Groundwater Resources Program

Hydrogeology and Hydrologic Conditions of the Northern 

Atlantic Coastal Plain Aquifer System From Long Island, 

New York, to North Carolina

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2013–5133
Front cover art. Modified animation-frame from National Aeronautics and Space Administration (NASA) Goddard Space Flight Center, Scientific 

Visualization Studio, accessed July 24, 2013, at http://svs.gsfc.nasa.gov/vis/a000000/a003400/a003472/. White boundary outlines study area. 

Back cover art. Lansat 7 mosaic of Chesapeake Bay region, from NASA Goddard Space Flight Center, Scientific Visualization Studio,

accessed July 24, 2013, from http://svs.gsfc.nasa.gov/vis/a000000/a003400/a003473/.
Hydrogeology and Hydrologic Conditions of 

the Northern Atlantic Coastal Plain Aquifer 

System From Long Island, New York, to 

North Carolina

By John P. Masterson, Jason P. Pope, Jack Monti, Jr., Mark R. Nardi, 

Jason S. Finkelstein, and Kurt J. McCoy

Scientific Investigations Report 2013–5133

U.S. Department of the Interior

U.S. Geological Survey

Groundwater Resources Program
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Masterson, J.P., Pope, J.P., Monti, Jack, Jr., Nardi, M.R., Finkelstein, J.S., and McCoy, K.J., 2013, Hydrogeology and 

hydrologic conditions of the Northern Atlantic Coastal Plain aquifer system from Long Island, New York, to North 

Carolina: U.S. Geological Survey Scientific Investigations Report 2013–5133, 76 p., http://dx.doi.org/10.3133/

sir20135133.

ISSN 2328–031X (print) 

ISSN 2328–0328 (online)

ISBN 978–1–4113–3688–9
iii

Acknowledgements

The authors wish to thank the following individuals for their assistance and guidance throughout 

this investigation: Martha Watt, Jennifer Bonin, Fredrick Stumm, Jessica Carpenter, and Tom 

Reilly (emeritus) of the U.S. Geological Survey. The authors also would like to thank David 

Andreasen of the Maryland Geological Survey and Randy McFarland and Howard Reeves of the 

U.S. Geological Survey for providing technical reviews for this report. The authors thank William 

Spitz and Dave Lengyell of the New York State Department of Environmental Conservation, 

Daniel P. DeSalvo of Suffolk County Water Authority, Richard Liebe and Loretta Dionisio of 

Nassau County Department of Public Works, Tom Tengleson of the New York City Department of 

Environmental Protection, and Stewart Lovell of Delaware Department of Natural Resources and 

Environmental Control for providing assistance on water-use information.
This page has been left blank intentionally.
v

Contents

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

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

Location and Physical Setting ............................................................................................................3

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

Hydrogeology..................................................................................................................................................8

Previous Investigations........................................................................................................................8

Depositional History .............................................................................................................................9

Northern Atlantic Coastal Plain Hydrologic Units.........................................................................12

Fluvial Deltaic Cretaceous Units in the Potomac Aquifer System.....................................14

Potomac-Patuxent Aquifer..............................................................................................14

Potomac-Patapsco Aquifer.............................................................................................18

Marine Cretaceous Units .........................................................................................................19

Magothy Aquifer...............................................................................................................19

Matawan Aquifer..............................................................................................................19

Monmouth-Mount Laurel Aquifer..................................................................................20

Marine Tertiary Units.................................................................................................................20

Aquia Aquifer.....................................................................................................................20

Piney Point Aquifer...........................................................................................................21

Chesapeake Bay Impact Crater and Associated Units .......................................................21

Transitional Tertiary and Quaternary Units............................................................................22

Lower Chesapeake Aquifer.............................................................................................22

Upper Chesapeake Aquifer.............................................................................................23

Surficial Aquifer................................................................................................................23

Hydraulic Properties...........................................................................................................................24

Hydrologic Conditions.................................................................................................................................26

Previous Investigations......................................................................................................................26

Sources of Water to the Northern Atlantic Coastal Plain Aquifer System ...............................26

Aquifer Recharge.......................................................................................................................27

Wastewater Return Flow..........................................................................................................28

Lateral Inflow..............................................................................................................................31

Loss of Water from the Northern Atlantic Coastal Plain Aquifer System .................................34

Groundwater Discharge to Streams.......................................................................................34

Groundwater Withdrawals.......................................................................................................34

Groundwater Discharge to Coastal Areas ............................................................................36

Groundwater in Storage ....................................................................................................................36

Hydrologic Budget for the Northern Atlantic Coastal Plain Aquifer System............................37

Changes in Hydrologic Conditions from 1900 to Present .............................................................39

Aquifer Recharge.......................................................................................................................39

Groundwater Withdrawals.......................................................................................................39

Shift in Groundwater Use: Western Long Island and East-Central New Jersey ...40

Shift in Groundwater Use: Southern Virginia...............................................................41

Regional Effects of Recent Pumping .............................................................................41
vi

Wastewater Return Flow..........................................................................................................41

Saltwater Intrusion....................................................................................................................48

Climate Change and Groundwater Resources ..............................................................................48

Summary and Conclusions.........................................................................................................................51

References Cited..........................................................................................................................................52

Appendix........................................................................................................................................................61

Figures

1. Map showing the location and extent of the Northern Atlantic Coastal Plain 

studyarea .......................................................................................................................................2

2. Maps showing population density by county in the area of the Northern Atlantic

Coastal Plain aquifer system for A, 2005 and B, changes in population density from

1985 to 2005....................................................................................................................................4

3. Map showing groundwater withdrawals by county in the Northern Atlantic Coastal

Plain for 2005 .................................................................................................................................5

4. Map showing the change in water levels in response to groundwater withdrawals

in the Potomac-Patapsco regional aquifer of the Northern Atlantic Coastal Plain

aquifer system for A, 1900 to 1980 and B, 1980 to 2000...........................................................6

5. Map showing land use and land cover for the Northern Atlantic Coastal Plain for

conditions in 2006 .........................................................................................................................7

6. Geologic time scale for the Northern Atlantic Coastal Plain aquifer system.....................8

7. Generalized section showing Northern Atlantic Coastal Plain aquifer system from

the Fall Zone to the coast............................................................................................................9

8. Schematic diagram showing the structural setting and inferred sediment sources

for the Northern Atlantic continental margin ........................................................................11

9. Correlation chart showing regional aquifer names and subregional aquifer names

for the Northern Atlantic Coastal Plain (NACP) aquifer system........................................13

10. Cross-sections A, A–A′ on Long Island, New York, B, B–B′ in New Jersey, 

C, C–C′ in Delaware and Maryland, and D, D–D′ in Virginia, showing the regional 

variations in the hydrogeologic framework of the Northern Atlantic Coastal Plain 

aquifer system.............................................................................................................................15

11. Cross-section showing distribution of the variable geometry and sediment

composition of the Potomac aquifer system across the southern half of the

Northern Atlantic Coastal Plain................................................................................................17

12. Schematic diagram showing the conceptual hydrogeologic model of the Northern

Atlantic Coastal Plain aquifer system ....................................................................................27

13. Schematic diagram showing the effects of pumping on groundwater flow in the

Northern Atlantic Coastal Plain aquifer system....................................................................28

14. Map showing the distribution of aquifer recharge for the Northern Atlantic 

Coastal Plain aquifer system ....................................................................................................29

15. Map showing the distribution of sewer and wastewater systems in 2005 in the

Northern Atlantic Coastal Plain................................................................................................30

16. Map showing the distribution of major streams and rivers that flow across the

Northern Atlantic Coastal Plain..............................................................................................32

17. Schematic cross-section showing groundwater flow near the Fall Zone of the

Northern Atlantic Coastal Plain................................................................................................33
vii

18. Chart showing the distribution of drinking water sources by State across the

Northern Atlantic Coast Plain...................................................................................................34

19. Charts showing the distribution of groundwater use by State across the Northern

Atlantic Coast Plain ....................................................................................................................36

20. Charts showing the distribution of groundwater withdrawals in 2005 by aquifer 

and by State in the Northern Atlantic Coastal Plain.............................................................37

21. Schematic showing hydrologic budget in millions of gallons per day (Mgal/d) for 

A, predevelopment (1900) and B, conditions in 2005 for the Northern Atlantic 

Coastal Plain aquifer system ....................................................................................................38

22. Plots showing observed and predicted changes in temperature and precipitation

over time for the Northern Atlantic Coastal Plain aquifer system.....................................40

23. Maps showing water levels in Kings and Queens Counties, New York, in A, 1903, 

B, 1936, C, 1951, and D, 2006......................................................................................................42

24. Map showing water-level recovery in east-central New Jersey in late 1980s and 

the surface contours of the top of the Middle Potomac-Raritan-Magothy (PRM) 

and Lloyd aquifers, Sandy Hook, New Jersey, and Coney Island, New York ..................44

25. Plots showing changes in water levels in the local Lloyd and Middle

Potomac-Raritan-Magothy (PRM) aquifers and changes in pumping in the 

A, Lloyd aquifer, B, Middle Potomac-Raritan-Magothy (PRM) aquifer, C, Lloyd 

and Middle Potomac-Raritan-Magothy (PRM) aquifers combined, and D, the Lloyd 

and Magothy aquifers combined .............................................................................................45

26. Hydrograph showing effects of pumping on groundwater levels in southeastern

Virginia from 1960 through 2012 ..............................................................................................46

27. Graph showing groundwater withdrawal rates from the Northern Atlantic Coastal

Plain aquifer system from 1900 to 2010 ...................................................................................46

28. Plot showing seasonal changes in groundwater withdrawals from 1985 through 

2005 in the Northern Atlantic Coastal Plain aquifer system..............................................47

29. Plot showing water-level response to increased sewering in Nassau County, 

New York, from 1940 through 2010..........................................................................................48

30. Map showing chloride distribution in the regional Potomac-Patapsco aquifer of 

the Northern Atlantic Coastal Plain aquifer system ............................................................49

31. Map showing trends in sea-level rise across the Northern Atlantic Coastal Plain

from 1902 to 2006.........................................................................................................................50

Tables

1. Mean annual inflows and outflows for major river systems in the Northern 

Atlantic Coastal Plain (NACP) for conditions in 2005............................................................33

2. Distribution of drinking water sources by State across the Northern Atlantic 

Coastal Plain................................................................................................................................35

3. Distribution of groundwater use by State across the Northern Atlantic 

Coastal Plain................................................................................................................................35

4. Percentage of groundwater withdrawals in 2005 by aquifer and by State in the

Northern Atlantic Coastal Plain................................................................................................36
viii

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2) 

Volume

gallon (gal) 0.003785 cubic meter (m3) 

million gallons (Mgal) 3,785 cubic meter (m3)

Flow rate

foot per day (ft/d) 0.3048 meter per day (m/d)

gallon per minute (gal/min) 0.06309 liter per second (L/s)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day (m/d)

Transmissivity*

foot squared per day (ft2/d) 0.09290 meter squared per day (m2/d) 

Leakance

inch per year per foot [(in/yr)/ft] 83.33 millimeter per year per meter [(mm/yr)/m]

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

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

(NGVD 29) and the North American Vertical Datum of 1988 (NAVD 88).

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.

Conversion Factors and Datum

Abbreviations

ASCII American Standard Code for Information Interchange 

NACP Northern Atlantic Coastal Plain

NAD 83 North American Datum of 1983

NGVD 29 National Geodetic Vertical Datum of 1929

NAVD 88 North American Vertical Datum of 1988

NHDPlus National Hydrography Dataset Plus

RASA Regional Aquifer Systems Analysis

SWB Soil-water balance

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey
Hydrogeology and Hydrologic Conditions of the Northern 

Atlantic Coastal Plain Aquifer System From Long Island, 

New York, to North Carolina

By John P. Masterson, Jason P. Pope, Jack Monti, Jr., Mark R. Nardi, Jason S. Finkelstein, and Kurt J. McCoy

part of the Potomac Formation now is considered part of the 

regional Potomac-Patapsco aquifer. This aquifer includes the 

Middle Potomac-Raritan-Magothy aquifer in New Jersey and 

the Lloyd aquifer on Long Island.

The name “Upper Potomac aquifer” has been removed 

as part of this regional framework revision. The local aquifer 

previously considered part of the Upper Potomac aquifer 

now are part of the regional Magothy aquifer. These units 

include the Upper Potomac-Raritan-Magothy aquifer in New 

Jersey, the Magothy aquifers on Long Island, Delaware, and 

Maryland, and the Virginia Beach aquifer in Virginia.

Updates to the regional hydrologic budget include revised 

estimates of aquifer recharge, water use and streamflow data. 

Inflow to the aquifer system of about 20,000 million gallons 

per day (Mgal/d) includes 19,600 Mgal/d from recharge 

from precipitation, 200 Mgal/d of recharge from wastewater 

via onsite domestic septic systems, and 200 Mgal/d from 

the release of water from aquifer storage. Outflow from the 

aquifer system includes groundwater discharge to streams 

(11,900 Mgal/d), groundwater withdrawals (1,500 Mgal/d), 

and groundwater discharge to coastal waters (6,600 Mgal/d). 

A numerical modeling analysis is required to improve this 

hydrologic budget calculation and to forecast future changes 

in water levels and aquifer storage caused by groundwater 

withdrawals, land-use changes, and the effects of climate 

variability and change.

Introduction

The Northern Atlantic Coastal Plain (NACP) aquifer 

system extends from Long Island, New York, to northeastern 

North Carolina (fig. 1), and includes aquifers primarily 

within New York, New Jersey, Delaware, Maryland, and 

Virginia. Although, the NACP aquifer system is one of the 

smallest of the 66 principal aquifer systems in the Nation 

recognized by the U.S. Geological Survey (USGS; Miller, 

2000), it ranks 13th overall in terms of total groundwater 

withdrawals (Reilly and others, 2008). Despite abundant 

precipitation [about 45 inches per year (in/yr)], the supply of 

Abstract

The seaward-dipping sedimentary wedge that underlies 

the Northern Atlantic Coastal Plain forms a complex 

groundwater system. This major source of water provides 

for public and domestic supply and serves as a vital source 

of freshwater for industrial and agricultural uses throughout 

the region. Population increases and land-use and climate 

changes, however, have led to competing demands for water. 

The regional response of the aquifer system to these stresses 

poses regional challenges for water-resources management at 

the State level because hydrologic effects often extend beyond 

State boundaries. In response to these challenges, the U.S. 

Geological Survey Groundwater Resources Program began 

a regional assessment of the groundwater availability of the 

Northern Atlantic Coastal Plain aquifer system in 2010.

The initial phase of this investigation included a 

refinement of the hydrogeologic framework and an updated 

hydrologic budget of this aquifer system from the last regional 

aquifer system assessment completed by the U.S. Geological 

Survey in the 1980s. Refinements to the hydrogeologic 

framework include revision of the regional aquifer names 

to be more consistent with local names in New York, New 

Jersey, Delaware, Maryland, and Virginia, the primary States 

included in the study area. Other revisions to the framework 

include characterization of the aquifers of the regional 

Potomac aquifer system. The regional Potomac aquifer system 

is subdivided for this report into two regional aquifers. These 

aquifers include the single Potomac aquifer in Virginia and 

two aquifers in Maryland, Delaware, and New Jersey, where 

the Potomac aquifer system thickens within the Salisbury 

Embayment. The two regional aquifers making up the 

Potomac aquifer system include the Potomac-Patapsco aquifer 

and the underlying Potomac-Patuxent aquifer.

The Potomac-Patuxent aquifer includes the Lower 

Potomac-Raritan-Magothy aquifer in southern New Jersey and 

the Patuxent aquifers in Delaware and Maryland. In northern 

New Jersey and on Long Island, New York, the PotomacPatuxent aquifer is absent, but the Late Cretaceous fluvialdeltaic aquifer that is laterally equivalent with the upper 
2 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Figure 1. Location and extent of the Northern Atlantic Coastal Plain study area.

Long Island

Trenton

New York City

Philadelphia

Dover Baltimore

Annapolis

Richmond

Greenville

Cape

May

Virginia

Beach

-77° -72°

41°

36°

DC

Base from U.S. Geological Survey digital data 1:2,000,000 series

Albers Equal Area Conic projection

Standard Parallels 34° 30' N and 41° 30' N; central meridian 75° 30' W 

North American Datum of 1983

Chesapeake Bay

Delaware Bay

Long Island Sound

Hu dson Riv er

Delaware River

Susquehanna

Potomac

James River

Tar River

Roanoke River

Schuykill

MA

RI

CONNECTICUT

NEW YORK

NEW

JERSEY

PENNSYLVANIA

VIRGINIA

WEST

MARYLAND

VIRGINIA

DELAWARE

NORTH CAROLINA

Pamlico Sound

Albemarle

Sound

ATLANTIC OCEAN

Continental Shelf margin

Fall Zone

River

River

River

0 25 50 100 MILES

0 25 50 100 KILOMETERS

EXPLANATION

Altitude, in feet above

 or below NAVD 88 

High : 0

Low : -7,105

Bathymetry 

Topography

High : 563

Low : 1

Study area

Study area
Introduction 3

fresh surface water in this region is limited because many of 

the coastal surface waters in this area are brackish estuaries. 

As a result, many communities in the NACP rely heavily 

on groundwater to meet their water demand. Water supply, 

however, can be limited by the amount of available drawdown, 

drought, saltwater intrusion, and agricultural and industrial 

contamination. Some communities also rely on surface water 

imported from outside the NACP, which, after used, may be 

discharged into shallow aquifers, streams, or coastal waters.

Increases in population and changes in land use during 

the past 100 years have resulted in diverse increased demands 

for fresh water throughout the NACP. Substantial groundwater 

withdrawals had begun in the northern part of the study area 

by the late 1800s. By 1900, about 100 Mgal/d of water was 

pumped from the NACP aquifer system, about half of which 

was from western Long Island, to provide for the New York 

City water-supply system (Buxton and Shernoff, 1999).

Groundwater serves as a vital source of drinking water 

for the nearly 20 million people who live in the region. 

Densely populated areas within the NACP (fig. 2) are 

generally those with the largest groundwater withdrawals 

(fig. 3) and, therefore, are most susceptible to effects from 

withdrawal over time. Total groundwater withdrawal in 2005 

was estimated to be about 1,500 Mgal/d and accounts for 

about 40 percent of the drinking water supply for the NACP 

(Kenny and others, 2009).

Water levels in many of the confined NACP aquifers 

are declining by up to 2 feet per year (ft/yr) in response to 

extensive development and subsequent increased withdrawals 

throughout the region. Total declines are more than a hundred 

feet in some aquifers from their predevelopment (1900) levels 

(fig. 4A–B; DePaul and others, 2008). In some areas, such as 

southeastern Virginia, declines greater than 200 feet (ft) result 

in water levels approximately 200 ft below the mean sea-level 

altitude relative to the National Geodetic Vertical Datum of 

1929 (NGVD 29; Heywood and Pope, 2009).

Water-level declines also extend across State lines 

and under the Chesapeake and Delaware Bays, creating the 

potential for interstate aquifer management issues. Regional 

water-resources managers in the NACP face challenges 

beyond those imposed by the competing local domestic, 

industrial, agricultural, and environmental demands for water. 

Large changes in regional water use have made the State-level 

management of aquifer resources increasingly more difficult 

because of hydrologic effects that extend beyond State 

boundaries. Understanding how groundwater flow is affected 

regionally by natural and human stresses is vital to managing 

and protecting the water resources of the NACP. Therefore, a 

comprehensive assessment of water availability in the NACP 

groundwater system is needed.

In 2010, the USGS Groundwater Resources Program 

began a 4-year regional assessment of the groundwater 

availability of the NACP aquifer system as part of its ongoing 

regional assessments of groundwater availability of the 

principal aquifers of the Nation (U.S. Geological Survey, 

2013). The primary goal of these regional assessments is 

to 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).

Location and Physical Setting

The portion of the NACP included in this investigation 

occupies a land area of more than 30,000 square miles (mi2) 

along the eastern seaboard of the United States from Long 

Island, New York, southward to the northeastern part of 

North Carolina (fig. 1). A seaward-dipping wedge of mostly 

unconsolidated stratified sediments comprising clay, silt, 

sand, and gravel underlies this area. This sedimentary wedge 

forms a complex groundwater system in which strata of sand 

and gravel function predominantly as aquifers, and those of 

silt and clay, as confining units. The aquifers of the NACP 

are major sources of water for public and domestic supply 

and serve as a vital source of freshwater for industrial and 

agricultural uses throughout the region.

Land use and land cover compose primarily a mosaic 

of forest, wetlands, and agriculture (Loveland and others, 

1999). Dominant land uses are farming and forestry, with 

locally dense urban development. More than half (57 percent) 

of the NACP area is undeveloped, nearly a third (30 percent) 

is agricultural, and 13 percent is developed. Land use varies 

from north to south with Long Island mostly (65 percent) 

developed, more than half (53 percent) of Delaware is 

agricultural, and Virginia is mostly (64 percent) undeveloped 

(fig. 5; Fry and others, 2011).

Land cover from the early 1900s has trended toward 

conversion of open space and agricultural land to residential, 

industrial, and commercial development (Brown and others, 

1972). The amount of land conversion was greatest from 1973 

to 2000 when forested lands and wetlands were converted for 

residential and urban uses throughout the NACP. Although 

total agricultural land cover has not changed appreciably, 

substantial changes in crop types have increased water-use 

demands (Loveland and others, 1999).

Purpose and Scope

Recent studies from the last USGS regional assessment 

conducted in the 1980s (Trapp and Meisler, 1992) have led 

to significant, but generally non-coordinated refinements of 

the hydrostratigraphic framework and hydrologic budgets 

throughout the NACP. These studies have provided local-scale 

improvements in the understanding of the groundwater-flow 

system, but do not provide a regional perspective of prevailing 

hydrogeologic conditions. This report synthesizes local-scale 

efforts as part of an updated regional characterization of 

the hydrogeologic conditions of the NACP aquifer system. 

This updated characterization includes (1) revisions to 

regional hydrogeologic units, (2) an updated hydrologic 

budget for conditions in 2005, and (3) use of local-scale 
4 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

studies to highlight issues related to changes in groundwater 

withdrawals, wastewater return flow, and the potential effects 

of climate change on groundwater flow in the NACP.

Refinements to the hydrogeologic framework include 

reclassification of the local aquifers of the regional Potomac 

aquifer system to reflect the more recent understanding of 

the extent and geometry of these aquifers across State lines 

and throughout the region. This refinement also includes a 

revision of the regional aquifer names more consistent with 

local names in New York, New Jersey, Delaware, Maryland, 

and Virginia. The study area for this investigation includes 

only the northeasternmost part of North Carolina because the 

main scope of this analysis is from Virginia to Long Island. 

A detailed investigation of the hydrogeologic framework and 

groundwater availability of the coastal plain of North Carolina 

was completed recently as part of the regional groundwater 

availability study for the Atlantic Coastal Plain of North and 

South Carolina (Campbell and Coes, 2010).

The regional hydrologic budget presented in this report 

includes a new estimate of aquifer recharge rates across 

the NACP from 2005 through 2009, updated estimates of 

streamflow, and a synthesis of water use for conditions in 

2005. The new methodology used to calculate recharge 

is documented in appendix 1 of this report. Streamflows 

were totaled by hydrologic basin using the USGS National 

Hydrography Dataset Plus (NHDPlus) attribute data in order 

to characterize the amount of surface water entering the 

NACP and the amount of groundwater discharging to streams 

leaving the NACP. Groundwater withdrawals and wastewater 

return flow estimates were compiled for conditions based on a 

detailed analysis of data from the USGS National Water-Use 

Information Program for 2005 (Kenny and others, 2009) and 

the U.S. census.

Examples of previous local-scale studies are presented 

in this report to help describe the hydrogeologic conditions 

throughout the NACP and to highlight local-scale or 

statewide issues that have the potential to affect the regional 

groundwater system in the future. These examples include the 

response of the groundwater system to large-scale pumping 

stresses in western Long Island, New Jersey, and Virginia 

from changes in public-supply and industrial uses throughout 

these areas. An example of changes in wastewater returnflow 

on Long Island also is presented to show how this change has 

affected water levels and streamflows and to illustrate how 

issues facing more urbanized areas may arise in less developed 

parts of the NACP with increased development.

Figure 2. Population density by county in the area of the Northern Atlantic Coastal Plain aquifer system for A, 2005 and 

B, changes in population density from 1985 to 2005. 

EXPLANATION

Change in population density

 between 1985 and 2005,

 in population per square mile

Study area

-1,661 to -1,000

-999 to -500

-499 to -250

-249 to -100

-99 to -1

0

1

2 to 100

101 to 250

251 to 500

1,001 to 3,056

501 to 1,000

QUEENS

KINGS COUNTY

COUNTY

39°

36°

-78° -75° -72°

39°

36°

-78° -75° -72°

Base from U.S. Geological Survey

digital data 1:2,000,000 series

Albers Equal Area Conic projection

Standard Parallels 34° 30' N and

41° 30' N; central median 75° 30' W

North American Datum of 1983

Base from U.S. Geological Survey

digital data 1:2,000,000 series

Albers Equal Area Conic projection

Standard Parallels 34° 30' N and

41° 30' N; central median 75° 30' W

North American Datum of 1983

EXPLANATION

Population density in 2005,

 in population per square mile

0 to 250

> 250 to 500

> 500 to 1,000

> 1,000 to 2,500

> 2,500 to 5,000

> 5,000 to 10,000

> 10,000 to 20,000

> 20,000 to 35,000

> 35,000 to 50,000

Study area

Albemarle

Sound

Pamlico Sound

ATLANTIC OCEAN NORTH CAROLINA DELAWARE

VIRGINIA

WEST

PENNSYLVANIA

VIRGINIA

MARYLAND

NEW

JERSEY

NEW

YORK CT

Delaware Bay

Chesapeake Bay

Long Island Sound

Albemarle

Sound

Pamlico Sound

ATLANTIC OCEAN

NORTH

CAROLINA

DELAWARE

DC

VIRGINIA

WEST

PENNSYLVANIA

VIRGINIA

MARYLAND

NEW

JERSEY

NEW

YORK

CT

Delaware Bay

Chesapeake Bay

LongIsland Sound

A B

QUEENS

KINGS COUNTY

COUNTY

DC

0 50 100 KILOMETERS

0 50 100 MILES

0 50 100 KILOMETERS

0 50 100 MILES
Introduction 5

Figure 3. Groundwater withdrawals by county in the Northern Atlantic Coastal Plain for 2005.

Chesapeake Bay

Delaware Bay

Long Island Sound

NEW CT

YORK

NEW

JERSEY

PENNSYLVANIA

MARYLAND VIRGINIA

WEST

DC

DELAWARE

VIRGINIA

NORTH CAROLINA

ATLANTIC OCEAN

Pamlico Sound

Albemarle Sound

0 35 70 140 KILOMETERS

0 25 50 100 MILES

-78° -72°

39°

36°

EXPLANATION

Total groundwater withdrawn in 2005,

 in million gallons per day (Mgal/d)

1 to 25

26 to 50

51 to 75

76 to 100

101 to 125

126 to 150

151 to 175

176 to 200

Study area

Base from U.S. Geological Survey digital data 1:2,000,000 series

Albers Equal Area Conic projection

Standard Parallels 34° 30' N and 41° 30' N; central meridian 75° 30' W 

North American Datum of 1983
6 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Figure 4. Change in water levels in response to groundwater withdrawals in the Potomac-Patapsco regional aquifer of the 

Northern Atlantic Coastal Plain aquifer system for A, 1900 to 1980 and B, 1980 to 2000. Modified from DePaul and others (2008).

Chesapeake Bay

ATLANTIC OCEAN

Delaware Bay

Albemarle Sound

Long Island Sound

NORTH CAROLINA

NEW

JERSEY

MARYLAND

VIRGINIA

NEW 

YORK

Raleigh

Durham

Fayetteville

Goldsboro

Wilmington

Rocky 

Mount

Greenville

Suffolk

Newport 

News

Norfolk

Virginia Beach

Richmond

Alexandria

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

-76°

-75°

-74°

-73°

-72°

42°

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 

Water-level rises,

 in feet 

EXPLANATION

Greater than 10 to 20 

Greater than 20 to 40 

Greater than 40 

Chesapeake Bay

ATLANTIC OCEAN

Delaware Bay

Albemarle Sound

Long Island Sound

Pamlico Sound

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

Potomac-Patapsco

Regional aquifer

NORTH CAROLINA

NEW

JERSEY

MARYLAND

VIRGINIA

NEW 

YORK

Raleigh

Durham

Fayetteville

Goldsboro

Wilmington

Rocky 

Mount

Greenville

Suffolk

Newport 

News

Norfolk

Virginia Beach

Richmond

Alexandria

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

42°

41°

40°

39°

38°

37°

36°

35°

-76°

-75°

-74°

-73°

-72°

80°

79°

78°

42°

0 25 50

0 25 50 100 MILES

100 KILOMETERS

42°

41°

40°

39°

38°

37°

36°

35°

80°

79°

78°

l

N

l

N

Control point

Potomac-Patapsco

Regional aquifer

Control point

DC DC

Fall Zone

Fall Zone

A B

Pamlico Sound

Study area
Introduction 7

Figure 5. Land use and land cover for the Northern Atlantic Coastal Plain for conditions in 2006. Modified from Fry and 

others (2011).

-78° -72°

39°

36°

Albemarle Sound

Pamlico Sound

ATLANTIC OCEAN

Chesapeake Bay

Delaware Bay

Long Island Sound

0

0 35 70 140 KILOMETERS

25 50 100 MILES

Developed

Undeveloped

Agricultural

EXPLANATION

Study area

New York

New Jersey

Maryland

Delaware

Virginia

Percentage of land usage

in NACP study area

North Carolina

Base from U.S. Geological Survey digital data 1:2,000,000 series

Albers Equal Area Conic projection

Standard Parallels 34° 30' N and 41° 30' N; central median 75° 30' W

North American Datum of 1983

NORTH CAROLINA

DELAWARE

DC

VIRGINIA

WEST

PENNSYLVANIA

VIRGINIA

MARYLAND

NEW

JERSEY

NEW

YORK

CT
8 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Hydrogeology

The NACP is underlain by a wedge of unconsolidated 

to partially consolidated sediments that range in age from 

Early Cretaceous to Holocene (fig. 6). These sediments 

unconformably overlie a basement of Precambrian to 

Paleozoic-age consolidated bedrock. The western limit of 

the Coastal Plain is the Fall Zone, the transition between the 

igneous and metamorphic rocks of the Piedmont Province 

and the sedimentary environment of the Coastal Plain (fig. 1). 

The coastal plain sedimentary wedge is aligned approximately 

parallel to the Fall Zone and dips and thickens to the east and 

south (fig. 7).

Sediments in the NACP are typically thousands of feet 

thick along the coastline with a maximum thickness of about 

10,000 ft beneath Cape Hatteras, North Carolina, at the 

southern edge of the study area (fig. 1). These sediments are 

several miles thick where they terminate to the east along the 

edge of the Continental Shelf. Coastal Plain sediments are 

continuous approximately from Newfoundland in the north 

to Honduras in the south, covering the Continental Shelf, but 

they are entirely submerged north of Cape Cod (Trapp, 1992). 

The eastern end of Long Island is considered the northern limit 

of their continuous exposure (fig. 1).

Previous Investigations

The USGS Regional Aquifer Systems Analysis 

(RASA) investigation of the NACP aquifer system, which 

was conducted from 1978 through 1987, provided a 

detailed review and summary of the literature describing 

the hydrogeologic setting of this region (Trapp, 1992). The 

RASA investigation (1) provided a summary of the regional 

geologic history, (2) described stratigraphic correlations 

across the States of this region, and (3) outlined the regional 

stratigraphic nomenclature.

The report by Trapp (1992) also presented a regional 

hydrogeologic framework for the NACP, which composed 

10 aquifers and 9 confining units, synthesized from Statelevel investigations for Long Island (Smolensky and others, 

1989), Virginia (Meng and Harsh, 1988), New Jersey 

(Zapecza, 1989), North Carolina (Winner and Coble, 1989), 

and Delaware and Maryland (Vroblesky and Fleck, 1991). 

The detailed summary and analysis of the hydrogeologic 

framework presented by Trapp is outlined only briefly in this 

report, with further emphasis on revisions to that framework 

from more recent studies.

During the past two decades, continued geologic and 

hydrogeologic investigations in the NACP by the USGS 

and cooperating agencies have built upon the information 

contained in the previous RASA reports. New information 

and interpretations have resulted in refinement and revision of 

the geology and the related hydrogeologic framework of the 

region. Many of these framework revisions concern relatively 

minor adjustments in altitudes of hydrogeologic units based 

Figure 6. Geologic time scale for the Northern Atlantic Coastal 

Plain aquifer system.

ERA/EON PERIOD EPOCH

Cenozoic

Quaternary

Tertiary

Pliocene

Late

Miocene

Late

Middle

Oligocene

Eocene

Late

Early

Paleocene

Late

Early

Mesozoic

Cretaceous

Late

Early

Holocene

Pleistocene

Early

Early

Late

Early

Middle

Paleozoic

Proterozoic

Jurassic

Triassic
Hydrogeology 9

on acquisition of additional data. However, groundwater 

investigations at the local, State, and subregional scale have 

increasingly recognized that understanding groundwater 

conditions at many locations in an interconnected system like 

the NACP often requires a regional approach that transcends 

jurisdictional boundaries. Consequently, recent State-level 

groundwater investigations in Virginia (McFarland and Bruce, 

2006; Heywood and Pope, 2009; McFarland, 2013), Maryland 

(Andreasen and others, 2013), and New Jersey (Martha 

Watt, U.S. Geological Survey, written commun., 2011) have 

involved substantial efforts to incorporate hydrogeologic 

data and interpretations from neighboring States. The refined 

interpretations from these studies have been immensely 

valuable in the creation of an updated regional hydrogeologic 

framework for the NACP.

In addition to the minor framework revisions mentioned 

above, recent investigations since the previous regional 

assessment have provided substantial advancements in the 

general understanding of geology and hydrogeology in the 

NACP. The most substantial revisions have resulted from the 

discovery of the buried Chesapeake Bay impact crater near 

the mouth of the Chesapeake Bay in southeastern Virginia 

(Powars and Bruce, 1999; Powars, 2000; McFarland and 

Bruce, 2006). Recent and ongoing work in the Virginia 

Coastal Plain also has resulted in a revised understanding 

of the Potomac aquifer system, the largest and most 

productive aquifer system in the NACP. This aquifer system 

is now considered a single heterogeneous aquifer (Potomac 

aquifer) in most of Virginia, except for an area along the 

Maryland border where confining units subdivide the 

system (McFarland, 2013). This new work has altered the 

previous understanding of the regional correlations between 

the Potomac and overlying Cretaceous-age aquifers across 

southern Maryland, Virginia, and northeastern North Carolina 

in the southern half of the NACP study area. These major 

revisions to the regional hydrogeology are incorporated 

now into the updated understanding of the regional 

hydrogeologic conditions.

Depositional History

Deposition of the sedimentary wedge forming the 

Continental Shelf and the Atlantic Coastal Plain began 

during the Triassic and Jurassic Periods with the opening 

Figure 7. Generalized section showing Northern Atlantic Coastal Plain aquifer system from the Fall Zone to the coast. From 

Leahy and Martin (1993).

EXPLANATION

Bedrock

Confining unit

Saline groundwater

Confined aquifer

Unconfined aquifer

Direction of groundwater flow

 under predevelopment conditions

Saprolite

ATLANTIC

OCEAN

SEA LEVEL

FEET

500 Fall Zone

−500

−1,000

−1,500

WEST EAST

10 20 KILOMETERS 

0 10 20 MILES

0
10 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

of the Atlantic Ocean in the rift zone separating the North 

American continent from the previous supercontinent (Trapp 

and Meisler, 1992). Since that time, sedimentary development 

of the NACP has continued along the edge of the North 

American continent with periods of deposition and erosion 

driven by global sea-level fluctuations across a tectonically 

dynamic region.

The basement surface, upon which NACP sediments 

have been deposited, generally dips eastward and southward 

from the Piedmont Province toward the coast and the edge 

of the Continental Shelf. However, a series of broad regional 

inflections in this sloping surface have contributed to 

substantial regional variability in the depositional environment 

across the NACP and have partially controlled the distribution 

and thickness of sedimentary units (McFarland and Bruce, 

2006). These inflections typically are described as a series 

of basins separated by intervening bedrock highs, also called 

arches or platforms (fig. 8; Owens and Gohn, 1985).

From north to south within the study area, the highs in the 

basement surface include the Long Island Platform, the South 

New Jersey Arch, the Norfolk Arch, and the Cape Fear Arch, 

whereas the lows in the basement surface include the Raritan, 

Salisbury, and Albemarle Embayments (fig. 8). The most 

substantial inflections in the basement surface that control 

the large-scale geologic structure of the NACP are associated 

with the Long Island Platform, a composite Salisbury-Raritan 

Embayment, and a composite Cape Fear-Norfolk Arch (Owens 

and Gohn, 1985).

Aside from these large regional features, the continental 

margin has been tectonically and structurally dynamic since 

its formation, as indicated by varied and shifting patterns of 

deposition among sedimentary units of different ages and 

spatial extents (McFarland and Bruce, 2006). This additional 

complexity is reflected in subregional variability in some 

geologic units that is not fully explained by the combination 

of seaward subsidence of the continental margin or by the 

pattern of topographic highs and lows in the bedrock surface. 

Nonetheless, the primary controls on the structure and 

characteristics of sedimentary deposits in the NACP appear to 

be (1) sea-level fluctuations driven by changes in climate and 

(2) geographic variability in the geometry of the depositional 

basin from the Early Cretaceous to present.

In Virginia, NACP sediments were altered dramatically 

during the late Eocene by the impact of an asteroid or comet 

near the mouth of the Chesapeake Bay (fig. 8; Powars and 

Bruce, 1999). The Virginia Coastal Plain was inundated at the 

time by a marine transgression, and the impact occurred in 

a shallow-shelf environment (Powars and Bruce, 1999). The 

resulting Chesapeake Bay impact crater is more than 50 miles 

(mi) in diameter and penetrates the sequence of preexisting 

NACP sediments to basement (fig. 8).

The crater contains a mixture of material deposited 

immediately after the impact. The crater fill contains slumped 

blocks of pre-impact sediments at the crater margin and a 

chaotic mix of impact-related sediments with a thickness 

reaching several thousands of feet within the crater, including 

variously sized clasts of displaced pre-impact sediments 

within a variable textured matrix of disaggregated and poorly 

sorted sands, silts, and clays (McFarland and Bruce, 2006). As 

marine sedimentation continued in the late Eocene, deposition 

into the submerged basin on the Continental Shelf left atop 

the previous crater-fill materials a layer of up to about 200 ft 

of low permeability, gray-brown silts and clays composing 

the Chickahominy Formation (McFarland and Bruce, 2006). 

Subsequent sediment deposition on the coastal plain of 

Virginia from the Eocene to the present has buried the crater 

and the crater-fill sediments about 1,000 ft below the presentday land surface.

Recognition of the recently discovered impact crater has 

led to a revised understanding of structural and stratigraphic 

relations among sediments deposited after the impact, 

including features anomalous to surrounding coastal plain 

depositional patterns (Powars, 2000; McFarland and Bruce, 

2006). The effects of the impact crater on the regional geology 

are an area of active research. The interior of the buried 

impact crater is known now to contain brines, and horizontal 

and vertical chloride concentration patterns in and around the 

crater impact area reflect complex structure and extremely 

varied hydraulic properties associated with impact-affected 

sediments (McFarland, 2010).

In areas outside the Chesapeake Bay impact crater 

zone, widespread sedimentary deposition on the NACP in 

the late Eocene through the early Miocene continued in a 

cyclical pattern controlled by marine transgressions and 

regressions (Trapp, 1992). Marine deposition later in the 

Miocene was more episodic and spatially variable, probably 

as the result of tectonic movement and evolving and shifting 

depositional environments.

During the Pleistocene Epoch of the Quaternary Period 

(fig. 6), NACP sediments on Long Island were affected 

substantially by several episodes of glaciation. Tertiary 

sediments of marine origin are absent onshore, and thick 

glacial deposits directly overlie severely eroded Cretaceous 

sediments (Smolensky and others, 1989). The glacial deposits 

crop out at land surface over most of Long Island, and two 

east-west moraine ridges and a gradually southward-sloping 

outwash plain dominate the topography of Long Island 

(Smolensky and others, 1989).

Much of the modern landscape of the NACP south of 

Long Island also was formed during the Pleistocene from 

repeated depositional and erosional events as coastal areas 

were alternately submerged and exposed during closely spaced 

sea-level fluctuations associated with continental glaciation. 

Three broad physiographic bands have been identified, each 

with a characteristic geomorphology related to its formation 

(Ator and others, 2005).

A band of relatively high relief along the Fall Zone from 

New Jersey to North Carolina contains some of the oldest 

and most deeply weathered landscapes of the Atlantic Coastal 

Plain (Ator and others, 2005). This area is underlain primarily 

by subcrops and outcrops of Cretaceous sediments near the 

western edge of their extent. The landscape exhibits a deeply 
Hydrogeology 11

Figure 8. Schematic diagram showing the structural setting and inferred sediment sources for the Northern Atlantic 

continental margin (modified from Poag, 1998).

Bedrock contour—Shows altitude of

bedrock surface. Contour interval 1,000 feet.

Datum is National Geodetic Vertical Datum

of 1929

-77° -72°

41°

36°

Base from U.S. Geological Survey digital data 1:2,000,000 series

Albers Equal Area Conic projection

Standard Parallels 34° 30' N and 41° 30' N; central meridian 75° 30' W 

North American Datum of 1983

Albemarle Sound

Pamlico Sound

Chesapeake Bay

ATLANTIC OCEAN

NORTH CAROLINA

DELAWARE

WEST

VIRGINIA

PENNSYLVANIA

VIRGINIA

MARYLAND

NEW

JERSEY

NEW

YORK CONNECTICUT 

Delaware Bay

Long Island Sound

RI

MA

Long Island

Trenton

New York

 City

Philadelphia

Baltimore Dover

Annapolis

Richmond

Greenville

Cape

May

Virginia

Beach

Impact

crater

site

APPALACHIAN MOUNTAINS

Erosion

Erosion

Erosion

Long Island

Platform

Edge of Continental Shel

W

f

estern edge of coastal plain

DC

Erosion

0 25 50 100 MILES

0 25 50 100 KILOMETERS

EOCENE ATLANTIC OCEAN 

Raritan Embayment

Salisbury Embayment

Albemarle Embayment

Cape Fear Arch

Norfolk Arch

South New Jersey Arch

Raritan Embayment

Salisbury Embayment

Albemarle Embayment

Cape Fear Arch

Norfolk Arch

South New Jersey Arch

-8,000

-2,000

-3,000

-6,000

0

-4,000

-1,000

-7,000

-5,000

-8,000

-2,000

-3,000

-6,000

0

-4,000

-1,000

-7,000

-5,000

EXPLANATION
12 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

incised drainage network, often with underfit streams in wide 

valleys. Land-surface altitudes in this band of high relief 

approach several hundred feet, the highest altitudes in the 

study area.

East of the Fall Zone is a broad, seaward-sloping plain 

bounded to the northwest by several scarps along the Inner 

Coastal Plain and to the southeast by the Suffolk Scarp (Ator 

and others, 2005). This plain is subdivided by several less 

prominent scarps into a series of stair-step terraces with 

progressively lower altitudes toward the coast. These terraces 

were formed by repeated transgressions and regressions of sea 

level from the Pliocene through the Pleistocene.

Lowland areas closest to the modern coastline include 

modern barrier islands and lagoons along the Atlantic Ocean, 

extensive tidal marshes along the coastal bays, and estuarine 

terraces that parallel the lower reaches of major rivers (Ator 

and others, 2005). This is an area of extremely low relief, 

with a poorly developed stream network, and the topographic 

boundary with the terraces to the west is very prominent. This 

lowland constitutes only a narrow fringe along the coast and 

major bays in the northern part of the study area but is more 

extensive east of the Suffolk Scarp in North Carolina and 

southeastern Virginia where it ranges from about 30 to 50 mi 

in width.

The sediments comprising the terraces and floodplains 

of major rivers throughout the NACP were deposited during 

the Holocene by fluvial and estuarine processes (Ator and 

others 2005). These landscapes are characterized as being 

of low relief and consisting of poorly drained soils. Marine 

transgression has dominated in the Holocene, drowning 

the lower reaches of the alluvial valleys (Trapp, 1992). 

Throughout most of the study area, the river valleys—

including the Hudson, Delaware, and Susquehanna—are tidal 

and estuarine to the Fall Zone (Ator and others, 2005). The 

rivers in the far southern part of the study area, south of the 

James River in Virginia, are wider and less deeply incised 

than the northern rivers and are nontidal about halfway across 

the NACP.

Northern Atlantic Coastal Plain Hydrologic Units

The sediments of the NACP have been divided for this 

study into 10 regional aquifers (fig. 9) and 9 regional confining 

units. These divisions are based on similarities and differences 

in hydrologic characteristics resulting from geologic origins 

of the units. The regional hydrogeologic units defined here are 

typically groupings of aquifers or confining units previously 

recognized at the State or local scale, with correlations 

across State boundaries based primarily on the continuity 

of hydraulic permeability. These regional delineations are 

based on previous work summarized in the RASA report on 

the NACP hydrogeologic framework (Trapp, 1992), with 

substantial updates interpreted from the numerous reports 

published during the two decades since the publication of 

the RASA report (Smolensky and others, 1989; Zapecza, 

1989; McFarland and Bruce, 2006; Gellici and Lautier, 2010; 

Andreasen and others, 2013; McFarland, 2013).

The revised regional aquifers are summarized in figure 9, 

and the units are described briefly below. The names of some 

of the regional aquifer units from the RASA report have been 

revised to reflect a decreased emphasis on the hydrogeology 

of the NACP in North Carolina given the change in the areal 

extent of the study area compared with the previous regional 

assessment in this area, as described in the Introduction (Trapp 

and Meisler, 1992). Consequently, several regional names 

have been changed to remove references to units in North 

Carolina and to emphasize nomenclature from the primary 

States of the study area: New York, New Jersey, Delaware, 

Maryland, and Virginia. The hydrogeologic units of North 

Carolina are included in this report to enable a description and 

analysis of hydrogeologic conditions across the Virginia-North 

Carolina border.

Revised names and delineations for other units reflect 

substantial revisions in the understanding of hydrogeologic 

correlations across the study area. A primary example is the 

reclassification of the aquifers that were considered part of the 

regional Potomac aquifer system, which are now considered 

a single aquifer unit in most of Virginia, four local aquifers 

in Maryland and Delaware, two local aquifers in New 

Jersey, and a single local aquifer in New York. These local 

aquifers have been classified into two regional aquifers, the 

Potomac-Patapsco and Potomac-Patuxent aquifers. These 

new designations approximately correspond to the Middle 

Potomac and Lower Potomac regional aquifers of Trapp 

(1992). Permeable sediments previously classified as the 

Upper Potomac aquifer have been regrouped with either 

the Potomac-Patapsco or the Magothy regional aquifers in 

this report. In another example, the revised nomenclature 

of the Monmouth-Mount Laurel aquifer reflects improved 

understanding of the local units composing this regional 

aquifer, as well as the elimination of the North Carolina 

reference from the regional name.

The other substantial changes from the previous regional 

study reflect recently improved understanding of the transition 

in geology and hydrogeology in southeastern Virginia 

and across the border with North Carolina. Whereas these 

revisions include alterations in geometry of hydrogeologic 

units and revisions to regional correlations with units in 

North Carolina, including the Lower Cretaceous, Cape Fear, 

and Black Creek aquifers, no changes were made in aquifer 

nomenclature.

The final substantive revision to the hydrogeologic 

framework of the NACP discussed in this report is the 

recognition of the Chesapeake Bay impact crater, which 

includes three hydrogeologic units described by McFarland 

and Bruce (2006)—the Chickahominy confining unit, 

the Exmore matrix confining unit, and the Exmore clast 

confining unit—that comprise the crater-fill and crater-cap 

sediments. These units do not affect regional hydrogeologic 

delineations beyond the vicinity of the crater, but it should 

be recognized that these units truncate and replace all units 
Figure 9. Correlation chart showing regional aquifer names and subregional aquifer names for the Northern Atlantic Coastal Plain (NACP) aquifer s

York are from Smolensky and others (1989); data for New Jersey are from Zapecza (1989); data for Delaware and Maryland are from Andreasen and oVirginia are from McFarland and Bruce (2006); and data for North Carolina are from Gellici and Lautier (2010) and McFarland (2013); *, aquifer and asare truncated in part of Virginia by sediments related to the Chesapeake Bay impact crater.

Surficial Upper Glacial Surficial Surficial Surficial 

Upper Chesapeake

absent

Lower Chesapeake Choptank, Calvert Saint Marys 

Mount Laurel Monmouth Peedee 

Virginia Beach Magothy Magothy Magothy Magothy

Potomac-Patapsco Lloyd Potomac

(undivided)*

absent

Piney Point

Aquia

Monmouth-Mount

Laurel

Matawan

Surficial

(Holly Beach)

Upper KirkwoodCohansey

Lower KirkwoodCohansey and

Confined Kirkwood

Piney Point

Vincentown

Wenonah-Mount

Laurel

Englishtown

Upper PotomacRaritan-Magothy 

Middle PotomacRaritan-Magothy

Lower PotomacRaritan-Magothy

Pocomoke,

Manokin 

Milford, Frederica,

Federalsburg,

Cheswold

Piney Point

Rancocas

Englishtown

Upper Patapsco,

Lower Patapsco

Patuxent,

Waste Gate

Pocomoke, Ocean

City, Manokin 

Piney Point

Aquia

Matawan

Upper Patapsco,

Lower Patapsco

Patuxent,

Waste Gate

YorktownEastover

Piney Point

Aquia*

NACP aquifer New Jersey Delaware Maryland Virginia 

New York (Long Island)

Potomac-Patuxent
14 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

older than the regional Piney Point aquifer within the impact 

area (fig. 10D). These crater-fill materials are extremely 

impermeable, represent a substantial discontinuity to the units 

they have replaced, and have been shown to influence regional 

groundwater flow in several important ways (McFarland and 

Bruce, 2006; Heywood and Pope, 2009).

Fluvial Deltaic Cretaceous Units in the Potomac 

Aquifer System

Unconsolidated Cretaceous-age sediments of fluvialdeltaic origin comprise an aquifer system that is the thickest, 

deepest, and most important source of groundwater throughout 

the NACP. This Potomac aquifer system (Potomac-Patuxent 

and Potomac-Patapsco aquifers, fig. 9) overlies basement 

bedrock throughout the NACP and ranges in thickness from 

a thin edge in the Fall Zone to thousands of feet offshore 

along the Atlantic Continental Shelf (fig. 10). The Potomac 

aquifer system takes its name from the Potomac Formation 

or Potomac Group ranging in age from Early Cretaceous to 

early Late Cretaceous Period. Consequently, the regional 

aquifers given the Potomac designation in this report include 

only fluvial-deltaic sediments of the Potomac Formation or 

the Potomac Group. These include sediments previously 

delineated as the Middle Potomac and Lower Potomac 

aquifers in the last regional study of this system (Trapp, 

1992). For the purpose of this investigation, the Lloyd aquifer 

of Long Island, as the Lloyd Sand Member of the Raritan 

Formation, is considered part of the regional PotomacPatapsco aquifer.

The highly heterogeneous sediments comprising the 

Potomac aquifer system have been subdivided in different 

parts of the NACP, depending on the presence and recognition 

of confining units or zones that are interpreted to define 

regionally continuous aquifers (fig. 9). In the previous RASA 

of the NACP aquifer system, three regional subdivisions of 

the Potomac aquifer system were recognized in Maryland, 

Virginia, and North Carolina: the Upper Potomac aquifer, 

the Middle Potomac aquifer, and the Lower Potomac aquifer 

(Trapp and Meisler, 1992; Leahy and Martin, 1993).

Two decades of subsequent investigation in Virginia and 

adjacent States have revealed that these three subdivisions 

are no longer valid in this area because of the absence of 

regionally extensive confining units or zones. Instead, a 

single, undivided Potomac aquifer is now recognized for the 

Virginia Coastal Plain, except for the area along the border 

with Maryland (fig. 11; McFarland and Bruce, 2006), and 

correlations with the Potomac aquifer system in adjacent 

States are better understood (McFarland, 2013). The Upper 

Potomac aquifer as previously defined by Meng and Harsh 

(1988) in Virginia and by Trapp and Meisler (1992) for the 

NACP no longer is recognized in Virginia or elsewhere 

(McFarland and Bruce, 2006). The top of the Potomac aquifer 

in Virginia correlates with the top of the Lower Cretaceous 

aquifer in North Carolina, and both units are overlain in 

southeastern Virginia and northeastern North Carolina by 

the distinct and regionally continuous upper Cenomanian 

confining unit (fig. 11; McFarland and Bruce, 2006; 

McFarland, 2013).

The top of the Potomac aquifer in Virginia now correlates 

with the top of the Upper Patapsco aquifer in Maryland, 

where the Potomac aquifer system includes the Upper 

Patapsco, Lower Patapsco, Patuxent, and Waste Gate aquifers 

(figs. 9, 11). This subdivision of the Potomac aquifer system 

into separate aquifers begins in northern Virginia south of the 

Potomac River with the occurrence of confining units as the 

system thickens into the Salisbury Embayment (McFarland, 

2013). However, the extent and geometry of these confining 

units have not yet been delineated in Virginia and remain 

uncertain. In Delaware, the Potomac aquifer system is 

considered to be a single aquifer with multiple zones (Benson, 

2006); however, in a regional refinement of the hydrogeologic 

framework in Maryland and Delaware, the division (Upper 

Patapsco, Lower Patapsco, Patuxent, and Waste Gate aquifers) 

of the Potomac aquifer system was extended north and east 

across Delaware (Andreasen and others, 2013). For this report, 

the local aquifers recognized in Delaware, Maryland, and 

northern Virginia are grouped regionally into the PotomacPatapsco and the Potomac-Patuxent aquifers, with the local 

Upper and Lower Patapsco aquifers grouped into a single 

regional Potomac-Patapsco aquifer (fig. 9).

In New Jersey, the Potomac aquifer system is subdivided 

into the Middle Potomac-Raritan-Magothy and Lower 

Potomac-Raritan-Magothy aquifers (figs. 9, 10B). These 

subdivisions of the Potomac aquifer system in Maryland and 

New Jersey are herein referred to as the regional PotomacPatapsco and Potomac-Patuxent aquifers. Farther north, the 

Potomac-Patapsco aquifer correlates with the Lloyd aquifer, 

which is the lowermost aquifer in Long Island (figs. 9, 10A). 

Possibly, because of the local upward slope in the bedrock 

surface (fig. 8), the Potomac-Patuxent aquifer was not 

deposited and therefore is not present in Long Island.

Potomac-Patuxent Aquifer

The Potomac-Patuxent aquifer is the lowermost 

(depth) regional aquifer of the NACP. This regional aquifer 

includes the Lower Cretaceous aquifer of North Carolina, the 

undifferentiated Potomac aquifer of Virginia, the Patuxent and 

Waste Gate aquifers of Maryland and Delaware, and the lower 

aquifer of the Potomac-Raritan-Magothy aquifer system of 

New Jersey (fig. 9). The Potomac-Patuxent aquifer is absent 

beneath Long Island (fig. 10B).

The Potomac-Patuxent aquifer is primarily of Early 

Cretaceous age and of fluvial-deltaic origin. It consists 

primarily of lenses of medium- to coarse-grained quartz sand 

with some gravel interbedded with lenses of clay and silt. It is 

highly heterogeneous, and the distribution of less permeable 

interbeds is highly variable, apparently controlled by regional 
Figure 10. Cross-sections A, A–A′ on Long Island, New York, B, B–B′ in New Jersey, C, C–C′ in Delaware and Maryland, and D, D–D′ in Virginia, sho

regional variations in the hydrogeologic framework of the Northern Atlantic Coastal Plain aquifer system. Cross-section D–D′ includes the impact craChesapeake Bay.

Area of saline groundwFreshwater aquifers (sBedrockFractures

EXPLANATION

Confining unitDirection of groundwa under predevelopme

Contact—Dashed whe

0 10 20 30 40

0 10 20 30 40 50 60 70

0

0 25 50 

25 

Fall Zone

Edge of Continen

41°39°37°

 35°

-78° -76° -74° 

B

B'

C

C'

D D'

A

A'

PA

NC

VA

NY

NJ

WV MD

DE

Area of impa

Fall Zone

B B'

Land surface

Kirkwood-Cohansey 

aquifer system ATLANTIC OCEAN

200

SEA LEVEL

−200−400−600−800

−1,000−1,200−1,400−1,600

FEET

200SEA LEVEL−200−400−600−800−1,000−1,200−1,400−1,600

FEET

WEST EAST

NORTH SOUTH

Lloyd aquifer

Magothy aquifer

Upper glacial aquifer

Gardiners clay Jameco aquifer ATLANTIC OCEAN

200

SEA LEVEL

−200−400−600−800

−1,000−1,200−1,400

200

FEET FEET

SEA LEVEL−200−400−600−800−1,000−1,200−1,400

A A'

LongIslandSound

A

Altitude in feet, relative to NGVD 29

VERTICAL SCALE GREATLY EXAGGERATED

Distance in miles Modified from Buxton and Shernoff (1999)

Altitude in feet, relative to NGVD29

Modified from DePaul and others (2008)

VERTICAL SCALE GREATLY EXAGGERATED

Distance in miles

B

aquifer

Raritan confining unit

Englishtown aquifer system

Vincentown aquifer 

Wenonah-Mount Laurel aquifer

Middle Potomac-Raritan-Magothy aquifer 

Upper Potomac-Raritan-Magothy aquifer
Figure 10. Cross-sections A, A–A′ on Long Island, New York, B, B–B′ in New Jersey, C, C–C′ in Delaware and Maryland, and D, D–D′ in Virginia, showing the regional variations in the hydrogeologic framework of the Northern Atlantic Coastal Plain aquifer system. Cross-section D–D′ includes the impact crater in the Chesapeake Bay. Area of saline groundwater Freshwater aquifer Bedrock Saprolite Fractures EXPLANATION Confining unit Direction of groundwater flow under predevelopment conditions Contact—Dashed where inferred

D D'

Aquia

aquifer

Yorktown-Eastover aquifer

Suffolk scarp Surficial aquifer

Yorktown confining zone

Nanjemoy-Marlboro confining unit

Potomac confining zone

Potomac aquifer

Calvert confining unit

Piney Point aquifer

Fall Zone

Exmore Matrix confining unit

Chickahominy 

confining unit

Exmore 

Clast 

confining 

unint

Exmore Matrix confining unit

Chickahominy 

confining unit

Chesapeake Bay 

impact crater

500250

−500−750−250

−1,000−1,250−1,500

500250

−500−750−250

−1,000−1,250−1,500

Chesapeake Bay

SEA LEVEL SEA LEVEL

FEET FEET

WEST EAST

Ocean City aquifer

Manokin aquifer

Surficial aquifer

Pocomoke aquifer

Surficial Upland aquifer

Chesapeake Bay

500

−500

−1,000−1,500−2,000−2,500

−3,000−3,500−4,000−4,500−5,000−5,500−6,000−6,500−7,000

SEA LEVEL

500−500−1,000−1,500−2,000−2,500−3,000−3,500−4,000−4,500−5,000−5,500−6,000−6,500−7,000SEA LEVEL

Upper Patapsco aquifer system

Magothy-Patapsco confining unit

Piney Point aquifer

Calvert aquifer system

Nan

Calvert confining unit

jemoy confining unit Marlboro Clay confining unit

Aquia aquifer

C C' FEET FEET

WEST EAST

Exmore 

Clast 

confining 

unint

Altitude in feet, relative to NGVD 29

Distance in miles

VERTICAL SCALE GREATLY EXAGGERATED

Modified from Andreasen

and others, (2013)

Upper Chesapeake

confining unit

Altitude in feet, relative to NGVD29

Distance in miles

VERTICAL SCALE GREATLY EXAGGERATED

Modified from McFarland and Bruce (2006)

0

0 25 50 75 100 KILOMETERS

25 50 75 100 MILES

Fall Zone

Edge of Continental Shelf

41°39°37°

 35°

-78° -76° -74° -72°

B

B'

C

C'

D D'

A

A'

PA

NC

VA

NY

NJ

CT

RI

WV MD

DE

MA

Area of impact crater

0 10 20 30 40 50 60 70 80 90 100 110 120

0 10 20 30 40 50 60 70 80 90

C

D

Lower Patapsco aquifer system

Patuxent aquifer system

Waste Gate aquifer system

Choptank aquifer

Mataw

Lower Chesapeake confining unit

an-Mag

M

oth

a

y confining unit

tawan aquifer

Magothy aquifer Monmouth aquifer

Lower Patapsco aquifer system

Patuxent aquifer system

Waste Gate aquifer system

Lower Ch Choptank aquifer

Mataw

esapeake confining unit

an-Mag

M

oth

a

y confining unit

tawan aquifer

Magothy aquifer Monmouth aquifer

Severn confining unit Severn confining unit

Matawan confining unit Matawan confining unit
Figure 11. Cross-section showing distribution of the variable geometry and sediment composition of the Potomac aquifer system across the southern half of the Northern Atlantic Coastal Plain.

EXPLANATION

Contact—Dashed where inferred

0 5 10 20 30 KILOMETERS

0 5 10 20 MILES

VERTICAL EXAGGERATION x80

other hydrogeologic units

SEA LEVEL

-500

-1,000

-1,500

500

-2,000

Upper Cretaceous sediments

Basement bedrock

Potomac aquifer

Hope Plantation Oak Grove James River Potomac River

E E'

SOUTH NORTH

Chowan

River

Albemarle Norfolk archembayment

Salisburyembayment

upper Cenomanianconfining unit 

Patuxent aquifer

Lower Patapsco aquifer

Arundel Clay

confining unit

Upper Patapsco

aquifer

West

Franklin Williamsburg Point

Patapsco confining unit

Fault line

NORTH CAROLINA | VIRGINIA VIRGINIA | MARYLAND 

41°39°37°

 35°

-78° -76° -74° -72°

Geologic cross section from McFarland (2013)

Fall Zone Edge of Continental Shelf

PA

NC

VA

NY

NJ

CT

RI

WV MD

DE

MA

E

E' Salisbury embayment

Norfolk arch

00 25 50 75 100 KILOMETERS

25 50 75 100 MILES

Area of impact crater
18 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

variations in the depositional environments under which the 

sediments were deposited (McFarland, 2013). The proportion 

of fine-grained sediments increases downdip toward the coast 

as well as laterally toward the Salisbury Embayment. The 

Potomac-Patuxent aquifer is a thick, productive aquifer and 

an important source of groundwater throughout most of its 

extent, except where it contains salty groundwater in downdip 

areas near the Atlantic coastline. The wedge shape of the 

aquifer is thinnest in the Fall Zone where it pinches out against 

bedrock of the Piedmont Province and thickest at and beyond 

the Atlantic coastline. The Potomac-Patuxent aquifer reaches 

a thickness of more than 1,000 ft in Maryland and Delaware 

(Trapp and Meisler, 1992). The aquifer thins northward in 

New Jersey where the average thickness penetrated by wells, 

mostly in the thinner updip section, is about 300 ft.

The Potomac-Patuxent aquifer (where present) is overlain 

by a confining unit separating this aquifer from the overlying 

Potomac-Patapsco aquifer. This regional confining unit, called 

the Potomac confining unit in the RASA study and known 

as the Arundel Clay confining unit in Maryland, composes 

hard clays and silts within the fluvial-deltaic Potomac and 

Raritan formations in New Jersey, Delaware, Maryland, and 

northern Virginia (Trapp, 1992). The confining unit ranges in 

thickness from a thin western edge in the Fall Zone to more 

than 1,000 ft along the Atlantic coast in Delaware and pinches 

out along the flank of the Norfolk Arch in northern Virginia, 

though this southern boundary of the unit is not yet well 

defined (McFarland, 2013). South of the southern edge of the 

confining unit, the Potomac-Patuxent regional aquifer is no 

longer differentiated from the overlying Potomac-Patapsco 

regional aquifer, and the Potomac aquifer system is considered 

to be a single heterogeneous aquifer.

Potomac-Patapsco Aquifer

The Potomac-Patapsco regional aquifer includes the 

Lloyd aquifer in New York (Long Island), the middle aquifer 

of the Potomac-Raritan-Magothy aquifer system in New 

Jersey, the Upper and Lower Patapsco aquifers of Delaware 

and Maryland, the undifferentiated Potomac aquifer of 

Virginia, and the Lower Cretaceous aquifer in North Carolina 

(fig. 9). In Maryland and in northern Virginia, the PotomacPatapsco regional aquifer consists of two local aquifers, 

the Upper Patapsco and Lower Patapsco, separated by an 

intervening confining unit (fig. 9; Andreasen and others, 2013). 

In most of Virginia and North Carolina, the Potomac aquifer 

system is considered to be a single heterogeneous aquifer, and 

the Potomac-Patapsco regional aquifer is not differentiated 

from the underlying Potomac-Patuxent regional aquifer.

The Potomac-Patapsco aquifer is similar to the 

underlying Potomac-Patuxent aquifer in that it consists 

primarily of lenses of medium- to coarse-grained quartz 

sand with some gravel, interbedded with lenses of clay and 

silt. This aquifer is highly heterogeneous, and the hydraulic 

properties vary considerably with the proportion of finegrained sediments. It is composed of fluvial-deltaic sediments 

of primarily Early Cretaceous age in Maryland and Delaware, 

and Late Cretaceous age in New Jersey and New York.

The Potomac-Patapsco regional aquifer is a thick, 

productive aquifer and an important source of groundwater 

throughout most of its extent. Similar to the underlying 

Potomac-Patuxent aquifer, the wedge shape of the aquifer 

is thinnest near the Fall Zone where it pinches out against 

bedrock of the Piedmont Province and thickest at and beyond 

the Atlantic coastline. In a north-south direction, the regional 

Potomac-Patapsco aquifer is thickest in southern Maryland 

(referred to locally as the Upper and Lower Patapsco aquifer 

system) where it is more than 500-ft thick (fig. 11). The 

aquifer thins northward into New Jersey and is thinnest 

in New York where the average onshore thickness of the 

Lloyd aquifer is less than 300 ft (fig. 10A; Smolensky and 

others, 1989).

In Virginia, the fluvial-deltaic sediments of Early 

Cretaceous age have been delineated as a single Potomac 

aquifer except in northern Virginia (McFarland, 2013). The 

Potomac aquifer is entirely truncated within the area of the 

Chesapeake Bay impact crater by crater-fill sediments (fig. 

10D) and contains tilted and faulted layers of fluvial-deltaic 

sediments at the edges of the impact zone (McFarland and 

Bruce, 2006). The crater-fill sediments are hydrogeologically 

quite distinct and are considered separately from the other 

regional hydrogeologic units discussed in this report.

The thickness of the undifferentiated Potomac and 

Lower Cretaceous aquifers in Virginia and North Carolina 

reaches about 1,500 ft, although the depth of production 

wells penetrating the aquifer is much less (Trapp and Meisler, 

1992). The average depth of production wells in this aquifer 

ranges from about 225 ft on Long Island to as much as 

900 ft in Maryland. The Potomac-Patapsco aquifer appears 

to be a somewhat more transmissive and productive aquifer 

than the underlying Potomac-Patuxent aquifer (Trapp and 

Meisler, 1992).

The Potomac-Patapsco aquifer is overlain by a regionally 

identified, but locally highly variable confining unit separating 

the primarily fluvial-deltaic Cretaceous units from units that 

are primarily marine in origin. This unit is referred to as the 

Raritan Clay confining unit in New York and New Jersey, the 

Magothy-Patapsco confining unit in Delaware and Maryland, 

the Potomac confining zone in northern and central Virginia, 

and the upper Cenomanian confining unit in southeastern 

Virginia and northeastern North Carolina. The transition from 

the fluvial-deltaic sediments of the Potomac aquifer system 

to the various overlying hydrogeologic units throughout the 

study area has resulted in a confining unit that is extremely 

variable in its properties and thickness (Trapp, 1992). The 

sediments comprising this regional unit in northern and 

central Virginia are referred to as a confining zone rather than 

a confining unit to reflect a high degree of heterogeneity and 

discontinuity, but in southeastern Virginia, the overlying upper 

Cenomanian confining unit is included in the regional unit 

and provides much greater hydrologic separation (McFarland 

and Bruce, 2006). In other parts of the study area, the local 
Hydrogeology 19

sediments comprising this unit also may provide substantial 

hydrologic separation between the regional Potomac-Patapsco 

aquifer and overlying units (Trapp, 1992; Andreasen and 

others, 2013).

Marine Cretaceous Units

After the fluvial-deltaic deposition that formed the 

Potomac aquifer system, a transition to a predominantly 

marine environment resulted in the deposition of sediments 

with substantially greater spatial homogeneity. These 

sediments form three primary aquifers—the Magothy, 

Matawan, and Monmouth-Mount Laurel aquifers—in the 

northern part of the study area (fig. 9), all pinching out in 

southern Maryland on the flank of the Salisbury Embayment 

(figs. 8, 10A, C). These aquifers are absent across almost all 

the Virginia Coastal Plain, likely because of structural control 

provided by the Norfolk Arch. However, other laterally 

discontinuous sediments of similar age are now recognized 

in far southeastern Virginia and form hydrogeologic units 

that appear to dip and thicken southward into the Albemarle 

Embayment and out of the study area to the south.

Magothy Aquifer

The Magothy regional aquifer composes the Magothy 

aquifer of New York, New Jersey, Delaware, and Maryland 

(fig. 9). On Long Island, this regional aquifer also may 

include sediments of the Matawan and Monmouth Groups 

(not included on fig. 9) as well as the Pleistocene-age Jameco 

Gravel and is only partially confined, primarily along the 

southern shore (fig. 10A). The southern boundary of the 

Magothy aquifer is north of the Potomac River in southern 

Maryland. Sediments of similar age and composition are 

identified as the Cape Fear aquifer in northeastern North 

Carolina, but this unit is entirely separate from the Magothy 

aquifer to the north, it is fluvial in deposition, and its 

importance as a groundwater resource is only in areas outside 

of the focus area of this investigation. The Cape Fear aquifer 

of North Carolina may be continuous with other permeable 

units in Virginia, such as the Virginia Beach aquifer, but 

the exact correlation has not been established definitively 

(McFarland, 2013).

The Magothy aquifer composes primarily sandy parts of 

the Magothy Formation, which were deposited in a transitional 

fluvial-marine environment during the Late Cretaceous 

(Andreasen and others, 2013). The Magothy aquifer consists 

of very fine to medium quartz sand, with discontinuous layers 

of carbonaceous clayey silt; it also contains coarse to very 

coarse sand and gravel, particularly in the thicker parts (Trapp, 

1992). The coarsest materials typically are found at the base of 

the aquifer, which fines upward into the clays of the overlying 

confining unit (Andreasen and others, 2013). The aquifer is 

confined except on Long Island and in subcrop areas near the 

western limit of the aquifer in the Fall Zone. Throughout much 

of Long Island, the Magothy aquifer is only partially confined 

or unconfined where it is in contact with surficial glacial 

deposits and may act as a continuation of the surficial aquifer. 

This aquifer is a very important water resource throughout 

most of its areal extent and particularly on Long Island.

The Magothy aquifer is typically less than 200 ft thick 

in New Jersey, Delaware, and Maryland, and intervals 

penetrated by production wells are less than 100 ft thick on 

average (Trapp, 1992). This aquifer is thickest on Long Island, 

however, where it reaches a thickness of more than 1,000 ft 

onshore, with an average of at least 500 ft (Smolensky and 

others, 1989). The average interval penetrated by production 

wells in this aquifer on Long Island is 460 ft (Trapp, 1992).

Except on Long Island, the Magothy regional aquifer 

is overlain by a confining unit that separates it from the 

overlying Matawan aquifer. This regional confining unit 

includes the Merchantville-Woodbury confining unit in 

New Jersey, consisting of glauconitic and micaceous clay 

and silt, and the Matawan-Magothy confining unit in 

Delaware and Maryland, consisting of silt and clay of the 

Magothy Formation and part of the overlying Matawan 

Group or Matawan Formation (Trapp, 1992; Andreasen and 

others, 2013).

Matawan Aquifer

The Matawan aquifer primarily consists of sands 

deposited by a marine transgression during the Late 

Cretaceous, including fine to medium quartz sand in New 

Jersey and fine silty to clayey sand in Delaware and Maryland. 

It may contain glauconite and mica, which distinguishes it 

from the underlying Magothy aquifer (Andreasen and others, 

2013). The Matawan aquifer is confined throughout most of its 

extent, except where it crops out or subcrops in the Fall Zone 

at the western edge of the aquifer. It is a locally important 

water resource in New Jersey and parts of Maryland. The 

regional Matawan aquifer as defined in this report includes 

a northern section in New Jersey, Delaware, and Maryland 

and a laterally discontinuous southern section in southeastern 

Virginia and northeastern North Carolina. The Virginia Beach 

aquifer may also be continuous with the underlying Upper 

and Lower Cape Fear aquifers in North Carolina (Magothy 

regional aquifer), but the nature of this relation has not been 

definitively established (McFarland, 2013).

The northern section of the Matawan aquifer includes the 

Englishtown aquifer in New Jersey, the Englishtown aquifer 

in Delaware, and the Matawan aquifer in Maryland. On 

Long Island, equivalent sediments of the Matawan Group are 

included with the underlying Magothy aquifer (fig. 9) (Trapp 

and Meisler, 1992). As defined, the extent of the Matawan 

aquifer is much larger to the east and south than previously 

indicated by Trapp (1992). The southern limit of the Matawan 

aquifer is north of the Maryland-Virginia State line and east of 

the Chesapeake Bay where it thins along the southern edge of 

the Salisbury Embayment (Andreasen and others, 2013).

In the northern section, the Matawan aquifer is 

overlain by a confining unit that separates it from the 
20 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Monmouth-Mount Laurel aquifer. This section of the regional 

confining unit includes the Marshalltown-Wenonah confining 

unit in New Jersey, composing glauconitic silt and fine sand, 

and the Matawan confining in Delaware and Maryland, 

composing clay and silt (Trapp, 1992).

In the southern part of the study area, sediments of 

similar age and composition to the northern section of the 

Matawan aquifer form a local unit known as the Virginia 

Beach aquifer in southeastern Virginia that may be the thin 

northern edge of the Black Creek aquifer of North Carolina 

(fig. 9) (McFarland and Bruce, 2006). The Virginia Beach and 

Black Creek aquifers are entirely separate, but equivalent to 

the Matawan aquifer of Maryland, Delaware, and New Jersey. 

The southern flank of the Norfolk Arch (fig. 8) controls the 

northern extent of these aquifers. The Virginia Beach aquifer 

is of limited local significance as a water resource, but the 

Black Creek aquifer becomes more important as it thickens 

southward into east-central North Carolina, beyond the focus 

area of this investigation. The Virginia Beach aquifer may also 

be continuous with the underlying Upper and Lower Cape 

Fear aquifers in North Carolina, but the nature of this relation 

has not been established (McFarland, 2013).

The southern section of the Matawan regional aquifer 

in Virginia and North Carolina is overlain by a confining unit 

that separates this aquifer from the Peedee aquifer above. This 

part of the regional confining unit includes the Virginia Beach 

confining zone in Virginia and the Black Creek confining unit 

in North Carolina. This confining unit in Virginia composes 

red beds consisting of sequences of interbedded oxidized 

clay, silty clay, and silty fine sand (McFarland and Bruce, 

2006). The unit’s designation as a confining zone reflects the 

considerable variability in the composition and configuration 

of this unit, as well as the sometimes indistinguishable 

relations between the underlying Virginia Beach aquifer and 

overlying aquifer units. This unit is reported to be as much 

as several tens of feet thick in Virginia and thickens into 

North Carolina.

Monmouth-Mount Laurel Aquifer

The Monmouth-Mount Laurel regional aquifer 

(previously referred to by Trapp and Meisler (1992) as the 

Peedee-Severn aquifer) includes the Wenonah-Mount Laurel 

aquifer in New Jersey, the Mount Laurel aquifer in Delaware, 

and the Monmouth aquifer in Maryland (fig. 9). On Long 

Island, laterally equivalent sediments are included as part of 

the regional Magothy aquifer.

The southern limit of the continuous Monmouth-Mount 

Laurel regional aquifer is north of the Maryland-Virginia State 

line and east of the Chesapeake Bay (Andreasen and others, 

2013). In the southern part of the study area, sediments of 

similar age form a unit known as the Peedee aquifer, which 

is not continuous with rest of the Monmouth-Mount Laurel 

aquifer but is considered part of the same regional aquifer. The 

Peedee aquifer has been identified in southeastern Virginia 

(McFarland and Bruce, 2006) where it is poorly constrained. 

From there, it extends southward into northeastern North 

Carolina, thickening into the Albemarle Embayment (Trapp 

and Meisler, 1992).

The Monmouth-Mount Laurel aquifer is the uppermost 

regional aquifer of Late Cretaceous age in the study area. 

It includes permeable parts of the marine Mount Laurel 

Formation, which is the lower part of the Monmouth Group 

(Andreasen and others, 2013). This aquifer consists of very 

fine to coarse, slightly glauconitic sand in New Jersey and 

fine glauconitic sand in Delaware and Maryland and ranges in 

thickness from about 10 to 120 ft in Delaware and Maryland 

and in southern New Jersey (Zapecza, 1989; Andreasen and 

others, 2013).

The Monmouth-Mount Laurel aquifer is confined 

throughout most of its extent, except where it crops out or 

subcrops near the Fall Zone at its westernmost extent. It is 

a locally developed and moderately productive aquifer in 

New Jersey, Delaware, and eastern Maryland. In the southern 

section of the Monmouth-Mount Laurel regional aquifer, the 

Peedee aquifer is unused as a groundwater resource in Virginia 

(McFarland and Bruce, 2006), but it is a more important 

groundwater resource in North Carolina outside of the focus 

area of this investigation.

The northern section of the Monmouth-Mount Laurel 

regional aquifer is separated from the Aquia regional aquifer 

above by an overlying confining unit that includes the 

Navesink-Hornerstown confining unit in New Jersey and 

the Severn confining unit in Delaware and Maryland. The 

confining unit consists of marine silt, clay, and silty and clayey 

glauconitic sand of primarily Cretaceous age (Trapp, 1992). Its 

thickness is generally less than 100 ft.

The southern section of this regional confining unit 

includes the Peedee confining zone in Virginia and the Peedee 

confining unit in North Carolina. The section’s designation as 

a confining zone in Virginia reflects the variable configuration 

and composition of the sediments above the Peedee aquifer 

and the transition to the overlying Aquia aquifer (McFarland 

and Bruce, 2006). The thickness of this section may be as 

great as several tens of feet in Virginia and up to 50 ft in 

North Carolina (McFarland and Bruce, 2006; Gellici and 

Lautier, 2010).

Marine Tertiary Units

Sediments deposited by continuing cyclical marine 

deposition in the Tertiary comprise aquifers and intervening 

confining units that are relatively homogeneous in composition 

and extend across most of the study area. The Aquia and 

Piney Point aquifers are thickest and most transmissive in 

the Salisbury Embayment, where they are most important to 

groundwater resources.

Aquia Aquifer

The Aquia regional aquifer (previously referred to by 

Trapp and Meisler (1992) as the Beaufort-Aquia aquifer) 
Hydrogeology 21

includes the Vincentown aquifer in New Jersey, the Rancocas 

aquifer in Delaware, the Aquia aquifer in Maryland and 

Virginia, and the Beaufort aquifer in North Carolina (fig. 9). 

The Aquia aquifer is not present on Long Island; it is uncertain 

if Tertiary-age marine sediments were deposited and 

subsequently eroded or never deposited here (Smolensky and 

others, 1989).

The sediments comprising the Aquia aquifer were 

deposited in a relatively uniform marine shallow-shelf 

environment, and therefore, this aquifer is widespread and 

relatively homogeneous throughout the study area. These 

sediments are reported to exist off the southeastern shore 

of Long Island in continuation with their extent in eastern 

New Jersey (Trapp, 1992). The Aquia aquifer forms only a 

narrow band in eastern New Jersey but is much more laterally 

extensive elsewhere, particularly in Maryland, near the 

Salisbury Embayment (fig. 8).

The Aquia aquifer composes permeable marine sediments 

of Paleocene age and consists primarily of medium- to coarsegrained glauconitic and fossiliferous quartz sands (Trapp, 

1992). In Virginia, the sediments that fill the Chesapeake 

Bay impact crater truncate the Aquia aquifer (McFarland 

and Bruce, 2006). In North Carolina and southern Virginia, 

the aquifer also includes thin shell and limestone beds. It is 

confined throughout most of its extent, except where it crops 

out or subcrops near the Fall Zone in a very narrow band at 

the western edge of the aquifer. The Aquia aquifer is most 

important as a groundwater resource in Maryland where it is 

as much as 250 ft thick (Andreasen and others, 2013).

The Aquia aquifer is overlain over most of its extent by 

a confining unit that separates it from the Piney Point aquifer 

above. The regional confining unit includes the VincentownManasquan confining unit in New Jersey; the NanjemoyMarlboro Clay confining unit in Delaware, Maryland, and 

Virginia; and the Beaufort confining unit in North Carolina. 

The confining unit overlaying the Aquia aquifer is made of 

marine silt, clay, and sandy clay ranging from a thickness of 

50 ft in North Carolina to more than 900 ft in New Jersey 

(Trapp, 1992). In contrast to the highly variable composition 

and configuration of several of the Cretaceous-age confining 

units, this regional confining unit extends over almost all the 

study area and substantially impedes regional groundwater 

flow (Trapp, 1992; McFarland and Bruce, 2006).

Piney Point Aquifer

The Piney Point regional aquifer (previously referred 

to by Trapp and Meisler (1992) as the Castle Hayne-Piney 

Point aquifer) includes the Piney Point aquifer of New Jersey, 

Delaware, Maryland, and Virginia, as well as the Castle 

Hayne aquifer of North Carolina (fig. 9). Composing geologic 

formations deposited in relatively uniform shallow-shelf and 

marginal marine conditions along the Continental Shelf, the 

Piney Point aquifer is somewhat consistent and homogeneous 

across the study area, though it is absent (truncated by younger 

units) along the western margin of the NACP and across Long 

Island (Trapp and Meisler, 1992).

The Piney Point aquifer consists of marine sediments of 

mostly Eocene to Oligocene age, though it also may include 

sediments of Miocene age in some locations (Trapp and 

Meisler, 1992). This aquifer generally consists of mostly 

glauconitic sand with shells from New Jersey to Virginia and 

limestone, sandy marl, and limey sand in North Carolina. 

However, a productive section of the Piney Point aquifer north 

of the James River and south of the Potomac River in Virginia 

composes calcite-cemented sands and moldic limestone 

(McFarland and Bruce, 2006). The Piney Point aquifer 

consists of sediments deposited both before and after the 

Chesapeake Bay impact, so its geometry and composition are 

altered by the presence of the impact crater; the Piney Point 

aquifer extends across the impact zone.

The part of the aquifer penetrated by production wells 

is thinnest in Virginia where it averages about 45 ft and 

thickest in Maryland and Delaware where it averages about 

120 ft (Trapp and Meisler, 1992). The aquifer is confined 

over its extent except for very limited subcrop areas along its 

western edge in the Fall Zone or where truncated by major 

river valleys. It is an important groundwater resource in New 

Jersey, Delaware, and Maryland. It is a moderately used 

resource in Virginia, but only over the consolidated part. The 

part of North Carolina where it is an important water resource 

is located mostly south of Albemarle Sound and outside the 

study area of this report.

The confining unit overlying the Piney Point regional 

aquifer and separating it from the Lower Chesapeake aquifer 

above includes the Basal Kirkwood confining unit in New 

Jersey, the Calvert confining unit in Delaware, Maryland, 

and Virginia, and the Castle Hayne confining unit in North 

Carolina. It consists primarily of marine clay and sandy clay 

of Miocene age, and its thickness increases northward from 

less than 50 ft in North Carolina to a range of 100 to 250 ft 

over the northern half of the study area (Trapp, 1992). As with 

the aquifers it separates, this confining unit is absent in New 

York. Otherwise, this unit extends across almost the entire 

study area and impedes groundwater flow regionally.

Chesapeake Bay Impact Crater and 

Associated Units

The Chesapeake Bay impact crater has had a profound 

effect on the hydrogeology of the NACP in southeastern 

Virginia (fig. 8). Consequently, an entirely separate description 

of the hydrogeologic units associated with the impact crater is 

warranted here, and special consideration of the crater area is 

required in the regional analysis of groundwater flow. Aquifer 

and confining unit sediments deposited before the impact 

were entirely truncated by the sediments filling the crater 

and altered around the crater edges (McFarland and Bruce, 

2006). The crater-fill sediments now are buried under about 

1,000 ft of sediments deposited in the 35 million years since 
22 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

the impact. They extend from a depth of about 1,000 ft to a 

maximum depth of about 6,000 ft where they directly overlie 

the basement bedrock in the inner part of the crater.

The previously described Potomac aquifer system 

(Potomac-Patuxent and Potomac-Patapsco regional aquifers) 

and the overlying confining unit are present within the outer 

part of the crater, but these units are disrupted at least partially 

by the crater (Powars and Bruce, 1999). Previously deposited 

sedimentary units within the inner part of the crater were 

removed entirely by the impact, which also substantially 

altered the regional bedrock basement. Most of the buried 

crater is filled by a chaotic mass referred to as tsunami-breccia 

that ranges in thickness from hundreds of feet around the 

outer part of the crater to almost 5,000 ft near the center. The 

sediments associated with the crater, aside from remnants 

of the regional aquifer and confining unit, are unique to 

the impact area and are limited to Virginia (McFarland and 

Bruce, 2006).

The Exmore clast confining unit is the lowermost unit 

of crater-fill material and extends across the entire impact 

crater area depicted in figure 8. The confining unit consists 

primarily of boulder-sized clasts of formations older than 

the impact; voids between the clasts are filled with poorly 

sorted sands, silts, and clays of various ages (McFarland and 

Bruce, 2006). In the inner part of the crater, the Exmore clast 

confining unit directly overlies the altered bedrock basement 

and reaches its maximum thickness of more than 4,500 ft. 

The Exmore clast confining unit overlies the local Potomac 

confining zone across most of the crater area, which in turn, 

overlies the Potomac aquifer. The portion of the Exmore clast 

confining unit that is outside the inner crater is several hundred 

feet thick on average and reaches a maximum thickness of 

almost 2,000 ft. The Exmore matrix confining unit directly 

overlies the Exmore clast confining unit over its entire areal 

extent within the impact crater area (fig. 1). It consists of 

pebble- to cobble-sized clasts of older sediments within a 

matrix of poorly sorted sands, silts, and clays. It reaches a 

maximum thickness of about 200 ft (McFarland and Bruce, 

2006). This unit is differentiated from the underlying Exmore 

clast confining unit by the relative abundance of matrix 

material. The Exmore clast and Exmore matrix confining 

units, composing primarily tsunami-breccia deposited during 

and after the impact are overlain by the Chickahominy 

confining unit, which composes primarily dense and very 

fine-grained silts and clays deposited in an abyssal basin 

on the Continental Shelf following the impact event. The 

Chickahominy confining unit is up to 200 ft thick and overlies 

the Exmore matrix confining unit throughout its extent, but 

also extends well beyond the impact area to the north and 

east, to approximately the Virginia-Maryland State line on the 

Eastern Shore (fig. 1).

The impact-related sediments described and identified as 

confining units by McFarland and Bruce (2006) individually 

and collectively impede horizontal and groundwater flow 

throughout the impact crater area. Information on the 

hydraulic properties of these units is limited because these 

units have only recently been identified, but hydraulic 

conductivity is known to vary widely with the highly variable 

composition and configuration of these units. The crater-fill 

sediments have been shown to substantially affect patterns of 

groundwater flow in the coastal plain of southeastern Virginia, 

especially under 2003 withdrawal conditions (Heywood and 

Pope, 2009). Nonetheless, the physical configuration of the 

impact crater and the hydraulic effects of the crater sediments 

continue to be the subject of active research.

Transitional Tertiary and Quaternary Units

The Lower Chesapeake and Upper Chesapeake regional 

aquifers described in this section compose sediments of 

Miocene and Pliocene (Tertiary) age. These aquifers are 

described here with the Quaternary units because they 

represent a transition from shallow marine to mixed estuarine 

and fluvial deposition; this transition is reflected in the 

increasingly heterogeneous character of the aquifer units they 

comprise. In addition, these units may, in some locations, 

form the surficial aquifer or be included with undifferentiated 

sediments forming the surficial aquifer.

On Long Island, the oldest unit in this group is the 

Gardiners Clay confining unit, which locally separates the 

surficial Upper Glacial aquifer from the underlying Magothy 

aquifer. The Tertiary units are not present on Long Island, 

suggesting that they were never deposited here or deposited 

and subsequently removed by erosion (Smolensky and 

others, 1989).

Lower Chesapeake Aquifer

The Lower Chesapeake regional aquifer includes the 

Lower Kirkwood-Cohansey aquifer system in New Jersey, 

the Milford, Frederica, Federalsburg, and Cheswold local 

aquifers in Delaware, the Choptank and Calvert local aquifers 

in Maryland, the Saint Marys aquifer in Virginia, and the 

Pungo River aquifer in North Carolina (fig. 9). The Lower 

Chesapeake aquifer is absent on Long Island (Smolensky 

and others, 1989). In Maryland, it is limited to the Delmarva 

Peninsula and a small section west of the Chesapeake Bay 

(Andreasen and others, 2013). In Virginia, it is mostly absent 

west of the Chesapeake Bay except for a small portion 

south of the James River (McFarland and Bruce, 2006). The 

southern part of the Lower Chesapeake aquifer, which is in 

North Carolina and south of the James River in Virginia, is 

entirely separate and discontinuous from the northern part of 

the aquifer.

The Lower Chesapeake aquifer consists primarily of 

marine sands ranging in age from Oligocene to Pliocene. In 

New Jersey, the aquifer includes interbedded sand and gravel 

(Trapp, 1992). In Maryland and Delaware, permeable zones 

of shelly sand are separated by less permeable silt and clay 

zones. The Lower Chesapeake aquifer includes fine, shelly 

sands in Virginia and phosphatic sands and limestone beds in 

North Carolina.
Hydrogeology 23

The Lower Chesapeake aquifer is confined over most 

of its extent, but it includes up-dip unconfined sections 

in New Jersey, Delaware, and Maryland where overlying 

confining units are absent and the aquifer is in direct hydraulic 

connection with the surficial aquifer (Andreasen and others, 

2013). The Lower Chesapeake aquifer is thickest in the 

Salisbury Embayment in Maryland and Delaware where its 

average thickness is about 300 ft. In New Jersey, the average 

thickness of the Lower Chesapeake aquifer is about 200 ft 

(Trapp, 1992). The Lower Chesapeake aquifer thins to the 

north and west. The southern section in Virginia and North 

Carolina is typically less than 50 ft thick (McFarland and 

Bruce, 2006; Trapp, 1992). This aquifer is an important 

groundwater supply in New Jersey, Delaware, and Maryland 

east of the Chesapeake Bay, but is sparsely used elsewhere 

(Martin, 1998; Andreasen and others, 2013).

The regional confining unit overlying the Lower 

Chesapeake aquifer includes an unnamed confining unit in 

New Jersey, the Saint Marys confining unit in Delaware, 

Maryland, and Virginia, and the Pungo River confining unit 

in North Carolina. This Miocene unit primarily composes 

silt and clay but is diatomaceous in New Jersey and silty and 

shelly in Delaware, Maryland, and Virginia (Trapp, 1992). The 

thickness of the unnamed confining unit is about 100 ft across 

much of its extent but may be as great as several hundred feet 

(Trapp, 1992; McFarland and Bruce, 2006). Despite some 

spatial variability in its configuration, this confining unit 

serves as a regional impediment to groundwater flow over a 

large part of its extent.

Upper Chesapeake Aquifer

The Upper Chesapeake aquifer includes the Upper 

Kirkwood-Cohansey aquifer in New Jersey, the Pocomoke and 

Manokin aquifers in Delaware, the Pocomoke, Ocean City, 

and Manokin aquifers in Maryland, the Yorktown-Eastover 

aquifer in Virginia, and the Yorktown aquifer in North 

Carolina (fig. 9). The Upper Chesapeake aquifer is not present 

on Long Island.

This aquifer consists of permeable sediments of the 

upper part of the Miocene to Pliocene-age Chesapeake Group. 

The aquifer composes primarily sands of marine origin in 

North Carolina and Virginia but transitions northward to New 

Jersey into coarser sands and gravels of fluvial origin (Trapp, 

1992). The permeable sands of the Upper Chesapeake aquifer 

contain substantial interbeds of less permeable silt and clay, 

particularly in Delaware, Maryland, and Virginia.

This regional aquifer is confined over most of its extent 

by an overlying clay layer and forms the uppermost confined 

aquifer in the NACP aquifer system (Trapp and Meisler, 

1992). The Upper Chesapeake aquifer subcrops beneath the 

surficial aquifer along the thin western margin of the surficial 

aquifer and where the overlying confining unit has been 

incised by major river valleys. The average thickness of the 

Upper Chesapeake aquifer ranges from less than 100 ft in 

North Carolina to about 400 ft in Maryland and Delaware and 

is between 100 and 200 ft in New Jersey and Virginia (Trapp, 

1992). It is a moderately important water resource over much 

of its extent.

Surficial fluvial-deltaic sediments, possibly of the Lower 

Chesapeake and Upper Chesapeake aquifers, were included 

in previous studies (Meng and Harsh, 1988; Trapp, 1992) 

in an unconfined part of the Yorktown-Eastover aquifer in 

the northwestern coastal plain of Virginia. These sediments 

are now considered part of the surficial aquifer in Virginia 

(McFarland and Bruce, 2006) and are included in the regional 

Surficial aquifer based on their hydraulic connection with 

other parts of the unconfined Surficial aquifer system. In 

contrast, unconfined Surficial sediments in the northwestern 

part of the Upper Kirkwood-Cohansey aquifer in New Jersey 

have previously been included in the Upper Chesapeake 

regional aquifer where a distinct Surficial aquifer was not 

identified separately, despite the presence of undifferentiated 

Quaternary sediments (Trapp, 1992).

The regional confining unit that overlies the Upper 

Chesapeake aquifer over most of its extent and underlies 

the unconfined Surficial aquifer includes the Holly Beach 

aquifer in New Jersey (only on the Cape May peninsula), the 

Upper Chesapeake confining unit in Delaware and Maryland, 

the Yorktown confining zone in Virginia, and the Yorktown 

confining unit in North Carolina. This regional unit consists of 

Pleistocene estuarine clays in New Jersey and clay, silty and 

sand clay, and shells of Miocene to Pliocene age in Delaware, 

Maryland, Virginia, and North Carolina (Trapp, 1992). Over 

much of its area, this confining unit is less than 20 ft thick, 

but it may be up to tens of feet thick in some locations (Trapp, 

1992; McFarland and Bruce, 2006). The spatially variable 

composition and configuration of this unit is responsible for 

highly variable interactions between the Upper Chesapeake 

regional aquifer and the overlying Surficial aquifer. In general, 

this confining unit impedes groundwater flow to a greater 

extent with increasing distance from the Fall Zone.

Surficial Aquifer

The Surficial aquifer is the uppermost aquifer in the 

NACP aquifer system, with its top altitude at land surface 

(fig. 9). It is unconfined, mostly shallow, and usually 

hydraulically continuous throughout the NACP but may 

exhibit lateral discontinuities and contain locally confined 

zones (Trapp, 1992).

Across most of the New Jersey Coastal Plain, permeable 

surficial sediments of Pleistocene to Holocene age that are 

hydraulically continuous with older, underlying sediments 

have been grouped with these older sediments into the 

unconfined upper part of the undifferentiated KirkwoodCohansey aquifer system (Upper Chesapeake regional aquifer) 

and have not been separately identified as the Surficial aquifer 

(Trapp, 1992). However, at least part of the unconfined 

Kirkwood-Cohansey aquifer is hydrologically equivalent to 

the Surficial aquifer as described in other States, and therefore 

could be grouped with the regional Surficial regional aquifer. 
24 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

In New Jersey, the Surficial aquifer also includes the Holly 

Beach local aquifer of Holocene age near Cape May; the 

Holly Beach aquifer is separated from the underlying Upper 

Kirkwood-Cohansey aquifer by the Cape May confining unit 

(Martin, 1998).

On Long Island, the Surficial aquifer is composed of 

mostly glacial deposits and is referred to as the Upper Glacial 

aquifer. Across much of its extent, the unconfined Upper 

Glacial aquifer is hydraulically continuous with the underlying 

Magothy aquifer (Smolensky and others, 1989).

The Surficial aquifer is of extremely heterogeneous 

composition due to the variable depositional origin of its 

sediments, which consist of unconsolidated sand and gravel 

in valley, terrace, dune, beach, marine, and glacial deposits 

ranging in age from Miocene to Holocene (Trapp, 1992). 

The thickness of the Surficial aquifer is highly variable but 

averages about 50 ft over most of its extent. The Upper Glacial 

portion of the Surficial aquifer on Long Island is much thicker, 

with an average of about 250 ft. The Surficial aquifer also 

may be as thick at 250 ft in buried channels on the Delmarva 

Peninsula in Delaware, Maryland, and Virginia where older 

marine sediments were eroded and replaced by Quaternary 

sediments of fluvial origin.

In most locations, the Surficial aquifer is a moderately 

important groundwater resource, providing the most accessible 

water supply for individual domestic users and small 

community systems. On the Delmarva Peninsula in Delaware 

and Maryland, the Surficial aquifer is an important supply of 

groundwater for agricultural irrigation and municipal supply. 

On Long Island, the Upper Glacial aquifer is a major water 

resource for many uses and, in combination with the Magothy 

aquifer, is part of an unconfined system that supplies most of 

the groundwater withdrawn.

Hydraulic Properties

A substantial amount of data exists on the hydrologic 

properties of the aquifers and confining units within the 

NACP. Hundreds of measurements of transmissivity, hydraulic 

conductivity (most commonly horizontal for aquifers and 

vertical for confining units), and specific storage from aquifer 

tests and laboratory permeameter samples are available 

throughout the study area. Even so, data are relatively sparse 

for the apparent heterogeneity of the hydrogeologic units, 

and the spatial distribution of measurements is extremely 

uneven and focused primarily around major pumping centers. 

Consequently, measurements of hydraulic properties are more 

readily available in areas of higher groundwater use and for 

those particular aquifer units receiving the most use.

Similarly, many more data are available of the hydraulic 

properties for aquifers than for confining units. For example, 

hundreds of measurements of hydraulic conductivity or 

transmissivity are available for the Potomac aquifer system, 

but data on other, less commonly used aquifers are much more 

limited, and for a few of the confining units, only a handful of 

measurements may be available throughout the entire study 

area.

Sources of published measurements of aquifer and 

confining unit properties summarized in this report include the 

NACP RASA hydrogeologic framework report (Trapp, 1992) 

and several State-level hydrogeologic framework reports and 

analyses (McClymonds and Franke (1972) for Long Island, 

Martin (1998) for New Jersey, Andreasen and others (2013) 

for Delaware and Maryland, and McFarland and Bruce 

(2006) for Virginia). In addition to hydraulic properties from 

published reports, unpublished data from more recent analyses 

were obtained for Delaware (John Callahan, Delaware 

Geological Survey, written commun., 2012), Maryland 

(Andreasen and others, 2013), and Virginia (McFarland, 

2013). However, these data have not been analyzed 

extensively for spatial trends or patterns and are discussed in 

this report only in terms of summary statistics.

As a result, data on hydraulic properties available 

for aquifers generally are not sufficient to determine 

consistent patterns of variation at either the local or the 

regional scale, and only a few studies have attempted to 

describe systematically these spatial variations. Notable 

examples include an analysis of hydraulic properties of 

all the aquifers on Long Island (McClymonds and Franke, 

1972), the analysis of transmissivity values in the Maryland 

Coastal Plain (Hansen, 1971), and a very recent analysis of 

the hydrogeologic characteristics of the Potomac aquifer 

in Virginia (McFarland, 2013). More commonly, available 

measurements of hydraulic properties serve only as starting 

points for groundwater modeling studies, and hydraulic 

properties for individual units then are determined through 

the model-calibration process. These estimates are somewhat 

dependent on the spatial and temporal discretization of the 

groundwater-flow models but probably are more representative 

of the regional hydraulic properties than estimates obtained 

from short-term pump tests or laboratory permeameter tests.

Published values of transmissivity and hydraulic 

conductivity from aquifer tests and laboratory studies for 

NACP aquifers and confining units reveal some general 

similarities and differences among various hydrogeologic 

units, though transmissivity variations generally appear to be 

more of a function of aquifer thickness than of large measured 

differences in the hydraulic conductivity of aquifer materials.

Published values of horizontal hydraulic conductivity 

for aquifers of the fluvial-deltaic Potomac aquifer system 

range from less than 1 foot per day (ft/d) to more than 500 ft/d 

within the individual States, whereas regionally the averages 

range from about 30 to about 200 ft/d. The large thickness 

of the Potomac aquifer system in Virginia has resulted in 

published transmissivity values as large as 58,000 square feet 

per day (ft2/d; McFarland, 2013).

Published values of hydraulic conductivity for the 

Magothy aquifer in New York, New Jersey, Delaware, and 

Maryland are very similar to those in the Potomac aquifer 

system. A horizontal hydraulic conductivity value of 642 ft/d 

for the Magothy aquifer on Long Island (McClymonds and 
Hydrogeology 25

Franke, 1972) may be the highest value reported for any 

aquifer in the NACP. Transmissivity values of 56,000 ft2/d 

reported on Long Island are similar to the values reported 

for the Potomac aquifer in Virginia despite the much thinner 

Magothy aquifer on Long Island.

The Tertiary and Cretaceous aquifers formed from 

marine transgressions and regressions tend to exhibit similar 

hydraulic properties to each other, based on a limited number 

of published values. These aquifers include the regional 

Piney Point, Aquia, Monmouth-Mount Laurel, and Matawan 

aquifers, all of which are considerably thinner than most of 

the other NACP aquifers. Horizontal hydraulic conductivity 

values of as much as several hundred feet per day have been 

reported in these aquifers, but average values range from about 

10 to about 40 ft/d across the NACP. Transmissivity values 

typically are less than 5,000 ft2/d and reflect both the lower 

hydraulic conductivities and thinner aquifer units.

Published values of horizontal hydraulic conductivity for 

the relatively clean and coarse marine sands of the surficial 

aquifer reveal a similar range to the Cretaceous fluvial-deltaic 

sands, with a minimum of less than 1 ft/d and a maximum 

of about 500 ft/d. The upper end of this range, however, 

is most typical of the thick Upper Glacial aquifer on Long 

Island, where transmissivity values as high as 27,000 ft2/d 

have been reported (McClymonds and Franke, 1972). Buried 

fluvial channels on the Delmarva Peninsula exhibit values 

approaching those observed on Long Island. The confining and 

Lower Chesapeake aquifers of Pliocene and Miocene age have 

a range of measured hydraulic conductivities typically lower 

than that of the Surficial aquifer, particularly in Maryland 

(Andreasen and others, 2013).

Hydraulic properties of the confining units separating 

these regional aquifers are known primarily from a very 

limited number of laboratory permeameter measurements and 

from estimates based on groundwater-flow model calibration. 

Consequently, the large range of values for vertical hydraulic 

conductivity may reflect the scales of those estimates and 

measurements in addition to the range of vertical hydraulic 

conductivity values in the confining units. Published values 

of vertical hydraulic conductivity for confining units range 

from about 3.0×10-9 to 0.1 ft/d in the NACP. The high value 

of 0.1 ft/d was reported for the confining unit above the 

Monmouth-Mount Laurel aquifer in New Jersey, and the low 

value of 3.0×10-9 ft/d was reported for the confining unit 

above the Middle Potomac aquifer in Maryland. The observed 

range of values within each hydrogeologic unit, however, is 

almost as large as the observed range reported here for all 

hydrogeologic units together, indicating substantial spatial 

variations in the properties of these confining units that are not 

yet fully understood at the regional scale.

Storage properties in the confined aquifers and confining 

units of the NACP aquifer system have been determined 

from aquifer tests conducted primarily for water-supply 

development throughout the study area. Measured values of 

elastic specific storage (Ss) in the confining units typically are 

in the range of 1.8×10-6 to 4.0×10-6 per foot (ft-1). However, 

values of one to two orders of magnitude greater have been 

reported throughout the study area, and these larger values 

appear to be more common in the shallow units (Martin, 1998; 

Heywood and Pope, 2009; Andreasen and others, 2013).

The large reported values of Ss are higher than expected 

for these types of materials and may reflect specific issues 

with the aquifer tests, such as short screens in thick aquifers, 

as well as interpretive problems arising from borehole storage 

effects, partial well penetration, or aquitard leakage (Heywood 

and Pope, 2009). Given the uncertainty in the measured 

values, model-calibrated values for Ss may reflect more 

realistically ambient aquifer conditions and behavior at the 

regional scale.

In previous regional and subregional simulations of the 

NACP aquifer system, a dimensionless storage coefficient 

value of 1.0×10-4 was used for all aquifers (Harsh and 

Laczniak, 1990; Leahy and Martin, 1993). This storage 

coefficient would translate to a range of Ss between 2.0×10-7

and 1.0×10-5 ft-1 for NACP aquifers over a thickness ranging 

from 10 to 500 ft. Most previous groundwater models of the 

NACP have not explicitly simulated storage in confining units, 

but a recent model of the Virginia Coastal Plain estimated 

values of specific storage ranging from about 5.0×10-4 to 

1.0×10-6 ft-1 (Heywood and Pope, 2009).

Actual measurements of Ss in confining units are limited 

to a few laboratory consolidation tests of clayey sediments. 

One such study conducted in Maryland reported Ss values 

ranging from 3.6×10-5 to 9.0×10-5 ft-1 for the marine Marlboro 

Clay confining unit and from 4.6×10-5 to 7.6×10-5 ft-1 for 

the Upper Potomac confining unit (Hansen, 1977). A study 

estimating aquifer depletion for the Virginia Coastal Plain 

assumed a range of values between 1.0×10-5 and 1.0×10-4 ft-1

for specific storage of confining layers (Konikow and 

Neuzil, 2007).

Another study applied analysis and modeling of 

extensometer data from the coastal plain of Virginia to 

estimate inelastic specific storage values ranging from 4.6×10-6

to 1.6×10-5 ft-1 for fine-grained interbeds within the Potomac 

aquifer system and from 3.0×10-5 to 4.6×10-5 ft-1 for postCretaceous fine-grained sediments (Pope and Burbey, 2004). 

This study also determined elastic specific storage values 

between 1.4×10-6 and 1.8×10-6 ft-1.

A comprehensive analysis of the updated information 

on hydrogeologic framework geometries since the previous 

regional assessment of the NACP aquifer system (Trapp 

and Meisler, 1992) may reveal spatial patterns in aquifer 

and confining unit hydraulic properties that have yet to 

be determined. In addition to descriptions of variations in 

hydraulic conductivity and transmissivity described in this 

report, a more comprehensive understanding of the storage 

properties of the confining units and aquifers is needed to 

properly characterize groundwater flow through each of these 

units; such an analysis was beyond the scope of the data 

synthesis presented in this report.
26 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Hydrologic Conditions

Continued land development and population growth 

in the NACP have created concerns regarding the supply 

of potable groundwater and the quality and quantity of 

water discharging to ponds, streams, and coastal waters. An 

important component of assessing groundwater availability 

is an understanding of the processes that govern how water 

enters, flows through, and exits aquifer systems.

Previous Investigations

Numerous studies have been conducted to characterize 

the hydrogeology of the NACP, some dating back to the early 

20th century (Crosby, 1900; Freeman, 1900; Sanford, 1911). 

These early studies were synthesized for the region-wide study 

of hydrologic and geologic constraints on groundwater flow 

and water availability conducted from 1978 through 1987 

(RASA program; Trapp and Meisler, 1992). Hydrogeologic 

data collected as part of the RASA effort served as the basis 

for the subsequent numerical models developed for the NACP 

aquifer system throughout the study area.

Numerical models provide a means to synthesize existing 

hydrogeologic information into an internally consistent 

mathematical representation of a real system and, during the 

past 30 years, have been instrumental in understanding the 

processes that affect groundwater flow and availability in the 

NACP aquifer system. A regional model (Leahy and Martin, 

1993) that extended from Long Island to South Carolina and 

subregional models of New Jersey (Martin, 1998), Maryland 

and Delaware (Fleck and Vroblesky, 1996), Virginia (Harsh 

and Laczniak, 1990), and North Carolina (Giese and others, 

1997) were used to assess water budgets from pumping 

conditions from predevelopment (pre-1900) through 1980. 

The regional and subregional models incorporated the 

hydrogeologic framework derived from the statewide RASA 

programs for New Jersey (Zapecza, 1989), Virginia (Meng 

and Harsh, 1988), Maryland and Delaware (Vroblesky and 

Fleck, 1991), and North Carolina (Winner and Coble, 1989). 

In addition to these statewide RASA studies, a separate 

analysis of groundwater availability was conducted for Long 

Island based on the regional model of the island (Reilly and 

Harbaugh, 1980).

Since the RASA study, more recent analyses have built 

upon the statewide NACP models, including updates to the 

hydrogeologic framework, finer grid discretization, and 

improved numerical codes. More recent modeling efforts 

in New Jersey include the use of the USGS numerical code 

SHARP (Essaid, 1990) to simulate the position and movement 

of the interface between freshwater and saltwater (Pope and 

Gordon, 1999) and later, a conversion of the model datasets 

(Voronin, 2005) to be compatible with the modular threedimensional (3-D) finite-difference groundwater flow model 

MODFLOW–96 (Harbaugh and McDonald, 1996). The 

updated model by Voronin (2005) also included a finer grid 

discretization, spatially variable recharge, and a more recent 

pumping period (1968–1998).

The first model for the Virginia Coastal Plain (Harsh and 

Laczniak, 1990) originally used a numerical finite-difference 

code developed by Trescott (1975) and was updated in the 

late 1990s to MODFLOW–96 (McFarland, 1998). The most 

recent model for the Virginia Coastal Plain (Heywood and 

Pope, 2009) was developed to include the latest interpretation 

of the hydrostratigraphy (McFarland and Bruce, 2006), most 

notably the presence of the large impact crater discovered 

in the Chesapeake Bay in the early 1990s (Powars and 

Bruce, 1999). This model was based on the numerical 

code SEAWAT (a computer program for simulation of 3-D 

variable-density groundwater flow and transport; Guo and 

Langevin, 2002) to simulate position and movement of the 

interface between freshwater and saltwater for the simulation 

period (1891–2003). The model for the Long Island region 

(Reilly and Harbaugh, 1980) also was updated, for use with 

MODFLOW–88 (McDonald and Harbaugh, 1988) for the 

averaged period between 1968 and 1983. The updated model 

for Long Island (Buxton and Smolensky, 1999) included an 

updated interpretation of the hydrostratigraphy of the island 

(Smolensky and others, 1989).

In addition to regional and statewide assessments, several 

small-scale studies have been done in the area that focus on 

the surficial unconfined and shallow, confined aquifers. These 

studies include areas such as in the Eastern Shore of Virginia 

where there has been concern about the effects of pumping on 

the position and movement of the interface between freshwater 

and saltwater (Richardson, 1994; Sanford and others, 2009). 

Ongoing work by the Chesapeake Bay Program in the 

Delmarva Peninsula is focused on understanding the transport 

of nutrients to the bay through the shallow groundwater flow 

system (Sanford and Pope, 2007; Sanford and others, 2012).

These recent studies have improved the understanding 

of the hydrogeologic and water-use conditions in the NACP, 

but the information obtained from these studies has not 

been synthesized into a regional hydrogeologic perspective 

similar to what had been done as part of the original RASA 

study for the NACP (Trapp and Meisler, 1992). A regionwide assessment of water-level changes (DePaul and others, 

2008) based on long-term and synoptic water-level data from 

predevelopment (1900) to 2000 shows significant changes in 

water levels since the RASA study and highlights the need 

for an updated regional assessment of hydrologic impacts 

resulting from changing stresses since 1980.

Sources of Water to the Northern Atlantic 

Coastal Plain Aquifer System

About 81,000 Mgal/d or about 29 trillion gallons per year 

(Tgal/yr) of freshwater enters the NACP area. The sources 

of water to the NACP aquifer system include recharge from 

precipitation, wastewater, leaky sewer and water-supply lines, 

and the eastward flow of groundwater and surface water 
Hydrologic Conditions 27

across the Fall Zone (fig. 12). Estimates of freshwater input 

from wastewater and leaky water-supply and sewer lines are 

described in the “Wastewater Return Flow” section.

Aquifer Recharge

The primary source of water into the NACP aquifer 

system is recharge from precipitation. The average annual rate 

of precipitation (2005–2009) was about 45 in/yr in the NACP 

study area (see appendix 1). Over the study area (30,000 mi2), 

this rate of precipitation is equal to about 62,000 Mgal/d or 

23 Tgal/yr of freshwater input. Although there is as much 

23 Tgal/yr of precipitation in the NACP study area, only about 

31 percent (19,600 Mgal/d) or about 7 Tgal/yr of it enters the 

underlying aquifer system as recharge. The majority of the 

precipitation in the NACP (69 percent) is lost to evaporation, 

transpiration, or surface runoff and, therefore, never reaches 

the underlying groundwater system. A detailed description of 

the SWB method used to calculate aquifer recharge for the 

NACP is included in appendix 1.

Nearly all the recharge that enters the NACP aquifer 

system does so in the shallow unconfined aquifers. Previous 

analyses (Leahy and Martin, 1993; McFarland, 1999) have 

determined that only a small percentage (less than 2 percent) 

of the available aquifer recharge enters the deeper, confined 

aquifer system under predevelopment conditions, where 

the deeper confined units crop out near the Fall Zone in the 

western part of the study area (fig. 7).

Recharge also can enter the underlying confined aquifers 

indirectly as induced infiltration of water from overlying 

aquifers and confining units in response to pumping in the 

deeper aquifers (fig. 13). Leahy and Martin (1993) calculated 

that the area contributing recharge to the underlying confined 

aquifer system increased from 25 percent to 45 percent of the 

land area in response to 1980 pumping conditions as compared 

to predevelopment.

The amount of recharge entering the NACP aquifer 

system in 2005 was determined for this investigation by the 

SWB method described in Westenbroek and others (2010) 

and presented in appendix 1 of this report. The results of this 

analysis indicate that aquifer recharge is about 13.9 in/yr, 

which results in about 19,600 Mgal/d of water entering the 

NACP aquifer system (fig. 14). The average recharge value 

of 13.9 in/yr is consistent with the range of reported recharge 

values (11.8 to 21.7 in/yr) throughout the study area. A more 

detailed description of the methodology used to calculate 

aquifer recharge from 2005 through 2009 and a listing the 

recharge rates reported throughout the NACP are included in 

appendix 1 of this report.

Figure 12. Schematic diagram showing the conceptual hydrogeologic model of the Northern Atlantic Coastal Plain aquifer 

system; modified from Edwin McFarland (U.S. Geological Survey, written commun., 2010).

Confining units

Confined aquifers

Unconfined

aquifer

Bedrock

Recharge

Streamflow

Inflow Outflow

Groundwater

pumping

Coastal

discharge

Streamflow

Subsea

discharge

Fall Zone
28 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Wastewater Return Flow

Recharge of wastewater from centralized sewage 

treatment facilities or from onsite domestic septic systems 

also can be an important source of water to the NACP 

aquifer system. This wastewater can originate as (1) fresh 

groundwater withdrawn from the underlying aquifer system, 

(2) treated saline groundwater withdrawn from the aquifer 

system, (3) surface-water sources within the NACP area, or 

from (4) surface waters imported into the NACP for drinking 

water. Regardless of the origin of this wastewater, eventually 

it all discharges to the NACP groundwater system or nearby 

surface waters. Nearly all the centralized wastewater treatment 

facilities in the NACP are located in close proximity to 

surface waters, and therefore, wastewater released from 

these facilities most likely discharges directly to these water 

bodies and reaches the coast without interacting with the 

groundwater system (fig. 15). However, in areas with aging 

public-supply distribution and sewer systems, such as the 

New York City area, leakage from these aging infrastructures 

can contribute as much as 70 Mgal/d of water to the shallow 

aquifer system from leaking lines throughout the area (Misut 

and Monti, 1999).

The importance of inflow of water from leaking sewer 

systems to the underlying aquifer system may not be as 

great, however, because sewer lines in low-lying coastal 

areas have the potential to be below the water table. If sewer 

lines are below the water table, then the potential exists for 

groundwater also to flow into the leaky sewer lines, thus 

offsetting the contribution of water to the aquifer system 

from leaks along sewer lines (Buxton and Smolensky, 

1999). A more comprehensive analysis than was possible 

for this investigation would be needed to assess fully the 

interaction of sewer systems with the shallow, unconfined 

aquifer system of the NACP and its relative importance to the 

hydrologic budget.

A potentially more substantial source of water to the 

aquifer system from wastewater comes from the about 

3.3 million residents who use onsite domestic septic 

systems in non-sewered areas. Assuming that as much as 

85 percent of the 70 gallons per day (gal/d) of the estimated 

Figure 13. Schematic diagram showing the effects of pumping on groundwater flow in the Northern Atlantic 

Coastal Plain aquifer system. From Barlow (2003).

Confining unit

Confining unit

Unconfined

aquifer

Pumping

well Pumping

well

Pumping

well Pumping

well Pumping

well

NOT TO SCALE

Stream

Major bay

Atlantic

Ocean

Water

table

SEA LEVEL

Saltwater

Confined

aquifers

Impermeable bedrock

Groundwater flow direction

Saltwater encroachment

Brackish

EXPLANATION

Salty water

Zone of mixing
Hydrologic Conditions 29

Figure 14. Distribution of aquifer recharge for the Northern Atlantic Coastal Plain aquifer system.

-78° -72°

39°

36°

EXPLANATION

Average recharge, in inches

0.01 to 3

>3 to 6

>6 to 9

>9 to 12

>12 to 15

>15 to 18

>18 to 21

>21 to 24

>24 to 27

>27 to 30

>30 to 33

> 33

Study area

Base from U.S. Geological Survey digital data 1:2,000,000 series

Albers Equal Area Conic projection

Standard Parallels 34o 30' N and 41o 30' N; central meridian 75o 30' W 

North American Datum of 1983

Delaware Bay

Long Island Sound

NEW CT

YORK

NEW

JERSEY

PENNSYLVANIA

MARYLAND VIRGINIA

WEST

DELAWARE

VIRGINIA

NORTH CAROLINA

ATLANTIC OCEAN

Pamlico Sound

Albemarle Sound

0 50 100 KILOMETERS

0 50 100 MILES

DC
30 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Figure 15. Distribution of sewer and wastewater systems in 2005 in the Northern Atlantic Coastal Plain.

-78° -72°

39°

36°

EXPLANATION

Percentage of housing units using sewers

0 to 20

21 to 40

41 to 60

61 to 80

81 to 100

Base from U.S. Geological Survey digital data 1:2,000,000 series

Albers Equal Area Conic projection

Standard Parallels 34o 30' N and 41o 30' N; central meridian 75o 30' W 

North American Datum of 1983

Onsite septic disposal

Public sewer disposal

Study area

Chesapeake Bay

Delaware Bay

Long Island Sound

NEW CT

YORK

NEW

JERSEY

PENNSYLVANIA

MARYLAND VIRGINIA

WEST

DELAWARE

VIRGINIA

NORTH CAROLINA

ATLANTIC OCEAN

Pamlico Sound

Albemarle Sound

0 35 70 105 140 KILOMETERS

0 25 50 75 100 MILES

DC
Hydrologic Conditions 31

per capita consumption of water (Shaffer and Runkle, 

2007) is discharged to domestic septic systems, a total of 

about 200 Mgal/d of wastewater would be estimated to be 

discharged to the shallow, unconfined aquifers throughout 

the NACP.

Wastewater returned to the aquifer system through 

domestic septic systems can mitigate the potentially adverse 

effects of pumping on the flow system depending where 

the water originated and ultimately where it is discharged. 

Groundwater withdrawn from deep confined aquifers that 

returns as wastewater to the shallow, unconfined aquifer 

system may represent only a 15 percent loss to the overall 

hydrologic budget; however, this redistribution of pumped 

water may actually represent a complete loss of water from the 

deep confined aquifer from which the water was withdrawn. 

This loss of water from the deeper system then may result 

in decreased deep subsea coastal discharge and increase the 

potential for saltwater intrusion (fig. 13).

Lateral Inflow

Another source of freshwater to the NACP is streamflow 

across the Fall Zone (fig. 1). Streamflow in the NACP can 

be grouped into three categories: (1) rivers that flow directly 

from upland areas to coastal water bodies along the Fall Zone, 

(2) rivers that flow eastward across the Fall Zone and through 

a portion of the NACP before discharging at the coast, and 

(3) rivers that originate entirely within the NACP. Regardless 

of origin, all the major rivers that flow through the NACP 

discharge to the coastal waters of the Atlantic Ocean (fig. 16).

Selected larger streams (Strahler stream order 3 or 

higher; Pierson and others, 2008) were evaluated spatially 

to determine their contributions to surface-water flow in the 

study area. Streamflows were totaled by period of record by 

hydrologic basin using the NHDPlus attribute data (table 1; 

U.S. Environmental Protection Agency, 2006). Stream 

segments crossing the Fall Zone, which defines the western 

boundary of the NACP, are considered surface-water inflows 

to the NACP, whereas segments intersecting the tidal zone 

downstream as delineated by Titus and Wang (2008) are 

considered part of the coastal discharge areas.

NHDPlus attribute data include computation of mean 

annual flow (unit runoff method; Research Triangle Institute, 

2001) and Strahler stream order (Pierson and others, 2008) 

for every segment in the stream network. These attributes 

were extremely useful in evaluating the surface-water network 

because they allowed for estimation of flow at any location 

in the stream network. Initial analysis determined that the 

large rivers account for the majority of the surface-water flow 

through the NACP, and therefore, a simple network of the 

large rivers and streams could be used to represent the flow 

network for the purposes of a hydrologic budget analysis. 

NHDPlus streams of order 3 and larger comprise a network of 

streams totaling about 4,000 mi in length (fig. 16) and account 

for about 98 percent of the mean annual flow into the NACP at 

the Fall Zone (table 1).

For some of the large rivers in the study area, tidal 

zones in the primary channels extend westward of the Fall 

Zone, and water flows directly from the Piedmont Province 

to coastal waters entirely bypassing the fresh surface-water 

and groundwater flow systems of the NACP. Flow from 

these rivers, including the Hudson, Delaware, Susquehanna, 

Potomac, and Roanoke (fig. 16), was not considered as either 

inflow or outflow to the NACP, despite the large total flow 

(about 47,000 Mgal/d).

For streams that originate in the Blue Ridge or Piedmont 

Provinces and flow into the NACP at the western boundary 

of the study area, the surface-water inflow to the NACP totals 

about 18,600 Mgal/d (table 1). Of this total amount of flow, 

most if not all flows through the NACP and discharges directly 

to the coast without contributing water to the underlying 

aquifer system.

Previous analyses in Virginia and New Jersey have 

examined the potential contribution of streamflow to the 

underlying groundwater system along the Fall Zone in the 

NACP. McFarland (1999) determined that rivers entering the 

Virginia Coastal Plain away from the influence of groundwater 

pumping stresses act as drains to the local groundwater system 

and, therefore, do not contribute water to the underlying 

groundwater system (fig. 17).

Navoy and Carleton (1995) determined that under certain 

pumping conditions, groundwater withdrawals in the western 

portion of the NACP in New Jersey resulted in induced 

infiltration of surface water from the Delaware River into the 

aquifer system and served as a source of water to pumping 

wells in Camden, N.J. Therefore, the amount of freshwater 

contribution from streamflow entering the NACP across the 

Fall Zone to the deep, confined aquifer system is difficult 

to quantify and may vary across the study area depending 

on local hydrogeologic conditions and on the amount and 

distribution of groundwater withdrawals.

Additional sources of freshwater into the NACP that 

are not readily quantifiable and are assumed to be of less 

importance are the subsurface flow across fractured rock in 

the upland Piedmont Province and the lateral flow across the 

southern boundary of the study area in North Carolina. In 

Virginia, the subsurface fractured rock flow from the upland 

Piedmont Province to the NACP deposits was about 13 Mgal/d 

(Heywood and Pope, 2009). Previous regional modeling 

efforts of the NACP aquifer system (Leahy and Martin, 1993) 

assumed that subsurface fractured rock flow was negligible.

The southern boundary of the study area for this 

investigation coincides with the east-west regional 

groundwater flow direction. Large changes in groundwater 

withdrawals can alter flow patterns near this boundary and 

locally affect the water budget to the NACP aquifer system 

(Heywood and Pope, 2009; Campbell and Coes, 2010); 

however, for the purpose of this analysis, it was assumed 

that any flow that either enters or exits across this southern 

boundary represents only a small portion of the total flow in 

this aquifer system.
32 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Figure 16. Distribution of major streams and rivers that flow across the Northern Atlantic Coastal Plain.

EXPLANATION

Nontidal streams, order greater

 than or equal to 3 

Tidal streams, order greater

 than or equal to 3

Study area

Base from U.S. Geological Survey digital data 1:2,000,000 series

Albers Equal Area Conic projection

Standard Parallels 34° 30' N and 41° 30' N; central meridian 75° 30' W 

North American Datum of 1983

41°

35 °

-78° -72°

Potomac River

James River

Tar River

RoanokeRive r

Hudson River

Delaware Ri ver

Schuykill River

Susquehanna River

Chesapeake Bay

Delaware Bay

Long Island Sound

ATLANTIC OCEAN

Pamlico Sound

Albemarle Sound

CT

NEW

YORK

NEW

JERSEY

PENNSYLVANIA

MARYLAND VIRGINIA

WEST

DELAWARE

VIRGINIA

NORTH CAROLINA

RI

MA

0 35 70 105 140 KILOMETERS

0 25 50 75 100 MILES

DC
Hydrologic Conditions 33

Figure 17. Schematic cross-section showing groundwater flow near the Fall Zone of the Northern Atlantic Coastal Plain.

Stream

Fall Zone

Basement

Aquifer

Confining unit

Confining unit

Aquifer

Unconfined aquifer

500

250

–500

–250

SEA LEVEL

WEST EAST

Basement

Coastal Plain Physiographic Province

FEET

SEA LEVEL

FEET

–250

–500

500

250

Piedmont

Physiographic

Province

Freshwater aquifer

Bedrock

Saprolite

Fractures

EXPLANATION

VERTICAL SCALE GREATLY EXAGGERATED

Modified from McFarland and Bruce (2006)

Confining unit

Direction of groundwater flow 

under predevelopment 

conditions 

River system

Surface-water 

inflow across 

Fall Zone

Groundwater discharge Total 

outflow to 

tidal waters

To rivers 

originating 

outside NACP

To rivers 

originating 

in NACP

Hudson River tributaries — — 0.2 0.2

Delaware River tributaries 8.4 0.1 1.2 9.7

Atlantic Ocean (New Jersey) — — 1.1 1.1

Upper Chesapeake Bay:

Upper Delmarva Peninsula 0.1 0 0.9 1

Patapsco River 0.4 0.4 — 0.8

Patuxent River 0.2 0.1 0.1 0.4

Middle Chesapeake Bay, Potomac River tributaries 0.2 0.5 0.5 1.2

Lower Chesapeake Bay:

Middle and lower James River 5.7 0.3 0.2 6.2

Piankatank River — — 0.1 0.1

Rappahannock River 1.2 0 0.2 1.4

York River 0.8 0.4 0.2 1.4

Roanoke River tributaries — — 0.5 0.5

Chowan River 0.9 0.8 1.5 3.2

Tar and Pamlico Rivers 0.7 1.2 1.4 3.3

Total 18.6 3.8 8.1 30.5

Table 1. Mean annual inflows and outflows for major river systems in the Northern Atlantic Coastal Plain (NACP) for conditions 

in 2005.

[All values are in billion gallons per day. NACP, North Atlantic Coastal Plain; —, no data reported]
34 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Figure 18. Distribution of drinking water sources by State 

across the Northern Atlantic Coast Plain.

Public surface water source

Public groundwater source

Domestic groundwater

Imported from surface waters

outside NACP

EXPLANATION

State potable sources of water

for residential purposes

Northern Atlantic Coastal Plain

North Carolina

District of

Columbia

New Jersey

Pennsylvania

New York

Delaware

Maryland

Virginia

Loss of Water from the Northern Atlantic Coastal 

Plain Aquifer System

For predevelopment conditions, all the water entering 

the aquifer system is balanced by water leaving the aquifer 

system. Water leaves the aquifer system as discharge to 

streams, discharge to shallow coastal waters, and deep subsea 

discharge farther offshore in the Atlantic Ocean. For postdevelopment conditions, an additional loss of water from the 

aquifer system is water removed by groundwater withdrawals. 

Depending on type of water use and whether pumped water 

is removed through sewers, a substantial amount of the water 

withdrawn from the aquifer system may be returned in the 

form of wastewater return flow.

Groundwater Discharge to Streams

About 30,500 Mgal/d of total streamflow leaves the 

NACP as surface-water discharge to coastal waters (table 1). 

Of the 30,500 Mgal/d of water that flows through the NACP, 

about 18,600 Mgal/d originated as streamflow west of the 

NACP and entered the study area across the Fall Zone 

(table 1). The difference between the total flow leaving 

the NACP at the coast (30,500 Mgal/d) and the amount 

streamflow entering the NACP area across the Fall Zone 

(18,600 Mgal/d) is the amount of groundwater that discharges 

to streams in the NACP (about 11,900 Mgal/d), of which about 

8,100 Mgal/d is to streams that originate within the NACP, and 

3,800 Mgal/d is discharge to streams that originate outside of 

the NACP. Consequently, there is a net loss of freshwater from 

the NACP aquifer of about 11,900 Mgal/d from groundwater 

discharge to streams.

Groundwater Withdrawals

Groundwater is withdrawn from the NACP aquifer 

system for multiple uses, including drinking water and for 

commercial, industrial, and agricultural purposes. In 2005, 

the total amount of groundwater withdrawn was about 

1,500 Mgal/d (Kenny and others, 2009). The distribution of 

water-use type varied spatially across the NACP, with drinking 

water being the primary use type in the northern part of the 

study area, agricultural use dominant in the middle part of the 

study area, and commercial and industrial uses dominant in 

the southern part of the study area.

The total drinking water use for the 20 million people 

living in the NACP in 2005 was about 2,500 Mgal/d, about 

39 percent of which (or about 1,000 Mgal/d) was derived from 

public and domestic groundwater sources in the underlying 

aquifer system (fig. 18, table 2; Kenny and others, 2009). 

The groundwater withdrawn for drinking water accounted for 

65 percent of all groundwater withdrawals in the study area 

(fig. 18). Most (70 percent) of these groundwater withdrawals 

occurred on Long Island and in New Jersey. Of the about 

1,000 Mgal/d of groundwater withdrawn for drinking water, 

85 percent was used for public supply, and 15 percent was 

from self-supplied domestic sources (table 2).

In the northern part of the study area (New York, 

Pennsylvania, New Jersey, and Delaware), nearly all 

(91 percent) the groundwater pumped for drinking water 

was derived from public-supply sources. In the southern part 

of the study area (Maryland, Virginia, and North Carolina), 

as much as 35 percent of the total groundwater pumped for 

drinking water was derived from self-supplied domestic 

sources (fig. 18). This change in the source of drinking water 

from north to south is reflected in the land use and land cover 

throughout the NACP (fig. 5); water demand is met by publicsupply systems in the more highly developed areas to the north 

and by domestic self-supplied sources in the less developed 

areas to the south.

Treated saline groundwater provides an additional source 

of drinking water in several coastal communities in which 

saltwater intrusion issues are of concern. Desalinization of 

saline groundwater is a significant source of drinking water 

in southeastern Virginia west of the Chesapeake Bay (fig. 1) 

where about 11 Mgal/d of treated saline groundwater is used 

for drinking water (Kenny and others, 2009). Cape May, N.J. 
Hydrologic Conditions 35

Table 2. Distribution of drinking water sources by State across the Northern Atlantic Coastal Plain.

[Values are in million gallons per day. Water use data are from Kenny and others (2009). Grey shaded box indicates that value is zero or rounded to zero. 

NACP, Northern Atlantic Coastal Plain]

State

Public 

groundwater 

source 

Public 

surface-water 

source

Domestic 

groundwater

Imported from 

surface waters 

outside NACP

Total

Distrtict of Columbia 0 0 0 70 70

Delaware 47 37 6 0 90

Maryland 83 165 42 35 325

North Carolina 332 9 7 0 48

New Jersey 260 109 41 0 410

New York 361 0 15 513 889

Pennsylvania 2 93 1 0 96

Virginia 38 430 36 26 530

Total 823 843 148 644 2,458

(fig. 1) is another location of where desalinization is used to 

augment drinking water supplies during the high-demand 

summer months.

The total agricultural water use in the NACP in 2005 

was estimated to be about 300 Mgal/d with about 60 percent 

(179 Mgal/d) derived from groundwater sources (table 3) 

(Kenny and others, 2009). Most (89 percent, or 160 Mgal/d) 

of the 179 Mgal/d of groundwater withdrawals was for crop 

irrigation, with the remaining 11 percent (20 Mgal/d) for 

livestock (15 Mgal/d) and aquaculture (5 Mgal/d) uses. More 

than half (56 percent) of the 2005 agricultural groundwater 

withdrawals from the NACP occurred in the Delmarva 

Peninsula. Agricultural groundwater withdrawals were the 

highest in Delaware (57 Mgal/d) and accounted for about 

47 percent of the total groundwater withdrawals in the State 

(fig. 19, table 3).

Commercial and industrial freshwater use in the NACP in 

2005 was about 800 Mgal/d, of which 43 percent (338 Mgal/d) 

was derived from groundwater sources (table 3; Kenny and 

others, 2009). Groundwater withdrawals in Virginia and 

North Carolina accounted for about 62 percent of the total 

commercial and industrial groundwater uses in the NACP. 

The primary use of this water was for the pulp and paper 

industry in southern Virginia and northern North Carolina. 

Groundwater withdrawals for this industrial purpose resulted 

in the large water-level declines throughout the southern 

portion of the NACP (fig. 4).

Thermoelectric power plants also use a substantial 

amount of water in the NACP. About 20,000 Mgal/d of water 

is used for cooling power plants; however, nearly all this 

water is derived from fresh and saline surface waters, with 

only a small amount coming from fresh groundwater sources. 

The total fresh groundwater use for thermoelectric power 

plants throughout the NACP is about 6 Mgal/d (Kenny and 

others, 2009).

Total rates of groundwater withdrawals for the NACP 

were apportioned by aquifer for the 10 principal (regional) 

aquifers shown in figure 9. This analysis was based on screen 

interval information and local aquifer designations associated 

with reported withdrawals across the study area (table 4, 

fig. 20). The largest withdrawals occur in the Potomac aquifer 

system, which accounts for about 57 percent of the total 

withdrawals in the NACP aquifer system. The Surficial and 

the Upper and Lower Chesapeake regional aquifers combined 

account for about 33 percent of the total use (table 4). 

The Surficial aquifer provides almost half (49 percent) the 

groundwater pumped in Delaware and almost a third of the 

water pumped in Maryland (30 percent). Withdrawals from 

the Lower Chesapeake aquifer account for only 15 percent 

of the total withdrawals in the NACP yet make up about half 

Table 3. Distribution of groundwater use by State across the 

Northern Atlantic Coastal Plain.

[Values are in million gallons per day. May not add to totals shown because of 

independent rounding. Water use data are from Kenny and others (2009). Grey 

shaded box indicates that value are is zero or rounded to zero]

State

Public 

and 

domestic 

supply 

Agricultural 

Commercial 

and

industrial 

Total

Delaware 53 57 12 122

Maryland 124 36 16 177

North Carolina 39 29 140 207

New Jersey 300 40 31 372

New York 376 9 68 454

Pennsylvania 4 0 1 4

Virginia 74 8 70 151

Total 971 179 338 1,488
36 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

(44 percent) of the total withdrawals in New Jersey where 

the NACP aquifer is the primary source of drinking water in 

southeastern coastal areas of the State. About only 6 percent of 

the total NACP groundwater is withdrawn from the Magothy 

regional aquifer, but it accounts for more than 77 percent of 

the total withdrawals on Long Island. The remaining 4 percent 

of the total withdrawals in the NACP was pumped from the 

Aquia, Matawan, Monmouth-Mount Laurel, and Piney Point 

regional aquifers (table 4).

Groundwater Discharge to Coastal Areas

Coastal waters that border the NACP are the ultimate 

freshwater discharge areas from the groundwater-flow system. 

Freshwater enters these coastal waters as either streamflow 

or as direct groundwater discharge through the shallow 

coastal seabed in the nearshore environment or farther 

offshore as deep subsea discharge from the deeper confined 

aquifers (fig. 13). Although it is not possible to make direct 

measurements to quantify coastal groundwater discharge, 

an estimate of this outflow from the aquifer system can be 

determined if all the other inflows and outflows to the aquifer 

systems can be quantified.

Groundwater in Storage

Aquifer storage can be considered both a source and 

sink of water for the aquifer system as water moves into 

and out of storage in response to changing stresses to the 

Regional Aquifer

Percentage withdraw from regional aquifer

New York New Jersey Delaware Maryland Virginia NACP

Surficial 21 0 49 30 0 15

Upper Chesapeake 0 2 7 8 6 3

Lower Chesapeake 0 44 7 1 0 15

Piney Point 0 2 5 2 4 1

Aquia 0 0 0 10 0 1

Monmouth Mt-Laurel 0 2 0 1 0 1

Matawan 0 2 0 0 0 1

Magothy 77 24 5 6 0 6

Potomac1 2 24 27 42 90 57

1

The Potomac aquifer includes the Potomac-Patuxent and Potomac-Patapsco regional aquifers.

Table 4. Percentage of groundwater withdrawals in 2005 by aquifer and by State in the Northern Atlantic Coastal Plain.

[Water use data are from Kenny and others (2009). Grey shaded boxes indicate that aquifer does not exist in this geographic location. NACP, Northern Atlantic 

Coastal Plain]

Figure 19. Distribution of groundwater use by State across the 

Northern Atlantic Coast Plain.

EXPLANATION

Public and domestic supply

Agriculture

Commercial and industrial

Northern Atlantic Coastal Plain

North Carolina

New Jersey

Pennsylvania

New York

Delaware

Maryland

Virginia
Hydrologic Conditions 37

Figure 20. Distribution of groundwater withdrawals in 2005 by 

aquifer and by State in the Northern Atlantic Coastal Plain.

Surficial

Upper Chesapeake

Lower Chesapeake

Piney Point

Aquia

Monmouth-Mount Laurel

Matawan

Magothy

Potomac*

EXPLANATION

Regional aquifer categories

New Jersey

New York

Delaware

Maryland

Virginia

Northern Atlantic Coastal Plain

*Note: Potomac aquifer includes

both the Potomac-Patuxent and

the Potomac-Patapsco aquifers

groundwater flow system. Increased population over time 

throughout the NACP has resulted in increased groundwater 

withdrawals from the aquifer system as well as increased 

return of wastewater to the aquifer system. Changes in these 

aquifer stresses can affect water levels and, therefore, the 

amount of water going into and out of storage. As groundwater 

withdrawals increase, water is removed from the aquifer 

system, resulting in water-level declines. One source of this 

pumped water is the release of water from aquifer storage. 

As groundwater withdrawals increase, so does the potential 

for the return of wastewater to the aquifer system. Enhanced 

recharge from the return of wastewater can result in increases 

in water levels as water goes into aquifer storage; in this 

case, storage serves as a sink for increased water added to the 

aquifer system.

The advent of the large-scale use of groundwater 

resources in the early 1900s has resulted in the depletion 

of aquifer storage from increased pumping, which in turn 

has resulted in decreases in water levels, streamflows, and 

discharge to coastal receiving waters, leading to saltwater 

intrusion along the coast (Konikow, 2013). Decreases in 

aquifer storage have imposed limitations on groundwater 

availability and resulted in shifts from groundwater resources 

to the importation of surface waters to meet the growing 

demand for water resources. Shifts in the use of groundwater 

throughout the NACP have resulted in changes in water 

moving both into and out of aquifer storage over time. 

Understanding the timing and magnitude of changes in aquifer 

storage is critically important in assessing groundwater 

availability in the NACP aquifer system.

Hydrologic Budget for the Northern Atlantic 

Coastal Plain Aquifer System

The sources and sinks of water to the NACP aquifer 

system can be depicted in terms of a hydrologic budget to 

illustrate how water enters, flows through, and exits this 

aquifer system (fig. 21). Under steady-state conditions, it is 

assumed that inflows are balanced by outflows; however, in 

thick, multilayered, aquifer systems such as the NACP, the 

response to hydrologic stresses can be slow (on the order of 

decades), and therefore, changes in the amount of water stored 

and released in the aquifer system may continue to occur until 

a steady-state condition is achieved.

From this analysis, it was determined that, for conditions 

in 2005, about 20,000 Mgal/d of freshwater enters the 

NACP aquifer system with nearly all this inflow from 

natural recharge (about 98 percent), with minor amounts 

from wastewater return flow, water released from confined 

aquifer storage, and subsurface fractured rock flow across 

the Fall Zone (fig. 21B). A substantial amount of freshwater 

does enter the NACP as streamflow across the Fall Zone 

(18,600 Mgal/d); however, it is assumed that, at a regional 

scale, this freshwater inflow discharges directly to the 

coast without interacting with the underlying groundwater 

flow system.

The largest component of outflow or loss of freshwater 

to the NACP aquifer system is from evapotranspiration 

(derived from evaporation and plant transpiration) and surface 

runoff. About 69 percent of the precipitation that enters the 

NACP is lost to evapotranspiration or runoff, and therefore, 

of the 61,800 Mgal/d of precipitation in the NACP area, only 

19,600 Mgal/d enters the groundwater flow system as aquifer 

recharge (see appendix 1).

Most of the 19,600 Mgal/d of aquifer recharge enters 

the shallow unconfined aquifer and discharges to streams 

that either originate within or flow through the NACP or 

discharges directly to the coastal receiving waters and never 

reaches the deeper confined aquifer system. The previous 

regional assessment (Trapp and Meisler, 1992) estimated 

that only 2 percent of the recharge that enters the aquifer 

system reaches the deeper confined system under unstressed 

conditions. For this analysis, 2 percent of the total aquifer 

recharge would result in about 400 Mgal/d of recharge to the 

confined aquifer system.
38 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

The primary loss of freshwater from the NACP aquifer 

system is from groundwater discharge to streams. About 

18,600 Mgal/d of freshwater enters the NACP across the 

Fall Zone as streamflow. Streamflow increases by about 

3,800 Mgal/d from additional groundwater discharge to 

streams as the streams flow toward the coast. Another loss 

of freshwater from the aquifer system is from groundwater 

discharge to streams that originate entirely within the NACP. 

About 8,100 Mgal/d of groundwater discharges to the 

streams that originate in the NACP area and discharge to the 

coastal receiving waters. In total, about 11,900 Mgal/d of 

groundwater discharges to streams and transported to coastal 

receiving waters and, therefore, is removed from the aquifer 

system. Groundwater discharge to streams accounts for about 

59 percent of the 20,000 Mgal/d of total outflow of freshwater 

from the groundwater flow system for conditions in 2005 

(fig. 21B).

In 2005, groundwater withdrawals totaled about 

1,500 Mgal/d and represented about 8 percent of the total 

outflow from the groundwater flow system. Most of this water 

(about 1,000 Mgal/d) was withdrawn from the deep confined 

aquifer system, suggesting that the amount of water entering 

the deep system as recharge under predevelopment conditions 

(about 400 Mgal/d) accounted for less than half of the total 

water pumped from the confined system. For the hydrologic 

budget to be in balance, this pumping rate must be offset 

by decreases in outflows from predevelopment conditions 

(as defined by total inflow from recharge) and (or) from the 

release of water from aquifer storage. Decreases in outflows 

from predevelopment conditions may include a reduction in 

deep subsea discharge from the confined system or decreases 

in streamflow and shallow coastal discharge resulting from 

an increase in downward leakage (deep recharge) from the 

shallow, unconfined system (fig. 21).

The previous regional assessment (Trapp and Meisler, 

1992) determined that the source of most of the water pumped 

from the deep confined aquifer system was derived from 

induced infiltration from the overlying shallow, unconfined 

aquifer system. For predevelopment conditions, the area that 

contributed recharge to the deep confined aquifer system 

was estimated to be about 26 percent of the total land area 

of the NACP. For 1980 pumping conditions, this area was 

estimated to increase to about 45 percent of the total NACP 

land area. The previous analysis also determined that this 

Figure 21. Schematic showing hydrologic budget in millions of gallons per day (Mgal/d) for A, predevelopment 

(1900) and B, conditions in 2005 for the Northern Atlantic Coastal Plain aquifer system.

Subsurface fractured

rock flow 

>10 (<1%) 

Streamflow and

coastal discharge 

19,600 (100%)

Total recharge

19,600 (>99%)

Unconfined aquifers

Confined aquifers

Deep recharge

400

(2% of total recharge)

Total recharge 

19,600 (<98%)

Subsurface fractured

rock flow 

>10 (<1%) 

Pumping 

1,500 (8%)

Wastewater 

200 (1%)

Streamflow 

11,900 (59%) and

coastal discharge

6,600 (33%)

Unconfined aquifers

Confined aquifers

Deep recharge

>400

(>2% of total recharge)

Storage release 

0 to 200 (0–1%)

A

B
Hydrologic Conditions 39

increase in downward percolation (and resulting reduction 

in streamflow and shallow coastal discharge) would account 

for 61 percent of the total loss of water from pumping. An 

additional 37 percent of the loss of water from the aquifer 

system from pumping was derived from a decrease in deep 

subsea discharge to coastal areas, which creates concerns for 

saltwater intrusion into the freshwater system (fig. 13).

The source of the remaining 2 percent of the water lost 

to the aquifer system from pumping was determined to be 

from the release of water from storage. This estimate from 

the previous analysis (Trapp and Meisler, 1992) was based 

on a quasi-3D modeling approach, in which confining units 

were not explicitly simulated but were represented instead 

by equivalent vertical conductance between model layers 

representing aquifers. Therefore, this approach did not 

consider potential storage release from the confining units.

More recent studies have determined that the release 

of water from confining unit storage potentially could be a 

substantial source of water to pumped wells in the confined 

aquifer system (Heywood and Pope, 2009; Konikow, 2013). 

Konikow (2013) determined that release of water from 

storage (that is, groundwater depletion) was about 200 Mgal/d 

for 2001–2008, about 13 percent of 2005 pumping rate 

(1,500 Mgal/d). Pope and Burbey (2004) determined that 

storage release from the confining units in response to large 

groundwater withdrawals in southern Virginia resulted in 

dewatering and compaction of these fine-grained sediments, 

explaining in part the increased rates of land subsidence and 

anomalously higher rates of sea-level rise observed in the 

lower Chesapeake Bay area.

The remaining loss of groundwater from the NACP 

aquifer system is from shallow and deep subsea coastal 

discharge. Unlike the rate of groundwater withdrawals and 

streamflow, the rate of groundwater discharge to the coast 

is impossible to measure directly for this regional system. 

Therefore, absent of a groundwater flow model, the rates of 

groundwater discharge to the coast can be estimated only by 

a process of elimination in order to balance out the known 

inflows to the flow system, assuming that the water released 

from storage can be quantified.

Assuming that about 13 percent (200 Mgal/d) of the 

groundwater pumping (1,500 Mgal/d) was derived from a 

release of water from storage, the amount of water estimated 

to discharge to the coast would be about 6,600 Mgal/d, or 

33 percent of the total outflow, from the groundwater system 

as shown in figure 21B. A numerical model that explicitly 

represents confining unit storage would be required to refine 

the estimates of the amount of water released from storage and 

the amount of shallow coastal and deep subsea discharge and 

would be instrumental in assessing the potential for decreases 

in groundwater discharge to the coastand the resulting effects.

Changes in Hydrologic Conditions from 1900 

to Present

The NACP aquifer system has undergone significant 

changes in the past 100 years in response to the large changes 

in population and the advent of large-scale public supply 

systems for the delivery of drinking water and the removal 

of wastewater. In additional to changes in groundwater 

withdrawals, changes also occurred in precipitation rates and 

temperature thereby affecting aquifer recharge and discharge 

to fresh and coastal receiving waters.

Aquifer Recharge

Aquifer recharge is a function of temperature, 

precipitation, and land use; therefore changes in any of these 

factors can affect the rate and distribution of recharge across 

the study area. An analysis presented in the report appendix 

of the historical changes in temperature and precipitation 

suggests that area the average-annual temperature has 

increased by 3 degrees Fahrenheit from 1895 to 2010 in the 

NACP while precipitation changes during the same period are 

much more variable (fig. 22). A limited analysis of recharge 

rates were calculated for the 5-year periods of 1895 to 1900 

and 2005 through 2009. This analysis determined that the 

average recharge rates for these two periods decreased from 

about 16.6 in/yr to 13.9 in/yr; however, a more complete 

analysis during the entire 115-year period of record would be 

needed to assess fully any possible trend in recharge over time.

Furthermore, land use and land cover has changed with 

increased population, resulting in more runoff and evaporation 

from impervious surfaces that can reduce aquifer recharge 

over time. In attempt to mitigate these effects, densely 

populated areas, such as on Long Island and in New Jersey, 

have installed recharge/catchment basins to recharge the 

surficial aquifer system from captured storm-water runoff on 

impervious surfaces. The effects of these infiltration basins 

on the water-levels in the shallow, unconfined groundwater 

systems is such that locally these basins may even increase 

recharge above predevelopment rates (Ku and others, 1992; 

Carleton, 2010).

Groundwater Withdrawals

Groundwater withdrawals in the NACP began in earnest 

in the late 1890s and account for about 1,500 Mgal/d of water 

removed from this aquifer system for conditions in 2005 

(table 3). Although pumping rates have generally increased 

for more than a century, notable large-scale decreases in 

groundwater withdrawals have occurred at different points 

in time in response to contamination concerns, changes in 

industrial water-use practices, or improved water conservation 

measures. In the northern part of the study area, there 

were large reductions in groundwater withdrawals in the 

late 1930s in western Long Island and in the late 1980s 
40 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

in east-central New Jersey in response to effects from 

overpumping on surface receiving waters and the threat of 

saltwater contamination to drinking water supplies. A more 

recent reduction in groundwater withdrawals also occurred in 

southern Virginia because of changes in water use by the pulp 

and paper industry.

Shift in Groundwater Use: Western Long Island and 

East-Central New Jersey

Before the mid-1930s, groundwater was the sole source 

of freshwater for the densely populated areas of Kings and 

Queens Counties in western Long Island (fig. 2). High rates 

of withdrawals resulted in large drawdowns of the water 

table and large declines in potentiometric heads of underlying 

aquifers in these areas. The development and completion 

of sanitary sewer systems during this period compounded 

the effect of withdrawals on the groundwater system by 

diverting wastewater directly to coastal water bodies instead 

of returning it as recharge to the underlying aquifer system. 

By 1936, water levels in northern Kings County had declined 

by as much as 45 ft from 1903 levels (fig. 23A, B), and by 

1951, water levels in southern Queens County had declined 

by as much as 35 ft from 1903 levels (fig. 23C; Buxton and 

Shernoff, 1999).

The decline in water levels caused the natural seaward 

groundwater flow gradient to reverse in some areas, inducing 

saltwater intrusion, which in turn necessitated the shutdown of 

all public-supply wells in Kings County and some in Queens 

County by the 1940s. From that point forward, water-supply 

needs in this area were met by water imported from a reservoir 

system in upstate New York. Once pumping in this area had 

stopped, the water table began to recover to predevelopment 

conditions (fig. 23D), and the rising water levels in some 

areas ultimately began to flood subterranean structures, such 

as basements and subway tunnels, that had been built during 

the period when the water table was much lower (Buxton and 

Shernoff, 1999).

Because of this water-level recovery from the cessation 

of pumping, many basements and subway tunnels in Kings 

County today require nearly continuous dewatering. The 

pumped water is directed to the combined sanitary and stormwater sewer system, which ultimately discharges to nearby 

coastal waters and thereby is removed from the groundwater 

Figure 22. Observed and predicted changes in temperature and precipitation over time for the Northern Atlantic Coastal Plain aquifer 

system. Data are for 5-year moving average values for 1895 through 2050 for the area near 74°39′39′′ W 39°54′40′′ N in Southampton 

Township, New Jersey. °F, degrees Fahrenheit; PRISM, Prism Climate Group (2007) data; R2, coefficient of determination; USFS, U.S. 

Forest Service.

R2 = 0.6115

R2 = 0.0903

A

B

Temperature, in degrees Fahrenheit Precipitation, in inches

75

70

65

60

55

55

45

40

35

30

50

PRISM

USFS Csiromk35 A1B climate model

emissions scenario

USFS Miroc32 A1B climate model

emissions scenario

USFS Cgcm31 A1B climate model

emissions scenario

Trend line (PRISM)

PRISM

USFS Csiromk35 A1B climate model

emissions scenario

USFS Miroc32 A1B climate model 

emissions scenario

USFS Cgcm31 A1B climate model 

emissions scenario

Trend line (PRISM)

Year

Annual average maximum temperature,

in degrees Fahrenheit

EXPLANATION

EXPLANATION

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020 2030 2040 2050

Annual average maximum precipitation,

in inches
Hydrologic Conditions 41

system. Dewatering accounts for at least 30 percent of the 

estimated 40 Mgal/d pumped for industrial use in Kings 

and western Queens Counties with the New York City 

Metropolitan Transit Authority alone, withdrawing as much as 

10 Mgal/d from subway tunnels in Kings County (Misut and 

Monti, 1999).

By the late 1980s, parts of east-central New Jersey 

experienced a similar response to overpumping as that 

observed in western Long Island, raising concerns of saltwater 

intrusion and reductions of groundwater flow to ecologically 

sensitive surface waters in what is referred to as Water 

Supply Critical Area 1 (Spitz and others, 2008). In this area, 

groundwater withdrawals generally were reduced by 25 to 

30 Mgal/d by the late 1980s because the nearby Manasquan 

Reservoir was used to meet water-supply needs. By 2003, 

reductions in pumping resulted in water-level increases in the 

local Middle Potomac-Raritan-Magothy aquifer (referred to 

regionally as the Potomac-Patapsco aquifer; fig. 9) by more 

than 80 ft near large pumping centers in northern Monmouth 

County and as much as 10 ft on Sandy Hook, N.J. (fig. 24).

Across the Raritan and Lower New York Bays, water 

levels in the local Lloyd aquifer also rose by as much as 5 ft 

on Coney Island, N.Y. (fig. 25A). The water-level changes in 

the local Lloyd aquifer occurred during a similar time in which 

water levels were recovering in the local Middle PotomacRaritan-Magothy aquifer at Fort Hancock near Sandy Hook 

(fig. 25B), the lateral equivalent to the local Lloyd aquifer. 

A comparison of changes in pumping on Long Island and 

in northern New Jersey indicate that pumping decreased in 

both areas from 1988 to 1996. Changes in water levels in the 

local Lloyd aquifer on Long Island appear to be affected less 

by changes in pumping in the local Lloyd aquifer than by 

changes in pumping in the overlying local Magothy aquifer 

on Long Island or possibly by decreases in pumping in New 

Jersey (fig. 25A–D). Further investigation would be needed 

to assess fully the potential hydrologic connection between 

the local Lloyd and Magothy aquifers on western Long Island 

or the possibility of a hydrologic connection between New 

Jersey and Long Island in the local Middle Potomac-RaritanMagothy and Lloyd aquifers (referred to as the PotomacPatapsco regional aquifer; fig. 9).

Shift in Groundwater Use: Southern Virginia

The pulp and paper industry in southeastern Virginia 

and northeastern North Carolina has been the largest user of 

groundwater in this region since the 1960s. The large amounts 

of water used by this industry have resulted in large-scale 

declines in water levels in this region (fig. 4). The recent 

(2009) economic downturn resulted in dramatically curtailed 

operations for this industry, which led to substantial reductions 

in groundwater withdrawals. As an example, a single pulp 

mill and paper manufacturing facility in southeastern Virginia 

pumped more than 30 Mgal/d until a reduction in operations 

in 2009 followed by a complete shutdown in early 2010 

(Katchmark, 2012). Since the shutdown, this cessation of 

withdrawals has resulted in a rapid recovery of water levels 

in the surrounding area (fig. 26). By April 2012, water levels 

in the Potomac aquifer near the well field recovered more 

than 100 ft, and water levels as much as 40 mi away from the 

facility recovered by about 4 ft. Since the recovery, the facility 

has resumed limited operations and water levels near the well 

field have begun to decline again.

Regional Effects of Recent Pumping

Since large-scale groundwater withdrawals began 

in the NACP at the turn of 20th century, about 110 cubic 

kilometers (km3) of freshwater have been withdrawn from the 

groundwater flow system. Half (about 55 km3) of this total 

volume of water was removed from the flow system during the 

80-year period from 1900 to 1980; the remaining half of the 

total withdrawals occurred only in the past 30 years (fig. 27). 

Konikow (2013) estimates that these groundwater withdrawals 

have resulted in about 10 km3 of groundwater depletion from 

1900–2008 with about 25 percent of this depletion (2.5 km3) 

occurring from 2000– 2008. Therefore, the long-term 

hydrologic effects of the large removal of groundwater from 

the NACP aquifer system during a relatively short period of 

time (30 years) may yet to have occurred, particularly with 

respect to decreases in groundwater discharge to streams and 

coastal receiving waters and in areas of suspected compaction 

of confining units and the accompanying land subsidence.

During the 20-year period from 1985 to 2005, publicsupply withdrawals have increased in the NACP on average by 

about 25 percent (fig. 28). During this same period, summer 

season (May-September) usage increased by about 50 percent, 

nearly three times the rate of winter season (October–April) 

increase (20 percent), suggesting that increased population 

and development throughout the NACP (fig. 2B) that requires 

greater water use during the summer season may account for 

most of the total increases during this 20-year period. Such 

increases in development in coastal areas create additional 

concerns for potential increases in saltwater intrusion at 

public-supply sources.

Increased pumping from the confined aquifer system 

in coastal areas can result in a decrease in the deep subsea 

discharge needed to balance the interface between freshwater 

and saltwater (figs. 13, 21B). Assuming that storage release 

from confining units can be a significant source of water 

pumped from the confined system (Konikow and Nuezil, 

2007), the response of the flow system to increased pumping 

could be delayed as water is released from storage and effects 

such as saltwater intrusion may yet to be fully realized for 

recent pumping conditions (fig. 13).

Wastewater Return Flow

In addition to concerns about changes in groundwater 

withdrawals, concerns also exist regarding the return of 

wastewater to the groundwater flow system from onsite 

domestic systems as well as from centralized treatment 
42 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York to North Carolina

Figure 23. Water levels in Kings and Queens Counties, New York, in A, 1903, B, 1936, C, 1951, and D, 2006.

40°45'

-74°00' -73°45'

Base modified from U.S. Geological Survey, 1:100,000;

Hydrography and cultural features modified to reflect

the period from 1903 through 1936

40°45'

-74°00' -73°45'

Base modified from U.S. Geological Survey, 1:100,000;

Hydrography and cultural features modified to reflect

the period from 1903 through 1936

ATLANTIC OCEAN

Flushing Bay

Jamaica Bay

Flushing

Creek

Little Neck Bay

ATLANTIC OCEAN

Flushing Bay

Jamaica Bay

Flushing

Creek

Little Neck Bay

KINGS

COUNTY

QUEENS

COUNTY

NASSAU

COUNTY

KINGS

COUNTY

QUEENS

COUNTY

NASSAU

COUNTY

5

5

5

5

10

10

10

10

20

20

20

20

30

30

30

40

50

40

40

40

50

EXPLANATION

5 Water-table contour—Shows altitude of 

 water table. Contour interval 5 and 10 feet. 

 Datum is sea level

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

A

B

10

10

10

20

20

30

40

0

0

-10

-20

-30

EXPLANATION

0 Water-table contour—Shows altitude of water 

 table. Contour interval 10 feet. Datum is sea 

 level. Hachures indicate depression

NY

NJ

CT

Area of

figure

NY

NJ

CT

Area of

figure
Hydrologic Conditions 43

Figure 23. Water levels in Kings and Queens Counties, New York, in A, 1903, B, 1936, C, 1951, and D, 2006.

-5

-5

ATLANTIC OCEAN

Flushing Bay FlushingCreek

Little Neck Bay

QUEENS

COUNTY

NASSAU

COUNTY

KINGS COUNTY

40°45'

-74°00' -73°45' EXPLANATION

0 Water-table contour—Shows altitude of water table. 

 Contour interval 5 feet. Datum is sea level. 

 Hachures indicate depression 

5

10

15

20

25 30

35

40

45

5

5

5

10

10

15

20

20

25

50

0

-10

0

-5

-15

-5

-5

15

Base modified from U.S. Geological Survey, 1:100,000;

Long Island West, New York-New Jersey- Connecticut, 1984;

Newark, New Jersey - New York, 1986

Jamaica Bay

EXPLANATION

0 Water-table contour—Shows altitude of water

 table in 2006. Contour interval 10 feet. 

 Datum is sea level. 

 Dashed where inferred

ATLANTIC OCEAN

Flushing Bay

Flushing

Creek

Little Neck Bay

QUEENS COUNTY

NASSAU

COUNTY

Jamaica Bay

KINGS COUNTY

40°45'

-74°00' -73°45'

Base modified from U.S. Geological Survey, 1:100,000;

Long Island West, New York-New Jersey- Connecticut, 1984;

Newark, New Jersey - New York, 1986

10

30

10

30

40

20

40

40

30

10

20

20

10

10

10

30

20

40

50

10

NY

NJ

CT

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

NY

NJ

CT

Area of

figure

Area of

figure

C

D
44 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Figure 24. Water-level recovery in east-central New Jersey in late 1980s (from Spitz and others, 2008) and the surface contours of 

the top of the Middle Potomac-Raritan-Magothy (PRM) and Lloyd aquifers, Sandy Hook, New Jersey, and Coney Island, New York. 

The observation wells are U.S. Geological Survey identification number (ID) 403451073585601 (local site designation K2858.1) on Coney 

Island, and 402705073595902 (local site designation Ft Hancock 5A) at Sandy Hook. NAVD 88, North American Vertical Datum of 1988 ; 

NGVD 29, National Geodetic Vertical Datum of 1929.

Ft Hancock 5A

402705073595902

K2858.1

403451073585601

-74°10' -74°00' -73°50'

40°40'

40°30'

40°20'

40°10'

Northern extent of aquifer unit

EXPLANATION

Sea floor altitude, in feet below NAVD 88

Pumping well—Indicates aquifer from

 which the well draws

Observation well—Top label is the local

 name, bottom label is the U.S. Geological

 Survey station identification number 

-405 to -400

-399 to -350

-349 to -300

-299 to -250

-249 to -200

-199 to -150

-149 to -100

-99 to -50

-49 to 0

Altitude of Middle PRM and Lloyd aquifers,

 in feet—Dashed where inferred. Queried

 where uncertain. Datum is NGVD 29

10 to 20

20 to 40

40 to 80

Greater than 80

Water level recovery, in feet

Middle PRM 

Magothy 

Lloyd

Northern extent of aquifer unit—Queried

 where uncertain

NEW YORK

NEW JERSEY

NEW JERSEY

NEW YORK

UNION 

COUNTY

KINGS

COUNTY

QUEENS 

COUNTY

NASSAU 

COUNTY

MONMOUTH 

COUNTY

MIDDLESEX

COUNTY

RICHMOND 

COUNTY

ESSEX 

COUNTY

HUDSON 

COUNTY

OCEAN

COUNTY

Sandy Hook

Bay

Raritan

Bay

Lower

Bay

Base from U.S. Geological Survey digital data 1:100,000 series

Albers Equal Area Conic projection

Standard Parallels 34o 30' N and 41o 30' N; central meridian 75o 30' W 

North American Datum of 1983

?

-800

-600

-400

-800

-600

-400

-1,000

-1,200

-1,200

-1,400

-200 ?

?

-100

-1,300

Sandy Hook

Coney Island

-1,000

Jamaica Bay

4

4

0 8 MILES

0 8 KILOMETERS
Hydrologic Conditions 45

Figure 25. Changes in water levels in the local Lloyd and Middle Potomac-Raritan-Magothy (PRM) aquifers and changes 

in pumping in the A, Lloyd aquifer, B, Middle PRM aquifer, C, Lloyd and Middle PRM aquifers combined, and D, the Lloyd and 

Magothy aquifers combined.

Year

19801981198219831984198519861987198819891990199119921993199419951996199719981999200020012002200320042005200620072008

45

40

35

30

25

20

15

10

5

0

10

8

6

4

2

0

-2

-4

-6

-8

-10

-12

35

30

25

20

15

10

5

0

10

8

6

4

2

0

-2

-4

-6

-8

-10

-12

30

25

20

15

10

5

0

10

8

6

4

2

0

-2

-4

-6

-8

-10

-12

8

6

4

2

0

Annual rate of withdrawals in million gallons per day

Water levels in feet above or below NGVD 29

10

8

6

4

2

0

-2

-4

-6

-8

-10

-12

Middle PRM

Lloyd

Magothy

402705073595902

(Ft. Hancock 5A)

403451073585601

(K2859.1)

EXPLANATION

Withdrawals from aquifer

U.S. Geological Survey

 well identification number

A

B

C

D
46 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Figure 26. Hydrograph showing effects of pumping on groundwater levels in southeastern Virginia from 1960 through 2012. Data 

are from U.S. Geological Survey well 36405907654901 (local site designation 55B 16) east of Franklin, Isle of White County. NGVD 29, 

National Geodetic Vertical Datum of 1929.

-50

-70

-90

-110

-130

-150

-170

-190

-210

-230

-250

1960 1970 1980 1990 2000 2010

Year

Water level elevation, in feet below NGVD 1929

ATLANTIC OCEAN

NC

DE

PA

VA

MD

NJ

NY CT

55B 16

N1614

(fig. 29)

DC

WV

39°

36°

-78° -72°

0 50 100 KILOMETERS

0 50 100 MILES

Figure 27. Groundwater withdrawal rates from the Northern Atlantic Coastal Plain aquifer system from 1900 to 2010.

0

200

400

600

800

1,000

1,200

1,400

1,600

Withdrawal rate, in million gallons per day

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Year
Hydrologic Conditions 47

Figure 28. Seasonal changes in groundwater withdrawals from 1985 through 2005 in the Northern Atlantic Coastal Plain 

aquifer system.

December

August

Annual

Winter

Summer

1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Year

Withdrawals, in billion gallons

280

260

240

220

200

180

160

140

120

100

80

60

40

20

0

EXPLANATION

300

Seasonal withdrawals

Trend

facilities. In 2005, more than 20 million people in the NACP 

use about 2,500 Mgal/d of drinking water from groundwater 

and surface water sources (Kenny and others, 2009). 

Depending on where this water is returned to the hydrologic 

system and the extent to which it is treated can affect the 

quantity and quality of the groundwater that discharges to 

ponds, streams, and coastal waters.

The Mid-Atlantic area includes many important coastal 

ecosystems, such as the New York-New Jersey Harbor 

Estuary, New Jersey Barnegat Bay, New Jersey Meadowlands, 

Delaware Inland Bays, Delaware Seashore, Delaware Estuary, 

and Chesapeake Bay (the largest estuary in the United States 

and the first in the Nation to be selected for restoration as an 

integrated watershed ecosystem; Chesapeake Bay Program, 

(2010)). Increased discharge of wastewater to the shallow 

flow system can increase nutrient loading and concerns about 

eutrophication in the near-shore ecosystems of these coastal 

receiving waters.

In response to increased concerns about nonpoint source 

contamination of the local groundwater system, Nassau 

County on Long Island began to transition in the mid-1950s 

from onsite domestic septic systems to more centralized 

wastewater treatment facilities (Buxton and Smolensky, 

1999). By 1983, nearly 66 percent of the 183 Mgal/d of 

groundwater pumped in Nassau County was discharged to 

coastal waters from sanitary sewers. The removal of this 

water from the groundwater system resulted in decreased 

water levels (fig. 29; Busciolano, 2005). Numerical modeling 

analyses by Buxton and Smolensky (1999) determined that 

water levels in this area are about 8 ft lower than they would 

have been had the practice of discharging wastewater to 

onsite domestic septic systems continued. The transition from 

onsite domestic systems to centralized wastewater treatment 

facilities also resulted in decreased flow to groundwaterfed streams. The most notable example of this hydrologic 

response is East Meadow Brook in southwest Nassau County, 

where flows have decreased by as much as 70 percent from 

predevelopment conditions (Scorca, 1997).

Although sanitary sewer systems were designed to reduce 

nonpoint contamination from septic systems, these systems 

are not 100 percent efficient. In 1983, recharge to the shallow 

aquifer system from leaky sewer and water-supply lines, as 

calculated from the number of miles of lines and the number 

of connections and multiplied by a leakage factor, is estimated 
48 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

to have been as much as 70 Mgal/d in Kings and Queens 

Counties on western Long Island (Buxton and Shernoff, 

1999). Additional leakage from combined sanitary and storm 

sewers has probably increased, because the lines have aged, 

and therefore leakage from sewer and supply lines has become 

an increasingly large component of the hydrologic budget in 

the urbanized areas of the NACP.

Saltwater Intrusion

The interface between fresh and saline groundwater 

generally appears to coincide with the coastline throughout 

most of the NACP in the Potomac aquifer (fig. 30) but 

potentially can be shifted inland locally in response to 

groundwater withdrawals (fig. 13). The previous regional 

analysis (Trapp and Meisler, 1992) indicated that a decrease in 

groundwater discharge to streams and coastal receiving waters 

in response to increased pumping from 1900 to 1980 created 

the potential for saltwater intrusion into this coastal aquifer 

system. It was in response to saltwater intrusion concerns in 

the mid-1930s and later in the late 1980s that initiated the 

reduction of pumping and increase in imported surface water 

in western Long Island and east-central New Jersey (see Shift 

in Groundwater Use: Western Long Island and East-Central 

New Jersey section).

More recent saltwater intrusion concerns in coastal areas 

such as Cape May, N.J. (fig. 1) have prompted the need for 

the desalinization of saline groundwater to help meet drinking 

water demand in these coastal communities. The effects of 

increased pumping on the position of the interface between 

freshwater and saltwater are expected to be exacerbated 

further by rising sea levels and land subsidence from the 

compaction of confining units, particularly in the lower 

Chesapeake Bay area.

Climate Change and Groundwater Resources

The potential effects of long-term climate change and 

variability on the hydrologic system and availability of 

water resources continue to be of serious societal concern 

Figure 29. Water-level response to increased sewering in Nassau County, New York, from 1940 through 2010. Data are from U.S. 

Geological Survey station identification 404446073392901 (local site designation N1614.1) for 1940 through 1950; 404446073392902 

(N1614.2) for 1951 through 1962; 404446073392903 (N1614.3) for 1963 through 1995; 404446073392904 (N1614.4) for 1966 through 2000; and 

404454073393001 (N1614.5) for 2001 through 2010.

45

50

55

60

65

70

75

1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Year

Water level elevation,

in feet below NGVD 1929

ATLANTIC OCEAN

NC

DE

PA

VA

MD

NJ

NY CT

55B 16

(fig. 26)

N1614

DC

WV

39°

36°

-78° -72°

0 50 100 KILOMETERS

0 50 100 MILES
Hydrologic Conditions 49

in the NACP. Most climate models (Coulson and others, 

2010) predict warmer temperatures with more variable and 

unreliable precipitation patterns (fig. 22).

Warmer temperatures potentially may have any number 

of effects that could act as drivers to increase summertime 

water demand. Increased evapotranspiration coupled with 

more highly variable precipitation patterns will likely prompt 

agricultural producers to rely more heavily on irrigation from 

groundwater sources to minimize drought risk. Livestock 

producers will likely demand more water for consumption and 

stock cooling. Increased demand for domestic and commercial 

cooling is likely to require increased electricity production 

and commensurate water use by thermoelectric power plants 

(Legesse and others, 2003).

Another concern facing coastal communities in the 

NACP is the potential effects of sea-level rise on coastal 

aquifer systems. Trends in relative sea-level rise from 1902 

to 2006 vary across the region from about 2.4 millimeters per 

year (mm/yr) on the northern shore of Long Island to more 

than 6.0 mm/yr at the Chesapeake Bay Bridge Tunnel (fig. 31; 

National Oceanic and Atmospheric Administration, 2012). 

Sea-level rise throughout the region averages about 3.5 mm/yr, 

or about 1 ft total for the past 100 years, a rate higher than the 

apparent average for the Atlantic Coast.

Recent analyses of the relative rates of sea-level rise 

along the Atlantic coast indicate that the Mid-Atlantic region 

represents a hot spot with anomalously higher rates of sealevel rise than observed elsewhere in the U.S. (Sallenger and 

others, 2012). In the lower Chesapeake Bay area, it is believed 

that the anomalous rates of observed sea-level rise are the 

result of eustatic sea-level rise coupled with land subsidence 

from the compaction of confining units caused by groundwater 

pumping (Pope and Burbey, 2004). These concerns are 

heightened because projected rates of sea-level rise could 

result in increases in mean sea level of 3 to 5 ft during the next 

100 years in parts of the NACP (Intergovernmental Panel on 

Climate Change, 2008) resulting in changes in groundwater 

levels, surface-water discharge, and increased potential for 

saltwater intrusion (Masterson and Garabedian, 2007).

Figure 30. Chloride distribution in the regional Potomac-Patapsco aquifer of the Northern Atlantic Coastal Plain aquifer system. 

Modified from Meisler (1989) by Frederick Stumm (U.S. Geological Survey, written commun., 2012).

1,000

EXPLANATION

Line of equal chloride concentration,

 in milligrams per liter—Dashed where

 approximately located. Interval is variable

Location of test or observation well.

NACP limit

0 25 50 100 MILES

0 25 50 100 KILOMETERS

DELAWARE

-80° -78° -76°

34°

-78°

-80°

36° 38° 40° 42°

? ? ? ?

NORTH CAROLINA

SOUTH CAROLINA

VIRGINIA

MARYLAND PENNSYLVANIA

NEW JERSEY

NEW YORK

CONNECTICUT

Fall Zone

ATLANTIC OCEAN

Pamlico

Sound

Albemarle Sound

Chesapeake Bay

Delaware Bay

Raritan Bay

Long Island Sound

1,000

5,000

10,000

18,000

Base from U.S. Geological Survey Digital Data 1:2,000,000 series

Universe Transverse Mercator Projection Zone 18N

Central meridian 75 W, Rotation 305 degrees

North American Datum of 1983

DC

l

N
50 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Figure 31. Trends in sea-level rise across the Northern Atlantic Coastal Plain from 1902 to 2006. From National Oceanic and 

Atmospheric Administration (2012).

-78° -72°

39°

36°

EXPLANATION

Study area

Sea-level rise trends—

 in millimeters per year

0.8 to 1.0

1.1 to 2.0

2.1 to 3.0

3.1 to 4.0

4.1 to 5.0

5.1 to 6.0

6.1 to 7.0

Chesapeake Bay

Delaware Bay

Long Island Sound

ATLANTIC OCEAN

Albemarle Sound

Pamlico Sound

DC

Base from U.S. Geological Survey digital data 1:2,000,000 series

Albers Equal Area Conic projection

Standard Parallels 34° 30' N and 41° 30' N; central meridian 75° 30' W 

North American Datum of 1983

0 25 50 100 MILES

0 25 50 100 KILOMETERS

NEW YORK CT

NEW

PENNSYLVANIA JERSEY

MARYLAND VIRGINIA

WEST

VIRGINIA

DELAWARE

NORTH CAROLINA
Summary and Conclusions 51

Summary and Conclusions

The Northern Atlantic Coastal Plain (NACP) occupies 

a land area of about 30,000 square miles along the eastern 

seaboard of the United States from Long Island, New York, 

southward to the northern part of North Carolina. A seawarddipping wedge of sediments comprising clay, silt, and sand 

and gravel underlies this area. This sedimentary wedge forms 

a complex groundwater system and constitutes a major source 

of public and domestic water supply as well as a vital source 

of freshwater for industrial and agricultural uses throughout 

the region.

Continued land development and population growth in 

the NACP during the past 100 years have resulted in increased 

and diverse demands for fresh water and have created 

concerns regarding the supply of potable groundwater and the 

quality and quantity of water discharging to ponds, streams, 

and coastal waters. Substantial withdrawals of groundwater 

from the underlying aquifer system had begun by the late 

1800s. Groundwater withdrawals have increased steadily 

throughout the NACP since then, and as of 2005, groundwater 

withdrawals have increased to about 1,500 million gallons per 

day (Mgal/d), serving as a vital source of drinking water for 

the region.

The need for water-resources management in this area 

creates hydrologic challenges beyond those imposed by 

the competing local domestic, industrial, agricultural, and 

environmental demands for water. Large changes in regional 

water use have made single-state management of aquifer 

resources increasingly more difficult because of hydrologic 

effects that extend beyond State boundaries. In response to 

this need, the U.S. Geological Survey (USGS) Groundwater 

Resources Program began a second regional assessment of 

the groundwater availability of the NACP aquifer system in 

2010. As part of the USGS ongoing regional assessments 

of groundwater availability of the principal aquifers of the 

Nation, the goal of this analysis is to provide an updated 

regional characterization of the hydrogeologic framework and 

conditions of the NACP aquifer system.

The areal extent of what was defined previously as the 

NACP aquifer system has been reduced from the area defined 

in the first analysis of the aquifer system to focus primarily 

from Long Island to Virginia to complement a recently 

completed regional assessment on the Atlantic Coastal Plain 

aquifer system of North and South Carolina (Campbell and 

Coes, 2010). Consequently, several regional aquifer names 

have been edited in this investigation to remove references to 

units in North Carolina and emphasize nomenclature from the 

primary States of the study area, which are New York, New 

Jersey, Delaware, Maryland, and Virginia.

Other revisions to the hydrogeologic framework reflect 

substantial revisions in the understanding of hydrogeologic 

correlations across the study area since the previous analysis. 

A primary example is the treatment of the aquifers of the 

regional Potomac aquifer system, which now is considered to 

be a single aquifer in Virginia. In Maryland, Delaware, and 

New Jersey, the Potomac aquifer system is subdivided into 

two aquifers as it thickens within the Salisbury Embayment. 

Regionally, the two parts of the Potomac aquifer system in 

these States are referred to as the Potomac-Patapsco and 

Potomac-Patuxent aquifers. Farther to the north in New York, 

the Potomac-Patapsco aquifer consists locally of the Lloyd 

aquifer, and the Potomac-Patuxent aquifer is absent.

The final substantial revision to the hydrogeologic 

framework of the NACP is the recognition of the Chesapeake 

Bay impact crater, which includes three hydrogeologic units—

the Chickahominy confining unit, the Exmore matrix confining 

unit, and the Exmore clast confining unit—that comprise the 

crater-fill materials.

The hydrologic budget also has been updated since the 

previous regional assessment to reflect the change in the study 

area extent and hydrologic data collection since the 1980s. 

The average annual rate of precipitation of about 45 inches 

per year (in/yr) resulted in about 62,000 Mgal/d or 23 trillion 

gallons per year (Tgal/yr) of freshwater input. Although there 

is as much 23 Tgal/yr of precipitation in the NACP study area, 

only 31 percent (19,600 Mgal/d or about 7 Tgal/yr) of it enters 

the underlying aquifer system as recharge, but accounts for 

more than 99 percent of the total inflow of freshwater to the 

NACP aquifer system.

Remaining inflows to aquifer system include wastewater 

from onsite domestic septic systems in nonsewered areas and 

leaking water-supply lines from water imported from outside 

the NACP. In 2005, about 3.3 million residents discharged as 

much as 200 Mgal/d of wastewater from onsite septic systems 

to the shallow, unconfined aquifers throughout the NACP.

Most (62 percent) of the 30,500 Mgal/d of streamflow 

enters the NACP across the Fall Zone on the western border 

of the NACP and discharges to the coast without interacting 

with the underlying aquifer system. The remaining 38 percent 

of the total streamflow that does interact with the groundwater 

system is groundwater discharge to streams that is lost from 

the aquifer system. This discharge (11,900 Mgal/d) accounts 

for about 59 percent of the total groundwater outflow from 

the system.

Groundwater withdrawals account for about 8 percent 

of the total outflow from the NACP aquifer system. In 2005, 

the total amount of groundwater withdrawn was about 

1,500 Mgal/d. The groundwater withdrawn for drinking water 

accounted for 65 percent of all groundwater withdrawals 

in the study area. Most (70 percent) of these groundwater 

withdrawals occurred on Long Island and in New Jersey. 

The total agricultural water use in the NACP in 2005 was 

estimated to be about 300 Mgal/d, with about 60 percent 

(179 Mgal/d) derived from groundwater sources Commercial 

and industrial freshwater use in the NACP in 2005 was about 

800 Mgal/d, of which, 43 percent (338 Mgal/d) was derived 

from groundwater sources. Groundwater withdrawals in 

Virginia and North Carolina, accounted for about 62 percent 

of the total commercial and industrial groundwater uses in 

the NACP.
52 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Although the NACP aquifer system consists of ten 

principal aquifers, the largest withdrawals occur in the Potomac 

aquifer system. Withdrawals from the Potomac aquifer system 

account for about 57 percent of the total withdrawals in the 

NACP aquifer system. The Surficial and the Upper and Lower 

Chesapeake regional aquifers combined account for about 33 

percent of the total use. The Surficial regional aquifer provides 

almost half (49 percent) the groundwater pumped in Delaware 

and almost a third of the water pumped in Maryland (30 

percent). Withdrawals from the Lower Chesapeake regional 

aquifer account for 15 percent of the total withdrawals in 

the NACP and make up about half (44 percent) of the total 

withdrawals in New Jersey; the Lower Chesapeake regional 

aquifer is the primary source of drinking water in southeastern 

and coastal areas of the State. About only 6 percent of the total 

groundwater in the NACP aquifer system is withdrawn from the 

Magothy regional aquifer and accounts for more than 77 percent 

of total withdrawals on Long Island. The remaining 4 percent of 

the total withdrawals in the NACP aquifer system were derived 

from the Piney Point, Aquia, Monmouth Mount-Laurel, and 

Matawan regional aquifers.

The remaining outflow component of the revised 

hydrologic budget for the NACP aquifer system is coastal 

discharge. Coastal discharge cannot be measured directly 

so it is determined indirectly by the process of balancing 

inflows and outflows to and from the NACP aquifer system. 

Assuming that the release of water from aquifer storage is about 

200 Mgal/d for pumping conditions in 2005, then the amount of 

groundwater discharge to the coast would be 6,600 Mgal/d, or 

33 percent of the total outflow from the groundwater system.

Effects of climate variability and change on the 

aquifer system were not considered as part of the previous 

RASA analysis. An analysis of changes in temperature and 

precipitation conducted as part of this investigation suggest 

that, in the NACP area, the average annual temperature has 

increased by 3 degrees Fahrenheit from 1895 to 2010, whereas 

precipitation changes during the same period are much more 

variable. A limited analysis of recharge rates calculated for 

the 5-year periods of 1895 to 1900 and 2005 through 2009 

indicate that the average recharge rates for these two periods 

decreased from about 16.6 in/yr to 13.9 in/yr.

Another set of climate-change effects not previously 

considered in the earlier analysis of the NACP aquifer system 

are the potential effects of sea-level rise on coastal aquifer 

systems. Regional trends in relative sea-level rise vary across 

the region from about 2.4 millimeter per year (mm/yr) on the 

northern shore of Long Island to more than 6.0 mm/yr at the 

Chesapeake Bay Bridge Tunnel. Sea-level rise throughout 

the region averages about 3.5 mm/yr, or about 1 foot (ft) in 

the past 100 years, a rate higher than the apparent average for 

the Atlantic Coast. These concerns are heightened because 

projected rates of sea-level rise could result in increases in 

mean sea level of 3 to 5 ft during the next 100 years in parts 

of the NACP, which would result in changes in groundwater 

levels, surface-water discharge, and increased potential for 

saltwater intrusion.

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Appendix 1. Soil-Water Balance Methodology 

and Analysis
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59

Contents

Introduction...................................................................................................................................................61

Data Requirements for the SWB Model ..................................................................................................61

Precipitation and Temperature.........................................................................................................61

Land Use Classification......................................................................................................................69

Hydrologic Soil Groups ......................................................................................................................69

Available Soil-Water Capacity..........................................................................................................69

Lookup Tables......................................................................................................................................69

Simulation Results .......................................................................................................................................72

References Cited..........................................................................................................................................75

Figures

1–1. Diagram of the interaction between the soil-water balance code and input data .........62

1–2. The soil-water balance (SWB) model grid for the Northern Atlantic Coastal Plain 

study area. ...................................................................................................................................63

1–3. Surface observation data for the Northern Atlantic Coastal Plain (NACP)......................64

1–4. Mean annual precipitation from 2005 through 2009 in the Northern Atlantic Coastal 

Plain (NACP)................................................................................................................................68

1–5. Hydrologic soil groups in the Northern Atlantic Coastal Plain; from U.S. Department 

of Agriculture (2006)...................................................................................................................70

1–6. Available water capacity for soils in the Northern Atlantic Coastal Plain........................71

1–7. Average annual recharge from 2005 through 2009 in the Northern Atlantic 

Coastal Plain................................................................................................................................73

Tables

1–1. National Oceanic and Atmospheric Administration (NOAA) surface stations used 

for precipitation and average temperatures in the Northern Atlantic Coastal Plain 

from 2005 through 2009. .............................................................................................................65

1–2. Distribution of soils in the Northern Atlantic Coastal Plain study area.............................72

1–3. Estimated available water capacities for various soil-drainage classifications.............72

1–4. Recharge rates from previous investigations in the Northern Atlantic Coastal Plain....74
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Appendix 1. Soil-Water Balance Methodology and Analysis

Introduction

Precipitation is the largest source of freshwater to the 

Northern Atlantic Coastal Plain (NACP). A proper accounting 

of the hydrologic budget for the NACP aquifer system requires 

an understanding of how much precipitation reaches the 

underlying aquifer system as groundwater recharge. Soil-water 

balance (SWB) models estimate shallow groundwater recharge 

by simulating the physical processes of the movement of water 

as it enters and moves through the soil column downward to 

the water table. Conditions such as the amount of precipitation 

and evapotranspiration and the capacity of soils to absorb 

and store water are important components used by the 

model when calculating recharge. The SWB model used 

for this investigation is based on a modified ThornthwaiteMather (Thornthwaite and Mather, 1957) approach that 

incorporates spatially distributed landscape, soils properties, 

and daily metrological data and produces model output of 

detailed spatial and temporal variability of recharge and 

evapotranspiration across the study area (Westenbroek and 

others, 2010). The aquifer recharge calculation described in 

this appendix section was critical for the development of the 

hydrologic budget analysis presented in the main section of 

the report.

Data Requirements for the SWB Model

The SWB model structure consists of tabular and gridded 

datasets of (1) precipitation and temperature, (2) land-use 

classification, (3) hydrologic soil group, and (4) available 

soil-water capacity, all of which are required to calculate 

recharge on a cell-by-cell basis as described in detail in the 

SWB model documentation (fig. 1–1, Westenbroek and others, 

2010). The SWB model grid for the NACP (fig. 1–2) extends 

along a north-south axis from southern Massachusetts to North 

Carolina and along a west-east axis from western Maryland 

to the end of Cape Cod. This area is much larger than the 

NACP study area (fig. 1–2, grey-shaded area) to minimize 

the potential that boundary effects could influence the model 

results in the area of interest. This grid consists of a uniform 

1-square mile (mi2) grid spacing of 518 rows and 554 columns 

for a total of 286,972 cells. Results reported for this analysis 

are limited to the NACP study area, an irregularly shaped area 

with about 74,000 model cells.

Precipitation and Temperature

The SWB model calculates recharge from weather data 

on a daily time step over a user-specified period (usually 

annually or monthly). Daily weather data can be input as either 

tabular time series or gridded datasets. Gridded data were 

developed for this analysis because of the large geographic 

size and latitudinal extent of the study area. Daily minimum 

and maximum temperature grids were created from tabular 

data of National Oceanic and Atmospheric Administration 

(NOAA) daily surface observations (National Oceanic and 

Atmospheric Administration, 2011). The tabular surface 

data were somewhat spatially and temporally discontinuous. 

The number of surface stations used in the temperature grid 

development varied depending on data availability for the 

period of interest (fig. 1–3; table 1–1). Statistical Analysis 

Systems routines (SAS version 4.2; 2008) were used to quality 

check the data and reformat and combine the data into files 

with daily data for each station, which then were verified and, 

along with station latitude and longitude, were combined into 

single files of daily data, one for each day of the processing 

period. The temperature files were processed into 1-mi2

gridded surfaces using Surfer, a semi-automated thin-plate 

spline method (Golden Software Inc., 2010). Data grids of 

precipitation were obtained from the NOAA National Climate 

Data Center (NCDC; National Oceanic and Atmospheric 

Administration, 2011) and were found to be spatially and 

temporally continuous. Esri ArcInfo grid functionality then 

was used to process all of the gridded precipitation and 

temperature data into SWB-compatible American Standard 

Code for Information Interchange (ASCII) data grids.

Precipitation grids for 2005 through 2009 were 

downloaded at a grid resolution of 0.25 degrees 

(approximately 13 miles at the latitude of the NACP 

study area) from the National Oceanic and Atmospheric 

Administration (2012) and were converted to a raster image 

for use in the SWB model using the Environmental Data 

Connector (EDC) (Applied Science Associates, undated). 

These precipitation grids were resampled to a grid resolution 

of 1 mi2 for input into the SWB model. Mean annual 

precipitation data from 2005 through 2009 for the NACP 

area is shown in figure 1–4 (Prism Climate Group, 2012). 

The highest rainfall trends toward the northeastern and 

southeastern coastal parts of the study area.
62 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Figure 1–1. Diagram of the interaction between the soil-water balance code and input data. From Westenbroek and others 

(2010).

Maximum soil-moisture capacity,

in inches

0

5

10

15

20

25

30

35

40

Accumulated potential water loss, in inches

Soil-moisture retained, in inches

2 4 6 8 10 12 14 16

Soil and land-use lookup table

Soil-Water-Balance code

Recharge to ground water

(gridded data)

Soil-water-retention table

(Thornthwaite-Mather, 1957)

Climate data

(tabular or gridded)

Landscape characteristics

• Land use

• Hydrologic soil type

• Flow direction

• Available water capacity
Data Requirements for the SWB Model 63

Figure 1–2. The soil-water balance (SWB) model grid for the Northern Atlantic Coastal Plain study area.

-78° -72°

42°

33°

ATLANTIC OCEAN

Chesapeake Bay

Delaware Bay

Long Island Sound RHODE ISLAND

MASSACHUSETS

Albemarle

Sound

Pamlico Sound

SWB Model Grid—each cell represents 100 cells used in analysis 

NACP model active cells

EXPLANATION

Study area

SOUTH

CAROLINA

Base from U.S. Geological Survey digital data 1:2,000,000 series

Albers Equal Area Conic projection

Standard Parallels 34o 30' N and 41o 30' N; central meridian 75o 30' W 

North American Datum of 1983

CONNECTICUT

NEW YORK

NEW

JERSEY

PENNSYLVANIA

MARYLAND

WEST

VIRGINIA DC

DELAWARE

VIRGINIA

NORTH CAROLINA

0 50 100

0 50 100

200 MILES

200 KILOMETERS
64 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Figure 1–3. Surface observation data for the Northern Atlantic Coastal Plain (NACP). National Oceanic and Atmospheric 

Administration (NOAA) weather stations are listed in table 1–1.

Base from U.S. Geological Survey digital data 1:2,000,000 series

Albers Equal Area Conic projection

Standard Parallels 34o 30' N and 41o 30' N; central meridian 75o 30' W 

North American Datum of 1983

-78° -72°

39°

36° EXPLANATION

45 NOAA weather station identifier

Study area

Other stations used outside NACP active cell model area

Urban areas

0

100

Color indicates of total study 

duration that the station 

was used, in percent.

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88

87

86

85

84

83

82

81

80

79

78 77

76

75

74

73

72

71

70

68

67

66

65

64

63

61

60

59

58

57

56

55

54

52

50

49 48

47

46

45

44

43

42

41

40

39

38

37

36

35

34

33

32

31

30

28

27

26

25

24

23

21

20

19

18

17

16

15

14

12

10

111

110

108

107

106

103 105

102

100

96 92

69

62

53

51

29

22

13 11

109

104

101

96 92

69

62

53

51

29

22

13 11

109

104

101

Delaware Bay

Long Island Sound

NEW CT

YORK

NEW

JERSEY

PENNSYLVANIA

MARYLAND VIRGINIA

WEST

DC

DELAWARE

VIRGINIA

NORTH CAROLINA

ATLANTIC OCEAN

Pamlico Sound

Albemarle Sound

0 50 100 KILOMETERS

0 50 100 MILES
Data Requirements for the SWB Model 65

Map identifier 

(fig. 1–1)

NOAA station 

identification number NCDC station name

1 71200 BEAR 2 SW

2 72730 DOVER

3 73595 GREENWOOD 2NE

4 75320 LEWES

5 76410 NEWARK UNIV FARM

6 79595 WILMINGTON NEW CASTLE CO AP

7 180015 ABERDEEN PHILLIPS FLD

8 180193 ANNAPOLIS POLICE BRKS

9 180335 ASSATEAGUE

10 180465 BALTIMORE WASH INTL AP

11 180700 BELTSVILLE

12 181750 CHESTERTOWN

13 183675 GLENN DALE BELL STN

14 185111 LAUREL 3 W

15 185718 MD SCI CTR BALTIMORE

16 185865 MECHANICSVILLE 5 NE

17 186350 NATL ARBORETUM DC

18 186800 OXON HILL

19 187230 PORT DEPOSIT 2 NE

20 187330 PRINCESS ANNE

21 187806 ROYAL OAK 2 SSW

22 188000 SALISBURY

23 188003 SALISBURY POLICE BRKS

24 188005 SALISBURY WICOMICO RGNL AP

25 188380 SNOW HILL 4 N

26 188405 SOLOMONS

27 189070 UPPER MARLBORO 3 NNW

28 189140 VIENNA

29 280311 ATLANTIC CITY INTL AP

30 280325 ATLANTIC CITY

31 280690 BELLEPLAIN STN FOREST

32 280990 BRANT BEACH BECH HAVEN

33 281351 CAPE MAY 2 NW

34 282805 ESTELL MANOR

35 283181 FREEHOLD-MARLBORO

36 283662 HAMMONTON 1 NE

37 283951 HIGHTSTOWN 2 W

38 284229 INDIAN MILLS 2 W

39 284987 LONG BRANCH OAKHURST

40 285346 MAYS LANDING 1 W

41 285581 MILLVILLE MUNI AP

Table 1–1. National Oceanic and Atmospheric Administration (NOAA) surface stations used 

for precipitation and average temperatures in the Northern Atlantic Coastal Plain from 2005 

through 2009.

[Data are from National Oceanic and Atmospheric Administration (2011). NCDC, National Climatic Data Center]
66 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Map identifier 

(fig. 1–1)

NOAA station 

identification number NCDC station name

42 285728 MOORESTOWN

43 287865 SANDY HOOK

44 287936 SEABROOK FARMS

45 288173 SOMERDALE 4 SW

46 288816 TOMS RIVER

47 288899 TUCKERTON 2 NE

48 300889 BRIDGEHAMPTON

49 301309 CENTERPORT

50 304130 ISLIP LI MACARTHUR AP

51 305377 MINEOLA

52 305796 NY AVE V BROOKLYN

53 305803 NEW YORK JFK INTL AP

54 305811 NEW YORK LAGUARDIA AP

55 306138 OCEANSIDE

56 307134 RIVERHEAD RSCH FM

57 307587 SEA CLIFF

58 307633 SETAUKET STRONG

59 308946 WANTAGH CEDAR CREEK

60 310375 AURORA 6 N

61 310674 BELHAVEN 3 NE

62 311458 CAPE HATTERAS AP

63 311606 CEDAR ISLAND

64 311949 COLUMBIA AG GUM NECK

65 312635 EDENTON

66 312719 ELIZABETH CITY

67 312827 ENFIELD

68 312940 FAIRFIELD

69 313333 FRISCO

70 313336 FRISCO 2NNE

71 314456 JACKSON

72 314962 LEWISTON

73 315303 MANTEO 2 WNW

74 315996 MURFREESBORO

75 316349 OCRACOKE

76 316853 PLYMOUTH 5 E

77 317395 ROCKY MOUNT

78 317400 ROCKY MT 8 ESE

79 317725 SCOTLAND NECK #2

80 318450 SWANQUARTER FERRY

81 318500 TARBORO 1 S

82 319100 WASHINGTON WWTP 4W

Table 1–1. National Oceanic and Atmospheric Administration (NOAA) surface stations used 

for precipitation and average temperatures in the Northern Atlantic Coastal Plain from 2005 

through 2009.—Continued

[Data are from National Oceanic and Atmospheric Administration (2011). NCDC, National Climatic Data Center]
Data Requirements for the SWB Model 67

Map identifier 

(fig. 1–1)

NOAA station 

identification number NCDC station name

83 319440 WILLIAMSTON 1 E

84 366889 PHILADELPHIA INTL AP

85 440327 ASHLAND

86 440385 BACK BAY WR

87 442009 CORBIN

88 442400 DISPUTANTA

89 442635 EASTVILLE

90 442790 EMPORIA 1 WNW

91 443204 FREDERICKSBURG SEWAGE

92 443713 HAMPTON UNIV

93 444044 HOLLAND 1 E

94 444101 HOPEWELL

95 444720 LANGLEY AFB

96 446139 NORFOLK INTL AP

97 446147 NORFOLK SOUTH

98 446161 NORTH

99 446475 PAINTER 2W

100 446656 PETERSBURG

101 447201 RICHMOND INTL AP

102 447541 SANDSTON

103 448129 STONY CREEK 2 N

104 448192 SUFFOLK LAKE KILBY

105 448800 WAKEFIELD 1NW

106 448829 WALKERTON 2 NW

107 448837 WALLACETON LK DRUMMOND

108 448894 WARSAW 2 NW

109 448906 WASHINGTON REAGAN AP

110 449025 WEST POINT 2 NW

111 449151 WILLIAMSBURG 2 N

Table 1–1. National Oceanic and Atmospheric Administration (NOAA) surface stations used 

for precipitation and average temperatures in the Northern Atlantic Coastal Plain from 2005 

through 2009.—Continued

[Data are from National Oceanic and Atmospheric Administration (2011). NCDC, National Climatic Data Center]
68 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Figure 1–4. Mean annual precipitation from 2005 through 2009 in the Northern Atlantic Coastal Plain (NACP).

Base from U.S. Geological Survey digital data 1:2,000,000 series

Albers Equal Area Conic projection

Standard Parallels 34o 30' N and 41o 30' N; central meridian 75o 30' W 

North American Datum of 1983

-78° -72°

39°

36°

Delaware Bay

Long Island Sound

NEW CT

YORK

NEW

JERSEY

PENNSYLVANIA

MARYLAND VIRGINIA

WEST

DC

DELAWARE

VIRGINIA

NORTH CAROLINA

ATLANTIC OCEAN

Pamlico Sound

Albemarle Sound

EXPLANATION

Study area

Annual average precipitation, in inches

High: 61

Low: 32

0 50 100 KILOMETERS

0 50 100 MILES
Data Requirements for the SWB Model 69

Land Use Classification

Land-use cover data are required in the SWB model for 

the calculation of net recharge to the groundwater flow system. 

Land use and land cover in the NACP is primarily a mosaic 

of forest, wetlands, and agriculture (Loveland and others, 

1999). Dominant land uses are farming and forestry, with 

locally dense urban development. More than half (57 percent) 

of the NACP area is undeveloped, nearly a third (30 percent) 

is agricultural, and 13 percent is developed (fig. 5). Grids 

of land-use classifications were developed from the MultiResolution Land Characteristics Consortium’s (MRLC) 

2001 National Land Cover Database (NLCD; LaMotte, 

2008a,b). The NLCD dataset includes 16 different land-cover 

classifications for 2001.

Hydrologic Soil Groups

The U.S. Department of Agriculture (USDA) Natural 

Resources Conservation Service (NRCS; undated) has 

classified more than 14,000 series of soils within the United 

States into one of four major hydrologic soils groups, 

which are assigned a letter designation (A–D) that describes 

soils with similar physical and runoff characteristics. This 

designation is focused specifically on water transmissivity, or 

infiltration capacity, through the soil. Soils designated as group 

A have a high capacity for infiltration (greater than 0.30 inch 

per hour (in/hr)) and low runoff potential, whereas those 

with a designation of group D have low infiltration capacity 

(less than 0.05 in/hr) and high runoff potential (Cronshey and 

others, 1986). The soils in almost half (about 43 percent) of 

the NACP, areas primarily in the southern Delmarva Peninsula 

in Delaware, Maryland, and Virginia and in northeastern North 

Carolina, are classified in group D (fig. 1–5; table 1–2). Areas 

with soils that have a high capacity for infiltration (groups 

A and B) are located in the northern portion of the study 

area (fig. 1–5). The data on the groups of hydrologic soils 

provided in the NRCS U.S. General Soil Map (STATSGO2) 

dataset (Natural Resources Conservation Service, undated a) 

were used to define gridded datasets of hydrologic soils and 

associated lookup tables to be used as input for hydrologic 

capacity in the SWB model.

Available Soil-Water Capacity

The SWB model uses land-use and land-cover 

information, in conjunction with available soil-water capacity 

and water interception coefficients of plants to calculate 

surface runoff and assign a maximum soil-moisture holding 

capacity for each grid cell in the model. Each soil type or soil 

series within the model area must be assigned an available 

water capacity. Available soil-water capacity is the water held 

in soil between its field capacity and the permanent wilting 

point (U.S. Department of Agriculture, 1998); it is often 

reported as inches of water per foot of soil thickness. Field 

capacity is the amount of water remaining in a soil 2 to 3 days 

after the soil has been saturated and allowed to drain freely. 

Permanent wilting point is the moisture content of a soil at 

which plants wilt and fail to recover when supplied with 

sufficient moisture (Tolk, 2003).

Available soil-water capacity is determined primarily 

by soil texture. Coarse soils have a lower field capacity 

than fine-textured soils because the large pore spaces 

between these coarse soils allow water to drain more freely 

than in the fine-textured soils. For this analysis, available 

soil-water capacity was assigned for the top 59 inches (or 

approximately 150 centimeters (cm)) of the soil column from 

the STATSGO2 database.

STATSGO2 reports available soil-water capacity for four 

depth classes:

• A, 0 to 25 cm

• B, 0 to 50 cm

• C, 0 to 100 cm

• D, 0 to 150 cm

The total soil-water capacity for each of the four classes 

was calculated and used as input for the SWB model. The 

SWB calculates the maximum soil-water capacity as the 

product of available soil water capacity and rooting depth, the 

depth of soil from which plant roots take up water. Within the 

NACP study area, soils with the highest available soil-water 

capacity tend to be located in low-lying areas along rivers 

and near the coast in the southern portion of the study area 

(fig. 1–6; table 1–3).

Lookup Tables

The SWB model uses a lookup-table scheme, consisting 

of two tables that contain text fields, to assign model cell 

properties related to soils and land use. The land-use table 

holds information on soil characteristics, such as rooting 

depth, runoff curve numbers, and maximum daily recharge 

based on land-use classifications.

Rooting depth is an important component of the recharge 

calculation because of the effect of rooting depth on water 

flow and transpiration. For example, Seyfried and Wilcox 

(2006) showed that recharge rates increase in areas where 

shallow rooted plants (crops) have replaced deep-rooted 

woody vegetation (forests). Rooting depths used for the 

calculation of recharge for the NACP area are derived from the 

Thornthwaite and Mather (1957) water-balance analysis that 

was conducted in central New Jersey.

The runoff curve value is an empirical parameter 

developed by the USDA Soil Conservation Service (SCS) that 

is used to estimate rainfall runoff or infiltration (Hjelmfelt, 

1991). The SWB model applies runoff curve values to soil 

types defined in the land-use lookup table; for this analysis, 

the values of land use were derived from Westenbroek and 

others (2010).
70 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Figure 1–5. Hydrologic soil groups in the Northern Atlantic Coastal Plain; from U.S. Department of Agriculture (2006).

-78° -72°

39°

36°

EXPLANATION

Study area 

Hydrologic Soils Group

A

B

C

D

Base from U.S. Geological Survey digital data 1:2,000,000 series

Albers Equal Area Conic projection

Standard Parallels 34o 30' N and 41o 30' N; central meridian 75o 30' W 

North American Datum of 1983

Delaware Bay

Long Island Sound

NEW CT

YORK

NEW

JERSEY

PENNSYLVANIA

MARYLAND VIRGINIA

WEST

DC

DELAWARE

VIRGINIA

NORTH CAROLINA

ATLANTIC OCEAN

Pamlico Sound

Albemarle Sound

0 50 100 KILOMETERS

0 50 100 MILES
Data Requirements for the SWB Model 71

-78° -72°

39°

36°

0 to 5

>5 to 10

>10 to 15

>15 to 20

>20 to 25

>25 to 30

>30 to 35

> 35

EXPLANATION

Study area

Available water capacity of upper 59 in. (150 cm) of soils

Base from U.S. Geological Survey digital data 1:2,000,000 series

Albers Equal Area Conic projection

Standard Parallels 34o 30' N and 41o 30' N; central meridian 75o 30' W 

North American Datum of 1983

Delaware Bay

Long Island Sound

NEW CT

YORK

NEW

JERSEY

PENNSYLVANIA

MARYLAND VIRGINIA

WEST

DC

DELAWARE

VIRGINIA

NORTH CAROLINA

ATLANTIC OCEAN

Pamlico Sound

Albemarle Sound

0 50 100 KILOMETERS

0 50 100 MILES

Figure 1–6. Available water capacity for soils in the Northern Atlantic Coastal Plain. From U.S. Department of Agriculture (2006).
72 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Table 1–3. Estimated available water capacities for various soildrainage classifications.

[Soil drainage is expressed in terms of hydrologic soil groups, as defined in 

Natural Resources Conservation Service (undated a,b). Values are rounded to 

the nearest tenth. NACP, North Atlantic Coastal Plain]

Soil drainage classification

Average 

available water 

capacity, in 

inches1

Soil type 

area, 

percentage 

of total NACP 

area

Excessively drained 8.6 5.7

Somewhat excessively drained 12.1 0.9

Well drained 16.8 39.9

Moderately well drained 18.0 11.8

Somewhat poorly drained 20.8 0.9

Poorly drained 19.0 23.6

Very poorly drained 23.6 17.3

1

Average available water capacity in top 59 inches (or approximately 

150 centimeters (cm)) of soil..

The maximum daily recharge rate is the measure of 

the maximum quantity at which soil is able to absorb water 

from rainfall or irrigation. This recharge value is influenced 

by soil characteristics, such as texture, organic matter, and 

compaction as well as land-cover conditions. The SWB model 

applies maximum daily recharge values to soil types defined in 

the land-use lookup table; for this analysis, the values of land 

use were derived from Westenbroek and others (2010).

The second lookup table is an extended version of the 

Thornthwaite-Mather soil-water retention tables that describe 

the ability of different soils to hold water (Thornthwaite and 

Mather, 1957). The ability of soil to hold water is strongly 

related to particle size, and water molecules hold more tightly 

to clay than they do coarse materials, such as sand grains. 

Generally, sandy soils will hold water through capillary 

binding whereas clay soils retain water through other physical 

binding processes. The soil-water retention tables are provided 

with the SWB model and require no user modification.

Simulation Results

The SWB model was run for conditions between 2005 

and 2009. Results of the simulation indicate that the average 

recharge rate for the study area is about 13.9 in/yr and varies 

from a minimum of 4.2 in/yr in the low lands of North 

Carolina and Virginia to a maximum of 37.6 in/yr on eastern 

Long Island (fig. 1–7). Simulation results compared favorably 

to previously published recharge rates within the NACP study 

area (table 1–4).

The SWB model used in this analysis calculates recharge 

as follows:

Rc = Pr – Inf – Ro – ETa, (1–1)

where

Pr is gross precipitation (including snowfall and 

snowmelt)

Inf is interception

Ro is runoff

ETa is actual evapotranspiration

Interception by plant foliage, runoff, and actual 

evapotranspiration (evaporation and plant transpiration) 

components are calculated by the SWB model from input 

datasets such as daily meteorological data, soils, and land use. 

In this analysis, the highest recharge rates occurred in areas 

with porous sandy soils where model results showed recharge 

rates of up to 50 percent of gross precipitation. Conversely, 

thick, clayey coastal soils and marshy areas showed recharge 

rates as low as 12 percent of gross precipitation. Differences 

attributable to soil properties drove model results especially 

with regard to evapotranspiration and runoff, both of which 

were seen to be high in the low-permeability soils typically 

found in flat low-lying coastal areas. If taken as a whole of 

the total gross precipitation that fell across the entire model 

area, the SWB model indicates the two largest components of 

water loss before recharge are evapotranspiration at 35 percent 

and runoff at 9 percent. The results presented here compare 

favorably with previous studies in the NACP in terms of 

both recharge rates and spatial distribution (Sanford and 

others, 2012).

Table 1–2. Distribution of soils in the Northern Atlantic Coastal 

Plain study area.

[Designations of groups of hydrologic soils are from U.S. Department of 

Agriculture (2006). NACP, Northern Atlantic Coastal Plain]

Hydrologic soil group Percentage of total NACP area

A 9.6

B 33.8

C 14.2

D 42.4
Simulation Results 73

-78° -72°

39°

36°

EXPLANATION

Average recharge, in inches

0.01 to 3

>3 to 6

>6 to 9

>9 to 12

>12 to 15

>15 to 18

>18 to 21

>21 to 24

>24 to 27

>27 to 30

>30 to 33

> 33

Study area

Base from U.S. Geological Survey digital data 1:2,000,000 series

Albers Equal Area Conic projection

Standard Parallels 34o 30' N and 41o 30' N; central meridian 75o 30' W 

North American Datum of 1983

Delaware Bay

Long Island Sound

NEW CT

YORK

NEW

JERSEY

PENNSYLVANIA

MARYLAND VIRGINIA

WEST

DC

DELAWARE

VIRGINIA

NORTH CAROLINA

ATLANTIC OCEAN

Pamlico Sound

Albemarle Sound

0 50 100 KILOMETERS

0 50 100 MILES

Figure 1–7. Average annual recharge from 2005 through 2009 in the Northern Atlantic Coastal Plain.
74 Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System, New York, to North Carolina

Geographic area Rate, in inches 

per year

Method Reference

Delaware, Coastal Plain 14–26 Base flow separation Johnston (1976)

Maryland:

Anne Arundel County Coastal Plain 14.4–21.2 Specific yield (Meinzer 1929) Achmad (1991)

Anne Arundel County Coastal Plain 20.1 Base flow separation Achmad (1991)

Wicomico County Coastal Plain 15–27 Physical methods (precipitation 

and well hydrograph analysis)

Rasmussen and Andreasen (1959)

North Carolina:

Coastal Plain 5–21 Unknown Heath (1994)

Southern Coastal Plain 5–40 Unknown North Carolina Department of 

Environment and Natural Resources 

(2003)

New Jersey:

Kirkwood-Cohansey aquifer system 15–20 Average precipitation 

and variances in 

evapotranspiration and runoff

Modica (1996)

Kirkwood-Cohansey aquifer system 

and Vincentown aquifer

19.4 Water-budget analyses (Watt and 

others, 1994)

Nicholson and Watt (1997)

Kirkwood-Cohansey Coastal Plain 16.2 Water-budget analyses (Charles 

and others, 2001)

Cauller and Carleton (2006)

Kirkwood-Cohansey Coastal Plain 13.03 Water-budget analyses (Charles 

and others, 1997)

Cauller and Carleton (2006)

Northern Atlantic Coastal Plain 2.5–15.7 Age dating McMahon and others (2011)

New York:

Coastal Plain 22 Unknown Wood (2006)

Long Island Brookhaven National Lab 23 Unknown Brookhaven National Laboratory (2001)

Long Island Coastal Plain 22 to 23 Unknown Wen (undated)

Virginia:

Coastal Plain 22.95 Zonebudget Heywood and Pope (2009)

Eastern Shore Coastal Plain 7.4–24 Inverse simulation and SF6 Sanford and others (2009)

Western Massachusetts, eastern New 

York, and northwestern Connecticut

17.5–22 Base flow separation Bent (1999)

Average 11.8–21.7

Table 1–4. Recharge rates from previous investigations in the Northern Atlantic Coastal Plain.
References Cited 75

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Prepared by the Pembroke Publishing Service Center.

For more information concerning this report, contact:

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Masterson and others—Hydrogeology and Hydrologic Conditions of the Northern Atlantic Coastal Plain Aquifer System—SIR 2013–5133

Printed on recycled paper

ISSN 2328–031X (print) ISSN 2328–0328 (online)

http://dx.doi.org/10.3133/sir20135133