U.S. Department of the Interior Page 1

U.S. Geological Survey

Fact Sheet 2011–3019

February 2011

Nationwide Assessment of 

Groundwater Availability

The U.S. Geological Survey’s 

Groundwater Resources Program is 

conducting an assessment of groundwater 

availability throughout the United 

States to gain a better understanding of 

the status of the Nation’s groundwater 

resources and how changes in land use, 

water use, and climate may affect those 

resources. The goal of this National 

assessment is to improve our ability to 

forecast water availability for future 

economic and environmental uses. 

Assessments will be completed for the 

Nation’s principal aquifer systems to help 

characterize how much water is currently 

available, how water availability is 

changing, and how much water we can 

expect to have in the future (Reilly and 

others, 2008).

The concept of groundwater 

availability is more than just how much 

water can be pumped from any given 

aquifer. Groundwater availability is 

a function of many factors, including 

the quantity and quality of water and 

the laws, regulations, economics, and 

environmental factors that control its use. 

The primary objective of the Northern 

Atlantic Coastal Plain groundwateravailability study is to identify spatial 

and temporal changes in the overall 

water budget by more fully determining 

the natural and human processes that 

control how water enters, moves through, 

and leaves the groundwater system. 

Development of tools such as numerical 

models can help hydrologists gain an 

understanding of this groundwater 

system, allowing forecasts to be made 

about the response of this system to 

natural and human stresses, and water 

quality and ecosystem health to be 

analyzed, throughout the region.

The Importance of Groundwater in the Northern Atlantic Coastal Plain

The Northern Atlantic Coastal Plain (NACP) occupies a land area of approximately 

34,000 square miles along the eastern seaboard of the United States from Long Island, 

N.Y., southward to the northern portion of North Carolina (fig. 1). This area is underlain 

by a seaward-dipping wedge of sediments comprising clay, silt, and sand and gravel 

(fig. 2). This sedimentary wedge forms a complex groundwater system in which the 

sands and gravels function as confined aquifers, and the silts and clays function as 

confining units. The confined aquifers of the NACP constitute a major source of water 

for public and domestic supply, as well as a vital source of freshwater for industrial and 

agricultural uses throughout the region.

USGS Groundwater Resources Program

Assessing Groundwater Availability in the Northern Atlantic 

Coastal Plain Aquifer System

Figure 1. Location of the Northern Atlantic Coastal Plain aquifer system, 

eastern United States.

FALL LINE

Base credit from U.S. Geological Survey

Digital data, elevation data from NASA-NGA Shuttle Radar

Topography Mission, Albers Equal-Area projection, NAD 83

77° 70°

Atlantic Ocean

35°

42°

Philadelphia

Baltimore

Washington, DC

Annapolis

Dover

Atlantic City

Trenton

Virginia Beach

Richmond

Raleigh Greenville

Fayetteville

Wilmington

NEW YORK

NEW

JERSEY

LONG ISLAND

PENNSYLVANIA

MARYLAND

VIRGINIA

NORTH CAROLINA

DELAWARE

Elevation, in feet

High : 459

Low : 0

EXPLANATION

0

0 100 KILOMETERS

50 100 MILES

50

Study area
Page 2

 Substantial withdrawals of groundwater from the NACP 

aquifer system had begun by the late 1800s. By 1900 about 

100 million gallons per day (Mgal/d) of water was being 

pumped from the NACP with about half the total from western 

Long Island for water supply for New York City. Groundwater 

withdrawals have increased steadily throughout the NACP since 

then; by 1980, total groundwater withdrawals were estimated to 

be about 1,200 Mgal/d. Between 1980 and 2005, the population 

increased by about 29 percent from 21 million to approximately 

27 million, whereas groundwater withdrawals increased by 

about 50 percent to about 1,800 Mgal/d (Forstall, 1996; Kenny 

and others, 2009). If the per capita rate of drinking water 

consumption is assumed to have remained the same during the 

period 1980–2005, this increase in withdrawals illustrates the 

importance of this vital resource for other uses besides simply 

being a source of fresh drinking water.

In response to the extensive development and increased 

withdrawals throughout the region, water levels in many of the 

confined NACP aquifers have been declining by up to 2 feet (ft) 

per year, resulting in total declines of tens to hundreds of feet 

from their predevelopment levels (figs. 3 and 4) (DePaul and 

others, 2008). In some areas, such as southeastern Virginia, 

these declines are greater than 100 ft, resulting in water-level 

altitudes of more than 100 ft below the mean sea-level elevation 

of 1929 (NGVD 29).

Water-resources management in the NACP creates 

hydrologic challenges beyond those imposed by the competing 

local domestic, industrial, agricultural, and environmental 

demands for water. This aquifer system extends across six 

State boundaries, which are typically used by water-resource 

managers in planning and protection efforts. As waterlevel declines extend under major coastal waters, such as 

the tidal portion of the Potomac River and Delaware Bay 

Figure 2. Schematic cross section of the Northern Atlantic Coastal Plain aquifer system (modified from Martin, 1998). 

Photograph courtesy of Flyin Phil’s Photos—http://www.FlyinPhilsPhotos.com

Poultry farm in Kent County, Delaware

Cape May, New Jersey

WEST

FEET

EAST

Bedrock

Saprolite

Confining unit

Confined aquifer

Unconfined aquifer

Direction of

groundwater flow

500

NGVD

1929

-500

-1,000

-1,500

Vertical scale greatly exaggerated

EXPLANATION

0

0 20 KILOMETERS

10 20 MILES

10

Atlantic

Ocean

Fall Line COASTAL PLAIN PHYSIOGRAPHIC PROVINCE

Saltwater interface
Page 3

Tidal creek in Sussex County, Delaware

Figure 3. Estimated water-level changes in the Middle 

Potomac aquifer from predevelopment to 1980 (modified from 

DePaul and others, 2008).

and across State lines (fig. 3), the potential for interstate 

aquifer-management issues and the need for a comprehensive 

regional assessment of the groundwater availability of this 

aquifer system will likely increase. Regional assessments 

of groundwater availability, such as those conducted by the 

USGS Groundwater Resources Program, provide consistent 

and integrated information that is useful to those who use and 

manage the resource across political boundaries at the State and 

local levels (Reilly and others, 2008).

Groundwater-Availability Assessment

USGS scientists will describe the hydrogeologic framework 

and quantify water-use and water-budget components (for 

example, recharge, withdrawals, evapotranspiration, groundwater 

discharge, and storage) of the aquifer system. Numerical models 

will be developed to forecast the effects of future groundwater 

development in the NACP aquifer system and to evaluate the 

adequacy of long-term monitoring networks to ensure that these 

data-collection efforts are designed properly to record potential 

changes in hydrologic conditions throughout the region. 

Hydrogeologic Conditions 

Numerous hydrogeologic data-collection efforts completed 

since the last regional assessment of the NACP aquifer system 

(Trapp and Meisler, 1992) have led to significant but generally 

disparate refinements of the hydrostratigraphic framework and 

hydrologic budgets throughout the region. These recent studies 

have provided local-scale improvements in the understanding 

of the groundwater-flow system, but the information obtained 

from these studies has not been synthesized into a regional 

hydrogeologic perspective. Synthesis of these hydrogeologic 

data-collection efforts into an internally consistent framework 

is needed to develop tools such as numerical models that can be 

used to forecast effects of future changes in water availability in 

the NACP.

Figure 4. Industrial users such as paper mills account for a large 

portion of groundwater withdrawals in the Virginia Coastal Plain. 

(Photograph courtesy of the Richmond Times-Dispatch.)

CHESAPEAKE BAY

ATLANTIC OCEAN

DELAWARE BAY

ALBEMARLE SOUND

LONG ISLAND SOUND

PAMLICO SOUND

FALL LINE

Greater than 100

Greater than 80 to 100

Greater than 60 to 80

Greater than 40 to 60

Greater than 20 to 40

Greater than 10 to 20

No significant change

Insufficient data

Water-level declines,

in feet 

EXPLANATION

Aquifer boundary

Control point

NORTH CAROLINA

NEW

JERSEY

MARYLAND

VIRGINIA

NEW YORK

Raleigh

Durham

Fayetteville

Goldsboro

Wilmington

Rocky Mount

Greenville

Suffolk

Newport 

News Norfolk

Virginia Beach

Richmond

Alexandria

Washington, DC

Baltimore

Annapolis

Harrisburg

Scranton

Wilmington

Camden

Philadelphia

Trenton

Newark

New 

York

Stamford

Milford

New 

Haven

Coram

Atlantic City

Vineland

Dover

Danville

Greensboro

Lynchburg

PENNSYLVANIA

DELAWARE

Base from U.S. Geological Survey digital data, 1972

1:2,000,000 Albers Equal-Area Conic projection 

Standard parallels 29°30'N and 45°30'N, central meridian 96°00'W

77°

42°

80°

79°

78°

41°

40°

39°

38°

37°

36°

35°

34°

35°

36°

76°

75°

74°

73°

72°

37°

38°

39°

41°

34°

80°

79°

78°

42°

72°

74°

75°

33°

33°

73°

76°

0 25 50

0 25 50 100 MILES

100 KILOMETERS

40°

DEPTH TO WATER, IN FEET BELOW LAND SURFACE

Water level declines in an 800-foot deep well in 

Potomac aquifer in King and Queen County, Virginia

YEAR

80

90

100

110

120

130

140

150

160

170

180

1970 1980 1990 2000 2010
Page 4

Numerical Models as Tools for Assessing Groundwater 

Availability

Numerical models provide a means to synthesize existing 

hydrogeologic information into an internally consistent 

mathematical representation of a real system or process and thus 

are useful tools for testing and improving conceptual models 

of groundwater-flow systems (Konikow and Reilly, 1999). The 

groundwater-flow model of the NACP will incorporate the 

latest knowledge of the hydrogeology of the aquifer system and 

withdrawal data collected as part of this study. 

With this model, it will be possible to simulate the changes 

that occurred in the past and then forecast future changes that 

may occur within the aquifer system—such as changes in 

water levels and aquifer storage—caused by pumping, landuse changes, and the effects of climate variability and change. 

A well constructed regional groundwater model will help 

local, State, and Federal agencies (1) assess the major inflows 

and outflows to the regional aquifer system; (2) understand, 

forecast, and manage groundwater depletion from changes 

in withdrawals; (3) estimate the effects of climate change 

on groundwater resources; (4) evaluate and refine existing 

long-term monitoring networks; and (5) assess the effects of 

proposed water-management decisions.

Vital Role of Partnerships

Multiple State and Federal agencies and nongovernmental 

organizations have responsibilities or interests in managing 

and protecting the groundwater resources of the Northern 

Atlantic Coastal Plain aquifer system. The USGS groundwateravailability study team is collaborating with many of these 

agencies and organizations to build a better understanding of 

the region’s water resources. This knowledge will help provide 

input for developing tools to better understand the aquifer and 

to make more informed decisions about the management of the 

groundwater resources of the Northern Atlantic Coastal Plain 

aquifer system.

Selected References 

DePaul, V.T., Rice, D.E., and Zapecza, O.S., 2008, Water-level changes 

in aquifers of the Atlantic Coastal Plain, predevelopment to 2000: 

U.S. Geological Survey Scientific Investigations Report 2007–5247, 

88 p. (Also available at http://pubs.usgs.gov/sir/2007/5247.)

Forstall, R.L., 1996, Population of states and counties of the United 

States—1790 to 1990, from the twenty-one decennial censuses: 

U.S. Bureau of the Census, Washington, D.C.

Kenny, J.F., Barber, N.L., Hutson, S.S., Linsey, K.S., Lovelace, 

J.K., Maupin, M.A., 2009, Estimated use of water in the United 

States in 2005: U.S. Geological Survey Circular 1344, 52 p. 

(Also available at http://pubs.usgs.gov/circ/1344.)

Konikow, L.F., and Reilly, T.E., 1999, Groundwater modeling, in 

Delleur, J.W., ed., The handbook of groundwater engineering: 

Boca Raton, Fla., CRC Press, 40 p.

Martin, Mary, 1998, Ground-water flow in the New Jersey Coastal 

Plain: U.S. Geological Survey Professional Paper 1404–H. 

(Also available at http://pubs.er.usgs.gov/publication/pp1404H.)

Reilly, T.E., Dennehy, K.F., Alley, W.M., and Cunningham, W.L., 

2008, Ground-water availability in the United States: U.S. 

Geological Survey Circular 1323, 70 p. (Also available online at 

http://pubs.usgs.gov/circ/1323.)

Trapp, Henry, Jr., and Meisler, Harold, 1992, The regional aquifer 

system underlying the Northern Atlantic Coastal Plain in parts of 

North Carolina, Virginia, Maryland, Delaware, New Jersey, and New 

York: U.S. Geological Survey Professional Paper 1404-A, 33 p., 

11 pls.

By John P. Masterson, Jason P. Pope, 

Jack Monti Jr., and Mark R. Nardi

For more information:

Visit the project Web site at: http://ny.water.usgs.gov/projects/NACP

Or contact John P. Masterson

508-490-5028 jpmaster@usgs.gov

Banner photograph courtesy of Chesapeake Bay Program