-22150 COMM 8/16/00 3.14

Upton S-66944 COMM 8/16/00 2.59

Wainscott PRIV 6/29/00 24.7

West Islip S-22389 COMM 3/1/01 4.1

Yaphank S-47438 COMM 9/26/00 2.28

Table 2. (Continued) Arsenic concentrations greater than 2 μg/L in ground-water samples collected 

from April 1997 through July 2001 and analyzed by Suffolk County Department of Health Services 

(SCDHS), Suffolk County, New York.

0 10 20 MILES

0 10 20 KILOMETERS

LONG ISLAND SOUND

ATLANTIC OCEAN

���������

���������

Well sampled by USGS from

June-August 2002

Arsenic concentration exceeded 10 �g/L

in 1997 through 2001

�����������

<1 to 4 dwelling unit per acre

5 to <11 dwelling unit per acre

Commercial

Commercial Recreational

Industrial

Institutional

Open space and recreational

Agricultural

Transportation and utilities

Vacant

Freshwater bodies

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Base from U.S. Geological Survey digital data, 1:24,000,

Universal Transverse Mercator projection, NAD27, Zone 18

Figure 2. Land-use categories in Suffolk County, N.Y., and locations of wells at which historical arsenic concentrations in ground water 

exceeded 10 micrograms per liter, and wells that were sampled during the June-August, 2002 investigation.

Occurrence of Arsenic in Ground Water 7
released from arsenic-bearing iron oxides under reducing 

conditions (Nickson and others, 1998) and can be weathered 

from arsenic-bearing sulfide minerals in oxidizing 

environments (Brown and Chute, 2002). Similar geochemical 

processes can occur in unconsolidated deposits containing 

these arsenic sources. Reducing conditions are most common 

in deep aquifers and near ground-water discharge zones, and 

oxidizing conditions are more typical of shallow aquifers and 

near pumping wells that draw oxygen-rich water into deeper 

zones. Runoff from sulfide-rich mine tailings is a source of 

arsenic in some parts of the United States.

Manmade sources of arsenic in ground water include 

industrial and agricultural products. Nationwide, 90 percent 

of the arsenic produced for industrial purposes is used in 

wood preservatives; the rest is used in paints, dyes and metals, 

drugs, soaps, and semiconductors (U.S. Environmental 

Protection Agency, 2001a). Agricultural sources include 

certain fertilizers and pesticides. 

Potential Sources of Arsenic in Suffolk County

Suffolk County has few of the potential sources of 

arsenic mentioned above. A bedrock source is unlikely 

because the depth to bedrock in the area of the 19 wells 

sampled in 2002 ranges from about 600 to 1,700 feet below 

sea level, and most of these wells are screened far above the 

bedrock. Other naturally occurring rock sources of arsenic 

are unlikely because most wells are screened in deposits 

of reworked fluviodeltaic sediments and outwash, where 

most reactive minerals have been removed by geochemical 

processes. Two possible exceptions are the SCWA supply 

wells S-76304 and S-76305, which penetrate till where 

cobbles and boulders could potentially provide a source of 

arsenic. Industrial sources are unlikely because past and 

current land-use practices tend toward residential, farming, 

and open space (fig. 2).

The most likely sources of arsenic in Suffolk County 

are geochemical reactions involving arsenic-bearing minerals 

within the unconsolidated deposits, and agricultural products. 

Sodium arsenite (an herbicide) and lead arsenate (an 

insecticide) were applied to farmland on Long Island until their 

use was curtailed in 1978, but most agricultural uses of arsenic 

compounds were not banned by the EPA until 1988. Therefore, 

the large areas of land in Suffolk County’s eastern end that 

were previously farmed, or that are still being farmed (fig. 2), 

could be or have been a source of arsenic in ground water. Soils 

collected from agricultural areas in Suffolk County in which 

arsenic-bearing pesticides were previously used were reported 

by the Cornell Cooperative Extension to contain arsenic 

concentrations ranging from 27.8 μg/L to 51 μg/L (Sanok and 

others, 1995). In contrast, an experiment in which recycled 

municipal waste, a known source of arsenic, was applied to sod 

farms in Suffolk County, found arsenic in the soil, but not in 

the ground water (W. J. Sanok, Cornell Cooperative Extension, 

oral commun., 2002). This may have been because arsenic 

tends to sorb onto organic particles and clays and can thereby 

be prevented from leaching into ground water. 

Concentrations and Distribution

Data on 24 selected wells are presented in table 3. 

Thirteen of these sampled from August 1997 through May 

2001 contained arsenic concentrations approaching or 

exceeding 10 μg/L. Data from 19 wells sampled in 2002 are 

also listed. In 2002 only 7 of the 13 wells sampled earlier 

were resampled because some wells had been shut down; 

7 other wells were sampled at locations close to previously 

sampled wells; and 5 wells were sampled in areas suspected 

of having arsenic contamination. The concentrations measured 

by SCWA and SCDHS at the selected wells during 1997 

through 2001 ranged from 8 μg/L to 93.5 μg/L, whereas those 

measured by USGS in 2002 ranged from less than 2 μg/L to 

11 μg/L (table 3). 

Only 2 of the 19 samples collected during 2002 had 

arsenic concentrations that approached the USEPA standard 

of 10 μg/L (table 3); these were the SCWA supply wells 

S-76304 (USGS well identifier 410406071523001) with an 

arsenic concentration of 11 μg/L, and S-76035 (USGS well 

identifier 410406071523101) with an arsenic concentration of 

7 μg/L. These wells are adjacent to each other and screened 

at similar depths (table 2) and, therefore, reflect conditions 

at one site. The reasons for the concentration discrepancies 

between the two sampling periods (table 3) are varied: (1) the 

elevated concentrations at the SCWA well S-15746 (USGS 

well identifier 404923073122401) and at the private supply 

well (USGS well identifier 410811072181701) in 1997 

through 2001 are attributed to improperly flushed carbon 

filters (carbon used in water filters is a source of arsenic); 

(2) the elevated arsenic concentration at the private supply 

well (USGS well identifier 410049072291401) in 1998 is a 

typographical error reported by SCDHS; and (3) the apparent 

changes at other wells (table 3) are unknown but may reflect 

the passing of localized contaminant plumes, although no data 

are available to support this. Future sampling could be done 

downgradient of these wells to test this hypothesis.

Locations of the five sites sampled in 2002 but not during 

1997 through 2001 are as follows: one is at a landfill, one is 

about 1500 feet downgradient from that landfill, one is near a 

ground-water-discharge zone in an agricultural area, and two 

are near a ground-water-discharge zone in till deposits similar 

to those at SCWA supply wells S-76304 and S-76305. None 

of these five wells showed arsenic contamination.

8 Occurrence of Arsenic in Ground Water of Suffolk County, New York, 1997-2002
Table 3. Arsenic concentrations at selected wells in Suffolk County, N.Y., 1997 through 2002.

[Lat.-Long., latitude and longitude in degrees and minutes; (mo/d/yr), month/day/year; μg/L, micrograms per liter; --, no measurement; <, less than.] 

1997-2001 2002

USGS

well

identifier Lat.- Long.1

Sample

 date(s)

(mo/d/yr)

Arsenic

concentration

(μg/L)

Well

status

Sample 

date

(mo/d/yr)

Arsenic 

concentration2

(μg/L)

404923073122401 4049 7312 5/7/01 3

28 offline 

indefinitely

no sample --

404921073122701 4049 7312 -- -- sampled (approx. same 

depth as #1)

8/6/02 <2

410406071523001 4104 7152 6/22/99 9.43 sampled 7/24/02 11 [9.4]

8/14/00 9.88 

12/12/00 8.73 

410406071523101 4104 7152 6/29/98 10.1 sampled 7/24/02 7 [7.8]

6/22/99 6.67 

12/12/00 6.53 

410308071542101 4103 7154 -- -- sampled 8/19/02 <2

410320071535901 4103 7153 -- -- sampled 8/19/02 <2

410009072175001 4100 7217 6/2/99 55.4 sampled 7/23/02 <2

6/17/99 22.9 

6/17/99 23.1 

410811072181701 4108 7218 6/8/00 3

93.5 sampled 6/19/02 <4

6/14/00 6.95 

410049072291401 4100 7229 3/13/98 †43.5 sampled 6/19/02 <4

404549072583401 4045 7258 4/27/00 9.82 sampled 7/11/02 <2

6/8/00 18.3 

7/20/00 8.76 

405359073111201 *4053 7311 8/6/97

10/1/97

10/1/97

 20

21

20.5

public water;

original well not

accessible

no sample --

405359073111101 4053 7311 -- -- sampled 7/9/02 <2

405235072254001 *4052 7225 4/7/98 34.3 public water; orig. well 

not accessible

no sample 

405254072252501 4052 7225 -- -- sampled 7/23/02 <2

405239072255601 4052 7225 -- -- sampled 7/31/02 <2

410222072310001 4102 7231 -- -- sampled 8/9/02 <2

410116072305101 *4101 7230 11/5/98 43.7 could not contact 

homeowner

no sample 

410049072300801 4100 7230 -- -- sampled 7/18/02 <2

405930072282401 4059 7228 -- -- sampled 7/18/02 <2

410143072295101 4101 7229 -- -- sampled 7/25/02 <2

404812072541301 *4048 7254 3/23/00 47.1 public water no sample --

4/17/00 82 public water no sample --

404812072541201 4048 7254 -- -- sampled 7/11/02 <2

404811072541301 4048 7254 -- -- sampled 7/11/02 <2

405652072142501 4056 7214 6/29/00 24.7 sampled 7/31/02 <4

8/17/00 2.5 

1 Coordinates with an asterisk (*) are approximate well locations

2 Numbers in brackets are from split samples collected and analyzed by Suffolk County Water Authority

3 Elevated concentration due to new, incompletely flushed carbon filter

†

 Typographical error in Suffolk County Department of Health Services data base 

Occurrence of Arsenic in Ground Water 9
Summary and Conclusions

Long-term exposure to arsenic in drinking water has 

been linked to several types of cancer and other health-related 

problems (National Research Council, 1999). On February 22, 

2002, the USEPA set a new standard for arsenic in drinking 

water to reduce public health risks; this new standard lowered 

the maximum concentration limit in drinking water from 

50 μg/L to 10 μg/L.

In 2001 the USGS on Long Island, N.Y., in cooperation 

with Suffolk County’s major water supplier, the Suffolk 

County Water Authority (SCWA), began an investigation 

to document the occurrence and concentrations of arsenic in 

ground water throughout Suffolk County. First, an inventory 

of data on arsenic concentrations in ground water from 1997 

through 2001 was made from data bases of the USGS, SCWA, 

SCDHS, the New York State Department of Environmental 

Conservation (NYSDEC), and the Cornell Cooperative 

Extension of Suffolk County to (1) define the occurrence 

of arsenic detections, and (2) determine whether further 

study was warranted. Secondly, based on the results from 

the inventory, nineteen wells were selected for additional 

sampling and water-quality analyses. 

Fourteen of the supply wells sampled from 1997 through 

2001 had a reported history of arsenic concentrations close 

to, or exceeding, 10 μg/L. An attempt was made to resample 

these wells in 2002. Seven were resampled; the other seven 

were no longer available for sampling, therefore, seven nearby 

wells were sampled. Five additional wells that were in areas 

suspected to have potential for elevated arsenic concentrations 

on the basis of their down-gradient position in relation to other 

arsenic-contaminated wells or agricultural areas where past 

arsenic use was likely were also sampled.

On the basis of the limited 2002 sampling data available 

for this study the concentrations of arsenic above drinking 

water standards (10 μg/L) do not appear to indicate a 

countywide problem in ground water in Suffolk County. 

Only two wells (located at one site) of 19 wells suspected of 

having arsenic contamination that were sampled in 2002 had 

arsenic concentrations near the EPA limit of 10 μg/L. These 

were SCWA supply wells S-76304 (USGS well identifier 

410406071523001) and S-76035 (USGS well identifier 

410406071523101). It should be noted that SCWA does 

not include a supply well in its distribution system unless 

arsenic concentrations are less than 5 μg/L (K. A. Randazzo, 

Laboratory Director, SCWA, oral commun., 2002). The 

paucity of wells with elevated arsenic concentrations in 

2002 does not rule out the possibility of unknown, localized 

arsenic concentrations that exceed 10 μg/L. Aquifer zones 

with arsenic-bearing iron-oxides or sulfide minerals can be 

a source of arsenic under oxidizing and reducing conditions, 

respectively. The geochemical data collected during this 

study are too limited to support or refute geochemical 

reactions within the aquifers as a source of arsenic. Future 

investigations could include sampling wells screened in 

aquifer zones that contain potentially arsenic-rich ferric 

hydroxides under reducing conditions, and iron sulfides under 

oxidizing conditions. 

The SCDHS’s sampling programs during 1997 through 

2001 were directed toward specific areas considered to be 

vulnerable to pesticide contamination; thus, the data from 

these sampling programs are biased and would be expected 

to indicate a greater number of arsenic detections than would 

be found through random sampling. The results from those 

programs, therefore, are not considered representative of 

ground-water quality throughout Suffolk County.

 The areal distribution of elevated concentrations detected 

during 1997 through 2001 and 2002 does not indicate a 

strong correlation with agricultural land use, contrary to 

expectation. This is consistent with a conclusion reported by 

SCDHS that very few of the low-level detections of arsenic 

can be attributed to the leaching of pesticides (NYSDEC web 

page—http://www.dec.state.ny.us/website/dshm/prl/scdhs1c.

htm; accessed 06/12/02). 

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M.J., 2000, Arsenic in ground-water resources of the 

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