U.S. Department of the Interior

U.S. Geological Survey

round-water resources along

the Atlantic coastal zone of the

United States are vulnerable to

saltwater intrusion and to contamination by nutrients. Saltwater intrusion,

the movement of saline water into freshwater aquifers (fig. 1), is most commonly caused by ground-water pumping

near the coast. Nutrient contamination

results from many human activities and

has caused widespread increases of

nitrate in shallow ground water nationwide (U.S. Geological Survey, 1999).

High salinity or nutrient concentrations

of the U.S. Geological Survey (USGS)

that provide scientific information

for sustainable management of groundwater resources in this coastal area.

Ground-water sustainability is defined

as developing and using ground water

in a way that can be maintained for an

indefinite time without causing unacceptable environmental, economic, or

social consequences (Alley and others,

1999).

Saltwater Intrusion

Freshwater aquifers along the Atlantic

coastal zone are among the most productive in the United States, supplying

drinking water to an estimated 30 million

people from Maine to Florida. These

freshwater aquifers are bounded at their

seaward margins by saltwater (fig. 2).

Because freshwater has a lower concentration of dissolved solids than does saltwater, it is less dense than saltwater and

tends to flow on top of surrounding or

underlying saltwater.

Under natural conditions, the

seaward flow of freshwater prevents

saltwater from encroaching coastal

USGS Fact Sheet 085–00

September 2000

Atlantic Coastal Zone

Ground water is among the Nation’s most important natural resources. It

provides drinking water to urban and rural communities, supports irrigation

and industry, sustains the flow of streams and rivers, and maintains riparian

and wetland ecosystems. In many areas of the Nation, the future sustainability of ground-water resources is at risk from overuse and contamination.

Because ground-water systems typically respond slowly to human actions, a

long-term perspective is needed to manage this valuable resource. This

publication is one in a series of fact sheets that describe ground-waterresource issues across the United States, as well as some of the activities of

the U.S. Geological Survey that provide information to help others develop,

manage, and protect ground-water resources in a sustainable manner.

GROUND-WATER RESOURCES FOR THE FUTURE

can make ground water unfit for public

consumption. Moreover, discharge of

nutrient-contaminated ground water to

coastal waters can trigger dense algal

blooms that result in habitat changes

and oxygen depletion, ultimately affecting the structure and function of coastal

ecosystems.

This fact sheet reviews some of

the issues associated with saltwater

intrusion and discharge of nutrientcontaminated ground water along

the Atlantic coastal zone. Also

described are some of the activities

Atlantic Ocean

Cape Cod,

Mass.

Long Island, N.Y.

Delaware River and

Bay, N.J., Delaware

Chesapeake Bay,

Maryland and Virginia

Northeastern

North Carolina

Beaufort-Hilton Head,

South Carolina

Jacksonville, Florida

Southeast Florida

Brunswick, Georgia

MAINE

MASS.

CONN.

NEW YORK

V I RGIN I A

FLORIDA

GEORGIA

N O R T H C A R O L I N A

SOUTH

CAROLINA

N . H . N .J. DEL.

MARYLAND

R.I.

Area of Map

Figure 1. Selected areas along the Atlantic

coast where saltwater has intruded into

freshwater aquifers.

Figure 2. Ground-water flow patterns and the zone of dispersion in an idealized coastal

aquifer. A circulation of saltwater from the sea to the zone of dispersion and then back

to the sea is induced by mixing of freshwater and saltwater in the zone of dispersion.

(Modified from Cooper, 1964.)

Land surface

Water table

Freshwater

Sea

Zone of dispersion

Saltwater

G
400

600 800

1,000

5,000

10,000

15,000

16,000

17,000

18,000

1,750 1,500 1,250 1,000 750 250 500 0 250 500

DISTANCE FROM SHORELINE, IN FEET

ALTITUDE, IN FEET ABOVE OR BELOW SEA LEVEL

Land surface

Water table

Base of Biscayne aquifer

Biscayne aquifer

Biscayne Bay

20

40

0

20

40

60

80

100

120

5,000 Line of equal chloride

 concentration, in parts

 per million

Bottom of well

EXPLANATION

Note: Concentrations expressed as parts 

per million can be converted to milligrams 

per liter by multiplying the concentrations 

by the density of water, in kilograms per 

liter. At low concentrations, such as that 

of freshwater, concentrations expressed 

as parts per million are nearly equal to 

those expressed as milligrams per liter. 

aquifers, and the boundary between

freshwater and saltwater is maintained

near the coast or far below land surface.

Typically, this boundary is not sharp

but rather is a diffuse zone where

freshwater and saltwater mix (figs. 2

and 3). The zone can be narrow, such

as the approximate 1,500-foot width

shown for the Biscayne aquifer in

figure 3, or as wide as several miles,

such as in parts of the New Jersey

Coastal Plain.

The pumping of ground water lowers

water levels and can cause saltwater to

be drawn toward the areas of stress

(figs. 4 and 5). As indicated by the

directions of ground-water flow in

figures 4 and 5, saltwater contamination

can occur by lateral flow of saltwater

from the sea or by vertical flow of

saltwater from deeper, more saline

zones of a ground-water system. Saltwater intrusion reduces fresh groundwater storage and can lead to the abandonment of supply wells when concentrations of dissolved ions exceed

drinking-water standards. For example,

the concentration of chloride in average

seawater is about 19,000 milligrams

per liter, whereas the recommended

drinking-water limit for chloride is

250 milligrams per liter.

Saltwater intrusion has been

documented throughout the Atlantic

coastal zone, but the degree of saltwater

intrusion varies widely among localities

and hydrogeologic settings. In many

cases, the area contaminated by saltwater is limited to small parts of

the aquifer and has little or no effect

on wells pumped for ground-water

supply. In other cases, contamination

is of regional extent, with substantial

effects on ground-water supplies. For

example, in Cape May County, N.J.,

more than 120 supply wells have been

abandoned since 1940 because of saltwater contamination (Lacombe and

Carleton, 1992).

Although most saltwater intrusion

along the Atlantic coast has been caused

by pumping, lowering of the water table

by drainage canals has led to saltwater

encroachment in a few locations, notably southeast Florida. Drainage canals

not only lower the water table but

can convey saltwater inland when the

canals lack salinity-control structures.

Salinity-control structures have been

effective in slowing or reversing saltwater encroachment along canals in

southeast Florida.

Many States and communities along

the Atlantic coast are taking action to

manage and prevent saltwater intrusion

so supplies can be maintained in the

future. These actions can be grouped

into three general categories: scientific

monitoring and assessment, engineering techniques, and regulatory actions.

A traditional engineering approach to

Land surface

Water table

Freshwater

Pumping well

Sea

Saltwater

Figure 3. The zone of dispersion in the Biscayne aquifer near Miami, Fla. Chloride concentrations in the zone of dispersion

range from freshwater containing 16 parts per million to seawater containing 19,000 parts per million. (Modified from Kohout,

1964.)

Figure 4. Ground-water flow patterns near a pumping well in an idealized coastal aquifer.

Ground-water levels around the well are lowered in response to pumping and the saltwater

zone moves toward the well.
reversing saltwater intrusion has been to

decrease or discontinue pumping from

coastal wells. In such cases, alternative

water sources are sought for coastal

communities. In New Jersey, for example, the Manasquan Reservoir was built

in 1989 as a supplemental water source

in response to declining ground-water

levels and saltwater intrusion in parts

of Middlesex, Monmouth, and Ocean

Counties.

As population increases along the

Atlantic coastal zone and new freshwater sources become more difficult

to obtain, innovative approaches for

managing saltwater intrusion are

becoming more widespread. Such

approaches include artificial-recharge

systems, desalination plants, and blending of waters of different quality. Some

States along the Atlantic coast also

have begun to regulate and restrict

withdrawals of coastal ground water

or to mandate or encourage water conservation. For example, the State of

Georgia has established an Interim

Strategy for Managing Saltwater

Contamination in the Upper Floridan

aquifer that caps ground-water use in

the Savannah and Brunswick areas at

1997 rates and encourages water conservation and reduced water use in areas

along the Georgia coastline.

Discharge of NutrientContaminated Ground Water

to Coastal Environments

Nutrients are essential for plant and

animal growth but in elevated concentrations can degrade water quality. Contamination of coastal ground water by

nutrients occurs as a consequence of

activities such as wastewater disposal

from septic systems, agricultural and

urban uses of fertilizer, and agricultural

use of manure. Nutrients carried by

ground water can be discharged to

coastal waters and wetlands or to

drinking-water supply wells.

Often, water-quality concerns for

coastal ecosystems have focused on

surface-water sources of nutrients and

other contaminants. Increasingly, however, the importance of ground water

as a source of contaminants to coastal

environments is becoming evident. To

design effective coastal water-quality

monitoring networks and to manage and

protect coastal environments, information is needed on the relative importance

of ground water as a nutrient source

compared to other sources. Because

ground water moves slowly, the flushing

of nutrient-contaminated ground water

from an aquifer can take many years,

even several decades.

In many coastal ecosystems, the

mechanisms and rates of groundwater discharge and nutrient loading

are not well understood. Knowing the

specific path by which ground water

discharges to a coastal ecosystem is

important because the natural removal

of dissolved nutrients from ground

water depends on the sediments and

geochemical conditions that the ground

water contacts before discharging to the

ecosystem. Figure 6 illustrates the variety of ground-water flow paths below

a tidal salt marsh in New England.

Brackish water

Saltwater

Direction of ground-water flow

Water table

Area of Major Pumping

Atlantic Ocean

Land Surface

Fracture

Sea Floor

SURFICIAL

AQUIFER

SYSTEM

SUB-FLORIDAN CONFINING UNIT

EXPLANATION

UNIT

UNIT

UNIT

MIDDLE SEMICONFINING

INTERMEDIATE CONFINING

UPPER FLORIDAN

UPPER ZONE OF

 LOWER FLORIDAN AQUIFER

FERNANDINA PERMEABLE ZONE

SEMICONFINING

AQUIFER

Upland

Shallow

flow path

Marsh

Tidal Creek

Intermediate flow path

Deep flow path

Marsh peat

Silt/clay

Fine and medium sand

Boundary seepage zone

Interior vegetated marsh zone

Sub-marsh aquifer zone

Creek bottom zone

1

1

2

3

4

2

3

4

EXPLANATION

Figure 5. Simplified model of saltwater leakage along fractures and other structural

deformities in the Floridan aquifer system in northeastern Florida and southeastern

Georgia. The fractures and other structural deformities provide a conduit between

freshwater zones and deeper, more saline zones. Saltwater flows upward through

the nearly vertical zones in response to ground-water pumping in the uppermost

aquifers. (Modified from Krause and Randolph, 1989; and Spechler, 1994.)

Figure 6. Diagram of ground-water flow paths and discharge locations below a tidal

salt marsh in New England. Ground-water discharge is concentrated at the boundary

between the marsh and upland areas and in the tidal creek. In these two zones, the

marsh peat is thinnest and the hydraulic properties of the marsh and aquifer materials

are most favorable for ground-water flow. (Modified from Howes and others, 1996.)

(Copyright John Wiley & Sons Limited. Reproduced with permission.)
Activities of the USGS

Through its national, State, and

local programs, the USGS provides

scientific information to help others

develop, manage, and protect groundwater resources in a sustainable manner.

Along the Atlantic coastal zone, the

USGS uses geophysical, geochemical,

and ground-water-level measurements

to define the boundary between freshwater and saltwater and to monitor

saltwater intrusion. Monitoring

networks of this type have been established by the USGS in collaboration

with State and local agencies in areas

of New Jersey, Maryland, Delaware,

Virginia, South Carolina, Georgia,

and Florida.

USGS scientists also are working

to better understand the geologic,

geochemical, and hydrologic controls

on saltwater intrusion and on groundwater discharge and nutrient loading

to coastal environments:

• In Virginia, USGS scientists are

working with others to understand how the recently discovered Chesapeake Bay impact

crater affects regional groundwater flow paths and groundwater salinity of a large part

of the Virginia Coastal Plain

(Poag, 1999; Powars and

Bruce, 1999).

• Near Savannah, Ga., and Hilton

Head Island, S.C., the USGS is

conducting a program of offshore

drilling to delineate the freshwater/saltwater boundary and

is developing computer models

to simulate and evaluate saltwater

movement in coastal aquifers.

• In several coastal environments

from Cape Cod, Mass., to the

Everglades in southern Florida,

USGS scientists are using field

measurements and computermodeling techniques to map

coastal sediments, quantify

ground-water discharge rates and

nutrient fluxes to coastal waters,

and predict the effects of management alternatives on groundwater discharge and nutrient

fluxes.

References

Alley, W.M., Reilly, T.E., and Franke,

O.L., 1999, Sustainability of groundwater resources: U.S. Geological

Survey Circular 1186, 79 p.

Cooper, H.H., Jr., 1964, A hypothesis

concerning the dynamic balance of

fresh water and salt water in a coastal

aquifer: U.S. Geological Survey

Water-Supply Paper 1613–C,

p. C1–C12.

Howes, B.L., Weiskel, P.K., Goehringer,

D.D., and Teal, J.M., 1996, Interception of freshwater and nitrogen

transport from uplands to coastal

waters—The role of saltmarshes, in

Nordstrom, K.F., and Roman, C.T.,

eds., Estuarine shores—Evolution,

environments and human alterations:

New York, John Wiley, p. 287–310.

Kohout, F.A., 1964, The flow of fresh

water and salt water in the Biscayne

aquifer of the Miami area, Florida:

U.S. Geological Survey WaterSupply Paper 1613–C, p. C12–C32.

Krause, R.E., and Randolph, R.B., 1989,

Hydrology of the Floridan aquifer

system in southeast Georgia and

adjacent parts of Florida and South

Carolina: U.S. Geological Survey

Professional Paper 1403–D, 65 p.

Lacombe, P.J., and Carleton, G.B., 1992,

Saltwater intrusion into fresh groundwater supplies, southern Cape May

County, New Jersey, 1890–1991, in

The future availability of ground

water resources: Proceedings of the

National Symposium, Raleigh, N.C.,

April 12–15, 1992, American Water

Resources Association, Bethesda,

Md., p. 287–297.

Poag, C.W., 1999, Chesapeake invader—

Discovering America’s giant meteorite crater: Princeton, N.J., Princeton

University Press, 183 p.

Powars, D.S., and Bruce, T.S., 1999, The

effects of the Chesapeake Bay impact

crater on the geological framework

and correlation of hydrogeologic

units of the lower York-James Peninsula, Virginia: U.S. Geological Survey Professional Paper 1612, 82 p.

Spechler, R.M., 1994, Saltwater intrusion and quality of water in the

Floridan aquifer system, northeastern

Florida: U.S. Geological Survey

Water-Resources Investigations

Report 92–4174, 76 p.

U.S. Geological Survey, 1999, The quality

of our Nation’s waters—Nutrients

and pesticides: U.S. Geological Survey Circular 1225, 82 p.

——Paul M. Barlow; edited by Mary A.

Kidd; graphics by Margo J. VanAlstine;

layout by Joy Monson

For more information on groundwater-resource issues along the

Atlantic coastal zone, please contact:

Chief, Office of Ground Water

U.S. Geological Survey

411 National Center

12201 Sunrise Valley Drive

Reston, VA 20192

(703) 648–5001

http://water.usgs.gov/ogw

Photograph courtesy of David Wilson, Maryland Coastal Bays Program.