1

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2010–1081

April 2010

Cooperator Note

Fact Sheet Title of Typical Length for USGS

Printed on recycled paper

• Simulations of groundwater discharge from the shallow 

aquifer indicate that nearly 80 percent of the total discharge 

enters the back-barrier estuaries; the rest discharges to the 

ocean or below the seabed as subsea outflow.

• The travel time of groundwater through the shallow aquifer 

to discharge zones varies with distance from the recharge 

area. Particle-tracking analysis indicates that the mean travel 

time is 3.4 years, and virtually all groundwater is younger 

than 20 years. 

• Further water-level and water-quality monitoring at 

selected locations would allow development of management 

strategies to protect back-barrier ecosystems from the effects 

of wastewater discharging from the aquifer.

Nitrogen Loads in Groundwater Entering Back Bays and Ocean 

from Fire Island National Seashore, Long Island, New York

• About 2.2 million people visit Fire Island each year. The 

arrival of summer residents and vacationers increases the 

population 50-fold. 

• Wastewater from most septic systems discharges directly 

into the shallow (water-table) aquifer. The associated 

nutrients, pathogens, and organic compounds can 

eventually seep into back-barrier estuaries and threaten their 

ecological health. 

• Elevated concentrations of nutrients in groundwater that 

discharges to surface waters can lead to increased production 

of phytoplankton and macroalgae; these, in turn, can cause 

oxygen depletion, declines in estuarine fish and shellfish 

communities, and loss of submerged seagrass habitat.

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. Groundwater discharge shown in the foreground can 

transport freshwater and contaminants from the island to 

the adjacent back bays and ocean. (Photograph from Paula 

Valentine, National Park Service)

Prepared in cooperation with the National Park Service
2

INTRODUCTION

Fire Island is a barrier island that lies south of central 

Long Island, N.Y. It is about 60 km (37 mi) long and 0.5 km 

(1/4 mi) wide and is bounded by the Great South Bay, Narrow 

Bay, and Moriches Bay estuaries to the north; by the Atlantic 

Ocean to the south; by Fire Island Inlet to the west; and by 

Moriches Inlet to the east (fig. 1). Fire Island National Seashore 

(FIIS) encompasses a 42-km (26-mi) length of Fire Island that 

is bordered by Robert Moses State Park to the west and Smith 

Point County Park to the east (fig. 2). Interspersed throughout 

FIIS are 17 residential beach communities that together contain 

about 4,100 homes.

The barrier island’s summer population increases 50-fold 

through the arrival of summer residents and vacationers. The 

National Park Service (NPS) has established several facilities 

on the island to accommodate visitors to FIIS. About 2.2 million 

people visit at least one of the 17 communities and (or) Smith 

Point County Park, the waterways surrounding Fire Island, or a 

FIIS facility annually (National Park Service, 2007). Combined 

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

100,000 (National Park Service, 2002).

Most homes and businesses in the 17 barrier-island 

communities discharge untreated wastewater directly to the 

shallow (water-table) aquifer through private septic systems 

and cesspools; the NPS facilities discharge wastewater to this 

aquifer through leach fields and cesspools. (The community 

of Ocean Beach (fig. 2) has a treatment plant that discharges 

to tidewater.) Contaminants in sewage entering the shallow 

groundwater move through the flow system and are ultimately 

discharged to adjacent marine surface waters, where they 

can pose a threat to coastal habitats. A contaminant of major 

concern is nitrogen, which is derived from fertilizers and 

human waste. The continuous inflow of nitrogen to surfacewater bodies can lead to increased production of phytoplankton 

and macroalgae, which in turn can cause oxygen depletion, 

decreases in size of estuarine fish and shellfish communities, 

and loss of submerged seagrass habitat through light limitation 

(Valiela and others, 1992).

The FIIS boundary extends roughly 1.2 km (0.8 mi) into 

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

Moriches Bay (fig. 1). Within this estuarine zone are extensive 

areas of seagrass, shellfish, and finfish habitat, as well as 

intense recreational activity (Bokuniewicz and others, 1993). 

Management strategies for protection of these habitats require 

data on (1) concentrations and movement of nutrients and other 

human-derived contaminants that enter the groundwater system 

from on-site septic systems, and (2) aquifer characteristics 

and groundwater flow patterns. These data can then be used in 

three-dimensional flow models of the shallow aquifer system to 

predict the rates of groundwater discharge to the marine surface 

waters that bound Fire Island and the concentrations of nitrogen 

entering these water bodies from the aquifer’s discharge zones.

Figure 1. Map shows the location of the Fire Island National Seashore study area in Suffolk County, N.Y.

#

#

#

#

#

#

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.

ISLIP LONG ISLAND MACARTHUR AIRPORT PATCHOGUE 2 N

73°15' 73° 72°45'

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

B A B Y L O N

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 2.

0 21 KILOMETERS

0 21 MILES

EXPLANATION

Township Boundary

 Trace of Hydrogeologic Section

X Precipitation-Measurement Station

# Tidal Benchmark

A'A
3

Figure 2. Maps show the type of land use and locations of the four local study areas—Kismet, Robbins Rest, Watch Hill, and 

National Wilderness—on Fire Island, Suffolk County, N.Y. (Locations of the areas shown in panels A, B, C, and D are shown in 

figure 1.)

73°02' 73° 72°58' 72°56' 72°54'

40°42'

40°41'

72°52' 72°50' 72°48' 72°46'

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 residential

Medium-density residential

High-density residential

Commercial

Industrial

Institutional

Recreation & open space

Agriculture

Vacant

Transportation

Utilities

Waste handling & management

Surface waters

Local Study Area--Indicates location of area

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

National

Wilderness

Watch Hill

(in panel C) shown in figure 5

73°18' 73°16' 73°14' 73°12'

40°37'

40°36'

73°10' 73°08' 73°06' 73°04'

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 KILOMETER

0 0.5 1 MILE

Robbins Rest

R o b e r t M o s e s S t a t e P a r k

Ocean Beach
4

A Fire Island land use—Percent of total area

Low ≤ 1

0.2%

Medium 2−4

3.8%

High ≥ 5 { 8.5%

B Fire Island residential density—

Dwellings per acre and percent of total area

Residential

12.5%

Other

3.4%

Vacant

3.1%

Recreation and

open space

81% 

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

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

(1) measure groundwater levels within four local study areas 

at FIIS, (2) collect groundwater samples from these areas for 

nutrient (nitrogen) analysis, (3) develop a three-dimensional 

model of the hydrologic system and adjacent saltwater bodies 

for groundwater-flow delineation and particle tracking, and 

(4) apply the results of groundwater-discharge simulations 

to calculate the annual nitrogen loads in these discharges, 

particularly those entering Great South Bay, which together 

with the other back bays receives an estimated 80 percent of the 

total groundwater discharge from Fire Island. 

• The four areas on which the investigation focused were the 

communities of Kismet and Robbins Rest, the NPS Visitor 

Center at Watch Hill, and the undeveloped Otis Pike Fire Island 

High Dune Wilderness (shown in panels A, B, C, and D in 

fig. 2); these were selected to represent the major land uses and 

corresponding nitrogen concentrations at FIIS. 

• Thirty-five observation wells were installed within FIIS to 

document the stratigraphy, establish a water-table-monitoring 

network, and collect water-quality data.

• A three-dimensional flow model was constructed to simulate 

groundwater discharge and to calculate the annual nitrogen 

loads in these discharges, particularly those entering backbarrier estuaries, from freshwater discharge zones. 

• The network of observation wells provides a foundation 

for long-term monitoring of groundwater levels and chemical 

quality throughout the National Seashore, and the model can be 

used in subsequent studies to simulate the transport of nitrogen 

and other contaminants through the aquifer system.

This paper describes the hydrogeologic system that 

underlies FIIS and presents results of the model simulations. 

It also (1) depicts the patterns of groundwater flow within the 

freshwater aquifer and adjacent saline subsurface, (2) presents 

the yearly amounts of freshwater discharged to the back 

bays and the ocean from the shallow aquifer, (3) presents a 

discussion on the quality of water in the freshwater aquifer and 

presents the annual nitrogen loads discharging to coastal waters, 

(4) provides the estimated travel time of water from recharge 

areas to these discharge zones, and (5) presents a discussion of 

model limitations and considerations for future groundwater 

monitoring. All information presented herein is given in greater 

detail in Schubert (2010).

Land Use

About 3 percent of the land area on Fire Island is vacant 

(Carl Lind, Suffolk County Department of Planning, written 

commun., 2006). About 81 percent is recreational and open 

space (fig. 3A), including a part of the island that extends from 

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

Visitor Center at Watch Hill (fig. 2) that Congress designated 

as a National Wilderness Area in 1980. About 12 percent of 

the land area is residential (fig. 3B), although most residences 

are occupied only during summer months (National Park 

Service, 2003). About 8 percent of the land area is classified 

as high-density 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 3 percent of the 

remaining land area is categorized as commercial, industrial, 

institutional, transportation, utilities, or waste handling and 

waste management.

Figure 3. Pie charts show (A) the percent of land used for 

recreation and open space, residential and other development, and 

vacant areas and (B) residential density, in number of dwellings 

per acre and the percent of total land use, in the Fire Island study 

area, Suffolk, County, N.Y. Photographs: (top) vegetation in Otis Pike 

Fire Island High Dune Wilderness (courtesy of Stevan/wikipedia.

org); (bottom) residential density in the Kismet area of Fire Island. 

(Photograph courtesy of the National Park Service.)
5

Water Supply 

Fire Island is served by 10 public water-supply systems. 

All public water supply for the barrier-island communities 

and for all NPS, State, and County park facilities is pumped 

from the underlying aquifer system; a small fraction of this 

pumpage occurs on the Long Island mainland. Virtually all 

water is withdrawn from the confined Magothy aquifer because 

the overlying upper glacial aquifer, which historically was 

used to supply summer residences, now contains elevated 

concentrations of chloride, nitrate, and coliform bacteria 

from septic waste-disposal systems and has largely been 

abandoned (Leggette, Brashears & Graham, Inc., 1996). 

The Magothy aquifer is protected from contamination in the 

surficial freshwater lens by saline water and a confining unit 

(Gardiners Clay) .

Most public-supply water on Fire Island is ultimately 

returned to the shallow aquifer, primarily through onsite 

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

from the water-distribution system; the remainder is lost as 

consumptive use (such as for irrigation). In unsewered areas of 

Long Island, about 85 percent of total public-supply pumpage 

is returned to the water table—about 70 percent through 

wastewater-disposal systems and 15 percent through leaky 

water mains and sewers (Franke and McClymonds, 1972). In 

sewered areas, wastewater generally is routed to a sewagetreatment plant, from which the effluent discharges to coastal 

water bodies.

HYDROGEOLOGIC SETTING

The hydrogeologic system was characterized from the 

results of exploratory drilling, geophysical surveys, water-level 

monitoring, and water sampling and analysis.

Aquifer System

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 (fig. 4). These deposits 

overlie a southward sloping basement complex of crystalline 

bedrock of Precambrian and (or) Paleozoic age. Fresh 

groundwater in the shallow (water-table) aquifer is contained 

within a hydraulically isolated flow system within the Holocene 

and Pleistocene units and is not connected with the underlying 

confined (Late Cretaceous) Magothy aquifer from which the 

island’s freshwater supply is obtained. The shallow aquifer 

extends laterally beneath adjacent marine water bodies and is 

bounded below by the Gardiners Clay confining unit. The watertable flow system is the unit of concern in this study because it 

receives the wastewater discharged from septic systems, which 

contains a variety of contaminants. The underlying Cretaceous 

units were not considered in this investigation. A generalized 

vertical section depicting the geometry of major hydrogeologic 

units underlying Fire Island is presented in figure 4.

Groundwater Levels, Quality, and Discharge

Water levels were measured on October 31 and November 

1, 2005, to depict the distribution of hydraulic head on Fire 

Island for flow-model calibration (see figures 5 and 6 for 

examples from Watch Hill study area). The measurements 

were made within 1 hour of the predicted time of low tide to 

minimize local differences resulting from tidal fluctuations. In 

addition, one well in each of the four local study areas (shown 

in panels A–D in fig. 2) was instrumented with a continuous 

water-level recorder to document the groundwater fluctuations 

caused by tides, recharge from precipitation, and other factors 

such as wave height and wastewater disposal.

Water levels were generally found to increase with 

proximity to the ocean and were highest at wells nearest 

the ocean or just inland from it. Water levels at inland wells 

increased with increasing barrier-island width because the 

widest areas receive the greatest amount of recharge. These 

results indicate that the island’s groundwater divide is skewed 

strongly toward the south (ocean) shore, probably in response 

to the combined effects of waves breaking (wave setup and 

setdown) and asymmetrical tidal infiltration and seepage (tidal 

pumping), both of which can elevate the water table along this 

shore to create a condition commonly referred to as water-table 

overheight (Nielsen, 1990; Hanslow and Nielsen, 1993; Turner 

and others, 1996; Nielsen, 1999). 

Water samples were collected from the 35 observation 

wells to define the spatial distribution of total nitrogen (TN) 

concentrations from wastewater, fertilizer, and (or) other 

human-derived sources. A subset of these wells was resampled 

during March, August, and September of 2006 to assess the 

seasonal variability of TN concentrations. The highest TN 

concentrations (up to 10 mg/L) were found downgradient 

from the Watch Hill leach field, which receives sewage from 

the park’s visitor center; the lowest (less than 0.1 mg/L) were 

within the National Wilderness (fig. 2). The primary form 

of nitrogen in groundwater in the Watch Hill vicinity was 

ammonia (fig. 5), whereas the primary form within the National 

Wilderness was nitrate. Both forms were common near Kismet 

and Robbins Rest.

Water in the freshwater flow system either discharges to 

shore and subsea-discharge areas or is lost through 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.

MODEL SIMULATIONS

A three-dimensional, variable-density flow model of the 

shallow-aquifer system and adjacent marine surface waters was 

developed to simulate the rates and directions of groundwater 

flow. Model development included the design of the model grid 

and the assignment of appropriate boundary conditions and 

aquifer properties. An example of vertical model discretization 

along column 569 (near location of section D–D ́ shown in 

fig. 5) is presented in figure 7. 

The model was calibrated to water levels measured in 

2005 and pumpage records provided by several local agencies. 

The variable-density groundwater flow model SEAWAT-2000 

(Langevin and others, 2003) was used in the analysis. The 

simulation provided estimates of groundwater discharges 

from the shallow aquifer system to back-barrier estuaries and 

the ocean; these discharge values were then multiplied by 

representative nitrogen concentrations to obtain the annual 

nitrogen loads in these discharges. 
6

Figure 4. Block diagrams show that Fire Island 

is underlain by an aquifer system that consists of 

(A) unconsolidated deposits of clay, silt, sand, and 

gravel along the southern shore of Long Island, and 

(B) the directions of fresh and brackish groundwater 

flows beneath Fire Island, N.Y. are toward the Atlantic 

Ocean and Great South Bay (General location of section 

A–A’ is shown in figure 1.)

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 4B

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
7

Figure 5. Map shows the location of cross section D–D’ drawn across Watch Hill, and observation wells drilled to 

document the geology, measure groundwater levels, and collect water-quality data in Suffolk County, N.Y.

!

!

!

!

!

!

!

!

! 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 6.

! 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

Prefix "S" denoting Suffolk County is omitted.
8

EXPLANATION

METERS 

-37.5

-30.0

-22.5

-15.0

-7.5

 0

7.5

125990, 125383

125382

125387

125380

D D'

SOUTHEAST NORTHWEST ATLANTIC OCEAN 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 5 --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)

Figure 6. Cross section of the Watch Hill study area (figure 5) shows the arrangement of layers of surficial, glacial, 

and nonglacial deposits beneath the land surface, where freshwater meets saltwater, the water table, and groundwater 

levels measured in observation wells from October 31 to November 1, 2005 on Fire Island, Suffolk County, N.Y. 
9

Figure 7. Cross section shows an example of how the shallow-aquifer system beneath the Fire Island study 

area can be recreated in a groundwater-flow model grid. Models can be used to represent geologic sediment 

layers and estimate rates and directions of groundwater flow through those layers to simulate groundwater flow.

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

 

1

3

6

9

12

 66 06 45 84 24 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

Column 569
10

Model Grid

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. The model was designed 

to simulate the steady-state groundwater flow directions, 

velocities, and discharge rates in the shallow flow system under 

2005 mean annual hydrologic conditions.

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 vertical layers. The grid has a 

uniform horizontal discretization of 50 m (164 ft) along rows 

and columns and is oriented 20.16 degrees counterclockwise 

from true north. The model represents the full thickness of 

the shallow aquifer system from the water table to the base of 

the upper glacial aquifer (fig. 4); layer thicknesses vary from 

cell to cell and generally were based on the geometry of the 

hydrogeologic units. The grid contains a total of 870,912 cells, 

of which 367,752 are active, and 503,160 are inactive. 

The model represents each of the Late Pleistocene and 

Holocene units. Layer 1 represents three surficial Holocene 

units (upper outwash, peat deposits, and barrier and shelf sands) 

in onshore areas and also represents surface-water bodies 

(figs. 6 and 7). 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. Layers 

3, 4, and 5 represent lagoonal deposits except along the Long 

Island mainland, where these layers represent the upper outwash 

of the upper glacial aquifer. Layers 6, 7, and 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. Layers 10 and 11 

represent the lower outwash. Layer 12 represents the finegrained, basal part of this unit.

Model Boundary Conditions

The boundary conditions specified in the flow model 

coincide with the hydraulic boundaries of the freshwater 

flow system under 2005 conditions. These boundaries were 

assumed to be constant through time and were used to simulate 

groundwater flow under steady-state, average annual conditions. 

A no-flow condition was specified at the lateral boundary 

between active and inactive model zones in all layers. This 

boundary condition was also applied at the bottom of the 

lowermost model layer (layer 12; fig. 7), which represents 

the base of the upper glacial aquifer. Poorly permeable units 

beneath this aquifer—the Gardiners Clay and Monmouth 

greensand (fig. 4)—hydraulically isolate the upper glacial 

aquifer from the underlying Magothy aquifer (Scorca and 

others, 1995).

Constant-head boundaries were applied in estuaries and 

the ocean to represent the flow of water from the shallow 

aquifer to marine surface waters (layer 1). Local measured or 

calculated differences in surface-water levels were extrapolated 

throughout the back bays and ocean to depict local mean sea 

level (MSL) to which the total water-table overheight, resulting 

from wave setup and setdown and tidal pumping, was added. 

Constant-concentration boundaries: Long-term salinitymeasurement records (R.M. Waters, Suffolk County Department 

of Health Services, written commun., 2006; National 

Oceanographic Data Center, 2006) were extrapolated to provide 

a time-averaged depiction of salinity throughout the backbarrier estuaries and the ocean.

A specified-flux boundary was applied at the water table 

(layer 1) to represent recharge from precipitation and the return 

flow of public-supply water. Inland surface-water bodies (layer 

1) were simulated as areas of high hydraulic conductivity 

(15,000 m/d or 50,000 ft/d) and porosity (1). These boundaries 

were simulated as described below.

• Recharge from precipitation was simulated by five zones that 

represented increasing rates of infiltration to the water table. 

The zone boundaries were delineated, and the values were 

assigned according to type of substrate, type of land cover, 

and land-surface elevation. Public-supply withdrawal was not 

simulated because virtually all water-supply pumpage is from 

the deeper (unmodeled) Magothy aquifer. 

• Recharge from the return flow of public-supply water was 

simulated by a series of rates that were assigned according 

to the water-supply system and the return-flow component 

(distribution or waste). The rates of public-supply return flow 

within the unsewered areas were calculated as 85 percent 

of the total 2005 pumpage for these areas, divided by the 

number of square kilometers (square miles) in each service 

area. The rate of public-supply return flow in the sewered 

community of Ocean Beach was calculated as 15 percent 

of the total 2005 pumpage for this system, divided by the 

number of square kilometers in its service area. Rates of 

public-supply return flow for the distribution systems and 

wastewater systems associated with Smith Point County Park 

and the NPS visitor centers at Sailors Haven, Watch Hill, 

and the National Wilderness generally were calculated as 

15 percent for the distribution systems and 70 percent for the 

wastewater systems.

Model Calibration

The flow model was calibrated through a quasi steady-state 

simulation of 2005 mean annual conditions. Simulated water 

levels were then compared with water-level measurements 

made at the 35 observation wells on October 31 and 

November 1, 2005. 

Simulated salinities (salt concentrations) were compared 

qualitatively with the positions of the freshwater-saltwater 

interface as estimated from salinity values measured at 21 wells 

during the study.

Horizontal and vertical hydraulic-conductivity values for 

the simulated hydrogeologic units were adjusted through trial 

and error until the differences between measured and simulated 

groundwater levels were minimized. The model was considered 

calibrated when the difference between measured and simulated 

values at wells representing the principal hydrogeologic units in 

each of the four study areas (fig. 2) were at their smallest.

Groundwater Levels and Divide

Simulated hydraulic heads in the unconfined flow system 

(and underlying confined flow systems) generally increase 
11

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 throughout 

Fire Island is strongly skewed toward the ocean shore in 

response to this water-table overheight; it also extends nearly 

to the base of the shallow aquifer system where the island is 

wide and thus probably restricts the exchange of subsurface 

flow between back-barrier estuaries and the ocean. Where the 

island is relatively narrow, however, the heads are lower, and 

the divide originates along the ocean shore. This presumably 

enhances downward movement of saline water such that it may 

subsequently migrate northward beneath the freshwater flow 

system to back-barrier estuaries. An example of simulated water 

levels, flow vectors, and extent of freshwater within the shallow 

aquifer system along column 569 (near location of section D–D ́ 

shown in fig. 5) is presented in figure 8.

Figure 8. Model grid shows simulated 2005 mean annual water levels, the location of the groundwater divide beneath the 

barrier island near the ocean shore toward the southeast (indicated by opposing flow arrows near row 54), and the direction of 

ground-water flow beneath most of the Fire Island study area toward the northwest and the back-barrier estuaries.

Simulated Water Level--Contour

 interval 0.15 meters. Datum is NGVD 29.

Simulated Flow Vector

Simulated salinity, in parts per thousand

Column 569

ROW METERS 

-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

 

1

3

6

9

12

 66 06 45 84 24 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

0.30
12

Discharge-Boundary Flows

Groundwater discharges to marine surface waters were 

estimated through analyses of discharge-boundary flows 

and areas that contribute recharge. Results of the dischargeboundary analyses were consistent with earlier conceptual and 

numerical model results of barrier-island groundwater flow. 

Uncertainties in the discharge-boundary estimates indicate, 

however, that the discharge values for the respective rechargecontributing areas probably should be considered qualitative 

(rather than quantitative) approximations of freshwater flow.

Contributing-Area Analysis

All freshwater discharge from the shallow aquifer system 

to back-barrier estuaries and the ocean originates at the water 

table in recharge areas. The areas that are sources of recharge 

entering these water bodies were delineated through the forward 

tracking of an individual particle placed at the center of each 

model cell that received recharge. The particle-tracking postprocessing package MODPATH (Pollock, 1994) was used in the 

analysis. Simulated contributing areas to marine surface waters 

under 2005 mean annual conditions are shown in figure 9. 

The simulated divide between areas that contribute recharge 

to back-barrier estuaries and those that contribute recharge to 

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. Recharge from areas that do not 

discharge to these water bodies was assumed to exit the system 

as subsea underflow.

Groundwater Budgets

Groundwater budgets were developed for recharge areas 

that contribute freshwater to back-barrier estuaries, the ocean, 

and subsea-discharge zones. These budgets were developed 

from results of the contributing-area analysis and indicate 

that the total freshwater discharge to back-barrier estuaries is 

about 43,500 m3

/d (1,540,000 ft3/d) or 79.8 percent of total 

freshwater discharge from the shallow-aquifer system. In 

contrast, discharge to the ocean is only about 25 percent of that 

amount—10,200 m3

/d (360,000 ft3/d) or 18.7 percent of total 

freshwater discharge from the shallow aquifer system. Another 

1.5 percent (836 m3

/d or 29,500 ft3/d) may exit the system as 

subsea underflow. 

The contributing-area analysis also provided estimates 

of the travel times for individual particles from their point 

of entry at the water table to their point of discharge at the 

shore. Results of the particle tracking indicate the median age 

of groundwater (travel time from the point of recharge to the 

point of discharge at back-barrier estuaries and the ocean) was 

3.4 years, depending on the distance between recharge point and 

the shore. Virtually all groundwater (95th percentile) was found 

to be younger than 20 years.

Total Nitrogen Concentrations and Loads

Freshwater entering the water table as precipitation that has 

infiltrated through the soil, and wastewater that enters the water 

table from on-site septic systems in unsewered areas, contain 

nitrogen in concentrations that vary with the location and the 

rate at which it is introduced, and also with the flow paths and 

travel time of water in which it moves. Nitrogen concentrations 

can be altered through physical processes such as dilution 

and dispersion; the form of nitrogen entering groundwater 

also can be altered through biogeochemical processes such as 

denitrification, whereby it is ultimately converted to nitrogen 

gas. The form of nitrogen that predominates in a given system, 

whether as 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. Ammonia is a reduced form of 

nitrate that strongly sorbs to aquifer sediments (Hem, 1985). 

Total nitrogen (TN) was computed in this study as the sum of 

the concentrations of ammonia and nitrite plus nitrate.

The TN loads entering back-barrier estuaries and the 

ocean as shallow groundwater discharge were calculated as 

the model-generated groundwater discharges multiplied by 

the representative TN concentrations for the principal land 

uses on Fire Island. The TN concentrations in undeveloped 

and developed park areas were assumed to be 0.29 and 

1.63 mg/L as N, respectively. These values were the median 

TN concentrations in water samples collected from 16 shallow 

wells in October 2005 (7 in undeveloped park areas and 9 in 

developed park areas). 

TN concentrations for the residential communities within 

FIIS were assumed to be comparable to published values for 

similar communities on the south shore of Suffolk County 

(Monti and Scorca, 2003). The nitrogen concentration for 

communities with mostly low- to medium-density residential 

development was estimated in this study to be generally 

equivalent to a median TN concentration of 1.19 mg/L, and that 

for communities consisting primarily of high-density residential 

development was 2.69 mg/L. The nitrogen concentration for 

the sewered community of Ocean Beach was estimated to 

be generally equivalent to a median TN value of 0.86 mg/L. 

The TN concentrations measured at four wells downgradient 

from the communities of Kismet and Robbins Rest showed 

reasonable agreement with the median TN concentrations 

calculated by Monti and Scorca (2003). Therefore, the values 

from that study probably can be considered reasonable 

approximations of the median TN concentrations for Fire Island 

communities, as summarized in the table:

 Median TN concentration 

estimated for south shore of 

Suffolk County mainland 

(Monti and Scorca, 2003)

Corresponding 

Fire Island 

community

1.19 mg/L for unsewered area 

east of Carmans River 

Unsewered low and medium 

density residential 

development

2.69 mg/L for unsewered area 

west of Carmans River 

Unsewered high density 

residential development

0.86 mg/L for Southwest Sewer 

District of Suffolk County 

Sewered community of 

Ocean Beach

Communities in the mainland areas, where residency 

is mostly year-round, probably generate a larger annual 

wastewater TN load than areas occupied mainly during summer. 

Nevertheless, the wastewater TN load produced by the intense 

summer visitation of Fire Island could exceed the summer load 

produced by a permanent population on the mainland. 
13

Figure 9. Maps of simulated recharge areas (numbered in white) to marine surface waters under 2005 mean annual 

conditions show that most freshwater from the shallow aquifer system discharges toward the back-barrier estuaries 

(recharge areas in blue) in the Fire Island study area, Suffolk County, N.Y. (Locations of areas are shown in panels A, B, C,

and D in figure 2.)

2

1

6 11 5 12 4 7 3

14 15 16 13 17

9

19

10 8

18

73°18' 73°16' 73°14' 73°12'

40°37'

40°36'

31

25 26

23

32 29

24

22

36 28 34 27 30 33

16

35

17 15 19 21

20 37

18

73°10' 73°08' 73°06' 73°04'

40°39'

A

B

0 0.5 1 KILOMETER

0 0.5 1 MILE

 

42

39

41

36 38

40

37

73°02' 73° 72°58' 72°56' 72°54'

40°42'

40°41'

46

42 45 43 44

72°52' 72°50' 72°48' 72°46'

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 Similar Land Use--Identifier described in Schubert (2010)

Simulated Contributing Area to Marine Surface Waters

Back-barrier estuaries

Ocean

Subsea-discharge zones(?)

42
14

The TN concentrations and loads calculated for 2005 mean 

annual conditions in areas that contribute fresh groundwater 

to back-barrier estuaries and the ocean were calculated for 

developed and undeveloped areas on Fire Island. Results 

indicate that, of the total annual TN load discharged from the 

shallow aquifer system, 82.2 percent (about 16,200 kg/yr or 

35,700 lb/yr) is distributed to shoreline discharge zones along 

the back bays (fig. 10A), and the remaining 17.8 percent (about 

3,500 kg/yr or 7,720 lb/yr) is to the ocean. TN loads entering 

back bays from the shallow aquifer system are presented below 

according to contributing land-use area (fig. 10B):

• Undeveloped park areas: 16.0 percent of total annual load 

(2,590 kg/yr or 5,710 lb/yr);

• Developed park areas: 29.2 percent of total annual load 

(4,730 kg/yr or 10,400 lb/yr);

• Unsewered residential areas: 

 low density: 0.4 percent of the total annual load 

(63 kg/yr or 140 lb/yr); 

 medium density: 7.3 percent of the total annual load 

(1,190 kg/yr or 2,620 lb/yr); 

 high density: 45.7 percent of the total annual load 

(7,400 kg/yr or 16,300 lb/yr); 

• Sewered residential area: 1.3 percent of the total annual load 

(203 kg/yr or 448 lb/yr).

The total TN load from Fire Island to marine surface 

waters is calculated to be 19,700 kg/yr (43,420 lb/yr), 

equivalent to roughly 6 percent of the annual TN load of 

about 345,000 kg/yr (761,000 lb/yr) that is discharged from 

shallow groundwater from the Suffolk County mainland into 

the South Shore Estuary Reserve (SSER) (Monti and Scorca, 

2003). In contrast to the total annual TN load from the entire 

1,420-km2

 SSER study area, which yields about 353 kg of 

nitrogen per square kilometer (300 lb/mi2

) annually, the total 

annual TN loading rate from Fire Island is about 890 kg/km2

(758 lb/mi2

)—nearly three times the annual loading rate from 

the SSER (fig. 11) and nearly twice the reported annual loading 

rate from shallow groundwater along the north shore of Nassau 

and Suffolk Counties (458 kg/km2

 or 390 lb/mi2) to Long Island 

Sound (Scorca and Monti, 2001).

Limitations of Analysis 

Simulations of groundwater discharge and calculations 

of nitrogen loads have several inherent limitations that are 

summarized briefly below:

1. 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. It also 

is based on a static shoreline position that, in reality, is 

dynamic, partly as a result of changes in wave climate that 

also may affect the amount of overheight.

2. Coarse model discretization requires 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; that is, variations in hydraulic conductivity and 

recharge are most likely inversely correlated (an increase 

in one produces the same change in 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. 

3. Variations in the no-flow condition specified for the base 

of the upper glacial aquifer could allow flow through the 

underlying, poorly permeable units and, thereby, affect 

model results but probably to a lesser extent than (2).

4. Simulations of variable-density groundwater flow were 

run to delineate the steady-state extent of the freshwater 

lens under 2005 conditions. The modeled hydrologic 

boundaries were assumed to be constant through time even 

though they can undergo large fluctuations from storm 

tides and other short-term factors such as storm overwash 

and coastal flooding. As a result, the model probably 

overestimated the depth of the freshwater lens, particularly 

in deeper, confined zones where travel times for 

freshwater recharge are extended. Consequently, transientstate modeling of historical conditions could provide 

information on the extent to which short-term fluctuations 

in hydrologic boundaries affect groundwater discharge and 

nitrogen loads.

Undeveloped Park

16%

Developed Park

29%

Unsewered Residential

Low Density

<1% 

Unsewered Residential

Medium Density

7% 

Unsewered Residential

High Density

46% 

Sewered Residential

1%

B Sources of total nitrogen load to

back bays by land use on Fire Island

Figure 10. Pie charts show total nitrogen load in groundwater 

discharge (A) distribution by percent and kilograms per year 

(kg/yr) to back bays and the ocean, and (B) sources of TN load 

to back bays by percent of contributing land-use area from Fire 

Island, Suffolk County, N.Y. 

Back Bays

82%

Ocean

18%

A Distribution and amount of annual total

nitrogen load from Fire Island

3,500 kg/yr 16,200 kg/yr
15

5. The use of TN concentrations to calculate nitrogen 

loads may overestimate mass-loading rates through the 

assumption of conservative transport in groundwater. 

Additional uncertainties in the estimated nitrogen loads 

from residential areas on Fire Island include (1) estimation 

of TN concentrations from values calculated for the south 

shore Suffolk County mainland and (2) reliance on median 

TN concentrations at four wells downgradient from two 

communities to assess the suitability of these surrogate 

values in calculating nitrogen loads.

Groundwater-Resource Concerns

The future of Fire Island and its surrounding water bodies 

will depend partly on whether human-derived contamination 

can be controlled. The major concerns, listed below, represent 

the primary groundwater-resource concerns identified in 

this study.

1. High-density residential land occupies only about 8 percent 

of Fire Island, yet its unsewered areas contribute almost 

half of the total TN load from the shallow aquifer system to 

back-barrier estuaries.

2. Water-table overheight directs about 80 percent of 

freshwater recharge on Fire Island to back-barrier 

estuaries through the combined effects of wave setup 

and tidal pumping along the ocean shore. As a result, the 

development of groundwater-protection strategies may 

warrant consideration of most of the island as a potential 

source area of nutrients entering the wetland and adjacent 

estuarine habitats within the boundaries of FIIS.

3. The water sampling that was done in this study 

typically showed extremely low TN concentrations in 

undeveloped park areas, yet TN concentrations were 

elevated locally in park areas downgradient from at 

least two communities, Kismet and Robbins Rest, and 

downgradient from the Watch Hill leach field. Although 

the elevated concentrations were within the range of those 

commonly found in developed areas of the Long Island 

mainland, they exceeded the background 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 could affect terrestrial and aquatic 

(freshwater and brackish) habitats and species that are 

adapted to the low nutrient concentrations generally found 

within and downgradient from undeveloped areas.

4. Elevated nutrient concentrations in groundwater underlying 

undeveloped areas indicate that recharge containing 

sewage, fertilizers, and (or) other human-derived products 

has infiltrated the water table in upgradient areas. This 

contaminated recharge could also contain pathogens and 

other manmade substances such as organic compounds 

that can move through the shallow aquifer and enter the 

surrounding water bodies, where they could threaten the 

ecological health of aquatic and coastal habitats.

Considerations for Future Monitoring

Groundwater discharge sustains freshwater ponds and 

wetlands; it also helps control salinity and maintain the 

biological diversity and productivity of estuaries and salt 

marshes (Masterson and Portnoy, 2005). Therefore, long-term 

water-level and water-quality monitoring is critical for the 

development of strategies to protect the groundwater system 

and the freshwater and estuarine ecosystems that depend on it. 

Continuous, monthly, and annual groundwater monitoring 

would provide a seasonal and long-term database for detection 

of current and future trends in water levels and in concentrations 

of selected constituents. This could be achieved through a 

network of 12 wells that bisect Fire Island in the four local areas 

detailed in this study. Future trends in nutrient concentrations 

in recharge entering the groundwater system and in discharge 

to the back bays and ocean could be monitored through annual 

collection of water samples from at least two of these wells 

in each of the four local study areas. The annual sampling for 

nutrient concentrations at these wells could include samples for 

organic-compound and (or) pathogen analysis. Development of 

a record of detections of a variety of constituents would enhance 

the database.

Nutrient concentrations measured during this study 

were low in undeveloped park areas but elevated locally in 

areas downgradient from communities and the NPS facility at 

Watch Hill. Therefore, any monitoring program designed to 

detect upward trends in these concentrations would include 

sampling in developed and undeveloped areas and analyses to 

identify contamination from sewage, fertilizers, and (or) other 

human-derived substances, including pathogens and organic 

compounds. An alternative approach could be the collection of 

water samples within the communities themselves. Thus, a plan 

for the future installation of additional monitoring wells in the 

target areas would be needed.

890

353

458

0

200

400

600

800

1,000

ANNUAL NITROGEN LOADING RATE FROM SHALLOW

GROUNDWATER TO MARINE SURFACE WATERS, IN KG/KM2

WATERSHED

Fire Island

SSER study

area 

North Shore

of Long Island 

Figure 11. Graph shows the rate of annual discharge 

of nitrogen from shallow groundwater to marine surface 

waters in the Fire Island, South Shore Estuary Reserve 

(SSER), and North Shore of Long Island watersheds 

on Long Island, N.Y. The rate of nitrogen discharge is 

estimated in kilograms per square kilometer (kg/km2

). 
References Cited

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eds., 1993, Estuarine resources of the Fire Island National 

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hydrologic situation on Long Island, N.Y., as a guide to 

water-management alternatives: U.S. Geological Survey 

Professional Paper 627–F, 59 p.

Hanslow, D., and Nielsen, P., 1993, Shoreline setup on 

natural beaches, in Short, A.D., ed., Beach and surfzone 

morphodynamics: Journal of Coastal Research, Special 

Issue 15, p. 1–11.

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characteristics of natural water (3d ed.): U.S. Geological 

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Klopfer, S.D., Olivero, Adele, Sneddon, Lesley, and Lundgren, 

Julie, 2002, Final report of the NPS Vegetation Mapping 

Project at Fire Island National Seashore: Blacksburg, Va., 

College of Natural Resources at Virginia Tech, Conservation 

Management Institute Report CMI–GRS–02–03, 205 p.

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ground-water flow and their effects on the ecology and water 

resources of the Cape Cod National Seashore, Massachusetts: 

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concentration and nitrogen loads entering the South Shore 

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24, 2007, at http://www.nps.gov/fiis/parkmgmt/statistics.htm

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By Christopher E. Schubert, M. Peter deVries, and Anne J. Finch

For more information, please contact:

U.S. Geological Survey

New York Water Science Center

2045 Route 112, Bldg. 4

Coram, NY 11727

(631) 736-0783

http://ny.water.usgs.gov

© 2004 Peter deVries