U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2009–5259

Prepared in cooperation with the National Park Service

Analysis of the Shallow Groundwater Flow System at

Fire Island National Seashore, Suffolk County, New York
Cover. Aerial photograph of Old Inlet vicinity of Otis Pike Fire Island High Dune Wilderness in Fire Island National Seashore, Suffolk County, 

N.Y. (Photograph courtesy of James Lynch, U.S. Geological Survey)
Analysis of the Shallow Groundwater Flow 

System at Fire Island National Seashore, 

Suffolk County, New York

By Christopher E. Schubert

Prepared in cooperation with the National Park Service

Scientific Investigations Report 2009–5259

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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Suggested citation:

Schubert, C.E., 2010, Analysis of the shallow groundwater flow system at Fire Island National Seashore, Suffolk 

County, New York: U.S. Geological Survey Scientific Investigations Report 2009–5259, 106 p. (Also available at 

http://pubs.usgs.gov/sir/2009/5259.)
iii

Contents

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

Introduction ....................................................................................................................................................2

Purpose and Scope ..............................................................................................................................4

Methods and Approach.......................................................................................................................4

Fire Island Study Area..........................................................................................................................4

Population and Land Use............................................................................................................4

Water Use .....................................................................................................................................5

Hydrogeologic Framework.........................................................................................................................17

Geologic Setting..................................................................................................................................17

Monmouth Group.......................................................................................................................17

Pleistocene Deposits ................................................................................................................17

Gardiners Clay...................................................................................................................18

Wisconsinan Deposits .....................................................................................................18

Lower Outwash ........................................................................................................19

“20-Foot” Clay ..........................................................................................................19

 Upper Outwash .......................................................................................................19

Holocene Deposits ....................................................................................................................19

Lagoonal Deposits ............................................................................................................19

Estuarine Mud ...................................................................................................................28

Seagrass Beds ..................................................................................................................28

Peat Deposits ....................................................................................................................28

Barrier Island and Shelf Sands ......................................................................................28

Hydrologic Setting ..............................................................................................................................29

Hydrologic Boundaries .............................................................................................................29

Precipitation and Recharge ............................................................................................29

Groundwater Discharge ..................................................................................................33

Local Mean Sea Level.............................................................................................34

Wave Setup and Setdown......................................................................................36

Tidal Pumping ...........................................................................................................36

Freshwater-Saltwater Interface.....................................................................................36

Directions of Groundwater Flow .............................................................................................37

Nitrogen Concentrations ..........................................................................................................37

Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore ...................39

Development of Flow Model .............................................................................................................39

Model Grid...................................................................................................................................39

Boundary Conditions.................................................................................................................39

Model Calibration ......................................................................................................................45

Simulation of Steady-State Conditions ...........................................................................................66

Groundwater Flow .....................................................................................................................66

Discharge-Boundary Analysis........................................................................................67

Contributing-Area Analysis.............................................................................................67

Nutrient Loads............................................................................................................................73
iv

Limitations of Analysis .......................................................................................................................89

Groundwater-Resource Concerns...................................................................................................89

Use of Long-Term Monitoring to Assess Water Resources ........................................................94

Groundwater Levels ..................................................................................................................94

Water-Quality Samples.............................................................................................................95

Summary........................................................................................................................................................95

Acknowledgements.....................................................................................................................................98

References Cited..........................................................................................................................................98

Glossary ......................................................................................................................................................103

References .................................................................................................................................................104

Appendix 1. Groundwater Levels Collected at Fire Island National Seashore, October 2005

through October 2006...............................................................................................................105

Appendix 2. Dissolved Inorganic Nitrogen (Ammonia, Nitrite, and Nitrite Plus Nitrate) and

Phosphorus (Orthophosphate) Concentrations in Groundwater Collected at Fire

Island National Seashore, October 2005 through September 2006..................................106

Figures

1. Map showing location of Suffolk County study area on Long Island, N.Y.,

tide-stage gaging station 01310740 Reynolds Channel at Point Lookout, N.Y., 

and selected tidal benchmarks in southeastern New York and northern 

New Jersey....................................................................................................................................2

2. Map showing location of Fire Island National Seashore, selected

precipitation-measurement stations and tidal benchmarks, vertical section A–A’,

and Fire Island study area in Suffolk County, N.Y. .................................................................3

3. Maps showing land use and locations of local study areas in the Fire Island study

area, Suffolk County, N.Y. ...........................................................................................................6

4. Maps showing locations of observation wells and vertical sections B–B’ through

E–E’ in local study areas and groundwater levels during October 31–November 1,

2005, on Fire Island, Suffolk County, N.Y. ..................................................................................8

5. Diagrams showing gamma-ray and electromagnetic-induction logs, salinity from

filter-press core samples, generalized sediment description from core samples,

and corresponding geologic units for borings at four wells on Fire Island, Suffolk

County, N.Y., (A) well 126753, (B) well 126000, (C) well 125990, and (D) well 124399........12

6. Maps showing locations of public water-supply systems and selected

wastewater-disposal systems on Fire Island, Suffolk County, N.Y.....................................14

7. Diagrams showing generalized vertical section A–A’ showing major

hydrogeologic units and approximate distribution and flow directions of fresh and

saline groundwater along (A) the southern shore of Long Island and (B) on Fire

Island, N.Y. ...................................................................................................................................18

8. Diagrams showing vertical sections B–B ́ through E–E ́ showing hydrogeologic

units, freshwater-saltwater interface, and wells with screened intervals in local

study areas, and water-table altitude and groundwater levels during October

31–November 1, 2005, on Fire Island, Suffolk County, N.Y. ..................................................20

9. Maps showing altitude of upper surface of Gardiners Clay and, where absent,

Monmouth greensand in the Fire Island study area, Suffolk County, N.Y.

(Upper-surface altitudes of Gardiners Clay and Monmouth greensand from

Smolensky and others, 1989, sheet 3.......................................................................................24
v

10. Maps showing altitude of upper surface of “20-foot” clay in the Fire Island study

area, Suffolk County, N.Y. .........................................................................................................26

11. Maps showing thickness of “20-foot” clay in the Fire Island study area, Suffolk

County, N.Y. .................................................................................................................................27

12. Maps showing generalized land cover and substrate on Fire Island and adjacent

islands within Fire Island National Seashore, and benthic habitats in back-bay

areas of the Fire Island study area, Suffolk County, N.Y. ....................................................30

13. Photograph of ocean shore between Watch Hill and Davis Park showing peat

deposits occasionally uncovered on the Atlantic Ocean beaches of Fire Island,

Suffolk County, N.Y......................................................................................................................32

14. Graph showing long-term mean monthly evaporation at land pans in Greenport

and Mineola, Long Island, N.Y. ................................................................................................33

15. Graphs showing continuous water-level records and synoptic water-level

measurements at selected wells on Fire Island, and daily higher high water values

at tide-stage gaging station 01310740 Reynolds Channel at Point Lookout, N.Y.,

during October 2005–April 2007, Long Island, N.Y.................................................................38

16. Maps showing total nitrogen concentrations at 35 wells sampled during October

2005 on Fire Island, Suffolk County, N.Y. ................................................................................40

17. Graph showing total nitrogen concentrations at four wells sampled during

October 2005 and March and August–September 2006 on Fire Island, Suffolk

County, N.Y. .................................................................................................................................44

18. Maps showing active and inactive areas and locations of columns 190, 255, 569,

and 790 in the model of Fire Island study area, Suffolk County, N.Y. .................................46

19. Graphs showing modeled hydrogeologic units for active model cells along four

selected columns in the Fire Island study area, Suffolk County, N.Y. (A) column

190, (B) column 255, (C) column 569, and (D) column 790 ....................................................48

20. Maps showing modeled hydrogeologic units for layer 1 in the Fire Island study

area, Suffolk County, N.Y. .........................................................................................................52

21. Maps showing modeled hydrogeologic units for layer 2 in the Fire Island study

area, Suffolk County, N.Y. ..........................................................................................................54

22. Maps showing modeled hydrogeologic units for layers 3–5 in the Fire Island study

area, Suffolk County, N.Y. ..........................................................................................................56

23. Maps showing modeled hydrogeologic units for layer 9 in the Fire Island study

area, Suffolk County, N.Y. ..........................................................................................................58

24. Maps showing modeled, constant hydraulic-head values in the Fire Island study

area, Suffolk County, N.Y. ..........................................................................................................60

25. Maps showing modeled, constant salt-concentration values in the Fire Island

study area, Suffolk County, N.Y. ...............................................................................................62

26. Maps showing zones and rates of recharge from precipitation simulated in the

model of Fire Island study area, Suffolk County, N.Y. ..........................................................64

27. Maps showing rates of recharge from the return flow of public-supply water

simulated in the model of Fire Island study area, Suffolk County, N.Y. .............................68

28. Maps showing simulated 2005 mean annual water-table altitude and extent of

unconfined freshwater flow system within surficial hydrogeologic units in the

model of Fire Island study area, Suffolk County, N.Y. ...........................................................74

29. Maps showing sSimulated 2005 mean annual potentiometric-surface altitude and

extent of confined freshwater flow system beneath peat deposits onshore and

surficial Holocene units offshore in the model of Fire Island study area, Suffolk

County, N.Y. ..................................................................................................................................76
vi

Tables

1. Monthly pumpage in 2005 for public water-supply systems on Fire Island, Suffolk

County, N.Y. ..................................................................................................................................16

2. Generalized description of geologic and hydrogeologic units in the Fire Island

study area, Suffolk County, N.Y.................................................................................................17

3. Annual and long-term mean precipitation amounts at Patchogue and Islip, Suffolk

County, N.Y. ..................................................................................................................................32

4. Published relations between tidal and fixed datums for 1983–2001 tidal epoch at

six active National Ocean Service tidal benchmark stations in southeastern New

York and northeastern New Jersey.........................................................................................34

5. Published relations and calculated difference between National Geodetic Vertical

Datum of 1929 and local mean low water datum for the 1941–59 and 1983–2001

tidal epochs at four active National Ocean Service tidal benchmark stations in

southeastern New York and northeastern New Jersey.......................................................34

6. Published and calculated relations between local mean low water and mean 

tide level datums and National Geodetic Vertical Datum of 1929 for the 1941–59 

tidal epoch, and estimated difference for and relation between mean tide level 

and National Geodetic Vertical Datum of 1929 for the 1983–2001 tidal epoch at 

selected discontinued National Ocean Service tidal benchmark stations, 

Suffolk County, N.Y......................................................................................................................35

7. Relations and calculated difference between mean sea level datum and National

Geodetic Vertical Datum of 1929 for the 1983–2001 tidal epoch and from the VDatum 

program at the National Ocean Service tidal benchmark station at Sandy Hook,

N.J. ................................................................................................................................................35

8. Simulated zones of recharge from precipitation in the model of Fire Island study

area, Suffolk County, N.Y. ..........................................................................................................66

9. Differences between measured and simulated water levels at wells in four areas

of Fire Island, Suffolk County, N.Y. ...........................................................................................70

10. Differences between measured and simulated water levels at wells screened in

four hydrogeologic units simulated in the model of Fire Island, Suffolk County, N.Y. .....71

11. Measured and simulated water levels and vertical hydraulic gradients at five

observation-well pairs on Fire Island, Suffolk County, N.Y. .................................................72

30. Maps showing simulated 2005 mean annual potentiometric-surface altitude and

extent of confined freshwater flow system beneath “20-foot” clay and (or) upper

outwash in the model of Fire Island study area, Suffolk County, N.Y.................................78

31. Diagrams showing simulated 2005 mean annual water levels, flow vectors, and

extent of freshwater within the shallow aquifer system along four selected

columns in the Fire Island study area, Suffolk County, N.Y. (A) column 190, 

(B) column 255, (C) column 569, and (D) column 790 ............................................................80

32. Maps showing simulated 2005 mean annual groundwater flows between the

shallow aquifer system and marine surface waters, and recharge areas for

fresh-groundwater budgets (tables 13 and 14) and total nitrogen loads (table 15) 

in the model of Fire Island study area, Suffolk County, N.Y. ...............................................86

33. Maps showing simulated contributing areas to marine surface waters under 

2005 mean annual conditions in the model of Fire Island study area, Suffolk 

County, N.Y. ..................................................................................................................................90
vii

12. Final horizontal and vertical hydraulic conductivities for hydrogeologic units

simulated in the model of Fire Island study area, Suffolk County, N.Y...............................72

13. Simulated 2005 mean annual fresh-groundwater budgets for recharge areas of

similar land use from the discharge-boundary analysis of Fire Island study area,

Suffolk County, N.Y......................................................................................................................84

14. Simulated 2005 mean annual fresh-groundwater budgets for recharge areas of

similar land use from the contributing-area analysis of Fire Island study area,

Suffolk County, N.Y......................................................................................................................88

15. Calculated total nitrogen loads to marine surface waters for recharge areas of

similar land use in the model of Fire Island study area, Suffolk County, N.Y. ...................92

16. Annual totals and rates of nitrogen loading from shallow groundwater to marine

surface waters from three watersheds on Long Island, N.Y...............................................93
viii

Conversion Factors, Datums, and Abbreviations

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

meter (m) 1.094 yard (yd) 

Area

square meter (m2) 0.0002471 acre 

hectare (ha) 2.471 acre

square hectometer (hm2) 2.471 acre

square kilometer (km2) 247.1 acre

square kilometer (km2) 0.3861 square mile (mi2)

Volume

liter (L) 33.82 ounce, fluid (fl. oz)

liter (L) 2.113 pint (pt)

liter (L) 1.057 quart (qt)

liter (L) 0.2642 gallon (gal)

Flow rate

centimeter per year (cm/yr) 0.3937 inch per year (in/yr)

meter per day (m/d) 3.281 foot per day (ft/d)

cubic meter per day (m3/d) 35.31 cubic foot per day (ft3/d) 

liter per second (L/s) 15.85 gallon per minute (gal/min) 

Mass

kilogram (kg) 2.205 pound avoirdupois (lb)

kilogram per year (kg/yr) 2.205 pound per year (lb/yr)

kilogram per square kilometer (kg/km2) 5.711 pound per square mile (lb/mi2)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Vertical coordinate information is referenced to National Geodetic Vertical Datum of 1929 

(NGVD 29).

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

Altitude, as used in this report, refers to distance above the vertical datum.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius 

(μS/cm at 25°C).

Concentrations of chemical constituents in water are given in milligrams per liter (mg/L).
ix

List of Acronyms

FIIS Fire Island National Seashore

GIS Geographic information system

MHHW Mean higher high water

MLW Mean low water

MLLW Mean lower low water

MSL Mean sea level 

MTL Mean tide level

NPS National Park Service

NCDC National Climatic Data Center

NOS National Ocean Service

SSER South Shore Estuary Reserve

SCDHS Suffolk County Department of Health Services

SCDP Suffolk County Department of Planning

SCWA Suffolk County Water Authority

TN Total nitrogen

USGS U.S. Geological Survey
x

This page has been left blank intentionally.
Analysis of the Shallow Groundwater Flow System at 

Fire Island National Seashore, Suffolk County, New York

By Christopher E. Schubert

were calculated from representative total nitrogen (TN) 

concentrations and simulated groundwater discharges to backbarrier estuaries and the ocean.

The model simulates groundwater directions, velocities, 

and discharge rates under 2005 mean annual conditions. 

Groundwater budgets were developed for recharge areas 

of similar land use that contribute freshwater to back-barrier 

estuaries, the ocean, and subsea-discharge zones. Total 

freshwater discharge from the shallow aquifer system is about 

43,500 cubic meters per day (m3/d) (79.8 percent) to backbarrier estuaries and about 10,200 m3

/d (18.7 percent) to the 

ocean; about 836 m3/d (1.5 percent) may exit the system as 

subsea underflow. The total contribution of fresh groundwater 

to shoreline discharge zones amounts to about 53,700 m3/d 

(98.5 percent). The median age of freshwater discharged to 

back-barrier estuaries and the ocean was 3.4 years, and the 

95th-percentile age was 20 years.

The TN concentrations and loads under 2005 mean 

annual conditions for areas that contribute fresh groundwater 

to back-barrier estuaries and the ocean were calculated for 

the principal land uses on Fire Island. The overall TN load 

from the shallow aquifer system to shoreline discharge zones 

is about 16,200 kilograms per year (kg/yr) (82.2 percent) to 

back-barrier estuaries and about 3,500 kg/yr (17.8 percent) 

to the ocean. The overall TN load to marine surface waters 

amounts to about 19,700 kg/yr—roughly 6 percent of the 

annual TN load from shallow groundwater entering the 

South Shore Estuary Reserve (SSER) from the Suffolk 

County mainland, which is about 345,000 kg/yr. In contrast 

to the TN load from shallow groundwater for the SSER 

watershed, which annually yields about 353 kilograms 

per square kilometer (kg/km2), the overall TN load from 

Fire Island produces nitrogen at an annual rate of about 

890 kg/km2—more than double the SSER loading rate from 

shallow groundwater.

Although high-density residential land use occupies about 

8 percent of Fire Island, unsewered areas contribute about 

46 percent of the overall TN load from groundwater to backbarrier estuaries. Because of the effects of wave setup and 

tidal pumping along the ocean shore, groundwater-protection 

strategies need to consider most of the island as a potential 

source area to the wetland and adjacent estuarine habitats 

within the boundaries of FIIS. TN concentrations in the park 

Abstract

Fire Island National Seashore (FIIS) occupies 

42 kilometers of the barrier island for which it is named 

that lies off the southern shore of Suffolk County, N.Y. 

Freshwater1 in the highly permeable, sandy aquifer

underlying Fire Island is bounded laterally by marine surface 

waters and at depth by saline groundwater. Interspersed 

throughout FIIS are 17 pre-existing residential communities 

that in summer months greatly increase in population through 

the arrival of summer residents and vacationers; in addition, 

the National Park Service (NPS) has established several 

facilities on the island to accommodate visitors to FIIS. The 

2.2 million people estimated by the NPS to visit Fire Island 

annually impact groundwater quality through the release of 

waste-derived contaminants, such as nutrients, pathogens, 

and organic compounds, into the environment. Wastecontaminated groundwater can move through the aquifer and 

threaten the ecological health of the adjacent back-barrier 

estuaries to which much of the groundwater ultimately 

discharges. In 2004, the U.S. Geological Survey (USGS), in 

cooperation with the NPS, began a 3-year investigation to 

(1) collect groundwater levels and water-quality (nutrient) 

samples, (2) develop a three-dimensional model of the 

shallow (water-table) aquifer system and adjacent marine 

surface waters, and (3) calculate nitrogen loads in simulated 

groundwater discharges from the aquifer to back-barrier 

estuaries and the ocean.

The hydrogeology of the shallow aquifer system 

was characterized from the results of exploratory drilling, 

geophysical surveying, water-level monitoring, and waterquality sampling. The investigation focused on four areas—

the communities of Kismet and Robbins Rest, the NPS 

Visitor Center at Watch Hill, and the undeveloped Otis Pike 

Fire Island High Dune Wilderness. Thirty-five observation 

wells were installed within FIIS to characterize subsurface 

hydrogeology and establish a water-table monitoring 

network in the four study areas. A variable-density model 

of the shallow aquifer system and adjacent marine surface 

waters was developed to simulate groundwater flow patterns 

and rates. Nitrogen loads from the shallow aquifer system 

1

Bold text is defined in the glossary.
2 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

downgradient from at least two communities—Kismet and 

Robbins Rest—and from the Watch Hill leach field exceeded 

the natural levels observed in other undeveloped areas of FIIS 

by a factor of 10 or more; the presence of excess nitrogen 

may have important consequences for species adapted to the 

low nutrient levels naturally occurring in aquatic and coastal 

habitats of FIIS. These elevated values also indicate that 

recharge containing fertilizers or human waste has infiltrated 

the water table in upgradient areas; this water may also contain 

pathogens and other harmful compounds.

Introduction 

Fire Island National Seashore (hereafter referred to 

as FIIS) occupies 42 km of the barrier island for which 

it is named that lies off the southern shore of western and 

central Suffolk County, Long Island, N.Y. (figs. 1 and 2). 

Interspersed throughout FIIS are 17 pre-existing residential 

beach communities that in summer months greatly increase 

in population through the arrival of summer residents and 

vacationers, and in addition, the National Park Service (NPS) 

has established several facilities on the island to accommodate 

visitors to FIIS.

Untreated wastewater from the numerous homes and 

businesses in the barrier-island communities generally is 

discharged directly into the shallow (water table) aquifer 

through the use of private septic systems and cesspools; 

wastewater from the NPS facilities also enters this aquifer 

through leach fields and cesspools. Contaminants entering 

the groundwater system can pose a threat to coastal habitats 

because the contaminants are transported by groundwater 

that discharges to coastal water bodies. In other parts of Long 

Island (Capone and Bautista, 1985) and elsewhere throughout 

coastal areas of the northeastern United States (Portnoy and 

others, 1998), there is concern about groundwater-derived 

nitrogen loading to shallow estuary systems. Excess levels 

of inorganic nitrogen can lead to increased production of 

phytoplankton and macroalgae, which in turn can lead to 

oxygen depletion, reductions in estuarine fish and shellfish 

communities, and loss of submerged seagrass habitat because 

of light limitation (Valiela and others, 1992).

The boundary of FIIS extends roughly 1 km into the 

back-barrier estuaries of Great South Bay, Narrow Bay, and 

Moriches Bay (fig. 2). Within this zone, there are extensive 

areas of seagrass habitat, shellfish and finfish resources, and 

intense recreational activity (Bokuniewicz and others, 1993). 

To better understand and protect the natural resources of FIIS, 

it is critical to determine the concentration and movement of 

nutrients and other contaminants being introduced into the 

shallow groundwater system and transported by freshwater 

that ultimately discharges to the surrounding coastal waters. 

Figure 1. Location of Suffolk County study area on Long Island, N.Y., tide-stage gaging station 01310740 

Reynolds Channel at Point Lookout, N.Y., and selected tidal benchmarks in southeastern New York and northern 

New Jersey. (NAD 83, North American Datum of 1983)

#

#

#

!

SANDY HOOK (FORT HANCOCK), N.J.

MONTAUK, FORT POND BAY, N.Y.

SHINNECOCK YACHT CLUB, PENNIMAN CREEK, N.Y.

01310740

74° 73° 72°

41°

40°30'

ATLANTIC OCEAN

Long Island Sound

LONG ISLAND

SUFFOLK CO.

NASSAU

CO. QUEENS

CO. KINGS

CO.

NEW

YORK

NEW

JERSEY

CONNECTICUT

STUDY AREA--Detail

is shown in figure 2.

Base from The National Map,

Universal Transverse Mercator projection, NAD 83, Zone 18

EXPLANATION

Southwest Sewer District

! Tide-Stage Gage

# Tidal Benchmark

0 5 10 KILOMETERS

0 5 10 MILES
Introduction 

3

#

#

#

#

#

#

X X

OAK BEACH, N.Y.

WEST FIRE ISLAND, N.Y.

FIRE ISLAND COAST GUARD STATION, N.Y.

BAY SHORE, WATCHOGUE CREEK ENTRANCE, N.Y.

EASTPORT, SEATUCK COVE, MORICHES BAY, N.Y.

SMITH POINT BRIDGE, NARROW BAY, N.Y.

PATCHOGUE 2 N ISLIP LONG ISLAND MACARTHUR AIRPORT

72°45' 73° 73°15'

40°45'

40°37'30"

Base from The National Map,

Universal Transverse Mercator projection, NAD 83, Zone 18

G R E A T S O U T H B A Y

NARROW BAY

A T L A N T I C O C E A N

Moriches Bay

Fire Island Inlet

Moriches

Inlet

A

B

C

D

NOLYBAB

I S L I P

B R O O K H A V E N

A

A'

Fire Island National Seashore

Fire Island

FIRE ISLAND STUDY AREA--Panels

A-D are shown enlarged in figure 3.

0 2 1 KILOMETERS0 2 1 MILES

EXPLANATION

Township Boundary

Trace of Hydrogeologic Section

X Precipitation-Measurement Station# Tidal Benchmark

A' A

Figure 2. Location of Fire Island National Seashore, selected precipitation-measurement stations and tidal benchmarks, vertical section A–A’, and Fire Island study area in Suffolk County, N.Y. (NAD 83, North American Datum of 1983)
4 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

The collection of geologic and hydrologic data and the 

synthesis of these data into a three-dimensional groundwater 

flow model of the shallow aquifer system are needed to best 

manage the groundwater and ecological resources of FIIS.

In 2004, the U.S. Geological Survey (USGS), in 

cooperation with the NPS, began a 3-year investigation to 

(1) collect groundwater levels and water-quality samples, 

(2) develop a three-dimensional model of the shallow aquifer 

system and adjacent marine surface waters, and (3) calculate 

nitrogen loads in simulated groundwater discharges from the 

aquifer to back-barrier estuaries and the ocean. An important 

component of this study is an assessment of nutrient loading 

to the ocean and back-barrier estuaries, particularly Great 

South Bay. The numerical model developed as part of this 

investigation can be used to simulate the transport of other 

contaminants during subsequent studies. In addition, the 

network of shallow observation wells installed as part of this 

study can provide the foundation for a long-term monitoring 

program of groundwater levels and water-quality data 

throughout the barrier-island seashore.

Purpose and Scope

This report presents the results of data collection and 

analysis done as part of the 3-year investigation. Specifically, 

the report presents (1) the boundaries of the shallow aquifer 

system, (2) results of numerical simulations that show 

groundwater flow patterns and rates within the freshwater 

aquifer and adjacent saline subsurface, (3) the amount of 

freshwater discharge from the shallow groundwater system, 

(4) the quality of water in the freshwater aquifer and nutrient 

loads from it to coastal waters, and (5) current water use and 

groundwater withdrawal from the aquifer system underlying 

the barrier island.

Methods and Approach

The shallow aquifer system was characterized from the 

results of exploratory drilling, geophysical surveying, waterlevel monitoring, and water-quality sampling conducted 

during this study. These results were used to develop a 

three-dimensional model of the shallow aquifer system, from 

which nitrogen loads were calculated to back-barrier estuaries 

and the ocean. The investigation focused on four areas—the 

communities of Kismet and Robbins Rest, the NPS Visitor 

Center at Watch Hill, and the undeveloped Otis Pike Fire 

Island High Dune Wilderness (fig. 3).

Thirty-five observation wells were installed within 

FIIS to characterize subsurface hydrogeology and establish 

a water-table monitoring network in the four study areas. 

During drilling, cores were collected almost continuously at 

four boreholes (S126753, S126000, S125990, and S124399; 

figs. 4 and 5) to depths ranging from 9.8 to 15.8 m below 

land surface; additional samples were collected at variable 

intervals at these sites to depths between 13.4 and 41.5 m. 

Filter-press samples were collected from selected cores. 

Geophysical logs—gamma-ray and electromagneticinduction—were collected from 26 and 21 of the newly drilled 

wells, respectively, including at the 4 sites where cores were 

collected. In addition to drilling done as part of this study, 

additional data regarding the hydrogeologic framework 

were obtained from previous investigations. These data 

included maps of the structure and configuration of regional 

hydrostratigraphic units (Smolensky and others, 1989, sheet 3; 

Doriski and Wilde-Katz, 1983, plates 7 and 8), as well as 

data from boreholes and wells that were published previously 

or are on file at the USGS Water Science Center office in 

Coram, N.Y.

The hydrologic setting was characterized from (1) filterpress samples and borehole electromagnetic-induction logs, 

(2) water-level monitoring and water-quality sampling, and 

(3) previous investigations of the Fire Island aquifer system 

and barrier-island aquifer systems elsewhere. Gamma-ray 

and electromagnetic-induction logs, filter-press samples, 

generalized descriptions of geologic cores, and corresponding 

hydrogeologic units for borings at four wells (S126753, 

S126000, S125990, and S124399; locations are shown in 

fig. 4) are shown in figure 5.

Fire Island Study Area

Fire Island is approximately 60 km long and 0.5 km wide 

and is bounded by the Great South Bay, Narrow Bay, and 

Moriches Bay estuaries to the north; the Atlantic Ocean to 

the south; and Fire Island Inlet and Moriches Inlet to the west 

and east, respectively (fig. 2). The 181 km2 Fire Island study 

area encompasses the entire barrier island, which lies within 

parts of the towns of Babylon, Islip, and Brookhaven and 

includes most of the adjacent marine surface waters within the 

boundaries of FIIS (fig. 2).

Population and Land Use

The 17 residential communities within FIIS contain over 

4,100 homes, and although they have a permanent resident 
Introduction 5

population of only about 400, the summer residency climbs to 

over 20,000 (National Park Service, 2003). About 2.2 million 

people are estimated to annually visit 1 of the 17 communities, 

Smith Point County Park (fig. 3), the waterways surrounding 

Fire Island, or a FIIS facility (National Park Service, 2007a). 

Combined visitation on a peak-season weekend day can be as 

high as 100,000 (National Park Service, 2002).

Land-use analyses done by the Suffolk County 

Department of Planning (SCDP) for the three townships 

(Babylon, Islip, and Brookhaven, fig. 2) that encompass 

parts of Fire Island indicated only about 3 percent of the 

land area (omitting surface waters) was vacant (Carl Lind, 

Suffolk County Department of Planning, written commun., 

2006). About 81 percent of the land area is recreational and 

open space, including a portion of the island stretching from 

just west of Smith Point County Park to just east of the NPS 

Visitor Center at Watch Hill (fig. 3) that Congress designated 

as a National Wilderness Area in 1980. About 12 percent 

of the land area is residential, although most residences are 

occupied only during summer months (National Park Service, 

2003). About 8 percent of the land area is classified as highdensity residential with at least five dwelling units per acre, 

4 percent is medium density (between one and five dwelling 

units per acre), and less than 1 percent is low density (one 

dwelling unit per acre or less). The approximately 3 percent 

of the land area remaining is distributed among six other land 

uses—commercial, industrial, institutional, transportation, 

utilities, and waste handling and management.

Water Use

The public water supply for the barrier-island 

communities and all NPS, State, and county park facilities is 

derived solely from wells that pump groundwater from the 

underlying aquifer system; a small fraction of this pumpage 

occurs on the Long Island mainland. Virtually all water 

supply currently is withdrawn from the uppermost Late 

Cretaceous (Magothy) aquifer, because freshwater in the 

overlying Pleistocene (upper glacial) aquifer and Holocene 

(recent) deposits is limited to a shallow lens. Historically, 

the shallow freshwater lens was used to supply summer 

residences; however, this supply has largely been abandoned 

because of elevated levels of chloride, nitrate, and coliform 

bacteria (Leggette, Brashears & Graham, Inc., 1996). The 

shallow freshwater lens is not in hydraulic connection with the 

regional aquifer system.

Currently (2005), Fire Island is served by 10 public 

water-supply systems (fig. 6). The service-area locations for 

the Kismet, Fair Harbor-Summer Club, Point O’Woods-Fire 

Island Pines, Davis Park, and Westhampton distribution zones 

of the Suffolk County Water Authority (SCWA) are based 

on a geographic information system (GIS) spatial data set 

assembled by the SCWA (Jeff Altorfer, Suffolk County Water 

Authority, written commun., 2005). Service-area locations 

for the water-supply systems operated by the communities 

of Saltaire, Ocean Beach, Seaview, and Ocean Bay Park 

were digitized from distribution-system maps on file at the 

Suffolk County Department of Health Services (SCDHS). The 

locations of buildings within Robert Moses State Park were 

digitized and used in this study to estimate the service-area 

location for this park’s water-supply system.

Monthly pumpage in 2005 (table 1) was compiled 

for the four distribution zones of the SCWA that withdraw 

groundwater from the aquifer system underlying Fire Island 

(Mike Boufis, Suffolk County Water Authority, written 

commun., 2006) and for the water-supply system at Robert 

Moses State Park (Amy Cooper on behalf of Elizabeth 

Wintenberger, New York State Office of Parks, Recreation 

and Historic Preservation, written commun., 2007). Monthly 

withdrawals in 2005 also were compiled for the community 

water-supply systems in Saltaire, Ocean Beach, Seaview, 

and Ocean Bay Park from pumpage records on file at the 

SCDHS. Quarterly meter readings in 2005 were compiled for 

the SCWA Westhampton distribution zone, which distributes 

water supply to the NPS Wilderness Visitor Center and 

to Smith Point County Park (figs. 3 and 6), but does not 

withdraw from the underlying aquifer system (Mike Boufis, 

Suffolk County Water Authority, written commun., 2006).

Most public-supply water on Fire Island is ultimately 

returned to the water table, primarily through on-site 

wastewater disposal systems and, to a lesser extent, as leakage 

from the water-distribution system; the remainder is lost 

as consumptive use. In unsewered areas of Long Island, 

the return flow of public-supply water to the water table 

is estimated to be about 85 percent of total public-supply 

pumpage—approximately 70 percent through wastewater 

disposal systems and 15 percent via leaky transmission 

systems (Franke and McClymonds, 1972). A small number 

of private and NPS dwellings not served by the public 

water-supply systems derive drinking water individually or 

collectively from single- or multiple-well systems. Information 

on the amount of drinking water derived from these wells 

typically is unavailable, however, because the pumping rates 

commonly are less than the New York State Department of 

Environmental Conservation required permitting and reporting 

level of 2.8 L/s.
6 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York Figure 3. Land use and locations of local study areas in the Fire Island study area, Suffolk County, N.Y. (Land use from C.G. Lind, Suffolk County Department of 

Planning, written commun., 2006. Locations of areas shown in panels A, B, C, and 

D are shown in figure 2. NAD 83, North American Datum of 1983)

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

73°04' 73°06' 73°08' 73°10'

40°39'

Base from New York State Department of Transportation planimetric quadrangles, 1991, 1:24,000,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

Kismet

Robbins Rest

0 0.5 1 KILOMETER0 0.5 1 MILE

Robbins Rest
Introduction 

7

72°54' 72°56' 72°58' 73° 73°02'

40°42'

40°41'

72°46' 72°48' 72°50' 72°52'

40°44'

40°43'

Base from New York State Department of Transportation planimetric quadrangles, 1991, 1:24,000,

Universal Transverse Mercator projection, NAD 83, Zone 18

EXPLANATION

Land Use

Low-density residentialMedium-density residentialHigh-density residentialCommercialIndustrialInstitutional

Recreation & open spaceAgricultureVacantTransportationUtilitiesWaste handling & managementSurface watersLocal Study Area--Indicates location of areas shown in figures 4 and 16

O t i s P i k e F i r e I s l a n d H i g h D u n e W i l d e r n e s s

S m i t h P o i n t C o u n t y P a r k

C

D

NationalWilderness

Watch Hill

Figure 3. Land use and locations of local study areas in the Fire Island study area, Suffolk County, N.Y. (Land use from C.G. Lind, Suffolk County Department of Planning, written commun., 2006. Locations of areas shown in panels A, B, C, and D are shown in figure 2. NAD 83, North American Datum of 1983)—Continued
8 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

!

!

!

!

!

!

!

!

! 125986

1.00

125984

0.90

125995

0.98

125981

1.05

125983

0.92

125370

0.52

126753

1.00

125985

0.92

125371

0.98

73°12'15" 73°12'

40°38'15"

40°38'

EXPLANATION

Trace of Hydrogeologic Section--Shows location of vertical section shown in figure 8A.

! Observation Well--Upper number is identifier. Lower number is water level, in meters above NGVD 29.

Base from New York State Department of Transportation planimetric quadrangles, 1991, 1:24,000; Universal Transverse Mercator projection, NAD 83, Zone 18

B'

B

125370

0.52

B B'

0 50 100 METERS

0 150 300 FEET

Figure 4. Locations of observation wells and vertical sections B–B’ through E–E’ in local study areas and groundwater 

levels during October 31–November 1, 2005, on Fire Island, Suffolk County, N.Y. (Well number is assigned by New York 

State Department of Environmental Conservation. Prefix “S” denoting Suffolk County is omitted. NGVD 29, National 

Geodetic Vertical Datum of 1929)
Introduction 9

!

!

!

!

!

!

! 126000

0.75

125982

0.75

125999

0.80

126001

0.69

125996

0.65

125997

0.71

125998

0.75

125994

0.77

73°10' 73°09'45"

40°38'45"

40°38'30"

EXPLANATION

Trace of Hydrogeologic Section--Shows location of vertical section shown in figure 8B.

! Observation Well--Upper number is identifier. Lower number is water level, in meters above NGVD 29.

Base from New York State Department of Transportation planimetric quadrangles, 1991, 1:24,000; Universal Transverse Mercator projection, NAD 83, Zone 18

C'

C

125996

0.65

C C'

0 50 100 METERS

0 150 300 FEET

Figure 4. Locations of observation wells and vertical sections B–B’ through E–E’ in local study areas and groundwater 

levels during October 31–November 1, 2005, on Fire Island, Suffolk County, N.Y. (Well number is assigned by New York 

State Department of Environmental Conservation. Prefix “S” denoting Suffolk County is omitted. NGVD 29, National 

Geodetic Vertical Datum of 1929)—Continued
10 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

!

!

!

!

!

!

!

!

! 125990

0.92

125384

0.91

125386

0.83

125385

0.94

125387

0.79

125382

0.83

125381

0.86

125383

0.93

125380

0.87

72°59'30" 72°59'15"

40°41'30"

40°41'15"

EXPLANATION

Trace of Hydrogeologic Section--Shows location of vertical section shown in figure 8C.

! Observation Well--Upper number is identifier. Lower number is water level, in meters above NGVD 29.

Base from New York State Department of Transportation planimetric quadrangles, 1991, 1:24,000; Universal Transverse Mercator projection, NAD 83, Zone 18

D'

D

125387

0.79

D D'

0 50 100 METERS

0 150 300 FEET

Figure 4. Locations of observation wells and vertical sections B–B’ through E–E’ in local study areas and groundwater 

levels during October 31–November 1, 2005, on Fire Island, Suffolk County, N.Y. (Well number is assigned by New York 

State Department of Environmental Conservation. Prefix “S” denoting Suffolk County is omitted. NGVD 29, National 

Geodetic Vertical Datum of 1929)—Continued
Introduction 11

!

!

! !

!

!

!

!

124400

0.96

125369

0.77

125368

0.69

124399

0.88

125365

0.61 125367

0.81 125366

0.66

125363

0.73

125364

0.71

72°52'15" 72°52'

40°44'

40°43'45"

EXPLANATION

Trace of Hydrogeologic Section--Shows location of vertical section shown in figure 8D.

! Observation Well--Upper number is identifier. Lower number is water level, in meters above NGVD 29.

Base from New York State Department of Transportation planimetric quadrangles, 1991, 1:24,000; Universal Transverse Mercator projection, NAD 83, Zone 18

E'

E

124400

0.96

E E'

0 50 100 METERS

0 150 300 FEET

Figure 4. Locations of observation wells and vertical sections B–B’ through E–E’ in local study areas and groundwater 

levels during October 31–November 1, 2005, on Fire Island, Suffolk County, N.Y. (Well number is assigned by New York 

State Department of Environmental Conservation. Prefix “S” denoting Suffolk County is omitted. NGVD 29, National 

Geodetic Vertical Datum of 1929)—Continued
12 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Geologic unit

and generalized

sediment desciption

from core samples

Lagoonal deposits--Gray to dark

gray and grayish brown, medium to

coarse sand, locally with shell

fragments, fine gravel, and layers of

dark gray, fine sand and silt.

Peat deposits

Barrier island and shelf sands

Geologic unit

and generalized

sediment description

from core samples

Gardiners Clay--Greenish black, fine

sand, silt, and clay with some medium

to coarse sand, fine gravel, and

lenses of black silt and clay.

Lower outwash--Very dark greenish

gray silt and clay with some fine to

coarse sand, fine gravel, and shell

fragments in basal 3 meters.

Upper outwash

Lagoonal deposits--Gray to dark

gray and dark reddish gray, fine to

coarse sand, locally with shell

fragments and some fine gravel.

Peat deposits

A Well S126753

Gamma

radiation,

in counts

per second

A

0 10 20 30 40 50 60 70 ltitude, in meters above

or below NGVD 1929

-43.5

-38.5

-33.5

-28.5

-23.5

-18.5

-13.5

-8.5

-3.5

1.5

Electromagnetic induction,

in millisiemens per meter

0 300 600 900 1,200

Depth, in meters below

 land surface

0

5

10

15

20

25

30

35

40

45

GAMMA RAY

ELECTROMAGNETIC

INDUCTION

EXPLANATION

0

B Well S126000

Gamma

radiation,

in counts

per second

A

0 10 20 30 40 50 60 ltitude, in meters above

or below NGVD 1929

-43.2

-38.2

-33.2

-28.2

-23.2

-18.2

-13.2

-8.2

-3.2

1.8

Electromagnetic induction,

in millisiemens per meter

0 300 600 900

Depth, in meters below

 land surface

0

5

10

15

20

25

30

35

40

45

GAMMA RAY

ELECTROMAGNETIC

INDUCTION

EXPLANATION

0

Salinity, in psu

0.37

0.30

0.31

0.28

2.7

Salinity, in psu

0.11

0.28

0.24

0.28

19

23

28

28

30

SCREENED

INTERVAL

SCREENED

INTERVAL

Figure 5. Gamma-ray and electromagnetic-induction logs, salinity from filter-press core samples, generalized sediment description 

from core samples, and corresponding geologic units for borings at four wells on Fire Island, Suffolk County, N.Y., (A) well 126753, 

(B) well 126000, (C) well 125990, and (D) well 124399. (Well locations are shown in figure 4. NGVD 29, National Geodetic Vertical Datum 

of 1929; psu, practical salinity units at 25 degrees Celsius)
Introduction 13

Figure 5. Gamma-ray and electromagnetic-induction logs, salinity from filter-press core samples, generalized sediment description 

from core samples, and corresponding geologic units for borings at four wells on Fire Island, Suffolk County, N.Y., (A) well 126753, 

(B) well 126000, (C) well 125990, and (D) well 124399. (Well locations are shown in figure 4. NGVD 29, National Geodetic Vertical Datum 

of 1929; psu, practical salinity units at 25 degrees Celsius)—Continued

Geologic unit

and generalized

sediment desciption

from core samples

Upper outwash--Gray to dark gray

and grayish brown, medium to coarse

sand, locally with fine to coarse

gravel; and layers of dark gray to

olive gray silt and clay, locally with

fine to coarse sand.

Peat deposits

Gardiners Clay--Dark greenish gray

to olive gray and brown silt and clay

with some mica.

Lagoonal deposits--Multicolored, medium 

to coarse sand, locally with fine gravel.

Barrier island and shelf sands--Multicolored, medium to coarse sand,

locally with fine gravel.

Lower outwash--Grayish brown, fine

to coarse sand, locally with abundant

mica and fine gravel; and light green

to pale green and dark greenish gray,

fine to medium sand and silt, locally

with abundant mica, clay, and some

coarse sand in basal 7 meters.

Geologic unit

and generalized

sediment description

from core samples

Lower outwash--Dark gray, fine sand

and silt with abundant mica and

lenses of dark gray clay; and gray to

dark gray, fine to coarse sand, locally

with abundant mica and fine gravel.

Upper outwash--Gray to dark gray,

fine to medium sand; and dark gray,

fine to medium sand with some shell

fragments, coarse sand, fine gravel,

and streaks of very dark gray silt and

clay in basal 1 meter.

Lagoonal deposits--Dark gray to

dark grayish brown, medium sand,

locally with coarse sand and streaks

of dark reddish brown, fine sand and

silt.

Barrier island and shelf sands

C Well S125990

Gamma

radiation,

in counts

per second

A

0 10 20 30 40 50 60 ltitude, in meters above

or below NGVD 1929

-43.2

-38.2

-33.2

-28.2

-23.2

-18.2

-13.2

-8.2

-3.2

1.8

Electromagnetic induction,

in millisiemens per meter

0 300 600 900 1,200

Depth, in meters below

 land surface

0

5

10

15

20

25

30

35

40

45

GAMMA RAY

ELECTROMAGNETIC

INDUCTION

EXPLANATION

0

D Well S124399

Gamma

radiation,

in counts

per second

A

10 20 30 40 50 60 70 80 ltitude, in meters above

or below NGVD 1929

-39.2

-34.2

-29.2

-24.2

-19.2

-14.2

-9.2

-4.2

0.8

5.8

Electromagnetic induction,

in millisiemens per meter

0 300 600 900 1,200

Depth, in meters below

 land surface

0

5

10

15

20

25

30

35

40

45

GAMMA RAY

ELECTROMAGNETIC

INDUCTION

EXPLANATION

0

Salinity, in psu

Salinity, in psu

0.24

0.20

0.22

0.29

6.4

18

23

27

29

0.10

0.18

0.16

0.48

14

24

31

9

11

SCREENED

INTERVAL

SCREENED

INTERVAL
14 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

3 7

5

8

7

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

9

2 4 1

9

7 7

9

73°04' 73°06' 73°08' 73°10'

40°39'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

0 0.5 1 KILOMETER0 0.5 1 MILE

Figure 6. Locations of public water-supply systems and selected wastewater-disposal systems on Fire Island, Suffolk County, N.Y. (Service-area locations for Suffolk County Water Authority from Jeff Altorfer, Suffolk County Water Authority, written commun., 2005. Service-area locations for Saltaire, Ocean Beach, Seaview, and Ocean Bay Park from distribution-system maps on file at the Suffolk County Department of Health Services. Locations of wastewater-disposal systems for National Park Service facilities from maps provided by Patricia Rafferty, National Park Service, written commun., 2007. Locations of wastewater-disposal systems for facilities at Smith Point County Park from maps provided by Eric Budres, Suffolk County Department of Parks, Recreation & Conservation, written commun., 2007. Locations of areas shown in panels A, B, C, and 

D are shown in figure 2. NAD 83, North American Datum of 1983)
Introduction 15

6

66

72°54' 72°56' 72°58' 73° 73°02'

40°42'

40°41'

10

10 10

72°52' 72°46' 72°48' 72°50'

40°44'

40°43'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

EXPLANATION

Map Number, System

1, Ocean Bay Park Distribution and Waste2, Ocean Beach Distribution3, Saltaire Distribution and Waste4, Seaview Distribution and Waste5, Robert Moses State Park Distribution and Waste

6, Davis Park Distribution and Waste6, Davis Park Distribution6, Watch Hill Waste7, Fair Harbor-Summer Club Distribution and Waste7, Fair Harbor-Summer Club Distribution8, Kismet Distribution and Waste9, Point O'Woods-Fire Island Pines Distribution and Waste

9, Sailors Haven Waste9, Point O'Woods-Fire Island Pines Distribution10, National Wilderness Waste10, Smith Point County Park Waste10, Westhampton Distribution

C

D

Fgure 6. Locations of public water-supply systems and selected wastewater-disposal systems on Fire Island, Suffolk County, N.Y. (Service-area locations for Suffolk County Water Authority from Jeff Altorfer, Suffolk County Water Authority, written commun., 2005. Service-area locations for Saltaire, Ocean Beach, Seaview, and Ocean Bay Park from distribution-system maps on file at the Suffolk County Department of Health Services. Locations of wastewater-disposal systems for National Park Service facilities from maps provided by Patricia Rafferty, National Park Service, written commun., 2007. Locations of wastewater-disposal systems for facilities at Smith Point County Park from maps provided by Eric Budres, Suffolk County Department of Parks, Recreation & Conservation, written commun., 2007. Locations of areas shown in panels A, B, C, and D are shown in figure 2. NAD 83, North American Datum of 1983)—Continued
16 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York Table 1. Monthly pumpage in 2005 for public water-supply systems on Fire Island, Suffolk County, N.Y. [Locations of water-supply systems shown in figure 6] Well field Withdrawal (thousands of liters) Map number Name January February March April May June July August September October November December Year OCEAN BAY PARK WATER DISTRICT

1 Subtotal 1,090 1,374 485 2,279 5,951 10,156 14,260 15,713 10,546 5,152 2,903 3,047 72,956

OCEAN BEACH WATER DISTRICT

2 Subtotal 15,842 12,401 13,964 17,489 28,939 20,555 40,368 40,250 29,640 14,903 14,604 12,178 261,133

SALTAIRE WATER DISTRICT

3 Subtotal 2,430 3,876 3,486 3,025 11,197 19,404 14,650 24,586 19,537 1,726 5,137 3,225 112,279

SEAVIEW WATER DISTRICT

4 Subtotal 0 0 0 8,589 19,094 24,064 35,931 28,315 15,452 0 0 0 131,445

ROBERT MOSES STATE PARK

5

Well number 2 265 140 0 23 0 0 121 572 0 72 0 0 1,192Well number 3 7,711 4,243 7,385 9,040 10,951 19,726 22,716 28,841 13,976 6,178 5,716 3,335 139,818Subtotal 7,976 4,384 7,385 9,062 10,951 19,726 22,837 29,413 13,976 6,250 5,716 3,335 141,010

SUFFOLK COUNTY WATER AUTHORITY—DAVIS PARK ZONE

6 Subtotal 19,400 8,252 4,690 6,878 14,912 14,866 17,655 14,813 9,720 7,974 3,264 2,736 125,161

SUFFOLK COUNTY WATER AUTHORITY—FAIR HARBOR-SUMMER CLUB ZONE

7

Bay Walk 102 8 32 160 8 9 13 207 2 45 118 58 761Center Walk 3,951 3,463 6,731 7,049 14,614 16,442 14,561 12,283 11,585 8,776 7,434 4,740 111,629Central Walk 3,112 179 5,978 1,721 6,380 16,059 26,980 37,036 25,212 16,253 3,712 671 143,294Robbins Walk 27 9 13 58 65 50 81 220 22 100 55 1 702Sandy Walk 0 0 0 0 0 0 0 0 0 0 72 1,832 1,904Subtotal 7,192 3,659 12,754 8,988 21,067 32,560 41,635 49,746 36,822 25,175 11,392 7,303 258,291

SUFFOLK COUNTY WATER AUTHORITY—KISMET ZONE

8 Subtotal 340 267 417 4,985 6,314 8,547 14,148 16,018 9,382 6,481 1,919 322 69,141

SUFFOLK COUNTY WATER AUTHORITY--POINT O’WOODS-FIRE ISLAND PINES ZONE

9

Bayview Walk 6,229 6,098 5,568 11,706 48,877 36,516 25,468 14,011 15,150 18,995 5,619 3,660 197,897Fishermans Path 8,324 10,377 11,357 22,398 4,096 41,112 82,148 103,249 76,180 29,262 21,600 12,650 422,751Harbor Walk 4 15 125 14 175 3,634 38 167 344 337 61 21 4,934Point Of Woods 0 0 0 6 642 23 0 0 0 0 118 225 1,013Subtotal 14,557 16,490 17,050 34,124 53,790 81,284 107,653 117,427 91,674 48,594 27,398 16,555 626,595TOTAL 68,826 50,703 60,232 95,418 172,215 231,163 309,136 336,281 236,748 116,255 72,332 48,702 1,798,011
Hydrogeologic Framework 17

Hydrogeologic Framework

The fresh groundwater reservoir on Fire Island consists 

of a freshwater flow system within a sequence of Holocene 

surficial deposits and Pleistocene glacial and nonglacial 

deposits that are underlain by Late Cretaceous Coastal 

Plain deposits.

Geologic Setting

Fire Island is underlain by a complex aquifer system 

consisting of unconsolidated glacial, lacustrine, deltaic, 

and marine deposits of clay, silt, sand, and gravel that range 

in age from Late Cretaceous to Holocene. These deposits 

overlie a southward-sloping basement complex of crystalline 

bedrock of Precambrian and (or) Paleozoic age. A generalized 

description of the uppermost Cretaceous, Pleistocene, and 

Holocene geologic units and their relation to hydrogeologic 

units is provided in table 2; the underlying Cretaceous units 

are omitted because they are hydraulically isolated from the 

shallow aquifer system by the low permeability of intervening 

units (Scorca and others, 1995) and, therefore, were not 

considered in this investigation. A generalized vertical section 

depicting the geometry of hydrogeologic units underlying Fire 

Island is presented in figure 7.

Monmouth Group

The uppermost unit of Cretaceous deposits on Long 

Island is the Monmouth Group, which constitutes the 

Monmouth greensand and unconformably overlies the 

Matawan Group and Magothy Formation, undifferentiated 

(Magothy aquifer). In Suffolk County, the Monmouth 

greensand consists primarily of interbedded marine deposits of 

greenish to black clay, silt, and sand containing large amounts 

of glauconite and lignite (Jensen and Soren, 1974). The unit 

does not appear to have been reached by any borings drilled 

during this study (fig. 8).

Pleistocene Deposits

Pleistocene deposits unconformably overlie Cretaceous 

deposits in the Fire Island study area and consist of the 

Gardiners Clay and overlying Wisconsinan deposits. The 

coarser-grained (mainly sand and gravel) Wisconsinan 

deposits are considered collectively as the upper 

glacial aquifer. 

Table 2. Generalized description of geologic and hydrogeologic units in the Fire Island study area, Suffolk County, N.Y.

[Descriptions of Upper Cretaceous deposits modified from Jensen and Soren (1974). Descriptions of selected Pleistocene deposits modified from Jensen and 

Soren (1974) and Doriski and Wilde-Katz (1983). Descriptions of selected Holocene deposits modified from Jones and Schubel (1980) and LoCicero (2006)]

Age Geologic unit Hydrogeologic unit Generalized description of deposits

Holocene Barrier island and shelf sands Fine to medium sand.

Peat deposits Black peat with some medium to fine sand.

Seagrass beds Inorganic sediments consist mostly of fine sand and silt with 

some medium sand.

Estuarine mud Clay and silt with some fine sand.

Lagoonal deposits Fine sand and silt with beds of medium to coarse sand and 

layers of clay and silt.

Pleistocene Upper outwash Upper glacial aquifer1 Brown, fine to coarse sand, locally with some gravel and 

streaks of silt and clay.

“20-foot” clay Grayish-green to gray clay that contains a few sand and silt 

beds, and has same fossil content as Gardiners Clay

Lower outwash Upper glacial aquifer1 Light brown, fine to medium sand, commonly with silt. Silt 

and clay with some fine to medium sand generally found 

in basal 6 to 9 m.

Gardiners Clay Greenish-gray to gray clay and silt with some interbedded 

sand and gravel. Locally contains diatoms, foraminifera, 

shell fragments of pelecypods and gastropods, and peat.

Upper Cretaceous Monmouth Group Monmouth greensand Greenish to black clay, silt, and sand containing large 

amounts of glauconite and lignite.

1

Coarse-grained deposits of post-Cretaceous age on Long Island are commonly considered one hydrologic unit and are referred to as the upper glacial aquifer. 

(See discussion in text.)
18 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Gardiners Clay

The lowermost unit of Pleistocene deposits beneath Fire 

Island is the Gardiners Clay. This unit generally is present 

throughout most of the study area (fig. 9), except along 

part of the southern shore of the Long Island mainland. In 

Suffolk County, the Gardiners Clay consists primarily of 

marine deposits of greenish-gray to gray clay and silt with 

some interbedded sand and gravel (Jensen and Soren, 1974). 

In southern Nassau and southwestern Suffolk Counties, the 

unit also contains diatoms, foraminifera, shell fragments of 

pelecypods and gastropods, and peat (Doriski and WildeKatz, 1983). Two borings drilled during this study reached 

the Gardiners Clay (S126753 and S124399, figs. 5 and 8), 

although neither boring penetrated its full thickness.

Wisconsinan Deposits

Late Pleistocene deposits in southern Nassau and 

southwestern Suffolk Counties are separated into three 

units by Doriski and Wilde-Katz (1983). These units are, in 

ascending order, a lower unit of Late Pleistocene outwash 

(hereafter referred to as the lower outwash), the “20-foot” 

clay, and an upper unit of Late Pleistocene outwash (hereafter 

referred to as the upper outwash).

Figure 7. Generalized vertical section A–A’ showing major hydrogeologic units and approximate distribution and 

flow directions of fresh and saline groundwater along (A) the southern shore of Long Island and (B) on Fire Island, N.Y. 

[(A) modified from Scorca and others, 1995 figure 2; (B) modified from Collier, 2006, figure 3. General location of section 

is shown in figure 2.]

A

Seaward extension of unconfined aquifer

containing salty groundwater

Confining unit (Gardiners Clay)

Schematic diagram, not to scale

Unconfined aquifer containing

fresh groundwater

(Upper glacial aquifer)

Confining unit (Monmouth greensand)

Zone of transition between fresh

groundwater and salty groundwater

(brackish water)

South North

A A’

Underlying confined aquifer (Magothy aquifer) contains fresh

groundwater. Saltwater interface is seaward of barrier island

ATLANTIC OCEAN

Water table

FIRE

ISLAND

LONG

ISLAND

GREAT

SOUTH

BAY

Area shown

in Figure 7b
Hydrogeologic Framework 19

Lower Outwash

The lower outwash constitutes the lowermost unit of the 

upper glacial aquifer. The lower outwash consists primarily 

of light brown, fine to medium sand (Doriski and WildeKatz, 1983), which in the study area commonly contains 

silt; the basal 6 to 9 m generally consist of silt and clay with 

some fine to medium sand. The lower outwash overlies the 

Gardiners Clay throughout most of the study area. Where 

the Gardiners Clay is absent, the lower outwash directly 

overlies the Monmouth Group (fig. 9D). At least four borings 

drilled during this study appear to have reached the lower 

outwash (S126753, S125381, S125990, and S124399; figs. 5 

and 8), two of which penetrated the full thickness (S126753 

and S124399).

“20-Foot” Clay

The “20-foot” clay is lithologically similar to the 

Gardiners Clay—typically grayish-green to gray clay that 

contains a few sand and silt beds—and has the same fossil 

content (Doriski and Wilde-Katz, 1983). The “20-foot” clay 

overlies the lower outwash in the southwestern part of the 

study area but is absent beneath the west-central part of Fire 

Island (Doriski and Wilde-Katz, 1983) (figs. 10 and 11). The 

unit does not appear to have been penetrated by any borings 

drilled during this study (fig. 8) and is assumed to be absent in 

the remainder of the study area (figs. 10 and 11).

 Upper Outwash

The upper outwash represents the uppermost Pleistocene 

unit of the upper glacial aquifer. The upper outwash consists 

primarily of brown sand that is coarser grained and contains 

more gravel than that of the lower outwash (Doriski and 

Wilde-Katz, 1983); in the current study, the upper outwash 

was found to generally consist of fine to coarse sand, locally 

with some gravel and streaks of silt and clay. The upper 

outwash overlies the “20-foot” clay in the southwestern part 

of the study area (Doriski and Wilde-Katz, 1983). Where the 

“20-foot” clay is absent, the upper outwash directly overlies 

the lower outwash. The upper outwash extends to land surface 

and constitutes the surficial hydrogeologic unit along the 

southern shore of the Long Island mainland and locally along 

the seabed in back-bay (estuary) areas (Goodbred and others, 

2005) and ocean areas offshore (Schwab and others, 2000). 

At least eight borings drilled during this study appear to have 

reached the upper outwash (S126753, S125380, S125381, 

S125382, S125990, S125385, S125369, and S124399; 

figs. 5 and 8), four of which appear to have penetrated the full 

thickness (S126753, S125381, S125990, and S124399).

Holocene Deposits

Holocene deposits that overlie Late Pleistocene deposits 

beneath back-bay areas, Fire Island, and most ocean areas 

are separated into an extensive sequence of lagoonal deposits 

overlain by four principally surficial units—estuarine mud, 

seagrass beds, peat deposits, and barrier island and shelf sands.

Lagoonal Deposits

Cores collected during this study, and those previously 

collected from Great South Bay (Goodbred and Usher, 

2001; Goodbred and others, 2005; LoCicero, 2006), indicate 

a sequence of fine- to coarse-grained marine deposits of 

Holocene age generally overlies Pleistocene deposits beneath 

Fire Island and adjacent back-bay areas. The Holocene 

deposits consist primarily of fine sand and silt with beds of 

medium to coarse sand and layers of clay and silt (LoCicero, 

2006). These deposits presumably include estuarine mud, 

relict seagrass beds, and peat deposits interbedded with floodtidal-shoal, inlet-fill, and overwash deposits. This sequence of 

fine- to coarse-grained deposits likely resulted from landward 

barrier-island migration and a gradually rising sea level and is 

hereafter collectively referred to as lagoonal deposits.

Figure 7. Generalized vertical section A–A’ showing major 

hydrogeologic units and approximate distribution and flow 

directions of fresh and saline groundwater along (A) the 

southern shore of Long Island and (B) on Fire Island, N.Y.

[(A) modified from Scorca and others, 1995 figure 2; 

(B) modified from Collier, 2006, figure 3. General location of 

section is shown in figure 2.]—Continued

Schematic diagram, not to scale

Unconfined aquifer containing

salty groundwater

Land surface Water table

Gardiners clay

Recharge area

GREAT

SOUTH

BAY

ATLANTIC

OCEAN

South North

Freshwater lens

Brackish water

B
20 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Figure 8. Vertical sections B–B ́ through E–E ́ showing hydrogeologic units, freshwater-saltwater interface, 

and wells with screened intervals in local study areas, and water-table altitude and groundwater levels during 

October 31–November 1, 2005, on Fire Island, Suffolk County, N.Y. (Section locations are shown in figure 4. 

NGVD 29, National Geodetic Vertical Datum of 1929)

EXPLANATION

METERS

-37.5

-30.0

-22.5

-15.0

-7.5

0

7.5

Gamma radiation, in counts per second

Electromagnetic induction, in millisiemens

 per meter

126753, 125986

125985

125370

125371

B B'

VERTICAL SCALE GREATLY EXAGGERATED

DATUM IS NGVD 29

SOUTHEAST NORTHWEST ATLANTIC

O

CEA

N

Great

South

Bay

0.98

1.00

1.00

0.92

0.52

Barrier island and shelf sands

Lagoonal deposits

Peat deposits

Upper outwash

(upper glacial aquifer)

Gardiners Clay

Lower outwash

(upper glacial aquifer)

? ?

?

?

"20-foot" clay ?

?

0 50 100 METERS

Well Shown in Figure 4A--Number is assigned 0 100 200 300 FEET

 by New York State Department of Environmental

 Conservation. Prefix "S" denoting Suffolk County

 is omitted.

Screened Interval--Number is water level, in

 meters above NGVD 29.

0 100

0 1,500

125371

0.98

Water table

Freshwater-saltwater

interface
Hydrogeologic Framework 21

Figure 8. Vertical sections B–B ́ through E–E ́ showing hydrogeologic units, freshwater-saltwater interface, 

and wells with screened intervals in local study areas, and water-table altitude and groundwater levels during 

October 31–November 1, 2005, on Fire Island, Suffolk County, N.Y. (Section locations are shown in figure 4. 

NGVD 29, National Geodetic Vertical Datum of 1929)—Continued

EXPLANATION

METERS

-37.5

-30.0

-22.5

-15.0

-7.5

0

7.5

126000, 125982

125997

125996

125994

C C'

SOUTHEAST NORTHWEST

ATLA

NTIC

O

CEA

N

Great South Bay

125998

Gamma radiation, in counts

 per second

Electromagnetic induction,

 in millisiemens per meter

0.77 0.75

0.75

0.75 0.71 0.65

Barrier island and shelf sands

Peat

Lagoonal deposits

deposits

Upper and lower outwash,

undifferentiated

(upper glacial aquifer)

Gardiners Clay

? ?

?

?

Water table

Freshwatersaltwater

interface

0 50 100 METERS

0 100 200 300 FEET 125994

Well Shown in Figure 4B--Number is

 assigned by New York State Department

 of Environmental Conservation. Prefix "S"

 denoting Suffolk County is omitted.

Screened Interval--Number is water

 level, in meters above NGVD 29. 0.77

0 100

0 1,500

VERTICAL SCALE GREATLY EXAGGERATED

DATUM IS NGVD 29
22 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Figure 8. Vertical sections B–B ́ through E–E ́ showing hydrogeologic units, freshwater-saltwater interface, 

and wells with screened intervals in local study areas, and water-table altitude and groundwater levels during 

October 31–November 1, 2005, on Fire Island, Suffolk County, N.Y. (Section locations are shown in figure 4. 

NGVD 29, National Geodetic Vertical Datum of 1929)—Continued

EXPLANATION

METERS

-37.5

-30.0

-22.5

-15.0

-7.5

0

7.5

125990, 125383

125382

125387

125380

D D'

SOUTHEAST NORTHWEST ATLA

NTIC 

O

CEA

N

Great South Bay

125381

Gamma radiation, in counts per second

Electromagnetic induction, in millisiemens

 per meter

0.87 0.93

0.92

0.86 0.83

0.79

Barrier island

and shelf sands

Peat

Lagoonal deposits

deposits

Upper outwash

(upper glacial

aquifer)

Gardiners Clay(?)

?

? ?

?

?

Water table

Freshwater-saltwater

interface

VERTICAL SCALE GREATLY EXAGGERATED

DATUM IS NGVD 29

0 50 100 METERS

0 100 200 300 FEET

125380

0.87

Well Shown in Figure 4A--Number is assigned

 by New York State Department of Environmental

 Conservation. Prefix "S" denoting Suffolk County

 is omitted.

Screened Interval--Number is water level, in

 meters above NGVD 29.

0 100

0 1,500

? ?

Lower outwash

(upper glacial aquifer)
Hydrogeologic Framework 23

Figure 8. Vertical sections B–B ́ through E–E ́ showing hydrogeologic units, freshwater-saltwater interface, 

and wells with screened intervals in local study areas, and water-table altitude and groundwater levels during 

October 31–November 1, 2005, on Fire Island, Suffolk County, N.Y. (Section locations are shown in figure 4. 

NGVD 29, National Geodetic Vertical Datum of 1929)—Continued

EXPLANATION

METERS

-37.5

-30.0

-22.5

-15.0

-7.5

0

7.5

125369, 125364

Gamma radiation, in counts

 per second

125366

125365

125363

E E'

SOUTHEAST NORTHWEST

ATLA

NTIC

O

CEA

N

Great

South

Bay

0.73 0.71

0.77

0.66

0.61

Barrier island and shelf sands

Peat

Lagoonal deposits

Upper and lower outwash,

undifferentiated

(upper glacial aquifer)

Gardiners Clay

? deposits ?

?

?

Water table

Freshwater-saltwater

interface

125363

0.73

Well Shown in Figure 4D--Number

 is assigned by New York State

 Department of Environmental

 Conservation. Prefix "S" denoting

 Suffolk County is omitted.

Screened Interval--Number is

 water level, in meters above

 NGVD 29.

VERTICAL SCALE GREATLY EXAGGERATED

DATUM IS NGVD 29

0 50 100 METERS

0 100 200 300 FEET

0 100
24 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

73°04' 73°06' 73°08' 73°10'

40°39'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

0 0.5 1 KILOMETER0 0.5 1 MILE

Figure 9.

Altitude of upper surface of Gardiners Clay and, where absent, Monmouth greensand in the Fire Island study area, Suffolk County, N.Y. (Upper-surface 

altitudes of Gardiners Clay and Monmouth greensand from Smolensky and others, 1989, sheet 3. Locations of areas shown in panels A, B, C, and 

D are shown in 

figure 2. NGVD 29, National Geodetic Vertical Datum of 1929; NAD 83, North American Datum of 1983)
Hydrogeologic Framework 25

72°54' 72°56' 72°58' 73° 73°02'

40°42'

40°41'

72°52' 72°46' 72°48' 72°50'

40°44'

40°43'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

EXPLANATION

Altitude, in meters. Datum is NGVD 29

-23.1 to -20.2-26.1 to -23.2-29.1 to -26.2-32 to -29.2

-35 to -32.1-38 to -35.1-41 to -38.1-44 to -41.1Updip limit of Gardiners Clay

C

D

Figure 9. Altitude of upper surface of Gardiners Clay and, where absent, Monmouth greensand in the Fire Island study area, Suffolk County, N.Y. (Upper-surface altitudes of Gardiners Clay and Monmouth greensand from Smolensky and others, 1989, sheet 3. Locations of areas shown in panels A, B, C, and D are shown in figure 2. NGVD 29, National Geodetic Vertical Datum of 1929; NAD 83, North American Datum of 1983)—Continued
26 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

73°04' 73°06' 73°08' 73°10'

40°39'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

E

X

P

L

A

N

ATIO

N

Altitude, in meters. Datum is N

G

V

D 29

-11.6 to -10.1-13.1 to -11.7-14.7 to -13.2-16.4 to -14.8

0 0.5 1 KILOMETER

0 0.5 1 MILE

Figure 10.

Altitude of upper surface of “20-foot” clay in the Fire Island study area, Suffolk County, N.Y. (Upper-surface altitude of “20-foot” clay from Doriski and 

Wilde-Katz, 1983, plate 7. “20-foot” clay absent from areas shown in panels C and D, which are omitted. Locations of areas shown in panels 

A and 

B are shown in 

figure 2. NGVD 29, National Geodetic Vertical Datum of 1929; NAD 83, North American Datum of 1983)
Hydrogeologic Framework 27

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

73°04' 73°06' 73°08' 73°10'

40°39'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

EXPLANATION

Thickness, in meters

7.6 to 96.1 to 7.54.6 to 63.1 to 4.51.6 to 30 to 1.5

0 0.5 1 KILOMETER

0 0.5 1 MILE

Figure 11. Thickness of “20-foot” clay in the Fire Island study area, Suffolk County, N.Y. (Thickness of “20-foot” clay from Doriski and Wilde-Katz, 1983, plate 8. “20-foot” clay absent from areas shown in panels C and D, which are omitted. Locations of areas shown in panels A and B are shown in figure 2. NGVD 29, National Geodetic Vertical Datum of 1929; NAD 83, North American Datum of 1983)
28 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Lagoonal deposits that constitute the surficial 

hydrogeologic unit appear to be present in back-bay areas 

characterized by unconsolidated sediments (fig. 12) and are 

presumed to be present in other areas west of Watch Hill 

and in main channels from this vicinity eastward that are 

characterized by unknown benthic habitat, as mapped by 

Greenhorne and O’Mara, Inc. (2003). The lagoonal deposits 

appear to have been penetrated by all but four borings drilled 

during this study (S125370, S125371, S125387, and S125365; 

figs. 5 and 8), and eight appear to have penetrated the full 

thickness (S126753, S125380, S125381, S125382, S125990, 

S125385, S125369, and S124399).

Estuarine Mud

Estuarine mud overlies the lagoonal deposits and 

constitutes the surficial hydrogeologic unit locally in backbay areas (fig. 12). This unit is present in embayments 

characterized by mud and is presumed to be found in other 

embayments and secondary channels from Watch Hill 

eastward that are characterized by unknown benthic habitat 

(Greenhorne and O’Mara, Inc., 2003); the unit also is inferred 

to be present in harbors at Sailors Haven, Fire Island Pines, 

and Davis Park (fig. 3). Cores of estuarine mud collected 

previously from various parts of Great South Bay indicate the 

estuarine mud consists primarily of clay and silt with some 

fine sand (Jones and Schubel, 1980; Greenhorne and O’Mara, 

Inc., 2003; LoCicero, 2006).

Seagrass Beds

Seagrass beds overlie the lagoonal deposits and constitute 

the surficial hydrogeologic unit in most back-bay areas 

from Watch Hill eastward and locally west of this vicinity 

(fig. 12). This unit is present in shallow areas characterized 

by submerged rooted vascular plants (hereafter referred to as 

SRV, Greenhorne and O’Mara, Inc., 2003). Cores collected 

from areas of Great South Bay mapped by Jones and Schubel 

(1980) or Greenhorne and O’Mara, Inc. (2003) as containing 

seagrass or SRV beds, respectively, suggest that the inorganic 

sediments associated with this unit consist mostly of fine sand 

and silt with some medium sand (LoCicero, 2006).

Peat Deposits

Peat outcrops on the beach foreshore are uncovered 

regularly along the back bays (Nordstrom and Jackson, 

2005) and in some places along the Atlantic Ocean (fig. 13); 

buried peat deposits also are reported to underlie areas of 

the Fire Island interior and those in the adjacent back bays 

(Bokuniewicz and Pavlik, 1990; Collier, 2006). The peat 

deposits that overlie the lagoonal deposits are described 

locally as bog in drillers’ logs for many borings on Fire Island 

and were correlated with cores from hand augering and 

gamma-ray borehole logging conducted during this study. Peat 

deposits consist primarily of black peat with some medium to 

fine sand. The deposits generally are present near local mean 

sea level throughout the areas where drilling was conducted. 

Peat deposits, which are inferred to be mostly continuous 

beneath Fire Island and adjacent back-bay islands, also are 

present in Fire Island wetland areas associated with reedgrass 

marsh, low salt marsh, high salt marsh, northern interdunal 

cranberry swale, and brackish interdunal swale (Klopfer 

and others, 2002). They also are presumed to be present in 

adjacent back-barrier intertidal zones characterized by tidal 

marsh as mapped by Greenhorne and O’Mara, Inc. (2003); in 

these areas, peat deposits constitute the surficial hydrogeologic 

unit (fig. 12). The peat deposits generally were penetrated in 

all borings drilled during this study, except at one (S125990, 

figs. 5 and 8) where the deposits seem to be locally absent. 

Whether the absence of this unit in the shallow subsurface 

indicates nondeposition, erosion, or manmade excavation 

(such as for buried utilities) is unknown.

Barrier Island and Shelf Sands

Coarse-grained deposits, which consist primarily of fine 

to medium sand, overlie the buried peat deposits beneath 

Fire Island (fig. 12) and generally constitute the surficial 

hydrogeologic unit. These sediments probably overlie the 

lagoonal deposits and underlying upper outwash in most 

ocean areas. They likely include contiguous lenses of beach, 

overwash, and eolian deposits onshore and inner-shelf deposits 

offshore. These lithologically similar deposits are difficult to 

distinguish and are undifferentiated in this report; hereafter, 

they are referred to as barrier island and shelf sands. This unit 

was fully penetrated in most borings with the exception of 11 
Hydrogeologic Framework 29

(S125995, S125984, S125985, S126753, S125370, S125986, 

S125996, S125997, S125381, S125382, and S125387; figs. 5 

and 8); the unit was not observed in these boreholes.

Hydrologic Setting

Fresh groundwater in the shallow aquifer system on 

Fire Island is contained within a hydraulically isolated 

freshwater flow system that extends through the Holocene and 

Pleistocene hydrogeologic units. This freshwater flow system 

is bounded laterally by marine surface waters and at depth by 

saline groundwater (figs. 5, 7, and 8). Hydraulic heads and the 

movement of fresh groundwater on Fire Island are controlled 

by the hydraulic boundaries and aquifer properties of the 

freshwater flow system (figs. 4 and 8).

Hydrologic Boundaries

The natural hydrologic boundaries of the fresh 

groundwater reservoir on Fire Island are the hydrologic 

features that bound the extent of the freshwater flow system 

and, hence, the hydraulic stresses that control the rate at which 

freshwater enters and exits the system. Recharge to the system 

occurs across the water table where freshwater enters through 

infiltration of precipitation and as return flow of publicsupply water (in unsewered areas). Discharge boundaries are 

near the shore where freshwater exits as seepage across the 

seabed into marine surface waters (shoreline underflow) or 

as seepage through confining layers into saline groundwater 

(subsea underflow). Discharge boundaries also are where the 

land surface intersects the water table and freshwater exits 

as wetland evapotranspiration. A flow-through condition 

is provided by topographic depressions in the land surface 

that intersect the water table to form ponds where shallow 

groundwater discharges to the pond at the upgradient 

side and pond water recharges the water-table aquifer 

at the downgradient side (Masterson and others, 1996). 

The freshwater-saltwater interface, where freshwater is 

separated from denser saltwater by a zone of diffusion, acts 

as a relatively impermeable boundary that moves gradually 

in response to changes in the balance between recharge 

and discharge.

Precipitation and Recharge

The natural source of freshwater to the water table 

is recharge from precipitation. The amount of recharge is 

determined by the pattern and rate of precipitation and by the 

amount of precipitation lost as evapotranspiration and, to a 

much lesser extent, as surface runoff. Although precipitation 

in Suffolk County is fairly evenly distributed throughout the 

year (Peterson, 1987), evapotranspiration is greatest during 

the summer; therefore, most recharge takes place during the 

fall, winter, and spring. Seasonal fluctuations in recharge—for 

example, between summer and winter—generally are greater 

than any annual to decadal fluctuations.

Long-term daily precipitation-measurement records were 

obtained from the National Climatic Data Center (NCDC) 

(National Climatic Data Center, 2006) for nearby discontinued 

and active stations at Patchogue and Islip, respectively (fig. 2), 

and were used to calculate the annual and long-term mean 

precipitation rates in table 3. Comparison with 1984–96 mean 

precipitation rates at Patchogue and Islip indicate that annual 

and mean rates for 2003–05 at Islip generally were consistent 

with the long-term (1938–96) mean rate at Patchogue 

(table 3).

Estimates of the percentage of precipitation that becomes 

recharge in land areas of Long Island were reviewed and 

summarized by Peterson (1987) and generally are consistent 

with a recharge rate equal to about 50 percent of mean annual 

precipitation. An alternative method of calculating recharge in 

these areas (Steenhuis and others, 1985) specifies an annual 

recharge rate equal to 75 to 90 percent of precipitation from 

October 15 through May 15. Values of recharge to the water 

table in land areas of Long Island that are calculated by both 

methods from the Patchogue and Islip records are shown in 

table 3.

Recharge to the water table through fresh surface-water 

bodies in Suffolk County has been calculated by Schubert 

(1998) and Misut and others (2004) as the difference between 

long-term mean annual precipitation (table 3) and annual 

lake evaporation. These studies determined the long-term 

mean rate of lake evaporation on Long Island to be about 

85.4 cm/yr, as calculated from an average pan coefficient of 

0.7, which relates the rate of annual lake evaporation to annual 

pan evaporation (Linsley and Franzini, 1979), and mean 
30 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

73°04' 73°06' 73°08' 73°10'

40°39'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

0 0.5 1 KILOMETER0 0.5 1 MILE

Figure 12. Generalized land cover and substrate on Fire Island and adjacent islands within Fire Island National Seashore, and benthic habitats in back-bay areas of the Fire Island study area, Suffolk County, N.Y. (Land cover and substrate modified from Klopfer and others, 2002. Benthic habitats modified from Greenhorne and O’Mara, Inc., 2003. Locations of areas shown in panels A, B, C, and 

D are shown in figure 2. NAD 83, North American Datum of 1983)
Hydrogeologic Framework 31 Figure 12. Generalized land cover and substrate on Fire Island and adjacent islands within Fire Island National Seashore, and benthic habitats in back-bay areas of the Fire Island study area, Suffolk County, N.Y. (Land cover and substrate modified from Klopfer and others, 2002. Benthic habitats modified from Greenhorne and O’Mara, Inc., 2003. Locations of areas shown in panels A, B, C, and D are shown in figure 2. NAD 83, North American Datum of 1983)—Continued

72°54' 72°56' 72°58' 73° 73°02'

40°42'

40°41'

72°52' 72°46' 72°48' 72°50'

40°44'

40°43'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

EXPLANATION

Benthic Habitat

MudSubmerged rooted vascular plantsTidal marshUnconsolidated sediments

Land Cover or Substrate

Developed featureInland surface waterLoam substratePeat substrateSand substrate or sparse vegetation

C

D
32 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Table 3. Annual and long-term mean precipitation amounts at Patchogue and Islip, Suffolk County, N.Y.

[Station locations are shown in figure 2. Data from National Climatic Data Center, 2006]

Period

Precipitation, in centimeters

Calendar year

Total 50 percent of total Total 75 percent of total 90 percent of total

PATCHOGUE

1

1938–96 119.6 59.79 71.22 53.41 64.11

1984–96 127.2 63.57 72.79 54.58 65.50

ISLIP

1984–96 116.1 58.04 65.58 49.20 59.03

2003 126.8 63.42 63.85 47.90 57.48

2004 99.59 49.81 57.15 42.87 51.43

2005 104.3 52.14 56.92 42.70 51.23

2003–05 110.2 55.12 59.31 44.47 53.39

1984–2005 115.2 57.58 65.94 49.45 59.33

1

Precipitation data are incomplete for 1947, 1959, and 1965–66; data for these years were not used to compute long-term mean value.

Figure 13. Photograph of ocean shore between Watch Hill and Davis Park showing peat deposits occasionally 

uncovered on the Atlantic Ocean beaches of Fire Island, Suffolk County, N.Y. (Photograph from Paula Valentine, 

National Park Service, written commun., 2010.)
Hydrogeologic Framework 33

2.5 2.5

7.7

11.4

15.1

17.4

20.0

16.0

12.7

8.6

5.3

2.8

13.2

14.5

15.7

13.2

9.7

7.6

0

5

10

15

20

25

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

EVAPORATION, IN CENTIMETERS

MONTH

MINEOLA GREENPORT

Figure 14. Long-term mean monthly evaporation at land pans in Greenport and Mineola, Long Island, N.Y. (Numbers 

above shaded bars are evaporation values. Evaporation data for Greenport from National Climatic Data Center, 2007; 

data for Mineola from Pluhowski and Kantrowitz, 1964)

annual evaporation of 122 cm from a land pan at Mineola—

located in central Nassau County (fig. 1)—during 1949–60 

(Pluhowski and Kantrowitz, 1964).

Maximum summer temperatures are lower along the 

southern shore of the Babylon-Islip area than in its northern 

and central parts, a factor that should decrease evaporation 

(Pluhowski and Kantrowitz, 1964). Thus, evaporation on Fire 

Island and in other nearshore areas probably is considerably 

less than in the Long Island interior. Daily evaporation records 

obtained from the NCDC (National Climatic Data Center, 

2007) for a discontinued land pan at Greenport—located near 

the end of the peninsula that constitutes Long Island’s North 

Fork (fig. 1)—likely provide a reasonable approximation of 

evaporation on Fire Island. These records, which generally 

are available only for the months of May through October 

during 1958–97, were combined with long-term mean values 

from Mineola for the remaining months (fig. 14) to estimate 

mean annual values. The resulting rates of long-term mean 

annual pan and annual lake evaporation at Greenport are about 

106 and 74.3 cm/yr, respectively. These decreased rates of 

evaporation are assumed to yield increased rates of recharge to 

the water table on Fire Island.

The sparse vegetation and sandy soils that constitute 

the surficial hydrogeologic unit on parts of Fire Island 

also likely result in decreased rates of groundwater 

evapotranspiration. Previous investigations found evaporation 

through unvegetated clay loam, such as is present on Fire 

Island, decreased from 25 percent to less than 10 percent of 

pan evaporation with a water-table drop from about 0.3 to 

0.9 m below land surface (White, 1932; Todd, 1980; Kontis 

and others, 2004). These same studies found groundwater 

evapotranspiration rates declined as the depths to the capillary 

fringe increased and were zero below extinction depths of less 

than 3.0 m to as much as 7.6 m. 

Groundwater Discharge

Groundwater in the freshwater flow system of Fire 

Island discharges to shore and subsea-discharge areas or is 

lost as wetland evapotranspiration. Most freshwater discharge 

occurs as shoreline and subsea underflow, but the diffuse 

character and location of this outflow make the rate difficult 

to measure. The discharge of fresh groundwater is controlled 

by the hydraulic properties of the freshwater flow system, 
34 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Table 5. Published relations and calculated difference between National Geodetic Vertical Datum of 1929 and local mean low water 

datum for the 1941–59 and 1983–2001 tidal epochs at four active National Ocean Service tidal benchmark stations in southeastern New 

York and northeastern New Jersey.

[All values are in meters. Locations of stations 8510560 and 8531680 are shown in figure 1. Locations of stations 8515102 and 8515186 are shown in figure 2. 

Data for 1941–59 tidal epoch from J.R. Hubbard, National Ocean Service, written commun., 1993. Published relations between North American Vertical 

Datum of 1988 (NAVD 88) and mean low water (MLW) for 1983–2001 tidal epoch at stations 8510560, 8515186, and 8531680 from National Ocean Service, 

2006a. Relations between NAVD 88 and National Geodetic Vertical Datum of 1929 (NGVD 29) at these stations and between fixed and MLW datums at 

station 8515102 from National Geodetic Survey, 2006]

Station NGVD 29 minus MLW for tidal epoch(s)

1941–59 1983–2001 1941–59 minus 1983–2001

8510560 Montauk, Fort Pond Bay, N.Y. 0.24 0.05 0.19

8515102 Bay Shore, Watchogue Creek Entrance, N.Y. -0.11 -0.22 10.10

8515186 Fire Island Coast Guard Station, N.Y. 0.09 0.02 10.08

8531680 Sandy Hook (Fort Hancock), N.J. 0.55 0.47 10.07

1

Amount may not equal the sum of values because of rounding to significant digits.

the leakance of the boundary, and the hydraulic gradient

across the boundary. The hydraulic gradient is determined 

largely by the time-averaged level of the surface waters that 

has been known to vary spatially because of differences in 

local mean sea level (MSL). The mean surface-water level 

also has been found to vary spatially and temporally near the 

shore because of the effects of breaking waves (wave setup 

and setdown) and asymmetrical tidal infiltration and seepage 

(tidal pumping), each of which can elevate the water table at 

this boundary creating a condition commonly referred to as 

water-table overheight (Nielsen, 1990; Hanslow and Nielsen, 

1993; Turner and others, 1996; Nielsen, 1999).

Local Mean Sea Level

Estimates of the difference between local MSL and 

NGVD 29 obtained or calculated from published and 

unpublished relations between tidal and fixed datums at 

active and discontinued National Ocean Service (NOS) 

tidal benchmark locations in southeastern New York and 

northeastern New Jersey (fig. 1) are summarized in tables 4–7 

(U.S. Department of Commerce, 1969; J.R. Hubbard, National 

Ocean Service, written commun., 1993; National Geodetic 

Survey, 2006; National Ocean Service, 2006a). These values 

were supplemented with estimates of the local MSL-NGVD 

29 difference calculated in this study for a series of points in 

adjacent ocean areas (fig. 1) using the VDatum2 program. A 

comparison of this VDatum program estimate for the active 

NOS station at Sandy Hook, N.J., with the local MSLNGVD 29 difference from this station for the most recent 

2

 A datum transformation software developed jointly by the National 

Oceanic and Atmospheric Administration’s Office of Coast Survey and 

National Geodetic Survey (Hess, 2001).

Table 4. Published relations between tidal and fixed datums for 1983–2001 tidal epoch at six active National Ocean Service tidal 

benchmark stations in southeastern New York and northeastern New Jersey.

[All values are in meters. Locations of stations 8510560, 8512769, and 8531680 are shown in figure 1. Locations of stations 8513825, 8515102, and 8515186 

are shown in figure 2. Published relations between tidal datums (MHHW, mean higher high water; MSL, mean sea level; and MLLW, mean lower low water) 

at all stations, and between tidal datums and North American Vertical Datum of 1988 (NAVD 88) at stations 8510560, 8515186, and 8531680 from National 

Ocean Service, 2006. Relations between NAVD 88 and National Geodetic Vertical Datum of 1929 (NGVD 29) at all stations, and between tidal and fixed 

datums at stations 8512769, 8513825 and 8515102, from National Geodetic Survey, 2006]

Station

MHHW minus datum MSL minus datum Datum minus MLLW

NAVD 88 NGVD 29 NAVD 88 NGVD 29 NAVD 88 NGVD 29

8510560 Montauk, Fort Pond Bay, N.Y. 0.37 0.66 -0.02 0.27 0.40 0.11

8512769 Shinnecock Yacht Club, Penniman Creek, N.Y. 0.37 0.65 -0.11 0.17 0.52 0.23

8513825 Smith Point Bridge, Narrow Bay, N.Y. 0.20 0.52 -0.02 0.30 0.25 -0.08

8515102 Bay Shore, Watchogue Creek Entrance, N.Y. 0.20 0.57 0.00 0.38 0.18 -0.20

8515186 Fire Island Coast Guard Station, N.Y. 0.30 0.65 -0.06 0.29 0.41 0.05

8531680 Sandy Hook (Fort Hancock), N.J. 0.73 1.06 -0.07 0.26 0.86 0.53
Hydrogeologic Framework 35

Table 6. Published and calculated relations between local mean low water and mean tide level datums and National Geodetic 

Vertical Datum of 1929 for the 1941–59 tidal epoch, and estimated difference for and relation between mean tide level and National 

Geodetic Vertical Datum of 1929 for the 1983–2001 tidal epoch at selected discontinued National Ocean Service tidal benchmark 

stations, Suffolk County, N.Y.

[All values are in meters. Station locations are shown in figure 2. Published relation between National Geodetic Vertical Datum of 1929 (NGVD 29) and mean 

low water (MLW) for 1941–59 tidal epoch from J.R. Hubbard, National Ocean Service, written commun., 1993; which also was used with data from National 

Ocean Service, 2006a; and National Geodetic Survey, 2006; to estimate relation between mean tide level (MTL) and MLW for 1941–59 at Moriches Coast 

Guard Station, N.Y. Relation between MTL and MLW for 1941–59 tidal epoch at all other stations from U.S. Department of Commerce, 1969]

Station(s)

Relations between datums 

for 1941–59 tidal epoch

Estimated MTL minus 

NGVD 29 for tidal epoch(s)

NGVD 29 

minus 

MLW

MTL 

minus 

MLW

MTL 

minus 

NGVD

1941–59 

minus 

1983–2001

1983–2001

Potunk Point, Moriches Bay, N.Y. -- 0.06

Eastport, Seatuck Cove, Moriches Bay, N.Y. 0.01 -- 10.15 20.08 30.24

Moriches Coast Guard Station, N.Y. -- 0.24

Wa Wa Yanda Yacht Club, Captree Island, Great South Bay, N.Y. -0.12 -- 0.21 4

0.10 30.30

West Fire Island, Great South Bay, N.Y. -- 0.09

Oakbeach, Great South Bay, N.Y. -0.10 0.09 0.18 40.10 30.30

1

Calculation is average for stations listed.

2

Estimate of difference between MTL minus NGVD relations for 1941–59 and 1983–2001 tidal epochs determined from calculated difference between these 

relations at Fire Island Coast Guard Station, N.Y., given in table 5.

3

Amount may not equal the sum of values because of rounding to significant digits.

4

Estimate of difference between MTL minus NGVD relations for 1941–59 and 1983–2001 tidal epochs determined from calculated difference between these 

relations at Bay Shore, Watchogue Creek Entrance, N.Y, given in table 5.

Table 7. Relations and calculated difference between mean sea level datum and National Geodetic Vertical Datum of 1929 for the 

1983–2001 tidal epoch and from the VDatum program at the National Ocean Service tidal benchmark station at Sandy Hook, N.J.

[All values are in meters. Station location is shown in figure1. Published relation between mean sea level (MSL) and North American Vertical Datum of 1988 

(NAVD 88) for 1983–2001 tidal epoch from National Ocean Service, 2006a. Relation between NAVD 88 and National Geodetic Vertical Datum of 1929 

(NGVD 29) from National Geodetic Survey, 2006] 

Station

MSL minus NGVD 29

VDatum 1983–2001 1983–2001 minus VDatum

8531680 Sandy Hook (Fort Hancock), N.J. 0.19 0.26 0.07
36 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

(1983–2001) tidal epoch suggests, however, that the VDatum 

values for this and other locations are derived from a different 

(previous or superseded) tidal epoch. This discrepancy was 

addressed in this study by applying the difference between 

the actual and VDatum values for Sandy Hook to the 

supplementary ocean points.

Wave Setup and Setdown

Collier (2006) found that wave setup and setdown along 

the Atlantic Ocean shore of Fire Island resulted in an increase 

(setup) of about 0.085 m in the mean surface-water level at 

the shore and a similar decrease (setdown) at the approximate 

position of the breaker zone and underlying bar about 200 m 

offshore. Empirical relations reported by Hanslow and Nielsen 

(1993) and Nielsen (1999) suggest the wave setup at the 

shoreline is considerably higher—on the order of 40 percent of 

the offshore wave height, on the basis of the relations in their 

simplified form. Wave Information Studies (WIS) produced 

by the U.S. Army Corps of Engineers that generate model 

hindcasts of wave climate for United States coastal waters 

indicate the 1980–99 mean annual wave height at an ocean 

location offshore of eastern Fire Island is 1.1 m (Coastal and 

Hydraulics Laboratory, 2006). On the basis of these empirical 

relations and offshore wave-height data, the maximum wave 

setup along the Atlantic Ocean shore of Fire Island was 

estimated to be 0.44 m.

Tidal Pumping

The asymptotic inland water-table overheight resulting 

from tidal pumping for the simplified case of a vertical beach 

face has been expressed by Nielsen (1990) as approximately:

η+

v

 = A2/4D (1)

where

η+

v = water-table overheight resulting from tidal 

pumping along a vertical beach face,

A = tidal amplitude, and

D = undisturbed aquifer thickness.

Along the Atlantic Ocean shore of Fire Island, the tidal 

amplitude is 0.44 m, on the basis of the predicted value for 

nearby Moriches Inlet (National Ocean Service, 2006b). 

The undisturbed aquifer thickness along this shore was 

approximated to be the difference between mean high water 

and the upper surface of Holocene peat deposits, which 

are inferred in this study to be mostly continuous beneath 

Fire Island; this approximation essentially equals the tidal 

amplitude. The resulting estimate of water-table overheight for 

the simplified case of a vertical beach face is about 0.11 m.

Solutions for the additional water-table overheight 

resulting from tidal pumping for the case of a sloping beach 

face have been derived by Nielsen (1990) and are as follows:

η+

β

 = 0.5∈A (2)

where

η+

β = water-table overheight resulting from tidal 

pumping along a sloping beach face,

A = tidal amplitude, and

	 ∈ = nondimensional perturbation parameter, 

which is given by:

∈ = kBA/tan β (3)

where

β = beach slope, and 

kB = Boussinesq wave number, expressed as

kB = (nw/2KD)1/2 (4)

where

n = porosity,

	 w = angular velocity of the tide, and

K = hydraulic conductivity.

The beach slope is presumed to annually average about 

2 degrees, on the basis of an analysis of the effects of offroad vehicles on beach slope (Anders and Leatherman, 1987) 

and from examination of selected beach profiles collected 

under the Atlantic Coast of New York Monitoring Program 

(New York Sea Grant, 2007) on Fire Island. The porosity and 

hydraulic conductivity of coarse-grained beach-face sediments 

are estimated to be about 0.3 and 60 m/d, respectively. The 

angular velocity of the tide is calculated to be 0.5059 radians 

per hour, on the basis of a tidal period of 12.42 hours. The 

resulting estimate of water-table overheight for the case of a 

sloping beach face is about 0.11 m—the same value estimated 

for a vertical beach face.

Thus, the sum of additive water-table overheight 

estimates for cases of a vertical and sloping beach face is 

roughly 0.22 m for the Atlantic Ocean shore of Fire Island. 

However, these terms do not account for the development of 

a seepage face that forms on all but the steepest beaches as 

the water table decouples from the falling tide (Turner and 

others, 1996). This development results in a further increase 

in the water-table overheight, but solutions to predict the 

amount of this increase do not appear to be readily available 

(Turner and others, 1996; Nielsen, 1999). Therefore, the 

total estimate of water-table overheight resulting from tidal 

pumping that was calculated in this study probably can be 

considered a conservative approximation. In combination with 

the maximum wave setup at the shore, the total water-table 

overheight resulting from wave setup and tidal pumping along 

the Atlantic Ocean shore of Fire Island is estimated to be about 

0.66 m. 

Freshwater-Saltwater Interface

The freshwater flow system of Fire Island is bounded 

in the subsurface by the freshwater-saltwater interface. 

The position of the freshwater-saltwater interface was 

estimated from filter-press core samples and borehole 
Hydrogeologic Framework 37

electromagnetic-induction logs. Filter-press core samples 

were collected by a method adapted from Lusczynski 

(1961). Samples with a salinity of approximately 0.4 part per 

thousand—corresponding to a chloride concentration of about 

250 mg/L—were considered to indicate the depth at which the 

freshwater-saltwater transition zone begins. Data from filterpress core samples and electromagnetic-induction logs were 

correlated to identify the position of the freshwater-saltwater 

interface (figs. 5 and 8).

Directions of Groundwater Flow

The movement of fresh groundwater on Fire Island is 

controlled by the distribution of hydraulic head, which in 

turn is controlled by the boundary conditions and hydraulic 

properties of the freshwater flow system. Groundwater levels 

on Fire Island vary in response to storm tides and other 

fluctuations in coastal water levels, to seasonal or annual 

variations in recharge from precipitation, and, to a much 

lesser extent, to changes in water use. Long-term (1950–1976) 

water-level records from wells in the isolated freshwater flow 

systems of eastern Long Island (Nemickas and Koszalka, 

1982; McNew-Cartwright, 1996; Simmons, 1986) indicate the 

water-table altitude generally declines from May through early 

October, when recharge is smallest and water use is greatest, 

and generally rises from the end of October through the end 

of April, when recharge is greatest and water use is smallest 

(Schubert, 1998).

Synoptic water-level measurements were made at 

35 observation wells during October 31–November 1, 2005, 

and March 28–29 and October 3–4, 2006 (appendix 1). Water 

levels measured during October 31–November 1, 2005, were 

used to depict the distribution of hydraulic head on Fire 

Island shown in figure 8. The water levels were measured 

in this study within 1 hour of the predicted time of low tide 

to minimize spatial differences in hydraulic head caused by 

tidal fluctuations. In addition, one well in each of four local 

study areas was instrumented with a continuous water-level 

recorder to document groundwater fluctuations caused by 

tides, natural recharge, and other factors such as wave height 

and wastewater disposal. Water-level records of 18-month 

duration were collected from one well each in the vicinity of 

Kismet (S125986) and the National Wilderness (S125364) 

during October 2005–April 2007 (figs. 15A and D). Records 

of 3-month duration were collected sequentially from six wells 

each in the vicinities of Robbins Rest (S125994, S125982, 

S125998, S125997, S125996, and S126000) and Watch 

Hill (S125380, S125383, S125381, S125382, S125387, and 

S125990) during the same 18-month period (figs. 15B and C). 

Coastal water levels also were monitored continuously during 

this period at the nearby USGS tide-stage gaging station on 

Reynolds Channel at Point Lookout, N.Y. (figs. 15A–D)

A map of the synoptic water-level measurements made at 

35 wells during October 31–November 1, 2005, is shown in 

figure 4. The water levels generally increased with proximity 

to the ocean shore; the highest water levels were at wells 

nearest the ocean or those just inland. Water levels in inland 

wells also increased with barrier-island width. The pattern 

was similar among synoptic water-level measurements made 

at the 35 wells during March 28–29 and October 3–4, 2006 

(appendix 1). These results indicate the groundwater divide

of Fire Island is skewed strongly toward the ocean shore, 

apparently because of the combined effects of wave setup 

and tidal pumping and the resulting water-table overheight 

along this shore. Water levels measured continuously during 

October 2005–April 2007 at the single wells in Kismet and the 

National Wilderness (figs. 15A and D, respectively) indicate 

that fluctuations caused by storm tides and other short-term 

factors generally equal or exceed any seasonal or longer-term 

fluctuations. The continuous water levels collected during 

this same period at 3-month intervals from six wells each in 

Robbins Rest and Watch Hill (figs. 15B and C, respectively) 

appear to show a similar pattern, although the short-term 

fluctuations at inland wells (S125997, S125998, S125381, and 

S125382, for example) are somewhat damped.

Nitrogen Concentrations

Freshwater entering the water table through infiltration 

of precipitation, and wastewater disposal in unsewered areas, 

contains nutrients in concentrations that vary spatially and 

temporally. The concentration of a solute, such as nitrogen, 

in the aquifer depends on the location and rate at which it 

is introduced and the flow paths and traveltime of water in 

which it moves. Nitrogen concentrations also can be altered 

by physical processes such as dilution and dispersion and 

biogeochemical processes such as denitrification. The primary 

form of nitrogen in natural waters—nitrate, ammonia, 

or organic nitrogen—is controlled by the pH and redox 

conditions in the water (Hem, 1985). Nitrate is an oxidized 

form of nitrogen that can be transported conservatively 

in groundwater, whereas ammonia is a reduced form that 

strongly sorbs to aquifer sediments (Hem, 1985).

Water samples were collected by the USGS during 

October 2005 from 35 observation wells to determine the 

spatial distribution of nutrients from wastewater and natural 

sources; analytes provided by the USGS National Water 

Quality Laboratory in Denver, Colo., included dissolved 

inorganic nitrogen (ammonia, nitrite plus nitrate, and nitrite) 

and phosphorus (orthophosphate). A subset of four wells—one 

from each local study area—was resampled during March 

2006 and August–September 2006 with an additional 21 wells 

to assess the seasonal variability of nutrient concentrations. 

Dissolved oxygen, pH, specific conductance, water 

temperature, and oxidation reduction potential were measured 

during well evacuation.

A map showing the distribution of total nitrogen (TN) 

concentrations, which were computed by summing the 

concentrations of ammonia and nitrite plus nitrate for the 

35 wells sampled during October 2005 (appendix 2), is 

provided in figure 16; seasonal TN concentrations for the 

4 wells resampled during March and August–September 
38 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

 0

0.5

1.0

1.5

2.0

10/1/05 12/31/05 4/1/06 7/1/06 9/30/06 12/30/06 3/31/07

W

ATER LEVEL, IN METERS

ABOVE NGVD 29

A Kismet

01310740 125986 MEASUREMENT

 0

0.5

1.0

1.5

2.0

10/1/05 12/31/05 4/1/06 7/1/06 9/30/06 12/30/06 3/31/07

W

ATER LEVEL, IN METERS

ABOVE NGVD 29

B Robbins Rest

01310740 125994 125982 125998 125997 125996 126000 MEASUREMENT

 0

0.5

1.0

1.5

2.0

10/1/05 12/31/05 4/1/06 7/1/06 9/30/06 12/30/06 3/31/07

W

ATER LEVEL, IN METERS

ABOVE NGVD 29

C Watch Hill

01310740 125380 125383 125381 125382 125387 125990 MEASUREMENT

 0

0.5

1.0

1.5

2.0

10/1/05 12/31/05 4/1/06 7/1/06 9/30/06 12/30/06 3/31/07

W

ATER LEVEL, IN METERS

ABOVE NGVD 29

D National Wilderness

01310740 125364 MEASUREMENT

Figure 15. Continuous water-level records and synoptic water-level measurements at selected wells on Fire Island, 

and daily higher high water values at tide-stage gaging station 01310740 Reynolds Channel at Point Lookout, N.Y., 

during October 2005–April 2007, Long Island, N.Y. (Well locations are shown in figure 4. Station location is shown in 

figure 1. Well number is assigned by New York State Department of Environmental Conservation. Prefix “S” denoting 

Suffolk County is omitted. NGVD 29, National Geodetic Vertical Datum of 1929)
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 39

2006 are shown in figure 17. The TN concentrations from 

most wells in undeveloped areas of the park near Kismet 

and Robbins Rest (figs. 16A and B, respectively) generally 

were comparable to those in the same areas of Watch Hill 

and the National Wilderness (figs. 16C and D, respectively). 

However, TN concentrations from four wells downgradient 

from the two communities (S126753, S125370, S125994, 

and S126000; figs. 16A and B) and those near the Watch Hill 

leach field were elevated above background (natural) levels 

observed in the undeveloped park areas by a factor of 10 or 

more. The pattern was similar among the 25 wells resampled 

during August–September 2006 (appendix 2). These results 

suggest that TN concentrations within the communities are 

comparable to those calculated by Monti and Scorca (2003) 

for similar settings along coastal areas of the southern 

Long Island mainland (Schubert, 2007). The highest TN 

concentrations—up to 10 mg/L—were measured immediately 

downgradient from the Watch Hill leach field; the lowest—

less than 0.1 mg/L—were within the National Wilderness. TN 

concentrations resampled from four wells during March and 

August–September 2006 (fig. 17) varied somewhat through 

time, although no consistent pattern emerged.

The primary form of nitrogen from wells near 

Kismet and Robbins Rest varied between ammonia and 

nitrate (appendix 2). Ammonia was the primary form in 

the vicinity of Watch Hill, whereas nitrate predominated 

within the National Wilderness. The presence in the shallow 

subsurface of peat containing organic carbon likely favors the 

denitrification of nitrate to nitrogen gas, although the small 

transport distances and attendant traveltime to receptors could 

limit this and other processes of attenuation. Use of the TN 

concentrations to calculate nitrogen loads may, therefore, 

result in a worst-case estimate of mass-loading rates; as such, 

it also would favor strategies to ensure protection of park 

natural resources.

Analysis of the Shallow Groundwater 

Flow System at Fire Island 

National Seashore

The analysis of the shallow groundwater flow system at 

FIIS entailed the development of a three-dimensional model of 

the shallow aquifer system and adjacent marine surface waters 

to simulate groundwater flow patterns and rates. Nitrogen 

loads from the shallow aquifer system were calculated from 

measured TN concentrations and simulated groundwater 

discharges to back-barrier estuaries and the ocean.

Development of Flow Model

Development of the flow model included the construction 

of the model grid and the assignment of appropriate boundary 

conditions and aquifer properties. The model was calibrated 

to observed water levels. The variable-density groundwater 

flow model SEAWAT-2000 (Langevin and others, 2003) was 

used in the analysis. SEAWAT outputs hydraulic heads that are 

referenced to the simulated density (a function of salinity) at a 

given location and time.

Model Grid

The model grid encompasses the full breadth of Fire 

Island (as mapped by Klopfer and others, 2002) and consists 

of 72 rows, 1,008 columns, and 12 layers. The grid has a 

uniform horizontal discretization of 50 m along both rows and 

columns and is oriented 20.16 degrees counterclockwise from 

true north (fig. 18). The model represents the full thickness of 

the shallow aquifer system (fig. 19), extending from the water 

table to the base of the upper glacial aquifer (fig. 9); layer 

thicknesses vary from cell to cell and generally are based on 

the geometry of hydrogeologic units. Overall, the grid contains 

870,912 cells, of which 367,752 are active and 503,160 

are inactive. 

Each of the Late Pleistocene and Holocene hydrogeologic 

units are represented in the model. Layer 1 represents three 

surficial Holocene units—upper outwash, peat deposits, and 

barrier and shelf sands—in onshore areas (fig. 20); surfacewater bodies also are represented in layer 1. Layer 2 represents 

an underlying lens of peat deposits in onshore areas, and five 

surficial Holocene units—lagoonal deposits, estuarine mud, 

seagrass beds, peat deposits, and barrier and shelf sands—in 

back-bay and ocean areas (fig. 21). Layers 3–5 represent 

lagoonal deposits, except along the Long Island mainland, 

where these layers represent the upper outwash of the upper 

glacial aquifer (fig. 22). Layers 6–8 represent upper outwash. 

Layer 9 represents the “20-foot” clay, except in areas where 

this unit is absent; in these areas, layer 9 represents the lower 

outwash of the upper glacial aquifer (fig. 23). Layers 10 and 

11 represent the lower outwash, and layer 12 represents the 

fine-grained, basal part of this unit.

Boundary Conditions

The boundary conditions specified in the flow model 

coincide with the hydrologic boundaries of the freshwater flow 

system under present (2005) conditions. These hydrologic 

boundaries are assumed to be constant through time and are 

used to simulate groundwater flow for steady-state, average 

annual conditions.

A no-flow condition is specified at the lateral boundary 

between active and inactive model zones in all layers (fig. 18). 

This boundary condition also applies at the bottom of the 

lowermost model layer (12), which represents the base of the 

upper glacial aquifer (fig. 9). Low-permeability units beneath 

this aquifer—the Gardiners Clay and Monmouth greensand—

constitute confining units that hydraulically isolate the upper 

glacial aquifer from the underlying Magothy aquifer (Scorca 

and others, 1995).
40 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

125984

0.09

.

125995

0 30

125981

0.13

125983

0.35

125370

1.32

125985

0.13

125371

0.77

125986

0.23 126753

5.34

73°12'15" 73°12'

40°38'15"

40°38'

EXPLANATION

Trace of Hydrogeologic Section--Shows location of vertical section shown in figure 8A.

Point of Observation-Well Sample--Size is proportional to total nitrogen concentration.

Base from New York State Department of Transportation planimetric quadrangles, 1991, 1:24,000; Universal Transverse Mercator projection, NAD 83, Zone 18

B'

B

B B'

125370

1.32

0 50 100 METERS

0 150 300 FEET

Upper number is identifier. Lower number is total nitrogen concentration, in mg/L.

Figure 16. Total nitrogen concentrations at 35 wells sampled during October 2005 on Fire Island, Suffolk County, N.Y. 

(Well number is assigned by New York State Department of Environmental Conservation. Prefix “S” denoting Suffolk 

County is omitted. mg/L, milligrams per liter; NAD 83, North American Datum of 1983)
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 41

126000

0.98

125999

0.44

126001

0.19

125996

0.28

125997

0.50

125994

0.87

125982

0.24

125998

0.10

73°10' 73°9'45"

40°38'45"

40°38'30"

EXPLANATION

Trace of Hydrogeologic Section--Shows location of vertical section shown in figure 8B.

Point of Observation-Well Sample--Size is proportional to total nitrogen concentration.

Base from New York State Department of Transportation planimetric quadrangles, 1991, 1:24,000; Universal Transverse Mercator projection, NAD 83, Zone 18

C'

C

C C'

125996

0.28

0 50 100 METERS

0 150 300 FEET

Upper number is identifier. Lower number is total nitrogen concentration, in mg/L.

Figure 16. Total nitrogen concentrations at 35 wells sampled during October 2005 on Fire Island, Suffolk County, N.Y. 

(Well number is assigned by New York State Department of Environmental Conservation. Prefix “S” denoting Suffolk 

County is omitted. mg/L, milligrams per liter; NAD 83, North American Datum of 1983)—Continued
42 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

125990

7.23

125384

9.46

125386

1.63

125385

0.88

125387

0.16

125382

0.70

125380

9.23

125381

0.77

125383

4.72

72°59'30" 72°59'15"

40°41'30"

40°41'15"

EXPLANATION

Trace of Hydrogeologic Section--Shows location of vertical section shown in figure 8C.

Point of Observation-Well Sample--Size is proportional to total nitrogen concentration.

Base from New York State Department of Transportation planimetric quadrangles, 1991, 1:24,000; Universal Transverse Mercator projection, NAD 83, Zone 18

D'

D

D D'

125387

0.16

Watch Hill leach field

0 50 100 METERS

0 150 300 FEET

Upper number is identifier. Lower number is total nitrogen concentration, in mg/L.

Figure 16. Total nitrogen concentrations at 35 wells sampled during October 2005 on Fire Island, Suffolk County, N.Y. 

(Well number is assigned by New York State Department of Environmental Conservation. Prefix “S” denoting Suffolk 

County is omitted. mg/L, milligrams per liter; NAD 83, North American Datum of 1983)—Continued
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 43

124400

0.71

125369

0.86

125368

0.28

125365

0.12 125367

0.29

125363

1.03

124399

0.18

125366

0.51

125364

0.12

72°52'15" 72°52'

40°44'

40°43'45"

EXPLANATION

Trace of Hydrogeologic Section--Shows location of vertical section shown in figure 8D.

Point of Observation-Well Sample--Size is proportional to total nitrogen concentration.

Base from New York State Department of Transportation planimetric quadrangles, 1991, 1:24,000; Universal Transverse Mercator projection, NAD 83, Zone 18

E'

E

E E'

125365

0.12

0 50 100 METERS

0 150 300 FEET

Upper number is identifier. Lower number is total nitrogen concentration, in mg/L.

Figure 16. Total nitrogen concentrations at 35 wells sampled during October 2005 on Fire Island, Suffolk County, N.Y. 

(Well number is assigned by New York State Department of Environmental Conservation. Prefix “S” denoting Suffolk 

County is omitted. mg/L, milligrams per liter; NAD 83, North American Datum of 1983)—Continued
44 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Constant-head and constant-concentration boundaries 

were used in estuaries and the ocean to represent the flow 

of water between the shallow aquifer and marine surface 

waters (layer 1). Local MSL-NGVD 29 differences obtained 

or calculated for NOS tidal benchmarks and calculated using 

the VDatum program at supplementary ocean points were 

extrapolated throughout the back-bays and ocean to depict 

local MSL, to which the total water-table overheight resulting 

from wave setup and setdown and to tidal pumping was added 

(fig. 24). Long-term salinity-measurement records obtained 

from the SCDHS (R.M. Waters, Suffolk County Department 

of Health Services, written commun., 2006) and National 

Oceanographic Data Center (National Oceanographic Data 

Center, 2006) were extrapolated to provide a time-averaged 

depiction of salinity throughout the back-barrier estuaries and 

the ocean (fig. 25).

Offshore, water-surface altitudes (fig. 24) are lower in the 

ocean than in the back-barrier estuaries, where levels generally 

increase with distance from inlets. Along the immediate ocean 

shore, however, altitudes are higher than along the adjacent 

back-barrier estuary shore; altitudes are lowest near the 

breaker zone and underlying bar. Salinities (fig. 25) are higher 

in the ocean than in the back-barrier estuaries, where they 

generally decrease with distance from inlets.

Recharge from precipitation and the return flow of publicsupply water is represented as a specified-flux boundary at the 

water table (layer 1). Inland surface-water bodies (layer 1) are 

simulated as areas of high hydraulic conductivity (15,000 m/d) 

and porosity (1).

Recharge from precipitation is simulated by five 

zones (fig. 26) that are numbered 1 through 5 and represent 

increasing rates of infiltration to the water table. The zones 

are assigned according to substrate, land cover, and altitude 

(table 8), the latter of which was used as a surrogate for 

water-table depth below land surface. Zone 1 consists of 

fresh surface-water bodies. Areas with developed features as 

mapped by Klopfer and others (2002) or where peat deposits 

constitute the surficial hydrogeologic unit are assigned by 

increasing land-surface altitude above NGVD 29 to zones 2 

(less than 2 m), 3 (between 2 and 4 m), or 4 (greater than 4 m). 

Vegetated areas associated with a loam substrate (Klopfer and 

others, 2002) are assigned by increasing land-surface altitude 

above NGVD 29 to zones 3 (less than 2 m), 4 (between 2 and 

4 m), or 5 (greater than 4 m). Areas associated with a sand 

substrate or sparse vegetation (Klopfer and others, 2002) are 

assigned by increasing land-surface altitude above NGVD 29 

to zones 4 (less than 2 m) or 5 (greater than or equal to 2 m). 

The applied rates of recharge to the five model zones were 

calculated as follows:

1. 1938–96 mean annual precipitation at Patchogue minus 

70 percent of long-term mean annual pan evaporation at 

Greenport (42 cm/yr);

2. 75 percent of 1938–96 mean annual precipitation at 

Patchogue from October 15 through May 15 plus 

55 percent of the difference between long-term mean 

annual pan evaporation at Mineola and Greenport 

(62 cm/yr);

0

1

2

3

4

5

Sep 2005 Mar 2006 Oct 2006

T

O

TAL

NIT

ROGE

N, IN

MILLIG

R

A

M

S

PER LITER

125370

125996

125387

Figure 17. Total nitrogen concentrations at four 

wells sampled during October 2005 and March and 

August–September 2006 on Fire Island, Suffolk County, 

N.Y. (Well locations are shown in figure 6. Well 

number is assigned by New York State Department 

of Environmental Conservation. Prefix “S” denoting 

Suffolk County is omitted.)
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 45

3. 90 percent of 1938–96 mean annual precipitation at 

Patchogue from October 15 through May 15 plus 

40 percent of the difference between long-term mean 

annual pan evaporation at Mineola and Greenport 

(76 cm/yr);

4. 1938–96 mean annual precipitation at Patchogue minus 

25 percent of long-term mean annual pan evaporation at 

Greenport (90 cm/yr); and

5. 1938–96 mean precipitation at Patchogue minus 

10 percent of long-term mean annual pan evaporation at 

Greenport (109 cm/yr).

Recharge from the return flow of public-supply water is 

simulated by a series of rates (fig. 27) assigned according to 

water-supply system and return-flow component (distribution 

or waste, fig. 6). The rates of public-supply return flow 

within the unsewered Kismet and Fair Harbor-Summer Club 

distribution zones of the SCWA and Robert Moses State Park, 

Saltaire, Seaview, and Ocean Bay Park water-supply systems 

were calculated as 85 percent of the total 2005 pumpage for 

these systems divided by their service areas. In the sewered 

community of Ocean Beach, the rate of public-supply return 

flow was calculated as 15 percent of the total 2005 pumpage 

for this system divided by its service area. Rates of publicsupply return flow for the distribution and wastewater systems 

associated with four park facilities served by the SCWA—

Smith Point County Park and the NPS Visitors Centers at 

Sailors Haven, Watch Hill, and the National Wilderness—

generally were calculated as 15 and 70 percent, respectively, 

of the total 2005 water supply to these facilities divided by 

the simulated areas of their corresponding systems. In the 

unsewered Point O’Woods-Fire Island Pines and Davis Park 

distribution zones of the SCWA, which supply water to the 

NPS facilities at Sailors Haven and Watch Hill, the rates 

of public-supply return flow generally were calculated as 

85 percent of the total 2005 pumpage for these systems minus 

the NPS water supply, divided by the portions of their service 

areas that exclude the respective NPS systems.

The model does not simulate public-supply withdrawal 

because virtually all water-supply pumpage is from the 

underlying regional (unmodeled) aquifer system. Although 

there are limited private and NPS withdrawals in areas not 

served by the public water-supply systems, these typically 

are at low pumping rates and, therefore, not accounted for in 

the model.

Model Calibration

The flow model was calibrated through a quasi steadystate simulation of 2005 mean annual conditions, the 

results of which were then compared with corresponding 

measured values that were assumed to estimate long-term 

conditions. The model was considered calibrated when the 

difference between measured and simulated groundwater 

levels was minimized at wells in each of four local study 

areas and four (screened) hydrogeologic units. Steady-state 

groundwater levels were simulated during two consecutive 

7,300-day (20-year) stress periods, the second of which 

was initiated with the results of the first; the same approach 

was followed to simulate steady-state salinities, except that 

the second stress period was 500 years. These procedures 

allowed model-calibration iterations to be run sufficiently 

quickly during simulation of steady-state groundwater 

levels, by only periodically performing the time-consuming 

simulation of steady-state salinities. Simulated groundwater 

levels were compared with water-level measurements at 

35 observation wells during October 31-November 1, 2005 

(figs. 4 and 8); simulated salinity concentrations were 

compared qualitatively with the positions of the freshwatersaltwater interface estimated at 21 wells during this study 

(figs. 5 and 8). Horizontal and vertical hydraulic conductivities 

that represent the water-transmitting properties of simulated 

hydrogeologic units were adjusted by trial and error to 

minimize the difference between measured and simulated 

groundwater levels. 

The differences between measured and simulated water 

levels grouped by local study area and by hydrogeologic 

unit are summarized in tables 9 and 10, respectively. Mean 

differences by local study area ranged from a low of -0.02 m 

at Kismet to a high of 0.03 m at the National Wilderness, and 

averaged 0.00 m; by hydrogeologic unit, the mean differences 

ranged from a low of -0.07 m in peat deposits to high values of 

0.05 m in barrier island and shelf sands and in upper outwash, 

and averaged 0.00 m. The measured and simulated water 

levels and vertical gradients at four observation-well pairs are 

shown in table 11. The final simulated horizontal and vertical 

hydraulic conductivities for hydrogeologic units represented in 

the model are given in table 12. A porosity of 0.3 was assumed 

for all unconsolidated sediments. Unconfined surficial 

sediments were assigned a specific yield of 0.15; values of 

specific storage in surficial units and in deeper confined units 

were assumed to be 2 x 10-6 and 1 x 10-5 ft-1, respectively. 

Fresh surface-water bodies had a simulated specific storage 

and porosity of 1.
46 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

73°04' 73°06' 73°08' 73°10'

40°39'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

255

190

0 0.5 1 KILOMETER0 0.5 1 MILE

Figure 18. Grid showing active and inactive areas and locations of columns 190, 255, 569, and 790 in the model of Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and 

D are shown in figure 2. NAD 83, North American Datum of 1983)
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 47

72°54' 72°56' 72°58' 73° 73°02'

40°42'

40°41'

72°52' 72°46' 72°48' 72°50'

40°44'

40°43'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

790

C

D

569

EXPLANATION

Inactive Model AreaActive Model Area

Select Model Column--Shows location of vertical sections shown in figures 19 and 31 790

Figure 18. Grid showing active and inactive areas and locations of columns 190, 255, 569, and 790 in the model of Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and D are shown in figure 2. NAD 83, North American Datum of 1983)—Continued
48 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

METERS ROW

-37.5

-30.0

-22.5

-15.0

-7.5

0

7.5

VERTICAL SCALE GREATLY EXAGGERATED

DATUM IS NGVD 29

SOUTHEAST NORTHWEST LAYER

1

EXPLANATION

A Column 190

3

6

9

12

65 59 53 47 41 35

Hydrogeologic Unit

Surface water

Barrier island and

 shelf sands

Peat deposits

Seagrass beds

Lagoonal deposits

Upper outwash

"20-foot" clay

Lower outwash

0 100 200 300 METERS

0 300 600 900 FEET

Figure 19. Modeled hydrogeologic units for active model cells along four selected columns in the Fire Island study 

area, Suffolk County, N.Y. (A) column 190, (B) column 255, (C) column 569, and (D) column 790. (Column locations are 

shown in figure 18. NGVD 29, National Geodetic Vertical Datum of 1929)
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 49

METERS ROW

-37.5

-30.0

-22.5

-15.0

-7.5

0

7.5

VERTICAL SCALE GREATLY EXAGGERATED

DATUM IS NGVD 29

SOUTHEAST NORTHWEST LAYER

EXPLANATION

B Column 255

1

3

6

9

12

67 61 55 49 43 41

Hydrogeologic Unit

Surface water

Barrier island and

 shelf sands

Peat deposits

Seagrass beds

Lagoonal deposits

Upper outwash

Lower outwash

0 100 200 300 METERS

0 300 600 900 FEET

Figure 19. Modeled hydrogeologic units for active model cells along four selected columns in the Fire Island study 

area, Suffolk County, N.Y. (A) column 190, (B) column 255, (C) column 569, and (D) column 790. (Column locations are 

shown in figure 18. NGVD 29, National Geodetic Vertical Datum of 1929)—Continued
50 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Figure 19. Modeled hydrogeologic units for active model cells along four selected columns in the Fire Island study 

area, Suffolk County, N.Y. (A) column 190, (B) column 255, (C) column 569, and (D) column 790. (Column locations are 

shown in figure 18. NGVD 29, National Geodetic Vertical Datum of 1929)—Continued

METERS ROW

-37.5

-30.0

-22.5

-15.0

-7.5

0

7.5

VERTICAL SCALE GREATLY EXAGGERATED

DATUM IS NGVD 29

SOUTHEAST NORTHWEST

LAYER

EXPLANATION

C Column 569

1

3

6

9

12

66 60 54 48 42 36 33

Hydrogeologic Unit

Surface water

Barrier island and

 shelf sands

Peat deposits

Seagrass beds

Estuarine mud

Lagoonal deposits

Upper outwash

Lower outwash

0 100 200 300 METERS

0 300 600 900 FEET
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 51

METERS ROW

-37.5

-30.0

-22.5

-15.0

-7.5

0

7.5

VERTICAL SCALE GREATLY EXAGGERATED

DATUM IS NGVD 29

SOUTHEAST NORTHWEST

LAYER

EXPLANATION

D Column 790

1

3

6

9

12

43 37 31 25 19 16

Hydrogeologic Unit

Surface water

Barrier island and

 shelf sands

Peat deposits

Seagrass beds

Lagoonal deposits

 Upper outwash

Lower outwash

0 100 200 300 METERS

0 300 600 900 FEET

Figure 19. Modeled hydrogeologic units for active model cells along four selected columns in the Fire Island study 

area, Suffolk County, N.Y. (A) column 190, (B) column 255, (C) column 569, and (D) column 790. (Column locations are 

shown in figure 18. NGVD 29, National Geodetic Vertical Datum of 1929)—Continued
52 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

73°04' 73°06' 73°08' 73°10'

40°39'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

0 0.5 1 KILOMETER0 0.5 1 MILE

Figure 20.

Modeled hydrogeologic units for layer 1 in the Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and 

D are shown 

in figure 2. NAD 83, North American Datum of 1983)
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 53 Figure 20. Modeled hydrogeologic units for layer 1 in the Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and D are shown in figure 2. NAD 83, North American Datum of 1983)—Continued

72°54' 72°56' 72°58' 73° 73°02'

40°42'

40°41'

72°52' 72°46' 72°48' 72°50'

40°44'

40°43'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

EXPLANATION

Active Model Area

Hydrogeologic Unit

Surface waterBarrier island and shelf sandsPeat depositsUpper outwash

C

D
54 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Figure 21.

Modeled hydrogeologic units for layer 2 in the Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and 

D are shown 

in figure 2. NAD 83, North American Datum of 1983)

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

73°04' 73°06' 73°08' 73°10'

40°39'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

0 0.5 1 KILOMETER0 0.5 1 MILE
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 55 Figure 21. Modeled hydrogeologic units for layer 2 in the Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and D are shown in figure 2. NAD 83, North American Datum of 1983)—Continued

72°54' 72°56' 72°58' 73° 73°02'

40°42'

40°41'

72°52' 72°46' 72°48' 72°50'

40°44'

40°43'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

EXPLANATION

Active Model Area

Hydrogeologic Unit

Barrier island and shelf sandsPeat deposits

Seagrass bedsEstuarine mudLagoonal depositsUpper outwash

C

D
56 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Figure 22.

Modeled hydrogeologic units for layers 3–5 in the Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and 

D are 

shown in figure 2. NAD 83, North American Datum of 1983)

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

73°04' 73°06' 73°08' 73°10'

40°39'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

0 0.5 1 KILOMETER0 0.5 1 MILE
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 57 Figure 22. Modeled hydrogeologic units for layers 3–5 in the Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and D are shown in figure 2. NAD 83, North American Datum of 1983)—Continued

72°54' 72°56' 72°58' 73° 73°02'

40°42'

40°41'

72°46' 72°48' 72°50' 72°52'

40°44'

40°43'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

EXPLANATION

Active Model Area

Hydrogeologic Unit

Lagoonal depositsUpper outwash

C

D
58 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Figure 23.

Modeled hydrogeologic units for layer 9 in the Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and 

D are shown 

in figure 2. NAD 83, North American Datum of 1983)

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

73°04' 73°06' 73°08' 73°10'

40°39'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

0 0.5 1 KILOMETER0 0.5 1 MILE
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 59 Figure 23. Modeled hydrogeologic units for layer 9 in the Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and D are shown in figure 2. NAD 83, North American Datum of 1983)—Continued

72°54' 72°56' 72°58' 73° 73°02'

40°42'

40°41'

72°46' 72°48' 72°50' 72°52'

40°44'

40°43'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

EXPLANATION

Active Model Area

Hydrogeologic Unit

"20-foot" clayLower outwash

C

D
60 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Figure 24.

Modeled, constant hydraulic-head values in the Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and 

D are 

shown in figure 2. NAD 83, North American Datum of 1983; NGVD 29, National Geodetic Vertical Datum of 1929)

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

73°04' 73°06' 73°08' 73°10'

40°39'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

0 0.5 1 KILOMETER0 0.5 1 MILE
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 61 Figure 24. Modeled, constant hydraulic-head values in the Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and D are shown in figure 2. NAD 83, North American Datum of 1983; NGVD 29, National Geodetic Vertical Datum of 1929)—Continued

72°54' 72°56' 72°58' 73° 73°02'

40°42'

40°41'

72°46' 72°48' 72°50' 72°52'

40°44'

40°43'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

EXPLANATION

Active Model Area

Water-surface altitude, in meters. Datum is NGVD 29

0.55 to 0.880.29 to 0.54

0.22 to 0.280.15 to 0.210.05 to 0.14

C

D
62 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Figure 25.

Modeled, constant salt-concentration values in the Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and 

D are 

shown in figure 2. NAD 83, North American Datum of 1983)

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

73°04' 73°06' 73°08' 73°10'

40°39'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

0 0.5 1 KILOMETER0 0.5 1 MILE
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 63 Figure 25. Modeled, constant salt-concentration values in the Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and D are shown in figure 2. NAD 83, North American Datum of 1983)—Continued

72°54' 72°56' 72°58' 73° 73°02'

40°42'

40°41'

72°46' 72°48' 72°50' 72°52'

40°44'

40°43'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

EXPLANATION

Active Model Area

Salinity, in parts per thousand

31.6 to 35.028.1 to 31.524.6 to 28.024.3 to 24.5

C

D
64 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

73°04' 73°06' 73°08' 73°10'

40°39'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

0 0.5 1 KILOMETER0 0.5 1 MILE

Figure 26. Zones and rates of recharge from precipitation simulated in the model of Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and 

D are shown in figure 2. NAD 83, North American Datum of 1983)
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 65

72°54' 72°56' 72°58' 73° 73°02'

40°42'

40°41'

72°46' 72°48' 72°50' 72°52'

40°44'

40°43'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

EXPLANATION

Active Model Area

Zone; Recharge, in meters per day

5; 0.002994; 0.00255

3; 0.001932; 0.001701; 0.00124

C

D

Figure 26. Zones and rates of recharge from precipitation simulated in the model of Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and D are shown in figure 2. NAD 83, North American Datum of 1983)—Continued
66 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Table 8. Simulated zones of recharge from precipitation in the model of Fire Island study area, Suffolk County, N.Y.

[Land-surface altitude in meters above National Geodetic Vertical Datum of 1929. Locations of recharge zones are shown in figure 26. Land cover and 

substrate information modified from Klopfer and others, 2002]

Land-surface altitude

Recharge zones for land cover or substrate

Fresh 

surface water

Peat substrate 

or developed feature Loam substrate Sand substrate or sparse vegetation

Less than 2 1 2 3 4

Between 2 and 4 Not present 3 4 5

Greater than 4 Not present 4 5 5

Simulation of Steady-State Conditions

Steady-state groundwater flow directions, velocities, 

and discharge rates under current (2005) conditions in the 

shallow aquifer system were generated from the numerical 

model. Cell-by-cell flows output from SEAWAT were used 

to create velocity vectors for the groundwater flow system. 

The simulated groundwater discharges were multiplied 

by representative TN concentrations in recharged water 

to calculate nitrogen loads to back-barrier estuaries and 

the ocean.

Groundwater Flow

The simulated water-table altitude under 2005 mean 

annual conditions is shown in figure 28 and can be compared 

with figure 4, which depicts the measured hydraulic heads 

in the four local study areas. The extent of the unconfined 

freshwater flow system—where salinity is less than 0.4 part 

per thousand—within the surficial hydrogeologic units as well 

as simulated salinities in the adjacent saline groundwater also 

are depicted in figure 28. The simulated 2005 mean annual 

potentiometric-surface altitude and extent of the confined 

freshwater flow system beneath the peat deposits onshore and 

surficial Holocene units offshore are shown in figure 29. The 

simulated hydraulic heads in the unconfined and underlying 

confined flow systems generally increase with proximity to 

the ocean shore, where the modeled effects of wave setup and 

tidal pumping result in a water-table overheight. These heads 

also increase with barrier-island width in inland areas because 

of the collectively greater amount of simulated recharge 

received in such areas. The simulated potentiometric-surface 

altitude and extent of the confined freshwater flow system 

beneath the “20-foot” clay or, where absent, the upper outwash 

under 2005 mean annual conditions is depicted in figure 30. 

Simulated salinity values indicate freshwater is present 

beneath these deposits in several isolated lens, at locations 

where the modeled barrier island is sufficiently wide to yield 

elevated hydraulic heads.

Similar patterns also are evident in figures 31A through 

D, which show the simulated 2005 mean annual water levels, 

flow vectors, and extent of freshwater within the shallow 

aquifer system in the four local study areas along model 

columns 190, 255, 569, and 790, respectively. These can 

be compared to figures 8A–D, which depict the measured 

hydraulic heads and freshwater-saltwater interface along 

vertical sections B–B′ through E–E′. The simulated water 

levels and resulting flow vectors shown along the four model 

columns indicate the groundwater divide is skewed strongly 

toward the ocean shore in response to the modeled water-table 

overheight; they also indicate this divide extends virtually to 

the base of the shallow aquifer system. Where the modeled 

barrier island is sufficiently wide to yield elevated hydraulic 

heads (figs. 31A and C), the groundwater divide originates 

within the freshwater flow system, which probably restricts 

the exchange of subsurface flow between back-barrier 

estuaries and the ocean. Where the barrier island is relatively 

narrow and heads are comparatively lower (figs. 31B and D), 

however, the divide originates along the ocean shore, which 

presumably enhances downward movement of saline water 

that may subsequently migrate beneath the freshwater flow 

system to back-barrier estuaries.
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 67

Discharge-Boundary Analysis

Simulated groundwater flows between the shallow 

aquifer system and marine surface waters (layer 1) under 2005 

mean-annual conditions are shown in figure 32. Negative 

groundwater flow from active model cells located along the 

immediate estuary shore into constant-head cells representing 

back-barrier estuaries simulates the shoreline underflow 

to these water bodies from the freshwater flow system 

(figs. 31A–D). Positive groundwater flow from constanthead cells located away from the immediate estuary shore 

into underlying active model cells simulates the recirculation 

of estuary water into the shallow aquifer system; this flow 

subsequently returns to the estuary along the freshwatersaltwater transition zone (figs. 31A–D). Negative groundwater 

flow from active model cells located beneath the modeled 

wave-setdown zone (about 200 m offshore) into constanthead cells representing the ocean simulates the shoreline 

underflow to this water body from the freshwater flow system; 

the negative flow also includes re-circulated seawater that 

previously infiltrated the shallow aquifer system along the 

immediate ocean shore and beyond 200 m offshore as positive 

flow (figs. 31A–D). Discharge to estuaries and the ocean from 

the freshwater flow system is calculated as the difference 

between the total discharge to each of these water bodies 

and the simulated groundwater flows from their respective 

constant-head cells into active model cells. Where the barrier 

island is relatively narrow, the resulting ocean discharge is 

positive and is assumed to represent the flow of a brackish 

mixture of freshwater and seawater beneath the freshwater 

flow system to back-barrier estuaries. In these locations, 

freshwater discharge to back-barrier estuaries is calculated as 

the difference between the total discharge to marine surface 

waters and the simulated groundwater flows from both estuary 

and ocean constant-head cells into active model cells.

Freshwater that exits the shallow aquifer system as 

seepage through confining layers (subsea underflow) cannot 

be directly measured in the model of this system because it 

does not explicitly represent subsea-discharge boundaries. 

Nevertheless, the simulated amount of freshwater seepage into 

saline groundwaters can be approximated from the difference 

between the total modeled recharge to the water table and 

freshwater discharge to marine surface waters. Groundwater 

budgets were developed for recharge areas of similar land 

use that contribute freshwater to back-barrier estuaries, the 

ocean, and subsea-discharge zones (table 13) on the basis 

of results of the discharge-boundary analysis (fig. 32). These 

budgets generally suggest that relatively minor amounts of 

freshwater may exit the shallow aquifer system as subsea 

underflow, except where the barrier island is relatively narrow. 

In these areas, the collectively smaller amount of simulated 

recharge to the water table limits the depth of the freshwater 

flow system, which extracts some flow from adjacent recharge 

areas just to sustain freshwater discharge directly to marine 

waters.

Results of the discharge-boundary analysis are consistent 

with the conceptual and numerical model of barrier-island 

groundwater flow. However, there is a large amount of 

uncertainty in model predictions of subsurface flow of saline 

groundwater. There also is uncertainty in the assumption 

that subsurface flows between back-barrier estuaries and the 

ocean do not cross budget-area boundaries. Consequently, 

the resulting estimates of groundwater discharge to these 

water bodies probably should be considered qualitative 

approximations of freshwater flow.

Contributing-Area Analysis

Areas that contribute recharge to back-barrier estuaries 

and the ocean are the only sources of freshwater discharge 

from the shallow aquifer system and were estimated by flowbased particle tracking. These contributing areas were defined 

by the forward-tracking of individual particles placed at the 

center of each model cell receiving recharge. The particle 

tracking post-processing package MODPATH (Pollock, 

1994) was used in the analysis. Simulated contributing areas 

to marine surface waters (layer 1) under 2005 mean annual 

conditions are shown in figure 33. The divide between 

simulated areas that contribute recharge to back-barrier 

estuaries and the ocean is skewed strongly toward the ocean 

shore, where the modeled effects of wave setup and tidal 

pumping result in the water-table overheight. Simulated areas 

that do not contribute recharge to these water bodies are 

assumed to provide a source of freshwater that may exit as 

subsea underflow.

Groundwater budgets were developed for recharge areas 

of similar land use that contribute freshwater to back-barrier 

estuaries, the ocean, and subsea-discharge zones (table 14) on 

the basis of results of the contributing-area analysis (fig. 33). 

These budgets indicate that total freshwater discharge from the 

shallow aquifer system is about 43,500 m3/d (79.8 percent) to 

back-barrier estuaries and about 10,200 m3/d (18.7 percent) to 

the ocean; about 836 m3/d (1.5 percent) may exit the system as 

subsea underflow. The total contribution of fresh groundwater 

to shoreline discharge zones amounts to about 53,700 m3/d 

(98.5 percent)—nearly the same as that resulting from the 

discharge-boundary analysis, which is about 52,800 m3/d 

(96.9 percent). In contrast to the generally consistent budget 

estimate for estuary zones from the discharge-boundary 

analysis, which is about 46,200 m3/d (84.8 percent), the 

ocean and subsea-discharge zone estimates from this analysis 

are only somewhat comparable, amounting to about 6,570 

(12.1 percent) and 2,760 m3/d (5.1 percent), respectively.
68 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

73°04' 73°06' 73°08' 73°10'

40°39'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

0 0.5 1 KILOMETER0 0.5 1 MILE

Figure 27. Rates of recharge from the return flow of public-supply water simulated in the model of Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and 

D are shown in figure 2. NAD 83, North American Datum of 1983)
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 69

72°54' 72°56' 72°58' 73° 73°02'

40°42'

40°41'

72°46' 72°48' 72°50' 72°52'

40°44'

40°43'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

EXPLANATION

Active Model Area

Return flow, in meters per day

0.00113 to 0.001560.000745 to 0.00112

0.000308 to 0.0007440.000178 to 0.0003070.0000808 to 0.000177

C

D

Figure 27. Rates of recharge from the return flow of public-supply water simulated in the model of Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and D are shown in figure 2. NAD 83, North American Datum of 1983)—Continued
70 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Table 9. Differences between measured and simulated water levels at wells in four areas of Fire 

Island, Suffolk County, N.Y.

[Water levels are in meters above National Geodetic Vertical Datum of 1929. Well locations are shown in figure 4. 

Well number is assigned by New York State Department of Environmental Conservation; prefix “S” denoting Suffolk 

County is omitted]

Well number

Groundwater level

Measured Simulated 1Difference

KISMET

125370 0.52 0.57 -0.05

125371 0.98 1.05 -0.06

125385 0.92 0.88 0.04

125981 1.05 1.00 0.05

125983 0.92 1.01 -0.09

125984 0.90 0.88 0.02

125986 1.00 1.02 -0.01

125995 0.98 1.03 -0.05

126753 1.00 1.01 -0.01

Mean -0.02

Standard deviation 0.05

ROBBINS REST

125982 0.75 0.80 -0.05

125994 0.77 0.86 -0.10

125996 0.65 0.53 0.12

125997 0.71 0.67 0.05

125998 0.75 0.74 0.01

125999 0.80 0.82 -0.03

126000 0.75 0.79 -0.04

126001 0.69 0.74 -0.06

Mean -0.01

Standard deviation 0.06

WATCH HILL

125380 0.87 0.92 -0.04

125381 0.86 0.84 0.02

125382 0.83 0.78 0.05

125383 0.93 0.90 0.03

125384 0.91 0.91 0.00

125385 0.94 0.92 0.02

125386 0.83 0.88 -0.05

125387 0.79 0.72 0.07

125990 0.92 0.88 0.04

Mean 0.01

Standard deviation 0.04

NATIONAL WILDERNESS

124399 0.88 0.83 0.05

124400 0.96 0.89 0.07

125363 0.73 0.78 -0.05

125364 0.71 0.71 0.00

125365 0.61 0.48 0.12

125366 0.66 0.63 0.03

125367 0.81 0.81 0.01

125368 0.69 0.77 -0.07

125369 0.77 0.70 0.07

Mean 0.03

Standard deviation 0.06

1

Amount may not equal the sum of values because of rounding to significant digits.
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 71

Table 10. Differences between measured and simulated water levels at wells screened in four 

hydrogeologic units simulated in the model of Fire Island, Suffolk County, N.Y.

[Water levels are in meters above National Geodetic Vertical Datum of 1929. Well locations are shown in figure 4. 

Well number is assigned by New York State Department of Environmental Conservation; prefix “S” denoting Suffolk 

County is omitted]

Well number

Groundwater level

Measured Simulated 1Difference

BARRIER ISLAND AND SHELF SANDS

124400 0.96 0.89 0.07

125365 0.61 0.48 0.12

125371 0.98 1.05 -0.06

125387 0.79 0.72 0.07

Mean 0.05

Standard deviation 0.07

PEAT DEPOSITS

125370 0.52 0.57 -0.05

125994 0.77 0.86 -0.10

125995 0.98 1.03 -0.05

Mean -0.07

Standard deviation 0.02

LAGOONAL DEPOSITS

125363 0.73 0.78 -0.05

125364 0.71 0.71 0.00

125366 0.66 0.63 0.03

125367 0.81 0.81 0.01

125368 0.69 0.77 -0.07

125380 0.87 0.92 -0.04

125381 0.86 0.84 0.02

125382 0.83 0.78 0.05

125383 0.93 0.90 0.03

125384 0.91 0.91 0.00

125385 0.94 0.92 0.02

125386 0.83 0.88 -0.05

125981 1.05 1.00 0.05

125982 0.75 0.80 -0.05

125983 0.92 1.01 -0.09

125984 0.90 0.88 0.02

125985 0.92 0.88 0.04

125986 1.00 1.02 -0.01

125996 0.65 0.53 0.12

125997 0.71 0.67 0.05

125998 0.75 0.74 0.01

125999 0.80 0.82 -0.03

126000 0.75 0.79 -0.04

126001 0.69 0.74 -0.06

126753 1.00 1.01 -0.01

Mean -0.00

Standard deviation 0.05

UPPER OUTWASH

124399 0.88 0.83 0.05

125369 0.77 0.70 0.07

125990 0.92 0.88 0.04

Mean 0.05

Standard deviation 0.01

1

Amount may not equal the sum of values because of rounding to significant digits.
72 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Table 11. Measured and simulated water levels and vertical hydraulic gradients at five observation-well pairs on Fire Island, 

Suffolk County, N.Y.

[Water levels are in meters above National Geodetic Vertical Datum of 1929. Well locations are shown in figure 4. Well number is assigned by New York State 

Department of Environmental Conservation; prefix “S” denoting Suffolk County is omitted. --, no value]

Statistic Well number

Local study area Kismet Robbins Rest Watch Hill National Wilderness

UPPER WELL 125986 125982 125383 125364 124400

Measured groundwater level 1.00 0.75 0.93 0.71 0.96

Simulated groundwater level 1.02 0.80 0.90 0.71 0.89

LOWER WELL 126753 126000 125990 125369 124399

Measured groundwater level 1.00 0.75 0.92 0.77 0.88

Simulated groundwater level 1.01 0.79 0.88 0.70 0.83

1

RELATION BETWEEN GROUNDWATER LEVELS IN UPPER AND LOWER WELLS

Difference between measured values 0.00 0.00 0.01 -0.06 0.08

Difference between simulated values 0.01 0.01 0.02 0.01 0.05

Measured gradient between upper and lower wells 0.00000 0.00000 0.00045 -0.01234 0.00383

Simulated gradient between upper and lower wells 0.00039 0.00117 0.00097 0.00167 0.00262

Ratio of simulated to measured gradient -- -- 2.170 -0.136 0.684

1

Amount may not equal the sum of values because of rounding to significant digits.

Table 12. Final horizontal and vertical hydraulic conductivities for hydrogeologic units simulated in the model of Fire Island 

study area, Suffolk County, N.Y.

[Values are in meters per day]

Hydrogeologic unit

Hydraulic conductivity

Horizontal Vertical Ratio of horizontal 

to vertical

Surficial Holocene units Barrier island and shelf sands 45 4.5 10:1

Peat deposits 2 0.02 100:1

Seagrass beds 4 0.04 100:1

Estuarine mud 0.3 0.003 100:1

Lagoonal deposits Western zone 25 1 25:1

Eastern zone 20 0.67 30:1

Upper outwash 30 1.5 20:1

“20-foot” clay 0.03 0.0003 100:1

Lower outwash Upper zone 20 0.67 30:1

Middle zone 10 0.1 100:1

Lower zone 3 0.03 100:1
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 73

The contributing-area analysis also provided estimates 

of the traveltimes for individual particles from their point of 

recharge at the water table to their point of discharge at the 

shore. Results of particle-traveltime tracking indicate the 

median age of freshwater discharged to back-barrier estuaries 

and the ocean was 3.4 years, and the 95th-percentile age 

was 20 years. Particles that do not discharge to these water 

bodies—assumed to represent a source of freshwater that 

may exit as subsea underflow—were excluded from particletraveltime tracking.

Nutrient Loads

Nitrogen loading to estuaries and the ocean can originate 

from various sources, including freshwater discharge from 

the shallow aquifer system as well as saline and transitional 

water discharge. Because the potential sources of nutrients in 

the freshwater system are probably greatest, nitrogen loads 

were calculated for freshwater discharge. Nitrogen loads from 

the shallow aquifer system to back-barrier estuaries and the 

ocean were calculated as the simulated discharges from the 

contributing-area analysis multiplied by the representative TN 

concentrations for the principal land uses on Fire Island.

The nitrogen concentrations in undeveloped and 

developed park areas on Fire Island are assumed to be 

0.29 and 1.63 mg/L as N, respectively. These are median 

TN concentrations found in groundwater from 16 shallow 

wells—7 in undeveloped and 9 in developed park areas—

sampled in October 2005.

Nitrogen concentrations for the residential communities 

within FIIS are presumed to be comparable to median TN 

values calculated by Monti and Scorca (2003) for 747 samples 

collected at 63 shallow wells within a 1.6-km-wide zone along 

the southern, mainland coastline of Suffolk County. This 

mainland zone includes areas of predominantly residential 

development similar to the Fire Island communities.

For communities consisting mainly of low- to mediumdensity residential development, the nitrogen concentration 

is estimated in this study to be generally equivalent to a 

median TN concentration of 1.19 mg/L calculated for the 

unsewered area east of Carmans River (fig. 2) during the 

period 1982–97 (Monti and Scorca, 2003). The concentration 

of TN in communities consisting primarily of high-density 

residential development is estimated to be generally equivalent 

to a median value of 2.69 mg/L calculated for the unsewered 

area west of Carmans River during 1982–97 (Monti and 

Scorca, 2003). For the sewered community of Ocean Beach, 

the concentration is estimated to be generally equivalent to a 

median TN value of 0.86 mg/L that Monti and Scorca (2003) 

calculated for the Southwest Sewer District (fig. 1) during this 

same period.

Residency in the mainland areas is mostly year-round, 

so these areas could generate a larger wastewater load 

annually than those areas occupied mainly during summer. 

However, the intense summer visitation of Fire Island could 

result in a higher seasonal wastewater load than would be 

produced by a permanent population in the same density of 

residential development. TN concentrations from four wells 

downgradient from two communities—Kismet and Robbins 

Rest—sampled during this study (S126753, S125370, 

S125994, and S126000; figs. 16A and B) show reasonable 

agreement with the median TN concentrations calculated 

by Monti and Scorca (2003); therefore, the values from the 

southern Suffolk County mainland probably can be considered 

reasonable approximations of the median TN concentrations 

within the Fire Island communities.

The TN concentrations and loads under 2005 mean 

annual conditions for areas that contribute fresh groundwater 

to back-barrier estuaries and the ocean were summarized 

for the principal land uses on Fire Island (table 15). The 

overall TN load from the shallow aquifer system to shoreline 

discharge zones is about 16,200 kg/yr (82.2 percent) to 

back-barrier estuaries and about 3,500 kg/yr (17.8 percent) 

to the ocean (table 15). TN loads to estuary zones are about 

2,590 (16.0 percent) and 4,730 kg/yr (29.2 percent) from 

undeveloped and developed park areas, respectively; about 

63 (0.4 percent), 1,190 (7.3 percent), and 7,400 kg/yr (45.7 

percent) from unsewered residential areas of low, medium, and 

high density, respectively; and about 203 kg/yr (1.3 percent) 

from the sewered residential area.

The nitrogen-loading calculations in table 15 indicate 

the overall TN load from Fire Island to marine surface waters 

amounts to about 19,700 kg/yr—roughly 6 percent of the 

annual TN load from shallow groundwater entering the South 

Shore Estuary Reserve (SSER) from the Suffolk County 

mainland, which is about 345,000 kg/yr (Monti and Scorca, 

2003). In contrast to the TN load from shallow groundwater 

for the entire 1,420-km2 SSER study area of Monti and Scorca 

(2003), which annually yields about 353 kg of nitrogen per 

square kilometer of watershed (table 16), the overall TN load 

from Fire Island produces nitrogen at an annual rate of about 

890 kg/km2—more than double the loading rate from shallow 

groundwater to the SSER from the southern mainland of 

Nassau and Suffolk Counties. Similarly, the loading rate for 

Fire Island is nearly twice the annual rate of nitrogen loading 

from shallow groundwater entering Long Island Sound from 

the North Shore of Long Island, which is about 458 kg/km2, 

as indicated by an earlier study (Scorca and Monti, 2001) that 

used load-calculation methods comparable to those of Monti 

and Scorca (2003) and this study.
74 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Figure 28. Simulated 2005 mean annual water-table altitude and extent of unconfined freshwater flow system within surficial hydrogeologic units in the model of Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and 

D are shown in figure 2. NAD 83, North American Datum 

of 1983; NGVD 29, National Geodetic Vertical Datum of 1929)

0.3 0.6 0.9

1 2.

0.3

0.3

0.9

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

0.6 0.3 0.9 0.3

0.3

0.9

73°04' 73°06' 73°08' 73°10'

40°39'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

0 0.5 1 KILOMETER0 0.5 1 MILE
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 75 Figure 28. Simulated 2005 mean annual water-table altitude and extent of unconfined freshwater flow system within surficial hydrogeologic units in the model of Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and D are shown in figure 2. NAD 83, North American Datum of 1983; NGVD 29, National Geodetic Vertical Datum of 1929)—Continued 0.6

0.

0.9

0.9

0 9.

72°54' 72°56' 72°58' 73° 73°02'

40°42'

40°41'

0.3 6

0.9

0.3

0.9 0.9

0 3.

0.3

3

. 0

0 3.

0 9.

0.9

72°46' 72°48' 72°50' 72°52'

40°44'

40°43'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

C

D

EXPLANATION

Active Model Area

Simulated salinity, in parts per thousand

0.4 7 14 21 28Less than 0.4

Simulated Water-Surface Altitude--Contour interval 0.15 meter. Datum is NGVD 29

35
76 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Figure 29. Simulated 2005 mean annual potentiometric-surface altitude and extent of confined freshwater flow system beneath peat deposits onshore and surficial Holocene units offshore in the model of Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and 

D are shown in figure 2. 

NAD 83, North American Datum of 1983; NGVD 29, National Geodetic Vertical Datum of 1929)

0.3 0.6 0.9

0.3

0.3

3. 0

0.9

0.3 0.9

0.9

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

0.6 0.3 0.9

0.3

0.3

0.3

0.9 0.9

73°04' 73°06' 73°08' 73°10'

40°39'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

0 0.5 1 KILOMETER0 0.5 1 MILE
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 77 0.6 0.3

0.9

0 3.

0

0.3 0.3

0.3

0.9

72°54' 72°56' 72°58' 73° 73°02'

40°42'

40°41'

0.3

.6 0.9

0

.3 0.3

0.9

0.3 0.3 0.3

0 3.

. 0 3

0 3.

0.9

72°46' 72°48' 72°50' 72°52'

40°44'

40°43'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

C

D

EXPLANATION

Active Model Area

Simulated salinity, in parts per thousand

0.4 7 14 21 28Less than 0.4

Simulated Potentiometric-Surface Altitude--Contour interval 0.15 meter. Datum is NGVD 29

35

Figure 29. Simulated 2005 mean annual potentiometric-surface altitude and extent of confined freshwater flow system beneath peat deposits onshore and surficial Holocene units offshore in the model of Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and D are shown in figure 2. NAD 83, North American Datum of 1983; NGVD 29, National Geodetic Vertical Datum of 1929)—Continued
78 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Figure 30. Simulated 2005 mean annual potentiometric-surface altitude and extent of confined freshwater flow system beneath “20-foot” clay and (or) upper outwash in the model of Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and 

D are shown in figure 2. NAD 83, North 

American Datum of 1983; NGVD 29, National Geodetic Vertical Datum of 1929)

0.3

0.6

0

9.

0.3

0.6

0.3

0.3 0.3

0.3

0.3

0.6

0.6

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

0.3 0.6

0.9

0.3

0.3

0.3

3. 0

0.6 0.6

0.9

73°04' 73°06' 73°08' 73°10'

40°39'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

0 0.5 1 KILOMETER0 0.5 1 MILE
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 79 0.3

0.6 0.9 0.3 0.3

3

. 00.3

0.9

0.6

0.

72°54' 72°56' 72°58' 73° 73°02'

40°42'

40°41'

0.3

0.6 0.9

0.6

0 3.

0.6 0.6

0 3.

0.

0

6

.9

0.9

72°46' 72°48' 72°50' 72°52'

40°44'

40°43'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

C

D

EXPLANATION

Active Model Area

Simulated salinity, in parts per thousand

0.4 7 14 21 28Less than 0.4

Simulated Potentiometric-Surface Altitude--Contour interval 0.15 meter. Datum is NGVD 29

35

Figure 30. Simulated 2005 mean annual potentiometric-surface altitude and extent of confined freshwater flow system beneath “20-foot” clay and (or) upper outwash in the model of Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and D are shown in figure 2. NAD 83, North 

American Datum of 1983; NGVD 29, National Geodetic Vertical Datum of 1929)—Continued
80 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Simulated Water Level--Contour

 interval 0.15 meters. Datum is

 NGVD 29.

Simulated Flow Vector

METERS ROW

-37.5

-30.0

-22.5

-15.0

-7.5

0

7.5

VERTICAL SCALE GREATLY EXAGGERATED

DATUM IS NGVD 29

SOUTHEAST NORTHWEST

0 100 200 300 METERS

0 300 600 900 FEET

LAYER

1

Column 190

3

6

9

12

65 59 53 47 41 35

0.400 6.167 11.93 17.70 23.47 29.23 35.00

0.60

0.60

0.30

0.3

0

0

6. 0

0.3

0

0.90

0.6

0

A

EXPLANATION

Simulated salinity, in parts per thousand

Figure 31. Simulated 2005 mean annual water levels, flow vectors, and extent of freshwater within the 

shallow aquifer system along four selected columns in the Fire Island study area, Suffolk County, N.Y. 

(A) column 190, (B) column 255, (C) column 569, and (D) column 790. (Column locations are shown in 

figure 18. NGVD 29, National Geodetic Vertical Datum of 1929)
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 81

Figure 31. Simulated 2005 mean annual water levels, flow vectors, and extent of freshwater 

within the shallow aquifer system along four selected columns in the Fire Island study area, Suffolk 

County, N.Y. (A) column 190, (B) column 255, (C) column 569, and (D) column 790. (Column locations 

are shown in figure 18. NGVD 29, National Geodetic Vertical Datum of 1929)—Continued

Simulated Water

 Level--Contour

 interval 0.15 meters.

 Datum is NGVD 29.

Simulated Flow Vector

Simulated salinity, in parts per thousand

METERS ROW

-37.5

-30.0

-22.5

-15.0

-7.5

0

7.5

VERTICAL SCALE GREATLY EXAGGERATED

DATUM IS NGVD 29

SOUTHEAST NORTHWEST

0 100 200 300 METERS

0 300 600 900 FEET

LAYER

Column 255

1

3

6

9

12

67 61 55 49 43 41

0.400 6.167 11.93 17.70 23.47 29.23 35.00

0.60

0.3

0

0.6

0

0.3

0

B

EXPLANATION
82 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Figure 31. Simulated 2005 mean annual water levels, flow vectors, and extent of freshwater within the shallow 

aquifer system along four selected columns in the Fire Island study area, Suffolk County, N.Y. (A) column 190, 

(B) column 255, (C) column 569, and (D) column 790. (Column locations are shown in figure 18. NGVD 29, National 

Geodetic Vertical Datum of 1929)—Continued

C

Simulated Water Level--Contour

 interval 0.15 meters. Datum is NGVD 29.

Simulated Flow Vector

Simulated salinity, in parts per thousand

METERS ROW

-37.5

-30.0

-22.5

-15.0

-7.5

0

7.5

VERTICAL SCALE GREATLY EXAGGERATED

DATUM IS NGVD 29

SOUTHEAST NORTHWEST LAYER

EXPLANATION

Column 569

1

3

6

9

12

66 60 54 48 42 36 33

0 100 200 300 METERS

0 300 600 900 FEET

0.30

0.6

0

0.400 6.167 11.93 17.70 23.47 29.23 35.00

0.30

0.3

0

0.

60

0.6

0

0

3. 0
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 83

Figure 31. Simulated 2005 mean annual water levels, flow vectors, and extent of freshwater within the 

shallow aquifer system along four selected columns in the Fire Island study area, Suffolk County, N.Y. 

(A) column 190, (B) column 255, (C) column 569, and (D) column 790. (Column locations are shown in 

figure 18. NGVD 29, National Geodetic Vertical Datum of 1929)—Continued

D

Simulated Water

 Level--Contour

 interval 0.15 meters.

 Datum is NGVD 29.

Simulated Flow Vector

Simulated salinity, in parts per thousand

METERS ROW

-37.5

-30.0

-22.5

-15.0

-7.5

0

7.5

VERTICAL SCALE GREATLY EXAGGERATED

DATUM IS NGVD 29

SOUTHEAST NORTHWEST

0 100 200 300 METERS

0 300 600 900 FEET

LAYER

EXPLANATION

Column 790

1

3

6

9

12

43 37 31 25 19 16

0

3. 0

0.60

0.400 6.167 11.93 17.70 23.47 29.23 35.00

0.60

0.30

0

3. 0
84 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Table 13. Simulated 2005 mean annual fresh-groundwater budgets for recharge areas of similar land use from the dischargeboundary analysis of Fire Island study area, Suffolk County, N.Y.—Continued

[Contributing-area locations are shown in figure 32]

Recharge area 

map identifier

Inflow, in cubic meters per day Outflow, in cubic meters per day

Shoreline discharge zones Subsea 

Total recharge discharge zones Lateral flow Estuary Ocean Total

UNDEVELOPED PARK

1 3,900 0 -2,440 -1,330 -3,770 -124

3 196 35 -179 -52 -231 0

4 333 0 -278 -28 -306 -27

6 799 54 -636 -218 -854 0

8 71 0 -45 0 -45 -26

10 127 74 -149 -52 -201 0

17 177 61 -238 0 -238 0

19 205 29 -234 0 -234 0

26 1,250 0 -1,220 0 -1,220 -39

28 394 0 -394 0 -394 0

30 417 74 -491 0 -491 0

32 554 54 -608 0 -608 0

34 387 5 -392 0 -392 0

36 491 0 -465 0 -465 -26

38 469 0 -417 0 -417 -51

40 315 93 -364 -44 -408 0

42 12,200 110 -10,300 -2,040 -12,300 0

44 375 0 -138 -52 -191 -185

46 8,270 0 -8,040 0 -8,040 -223

Subtotal 30,900 589 -27,000 -3,820 -30,800 -701

DEVELOPED PARK

2 6,130 0 -4,670 -1,150 -5,820 -310

5 538 0 -377 -136 -514 -24

15 296 0 -196 0 -196 -100

27 400 0 -399 0 -399 -1

33 215 0 -103 0 -103 -113

41 717 0 -489 -53 -542 -175

43 177 15 -193 0 -193 0

45 1,530 0 -1,510 0 -1,510 -21

Subtotal 10,000 15 -7,940 -1,340 -9,280 -744
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 85

Table 13. Simulated 2005 mean annual fresh-groundwater budgets for recharge areas of similar land use from the dischargeboundary analysis of Fire Island study area, Suffolk County, N.Y.—Continued

[Contributing-area locations are shown in figure 32]

Recharge area 

map identifier

Inflow, in cubic meters per day Outflow, in cubic meters per day

Shoreline discharge zones Subsea 

Total recharge discharge zones Lateral flow Estuary Ocean Total

LOW-DENSITY RESIDENTIAL

37 145 59 -204 0 -204 0

MEDIUM-DENSITY RESIDENTIAL

11 1,580 0 -1,250 -239 -1,490 -89

20 146 26 -126 -46 -172 0

25 1,860 0 -1,370 -350 -1,720 -143

Subtotal 3,580 26 -2,740 -634 -3,380 -231

HIGH-DENSITY RESIDENTIAL

7 410 0 -211 -136 -347 -62

9 140 43 -119 -64 -183 0

12 867 0 -645 -18 -664 -203

13 304 15 -283 -36 -319 0

14 296 58 -353 -2 -354 0

16 211 76 -287 0 -287 0

18 88 11 -99 0 -99 0

21 236 39 -172 -103 -275 0

23 1,100 72 -1,000 -170 -1,170 0

24 820 25 -795 -51 -845 0

29 846 0 -739 0 -739 -108

31 2,380 0 -2,160 0 -2,160 -218

35 172 0 -127 0 -127 -46

39 1,080 0 -978 0 -978 -103

Subtotal 8,950 339 -7,970 -580 -8,550 -741

SEWERED RESIDENTIAL

22 920 0 -379 -195 -575 -345

OVERALL

Total 54,500 1,030 -46,200 -6,570 -52,800 -2,760

Percent 100.0 1.9 -84.8 -12.1 -96.9 -5.1
86 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

2

1

6 11 5 12 4 7 3

15 14 16 13 17

9

19

10 8

18

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

31

25 26

23

32 29

24

22

36 34 28 27 30 33

16

35

17 19 15 21

20 37

18

73°04' 73°06' 73°08' 73°10'

40°39'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

A

B

0 0.5 1 KILOMETER0 0.5 1 MILE

Figure 32. Simulated 2005 mean annual groundwater flows between the shallow aquifer system and marine surface waters, and recharge areas for freshgroundwater budgets (tables 13 and 14) and total nitrogen loads (table 15) in the model of Fire Island study area, Suffolk County, N.Y. (Negative values denote flow from the shallow aquifer system to marine surface waters. Positive values denote flow from marine surface water into the shallow aquifer system. Locations of areas shown in panels A, B, C, and 

D are shown in figure 2. NAD 83, North American Datum of 1983)
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 87

42

39

41

36 38

40

37

72°54' 72°56' 72°58' 73° 73°02'

40°42'

40°41'

46

42 45 44 43

72°46' 72°48' 72°50' 72°52'

40°44'

40°43'

Base from National Park Service and U.S. Geological Survey digital data,

Universal Transverse Mercator projection, NAD 83, Zone 18

C

D

EXPLANATION

Recharge Area of SimilarLand Use--Identifier corresponds to data in tables 14-16

Simulated groundwater flow, in cubic meters per day

66.5 to 114.142.6 to 66.41.1 to 42.5

-1.0 to 1.0-26.7 to -1.1-47.5 to -26.8-161.0 to -47.6

42

Figure 32. Simulated 2005 mean annual groundwater flows between the shallow aquifer system and marine surface waters, and recharge areas for freshgroundwater budgets (tables 13 and 14) and total nitrogen loads (table 15) in the model of Fire Island study area, Suffolk County, N.Y. (Negative values denote flow from the shallow aquifer system to marine surface waters. Positive values denote flow from marine surface water into the shallow aquifer system. Locations of areas shown in panels A, B, C, and D are shown in figure 2. NAD 83, North American Datum of 1983)—Continued
88 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Table 14. Simulated 2005 mean annual fresh-groundwater budgets for recharge areas of similar land use from the contributing-area 

analysis of Fire Island study area, Suffolk County, N.Y.—Continued

[Contributing-area locations are shown in figure 32]

Recharge area 

map 

identifier

Total recharge, 

in cubic meters 

per day

Outflow, in cubic meters per day

Shoreline discharge zones Subsea 

Estuary Ocean Total discharge zones

UNDEVELOPED PARK

1 3,900 -2,570 -1,270 -3,840 -63

3 196 -136 -59 -196 0

4 333 -233 -101 -333 0

6 799 -555 -244 -799 0

8 71 -48 -23 -71 0

10 127 -87 -39 -127 0

17 177 -177 0 -177 0

19 205 -165 -40 -205 0

26 1,250 -1,060 -188 -1,250 -7

28 394 -394 0 -394 0

30 417 -381 -28 -410 -7

32 554 -554 0 -554 0

34 387 -387 0 -387 0

36 491 -491 0 -491 0

38 469 -469 0 -469 0

40 315 -260 -50 -310 -6

42 12,200 -9,190 -2,700 -11,900 -327

44 375 -302 -66 -368 -7

46 8,270 -7,020 -1,100 -8,120 -142

Subtotal 30,900 -24,500 -5,900 -30,400 -561

DEVELOPED PARK

2 6,130 -4,620 -1,480 -6,100 -35

5 538 -394 -144 -538 0

15 296 -278 -17 -296 0

27 400 -400 0 -400 0

33 215 -215 0 -215 0

41 717 -555 -141 -696 -21

43 177 -155 -22 -177 0

45 1,530 -1,320 -179 -1,500 -27

Subtotal 10,000 -7,940 -1,980 -9,920 -83

LOW-DENSITY RESIDENTIAL

37 145 -145 0 -145 0

MEDIUM-DENSITY RESIDENTIAL

11 1,580 -1,270 -283 -1,550 -27

20 146 -109 -36 -146 0

25 1,860 -1,370 -477 -1,840 -17

Subtotal 3,580 -2,740 -797 -3,540 -44
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 89

Limitations of Analysis

Several limitations apply to simulations of groundwater 

discharge and calculations of nitrogen loads.

The total estimate of water-table overheight resulting 

from wave setup and tidal pumping along the Atlantic Ocean 

shore of Fire Island relies on simplified terms and probably 

can be considered a conservative approximation. The estimate 

also is based on a static shoreline position that in reality is 

dynamic, due partly to changes in wave climate that also may 

affect the overheight amount.

Coarse model discretization results in averaging over 

large spatial and temporal scales and, therefore, limits the 

precision of simulated groundwater flow patterns and rates 

and the resulting nitrogen-loading calculations. Because the 

calibration was limited to groundwater levels, the model 

is nonunique—variations in hydraulic conductivity and 

recharge are most likely inversely correlated (an increase 

in one has the same result on hydraulic head as a decrease 

in the other). Although no formal sensitivity analysis was 

done, addition of the water-table overheight condition during 

model development strongly affected simulated water levels, 

particularly along the ocean shore. Variations in the no-flow 

condition specified at the base of the upper glacial aquifer that 

could allow flow across underlying low-permeability units (the 

Gardiners Clay and Monmouth greensand) also may affect 

results, but probably to a lesser extent.

Simulations of variable-density groundwater flow were 

run to achieve a steady-state extent of freshwater under 

current (2005) conditions. Although the modeled hydrologic 

boundaries are assumed to be constant through time, they can 

undergo significant fluctuations due to storm tides and other 

short-term factors. For example, the barrier-island aquifer 

system is periodically recharged by saline groundwater 

through storm overwash and coastal flooding. As a result, the 

model probably overestimates the depth of freshwater extent, 

particularly in deeper, confined zones with longer traveltimes 

for freshwater recharge. Consequently, transient modeling 

of historical conditions may be required to better understand 

how short-term fluctuations in hydrologic boundaries affect 

groundwater discharge and nitrogen loads.

Table 14. Simulated 2005 mean annual fresh-groundwater budgets for recharge areas of similar land use from the contributing-area 

analysis of Fire Island study area, Suffolk County, N.Y.—Continued

[Contributing-area locations are shown in figure 32]

Recharge area 

map 

identifier

Total recharge, 

in cubic meters 

per day

Outflow, in cubic meters per day

Shoreline discharge zones Subsea 

Estuary Ocean Total discharge zones

HIGH-DENSITY RESIDENTIAL

7 410 -281 -129 -410 0

9 140 -99 -41 -140 0

12 867 -693 -150 -843 -24

13 304 -248 -57 -304 0

14 296 -252 -38 -291 -5

16 211 -211 0 -211 0

18 88 -81 -7 -88 0

21 236 -175 -61 -236 0

23 1,100 -826 -232 -1,060 -41

24 820 -647 -164 -812 -8

29 846 -816 -25 -841 -6

31 2,380 -2,010 -349 -2,360 -18

35 172 -172 0 -172 0

39 1,080 -1,020 -45 -1,070 -13

Subtotal 8,950 -7,530 -1,300 -8,830 -116

SEWERED RESIDENTIAL

22 920 -648 -241 -888 -31

OVERALL

Total 54,500 -43,500 -10,200 -53,700 -836

Percent 100.0 -79.8 -18.7 -98.5 -1.5
90 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Figure 33. Simulated contributing areas to marine surface waters under 2005 mean annual conditions in the model of Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and 

D are shown in figure 2. NAD 83, North American Datum of 1983)

2

1

6 11 5 12 4 7 3

15 14 16 13 17

9

19

10 8

18

73°12' 73°14' 73°16' 73°18'

40°37'

40°36'

31

25 26

23

32 29

24

22

36 34 28 27 30 33

16

35

17 19 15 21

20 37

18

73°04' 73°06' 73°08' 73°10'

40°39

A

B

'

0 0.5 1 KILOMETER0 0.5 1 MILE

Base from New York State Department of Transportation planimetric quadrangles, 1991, 1:24,000,

Universal Transverse Mercator projection, NAD 83, Zone 18
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 91 Figure 33. Simulated contributing areas to marine surface waters under 2005 mean annual conditions in the model of Fire Island study area, Suffolk County, N.Y. (Locations of areas shown in panels A, B, C, and D are shown in figure 2. NAD 83, North American Datum of 1983)—Continued

42

39

41

36 38

40

37

72°54' 72°56' 72°58' 73° 73°02'

40°42'

40°41'

46

42 45 44 43

72°46' 72°48' 72°50' 72°52'

40°44'

40°43

C

D

'

Base from New York State Department of Transportation planimetric quadrangles, 1991, 1:24,000,

Universal Transverse Mercator projection, NAD 83, Zone 18

EXPLANATION

Recharge Area of SimilarLand Use--Identifier corresponds to data in tables 14-16

Simulated Contributing Area to Marine Surface Waters

Back-barrier estuariesOceanSubsea-discharge zones(?)

42
92 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Table 15. Calculated total nitrogen loads to marine surface waters for recharge areas of similar 

land use in the model of Fire Island study area, Suffolk County, N.Y.—Continued

[Contributing-area locations are shown in figure 32. kg/yr, kilograms per year]

Recharge area 

map identifier

Total nitrogen load to shoreline discharge zones from fresh ground water, in kg/yr

Estuary Ocean Total

UNDEVELOPED PARK

1 272 134 406

3 14 6 21

4 25 11 35

6 59 26 85

8 5 2 8

10 9 4 13

17 19 0 19

19 17 4 22

26 112 20 132

28 42 0 42

30 40 3 43

32 59 0 59

34 41 0 41

36 52 0 52

38 50 0 50

40 28 5 33

42 973 286 1,260

44 32 7 39

46 744 116 860

Subtotal 2,590 625 3,220

DEVELOPED PARK

2 2,750 881 3,630

5 235 86 320

15 166 10 176

27 238 0 238

33 128 0 128

41 330 84 415

43 92 13 106

45 788 106 895

Subtotal 4,730 1,180 5,910

LOW-DENSITY RESIDENTIAL

37 63 0 63

MEDIUM-DENSITY RESIDENTIAL

11 551 123 674

20 47 16 63

25 594 207 801

Subtotal 1,190 346 1,540
Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore 93

Table 16. Annual totals and rates of nitrogen loading from shallow groundwater to marine surface 

waters from three watersheds on Long Island, N.Y.

[km2, square kilometers; kg, kilograms; kg/km2, kilograms per square kilometer]

Watershed 

area, 

in km2

Annual nitrogen loading from shallow groundwater to marine surface waters

Total, in kg Rate, in kg/km2

FIRE ISLAND1

22 19,700 2890

SOUTH SHORE ESTUARY RESERVE STUDY AREA3

1,420 501,000 353

NORTH SHORE OF LONG ISLAND4

927 425,000 458

1

As mapped by Klopfer and others (2002).

2

Amount may not equal the sum of values because of rounding to significant digits.

3

As defined by Monti and Scorca (2003).

4

As defined by Scorca and Monti (2001).

Table 15. Calculated total nitrogen loads to marine surface waters for recharge areas of similar 

land use in the model of Fire Island study area, Suffolk County, N.Y.—Continued

[Contributing-area locations are shown in figure 32. kg/yr, kilograms per year]

Recharge area 

map identifier

Total nitrogen load to shoreline discharge zones from fresh ground water, in kg/yr

Estuary Ocean Total

HIGH-DENSITY RESIDENTIAL

7 276 126 403

9 97 41 138

12 681 147 828

13 243 56 299

14 248 38 286

16 207 0 207

18 79 7 86

21 172 60 232

23 812 228 1,040

24 636 161 797

29 801 25 826

31 1,970 342 2,320

35 169 0 169

39 1,000 45 1,050

Subtotal 7,400 1,280 8,680

SEWERED RESIDENTIAL

22 203 76 279

OVERALL

Total 16,200 3,500 19,700

Percent 82.2 17.8 100.0
94 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Use of the TN concentrations to calculate nitrogen loads 

may overestimate mass-loading rates through the assumption 

of conservative transport in groundwater. Additional 

uncertainties in nitrogen loads from residential areas on 

Fire Island include (1) estimation of TN concentrations 

using values calculated by Monti and Scorca (2003) for 

the southern Suffolk County mainland, and (2) reliance on 

TN concentrations from four wells downgradient from two 

communities to assess the suitability of these surrogate values.

Groundwater-Resource Concerns

Although high-density residential land use occupies only 

about 8 percent of Fire Island, its unsewered areas contribute 

almost half of the overall TN load from the shallow aquifer 

system to back-barrier estuaries.

Because of the combined effects of wave setup and tidal 

pumping along the ocean shore, the resulting water-table 

overheight directs about 80 percent of freshwater recharge on 

Fire Island to back-barrier estuaries. As a result, groundwaterprotection strategies need to consider most of the island as 

a potential source area to the wetland and adjacent estuarine 

habitats within the boundaries of FIIS.

Water-quality sampling conducted during this study 

typically showed very low TN concentrations in undeveloped 

park areas. However, TN concentrations were elevated 

locally in these areas that were downgradient from at least 

two communities—Kismet and Robbins Rest—and from the 

Watch Hill leach field. Although the elevated concentrations 

were within the range of those commonly measured in 

developed areas of the Long Island mainland, they exceeded 

the background (natural) levels observed in undeveloped park 

areas at FIIS by a factor of 10 or more. This considerable 

increase in TN concentrations associated with human 

activities may have important consequences for terrestrial and 

aquatic (freshwater and brackish) species adapted to the low 

nutrient levels generally within and downgradient from these 

undeveloped areas.

Elevated nutrient concentrations in groundwater 

underlying undeveloped areas indicate that recharge 

containing sewage, fertilizers, and (or) other human products 

has infiltrated the water table in upgradient areas. This 

contaminated recharge could also contain pathogens and other 

human-derived substances such as organic compounds that 

can move through the shallow aquifer and, in turn, threaten the 

ecological health of downgradient aquatic and coastal habitats.

Use of Long-Term Monitoring to Assess 

Water Resources

Natural groundwater discharge sustains ponds, wetlands, 

and salinity in back-barrier estuaries at FIIS. Knowledge of 

the quantity and quality of groundwater flow is an essential 

step in evaluating the availability and suitability of water for 

aquatic ecosystems (Schubert and others, 1997). Furthermore, 

long-term hydrologic monitoring is critical for understanding 

the effects of stresses on the groundwater system and on 

the freshwater and estuarine ecosystems that depend on it 

(McCobb and Weiskel, 2003).

The network of shallow observation wells installed at 

FIIS during this study is to be used in a long-term program of 

groundwater-level and water-quality-sample collection. The 

groundwater monitoring protocol established by the USGS 

for the Cape Cod National Seashore Long-Term Ecosystem 

Monitoring Program (McCobb and Weiskel, 2003) provides 

a guide for the implementation of the groundwater-level 

monitoring program. The National Field Manual for the 

Collection of Water-Quality Data (U.S. Geological Survey, 

variously dated) describes USGS-accepted methods for 

water-quality data collection that form the basis for the waterquality-sampling program.

Groundwater Levels

Groundwater levels are an intrinsic indicator of the 

quantity of water in an aquifer system and of the ability of 

water and chemicals to enter, transit, and exit this system. The 

minimum measurement frequency of groundwater levels is 

dependent on the factor(s) of interest. Yearly measurements 

may be adequate to describe inter-annual changes in aquifer 

conditions caused by climate variability; however, monthly 

measurements typically are required to characterize seasonal 

variations. Continuous recorders that collect measurements 

several times an hour can document groundwater fluctuations 

caused by the tides and by short-term changes in other 

factors such as natural recharge, wastewater disposal, and 

wave climate. The optimal network design would utilize a 

range of measurement intervals to address the monitoring 

objectives efficiently.

In this study, the directions of fresh groundwater 

movement throughout FIIS were controlled primarily by 

the combined effect of wave setup and tidal pumping, which 

together result in a water-table overheight condition along the 

ocean shore. Thus, monitoring intended to assess long-term 

changes in this movement would need to include groundwater 

measurements collected continuously to resolve short-term 

fluctuations caused by these factors. Moreover, because the 

shallow aquifer system contains a relatively minor amount of 

freshwater storage that can buffer these fluctuations, shortterm and seasonal (between summer and winter) water-table 

variations may equal or exceed annual and longer-term 

changes. As a result, monitoring resources would probably 

be used most beneficially if they were allocated primarily to 

a relatively small number of sites measured at comparatively 

high frequencies.

Operating at least two continuous water-level recorders 

at FIIS would help resolve short-term water-table fluctuations 

in the barrier-island groundwater flow system. One recorder 

could be situated in a developed area to evaluate the effects of 

urbanization on groundwater resources; the other could be in 

an area isolated from the effects of development. Water-table 
Summary 95

observation wells S125986 in Kismet and S125364 in 

the National Wilderness (fig. 4) essentially fulfill these 

requirements, and the water levels in these wells have been 

monitored continuously since October 2005 (figs. 15A and D).

Seasonal variations in the directions of groundwater flow 

could be characterized through the monthly measurement 

of water levels at a pair of water-table observation wells 

downgradient from each recorder well. These measurements 

would be made at wells near ocean and estuary discharge 

locations and together with the recorder well would form a 

water-table transect that could be used to identify changes in 

the patterns and rates of groundwater flow to coastal habitats 

caused by barrier-island and inlet dynamics, climate change, 

and sea-level rise. In the Kismet vicinity of recorder well 

S125986, observation wells S125371 and S125370 are closest 

to the marine surface waters along this transect, as are wells 

S125363 and S125365 in the National Wilderness vicinity of 

recorder well S125364 (fig. 4).

Long-term changes in the regional patterns and rates of 

groundwater flow caused by climatic fluctuations could be 

monitored by collecting yearly water-level measurements 

at three observation wells in each of the remaining local 

study areas. Monitoring could be conducted along a watertable transect between the ocean and estuary in Robbins 

Rest at wells S125994, S125982, and S125996 and along a 

comparable transect in Watch Hill at wells S125380, S125383, 

and S125387 (fig. 4). Overall, continuous, monthly, and 

annual water-level monitoring would encompass a network of 

12 wells that bisects Fire Island at four locations and quantifies 

the condition of the aquifer system along a continuum of 

time scales.

Water-Quality Samples

Water samples provide a discrete measure of the quality 

of water in an aquifer system and of the occurrence and 

distribution of chemical constituents transported by water 

through the system. Information on the presence of potential 

contamination within an aquifer system is determined by the 

suite of constituents chosen for collection and analysis. These 

constituents can be fixed during repeated sampling to assess 

changes in concentrations of selected constituents over time or 

can be varied to investigate multiple classes of contaminants 

during the same period. The approach taken in a sampling 

program depends on the chemical properties of a given 

constituent, the manner and rate at which this constituent 

may be introduced to the hydrologic system, and the potential 

interaction of these properties with other factors spatially and 

temporally within this system.

Nutrient concentrations sampled during this study 

were low in undeveloped park areas but elevated locally in 

areas downgradient from communities and from the NPS 

facility at Watch Hill. Therefore, monitoring focused on 

detecting long-term changes in these concentrations would 

need to include water samples collected from developed 

and undeveloped areas to recognize nutrient contamination 

from sewage, fertilizers, and (or) other human products. 

These point and nonpoint sources of contamination could 

include pathogens and other human-derived substances such 

as organic compounds. To begin developing a database of 

occurrences for these and other contaminants, the monitoring 

program would need to focus on a variety of constituent suites. 

Alternatively, collection of water samples from within the 

communities themselves to better understand the occurrence 

and distribution of elevated nutrient concentrations and other 

potential contaminants would be quite useful. As a result, the 

water-quality monitoring program should consider having 

a contingency for the future installation and sampling of 

additional wells in these areas.

To monitor long-term changes in nutrient concentrations 

within groundwater recharge and discharge, annual water 

samples would need to be collected and analyzed for this 

constituent grouping from at least two observation wells 

in each of the four local study areas. Sampling could be 

conducted in the interior of the island at the deep watertable observation well—from which nutrient concentrations 

representative of upgradient land use have previously been 

measured—and at the shallow well situated near the estuary 

discharge location in each area. These wells include S126753 

and S125370 in Kismet, S126000 and S125996 in Robins 

Rest, S125990 and S125387 in Watch Hill, and S125369 and 

S125365 in the National Wilderness (fig. 4). Collection of 

physical characteristics (for example, specific conductance) 

during sampling of the deep wells would provide additional 

information that could be used to indicate the movement of the 

freshwater-saltwater transition zone caused by climate change, 

sea-level rise, and changes in water use.

The annual collection and analysis of water samples 

for nutrient concentrations could also include monitoring 

for the occurrence of organic compounds. A different 

suite of constituents could be targeted each year during a 

3-year cycle. Year 1 could examine a range of pesticides 

using extremely low (parts per trillion) detection limits and 

volatile organic compounds. The second year could focus on 

assessing wastewater compounds and an alternate group of 

low-level (parts per trillion) pesticides. Year 3 could evaluate 

pharmaceuticals and other hormone-mimicking compounds. 

At the conclusion of the 3-year cycle, the need to monitor 

these and other types of emerging contaminants could be 

re-assessed and updated to improve the efficacy of future 

monitoring efforts.

Summary

Fire Island National Seashore (FIIS) occupies 42 km 

of the barrier island for which it is named that lies off the 

southern shore of western and central Suffolk County, Long 

Island, N.Y. Interspersed throughout FIIS are 17 pre-existing 

residential beach communities that in summer months greatly 

increase in population through the arrival of summer residents 
96 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

and vacationers. In addition, the National Park Service (NPS) 

has established several facilities on the island to accommodate 

visitors to the National Seashore.

Untreated wastewater from the numerous homes and 

businesses in the barrier-island communities generally is 

discharged directly into the shallow (water table) aquifer 

through use of private septic systems and cesspools; 

wastewater from the NPS facilities also enters this reservoir 

through leach fields and cesspools. Contaminants entering the 

groundwater system can pose a threat to coastal habitats, as 

they are transported by groundwater that discharges to coastal 

water bodies. The boundary of FIIS extends roughly 1 km into 

the back-barrier estuaries of Great South Bay, Narrow Bay, 

and Moriches Bay. Within this zone, there are extensive areas 

of seagrass habitat, shellfish and finfish resources, and intense 

recreational activity. To better understand and protect park 

natural resources, it is critical to determine the concentration 

and movement of nutrients and other contaminants being 

introduced into the shallow groundwater system and 

transported by freshwater that ultimately discharges to the 

surrounding coastal waters.

In 2004, the U.S. Geological Survey (USGS), in 

cooperation with the NPS, began a 3-year investigation to 

(1) collect groundwater levels and water-quality samples, 

(2) develop a three-dimensional model of the shallow aquifer 

system and adjacent marine surface waters, and (3) calculate 

nitrogen loads in simulated groundwater discharges from the 

aquifer to back-barrier estuaries and the ocean.

The shallow aquifer system was characterized from the 

results of exploratory drilling, geophysical surveying, waterlevel monitoring, and water-quality sampling conducted 

during this study. The investigation focused on four areas—

the communities of Kismet and Robbins Rest, the NPS 

Visitor Center at Watch Hill, and the undeveloped Otis Pike 

Fire Island High Dune Wilderness. Thirty-five observation 

wells were installed within FIIS to characterize subsurface 

hydrogeology and establish a water-table monitoring network 

in the four study areas.

Fire Island is underlain by a complex aquifer system 

consisting of unconsolidated glacial, lacustrine, deltaic, and 

marine deposits of clay, silt, sand, and gravel that range in age 

from Late Cretaceous to Holocene. These deposits overlie a 

southward-sloping basement complex of crystalline bedrock 

of Precambrian and (or) Paleozoic age. Fresh groundwater in 

the shallow aquifer system on Fire Island is contained within 

a hydraulically isolated freshwater flow system that extends 

through the Holocene and Pleistocene hydrogeologic units. 

This freshwater flow system is bounded laterally by marine 

surface waters and at depth by saline groundwater.

Three sets of water-level altitudes were collected at 

all wells to depict the distribution of hydraulic head and 

provide data for flow-model calibration. One well in each of 

four local study areas was instrumented with a continuous 

water-level recorder to document groundwater fluctuations 

caused by natural recharge, tides, and other factors such as 

wave height and wastewater disposal. The water levels in all 

wells generally increased with proximity to the ocean shore; 

the highest values are at wells nearest the ocean or those 

just inland. Water levels in inland wells also increased with 

barrier-island width. These results indicate the groundwater 

divide of the island is skewed strongly toward the ocean shore, 

apparently because of the combined effects of wave setup and 

tidal pumping and the resulting water-table overheight along 

this shore.

Water samples were collected from 35 observation wells 

to determine the spatial distribution of total nitrogen (TN) 

concentrations from wastewater and natural sources. A subset 

of these wells was resampled to assess the seasonal variability 

of TN concentrations. The highest TN concentrations—up to 

10 mg/L—were downgradient from the Watch Hill leach field; 

the lowest—less than 0.1 mg/L—were within the National 

Wilderness. The primary form of nitrogen in groundwater 

from wells near Kismet and Robbins Rest varied between 

ammonia and nitrate. Ammonia was the primary form in the 

vicinity of Watch Hill, whereas nitrate predominated within 

the National Wilderness.

The analysis of the shallow groundwater flow system at 

FIIS entailed the development of a three-dimensional model of 

the shallow aquifer system and adjacent marine surface waters 

to simulate groundwater flow patterns and rates. Development 

of the variable-density flow model included the construction 

of the model grid and the assignment of appropriate boundary 

conditions and aquifer properties. The model was calibrated 

to observed water levels. Nitrogen loads from the shallow 

aquifer system were calculated from TN concentrations and 

simulated groundwater discharges to back-barrier estuaries 

and the ocean.

The model simulates steady-state groundwater flow 

directions, velocities, and discharge rates under 2005 mean 

annual conditions in the shallow aquifer system. Simulated 

hydraulic heads in the unconfined and underlying confined 

flow systems generally increase with proximity to the ocean 

shore, where the modeled effects of wave setup and tidal 

pumping result in a water-table overheight. The simulated 

groundwater divide is skewed strongly toward the ocean 

shore in response to the modeled water-table overheight; 

it also extends virtually to the base of the shallow aquifer 

system. Where the modeled barrier island is sufficiently wide 

to yield elevated hydraulic heads, the groundwater divide 

originates within the freshwater flow system, which probably 

restricts the exchange of subsurface flow between back-barrier 

estuaries and the ocean. Where the barrier island is relatively 

narrow and heads are comparatively lower, however, the 

divide originates along the ocean shore, which presumably 

enhances downward movement of saline water that may 

subsequently migrate beneath the freshwater flow system to 

back-barrier estuaries.

Groundwater discharges to these water bodies were 

estimated through discharge-boundary and contributingarea analyses. Results of the discharge-boundary analysis 

are consistent with the conceptual and numerical model 

of barrier-island groundwater flow. Uncertainties in the 
Summary 97

discharge-boundary estimates, however, suggest they probably 

should be considered qualitative approximations of freshwater 

flow. Areas that contribute recharge to back-barrier estuaries 

and the ocean are the only sources of freshwater discharge 

from the shallow aquifer system and were estimated by 

flow-based particle tracking. These contributing areas were 

defined by the forward-tracking of individual particles placed 

at the center of each model cell receiving recharge. The divide 

between simulated areas that contribute recharge to backbarrier estuaries and the ocean is skewed strongly toward the 

ocean shore, where the modeled effects of wave setup and 

tidal pumping result in the water-table overheight. Simulated 

areas that do not contribute recharge to these water bodies are 

assumed to provide a source of freshwater that may exit as 

subsea underflow.

Groundwater budgets were developed on the basis of 

results of the contributing-area analysis for recharge areas 

of similar land use that contribute freshwater to back-barrier 

estuaries, the ocean, and subsea-discharge zones. These 

budgets indicate that total freshwater discharge from the 

shallow aquifer system is about 43,500 m3/d (79.8 percent) to 

back-barrier estuaries and about 10,200 m3/d (18.7 percent) to 

the ocean; about 836 m3/d (1.5 percent) may exit the system as 

subsea underflow. The total contribution of fresh groundwater 

to shoreline discharge zones amounts to about 53,700 m3/d 

(98.5 percent)—nearly the same as that resulting from the 

discharge-boundary analysis, which is about 52,800 m3/d 

(96.9 percent). In contrast to the generally consistent budget 

estimate for estuary zones from the discharge-boundary 

analysis, which is about 46,200 m3/d (84.8 percent), the 

ocean and subsea-discharge zone estimates from this analysis 

are only somewhat comparable, amounting to about 6,570 

(12.1 percent) and 2,760 m3/d (5.1 percent), respectively.

The contributing-area analysis also provided estimates 

of the traveltimes for individual particles from their point of 

recharge at the water table to their point of discharge at the 

shore. Results of particle-traveltime tracking indicate the 

median age of freshwater discharged to back-barrier estuaries 

and the ocean was 3.4 years, and the 95th-percentile age was 

20 years.

Simulated groundwater discharges were multiplied by 

TN concentrations in recharged water to calculate nitrogen 

loads to back-barrier estuaries and the ocean. The nitrogen 

concentrations in undeveloped and developed park areas 

on Fire Island are assumed to be 0.29 and 1.63 mg/L as 

N, respectively. These are median TN concentrations in 

groundwater from 16 shallow wells—7 in undeveloped and 9 

in developed park areas—sampled in October 2005. Nitrogen 

concentrations for the residential communities within FIIS 

are presumed to be comparable to median TN concentrations 

previously published for the southern, mainland coastline 

of Suffolk County. For communities consisting mainly of 

low- to medium-density residential development, the nitrogen 

concentration is estimated in this study to be generally 

equivalent to a median TN concentration of 1.19 mg/L. The 

concentration of TN in communities consisting primarily 

of high-density residential development is estimated to be 

generally equivalent to a median value of 2.69 mg/L. For 

the sewered community of Ocean Beach, the concentration 

is estimated to be generally equivalent to a median TN 

value of 0.86 mg/L. Total nitrogen concentrations from four 

wells downgradient from two communities sampled during 

this study show reasonable agreement with the previously 

published values; therefore, the values from the southern 

Suffolk County mainland probably can be considered 

reasonable approximations of the median TN concentrations 

within the Fire Island communities.

The TN concentrations and loads under 2005 mean 

annual conditions for areas that contribute fresh groundwater 

to back-barrier estuaries and the ocean were calculated for the 

principal land uses on Fire Island. Results indicate that the 

overall TN load from the shallow aquifer system to shoreline 

discharge zones is about 16,200 kg/yr (82.2 percent) to 

back-barrier estuaries and about 3,500 kg/yr (17.8 percent) 

to the ocean. Overall TN loads to estuary zones are about 

2,590 (16.0 percent) and 4,730 kg/yr (29.2 percent) from 

undeveloped and developed park areas, respectively; about 

63 (0.4 percent), 1,190 (7.3 percent), and 7,400 kg/yr 

(45.7 percent) from unsewered residential areas of low, 

medium, and high density, respectively; and about 203 kg/yr 

(1.3 percent) from the sewered residential area.

The overall TN load from Fire Island to marine surface 

waters amounts to about 19,700 kg/yr—roughly 6 percent of 

the annual TN load from shallow groundwater entering the 

South Shore Estuary Reserve (SSER) from the Suffolk County 

mainland, which is about 345,000 kg/yr. In contrast to the 

TN load from shallow groundwater for the entire 1,420-km2

SSER study area, which annually yields about 353 kg of 

nitrogen per square kilometer of watershed, the overall TN 

load from Fire Island produces nitrogen at an annual rate of 

about 890 kg/km2—more than double the loading rate from 

shallow groundwater to the SSER from the southern mainland 

of Nassau and Suffolk Counties. Similarly, the loading rate for 

Fire Island is nearly twice the annual rate of nitrogen loading 

from shallow groundwater entering Long Island Sound from 

the North Shore of Long Island, which is about 458 kg/km2.

The results of this study have identified several factors 

that may affect groundwater-resource management within the 

park. Although high-density residential land use occupies only 

about 8 percent of Fire Island, the unsewered areas contribute 

almost half of the overall TN load from groundwater to backbarrier estuaries. Because of the effects of wave setup and 

tidal pumping along the ocean shore, groundwater protection 

strategies need to consider most of the island as a potential 

source area to the wetland and adjacent estuarine habitats 

within the boundaries of FIIS. TN concentrations in the park 

downgradient from at least two communities—Kismet and 

Robbins Rest—and from the Watch Hill leach field exceeded 

the background (natural) levels observed in other undeveloped 

park areas at FIIS by a factor of 10 or more. This considerable 

increase in TN concentrations associated with human activities 

may have important consequences for species adapted to the 
98 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

low nutrient levels generally within and downgradient from 

these undeveloped areas. The elevated nutrient concentrations 

indicate that recharge containing sewage, fertilizers, and 

(or) other human products has infiltrated the water table 

in upgradient areas and could also contain pathogens and 

other human-derived substances such as organic compounds 

that could move through the shallow aquifer and in turn 

threaten the ecological health of downgradient aquatic and 

coastal habitats.

Natural groundwater discharge sustains ponds, wetlands, 

and salinity in back-barrier estuaries at FIIS; long-term 

hydrologic monitoring is critical for understanding the effects 

of stresses on the groundwater system and on the freshwater 

and estuarine ecosystems that depend on it. Continuous, 

monthly, and annual water-level monitoring to quantify the 

condition of the aquifer system along a continuum of time 

scales could be accomplished through a network of 12 wells 

that bisects Fire Island at four locations. Long-term changes 

in nutrient concentrations within groundwater recharge and 

discharge could be monitored by annually collecting water 

samples from at least two observation wells in each of the four 

local study areas and analyzing for this constituent grouping. 

Monitoring the occurrence of organic compounds could 

coincide with the annual sampling of nutrient concentrations 

at these same observation wells.

Acknowledgements

The author thanks the USGS Biological Resources 

Discipline and NPS for support and cooperation under the 

Research Natural Resources Preservation Program. The 

assistance of many individuals is greatly appreciated. In 

particular, valuable coordination and assistance during the 

investigation were provided by former FIIS Superintendent 

Michael Reynolds, FIIS Chief of Resources Management & 

Planning Michael Bilecki, and NPS Coastal Marine Ecologist 

Patricia Rafferty. Generous assistance with exploratory drilling 

and well installation was provided by Ronald Paulsen, Frank 

Basile, James DeLise, Frank Iannazzo, Ralph Milito, Dave 

Pertyshin, and Jonathan Wanlass of the SCDHS and William 

D. Capurso, Agnes P. Cwalina, Robert E. Pearsall, and Thomas 

F. Fitzpatrick of the USGS. Assistance with water-quality 

sampling was provided by Irene Abbene, Michele Abbene, 

Agnes P. Cwalina, Kurt T. Goetz, Thomas F. Fitzpatrick, 

and Erin K. O’Sullivan of the USGS. Frederick Stumm and 

Anthony Chu of the USGS assisted with borehole geophysical 

logging, and Lowell Abbadini and Agnes P. Cwalina of the 

USGS assisted with wellhead surveying. Helpful logistical 

support was provided by Steve Singler, Joe Heinrich, and 

Jim Dumphy of FIIS. Assistance with data compilation was 

provided by Diane Abell of FIIS; Henry Bokuniewicz of 

Stony Brook University (SBU); Mike Boufis, Paul Kuzman, 

and Jeff Altorfer of the SCWA; Eric Budres of the Suffolk 

County Department of Parks, Recreation & Conservation; 

Amy Cooper and Elizabeth Wintenberger of the New York 

State Office of Parks, Recreation and Historic Preservation; 

Robert Farmer, Andrew Rapiejko, and Robert Waters of the 

SCDHS; Carl Lind of the SCDP; Jack Monti, Jr. of the USGS; 

and Jay Tanski of New York Sea Grant. James Rumbaugh of 

Environmental Simulations, Inc., provided generous help with 

computer software support. Thoughtful comments on this 

manuscript were provided by Henry Bokuniewicz of SBU, 

Patricia Rafferty and four anonymous NPS reviewers, and 

Paul E. Misut and Donald A. Walter of the USGS.

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Portnoy, J.W., Nowicki, B.L., Roman, C.T., and Urish, D.W., 

1998, The discharge of nitrate-contaminated groundwater 

from developed shoreline to marsh-fringed estuary: Water 

Resources Research, v. 34, p. 3095–3104. 

Schubert, C.E., 1998, Areas contributing ground water to the 

Peconic Estuary and ground-water budgets for the North 

and South Forks and Shelter Island, eastern Suffolk County, 

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

Investigations Report 97–4136, 36 p., 1 pl.

Schubert, C.E., 2007, Analysis of the shallow ground-water 

flow system at Fire Island National Seashore, Suffolk 

County, New York [abs.]: in 6th Biennial Fire Island 

National Seashore Planning, Science and Research 

Conference, Abstracts of Presentations, May 9–10, 2007, 

p. 7.

Schubert, C.E., Buxton, H.T., and Monti, Jack, Jr., 1997, 

Ground-water resource evaluation on Long Island, New 

York, using flow models and a geographic information 

system: U.S. Geological Survey Fact Sheet FS–239–96, 

4 p.

Schwab, W.C., Thieler, E.R., Allen, J.R., Foster, D.S., 

Swift, B.A., and Denny, J.F., 2000, Influence of innercontinental shelf geologic framework on the evolution 

and behavior of the barrier-island system between Fire 

Island Inlet and Shinnecock Inlet, Long Island, New York: 

Journal of Coastal Research, v. 16, no. 2, p. 408–422.

Scorca, M.P., and Monti, Jack, Jr., 2001, Estimates of nitrogen 

loads entering Long Island Sound from ground water 

and streams on Long Island, New York, 1985–96: U.S. 

Geological Survey Water-Resources Investigations Report 

00–4196, 29 p.

Scorca, M.P., Reilly, T.E., and Franke, O.L., 1995, Selected 

hydrogeologic and water-quality data from Jones Beach 

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

Water-Resources Investigations Report 92–4171, 21 p.

Simmons, D.L., 1986, Geohydrology and ground-water 

quality on Shelter Island, Suffolk County, Long Island, 

New York, 1983–1984: U.S. Geological Survey WaterResources Investigation Report 85–4165, 39 p.

Smolensky, D.A., Buxton, H.T., and Shernoff, P.K., 1989, 

Hydrologic framework of Long Island, New York: U.S. 

Geological Survey Hydrologic Investigations Atlas 

HA–709, 3 sheets, scale 1:250,000.
References Cited 101

Steenhuis, T.S., Jackson, C.D., Kung, S.K.J., and Brutsaert, 

Wilfried, 1985, Measurement of groundwater recharge 

on eastern Long Island, New York, USA: Journal of 

Hydrology, v. 79, p. 145–169.

Todd, D.K., 1980, Groundwater hydrology (2d ed.): New 

York, John Wiley and Sons, 535 p.

Turner, I.L., Coates, B.P., and Acworth, R.I., 1996, The effects 

of tides and waves on water-table elevations in coastal 

zones: Hydrogeology Journal, v. 4, no. 2, p. 51–69.

U.S. Department of Commerce, 1969, Tide tables 1970 high 

and low water predictions—East Coast of North and South 

America including Greenland: Environmental Science 

Services Administration, Coast and Geodetic Survey, 290 p.

U.S. Geological Survey, variously dated, National field manual 

for the collection of water-quality data: U.S. Geological 

Survey Techniques of Water-Resources Investigations, 

book 9, chaps. A1–A9, available at http://pubs.water.usgs.

gov/twri9A. 

Valiela, Ivan, Foreman, Kenneth, LaMontagne, Michael, 

Hersh, Douglas, Costa, Joseph, Peckol, Paulette, DeMeoAndreson, Barbara, D’Avanzo, Charlene, Babione, Michele, 

Sham, Chi-Ho, Brawley, John, and Lajtha, Kate, 1992, 

Couplings of watersheds and coastal waters: sources 

and consequences of nutrient enrichment in Waquoit 

Bay, Massachusetts: Estuaries and Coasts, v. 15, no. 4, 

p. 443–457.

White, W.N., 1932, A method of estimating groundwater 

supplies based on discharge by plants and evaporation from 

soil: U.S. Geological Survey Professional Paper 659–A, 

p. 1–105.
102 

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Glossary 103

Glossary 

A

aquifer An underground layer of waterbearing permeable rock or unconsolidated 

materials (gravel, sand, silt, or clay) from 

which groundwater can be usefully extracted 

using a water well. 

aquifer system A heterogeneous body of 

permeable and less permeable material that 

acts as a water-yielding hydraulic unit of 

regional extent. 

B

barrier island A long, narrow, sandy coastal 

island, representing a broadened barrier 

beach that is above high tide and parallel to 

the shore and that commonly has dunes and 

marshy terranes extending landward from 

the beach. 

C

consumptive use Removal of water from 

the water environment by evaporation, 

transpiration, incorporation into products or 

crops, or consumption by man or livestock 

(Solley and others, 1983). 

contributing area The land area through 

or over which rainwater and other surface 

water soaks through the ground to 

replenish an aquifer, stream, river, lake, 

wetland, or estuary. This can also be called 

“recharge area.” 

D

discharge zone An area in which subsurface 

water, including both groundwater and vadose 

water, is discharged to the land surface, to 

bodies of surface water, or to the atmosphere.

E

evaporation The process, also called 

vaporization, by which a substance 

passes from the liquid or solid state to the 

vapor state. 

evapotranspiration Loss of water from 

a land area through transpiration of plants 

and evaporation from the soil and surfacewater bodies. 

F

freshwater Water containing less than 

1,000 milligrams per liter of dissolved solids; 

generally, water with more than 500 mg/L 

is undesirable for drinking and for many 

industrial uses (Solley and others, 1983). 

G

geophysical surveying The use of one or 

more geophysical techniques in geophysical

exploration, such as earth currents, electrical, 

gravity, or electromagnetic methods, heat 

flow, magnetics, radioactivity, and seismics. 

groundwater budget A numerical account 

of the recharge, discharge, and changes in 

storage of an aquifer, part of an aquifer, or 

system of aquifers. 

groundwater divide A ridge in the water 

table or other potentiometric surface from 

which the groundwater represented by that 

surface moves away in both directions. Water 

in other aquifers above or below, and even in 

the lower part of the same aquifer, may have a 

potentiometric surface lacking the ridge, and 

so may flow past the divide. 

groundwater level The elevation of the 

water table or another potentiometric surface 

at a particular place or in a particular area, 

as represented by the level of water in 

wells or other natural or artificial openings 

or depressions communicating with the 

saturated zone.

groundwater movement The movement, or 

flow, of water in the saturated zone, whether 

naturally or artificially induced. 

groundwater reservoir An underground 

layer of water-bearing permeable rock or 

unconsolidated materials (gravel, sand, silt, 

or clay) in which groundwater is stored. The 

upper surface of the groundwater reservoir 

is known as the water table. Changes in the 

water table represent a change in the storage 

of the reservoir. 
104 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

H

hydraulic conductivity The rate at which 

water can move through an aquifer or other 

permeable medium.

hydraulic gradient In an aquifer, the rate of 

change of total head per unit of distance of 

flow at a given point and in a given direction. 

hydraulic head The height of the free 

surface of a body of water above a given 

subsurface point. 

hydrogeology The science that deals with 

subsurface waters and with related geologic 

aspects of surface waters. 

I

infiltration The process by which water 

penetrates through the ground surface into and 

through the porous medium (evolved from 

consolidated rocks, sediments, glacial tills, 

and organic matter).

L

lacustrine Pertaining to, produced by, 

or formed in a lake or lakes; for example, 

“lacustrine sands” deposited on the bottom of 

a lake, or a “lacustrine terrace” formed along 

its margin. 

O

observation well A nonpumping well that 

is used to observe the elevation of the water 

table or other potentiometric surface. It may 

be used temporarily during a pumping test or 

as a long-term monitoring well.

R

recharge The processes involved in the 

addition of water to the saturated zone, 

naturally by precipitation or runoff, or 

artificially by spreading or injection; also, the 

amount of water added. 

recharge area An area beneath which 

water reaches the saturated zone following 

infiltration and percolation. Beneath it, 

downward components of hydraulic head 

exist and groundwater moves downward into 

deeper parts of the aquifer. 

S

shallow well A water well that taps the 

shallowest aquifer in the vicinity. The water is 

generally unconfined groundwater. 

surface runoff Water that flows across 

surfaces, rather than soaking into the ground, 

and eventually enters a stream, river, lake, 

wetland, or estuary. 

T

tidal pumping The action of tides on sloping 

sand beaches that causes the water-table 

height (elevation) to rise above mean sea 

level. 

W

water table The surface between the 

saturated zone and the unsaturated zone; that 

surface of a body of unconfined groundwater

at which the pressure is equal to that of 

the atmosphere.

water-table overheight The difference 

between the average water-table height 

(elevation) and the mean sea level, caused by 

the action of tides on sloping sandy beaches 

(tidal pumping) and wave setup. 

wave setdown A decrease in the mean 

water level in the region in which breakers 

form near the seashore, caused by the 

presence of a pressure field. 

wave setup An increase in the mean water 

level shoreward of the region in which 

breakers form at the seashore, caused by the 

onshore flux of momentum against the beach. 

References 

Solley, W.B., Chase, E.B., and Mann, W.B., 

IV, 1983, Estimated use of water in the 

United States in 1980: U.S. Geological 

Survey Circular 1001, 56 p.
Appendix 1. Groundwater levels collected at Fire Island National Seashore, October 2005 through October 2006.

[Water levels are in meters above National Geodetic Vertical Datum of 1929. Well locations are shown in figure 4. Well number is assigned by New York 

State Department of Environmental Conservation; prefix “S” denoting Suffolk County is omitted]

Local well 

number

October–November 2005 water level March 2006 water level October 2006 water level

Date Level Date Level Date Level

KISMET

125370 11/1/2005 0.52 3/29/2006 0.47 10/4/2006 0.48

125371 11/1/2005 0.98 3/29/2006 1.19 10/4/2006 1.10

125981 11/1/2005 1.05 3/29/2006 1.03 10/4/2006 1.07

125983 11/1/2005 0.92 3/29/2006 1.02 10/4/2006 1.03

125984 11/1/2005 0.90 3/29/2006 0.82 10/4/2006 0.90

125985 11/1/2005 0.92 3/29/2006 0.85 10/4/2006 0.91

125986 11/1/2005 1.00 3/29/2006 1.07 10/4/2006 1.06

125995 11/1/2005 0.98 3/29/2006 1.05 10/4/2006 1.03

126753 11/1/2005 1.00 3/29/2006 1.07 10/4/2006 1.06

ROBBINS REST

125982 11/1/2005 0.75 3/29/2006 1.02 10/4/2006 0.73

125994 11/1/2005 0.77 3/29/2006 1.11 10/4/2006 0.75

125996 11/1/2005 0.65 3/29/2006 0.73 10/4/2006 0.59

125997 11/1/2005 0.71 3/29/2006 0.83 10/4/2006 0.67

125998 11/1/2005 0.75 3/29/2006 0.96 10/4/2006 0.71

125999 11/1/2005 0.80 3/29/2006 0.95 10/4/2006 0.78

126000 11/1/2005 0.75 3/29/2006 1.02 10/4/2006 0.73

126001 11/1/2005 0.69 3/29/2006 0.93 10/4/2006 0.69

WATCH HILL

125380 10/31/2005 0.87 3/28/2006 1.03 10/3/2006 1.13

125381 10/31/2005 0.86 3/28/2006 0.86 10/3/2006 0.92

125382 10/31/2005 0.83 3/28/2006 0.83 10/3/2006 0.84

125383 10/31/2005 0.93 3/28/2006 0.97 10/3/2006 1.05

125384 10/31/2005 0.91 3/28/2006 0.91 10/3/2006 1.01

125385 10/31/2005 0.94 3/28/2006 0.91 10/3/2006 1.04

125386 10/31/2005 0.83 3/28/2006 0.94 10/3/2006 1.02

125387 10/31/2005 0.79 3/28/2006 0.78 10/3/2006 0.75

125990 10/31/2005 0.92 3/28/2006 0.97 10/3/2006 1.05

NATIONAL WILDERNESS

124399 10/31/2005 0.88 3/28/2006 0.90 10/3/2006 0.91

124400 10/31/2005 0.96 3/28/2006 0.82 10/3/2006 0.90

125363 10/31/2005 0.73 3/28/2006 0.95 10/3/2006 0.83

125364 10/31/2005 0.71 3/28/2006 0.87 10/3/2006 0.77

125365 10/31/2005 0.61 3/28/2006 0.66 10/3/2006 0.56

125366 10/31/2005 0.66 3/28/2006 0.78 10/3/2006 0.68

125367 10/31/2005 0.81 3/28/2006 0.96 10/3/2006 0.88

125368 10/31/2005 0.69 3/28/2006 0.93 10/3/2006 0.78

125369 10/31/2005 0.77 3/28/2006 0.87 10/3/2006 0.77

Appendix 1 105
106 Analysis of the Shallow Groundwater Flow System at Fire Island National Seashore, Suffolk County, New York

Appendix 2. Dissolved inorganic nitrogen (ammonia, nitrite, and nitrite plus nitrate) and phosphorus (orthophosphate) 

concentrations in groundwater collected at Fire Island National Seashore, October 2005 through September 2006.—Continued

[Well locations are shown in figure 4. Well number is assigned by New York State Department of Environmental Conservation; prefix “S” denoting 

Suffolk County is omitted. E, estimated; mg/L, milligrams per liter; N, nitrogen; P, phosphorus; <, less than]

Local well 

number Date of sample Ammonia, mg/L as N

Nitrite plus nitrate, 

mg/L as N

Nitrite, 

mg/L as N

Orthophosphate, 

mg/L as P

KISMET

125370 10/20/2005 1.29 E0.03 E0.006 0.57

3/1/2006 3.63 <0.06 0.021 1.6

8/30/2006 0.527 <0.06 0.008 0.469

125371 10/20/2005 E0.03 0.74 0.021 0.14

8/30/2006 0.023 0.1 0.014 0.18

125981 10/20/2005 0.07 <0.06 <0.008 <0.02

8/30/2006 0.091 <0.06 E0.002 0.02

125983 10/24/2005 0.29 <0.06 0.009 0.8

125984 10/24/2005 E0.03 <0.06 0.013 0.35

125985 10/24/2005 0.07 <0.06 <0.008 1.05

8/30/2006 E0.01 <0.06 E0.001 0.452

125986 10/24/2005 0.17 <0.06 0.009 0.08

8/30/2006 0.436 <0.06 0.005 0.107

125995 10/24/2005 0.24 <0.06 0.014 0.04

126753 10/24/2005 0.84 4.5 0.066 0.13

8/30/2006 0.039 1.89 0.024 0.131

ROBBINS REST

125982 10/31/2005 0.18 <0.06 E0.006 0.1

8/23/2006 0.116 <0.06 E0.002 0.088

125994 10/31/2005 <0.04 0.83 <0.008 0.04

8/23/2006 <0.01 1.19 <0.002 0.034

125996 10/31/2005 0.22 <0.06 <0.008 0.28

3/1/2006 <0.04 <0.06 <0.008 0.09

8/23/2006 0.016 <0.06 0.004 0.199

125997 10/31/2005 0.44 <0.06 0.016 0.19

8/23/2006 0.053 <0.06 0.003 0.151

125998 10/31/2005 E0.04 <0.06 <0.008 0.12

8/23/2006 0.019 <0.06 0.003 0.131

125999 10/31/2005 0.38 <0.06 E0.005 0.07

126000 10/31/2005 0.11 0.87 0.133 0.06

8/23/2006 0.094 <0.06 0.007 0.101

126001 10/31/2005 0.13 <0.06 <0.008 <0.02
Appendix 2 107

Appendix 2. Dissolved inorganic nitrogen (ammonia, nitrite, and nitrite plus nitrate) and phosphorus (orthophosphate) 

concentrations in groundwater collected at Fire Island National Seashore, October 2005 through September 2006.—Continued

[Well locations are shown in figure 4. Well number is assigned by New York State Department of Environmental Conservation; prefix “S” denoting 

Suffolk County is omitted. E, estimated; mg/L, milligrams per liter; N, nitrogen; P, phosphorus; <, less than]

Local well 

number Date of sample Ammonia, mg/L as N

Nitrite plus nitrate, 

mg/L as N

Nitrite, 

mg/L as N

Orthophosphate, 

mg/L as P

WATCH HILL

125380 10/17/2005 9.17 <0.06 E0.004 0.94

9/20/2006 10.1 <0.06 E0.002 1.06

125381 10/11/2005 0.74 E0.03 <0.008 0.14

125382 10/17/2005 0.64 <0.06 <0.008 0.24

125383 10/11/2005 4.66 <0.06 0.008 0.66

9/20/2006 5.27 <0.06 0.003 0.427

125384 10/11/2005 9.4 <0.06 <0.008 1.97

9/20/2006 5.1 <0.06 0.002 1.05

125385 10/17/2005 0.82 <0.06 <0.008 0.13

125386 10/11/2005 1.57 <0.06 <0.008 0.29

9/20/2006 1.21 <0.06 0.003 0.145

9/20/2006 1.23 <0.06 0.003 0.134

125387 10/17/2005 0.08 0.08 0.034 0.05

3/1/2006 E0.04 <0.06 0.015 0.15

9/20/2006 0.139 E0.05 0.028 0.085

125990 10/11/2005 7.17 <0.06 <0.008 1.64

9/20/2006 6.6 <0.06 0.003 2.51

NATIONAL WILDERNESS

124399 10/13/2005 0.12 <0.06 <0.008 0.09

124400 10/18/2005 <0.04 0.67 E0.005 <0.04

125363 10/18/2005 E0.04 0.99 <0.008 <0.02

8/17/2006 E0.009 0.25 <0.002 0.015

125364 10/18/2005 <0.04 0.08 <0.008 0.07

8/17/2006 0.012 0.17 <0.002 0.081

125365 10/19/2005 <0.04 0.08 <0.008 0.03

3/1/2006 <0.04 0.12 <0.008 <0.02

8/17/2006 0.034 0.51 0.004 0.058

125366 10/19/2005 <0.04 0.47 <0.008 0.07

8/17/2006 E0.009 0.3 <0.002 0.05

125367 10/18/2005 E0.03 0.26 <0.008 <0.02

8/17/2006 E0.009 E0.06 E0.001 E0.006

125368 10/19/2005 E0.03 0.25 <0.008 <0.02

8/17/2006 0.023 0.08 <0.002 0.008

125369 10/18/2005 <0.04 0.82 <0.008 0.02

8/17/2006 0.011 0.33 E0.001 0.089
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