U.S. Department of the Interior

U.S. Geological Survey

Water Availability and Use Science Program

Assessment of Groundwater Availability in the 

Northern Atlantic Coastal Plain Aquifer System 

From Long Island, New York, to North Carolina

Professional Paper 1829
Cover. Block diagram of the Northern Atlantic Coastal Plain aquifer system, which includes the 

areas east of the Fall Line, from Long Island, New York, to northern North Carolina. This multilayer 

system consists of confined aquifers and confining units capped by an unconfined, surficial aquifer. 

Groundwater flows predominantly from west to east from the Fall Line to the Atlantic Ocean, except 

where high-capacity pumping wells alter this regional flow pattern.
Assessment of Groundwater Availability in 

the Northern Atlantic Coastal Plain Aquifer 

System From Long Island, New York, to 

North Carolina

By John P. Masterson, Jason P. Pope, Michael N. Fienen, Jack Monti, Jr., 

Mark R. Nardi, and Jason S. Finkelstein

Water Availability and Use Science Program

Professional Paper 1829

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

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Suggested citation:

Masterson, J.P., Pope, J.P., Fienen, M.N., Monti, Jack, Jr., Nardi, M.R., and Finkelstein, J.S., 2016, Assessment 

of groundwater availability in the Northern Atlantic Coastal Plain aquifer system from Long Island, New York, to 

North Carolina: U.S. Geological Survey Professional Paper 1829, 76 p., http://dx.doi.org/10.3133/pp1829.

Library of Congress Cataloging-in-Publication Data

Names: Masterson, John P., author. | Geological Survey (U.S.), issuing body.

 | Water Availability and Use Science Program, author.

Title: Assessment of groundwater availability in the northern Atlantic

 coastal plain aquifer system from Long Island, New York, to North Carolina

 / by John P. Masterson [and five others].

Other titles: U.S. Geological Survey professional paper ; 1829.

Description: Reston, Virginia : U.S. Department of the Interior, U.S.

 Geological Survey, 2016. | Series: Professional paper, ISSN 1044-9612 ;

 1829 | At head of title: Water Availability and Use Science Program.

 Includes bibliographical references.

Identifiers: LCCN 2016028190 | ISBN 9781411340572 (paperback)

Subjects: LCSH: Groundwater--Atlantic Coast (U.S.) | Aquifers--Atlantic Coast

 (U.S.)

Classification: LCC GB1016 .M37 2016 | DDC 551.490975--dc23 | SUDOC I

 19.16:1829

LC record available at https://lccn.loc.gov/2016028190

ISSN 1044-9612 (print)

ISSN 2330-7102 (online)
iii

Foreword

Although often overlooked, groundwater is increasingly important to all our lives. Groundwater 

is the Nation’s principal reserve of freshwater. It provides half our drinking water, is essential to 

U.S. food production, and facilitates business and industry in developing economic wellbeing. 

Groundwater is also an important source of water for sustaining the ecosystem health of rivers, 

wetlands, and estuaries throughout the country.

Large-scale development of groundwater resources with accompanying decreases in 

groundwater levels and other effects of pumping have led to concerns about the future 

availability of groundwater to meet all our Nation’s needs. The depletion of groundwater to 

satisfy the country’s thirst and the compounding effects of recent droughts emphasize the need 

for an updated status of the Nation’s groundwater resources. Assessments of groundwater 

availability provide the science and information needed by the public and decision makers to 

manage water resources and use them responsibly. Adding to this already complex task of 

resource assessment is the analysis of potential future effects due to climate variability, which 

can further exacerbate an already challenging situation.

The U.S. Geological Survey’s Water Availability and Use Science Program is conducting largescale multidisciplinary regional studies of groundwater availability, such as this study of the 

Northern Atlantic Coastal Plain aquifer system. These regional studies are intended to provide 

citizens, communities, and natural resource managers with clearer knowledge of the status 

of the Nation’s groundwater resources and how changes in land use, water use, and climate 

have affected those resources and to develop tools that enable scientists to forecast how these 

resources may change in the future. Over time, the findings from these individual regionally 

integrated groundwater assessments of principal aquifers will be combined to provide a national 

assessment. Results derived from these studies will provide much needed answers to basic 

questions about the Nation’s ability to meet current and future demands for groundwater.

Donald Cline

Associate Director for Water

v

Contents

Foreword ........................................................................................................................................................iii

Executive Summary.......................................................................................................................................1

Introduction.....................................................................................................................................................3

Purpose and Scope ..............................................................................................................................7

Description of Study Area ...................................................................................................................7

Hydrogeologic Setting ................................................................................................................7

Land Use......................................................................................................................................12

Groundwater Use.......................................................................................................................14

Previous Investigations......................................................................................................................16

Methods.........................................................................................................................................................16

Data Compilation.................................................................................................................................17

Numerical Model ................................................................................................................................17

Conceptualization of the Hydrologic System ..........................................................................................17

Simulation of the Hydrologic System .......................................................................................................19

Comparison of the Conceptualized and Simulated Hydrologic Systems ..................................20

Comparison of Predevelopment Conditions..........................................................................20

Comparison of Postdevelopment Conditions ........................................................................20

Hydrologic Budget..............................................................................................................................22

Changes in Hydrologic Budgets Over Time...........................................................................22

Comparison of Predevelopment and Current [2013] Conditions ........................................38

Comparison of Current [2013] and Future [2043] Conditions...............................................42

Changes in Groundwater Storage and Water Levels ...................................................................44

Comparison of Predevelopment and Current [2013] Conditions ........................................50

Comparison of Current [2013] and Future [2043] Conditions...............................................52

Changes in the Boundary Between the Fresh and Saline Groundwater Flow Systems.........56

Reduction in Groundwater Discharge to Saline Coastal Waters ......................................56

Landward Movement of Saline Groundwater.......................................................................58

Simulation of Effects of Climate Change..................................................................................................60

Effects of Drought Conditions on Water Resources .....................................................................60

Effects of Sea-Level Rise on the Groundwater Flow System......................................................66

Use of Numerical Models to Inform Groundwater Monitoring Networks .........................................68

Challenges for Future Groundwater Availability Assessments—Lessons Learned.........................69

Acknowledgments.......................................................................................................................................71

References Cited..........................................................................................................................................72
vi

Figures

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

aquifer system...............................................................................................................................4

2. Map showing groundwater withdrawals by county in the Northern Atlantic

Coastal Plain aquifer system in 2010 .........................................................................................5

3. Maps showing changes in water levels in the Potomac-Patapsco 

regional aquifer of the Northern Atlantic Coastal Plain aquifer system

for 1900 to 1980 and 1980 to 2000................................................................................................6

4. Generalized cross section showing generalized geology of the Northern Atlantic 

Coastal Plain aquifer system from the Fall Zone to the coast of the Atlantic Ocean........8

5. Correlation chart showing regional hydrogeologic units and corresponding

state-level hydrogeologic units for the Northern Atlantic Coastal Plain

aquifer system...............................................................................................................................9

6. Cross sections showing regional variations in the hydrologic framework of

the Northern Atlantic Coastal Plain aquifer system .................................................................10

7. Map and pie charts showing land use and land cover for the northern

Atlantic Coastal Plain physiographic province for conditions in 2006...............................13

8. Pie charts showing distribution of drinking water sources by State across

the Northern Atlantic Coastal Plain aquifer system .............................................................14

9. Pie charts showing distribution of groundwater use by State across the

Northern Atlantic Coastal Plain aquifer system....................................................................14

10. Schematic diagram showing the conceptual hydrogeologic model of the

Northern Atlantic Coastal Plain aquifer system....................................................................18

11. Schematic diagram showing hydrologic budget for predevelopment 

(before 1900) and average current conditions for the conceptual model

of Masterson and others and the numerical analysis in this report for the

Northern Atlantic Coastal Plain aquifer system....................................................................21

12. Map showing geographic areas delineated for hydrologic budget computations

within the Northern Atlantic Coastal Plain aquifer system .................................................23

13. Graph showing groundwater withdrawal rates from the Northern Atlantic 

Coastal Plain aquifer system from 1900 to 2010.....................................................................24

14. Graph showing groundwater withdrawals for the Northern Atlantic 

Coastal Plain aquifer system by geographic area for the historical period,

the period of emphasis, and the future period.......................................................................24

15. Graphs showing groundwater withdrawals for the Northern Atlantic 

Coastal Plain aquifer system and regional aquifers in New York, New Jersey,

Delmarva, Maryland, Virginia, and North Carolina geographic areas for the

historical period, the period of emphasis, and a future period...........................................25

16. Pie charts showing distribution of groundwater withdrawals in 2008

by regional aquifer for each geographic area in the Northern Atlantic

Coastal Plain aquifer system.......................................................................................................28

17. Graph showing hydrologic budget for the historical period, period of emphasis,

and a future period for the Northern Atlantic Coastal Plain aquifer system....................29

18. Graphs showing changes in hydrologic budget components as a function

of pumping for the historical period, period of emphasis, and a future period

for the Northern Atlantic Coastal Plain aquifer system and regional aquifers

in the New York, New Jersey, Delmarva, Maryland, Virginia, and 

North Carolina geographic areas ............................................................................................31
vii

19. Graphs showing cumulative changes in hydrologic budget components

in response to pumping for the historical period, period of emphasis, and

a future period for the Northern Atlantic Coastal Plain aquifer system and

regional aquifers in the New York, New Jersey, Delmarva, Maryland, 

Virginia, and North Carolina geographic areas.....................................................................35

20. Pie charts showing distribution of the cumulative response of the 

hydrologic budget components to changes in groundwater pumping for the

historical period, period of emphasis, and a future period for the Northern Atlantic 

Coastal Plain aquifer system and regional aquifers in the New York, New Jersey, 

Delmarva, Maryland, Virginia, and North Carolina geographic areas ..............................40

21. Pie charts showing changes in hydrologic budget components with the 

removal of wastewater return flow for current conditions across the 

Northern Atlantic Coastal Plain aquifer system....................................................................43

22. Graphs showing change in aquifer storage by hydrogeologic unit for the

historical period, the period of emphasis, and a future period for the 

Northern Atlantic Coastal Plain aquifer system and regional aquifers in

the New York, New Jersey, Delmarva, Maryland, Virginia, and North Carolina

geographic areas........................................................................................................................44

23. Graphs showing cumulative change in aquifer storage by hydrogeologic unit

for the historical period, the period of emphasis, and a future period for the

Northern Atlantic Coastal Plain aquifer system and regional aquifers in the

New York, New Jersey, Delmarva, Maryland, Virginia, and North Carolina

geographic areas........................................................................................................................47

24. Pie charts showing distribution of cumulative aquifer storage change by 

hydrogeologic unit for the historical period, period of emphasis, and a 

future period for the Northern Atlantic Coastal Plain aquifer system and 

regional aquifers in the New York, New Jersey, Delmarva, Maryland, 

Virginia, and North Carolina geographic areas.....................................................................50

25. Map showing change in water levels in response to groundwater withdrawals

from 1900 to 1985 in the Potomac-Patapsco aquifer of the Northern Atlantic

Coastal Plain aquifer system ....................................................................................................53

26. Map showing change in water levels in response to groundwater withdrawals

from 1986 to 2013 in the Potomac-Patapsco aquifer of the Northern Atlantic 

Coastal Plain aquifer system ....................................................................................................54

27. Map showing change in water levels in projected response to groundwater

withdrawals from 2013 to 2043 in the Potomac-Patapsco aquifer of the 

Northern Atlantic Coastal Plain aquifer system....................................................................55

28. Map showing change in water levels in the surficial aquifer on Long Island,

New York, of the Northern Atlantic Coastal Plain aquifer system with the

removal of wastewater return flow for current conditions .................................................57

29. Graph showing change in coastal groundwater discharge in the 

Northern Atlantic Coastal Plain aquifer system in response to groundwater

pumping for the historical period, the period of emphasis, and a future period

by geographic area ....................................................................................................................57

30. Schematic diagram showing the effects of pumping on groundwater flow

in the Northern Atlantic Coastal Plain aquifer system.........................................................58

31. Pie charts showing distribution of change in coastal groundwater discharge as

inflow from saline groundwater in response to pumping for the surficial aquifer

system in 2013 and 2043, and the confined aquifer system in 2013 and 2043...................59
viii

32. Pie charts showing distribution of change in coastal groundwater discharge

as a net reduction in total groundwater discharge to the coast for in response

to pumping for the surficial aquifer system in 2013 and 2043, and the confined

aquifer system in 2013 and 2043...............................................................................................59

33. Pie charts showing distribution of changes in hydrologic budget components

in response to a simulated 8-year drought for pumping conditions in 2013

across the Northern Atlantic Coastal Plain aquifer system................................................60

34. Pie charts showing distribution of the change as a percentage of the total 

in reduction in recharge and increase in irrigation withdrawals in response to

a simulated 8-year drought for current conditions by geographic area across

the Northern Atlantic Coastal Plain aquifer system .............................................................61

35. Map showing difference between average recharge and average drought 

recharge in inches per year calculated by the Soil Water Balance model for 

the Delmarva geographic area of the Northern Atlantic Coastal Plain 

aquifer system.............................................................................................................................62

36. Graph showing distribution of monthly precipitation in inches for the 

growing season months for the drought of 1960–1967 and average conditions 

from 1980 to 2010 for the Delmarva geographic area of the Northern Atlantic

Coastal Plain aquifer system ....................................................................................................63

37. Pie chart showing increase in withdrawals for irrigation, in million gallons

per day, in response to drought conditions in the Delmarva geographic area

of the Northern Atlantic Coastal Plain aquifer system.........................................................63

38. Maps showing simulated water-level response of the surficial aquifer in the

Delmarva geographic area of the Northern Atlantic Coastal Plain aquifer system 

to drought conditions as decrease in water levels and percentage of water-level 

decreases attributed to increased withdrawals for irrigation............................................64

39. Maps showing simulated water-level response of Piney Point aquifer to 

drought conditions as decreases in water levels and percentage of water-level 

decreases attributed to increased withdrawals for irrigation............................................65

40. Map showing model-simulated increase in water levels in the surficial aquifer 

in response to a 3-foot increase in the current sea-level position across the 

Northern Atlantic Coastal Plain aquifer system....................................................................67

41. Pie charts showing distribution of simulated increase in groundwater discharge

to streams in response to sea-level rise and net groundwater discharge to 

streams in 2013 across the Northern Atlantic Coastal Plain aquifer system ...................68

42. Map showing variance of water-level change from the difference between 

annually varying and average annual recharge from 1986 to 2008 for the 

Potomac-Patapsco aquifer of the Northern Atlantic Coastal Plain aquifer system........70

Tables

1. Distribution of drinking water sources in 2010 by State in the 

Northern Atlantic Coastal Plain aquifer system....................................................................15

2. Distribution of groundwater use in 2010 by State in the 

Northern Atlantic Coastal Plain aquifer system....................................................................15

3. Percentage of groundwater withdrawals in 2008 by aquifer and 

geographic area in the Northern Atlantic Coastal Plain......................................................28

4. Comparison of total net volumetric groundwater depletion for the 

Northern Atlantic Coastal Plain aquifer system from 1900 to 2000

and from 1900 to 2008.................................................................................................................39
ix

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2) 

Volume

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

trillion gallons (Tgal) 3.7854 cubic kilometer (km3)

Flow rate

foot per year (ft/yr) 0.3048 meter per year (m/yr)

gallon per day (gal/d) 0.003785 cubic meter per day (m3/d)

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)

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.

Datum

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).

Elevation, as used in this report, refers to distance above or below the vertical datum.

Abbreviations

3D three dimensional

MRLC Multi-Resolution Land Characteristics Consortium

NACP Northern Atlantic Coastal Plain

NLCD National Land Cover Database

NWIS National Water Information System

RASA Regional Aquifer-System Analysis

SWB soil water balance

USGS U.S. Geological Survey

Assessment of Groundwater Availability in the 

Northern Atlantic Coastal Plain Aquifer System 

From Long Island, New York, to North Carolina

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

and Jason S. Finkelstein

Executive Summary

The U.S. Geological Survey began a multiyear regional 

assessment of groundwater availability in the Northern 

Atlantic Coastal Plain (NACP) aquifer system in 2010 as 

part of its ongoing regional assessments of groundwater 

availability of the principal aquifers of the Nation. The goals 

of this national assessment are to document effects of human 

activities on water levels and groundwater storage, explore 

climate variability effects on the regional water budget, and 

provide consistent and integrated information that is useful to 

those who use and manage the groundwater resource. As part 

of this nationwide assessment, the USGS evaluated available 

groundwater resources within the NACP aquifer system from 

Long Island, New York, to northeastern North Carolina.

The northern Atlantic Coastal Plain physiographic 

province depends heavily on groundwater to meet agricultural, 

industrial, and municipal needs. The groundwater assessment 

of the NACP aquifer system included an evaluation of how 

water use has changed over time; this evaluation primarily 

used groundwater budgets and development of a numerical 

modeling tool to assess system responses to stresses from 

future human uses and climate trends.

This assessment focused on multiple spatial and temporal 

scales to examine changes in groundwater pumping, storage, 

and water levels. The regional scale provides a broad view 

of the sources and demands on the system with time. The 

sub-regional scale provides an evaluation of the differing 

response of the aquifer system across geographic areas 

allowing for closer examination of the interaction between 

different aquifers and confining units and the changes in these 

interactions under pumping and recharge conditions in 2013 

and hydrologic stresses as much as 45 years in the future. By 

focusing on multiple scales, water-resource managers may 

utilize this study to understand system response to changes 

as they affect the system as a whole.

The NACP aquifer system extends from Long Island to 

northeastern North Carolina, and includes aquifers primarily 

within New York, New Jersey, Delaware, Maryland, Virginia, 

and North Carolina. The seaward-dipping sedimentary 

wedge that underlies the northern Atlantic Coastal Plain 

physiographic province forms a complex groundwater 

system. Although the NACP aquifer system is recognized 

by the U.S. Geological Survey as one of the smallest of 

the 66 principal aquifer systems in the Nation, it ranks 

13th overall in terms of total groundwater withdrawals and 

is 7th in population served. Despite abundant precipitation 

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

water in this region is limited because many of the surface 

waters in this area are brackish estuaries, contributing to why 

many communities in the northern Atlantic Coastal Plain 

physiographic province rely heavily on groundwater to meet 

their water needs.

Increases in population and changes in land use during 

the past 100 years have resulted in diverse increased demands 

for freshwater throughout the northern Atlantic Coastal Plain 

physiographic province with groundwater serving as a vital 

source of drinking water for the nearly 20 million people who 

live in the region. Total groundwater withdrawal in 2013 was 

estimated to be about 1,300 million gallons per day (Mgal/d) 

and accounts for about 40 percent of the drinking water supply 

with the densely populated areas tending to have the highest 

rates of withdrawals and, therefore, being most susceptible to 

effects from these withdrawals over time.

Water levels in many of the confined aquifers are 

decreasing by as much as 2 feet per year (ft/yr) in response 

to extensive development and subsequent increased withdrawals throughout the region. Total water-level decreases 

(drawdowns) are more than 100 feet (ft) in some aquifers from 

their predevelopment (before 1900) levels. These drawdowns 

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 northern Atlantic Coastal Plain physiographic province 

face challenges beyond 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 more difficult because of 

hydrologic effects that extend beyond State boundaries.
2 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

The northern Atlantic Coastal Plain physiographic 

province is underlain by a wedge of unconsolidated to 

partially consolidated sediments that are typically thousands 

of feet thick along the coastline with a maximum thickness 

of about 10,000 ft near the edge of the continental shelf. The 

NACP aquifer system consists of nine confined aquifers and 

nine confining units capped by an unconfined surficial aquifer 

that is bounded laterally from the west by the contact between 

Coastal Plain sediments and the upland Piedmont bedrock. 

This aquifer system extends to the east to the limit of the 

Continental Shelf, however, the boundary between fresh and 

saline groundwater is considered to be much closer to the 

shoreline and varies vertically by aquifer.

Precipitation over the region for average conditions from 

2005 to 2009 is about 61,800 Mgal/d, but about 70 percent of 

it is lost to evapotranspiration resulting in an inflow of about 

19,600 Mgal/d entering the groundwater system as aquifer 

recharge. Most of this recharge enters the aquifer system 

and flows through the shallow unconfined aquifer and either 

discharges to streams or directly to coastal waters without 

reaching the deep, confined aquifer system. In addition to 

recharge from precipitation, other sources of water include 

the return of wastewater from domestic septic systems of 

about 230 Mgal/d, about 60 Mgal/d of water released from 

storage in the confined system, and about 30 Mgal/d of lateral 

inflow at the boundary between freshwater and saltwater in 

response to pumping for conditions in 2013.

The outflow needed to balance the inflows was 

subdivided between streamflow, discharge to tidal portions 

of streams, and coastal discharge. The hydrologic budget 

developed for current [2013] conditions determined that 

93 percent of the total outflow was to surface waters with 

about 70 percent divided evenly between streamflow and 

shallow coastal discharge and 23 percent as discharge to 

tidal waters. The remaining 7 percent of the total outflow 

components include withdrawals from both the surficial 

and confined aquifers of the groundwater system.

The groundwater availability assessment of the 

NACP aquifer system highlights the importance of analyses at 

both the regional and local scales to understand how changes 

in land use, water use, and climate have affected groundwater 

resources and how these resources may change in the future. 

The investigation included assessments of the regional 

changes in water levels and budgets across State lines, the 

importance of considering storage change in the confining 

units, the response of the aquifer system to a continuation 

of current [2013] hydrologic stresses into the future, and the 

potential effects of climate change and sea-level rise on the 

aquifer system.

The Potomac aquifer group includes two of the most 

widely used aquifers in the NACP aquifer system, the 

Potomac-Patapsco and Potomac-Patuxent regional aquifers, 

providing about 24 percent of the total groundwater used in 

the region. Withdrawals from large pumping centers in this 

deep, confined aquifer group have resulted in substantial 

decreases in water-levels across state lines, particularly 

between southern Virginia and northeastern North Carolina as 

well as between southern New Jersey and northern Delaware 

where water levels in the Potomac-Patapsco aquifer have 

decreased by as much as 200 ft and 50 ft, respectively from 

predevelopment to current [2013] conditions. This response 

in water levels also is reflected in changes in water budgets 

where, for example, about 20 percent of the total response to 

pumping in Virginia is met by inducing flow from adjacent 

States. Understanding and quantifying these hydrologic effects 

that extend beyond State boundaries is critical for the Stateand regional-level management of aquifer resources.

The cumulative storage loss from the intervening 

confining units throughout the entire NACP aquifer system 

was about 35 percent of the total storage loss from predevelopment to current [2013] conditions. In geographic areas such 

as the Delmarva Peninsula, Maryland, and New Jersey, the 

water released from storage in the confining units makes up 

the majority of the total storage release from the groundwater 

system and is becoming proportionally more important over 

time as the surficial aquifer approaches equilibrium with 

respect to pumping and recharge stresses as of 2013.

Storage loss from the confining units is of particular 

concern because, unlike in the sands that comprise the 

confined aquifers, water removed from the clayey confining 

unit sediments cannot be replenished as these units gradually 

compress. This nonrecoverable storage loss, if great enough, 

can result in land subsidence where these units are thick and 

the release from storage is relatively large and contributes to 

increased concerns for sea-level rise in areas such as the lower 

portion of the Chesapeake Bay.

Groundwater usage increased dramatically in the 

NACP aquifer system during post-World War II era from the 

mid-1940s to early the 1980s, with withdrawals increasing 

from about 400 Mgal/d to more than 1,300 Mgal/d. Although 

groundwater withdrawals have been relatively constant since 

the early 1980s, about half of the total groundwater withdrawn 

from the NACP aquifer system since 1900 was withdrawn in 

the past 30 years. An analysis of the response of the groundwater system to a continuation of the current [2013] pumping 

for an additional 30 years into the future shows that the flow 

system continues to adjust in terms of changes in water budget 

components, water levels, and the boundary between freshwater and saltwater as it approaches equilibrium. The largest 

change in water budget components over this timeframe is the 

reduction in the amount of water released from storage.

Across the entire NACP aquifer system, the reduction 

of storage release from 7 to 4 percent of the total water 

budget change is accounted for by reductions in groundwater 

discharge to streams and coastal waters. Locally, a similar 

response is calculated for each of the geographic areas 

except for Virginia where the amount of water released from 

storage accounts for about 25 percent of the total change in 

water budget. This finding suggests that the groundwater 

flow system in Virginia is not approaching equilibrium under 

the current [2013] stresses and, therefore, water levels will 

continue to decrease even if the pumping remains constant.
Introduction 3

An analysis of the change in water levels in the 

Potomac-Patapsco aquifer as pumping is continued 30 years 

into the future reveals that the largest decreases in water 

levels throughout the NACP aquifer system will occur in the 

southern Virginia and northeastern North Carolina parts of the 

study area. It is these areas that also see the greatest potential 

for increased lateral movement of saline groundwater in the 

deep, confined portion of the groundwater flow system in 

response to a continuation of the current [2013] pumping rates.

The potential effects of long-term climate change and 

variability on the hydrologic system and availability of water 

resources in the NACP aquifer system continue to be of 

serious societal concern. These concerns include the effects 

of changes in aquifer recharge and in sea-level rise on the 

groundwater flow system. An assessment of the potential 

effects of a prolonged drought during current [2013] pumping 

conditions indicated that the reductions in recharge associated 

with droughts, including additional irrigation withdrawals 

required to meet increased crop water demand, have the 

greatest effects on water levels and streamflows in the surficial 

aquifer, and changes in water levels in the confined aquifers 

primarily resulted from the increased withdrawals associated 

with increased irrigation pumping; this response was most 

apparent in the Delmarva Peninsula. These results suggest that 

water levels may not be susceptible to the effects of droughts 

in the confined aquifers of the NACP aquifer system not used 

for irrigation, unlike in the unconfined surficial aquifer.

A second analysis also was conducted to assess the 

effects of sea-level rise on the groundwater system throughout 

the northern Atlantic Coastal Plain physiographic province 

because 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 

sea-level rise than observed elsewhere in the United States. 

Groundwater levels rose from 0 to 3 ft in response to a 3-ft 

simulated change in sea-level position, with the largest 

response occurring along the shoreline and away from nontidal streams. About 37 percent (or 10,000 square miles) of 

the area of the northern Atlantic Coastal Plain physiographic 

province may experience about a 0.5-ft or more increase 

in water levels with the 3-ft increase in sea-level position, 

whereas about 18 percent (almost 5,000 square miles) of land 

of the northern Atlantic Coastal Plain physiographic province 

may experience a 2-ft or more increase in water levels with 

the 3-ft increase in sea-level position.

These increases in the water table are of particular 

concern in low-lying areas where the unsaturated (vadose) 

zone is already thin, thus creating concerns for groundwater 

inundation of subsurface infrastructure, such as basements, 

septic systems, and subway systems. This increase in the water 

table also will likely alter the distribution of groundwater 

discharge to surface-water bodies thus increasing groundwater 

flow to streams that would have otherwise discharged directly 

to coastal waters. Throughout the NACP aquifer system, this 

redistribution of groundwater discharge results in an additional 

2 percent of baseflow in streams. Although the increases 

in groundwater discharge to streams (and corresponding 

decreases in discharge to coastal waters) calculated for the 

entire NACP aquifer system and its geographic areas represent 

only a small increase compared with current [2013] conditions, this redistribution of groundwater discharge from the 

coast to streams locally can alter the delivery of freshwater 

input to coastal receiving waters and have ecohydrological 

implications on the sensitive ecosystems which rely on a 

balance of groundwater discharge and surface-water flow.

Introduction

The seaward-dipping sedimentary wedge that underlies 

the embayed section of the Coastal Plain physiographic 

province of the Atlantic Plain (referred to as the northern 

Atlantic Coastal Plain province in this report; Fenneman, 

1917) forms the Northern Atlantic Coastal Plain (NACP) 

aquifer system. The 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, Virginia, and North Carolina. Although 

the NACP aquifer system is recognized by the U.S. Geological 

Survey (USGS) as one of the smallest in area of the 

66 principal aquifer systems in the Nation (Miller, 1999), it 

ranks 13th overall in terms of total groundwater withdrawals 

(Reilly and others, 2008) and 7th in population served 

(Maupin and Barber, 2005).

Many communities in the northern Atlantic Coastal Plain 

province rely heavily on groundwater to meet their water 

needs because of the limited supply of fresh surface water in 

this region. This major source of water provides public and 

domestic supply and serves as a vital source of freshwater for 

agricultural, commercial, and industrial uses throughout the 

region. However, water supplies in this region can be limited 

by the amount of allowable drawdown, droughts, saltwater 

intrusion, and contamination by agricultural and industrial 

sources. Some communities also rely on surface water 

imported from outside the northern Atlantic Coastal Plain 

province, which may be discharged into shallow aquifers, 

streams, or coastal waters after use.

Increases in population and changes in land use during 

the past 100 years have resulted in diverse increased demands 

for freshwater throughout the northern Atlantic Coastal Plain 

province. Substantial groundwater withdrawals had begun 

in the northern part of the study area by the late 1800s. By 

1900, about 100 million gallons per day (Mgal/d) of water was 

pumped from the NACP aquifer system, about half of which 

was from western Long Island to provide for the water-supply 

system of New York City (Buxton and Shernoff, 1999).

Groundwater still serves as a vital source for drinking 

water for the people who live in the region; densely populated 

areas tend to have higher rates of groundwater withdrawals 

than less densely populated areas (fig. 2) and, therefore, 

are most susceptible to effects from withdrawal over time. 
4 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

James River

Potom

R

ac

iver

masterson_nacp_fig 01_v3

Long Island

Trenton

New York City

Philadelphia

Dover Baltimore

Annapolis

Richmond

Cape

May

Virginia

Beach

DC

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

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

Susquehanna

MA

RI

CONNECTICUT

NEW YORK

NEW

JERSEY

PENNSYLVANIA

VIRGINIA

Pamlico Sound

Albemarle

Sound

ATLANTIC OCEAN

Continental Shelf margin

River

Delmarva

Peninsula

0 50 100 MILES

0 50 100 KILOMETERS

Study

area

Raritan Bay

Fall Zone

WV

James River

Potomac River

MARYLAND

DELAWARE

NORTH CAROLINA

Greenville

EXPLANATION

Elevation, in feet above or

 below NAVD 88 

High : 0

Low : –7,105

Bathymetry—Data from National

 Oceanic and Atmospheric

 Administration (undated) and

 Amante and Eakins (2009)

Topography—Data from

 U.S. Geological Survey

 (undated) 

High : 563

Low : 1

Northern Atlantic Coastal Plain

 study area

Chesapeake Bay impact crater

41°

38°

35°

78° 75° 72°

Figure 1. Location and extent of the Northern Atlantic Coastal Plain aquifer system.
Introduction 5

masterson_nacp_fig02_new

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

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

NEW CT

YORK

NEW

JERSEY

PENNSYLVANIA

MARYLAND

DC DELAWARE

VIRGINIA

NORTH CAROLINA

ATLANTIC OCEAN

Pamlico Sound

Albemarle Sound

WV

EXPLANATION

Total groundwater withdrawn

 in 2010, 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 250

Study area

0 100 KILOMETERS

0 50

50

100 MILES

78° 75° 72°

41°

38°

35°

Figure 2. Groundwater withdrawals by county in the Northern Atlantic Coastal Plain aquifer system in 2010. 

Modified from Maupin and others (2014).
6 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

Total groundwater withdrawal in 2010 was estimated to be 

about 1,500 Mgal/d and accounts for about 40 percent of the 

drinking water supply in the areas served by the NACP aquifer 

system (Maupin and others, 2014).

Water levels in many of the confined NACP aquifers 

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

extensive development and subsequent increased withdrawals 

throughout the region. Total water-level decreases (drawdowns) are more than 100 feet (ft) in some aquifers from their 

predevelopment (before 1900) levels (fig. 3A–B; dePaul and 

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

cumulative decreases greater than 200 ft resulted in water 

levels approximately 200 ft below land surface (Heywood 

and Pope, 2009).

>100

81 to 100

61 to 80

41 to 60

21 to 40

11 to 20

No significant change

Insufficient data

Water-level declines,

 in feet 

EXPLANATION

Potomac-Patapsco

regional aquifer

Control point Potomac-Patapsco

regional aquifer

Control point

Water-level declines,

 in feet

 >100

81 to 100

 61 to 80

 41 to 60

 21 to 40

 11 to 20 

Water-level rises,

 in feet

EXPLANATION

 11 to 20 

 21 to 40 

 >40 

 No significant change

 Insufficient data

A B

Pamlico Sound

Chesapeake Bay

ATLANTIC OCEAN

Delaware Bay

Albemarle Sound

Long Island Sound

Pamlico Sound

NORTH CAROLINA

NEW

JERSEY

MARYLAND

VIRGINIA

NEW 

YORK

PENNSYLVANIA

DELAWARE

42°

41°

40°

39°

38°

37°

36°

35°

76°

75°

74°

73°

72°

80°

79°

78°

42°

0 50

0 50 100 MILES

100 KILOMETERS

42°

41°

40°

39°

38°

37°

36°

35°

80°

79°

78°

l

N

l

N

DC

Chesapeake Bay

ATLANTIC OCEAN

Delaware Bay

Albemarle Sound

Long Island Sound

NORTH CAROLINA

NEW

JERSEY

MARYLAND

VIRGINIA

NEW 

YORK

PENNSYLVANIA

DELAWARE

76°

75°

74°

73°

72°

42°

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

Albers Equal-Area Conic projection

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

North American Datum of 1983

DC

Figure 3. Changes in water levels 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). >, greater than.
Introduction 7

Drawdowns also extend across State lines and under 

the Chesapeake and Delaware Bays, creating the potential 

for interstate aquifer management issues. Regional waterresources managers in the northern Atlantic Coastal Plain 

province face challenges beyond those imposed by competing 

local agricultural, domestic, and industrial demands for water 

and the various environmental issues faced by water-resource 

managers nationwide. Large changes in regional water use 

have made the state-level management of groundwater 

resources 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 groundwater resource, 

leading to the need for a comprehensive assessment of water 

availability in the NACP aquifer system.

In 2010, the USGS began a multiyear regional assessment 

of groundwater availability in the NACP aquifer system as part 

of ongoing regional assessments of groundwater availability of 

the principal aquifers of the Nation (Alley and others, 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 groundwater resource (Reilly and others, 2008).

The initial phase of the assessment of the NACP aquifer 

system (Masterson and others, 2013b) included revisions 

to the regional aquifer nomenclature and hydrogeologic 

correlations and an updated hydrologic budget of this aquifer 

system from the previous regional aquifer system assessment 

that had been completed by the USGS in the 1980s (Trapp and 

Meisler, 1992). The final phase of the assessment in this report 

included a comprehensive analysis of groundwater availability 

of the NACP aquifer system.

Purpose and Scope

The purpose of this report is to describe the groundwater 

availability of the Northern Atlantic Coastal Plain aquifer 

system by quantifying the resource for current [2013] conditions, describing how this resource has changed since 1900 

(predevelopment conditions), and assessing how the resource 

may change in response to future anthropogenic and environmental stresses. Results of a numerical analysis to assess 

groundwater availability from New York to North Carolina are 

presented in this report both in terms of changes to hydrologic 

budgets and as water-level change maps.

The spatial 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 (Trapp and 

Meisler, 1992) for this report to focus primarily on the area 

from Long Island to northeastern part of North Carolina. 

This approach complements a recently completed regional 

groundwater availability assessment of the Atlantic Coastal 

Plain aquifer system of North and South Carolina (Campbell 

and Coes, 2010). Consequently, several regional aquifer names 

have been revised in this report 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; the hydrogeologic 

units of North Carolina are included in the discussion in this 

report to provide the information needed to correlate hydrogeologic units across the Virginia-North Carolina border.

The description of the groundwater model developed and 

used for this analysis is presented in Masterson and others 

(2016). A detailed description of the hydrogeologic conditions 

of the NACP aquifer system is presented in Masterson and 

others (2013b). Digital data on the extents and elevations of 

the units composing the revised hydrogeologic framework of 

the NACP can be found in Pope and others (2016).

Description of Study Area

The part of the northern Atlantic Coastal Plain province included in this investigation occupies a land area of 

approximately 27,000 square miles (mi2) along the eastern 

seaboard of the United States from Long Island southward to 

the northeastern part of North Carolina (fig. 1). A seawarddipping 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 function as confining 

units. The aquifers within the NACP aquifer system are major 

sources of water for public and domestic supply and serve 

as a vital source of water for agricultural, commercial, and 

industrial uses throughout the region.

Hydrogeologic Setting

The NACP aquifer system is composed of unconsolidated 

to partially consolidated sediments that range in age from 

Early Cretaceous to Holocene. These sediments unconformably overlie a basement of Precambrian to Paleozoic-age 

consolidated bedrock. The western limit of the NACP aquifer 

system is the Atlantic seaboard Fall Zone (fig. 1), which is 

the transition from the igneous and metamorphic rocks of the 

Piedmont physiographic province to the sedimentary environment of the Coastal Plain physiographic region.

The sedimentary wedge of the Coastal Plain is aligned 

approximately parallel to the Fall Zone and dips and thickens 

to the east and south. It is typically thousands of feet thick at 

the coastline and as much as 2 miles thick where it terminates 

to the east along the edge the Continental Shelf margin (fig. 4). 

The Coastal Plain sediments are continuous approximately 

from Newfoundland in Canada 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 to be the northern limit of their 

continuous exposure (fig. 1).

The sediments that underlie the northern Atlantic Coastal 

Plain province have been divided into 10 regional aquifers 

and 9 interlayered regional confining units. These regional 
8 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

divisions of the NACP aquifer system were based on previous 

work summarized in Trapp (1992), which was part of the 

USGS Regional Aquifer-System Analysis (RASA) Program. 

These divisions were based primarily on similarities and 

differences in hydrologic characteristics resulting from the 

geologic origins of the units. The regional hydrogeologic units 

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.

A more recent investigation has incorporated the substantial revisions to the hydrostratigraphy at the State level into 

an updated regional framework for the NACP aquifer system 

(Pope and others, 2016). The State-level revisions that have 

been published in the two decades since the publication of 

Trapp (1992) are documented in Voronin (2003), McFarland 

and Bruce (2006), Gellici and Lautier (2010), Andreasen and 

others (2013), and McFarland (2013).

The revised delineations of the regional aquifers and 

confining units documented in Pope and others (2016) are 

summarized in figure 5 and described in detail in Masterson 

and others (2013b). The names of some of the regional aquifer 

units from Trapp (1992) were revised in Masterson and others 

(2013b) to reflect a decreased emphasis on the hydrogeology 

of the NACP aquifer system in North Carolina, given the 

change in the extent of the study area compared with the 

previous regional assessment in this area (Trapp and Meisler, 

1992). Consequently, several regional names were 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 were included in 

the chart shown in figure 5 to provide the information needed 

to correlate hydrogeologic units across the Virginia-North 

Carolina border.

Revised names and delineations for other units reflect 

substantial revisions in hydrogeologic correlations (fig. 5) 

across the study area. A primary example is the reclassification of the Upper, Middle, and Lower Potomac aquifers that 

previously were considered part of the regional Potomac 

aquifer system, which are now considered a single Potomac 

aquifer in most of Virginia (fig. 6A, E), four local aquifers in 

Maryland and Delaware (Upper Patapsco, Lower Patapsco, 

Patuxent, and Waste Gate; fig. 6B), two local aquifers in 

New Jersey (Middle Potomac-Raritan-Magothy and Lower 

Potomac-Raritan-Magothy; fig. 6C), and a single local 

aquifer (Lloyd) in New York (fig. 6D). These local aquifers 

all have been reclassified by Masterson and others (2013b) 

SEA LEVEL

500

−500

−1,000

−1,500

10 20 KILOMETERS 

0 10 20 MILES

0

Fractures

EXPLANATION

Bedrock

Confining unit

Saline groundwater

Confined aquifer

Unconfined aquifer

Direction of groundwater flow

 under predevelopment conditions

Land surface

Saprolite

Contact—Dashed where inferred

ATLANTIC

OCEAN

Fall Zone

WEST EAST

VERTICAL SCALE GREATLY EXAGGERATED

masterson_nacp_fig04

Figure 4. Generalized geology of the Northern Atlantic Coastal Plain aquifer system from the Fall Zone to the 

coast of the Atlantic Ocean. Modified from Leahy and Martin (1993). The Fall Zone (fig. 1) is the transition from 

the igneous and metamorphic rocks of the Piedmont physiographic province to the sedimentary environment 

of the Coastal Plain physiographic region.
Introduction 9

into two regional aquifers, the Potomac-Patapsco and 

Potomac-Patuxent aquifers (fig. 5). These new designations 

approximately correspond to the Middle Potomac and Lower 

Potomac regional aquifers of Trapp (1992). In another 

example, permeable sediments previously classified as the 

Upper Potomac aquifer have been regrouped with either 

the Potomac-Patapsco or the Magothy regional aquifers by 

Masterson and others (2013b). In a further example, the 

Peedee regional aquifer is now referred to as the MonmouthMount Laurel aquifer to emphasize the primary local units 

comprising this regional aquifer in the study area.

The other substantial changes reported in Masterson and 

others (2013b) from Trapp (1992) reflect recently improved 

understanding of the transition in geology and hydrogeology in southeastern Virginia and across the border with 

North Carolina (McFarland and Bruce, 2006). Although 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 regional 

aquifer nomenclature as part of that effort.

The final substantive revision to the hydrogeologic 

framework of the northern Atlantic Coastal Plain province 

discussed in Masterson and others (2013b) is the recognition 

of the Chesapeake Bay impact crater, which includes three 

local hydrogeologic units described by McFarland and Bruce 

(2006)—the Chickahominy confining unit, the Exmore matrix 

confining unit, and the Exmore clast confining unit—that 

compose the crater-fill and crater-cap sediments (fig. 6A). 

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 older than the 

regional Piney Point aquifer within the impact crater area 

(fig. 6A). These crater-fill materials are considered to be of 

very low permeability, 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).

NACP New York

(Long Island) New Jersey Delaware Maryland Virginia North Carolina

surficial upper glacial Holly Beach,

unconfined upper Kirkwood-Cohansey

surficial

upper Chesapeake Holly Beach upper Chesapeake Yorktown Yorktown

upper Chesapeake confined upper Kirkwood-Cohansey

Pocomoke,

Manokin

Pocomoke,

Ocean City,

Manokin

Yorktown-Eastover Yorktown

lower Chesapeake Saint Marys Pungo River

lower Chesapeake lower Kirkwood-Cohansey,

confined Kirkwood

Milford,

Frederica,

Federalsburg,

Cheswold

Choptank,

Calvert Saint Marys Pungo River

Calvert basal Kirkwood Calvert Castle Hayne

Piney Point Piney Point Castle Hayne

Nanjemoy-Marlboro Vincentown-Manasquan Nanjemoy-Marlboro* Beaufort

Aquia Vincentown Rancocas Aquia* Beaufort

Monmouth-Mount Laurel Navesink-Hornerstown Severn Peedee

Monmouth-Mount Laurel Wenonah-Mount Laurel Mount Laurel Monmouth Peedee

Matawan Marshalltown-Wenonah Matawan Virginia Beach Black Creek

Matawan Englishtown Matawan Virginia Beach Black Creek

Magothy Merchantville-Woodbury Matawan-Magothy Cape Fear

Magothy Magothy upper Potomac-Raritan-Magothy Magothy Magothy upper Cape Fear,

lower Cape Fear

Potomac Raritan Magothy-Patapsco Potomac,*

upper Cenomanian upper Cenomanian

Potomac-Patapsco Lloyd middle Potomac-Raritan-Magothy upper Patapsco,

lower Patapsco Potomac* Lower Cretaceous

Potomac-Patuxent Arundel Clay

Potomac-Patuxent lower Potomac-Raritan-Magothy

Patuxent,

Waste Gate

Patuxent,

Waste Gate

bedrock basement

absent

unnamed

unnamed

unnamed

absent absent

absent

absent

masterson_nacp_fig 05_new

Figure 5. Regional hydrogeologic units and corresponding state-level hydrogeologic units for the Northern Atlantic Coastal Plain 

(NACP) aquifer system. Unit names for New York are from Smolensky and others (1989); for New Jersey, from the U.S. Geological Survey 

New Jersey Water Science Center (unpub. data, 2011); for Virginia, from McFarland and Bruce (2006); and for North Carolina, from 

Gellici and Lautier (2010). Units for Delaware and Maryland are from Andreasen and others (2013), but the units in Delaware were not 

the primary focus of that report. Regional units are from Masterson and others (2013b), as modified by Pope and others (2016). Units 

shaded in gray are confining units; units that are unshaded are aquifer units. Aquifers and associated confining units indicated with 

an asterisk are truncated in part of Virginia by sediments related to the Chesapeake Bay impact crater. Hydrogeologic units shown in 

bold italic font are those in southern Maryland that extend across the State border into northern Virginia. Unit name descriptors such 

as “lower” and “upper” are shown without capital letters for internal consistency in this report, except for Lower Cretaceous, which 

corresponds to a formal geologic unit name. This format may differ from the source references.
10 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

Figure 6. Cross sections showing regional variations in the hydrologic framework of the Northern Atlantic Coastal Plain 

aquifer system. A, A–A' in Virginia modified from McFarland and Bruce (2006); B, B–B' in Delaware and Maryland modified 

from Andreasen and others (2013); C, C–C' in New Jersey modified from dePaul and others (2008); D, D–D' on Long Island, 

New York, modified from Buxton and Shernoff (1999); and E, E–E' in Maryland, Virginia, and North Carolina modified from 

McFarland (2013). Cross-section A–A' includes the impact crater in the Chesapeake Bay. Unsaturated zone in surficial 

aquifer is not represented on sections.

Fall Zone

Edge of Continental Shelf

Area of impact crater

C

C'

B B'

D

D'

A A'

0

0 50 100 KILOMETERS

50 100 MILES

41°

39°

37°

 35°

78° 76° 74° 72°

PA

NC

VA

NY

NJ

CT RI

WV MD

DE

MA

E

E'

Area of saline groundwater

Freshwater aquifers

Bedrock

Fractures

EXPLANATION

Confining unit

Direction of groundwater flow

 under predevelopment (1900) 

 conditions 

Contact—Dashed where inferred

Land surface

Saprolite

SECTION 

E–E'

SECTION 

E–E'

Lower Patapsco aquifer system

Upper Patapsco 

aquifer system

Choptank aquifer

Lower 

Chesapeake 

confining unit

Matawan 

confining unit

Monmouth aquifer

Matawan-Magothy 

confining unit

Matawan aquifer

Severn 

confining 

unit

Magothy 

aquifer

Patuxent aquifer system

Waste Gate aquifer system

A A'

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

Chickahominy 

confining unit

Chesapeake Bay 

impact crater

500

250

−500

−750

−250

−1,000

−1,250

−1,500

Chesapeake Bay

0

FEET

WEST EAST

Ocean City 

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

0

B B'

FEET

WEST EAST

Upper 

Chesapeake

confining unit

DATUM IS NATIONAL GEODETIC VERTICAL DATUM OF 1929

VERTICAL SCALE GREATLY EXAGGERATED

DATUM IS NATIONAL GEODETIC VERTICAL DATUM OF 1929

VERTICAL SCALE GREATLY EXAGGERATED

Manokin aquifer Calvert aquifer system

Nanjemoy confining unit

Piney Point aquifer

Marlboro Clay confining unit

Aquia aquifer

Calvert

confining

unit

Magothy-Patapsco 

confining unit

Exmore matrix 

confining unit

Exmore clast 

confining unit

MARYLANDDELAWARE

0 10 20 KILOMETERS

0 10 20 MILES

0 10 20 KILOMETERS

0 10 20 MILES

A

B
Introduction 11

Figure 6 (see page 10).—Continued 

Area of saline 

 groundwater

Freshwater aquifer

Bedrock

Fractures

EXPLANATION

Confining unit

Direction of groundwater 

 flow under predevelopment 

 (1900) conditions

Contact—Dashed 

 where inferred

Land surface

D

masterson_nacp_fig 6cd

Fall Zone

Edge of Continental Shelf

Area of impact crater

C

C'

B B'

D

D'

A A'

0

0 50 100 KILOMETERS

50 100 MILES

41°

39°

37°

35°

78° 76° 74° 72°

PA

NC

VA

NY

NJ

CT RI

WV MD

DE

MA

E

E'

C C'

Kirkwood-Cohansey aquifer

200

0

−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

ATLANTIC 

OCEAN

200

0

−200

−400

−600

−800

−1,000

−1,200

−1,400

FEET

D D'

Long

Island

Sound

Raritan confining unit

Englishtown aquifer system

Wenonah-Mount Laurel aquifer

Upper Potomac-Raritan-Magothy aquifer

Middle Potomac-Raritan-Magothy aquifer 

Vincentown aquifer 

PENNSYLVANIA

NEW JERSEY

Fall Zone

DATUM IS NATIONAL GEODETIC VERTICAL DATUM OF 1929

VERTICAL SCALE GREATLY EXAGGERATED

DATUM IS NATIONAL GEODETIC VERTICAL DATUM OF 1929

VERTICAL SCALE GREATLY EXAGGERATED

0 5 10 KILOMETERS

0 5 10 MILES

0 10 20 KILOMETERS

0 10 20 MILES

C

D
12 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

Land Use

Land use and land cover in the study area are a mosaic of 

forest, wetlands, and agriculture (Loveland and others, 1999). 

Dominant land uses are farming and forestry, with locally 

dense urban and suburban development. More than half 

(57 percent) of the northern Atlantic Coastal Plain province 

is classified as undeveloped, nearly a third (30 percent) is 

agricultural, and 13 percent is developed. Land use varies 

substantially from north to south. Long Island is mostly 

(65 percent) developed, more than half (53 percent) of 

Delaware is agricultural, and Virginia is mostly (64 percent) 

undeveloped (fig. 7; Fry and others, 2011).

Land cover since the early 1900s has trended toward 

conversion of open space and agricultural land to commercial, industrial, and residential 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 

northern Atlantic Coastal Plain province. Although total 

agricultural land cover has not changed appreciably, substantial changes in crop types have increased water-use demands 

(Loveland and others, 1999).

FEET

masterson_nacp_fig 6e

Other hydrogeologic units 0

–500

–1,000

–1,500

500

–2,000

Upper Cretaceous sediments

Potomac aquifer

Hope

Plantation

Oak

Grove James

River Potomac River

E E'

SOUTH NORTH

Chowan

River

Norfolk arch Salisbury embayment

Upper Cenomanian

confining unit 

Patuxent aquifer

Lower Patapsco aquifer

Arundel Clay

confining unit

Upper Patapsco

aquifer

West

Franklin Williamsburg Point

Patapsco confining unit

Fault line

VIRGINIA 

0 10 20 30 KILOMETERS

0 10 20 30 MILES

NORTH CAROLINA

VIRGINIAMARYLAND

FEET Albemarle embayment

SECTION 

A–A '

SECTION 

B–B'

Freshwater aquifer

Bedrock

EXPLANATION

Confining unit

Contact—Dashed where inferred

Land surface Fall Zone

Edge of Continental Shelf

Area of impact crater

C

C'

B B'

D

D'

A A'

0

0 50 100 KILOMETERS

50 100 MILES

41°

39°

37°

 35°

78° 76° 74° 72°

PA

NC

VA

NY

NJ

CT RI

WV MD

DE

MA

E

E'

Fault—Arrows show relative

 movement. Dashed where

 inferred

E

DATUM IS NATIONAL GEODETIC VERTICAL DATUM OF 1929

VERTICAL SCALE GREATLY EXAGGERATED

Figure 6. Cross sections showing regional 

variations in the hydrologic framework of the 

Northern Atlantic Coastal Plain aquifer system. 

A, A–A' in Virginia modified from McFarland 

and Bruce (2006); B, B–B' in Delaware and 

Maryland modified from Andreasen and others 

(2013); C, C–C' in New Jersey modified from 

dePaul and others (2008); D, D–D' on Long 

Island, New York, modified from Buxton and 

Shernoff (1999); and E, E–E' in Maryland, 

Virginia, and North Carolina modified from 

McFarland (2013). Cross section A–A' includes 

the impact crater in the Chesapeake Bay. 

Unsaturated zone in surficial aquifer is 

not represented on sections.—Continued
Introduction 13

Developed

Undeveloped

Agricultural

EXPLANATION

Study area

Land use

57 Percentage of total

65

59

6

29

21

20

8

13

13

30

57

49 38

8

3

65

32

64

28

39 53

NORTH CAROLINA

DELAWARE

DC

PENNSYLVANIA

VIRGINIA

MARYLAND

NEW

JERSEY

NEW

YORK

CT

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

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 50 100 MILES

0 50 100 KILOMETERS

masterson_nacp_fig 07

78° 75° 72°

39°

36°

Albemarle Sound

Pamlico Sound

ATLANTIC OCEAN

Chesapeake Bay

Delaware Bay

Long Island Sound

New York

New Jersey

Maryland

Delaware

Virginia

Northern Atlantic Coastal Plain

North Carolina

WV

Figure 7. Land use and land cover for the northern Atlantic Coastal Plain physiographic province for conditions in 

2006. Modified from Fry and others (2011).
14 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

Groundwater Use

Groundwater is withdrawn from the NACP aquifer 

system for multiple uses, including agricultural, commercial, 

domestic, and industrial purposes (fig. 8). In 2010, the total 

amount of groundwater withdrawn was about 1,500 Mgal/d 

(Maupin and others, 2014). The distribution of dominant 

water-use type varies spatially across the northern Atlantic 

Coastal Plain province, with public and domestic supply the 

primary use 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 (fig. 9).

The total public and domestic water use (groundwater 

and surface water) for the nearly 21 million people living 

in the northern Atlantic Coastal Plain province in 2010 was 

about 2,334 Mgal/d, about 44 percent of which (or about 

1,026 Mgal/d) was derived from groundwater (fig. 8; table 1; 

Maupin and others, 2014) and accounted for 69 percent of 

all groundwater withdrawals (1,489 Mgal/d) in the study 

area (fig. 8; table 2). Most (71 percent) of these groundwater 

withdrawals occurred on Long Island and in New Jersey.

Of the estimated 1,026 Mgal/d of groundwater withdrawn 

for public and domestic use, 84 percent was supplied through 

public water-supply systems, and 16 percent was self-supplied 

from private wells (table 1). In the northern part of the study 

area (New York, New Jersey, Pennsylvania, and Delaware), 

nearly all (90 percent) the groundwater pumped for drinking 

water was obtained from public water-supply sources. In the 

southern part of the study area (Maryland, Virginia, and the 

northernmost part of North Carolina), as much as 38 percent of 

the total groundwater pumped for drinking water was obtained 

from self-supplied domestic sources (fig. 8; table 1). This 

change in the source of drinking water from north to south is 

reflected in the land use and land cover throughout the northern 

Atlantic Coastal Plain province (fig. 7); water demand is met by 

public water-supply systems in the more highly populated areas 

in the north and by domestic self-supplied sources in the less 

densely populated areas in the south.

Treated saline groundwater provides an additional source 

of drinking water in several coastal communities where the 

availability of fresh surface water and fresh groundwater is 

limited. Desalinization of saline groundwater is a significant 

source of drinking water in southeastern Virginia where more 

than 10 Mgal/d of saline groundwater treated at five separate 

facilities is used for drinking water (Maupin and others, 2014). In 

Cape May, New Jersey, desalinization is used to augment drinking 

water supplies during the high-demand summer months.

The total agricultural water use in the northern Atlantic 

Coastal Plain province in 2010 was estimated to be about 

Public surface water source

Public groundwater source

Domestic groundwater

Imported from surface waters

outside the NACP

EXPLANATION

Source of potable water for

residential purposes

Percentage of total

Northern Atlantic Coastal Plain

North Carolina District of

Columbia

Delaware Pennsylvania New York New Jersey

Virginia Maryland

100 69

15

16

69

69

18

7

6

54

15 23

8

50

33

17

77

20

25 61

14

48 51

37 25

31

7

EXPLANATION

Public and domestic supply

Agriculture

Commercial and industrial

Northern Atlantic Coastal Plain

Groundwater use

Percentage of total

North Carolina

Pennsylvania New York New Jersey

Virginia Maryland Delaware

50

30

20

51 38

12

63

9

28

57

6

36

9

55

36 16

78

78

6

3

69

13

18

Figure 8. Distribution of drinking water sources by State across

the Northern Atlantic Coastal Plain (NACP) aquifer system. Modified

from Maupin and others (2014).

Figure 9. Distribution of groundwater use by State across the 

Northern Atlantic Coastal Plain aquifer system. Modified from 

Maupin and others (2014).
Introduction 15

467 Mgal/d, with about 58 percent (271 Mgal/d) obtained 

from groundwater sources (table 2; Maupin and others, 2014). 

Kenny and others (2009) reported total agricultural use in 

2005 to be 300 Mgal/d with about 180 Mgal/d derived from 

groundwater. The 50 percent increase in agricultural water 

use from 2005 to 2010 may be the result of more demand 

because of a precipitation deficit, a trend toward more 

irrigation, or better reporting of water-use information. Most 

(92 percent, or 249 Mgal/d) of the 271 Mgal/d of groundwater 

withdrawals was for crop irrigation, with the remaining 

8 percent (22 Mgal/d) for livestock and aquaculture uses 

(13 and 9 Mgal/d, respectively). More than half (53 percent; 

144 Mgal/d) of the groundwater withdrawals from the 

NACP aquifer system for agriculture in 2010 was in the 

Delmarva Peninsula. Agricultural groundwater withdrawals 

were the highest in Delaware (87 Mgal/d) and accounted for 

about 58 percent of the total groundwater withdrawals in the 

State (fig. 9; table 2).

Use of freshwater for commercial and industrial purposes 

in the northern Atlantic Coastal Plain province in 2010 was 

about 940 Mgal/d, of which 20 percent (192 Mgal/d) was 

derived from groundwater sources (table 2; Maupin and 

others, 2014). Groundwater withdrawals in Virginia and 

northern North Carolina accounted for about 69 percent of the 

total groundwater used for commercial and industrial purposes 

in the northern Atlantic Coastal Plain province in 2010. The 

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

in the Coastal Plain of Virginia and North Carolina; groundwater withdrawals for this industrial purpose accounted for 

much of the large water-level decrease throughout the southern 

portion of the northern Atlantic Coastal Plain province from 

1900 to 2000 (fig. 3).

Thermoelectric powerplants in the northern Atlantic 

Coastal Plain province also use a substantial amount of 

water. About 18,000 Mgal/d of water was used for cooling 

powerplants in 2010; 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 powerplants throughout 

the northern Atlantic Coastal Plain province is about 7 Mgal/d 

(Maupin and others, 2014).

Pennsylvania is included in the tabular water-use 

summaries in tables 1 and 2 because a small area with a 

substantial population in the eastern part of the State falls 

within the northern Atlantic Coastal Plain province. However, 

most of the water supply for this area is imported from 

outside the study area, and the small amount of groundwater 

withdrawn from the NACP aquifer system in Pennsylvania 

is a negligible part of the total withdrawals from this system. 

Therefore, Pennsylvania is not included in subsequent analysis 

of groundwater availability in the NACP aquifer system.

Table 1. Distribution of drinking water sources in 2010 by State in the Northern Atlantic Coastal Plain aquifer system.

[Units are million gallons per day. Water-use data are from Maupin and others (2014). Gray shaded box indicates that value is zero or rounded 

to zero. NACP, Northern Atlantic Coastal Plain]

State

Public

groundwater

source

Public

surface-water

source

Self-supplied

domestic

groundwater

Imported from

surface waters

outside NACP

Total

New York 414 0 3 389 806

New Jersey 251 105 57 0 413

Pennsylvania 2 93 2 24 121

Delaware 41 27 14 0 82

District of Columbia 0 0 0 63 63

Maryland 78 180 51 26 335

Virginia 28 312 33 79 452

North Carolina 43 10 9 0 62

Total 857 727 169 581 2,334

Table 2. Distribution of groundwater use in 2010 by State in the 

Northern Atlantic Coastal Plain aquifer system.

[Units are million gallons per day. Values may not sum to totals shown 

because of independent rounding. Water-use data are from Maupin and others 

(2014). Gray shaded box indicates that value is zero or rounded to zero]

State

Public 

and 

domestic

Agricultural

Commercial and 

industrial

Total

New York 417 14 2 433

New Jersey 308 64 25 397

Pennsylvania 4 1 6 11

Delaware 55 87 9 151

District of Columbia 0 0 0 0

Maryland 129 57 18 204

Virginia 61 14 45 120

North Carolina 52 34 87 173

Total 1,026 271 192 1,489
16 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

Previous Investigations

Numerous studies have been conducted to characterize 

the hydrogeology of the northern Atlantic Coastal Plain 

province, 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 (Trapp and 

Meisler, 1992) under the auspices of the RASA Program. 

Hydrogeologic data collected as part of the RASA Program’s 

assessment served as the basis for the subsequent numerical 

models developed for the NACP aquifer system.

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 Virginia (Harsh and Laczniak, 

1990), Maryland and Delaware (Fleck and Vroblesky, 1996), 

North Carolina (Giese and others, 1997), and New Jersey 

(Martin, 1998) were used to assess water budgets from predevelopment through 1980. The regional and subregional models 

incorporated the hydrogeologic framework derived from the 

statewide components of the RASA Program’s assessment 

for Virginia (Meng and Harsh, 1988), New Jersey (Zapecza, 

1989), North Carolina (Winner and Coble, 1989), and Maryland and Delaware (Vroblesky and Fleck, 1991). In addition 

to these statewide studies, a separate analysis of groundwater 

availability was conducted for Long Island based on the 

regional model of the island (Reilly and Harbaugh, 1980).

Analyses since Trapp and Meisler (1992) have built upon 

and enhanced the statewide NACP aquifer system 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 a conversion of the model 

datasets (Voronin, 2003) to be compatible with the modular 

three-dimensional (3D) finite-difference groundwater flow 

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

updated model by Voronin (2003) also included a finer grid 

discretization, spatially variable recharge, and a more recent 

pumping period (1968–1998).

The first groundwater model for the Virginia Coastal 

Plain physiographic province (Harsh and Laczniak, 1990) 

originally used a numerical finite-difference code developed 

by Trescott (1975) and was updated in the late 1990s (McFarland, 1998) to MODFLOW–96 (Harbaugh and McDonald, 

1996). The most recent model for the Virginia Coastal Plain 

(Heywood and Pope, 2009) was developed to include the 

latest interpretation of the hydrostratigraphy of the study area 

(McFarland and Bruce, 2006), most notably the presence of 

the large impact crater discovered beneath 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 3D 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 groundwater model for Long Island (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 completed in the area that focus 

on the surficial unconfined and shallow, confined aquifers. 

These studies include a simulation of groundwater flow on the 

Eastern Shore of Virginia on the Delmarva Peninsula 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 Chesapeake 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 conditions in the northern Atlantic 

Coastal Plain province, but the information obtained from 

these studies had not been synthesized into a regional hydrogeologic perspective similar to Trapp and Meisler (1992). A 

region-wide assessment of water-level changes (dePaul and 

others, 2008) based on long-term and synoptic water-level 

data from predevelopment to 2000 shows significant changes 

in water levels since the RASA Program study (Trapp and 

Meisler, 1992) and highlights the need for an updated regional 

assessment of hydrologic effects from changing stresses in the 

groundwater system since 1980.

Methods

Pertinent hydrogeologic information needed to develop 

a conceptual model of how water enters, moves through, 

and ultimately leaves the aquifer system was collected. The 

conceptual model then was used as the foundation for the 

construction of a 3D numerical model capable of simulating 

groundwater flow conditions throughout the NACP aquifer 

system (Masterson and others, 2016). The numerical modeling 

tool then was used to analyze groundwater availability in the 

aquifer system.
Conceptualization of the Hydrologic System 17

Data Compilation

The collection of digital hydrogeologic data in the study 

area necessary for the model development included elevations 

and extents of hydrostratigraphic units (Pope and others, 2016), 

groundwater salinity distribution (Charles, 2016), groundwater 

withdrawals, temperature and precipitation distributions, and 

information necessary for parameter estimation including 

groundwater levels and streamflow. The hydrostratigraphic 

data were compiled from data of the USGS water science 

centers in Delaware, Maryland, New York, New Jersey, 

North Carolina, and Virginia. The data were synthesized, 

generalized, and joined across State lines, providing the 

information necessary to develop the layering scheme used 

in the numerical model (Masterson and others, 2016).

The groundwater withdrawal information for this study 

was compiled from several sources. The primary sources of 

the data were the water-use programs at individual USGS 

water science centers within the study area, which in turn 

collected and compiled data from State and local agencies. 

Water-use information before 1980 that was not available 

through the water-use program was derived from the previous 

regional groundwater model of the NACP aquifer system 

(Leahy and Martin, 1993). Additional estimates for domestic 

self-supply and irrigation withdrawals are described in more 

detail in the documentation for the groundwater model 

(Masterson and others, 2016).

Information on aquifer recharge was developed from the 

Soil Water Balance (SWB) model (Westenbroek and others, 

2012) based on temperature and precipitation data obtained 

from the Oak Ridge National Laboratory’s Daymet dataset, 

which included daily surface weather data on a 1-kilometer 

(km; 0.62-mile [mi])×1-km grid for North America from 

1980 to 2012 (Thornton and others, 2012); existing land-use 

information from the Multi-Resolution Land Characteristics 

Consortium (MRLC) 2001 National Land Cover Database 

(NLCD; LaMotte, 2008a,b); and available water capacity 

of soils classified by the U.S. Department of Agriculture 

Natural Resources Conservation Service (undated). The 

SWB modeling approach used in the initial development of 

the recharge arrays needed for the numerical model in this 

study is described in Masterson and others (2013b).

The groundwater salinity information used to delineate 

the offshore extent of freshwater in the numerical model was 

compiled from available chloride data in the USGS National 

Water Information System (NWIS) database (U.S. Geological 

Survey, 2011). Isochlors were constructed from the available 

data for each of the nine regional confined aquifers in the 

NACP aquifer system (Charles, 2016).

Numerical Model

Numerical models provide a means to synthesize existing 

hydrogeologic information into an internally consistent 

mathematical representation of a real-world system or process 

and thus are useful tools for testing and improving conceptual 

models or hypotheses of groundwater-flow systems (Konikow 

and Reilly, 1999). A transient, 3D groundwater model was 

developed for the NACP aquifer system from Long Island 

to northeastern North Carolina to quantify current [2013] 

groundwater resources, evaluate how groundwater resources 

have changed over time, and forecast the responses of the 

aquifer system to future stresses. The numerical code used for 

the model in this study was MODFLOW–NWT (Niswonger 

and others, 2011) and the model was calibrated using 

PEST (Doherty, 2010). Detailed descriptions of the model 

development and calibration are presented in Masterson and 

others (2016).

Conceptualization of the 

Hydrologic System

The NACP aquifer system consists of nine confined 

aquifers and nine confining units capped by an unconfined 

surficial aquifer. This aquifer system is bounded laterally from 

the west by the contact between Coastal Plain sediments and 

the upland Piedmont bedrock at the Fall Zone (fig. 6A–E) 

and extends to the east to the limit of the Continental Shelf; 

however, the boundary between fresh and saline groundwater 

is considered to be much closer to the shoreline (Charles, 

2016). A detailed description of the hydrogeologic framework 

can be found in Masterson and others (2013b), and the digital 

data and documentation on the configuration of the hydrogeologic units can be found in Pope and others (2016).

The sources (or inflows) and sinks (or outflows) of water 

to the NACP aquifer system can be illustrated schematically 

(fig. 10) to illustrate how water enters, flows through, and exits 

this aquifer system. The sources of water to the NACP aquifer 

system for predevelopment conditions include recharge from 

precipitation and the return of wastewater, and the eastward 

flow of groundwater and surface water across the Fall Zone. 

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. 

An additional loss of water from the aquifer system for postdevelopment conditions 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.

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 until a steady-state condition 

is achieved.
18 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

The Soil-Water Balance model (SWB; Westenbroek 

and others, 2012) used in Masterson and others (2013b) 

determined that, for conditions in 2005, about 68 percent 

of the precipitation that entered the NACP aquifer system 

was assumed to be lost to evapotranspiration or runoff, 

and therefore, of the 61,800 Mgal/d of precipitation, only 

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

recharge. Most of this recharge entered the aquifer system and 

flowed through the shallow unconfined aquifer and discharged 

either to streams or directly to coastal waters without reaching 

the deep confined aquifer system. The regional assessment of 

the NACP aquifer system of Trapp and Meisler (1992) estimated that, under predevelopment conditions, only 2 percent 

of the recharge that entered the aquifer system reached the 

deep confined system. Based on the more recent analysis by 

Masterson and others (2013b), 2 percent of the total groundwater recharge would result in about 400 Mgal/d of recharge 

to the confined aquifer system.

Masterson and others (2013b) estimated a total input of 

19,800 Mgal/d of freshwater entered the NACP aquifer system 

based on 2005 conditions, and nearly all (about 98 percent) of 

this inflow was from the infiltration of precipitation (referred 

to as natural recharge). The remaining 2 percent was assumed 

to be derived from wastewater return flow, water released 

from storage in aquifers and confining units, and subsurface 

fractured rock flow across the Fall Zone. A substantial 

amount of freshwater does enter the NACP aquifer system as 

streamflow across the Fall Zone (18,600 Mgal/d); however, 

it is assumed that, at a regional scale, most of this streamflow 

discharges directly to the coast without interacting with the 

groundwater flow system.

The largest components of outflow (or loss) of freshwater 

from the NACP aquifer system are from evapotranspiration 

(derived from evaporation and plant transpiration) and surface 

runoff, but only the resulting net recharge from precipitation 

is considered in the hydrologic budget of the aquifer system. 

An additional loss of freshwater may occur locally from 

groundwater evapotranspiration at the water table where the 

depth to water is shallow; however, this loss is considered to 

be negligible at the regional scale. Therefore, the primary loss 

of freshwater from the NACP aquifer system is from groundwater discharge to streams. Masterson and others (2013b) 

estimated that about 18,600 Mgal/d of freshwater entered the 

study area from streams flowing across the Fall Zone. Flow in 

these streams increased by about 3,800 Mgal/d from additional 

groundwater discharge as the streams flowed toward the coast. 

Further loss of freshwater from the NACP aquifer system 

was from groundwater discharge to streams that originated 

entirely within the northern Atlantic Coastal Plain province. 

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

masterson_nacp_fig 10

Fall Zone

Fractures

Inflow

Outflow

EXPLANATION

Bedrock

Confining unit

Confined aquifer

Unconfined aquifer

Land surface

Contact

Confining unit

Confined aquifer

Unconfined

aquifer

Unconfined

aquifer

Bedrock NOT TO SCALE

Recharge

Fall Zone

Inflow Outflow

Coastal

discharge

Streamflow

Subsea

discharge

Groundwater

pumping

Streamflow

Figure 10. The conceptual hydrogeologic 

model of the Northern Atlantic Coastal 

Plain aquifer system; modified from 

Masterson and others (2013b).
Simulation of the Hydrologic System 19

that originated in this area and then discharges to the coastal 

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

discharges to streams and then was transported to coastal 

receiving waters and, therefore, was lost from the aquifer 

system. Groundwater discharge to streams accounted for about 

60 percent of the 19,800 Mgal/d of total outflow of freshwater 

from the groundwater flow system for conditions in 2005.

In 2005, groundwater withdrawals totaled about 

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

outflow from the groundwater flow system (Masterson and 

others, 2013b). Most of this water (about 1,000 Mgal/d) was 

withdrawn from the 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, the 

groundwater withdrawal 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 

storage. Decreases in outflows from predevelopment conditions may include a reduction in deep subsea discharge from 

the confined system or reductions in streamflow and shallow 

coastal discharge resulting from an increase in downward 

leakage (deep recharge) from the shallow unconfined system.

The regional assessment of groundwater flow in the 

NACP aquifer system of 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 northern Atlantic 

Coastal Plain province. For 1980 pumping conditions, this 

area was estimated to increase to about 45 percent of the total 

land area of northern Atlantic Coastal Plain province. The 

analysis of Trapp and Meisler (1992) also determined that this 

increase in downward percolation (and the resulting reduction in streamflow and shallow coastal discharge) accounted 

for 61 percent of the total groundwater removed from the 

system by 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 about saltwater intrusion into the 

freshwater system.

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 analysis 

of 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, the 

approach of Trapp and Meisler (1992) did not consider potential 

storage release from the confining units.

The more recent studies of Heywood and Pope (2009) 

and Konikow (2013) 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. Konikow (2013) determined that release of water from 

storage (that is, groundwater storage depletion) was about 

200 Mgal/d for 2001 to 2008, which was about 13 percent of 

the pumping rate in 2005 (1,500 Mgal/d).

Storage release from the confining unit sediments creates 

the potential for increased risk of land subsidence. Pope and 

Burbey (2004) determined that storage release from the finegrained sediments that compose the confining units in southern 

Virginia in response to large groundwater withdrawals has 

resulted in compaction of these sediments, explaining in part 

the increased rates of land subsidence and anomalously high 

rates of sea-level rise observed in the lower Chesapeake Bay 

area (Eggleston and Pope, 2013).

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, without a groundwater flow model and based on 

the assumption that the water released from storage can be 

quantified, the rates of groundwater discharge to the coast can 

only be estimated as a residual term in order to balance out 

known inflows to the system.

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. 

A numerical model that explicitly represents confining unit 

storage is necessary to refine the estimates of the amount of 

water released from storage and the amount of shallow coastal 

and deep subsea discharge and is instrumental in assessing the 

potential for decreases in groundwater discharge to the coast 

as well as the resulting effects.

Simulation of the Hydrologic System

A numerical model was developed to improve the understanding of the conceptual model of groundwater flow in the 

NACP aquifer system and to assess the effects of development 

on water availability from predevelopment to future conditions 

(Masterson and others, 2016). The model was used to assess 

changes in water levels, storage, and hydrologic budgets over 

time at the regional scale and for selected subregions that 

roughly coincide with State boundaries.

The total simulation time from 1900 to 2058 was subdivided into three periods. A historical period (1900 to 1985) 

was selected to coincide with the previous regional assessment 

of the NACP aquifer system (Trapp and Meisler, 1992). The 

period of emphasis described in this report was from 1986 to 

2013. Current [2013] conditions are defined as the 5-year time 

span of 2009–2013 and included a pumping stress equal to the 

pumping in 2008, not the average of the actual pumping rates 

for this time span. The current [2013] conditions recharge rate 
20 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

is based on long-term average recharge conditions from 1980 to 

2008. Future conditions were simulated by continuing current 

[2013] conditions into the future to assess how the system 

continues to adjust to these stresses after 30 years [2043] and 

45 years [2058] from present [2013]. Additional analyses 

include the effects of sea-level rise on water availability and 

the effects of a prolonged drought on irrigation withdrawals 

of groundwater and water levels in agricultural areas.

Comparison of the Conceptualized and 

Simulated Hydrologic Systems

The conceptual model of the hydrologic budget 

developed by Masterson and others (2013b) described in 

the “Conceptualization of the Hydrologic System” section 

provided a general understanding of the regional water budget 

and served as the starting point for the assessment of groundwater availability in the NACP aquifer system in this report. 

A comparison of this conceptual model and the results of the 

investigation in this report are provided here to illustrate how 

the results of the investigation in this report can be used to 

improve the understanding of the regional hydrologic budget 

for the groundwater flow system of the NACP aquifer system.

Comparison of Predevelopment Conditions

The conceptualization of the hydrologic budget of 

Masterson and others (2013b) did not consider streamflow and 

coastal discharge as separate components of the total outflow 

from the system for predevelopment conditions, and therefore, 

the total discharge to these surface receiving waters balanced 

the total 19,600 Mgal/d of inflow from recharge (fig. 11A). The 

simulated hydrologic budget developed from the investigation 

in this report determined that for predevelopment conditions 

the total inflow from recharge was about 18,100 Mgal/d, 

about 8 percent less than the assumed recharge in the predevelopment conceptual hydrologic budget of Masterson and 

others (2013b). In both investigations, it was assumed that 

the recharge rates for pre- and postdevelopment conditions 

were equal. The recharge rate in the conceptual hydrologic 

budget in Masterson and others (2013b; fig. 11A) was calculated by the SWB model (Westenbroek and others, 2012) 

for 2005–2009 conditions, whereas for the analysis in this 

report, the recharge rate shown in figure 11B was estimated as 

part of the model parameter estimation process described in 

Masterson and others (2016).

The outflow needed to balance the inflow from recharge 

for the analysis in this report was subdivided between streamflow, discharge to tidal portions of streams (referred to as “Tidal 

waters” in fig. 11), and coastal discharge. The model-calculated 

distribution of the groundwater discharge to these surface water 

receptors indicates that about 76 percent of the total discharge is 

distributed nearly equally between streamflow and direct coastal 

discharge, with the remaining 24 percent discharging to the 

tidal portions of rivers at the headwaters of coastal estuaries.

The regional assessment of Trapp and Meisler (1992) 

assumed that, for predevelopment conditions, up to 2 percent 

(400 Mgal/d) of the total aquifer recharge reached the deep 

underlying confined aquifer system. The analysis in this 

report calculated no net vertical flow between the surficial 

aquifer and the confined aquifer system, even though as much 

60 percent of the total recharge is exchanged between the 

surficial and underlying confined aquifers.

The differences between the conceptual models of the 

regional assessment of Trapp and Meisler (1992) and the 

analysis in this report include the large differences in what 

is considered deep recharge. The analysis in this report only 

considers the water-table aquifers to be part of the surficial 

aquifer system; therefore, unconfined aquifers, such as large 

parts of the Kirkwood-Cohansey aquifer in New Jersey and 

the Magothy aquifer on Long Island, are grouped with the 

confined aquifers for the purpose of this report. Because 

these local confined aquifers are not separated from the 

surficial aquifer by an intervening confining unit, vertical flow 

exchange occurs between these aquifers and the overlying 

surficial aquifer; however, the upward and downward fluxes 

balance, resulting in a net vertical flow flux of zero (fig. 11B).

Comparison of Postdevelopment Conditions

The conceptualization of the hydrologic budget of 

Masterson and others (2013b) and the analysis in this 

report assumed that the natural recharge was the same for 

predevelopment and postdevelopment conditions. The return 

of wastewater by onsite domestic septic systems also was 

considered in both analyses and equaled about 1 percent of 

the total recharge in both cases (fig. 11).

Additional sources of water since 1900 were the release 

of water from storage and inflow from the subsea coastal 

boundary between freshwater and saltwater (fig. 11B) in 

response to groundwater withdrawals. The storage release in 

the conceptual model of Masterson and others (2013b) ranged 

from 0 to 200 Mgal/d based on previous analyses and was 

not differentiated between the surficial and confined systems 

(Trapp and Meisler, 1992; Konikow, 2013). The release from 

aquifer storage in the analysis in this report was calculated for 

postdevelopment [2003–2008] conditions to be about 60 Mgal/d 

with all the release derived from the confined system, nearly half 

of which was from the confining units. The total storage release 

from the surficial aquifer was close to zero for this period.

The lateral inflow from the subsea coastal discharge occurs 

in response to overpumping in the confined aquifer system and 

accounts for about 30 Mgal/d of water, or about 2.3 percent 

of the total groundwater withdrawals of 1,330 Mgal/d, but is 

negligible when considering the total inflow into the aquifer 

system (fig. 11B). Although, this inflow is small relative to total 

pumping and total inflow into the aquifer system as a whole, it 

represents a reversal in the gradient of groundwater discharge 

in the subsurface at the freshwater-saltwater interface, and is an 

indication of the lateral landward movement of saline groundwater and, therefore, may be important at the local scale.
Simulation of the Hydrologic System 21 masterson_nacp_fig11

Subsurface 

fractured

rock flow 

>10 (<1%) 

Streamflow and

coastal discharge 

19,600 

(100%)

Natural recharge

19,600 

(>99%)

Surficial aquifer

Confined aquifers

Deep recharge

400

(2% of total recharge)

Natural

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%)

Confined aquifers

Deep recharge

>400

(>2% of total recharge)

Storage release 

0 to 200 (0–1%)

Natural recharge

18,070 

(>99%)

Surficial aquifer

Surficial aquifer Surficial aquifer

Confined aquifers

No net vertical flow

Natural

recharge 

18,070 (<99%)

Pumping 

240 (<1%)

Pumping 

1,090 (6%)

Wastewater 

230 (1%)

Streamflow 

6,370 (35%) 

Subsea

coastal 

inflow

Shallow

coastal 

30 (<1%)

discharge

6,520 (35%)

Shallow

coastal

discharge

7,050 (39%)

Streamflow 

6,660 (37%)

Tidal waters

4,360 (24%)

Confined aquifers

Net downward flow

1,000

(5% of total recharge)

Storage release 

60 (<1%)

Tidal waters

4,170 (23%)

Masterson and others (2013b) This report

A. Predevelopment (before 1900)B. Postdevelopment

Figure 11. Hydrologic budget for A, predevelopment (before 1900) and B, average current (postdevelopment; 2013) conditions for the conceptual model of Masterson and others (2013b) and the numerical analysis in this report for the Northern Atlantic Coastal Plain aquifer system. Values are in million gallons per day; percentages (%) are of total recharge. >, greater than; <, less than.
22 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

The inflows in the conceptualization of the hydrologic 

budget of Masterson and others (2013b) for the postdevelopment condition are balanced by streamflow, groundwater 

discharge to the coast, and groundwater removed by pumping 

(fig. 11B). The streamflows were totaled by period of record 

by hydrologic basin using NHDPlus geospatial data on 

surface-water hydrology (Dewald and others, 2010). The 

groundwater withdrawals were based on estimates of reported 

pumping for conditions in 2005. The discharge to coastal 

waters was calculated as a residual to balance the total inflows 

with outflows. For the conceptual hydrologic budget of 

Masterson and others (2013b), the distribution of the outflows 

indicates that streamflow accounts for about 59 percent of 

the outflow and groundwater withdrawals represent about 

8 percent of the outflow; the remaining 33 percent was 

assumed to be groundwater discharge to the coast (fig. 11B).

The hydrologic budget developed for postdevelopment 

conditions for this report had 93 percent of the total outflow 

allocated to surface receiving waters, with 70 percent divided 

evenly between streamflow and shallow coastal discharge and 

the remaining 23 percent of the total outflow discharging to 

tidal waters (fig. 11B). The remaining 7 percent of the total 

outflow includes withdrawals from the surficial and confined 

aquifers of the groundwater system.

The main difference between the postdevelopment 

conceptual hydrologic budget of Masterson and others (2013b) 

and the hydrologic budget calculated with the numerical 

model in this report is the refinement of the discharge to 

surface receiving waters. The conceptual hydrologic budget 

(Masterson and others, 2013b) calculated streamflow from 

the NHDPlus attribute data (Dewald and others, 2010) and 

assumed the remainder of the inflow was balanced by coastal 

discharge without considering discharge to tidal parts of 

rivers; all nonstreamflow was assumed to be coastal discharge 

(fig. 11A). Therefore, the largest difference in the hydrologic 

budget components is the discharge to the streams that cannot 

be accounted for by discharge to tidal waters.

Average streamflow for postdevelopment conditions 

totaled 11,900 Mgal/d in the conceptual hydrologic budget 

(Masterson and others, 2013b); whereas the groundwater 

discharge to streams calculated with the numerical model 

(this report) totaled 6,370 Mgal/d. This difference can be 

explained by the fact that the NHDPlus data provided average 

streamflows, not average base flows. When the base flow 

correction of 59 percent determined as part of this investigation (Masterson and others, 2016), which is consistent 

with base flow analyses published for other aquifers in the 

Coastal Plain physiographic province (Priest, 2004; Sanford 

and others, 2012), was applied to the NHDPlus estimates, 

the base flow estimate of 7,020 Mgal/d is more in line with 

the 6,370 Mgal/d calculated for this report. Reducing the 

streamflow estimate in the conceptual hydrologic budget 

from 11,900 Mgal/d to 7,020 Mgal/d then would require more 

discharge to the coastal waters outflow component or the 

addition of a tidal waters component of 4,880 Mgal/d.

Hydrologic Budget

Hydrologic budgets account for the inputs to, outputs 

from, and changes in the amount of water in the various 

components of the water cycle and provide the hydrologic 

foundation for analysis of water availability. Basic components of water budgets are precipitation, evapotranspiration, 

surface-water and groundwater flow into and out of an aquifer 

system, change in surface-water and groundwater storage, and 

human withdrawals and interbasin transfers. Knowing where, 

when, and how much water is flowing into or out of a hydrologic budget can illuminate how much is left for other uses 

(water availability) and reveal where stresses to the hydrologic 

budget exist or are developing (Alley and others, 2013). 

Hydrologic budget analyses are necessary to understanding 

the sources of water to a groundwater system and how water 

withdrawals change the components of the hydrologic cycle 

(Reilly and others, 2008).

The changes to the hydrologic budget for the entire 

NACP aquifer system and to six subregions (geographic areas) 

within the NACP aquifer system (fig. 12) were calculated 

using the numerical model to simulate changes in pumping and 

recharge conditions from 1900 to 2013 and projected to 2058. 

Analyses of hydrologic budgets in this report are presented 

both as total changes over time and as the changes over time 

in hydrologic budget components as a function of pumping.

Changes in Hydrologic Budgets Over Time

The initiation of large-scale groundwater withdrawals in 

the NACP aquifer system in the early 1900s resulted in shifts 

in the hydrologic budget components to balance the effects of 

this new stress on the system. When groundwater is removed, 

the aquifer system adjusts accordingly. This adjustment can 

consist of reductions in discharge to surface receiving waters 

(streams, tidal waters, and coastal waters), reductions in 

aquifer storage, induced infiltration from surface waters, and 

in some areas, the addition of a new source of water from the 

return of wastewater from septic systems.

Groundwater withdrawal in the NACP aquifer system 

increased substantially from 1900 to 2013, with the largest 

increases in pumping occurring during the post-World 

War II era from the mid-1940s to the early 1980s (fig. 13). 

Although pumping rates have been relatively constant 

from 1986 to 2013, about half the total volume of water 

removed from the aquifer system (13.4 trillion gallons [Tgal] 

of 27 Tgal) since large-scale groundwater withdrawals began 

around 1900, was removed in just the past 28 years. Therefore, 

the long-term hydrologic effects of the large removal of 

groundwater from the NACP aquifer system during a relatively short time span (28 years) may not have yet 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 resulting 

land subsidence.
Simulation of the Hydrologic System 23

masterson_nacp_fig 12

0 100 KILOMETERS

0 50

50

100 MILES

North Carolina

Study area

Geographic region

Maryland

Virginia

Delmarva

New York

New Jersey

EXPLANATION

Chesapeake Bay

Delaware Bay

Long Island Sound

NEW CT

YORK

NEW

JERSEY

PENNSYLVANIA

MARYLAND

DC DELAWARE

VIRGINIA

NORTH CAROLINA

ATLANTIC OCEAN

Pamlico Sound

Albemarle Sound

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

Albers Equal-Area Conic projection

Standard parallels 34°30' N and 41°30' N

Central meridian 75°30' W 

North American Datum of 1983

WV

78° 75° 72°

39°

36°

Figure 12. Geographic areas delineated for hydrologic budget computations within the Northern Atlantic Coastal 

Plain aquifer system.
24 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

An understanding of the geographic distribution (fig. 14) 

and from which aquifers the water is being withdrawn (fig. 15),

is required to better assess the potential effects of groundwater 

withdrawals for the entire NACP aquifer system. Withdrawal 

totals were apportioned in this report by regional aquifer to 

characterize the pumping distribution in the NACP aquifer 

system as a whole and by geographic area (fig. 15). This 

analysis was based on screen interval information and local 

aquifer designations associated with reported withdrawals 

across the study area.

Figure 14. Groundwater withdrawals for the Northern Atlantic Coastal Plain 

aquifer system by geographic area for the historical period (1900–1985), the 

period of emphasis (1986–2013), and the future period (2014–2058). Geographic 

areas are shown in figure 12.

masterson_nacp_fig 13

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

Historical period Period of emphasis Future period

1920

1939

1945

1952

1957

1964

1967

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Pre-1900

Year

Groundwater withdrawals, in million gallons per day 1,400

1,200

1,000

800

600

400

200

0

1,600

North Carolina

Maryland

Virginia

Delmarva

New York

New Jersey

EXPLANATION

Geographic area

Figure 13. Groundwater withdrawal rates from the Northern Atlantic Coastal Plain aquifer system from 1900 to 2010.
Simulation of the Hydrologic System 25

Figure 15. Groundwater withdrawals for A, the Northern Atlantic Coastal 

Plain aquifer system and regional aquifers in B, New York, C, New Jersey, 

D, Delmarva, E, Maryland, F, Virginia, and G, North Carolina geographic areas 

for the historical period (1900–1985), the period of emphasis (1986–2013), 

and a future period (2014–2058). The Potomac aquifer includes the PotomacPatuxent and Potomac-Patapsco regional aquifers. Geographic areas are 

shown in figure 12.

masterson_nacp_fig15ab

Historical period Future period

Historical period Future period

Historical period Period of emphasis Future period

1,400

1,200

1,000

800

600

400

200

0

1,600

200

150

100

 50

0

500

450

400

350

300

250

200

150

100

 50

0

400

350

300

250

450

Groundwater withdrawals, in million gallons per day 

1920

1939

1945

1952

1957

1964

1967

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Pre-1900

Year

Period of emphasis

Period of emphasis

Monmouth-

 Mount Laurel

Matawan

Magothy

Potomac

Surficial

Upper Chesapeake

Lower Chesapeake

Piney Point

Aquia

EXPLANATION

A

B

Aquifer

C
26 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

Figure 15. Groundwater withdrawals for A, the Northern Atlantic Coastal 

Plain aquifer system and regional aquifers in B, New York, C, New Jersey, 

D, Delmarva, E, Maryland, F, Virginia, and G, North Carolina geographic areas 

for the historical period (1900–1985), the period of emphasis (1986–2013), and 

a future period (2014–2058). The Potomac aquifer includes the PotomacPatuxent and Potomac-Patapsco regional aquifers. Geographic areas are 

shown in figure 12.—Continued

Period of emphasis

Period of emphasis

Historical period Future period

Historical period Future period

Historical period Period of emphasis Future period

 50

0

 90

 80

 70

 60

 40

 30

 20

 10

 100

20

100

0

80

60

40

160

140

120

200

150

100

 50

0

300

250

1920

1939

1945

1952

1957

1964

1967

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Pre-1900

Year

Groundwater withdrawals, in million gallons per day 

E

F

Monmouth-

 Mount Laurel

Matawan

Magothy

Potomac

Surficial

Upper Chesapeake

Lower Chesapeake

Piney Point

Aquia

EXPLANATION

Aquifer

D
Simulation of the Hydrologic System 27

Historical period Period of emphasis Future period

0

 80

 60

 40

 20

 100

 120

Year

Monmouth-

 Mount Laurel

Matawan

Magothy

Potomac

Surficial

Upper Chesapeake

Lower Chesapeake

Piney Point

Aquia

EXPLANATION

Aquifer

1920

1939

1945

1952

1957

1964

1967

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Pre-1900

G

Groundwater withdrawals, in million gallons per day

Figure 15. Groundwater withdrawals for A, the Northern Atlantic Coastal Plain aquifer system and regional 

aquifers in B, New York, C, New Jersey, D, Delmarva, E, Maryland, F, Virginia, and G, North Carolina geographic 

areas for the historical period (1900–1985), the period of emphasis (1986–2013), and a future period (2014–2058). The 

Potomac aquifer includes the Potomac-Patuxent and Potomac-Patapsco regional aquifers. Geographic areas are 

shown in figure 12.—Continued
28 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

Regionally, the largest withdrawals occurred in the 

Magothy aquifer, which accounted for about 28 percent 

of all withdrawals in the NACP aquifer system (fig. 16; 

table 3). The surficial and the Upper and Lower Chesapeake 

regional aquifers combined account for about 37 percent 

of the total use of water in the northern Atlantic Coastal 

Plain province. The surficial aquifer provides almost half 

(47 percent) of the groundwater pumped in the Delmarva 

geographic area and more than a quarter of the groundwater 

pumped in the New York geographic area (27 percent). 

Withdrawals from the Lower Chesapeake aquifer account 

for only 13 percent of the total withdrawals in NACP aquifer 

system yet make up almost half (42 percent) of the total 

withdrawals in the New Jersey geographic area, where this 

aquifer is the primary source of drinking water in southeastern coastal areas of the State.

Withdrawals from the Potomac-Patapsco and PotomacPatuxent aquifers are combined into a Potomac aquifer 

system for this water-use analysis because the Potomac 

aquifer is not subdivided in the Virginia and North Carolina 

geographic areas (fig. 5). About 24 percent of the groundwater withdrawn from the NACP aquifer system is withdrawn from the Potomac aquifer system, which accounts 

for about 87 percent of the total withdrawals in the Virginia 

geographic area alone. The remaining 11 percent of the 

total groundwater withdrawal in the NACP aquifer system 

is pumped from the Aquia, Matawan, Monmouth-Mount 

Laurel, and Piney Point regional aquifers (table 3).

Table 3. Percentage of groundwater withdrawals in 2008 by aquifer and geographic area in the Northern Atlantic Coastal Plain.

[Gray shaded box indicates that aquifer does not exist in this geographic location. NACP, Northern Atlantic Coastal Plain]

Regional aquifer Percentage withdrawn from regional aquifer

New York New Jersey Delmarva Maryland1 Virginia2 North Carolina NACP total

Surficial 27 3 47 2 2 12 19

Upper Chesapeake 0 1 18 0 4 14 5

Lower Chesapeake 0 42 8 0 0 0 13

Piney Point 0 3 7 2 5 47 6

Aquia 0 1 5 17 2 2 3

Monmouth-Mount Laurel 0 4 0 0 0 0 1

Matawan 0 3 0 0 0 3 1

Magothy 71 17 4 12 0 2 28

Potomac 2 26 11 67 87 20 24

1

Maryland geographic area does not include the part of Maryland on the Delmarva Peninsula.

2

Virginia geographic area does not include the part of Virginia on the Delmarva Peninsula.

EXPLANATION

Regional aquifer

Percentage of total Northern Atlantic Coastal Plain

New York New Jersey Delmarva

Maryland Virginia North Carolina

47

47

14

20 12

2

3

2

2

87

5 2 4

17

12 67

2 2

11

18

47

8

7

5

4

17

42

3 4 3

26

3 1

1

71

27

2

13

24

28

19

5

6

3

11

masterson_nacp_fig16_new

Surficial

Upper Chesapeake

Lower Chesapeake

Piney Point

Aquia 

Monmouth-Mount Laurel

Matawan

Magothy

Potomac

Figure 16. Distribution of groundwater withdrawals in 2008 by 

regional aquifer for each geographic area in the Northern Atlantic 

Coastal Plain aquifer system. The Potomac aquifer includes the 

Potomac-Patuxent and Potomac-Patapsco regional aquifers. 

Geographic areas are shown in figure 12.
Simulation of the Hydrologic System 29

Hydrologic budgets developed from the numerical simulation results of the response of the flow system to changes in 

groundwater withdrawals and aquifer recharge are presented 

graphically to illustrate how streamflow, coastal discharge, 

and storage change over time (fig. 17). In years with greater 

recharge, such as 2003, corresponding increases are seen in 

discharge to the coast and the streams and water going into 

storage (referred to as outflow in fig. 17). In years with less 

recharge, such as 2007, more water is released from storage 

(referred to as inflow in fig. 17).

Changes in groundwater withdrawals also affect changes 

in coastal discharge, streamflow, and flow of water into and 

out of storage; however, total withdrawals are on average less 

than 10 percent of total recharge. Although the amounts of 

withdrawals are small relative to those of recharge, a substantial 

amount of pumping in the NACP aquifer system occurs in the 

deep parts of the aquifer system that are not connected to the 

surficial aquifer. Therefore, diagrams of the total groundwater 

budget, such as the one shown in figure 17, do not provide the 

information needed to determine how much these withdrawals 

affect the deep groundwater flow system. A comparison of flowsystem simulations with and without pumping and wastewater 

return flow was used to isolate the effects of groundwater 

development (pumping and wastewater return flow) on the 

regional aquifers of the NACP aquifer system in order to 

assess how the groundwater flow system is affected only by 

changes in these stresses. The natural recharge is the same in 

all simulations, but varies with time as previously described.

1920

1939

1945

1952

1957

1964

1967

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Pre-1900 

Year

–30,000

–20,000

–10,000

0

10,000

20,000

30,000

Rate, in million galllons per day

Inflow Outflow

Recharge in

Inflow Outflow

Storage release

Wells

Out to coast

Out to streams

Storage replenishment

EXPLANATION

pp_masterson_nacp_fig17_new

Historical period Period of emphasis Future period

Figure 17. Hydrologic budget for the historical period (1900–1985), 

period of emphasis (1986–2013), and a future period (2014–2058) for 

the Northern Atlantic Coastal Plain aquifer system.
30 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

Once the effects of pumping and wastewater return 

flow on the aquifer system are determined independently of 

changes in recharge, a hydrologic budget can be presented 

that shows how the aquifer system adjusts to these stresses 

alone (fig. 18). The hydrologic budget diagrams presented 

in figure 18 include only the changes to the flow budget 

components that occur in response to the simulated pumping 

and wastewater return flow stresses; thus, the magnitude of 

total change for each time span should equal the total amount 

of groundwater withdrawn for that time span. Each hydrologic 

budget component represents the absolute change from a 

nonpumping condition; for instance, the streamflow reduction includes the total decrease in groundwater flow to the 

stream as well as any induced infiltration from the stream in 

response to pumping. In the end, all changes in the hydrologic 

budget terms must balance the total pumping. The return of 

wastewater from domestic septic systems is also included 

in this calculation because wastewater return from domestic 

septic systems was derived mostly from pumped groundwater 

that was subsequently returned to the aquifer as recharge, thus 

mitigating some of the effects of pumping.

The hydrologic budget response to pumping before 1957 

appears to be mostly as reductions in coastal discharge for 

the NACP aquifer system (fig. 18A). The reduction in coastal 

discharge becomes a smaller portion of the total response 

as changes to streamflow and storage increase over time. 

These overall changes appear to coincide with the distribution of pumping over time by geographic area (fig. 15). As 

pumping in the New York geographic area accounts for less 

of the total pumping in the entire NACP aquifer system, the 

primary response in the New York geographic area to pumping 

(reduction in coastal discharge; fig. 18B) becomes a smaller 

percentage of the total response of the NACP aquifer system 

to pumping.

The changes in the streamflow and storage hydrologic 

components for the entire NACP aquifer system (fig. 18A) 

can be assessed better by examining the hydrologic budgets 

at the geographic-area scale. The large change in streamflow 

in the New Jersey geographic area accounts for a large part 

of the streamflow response to pumping in the NACP aquifer 

system in total (fig. 18C). The change in storage in response to 

pumping in the Delmarva, Maryland, and Virginia geographic 

areas (fig. 18D–F) accounts for much of the storage response 

seen in the entire NACP aquifer system (fig. 18A) because 

most of the pumping in these geographic areas is from the 

deep confined aquifers compared with pumping in the shallow 

aquifers in the New York (fig. 18B) and New Jersey (fig. 18C) 

geographic areas.

An additional hydrologic budget component reported for 

the individual geographic areas (fig. 18B–G) is the flow across 

the geographic area boundaries. This component represents 

the amount of flow to and from adjacent geographic areas in 

response to changes in pumping and is referred to on figure 18 

as “net flow in from other geographic areas” and “net flow out 

to other geographic areas.” In most of the geographic areas, 

flow to and from adjacent geographic areas is small compared 

to the other hydrologic budget components; however, because 

there is flow among the geographic areas during pumping, 

it is not possible to attribute the response of the hydrologic 

budget components for a given geographic area to only 

the pumping in that area. The geographic areas where this 

effect is most prevalent are the Virginia (fig. 18F ) and North 

Carolina (fig. 18G) geographic areas. The net flow in from 

other geographic areas in Virginia (fig. 18F ) and the net flow 

out to other geographic areas for North Carolina (fig. 18G) 

indicate that groundwater is flowing from the North Carolina 

geographic area to the Virginia geographic area in response to 

pumping in the Virginia geographic area.

The long-term response of the groundwater flow system 

to pumping can be assessed by accumulating the changes in 

the hydrologic budget components shown in figure 18. The 

budget components shown for each time span in the cumulative plots are the summation of each budget component since 

1900 (fig. 19). The largest long-term effects from pumping at 

the regional scale are the reduction of groundwater discharge 

to the coast and, to a lesser extent, a reduction in discharge 

to streams. The loss of storage appears to be minor compared 

with the reductions in groundwater discharge to the coast 

and streams at the regional scale (fig. 19A); however, in the 

Virginia geographic area, the storage loss is a much larger 

percentage of the total change in the local hydrologic budget 

(fig. 19F ) than in the total NACP aquifer system. Also, the 

cumulative effect of flow from the adjacent geographic areas 

for the period of emphasis in the Virginia geographic area 

appears to be greater than the effect on the reductions in 

streamflow and coastal discharge combined. A comparison 

of the changes in hydrologic budget components among 

the geographic areas shows the differences in how these 

geographic areas respond to the long-term effects of groundwater withdrawals and highlights the importance of local- and 

regional-scale analyses to fully assessing the NACP aquifer 

system-wide response to regional groundwater withdrawals 

(fig. 19A).

The predominant trends in the cumulative hydrologic 

budget components for the region are an increase in the 

amount of streamflow reduction and a decrease in the amount 

of coastal discharge reduction (fig. 19A). These streamflow 

reduction trends are most likely attributed to the effects of 

pumping on streamflow at the geographic-area scale in the 

New Jersey, Delmarva, and Maryland geographic areas 

(fig. 19C–E) rather than in the New York geographic area 

where streamflow is a minor component of the total hydrologic 

budget and in the Virginia geographic area where most of the 

groundwater pumping occurs in the deep, confined aquifers.

This method of analysis also highlights the relative 

importance of the return of wastewater to the aquifer system; 

however, it can be misleading in that it does not take into 

account where the water is returned to the system and how it 
Simulation of the Hydrologic System 31

Period of emphasis Historical period Future period

Period of emphasis Historical period Future period

Change in hydrologic budget component flow, 

in million gallons per day

Increase in inflow Decrease in outflow Increase in inflow Decrease in outflow

Change in hydrologic budget component flow, 

in million gallons per day

A. Northern Atlantic Coastal Plain B. New York

EXPLANATION

Net flow in from other geographic areas

Net flow out to other geographic areas

Net replenishment to storage

Net reduction in streamflow Net release of storage

Net increase in streamflow

Net reduction in coastal discharge

Net increase in coastal discharge

Net wastewater return flow

Pre-1900

1920

1940

1945

1952

1957

1964

1968

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Year

Pre-1900

1920

1940

1945

1952

1957

1964

1968

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Year

600

400

200

0

200

400

600

800

1,000

1,200

200

100

0

100

200

300

400

500

pp_masterson_nacp_fig18ab

Figure 18. Changes in hydrologic budget components as a function of pumping for the historical period

(1900–1985), period of emphasis (1986–2013), and a future period (2014–2058) for A, the Northern Atlantic 

Coastal Plain aquifer system and regional aquifers in the B, New York, C, New Jersey, D, Delmarva, 

E, Maryland, F, Virginia, and G, North Carolina geographic areas. Values to the left of zero indicate an 

increase from the nonpumping condition and values to the right of zero indicate a decrease from the 

nonpumping condition. Geographic areas are shown in figure 12.
32 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

Period of emphasis Historical period Future period

Period of emphasis Historical period Future period

Change in hydrologic budget component flow, 

in million gallons per day

Change in hydrologic budget component flow, 

in million gallons per day

C. New Jersey D. Delmarva

EXPLANATION

Net flow in from other geographic areas

Net flow out to other geographic areas

Net replenishment to storage

Net reduction in streamflow Net release of storage

Net increase in streamflow

Net reduction in coastal discharge

Net increase in coastal discharge

Net wastewater return flow

150

100

50

0

50

100

150

200

250

300

350

Pre-1900

1920

1940

1945

1952

1957

1964

1968

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Year

Pre-1900

1920

1940

1945

1952

1957

1964

1968

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Year

100

50

0

50

100

150

200

250

300

pp_masterson_nacp_fig18cd_new

Increase in inflow Decrease in outflow Increase in inflow Decrease in outflow

Figure 18. Changes in hydrologic budget components as a function of pumping for the historical period

(1900–1985), period of emphasis (1986–2013), and a future period (2014–2058) for A, the Northern Atlantic 

Coastal Plain aquifer system and regional aquifers in the B, New York, C, New Jersey, D, Delmarva, 

E, Maryland, F, Virginia, and G, North Carolina geographic areas. Values to the left of zero indicate an 

increase from the nonpumping condition and values to the right of zero indicate a decrease from the 

nonpumping condition. Geographic areas are shown in figure 12.—Continued
Simulation of the Hydrologic System 33

Period of emphasis Historical period Future period

Period of emphasis Historical period Future period

Change in hydrologic budget component flow, 

in million gallons per day

Change in hydrologic budget component flow, 

in million gallons per day

E. Maryland F. Virginia

EXPLANATION

Net flow in from other geographic areas

Net flow out to other geographic areas

Net replenishment to storage

Net reduction in streamflow Net release of storage

Net increase in streamflow

Net reduction in coastal discharge

Net increase in coastal discharge

Net wastewater return flow

Pre-1900

1920

1940

1945

1952

1957

1964

1968

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Year

Pre-1900

1920

1940

1945

1952

1957

1964

1968

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Year

60

40

20

0

20

40

60

80

140

120

100

80

60

40

20

0

20

40

60

pp_masterson_nacp_fig18ef

Increase in inflow Decrease in outflow Increase in inflow Decrease in outflow

Figure 18. Changes in hydrologic budget components as a function of pumping for the historical period

(1900–1985), period of emphasis (1986–2013), and a future period (2014–2058) for A, the Northern Atlantic 

Coastal Plain aquifer system and regional aquifers in the B, New York, C, New Jersey, D, Delmarva, 

E, Maryland, F, Virginia, and G, North Carolina geographic areas. Values to the left of zero indicate an 

increase from the nonpumping condition and values to the right of zero indicate a decrease from the 

nonpumping condition. Geographic areas are shown in figure 12.—Continued
34 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System Period of emphasis Historical period Future period

Change in hydrologic budget component flow, 

in million gallons per day

G. North Carolina

EXPLANATION

Net flow in from other

 geographic areas

Net flow out to other

 geographic areas

Net replenishment to storage

Net reduction in streamflow Net release of storage

Net increase in streamflow

Net reduction in coastal discharge

Net increase in

 coastal discharge

Net wastewater return flow

60

40

20

0

20

40

60

80

100

120

140

Pre-1900

1920

1940

1945

1952

1957

1964

1968

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Year

pp_masterson_nacp_fig18g_new

Increase in inflow Decrease in outflow

Figure 18. Changes in hydrologic budget components as a function of pumping for the historical 

period (1900–1985), period of emphasis (1986–2013), and a future period (2014–2058) for A, the Northern 

Atlantic Coastal Plain aquifer system and regional aquifers in the B, New York, C, New Jersey, 

D, Delmarva, E, Maryland, F, Virginia, and G, North Carolina geographic areas. Values to the left of 

zero indicate an increase from the nonpumping condition and values to the right of zero indicate a 

decrease from the nonpumping condition. Geographic areas are shown in figure 12.—Continued
Simulation of the Hydrologic System 35

Period of emphasis Historical period Future period

Period of emphasis Historical period Future period

Cumulative change in hydrologic budget component flow,

in trillion gallons

Cumulative change in hydrologic budget component flow,

in trillion gallons

A. Northern Atlantic Coastal Plain B. New York

EXPLANATION

Net flow in from other geographic areas

Net flow out to other geographic areas

Net replenishment to storage

Net reduction in streamflow Net release of storage

Net increase in streamflow

Net reduction in coastal discharge

Net increase in coastal discharge

Net wastewater return flow

Pre-1900

1920

1940

1945

1952

1957

1964

1968

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Year

Pre-1900

1920

1940

1945

1952

1957

1964

1968

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Year

20

10

0

10

20

30

40

6

4

2

0

2

4

6

8

10

12

14

16

pp_masterson_nacp_fig19ab_new

Increase in inflow Decrease in outflow Increase in inflow Decrease in outflow

Figure 19. Cumulative changes in hydrologic budget components in response to pumping for the historical 

period (1900–1985), period of emphasis (1986–2013), and a future period (2014–2058) for A, the Northern Atlantic 

Coastal Plain aquifer system and regional aquifers in the B, New York, C, New Jersey, D, Delmarva, E, Maryland, 

F, Virginia, and G, North Carolina geographic areas. Values to the left of zero indicate an increase from the 

nonpumping condition and values to the right of zero indicate a decrease from the nonpumping condition. 

Geographic areas are shown in figure 12.
36 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

pp_masterson_nacp_fig19cd_new

C. New Jersey

2058

2053

2048

2043

2038

2033

2028

2023

2018

2013

2008

2007

2006

2005

2004

2003

2002

2001

2000

1999

1998

1997

1996

1995

1994

1993

1992

1991

1990

1989

1988

1987

1986

1985

1980

1977

1972

1967

1964

1957

1952

1945

1939

1920

Pre-1900

D. Delmarva

2058

2053

2048

2043

2038

2033

2028

2023

2018

2013

2008

2007

2006

2005

2004

2003

2002

2001

2000

1999

1998

1997

1996

1995

1994

1993

1992

1991

1990

1989

1988

1987

1986

1985

1980

1977

1972

1967

1964

1957

1952

1945

1939

1920

Pre-1900

YearYear

Period of emphasis Historical period Future period

Historical period Future period Period of emphasis

EXPLANATION

Net flow in from other geographic areas

Net flow out to other geographic areas

Net replenishment to storage

Net reduction in streamflow Net release of storage

Net increase in streamflow

Net reduction in coastal discharge

Net increase in coastal discharge

Net wastewater return flow

4

2

0

2

4

6

8

10

12

3

2

1

0

1

2

3

4

5

6

7

Increase in inflow Decrease in outflow Increase in inflow Decrease in outflow

Cumulative change in hydrologic budget component flow,

in trillion gallons

Cumulative change in hydrologic budget component flow,

in trillion gallons

Figure 19. Cumulative changes in hydrologic budget components in response to pumping for the historical 

period (1900–1985), period of emphasis (1986–2013), and a future period (2014–2058) for A, the Northern Atlantic 

Coastal Plain aquifer system and regional aquifers in the B, New York, C, New Jersey, D, Delmarva, E, Maryland, 

F, Virginia, and G, North Carolina geographic areas. Values to the left of zero indicate an increase from the 

nonpumping condition and values to the right of zero indicate a decrease from the nonpumping condition. 

Geographic areas are shown in figure 12.—Continued
Simulation of the Hydrologic System 37

Period of emphasis Historical period Future period

Period of emphasis Historical period Future period

E. Maryland F. Virginia

EXPLANATION

Net flow in from other geographic areas

Net flow out to other geographic areas

Net replenishment to storage

Net reduction in streamflow Net release of storage

Net increase in streamflow

Net reduction in coastal discharge

Net increase in coastal discharge

Net wastewater return flow

Pre-1900

1920

1940

1945

1952

1957

1964

1968

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Year

Pre-900

1920

1940

1945

1952

1957

1964

1968

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Year

1.5

1.0

0.5

0

0.5

1.0

1.5

2.0

2.5

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

0.5

1.0

1.5

pp_masterson_nacp_fig19ef_new

Increase in inflow Decrease in outflow Increase in inflow Decrease in outflow

Cumulative change in hydrologic budget component flow,

in trillion gallons

Cumulative change in hydrologic budget component flow,

in trillion gallons

Figure 19. Cumulative changes in hydrologic budget components in response to pumping for the historical 

period (1900–1985), period of emphasis (1986–2013), and a future period (2014–2058) for A, the Northern Atlantic 

Coastal Plain aquifer system and regional aquifers in the B, New York, C, New Jersey, D, Delmarva, E, Maryland, 

F, Virginia, and G, North Carolina geographic areas. Values to the left of zero indicate an increase from the 

nonpumping condition and values to the right of zero indicate a decrease from the nonpumping condition. 

Geographic areas are shown in figure 12.—Continued
38 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

may alter the way the aquifer system responds to groundwater 

pumping. For example, in the Virginia geographic area, 

wastewater is returned to the surficial aquifer, but the majority 

of the pumping in this geographic area occurs in the deep, 

confined aquifers (fig. 16). Therefore, this additional source of 

water at the surface may result in increases in streamflow and 

discharge to the coast when wastewater returnflow is added 

in the in the Virginia geographic area (figs. 18F and 19F), 

particularly during the historical period when pumping from 

the confined aquifers has not yet affected flow in the overlying 

surficial aquifer.

Another explanation for the additional flow to streams 

and discharge to the coast during the historical period in the 

Delmarva, Virginia, and North Carolina geographic areas 

shown in figures 18 and 19 is that wastewater return flow is 

calculated independently of groundwater withdrawals and that 

self-supplied domestic withdrawals were not included in the 

numerical simulation for the historical period (Masterson and 

others, 2016). Therefore, the wastewater return flow recharge 

appears as a larger percentage of the simulated pumping 

than it would be had domestic pumping been simulated for 

the historical period. Although, the relative contribution of 

wastewater return flow appears large for this period, it is 

still a relatively small amount of the total hydrologic budget 

compared with the period of emphasis (1986–2013) when both 

wastewater return flow and domestic pumping were included 

in the analysis.

Comparison of Predevelopment and Current 

[2013] Conditions

Large-scale groundwater withdrawals began at the 

turn of the twentieth century and increased steadily until 

the early 1980s (fig. 13). From the early 1980s until present 

[2013], pumping remained generally constant at the regional 

scale, with some geographic areas experiencing increased 

withdrawals and others experiencing decreases (figs. 13–15). 

Therefore, an assessment was made of the cumulative response 

Period of emphasis Historical period Future period

G. North Carolina

EXPLANATION

Net flow out to other

 geographic areas

Net flow in from other

 geographic areas

Net replenishment to storage

Net reduction in streamflow Net release of storage

Net increase in streamflow

Net reduction in coastal discharge

Net increase in

 coastal discharge

Net wastewater return flow

Pre-1900

1920

1940

1945

1952

1957

1964

1968

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Year

2.0

1.5

1.0

0.5

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0

fig19g_new

Increase in inflow Decrease in outflow

Cumulative change in hydrologic budget component flow,

in trillion gallons

Figure 19. Cumulative changes in hydrologic budget 

components in response to pumping for the historical period

(1900–1985), period of emphasis (1986–2013), and a future 

period (2014–2058) for A, the Northern Atlantic Coastal Plain 

aquifer system and regional aquifers in the B, New York, 

C, New Jersey, D, Delmarva, E, Maryland, F, Virginia, and 

G, North Carolina geographic areas. Values to the left of 

zero indicate an increase from the nonpumping condition 

and values to the right of zero indicate a decrease from 

the nonpumping condition. Geographic areas are shown in 

figure 12.—Continued
Simulation of the Hydrologic System 39

of the NACP regional aquifer system and its geographic 

regions for the historical period and the period of emphasis 

to properly assess the differences in long-term groundwater 

system response for these two time periods (fig. 20).

The total hydrologic budget change for the historical 

period was about 14.4 Tgal for the entire NACP aquifer 

system compared with about 13.4 Tgal for the period of 

emphasis. Although a similar amount of groundwater was 

withdrawn during these two periods, the flow system response 

differed between the two periods. For instance, the relative 

importance of the net release from storage and wastewater 

return flow is diminished between periods as the net reduction in streamflow becomes more important over time as the 

regional groundwater system approaches a new equilibrium 

with respect to pumping (fig. 20A).

The hydrologic budget response to pumping varies by 

geographic area from north to south. Similar responses in 

the New York and New Jersey geographic areas indicate 

that the release of groundwater from storage is less for the 

period of emphasis than for the historical period, and the 

largest hydrologic budget change is the increase in streamflow 

reduction (fig. 20B–C). Also, in New York and New Jersey, 

unlike farther to the south in the Delmarva, Maryland, and 

Virginia geographic areas (fig. 20D–F ), the total cumulative 

withdrawals and the resulting system adjustment to pumping 

are less for the period of emphasis than for the historical 

period, indicating that groundwater withdrawals started to 

level off after 1985 in these areas, and the hydrologic budgets 

in these geographic areas are approaching equilibrium with 

respect to pumping.

Cumulative withdrawals increased for the period of 

emphasis in the Delmarva, Maryland, and Virginia geographic 

areas, compared with those in the historical period, with 

the largest increase in the Delmarva geographic area where 

cumulative withdrawals more than doubled from the historical 

period to the period of emphasis (fig. 20D). Cumulative 

withdrawals in the Maryland and Virginia geographic 

areas increased only slightly for the period of emphasis 

compared with the historical period (fig. 20E–F ). In the 

Maryland geographic area, the amounts of storage release and 

wastewater return flow increased for the period of emphasis 

compared with the historical period, whereas the reduction 

in streamflow remained nearly constant and the reduction 

in coastal discharge decreased. The decreased reduction in 

coastal discharge in the Maryland geographic area (fig. 20E ) 

can be attributed to the increase in wastewater return flow 

for the period. In the Virginia geographic area, less water is 

captured from the adjacent geographic areas and reduction is 

streamflow and coastal discharge becomes greater over time 

(fig. 20F ).

The total model-calculated groundwater depletion 

derived from groundwater withdrawal related storage release 

Table 4. Comparison of total net volumetric groundwater 

depletion for the Northern Atlantic Coastal Plain aquifer system 

from 1900 to 2000 and from 1900 to 2008.

[Mgal, million gallons]

Geographic

area

Net volumetric groundwater depletion (Mgal)

1900–2000 1900–2008

Konikow 

(2013)

This

report

Konikow 

(2013)

This

report

New York 423,000 502,000 291,000 502,000

Maryland and 

Delaware1

423,000 317,000 502,000 396,000

New Jersey 317,000 211,000 317,000 264,000

Virginia 660,000 423,000 1,189,000 502,000

North Carolina2 317,000 396,000 423,000 423,000

Total 2,140,000 1,849,000 2,721,000 2,061,000

1

Includes the Maryland and Delmarva geographic areas in the data for 

this report.

2

Includes only the northern half of North Carolina in the data for this report.

in the NACP aquifer system was about 2.2 Tgal [8.3 cubic 

kilometers (km3)] by the end of the period of emphasis, which 

is about 8 percent of the 27.8 Tgal (105 km3) of groundwater 

pumped from the aquifer system. A comparison of results in 

this report to the net groundwater depletion volume calculations from Konikow (2013) shows that the results from the 

two analyses appear to be generally in good agreement from 

1900 to 2000, but appear to diverge when the cumulative 

groundwater depletion volume through 2008 is considered 

(table 4).

The largest differences between the two analyses 

occur in the New York and Virginia geographic areas. In the 

New York geographic area, Konikow (2013) calculated a 

130,000 Mgal (0.5 km3) replenishment of groundwater storage 

from 2000 to 2008; however, in the analysis in this report, 

there is no net change in groundwater storage for the same 

period. The replenishment in groundwater storage in Konikow 

(2013) is derived from the integration of changes in groundwater measurements from 2005 to 2008 when precipitation 

rates were above average. The cumulative change in storage 

for the analysis in this report considers only the groundwater 

depletion or storage release attributed to groundwater withdrawals and does not include changes in storage associated 

with changes in aquifer recharge. Therefore, the replenishment 

of storage from periods of high aquifer recharge (or groundwater depletion from periods of low aquifer recharge) is not 

calculated for the analysis in this report, only the change in 

storage attributed to groundwater withdrawals.
40 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

3,035,661

(21%)

8,568,821,

(59%)

1,553,413

(11%)

1,255,579

(9%)

1900–1985

2,412,766

(18%)

7,317,025

(54%)

2,769,850

(21%)

924,913

 (7%)

1986–2013

2,583,946

(18%)

8,102,309

(55%)

3,368,593

(23%)

587,294 (4%) 2014–2043

A. Northern Atlantic Coastal Plain

pp_masterson_nacp_fig20abcd_new

851,964,

(12%)

5,488,554

(79%)

197,087 (3%)

423,055

 (6%)

8,876 (0%)

B. New York geographic area

760,912

(18%)

3,150,334

(73%)

360,710

 (8%)

63,684 (1%) 10,510 (0%)

814,866

(18%)

3,290,929

(72%)

433,843

(10%)

9,435 (0%) 12,222 (0%)

Net flow out to other geographic areas

Net replenishment to storage

Net reduction in streamflow

Net reduction in coastal discharge

Net wastewater return flow

Net increase in coastal discharge

Net increase in streamflow

Net release from storage

Net flow in from other geographic areas

Cumulative totals of hydrologic budget components

EXPLANATION

3,290,929

(72%)

657,425

(16%)

2,214,588

(54%)

973,384

(24%)

146,591 (4%)

73,396 (2%)

C. New Jersey geographic area

476,152

(13%)

1,719,123

(48%)

1,175,945

(32%)

135,054 (4%)

112,023 (3%)

510,076

(12%)

2,007,506

(49%)

1,458,634

(35%)

49,871 (1%) 129,860 (3%)

519,789

(43%)

349,225

(29%)

129,622

(11%)

113,943

(10%)

82,334

(7%)

D. Delmarva geographic area

350,487

(13%)

1,436,118

(54%)

588,202

(22%)

166,746 (6%) 121,416 (5%) 

375,556

(12%)

1,769,678

(59%)

656,613

(22%)

83,237 (3%)

126,371 (4%) 

In millions of gallons

As percentage of total

Figure 20. Distribution of the cumulative response of the hydrologic budget components to changes in groundwater pumping for the 

historical period (1900–1985), period of emphasis (1986–2013), and a future period (2014–2043) for A, the Northern Atlantic Coastal Plain 

aquifer system and regional aquifers in the B, New York, C, New Jersey, D, Delmarva, E, Maryland, F, Virginia, and G, North Carolina 

geographic areas. Geographic areas are shown in figure 12. 
Simulation of the Hydrologic System 41

1900–1985

1986–2013

2014–2043

pp_masterson_nacp_fig20efg_new

172,540

(20%)

338,633

(40%)

232,855

(27%)

61,034

 ( 7%)

48,397

 (6%)

E. Maryland geographic area

288,184

(34%)

262,573

(31%)

235,644

(27%)

73,004

(8%)

453 (0%)

308,678

(32%)

326,179

(34%)

284,055

(30%)

32,632 (3%) 12,659 (1%)

463,215

(42%)

42,020 (4%) 15,278 (1%)

235,461

0 (0%) (21%)

358,379

(32%)

F. Virginia geographic area

367,978

(28%)

177,974

(14%) 194,159

(15%)

325,086

(25%)

229,687

(18%)

394,218

(30%)

215,764

(16%)

215,764

(16%)

244,190

(19%)

325,330

(25%)

135,356

(10%)

370,726

(31%)

219,841

(19%)

5,187 (1%)

275,494

(23%)

309,902

(26%)

G. North Carolina gegraphic area

169,051

(13%)

570,902

215,190 (43%)

(16%)

161,340

(12%)

210,239

(16%)

180,551

(15%)

492,254

(41%)

291,257

(24%)

105,659

(9%)

139,286

(11%)

Net flow out to other geographic areas

Net replenishment to storage

Net reduction in streamflow

Net reduction in coastal discharge

Net wastewater return flow

Net increase in coastal discharge

Net increase in streamflow

Net release from storage

Net flow in from other geographic areas

Cumulative totals of hydrologic budget components

EXPLANATION

In millions of gallons

As percentage of total

Figure 20. Distribution of the cumulative response of the hydrologic budget components to changes in groundwater pumping 

for the historical period (1900–1985), period of emphasis (1986–2013), and a future period (2014–2043) for A, the Northern 

Atlantic Coastal Plain aquifer system and regional aquifers in the B, New York, C, New Jersey, D, Delmarva, E, Maryland, 

F, Virginia, and G, North Carolina geographic areas. Geographic areas are shown in figure 12.—Continued
42 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

The difference in calculated volumetric groundwater 

depletion between the two analyses is largest in the Virginia 

geographic area (table 4). The analysis in Konikow (2013) for 

1900 to 2000 calculates groundwater depletion to be about 

660,000 Mgal (2.5 km3), which is about 60 percent greater 

than the 423,000 Mgal (1.6 km3) calculated for the analysis in 

this report. This difference becomes more pronounced when 

considering the additional groundwater depletion calculated 

from 2000 to 2008. The analysis in Konikow (2013) calculates an additional 529,000 Mgal (2.0 km3

) of groundwater 

depletion for this 9-year period compared with the additional 

79,000 Mgal (0.3 km3) groundwater depletion calculated in 

the analysis of this report (table 4). This large discrepancy in 

depletion totals between the two analyses for this period can 

be attributed to the Konikow (2013) analysis including storage 

changes in response to both pumping and recharge whereas in 

the analysis in this report only the changes in storage associated with groundwater withdrawals are considered.

Cumulative changes in groundwater storage in the 

New York geographic area that consider changes in both 

recharge and groundwater withdrawals may better reflect the 

total response of the aquifer system to changes in stresses over 

time because pumping from the Magothy aquifer accounts for 

more than 70 percent of the total withdrawals (table 3) and the 

Magothy is in good hydrologic connection with the overlying 

surficial (upper glacial) aquifer (fig. 6D), which is the aquifer 

most affected by changes in recharge. Conversely, in the 

Virginia geographic area, nearly 90 percent of the groundwater withdrawals come from the Potomac aquifer (table 3), 

which is not in good connection with the surficial aquifer 

(fig. 6A), and therefore, the storage changes in the surficial 

aquifer from changes in recharge should not be grouped with 

the storage changes from pumping that occur in the deep, 

confined aquifers in the assessment of the potential effects 

of groundwater withdrawals on changes in aquifer storage 

in the primary aquifers for this geographic area. However, 

despite the differences in methodologies between the two 

analyses, the differences in the cumulative net groundwater 

depletion volumes are less than 15 percent from 1900 to 2000 

and less than 25 percent from 1900 to 2008 for the entire 

NACP aquifer system.

Comparison of Current [2013] and 

Future [2043] Conditions

Two hypothetical conditions were considered in order 

to assess how the groundwater flow system may respond in 

the future. Those conditions included first maintaining the 

current [2013] pumping and recharge stresses for an additional 

30 years. The second condition assumed that the wastewater 

return flow from all domestic onsite septic systems was 

captured by sewers and treated at centralized wastewater 

treatment facilities along the coast, effectively removing all 

wastewater from the groundwater system.

In the first condition, the same stresses were continued 

for 30 years into 2043 to determine how the system would 

respond with a continuation of the current [2013] stresses; 

the 30-year period was chosen because nearly half of the total 

groundwater withdrawn from the groundwater system in the 

113-year period from 1900 to 2013, was withdrawn in just 

the past 30 years. The results indicate that the system-wide 

distribution of the hydrologic budget components appears 

similar from 2014 to 2043 compared with that from 1986 to 

2013, with the only differences being less release from storage 

and greater reductions in discharges to streams and the coast.

A comparison between the New York (Long Island) 

and Virginia geographic areas illustrates the differences in 

hydrologic responses between an area where groundwater 

withdrawals are primarily from the unconfined part of the 

flow system (Long Island) and an area where withdrawals are 

primarily from the deep, confined portion of the flow system 

(Virginia). In the case of the New York geographic area 

(fig. 20B), the flow system has achieved approximate equilibrium with respect to pumping because the change in storage 

by the end of 2043 is zero. A zero change in storage indicates 

that the withdrawals in this geographic area are balanced 

by corresponding reductions in discharge to streams and 

the coast as well as the addition of the return of wastewater. 

This response in the New York geographic area is because 

nearly all the groundwater withdrawals are from the surficial 

(locally referred to as the upper glacial) and Magothy aquifers 

(figs. 15B and 16) that either include the water table or are 

not separated to any large extent by confining units from the 

aquifer including the water table (fig. 6D).

The flow system in the Virginia geographic area, in 

contrast to the New York geographic area, is still responding 

by the end of 2043 to the cumulative effects of groundwater 

withdrawals because the amount of water released from 

storage continues to account for nearly 25 percent of the 

flow system response to withdrawals in that area (fig. 20F ). 

Also, the amount of water derived from other geographic 

areas continues to decrease over time, indicating that, as 

groundwater withdrawals increase in adjacent areas such as 

North Carolina, the amount of water flowing into Virginia 

becomes less of a contribution to the total response to groundwater withdrawals within this area.

A comparison of the geographic areas in New Jersey 

(fig. 20C ), Delmarva (fig. 20D), and Maryland (fig. 20E ) 

shows that the general adjustment of the flow system to 

the continuation of current [2013] groundwater withdrawal 

rates is a decrease in the amount of water released from 

storage, which is then compensated for by more reductions 

in discharge to streams and coastal waters. The flow systems 

in these geographic areas are more similar to those in the 
Simulation of the Hydrologic System 43

New York geographic area than in the Virginia geographic 

area. For the simulation for future conditions, the water 

released from storage approaches zero as these systems 

approach equilibrium relative to current [2013] pumping and 

recharge stresses.

The second condition that was used to assess the future 

response of the flow system of the NACP aquifer system was 

the removal of all 236 Mgal/d of wastewater return flow that 

was assumed to enter the groundwater system under current 

[2013] conditions. This simulation represents a change from 

onsite disposal of domestic septic to centralized wastewater 

treatment facilities across the study area, with the assumption 

that the treated water discharged from the centralized wastewater treatment facilities is discharged directly to surface 

receiving waters, unlike onsite septic systems that can be an 

additional source of recharge the aquifer system.

The estimate of 236 Mgal/d of wastewater return 

flow was based on the assumptions that about 8 million 

people in the northern Atlantic Coastal Plain province use 

groundwater for drinking water and roughly half of them 

use onsite domestic septic systems for wastewater disposal. 

Assuming that per capita consumption is about 75 gallons 

per day, and the consumptive loss is about 20 percent, then 

about 18 percent of the total groundwater withdrawals in 

the NACP aquifer system (1,330 Mgal/d) is returned to the 

aquifer system as additional recharge from wastewater. The 

distribution of this wastewater return flow varies across the 

NACP aquifer system, with the largest amounts from the more 

densely populated areas. Therefore, the flow system response 

to removing wastewater return flow also will vary across the 

NACP aquifer system and be most pronounced in the most 

densely populated areas, such as Long Island.

Removing wastewater return flow as an additional source 

of recharge to the flow system creates a reduction in groundwater discharge to streams and to coastal receiving waters 

(fig. 21). The removal of the current [2013] wastewater return 

flow across the entire NACP aquifer system would result in 

a loss of about 236 Mgal/d of water from the groundwater 

system, with about 66 percent (155 Mgal/d) of that total being 

a reduction in coastal discharge. The individual values across 

the NACP aquifer vary; for example, more than half of the 

46 Mgal/d of wastewater not returned to the flow system 

in the New Jersey geographic area is water that otherwise 

would have discharged to streams, whereas in the New York 

geographic area, most of the removed wastewater return flow 

(77 percent) is water that otherwise would have discharged 

directly to the coast.

In addition to the removal of wastewater by centralized 

wastewater treatment facilities reducing the quantity of flow 

to surface receiving waters, this removal also would reduce 

the contaminants associated with this water source and the 

associated concerns about eutrophication in the near-shore 

ecosystems of coastal receiving waters, such as the New YorkNew Jersey harbor estuary, Barnegat Bay in New Jersey, 

the meadowlands in Hackensack, N.J., inland bays and the 

sea shore of Delaware, the Delaware River Estuary, and the 

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). Therefore, the balance between decreased quantity 

in surface-water flow and improved water quality must be 

assessed when large-scale centralized wastewater treatment 

facilities projects are considered.

EXPLANATION

Rate, in million gallons

 per day

57.0 

154.7

80.8

0.7

16.7

57.0

22.2 24.0

0.3 0.1

8.7

25.7

0.1

18.7

0.4 0.3

8.8

15.8

20.2

0.2

6.2

11.6

10.8

North Carolina

geographic

area

New Jersey

geographic

area

New York

geographic

area

Delmarva

geographic

area

Maryland

geographic

area

Virginia

geographic

area

Northern Atlantic Coastal Plain

Inflow from other 

 geographic areas

Release from storage

Net reduction in discharge

 to streams

Net reduction in discharge

 to coast

pp_masterson_nacp_fig21_new

Figure 21. Changes in hydrologic budget components with the 

removal of wastewater return flow for current [2013] conditions 

across the Northern Atlantic Coastal Plain aquifer system. 

Geographic areas are shown in figure 12.
44 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

Changes in Groundwater Storage and 

Water Levels

Continued groundwater withdrawals in the study area 

have resulted in the loss of water from storage and a subsequent lowering of water levels in response to this storage 

loss (fig. 3). The change in storage over time was calculated 

for each of the three hydrogeologic unit groups: the surficial 

aquifer, confining units, and confined aquifers (fig. 22). A 

comparison of changes in pumping (fig. 15) with changes 

in storage in the three hydrologic unit groups indicates that 

during times of decreased pumping, water replenishes storage 

rather than being released from it (shown in fig. 22 as a 

negative release). For example, a decrease in pumping from 

2002 to 2003 resulted in more water going into storage in the 

surficial aquifer compared with that from 2000 to 2002 when 

increased pumping resulted in water being released from 

storage from the hydrologic unit groups (fig. 22A).

An analysis of storage change by geographic area indicates 

that more water goes into storage not only in the surficial aquifer, 

but also in the confined aquifers during periods of lower pumping 

in areas such as the New York and New Jersey geographic areas 

(fig. 22B–C), suggesting that the local confined aquifers in these 

areas are in better hydrologic connection with the surficial aquifer 

than in the Virginia geographic area (fig. 22F ). These aquifers in 

the New Jersey (Lower Chesapeake) and New York (Magothy) 

geographic areas are, for the most part, in direct contact with 

the overlying surficial aquifer (fig. 6B–C ).

Historical period Period of emphasis Future period

150

100

 50

0

–50

–100

Changes in storage, in million gallons per day

200

Confined aquifer depletion 

Confining unit depletion

Surficial aquifer depletion

Confined aquifer replenishment 

Confining unit replenishment 

Surficial aquifer replenishment 

Historical period Period of emphasis Future period

50

30

 10

0

 –10

–20

60

40

20

–30

–40

Year

EXPLANATION

A

B

1920

1939

1945

1952

1957

1964

1967

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Pre-1900

Figure 22. Change in aquifer storage by hydrogeologic unit for the 

historical period (1900–1985), the period of emphasis (1986–2013), 

and a future period (2014–2058) for A, the Northern Atlantic Coastal 

Plain aquifer system and regional aquifers in the B, New York, 

C, New Jersey, D, Delmarva, E, Maryland, F, Virginia, and G, North 

Carolina geographic areas. Geographic areas are shown in 

figure 12. Negative values indicate storage replenishment.
Simulation of the Hydrologic System 45

pp_masterson_nacp_fig22ef_new Changes in storage, in million gallons per day

Confined aquifer depletion 

Confining unit depletion

Surficial aquifer depletion

Confined aquifer replenishment 

Confining unit replenishment 

Surficial aquifer replenishment 

Year

Historical period Period of emphasis Future period

10

5

 0

–5

15

Historical period Period of emphasis Future period

60

50

40

30

20

 10

0

 –10

–20

–30

EXPLANATION

1920

1939

1945

1952

1957

1964

1967

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Pre-1900

E

D

Historical period Period of emphasis Future period

30

20

 10

0

–10

–20

40

C

Figure 22. Change in aquifer storage by hydrogeologic unit 

for the historical period (1900–1985), the period of emphasis

(1986–2013), and a future period (2014–2058) for A, the Northern 

Atlantic Coastal Plain aquifer system and regional aquifers 

in the B, New York, C, New Jersey, D, Delmarva, E, Maryland, 

F, Virginia, and G, North Carolina geographic areas. Geographic 

areas are shown in figure 12. Negative values indicate 

storage replenishment.—Continued
46 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

pp_masterson_nacp_fig22g _new Changes in storage, in million gallons per day

Confined aquifer depletion 

Confining unit depletion

Surficial aquifer depletion

Confined aquifer replenishment 

Confining unit replenishment 

Surficial aquifer replenishment 

Year

EXPLANATION

1920

1939

1945

1952

1957

1964

1967

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Pre-1900

Historical period Period of emphasis Future period

30

20

 10

0

–10

–20

40

G

Historical period Period of emphasis Future period

60

50

40

 30

20

10

0

–10

F

Water appears to flow in and out of storage in response 

to annual changes in pumping, and yet, the net cumulative 

storage change over time is almost always a positive release 

(or loss) of water in response to long-term groundwater 

withdrawals (fig. 23). The one exception is in the Delmarva 

geographic area, where there is a net gain in storage (shown 

as a negative change in storage in figure 23D) in the surficial 

aquifer from 1964 to 1980. This gain may be an artifact of 

the numerical analysis where wastewater returnflow was 

simulated during that period, but not domestic pumping. In 

areas such as the Delmarva geographic area where domestic 

pumping may have been a locally important component of 

the total pumping for this period, the exclusion of domestic 

pumping may result in more water reported as replenishing 

storage than would have be recorded had domestic withdrawals been accounted for in this analysis (fig. 23D).

Figure 22. Change in aquifer storage by hydrogeologic unit 

for the historical period (1900–1985), the period of emphasis

(1986–2013), and a future period (2014–2058) for A, the Northern 

Atlantic Coastal Plain aquifer system and regional aquifers 

in the B, New York, C, New Jersey, D, Delmarva, E, Maryland, 

F, Virginia, and G, North Carolina geographic areas. Geographic 

areas are shown in figure 12. Negative values indicate 

storage replenishment.—Continued
Simulation of the Hydrologic System 47

The long-term trend in this storage release suggests 

that the release of water from the confining units becomes 

proportionally more important over time in the New 

Jersey, Delmarva, Maryland, and Virginia geographic areas 

(fig. 23C–F ). Storage loss from the confining units is of 

concern because water removed from clayey sediments 

cannot be replenished as these units gradually compress, a 

process referred to as inelastic or nonrecoverable storage loss 

(not represented in this numerical analysis). This nonrecoverable storage loss can result in land subsidence where these 

Changes in storage, in billion gallons

3,500

3,000

2,500

 2,000

1,500

1,000

500

0

600

500

 400

300

200

100

0

Year

Historical period Period of emphasis Future period

Historical period Period of emphasis Future period

Confined aquifer depletion 

Confining unit depletion

Surficial aquifer depletion

Confined aquifer replenishment 

Confining unit replenishment 

Surficial aquifer replenishment 

EXPLANATION

1920

1939

1945

1952

1957

1964

1967

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Pre-1900

A

B

confining units are thick and the release from storage is 

relatively large. The storage loss from the confining units 

in the New York geographic area (fig. 23B) is negligible 

compared with the storage loss from the surficial aquifer 

there, indicating that the aquifer system response to pumping 

will occur much more quickly in this geographic area than 

in others where storage loss from the confining units is more 

prevalent, such as in the Delmarva geographic area where 

most of the storage loss occurred in the confining units 

(fig. 23D).

Figure 23. Cumulative change in aquifer storage by hydrogeologic unit 

for the historical period (1900–1985), the period of emphasis (1986–2013), 

and a future period (2014–2058) for A, the Northern Atlantic Coastal Plain 

aquifer system and regional aquifers in the B, New York, C, New Jersey, 

D, Delmarva, E, Maryland, F, Virginia, and G, North Carolina geographic 

areas. Geographic areas are shown in figure 12. Negative values indicate 

storage replenishment.
48 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

Figure 23. Cumulative change in aquifer storage by hydrogeologic unit 

for the historical period (1900–1985), the period of emphasis (1986–2013), 

and a future period (2014–2058) for A, the Northern Atlantic Coastal Plain 

aquifer system and regional aquifers in the B, New York, C, New Jersey, 

D, Delmarva, E, Maryland, F, Virginia, and G, North Carolina geographic 

areas. Geographic areas are shown in figure 12. Negative values indicate 

storage replenishment.—Continued

pp_masterson_nacp_fig23ef_new

1920

1939

1945

1952

1957

1964

1967

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Pre-1900

Changes in storage, in billion gallons

Year

200

180

140

160

120

 100

80

60

40

20

0

Historical period Period of emphasis Future period

450

400

300

350

250

 200

150

100

50

0

–50

Historical period Period of emphasis Future period

E

D

400

350

300

250

 200

150

100

50

0

C Historical period Period of emphasis Future period

Confined aquifer depletion 

Confining unit depletion

Surficial aquifer depletion

Confined aquifer replenishment 

Confining unit replenishment 

Surficial aquifer replenishment 

EXPLANATION
Simulation of the Hydrologic System 49

pp_masterson_nacp_fig23g _new Changes in storage, in billion gallons

600

500

 400

300

200

100

0

Year

G

1920

1939

1945

1952

1957

1964

1967

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Pre-1900

Historical period Period of emphasis Future period

1,200

1,000

 800

600

400

200

0

Historical period Period of emphasis Future period F

Confined aquifer depletion 

Confining unit depletion

Surficial aquifer depletion

Confined aquifer replenishment 

Confining unit replenishment 

Surficial aquifer replenishment 

EXPLANATION Figure 23. Cumulative change in aquifer storage by hydrogeologic unit 

for the historical period (1900–1985), the period of emphasis (1986–2013), 

and a future period (2014–2058) for A, the Northern Atlantic Coastal Plain 

aquifer system and regional aquifers in the B, New York, C, New Jersey, 

D, Delmarva, E, Maryland, F, Virginia, and G, North Carolina geographic 

areas. Geographic areas are shown in figure 12. Negative values indicate 

storage replenishment.—Continued
50 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

Comparison of Predevelopment and 

Current [2013] Conditions

The large-scale groundwater withdrawals that started 

at the beginning of the 20th century increased steadily until 

the early 1980s (fig. 13). From the early 1980s until the 

present [2013], pumping has remained generally constant at 

the regional scale, with some geographic areas experiencing 

increased withdrawals and others experiencing decreases 

(figs. 14 and 15). To assess the long-term groundwater 

system response from predevelopment to current [2013] 

conditions, the cumulative response of the NACP regional 

aquifer system and its geographic regions can be assessed 

in terms of both storage change and change in water levels 

for the historical period and the period of emphasis. During 

the historical period, storage depletion in the surficial 

aquifer represented more than a third (34 percent) of total 

storage depletion calculated for the NACP regional aquifer 

1900–1985

1986–2013

2014–2043

A. Northern Atlantic Coastal PlainB. New York geographic area

Storage depletion from surficial aquifer

Storage depletion from confined aquifers

Storage depletion from confining units

Storage increase in surficial aquifer

Storage increase in confined aquifers

Storage increase in confining units

Cumulative storage change

 

EXPLANATION

45,817

(92%)

C. New Jersey geographic area D. Delmarva geographic area

421,665

(34%)

490,181.7

(39%)

343,732

(27%)

92,677

(10%)

412,572

(45%)

419,663

(45%)

296,269

(50%) 290,835

(50%)

357,027.5

(84%)

59,404

(14%)

6,624 (2%)

45,726

(72%)

15,739

(25%)

2,218 (3%)

7,335

(78%)

2,022

(21%)

78 (1%)

36,327

(25%)

30,312

(21%)

79,952

(54%)

41,999

(31%)

5,570

(4%)

87,485

(65%)

2,309 (5%)

1,746 (3%)

45,817

(92%)

1,480 (1%)

30,882

(27%)

81,580

(72%)

19,112

(11%)

27,750

(17%)

119,883

(72%)

311 (1%)

7,759

(9%)

75,166

(90%)

In million gallons

As percentage of total

Figure 24. Distribution of cumulative aquifer storage change by hydrogeologic unit for 

the historical period (1900–1985), period of emphasis (1986–2013), and a future period

(2014–2043) for A, the Northern Atlantic Coastal Plain aquifer system and regional 

aquifers in the B, New York, C, New Jersey, D, Delmarva, E, Maryland, F, Virginia, 

and G, North Carolina geographic areas. Geographic areas are shown in figure 12. 
Simulation of the Hydrologic System 51

1900–1985

1986–2013

2014–2043

E. Maryland geographic area F. Virginia geographic area G. North Carolina geographic area

5,310

(9%)

31,337

(51%)

24,387

(40%)

38,922

(49%)

37,654

(47%)

3,571 (4%) 

811 (2%)

8,817

(27%)

23,003

(71%)

8,937 (4%)

124,217

(53%)

102,307

(43%)

203,667

(61%)

124,451

(38%)

3,032 (1%)

2,297 (1%)

223,204

(68%)

99,829

(31%)

12,584 (4%)

214,029

(78%)

48,881

(18%)

120,924

(69%)

47,972

(27%)

7,556 (4%)

1,796 (2%)

56,765

(54%)

47,098

(44%)

Storage depletion from surficial aquifer

Storage depletion from confined aquifers

Storage depletion from confining units

Storage increase in surficial aquifer

Storage increase in confined aquifers

Storage increase in confining units

Cumulative storage change

 

EXPLANATION

47,098

(44%)

In million gallons

As percentage of total

system (fig. 24). In the model simulations, this value 

decreased to 10 percent during the period of emphasis, and 

to 0 percent during a future [2014 –2043] period. Over the 

same timeframe, storage depletion in the confined aquifers 

increased from 39 to 50 percent of total storage depletion 

and storage depletion in the confining units increased 

from 27 to 50 percent.

It is important to examine the storage changes at the scale 

of the individual geographic areas to understand the complex 

response of the regional groundwater system. For example, 

during the historical period, almost 85 percent of the nearly 

422,000 Mgal/d of water released from storage in the surficial 

aquifer for the entire NACP aquifer system occurred in the 

New York geographic area (fig. 24B). The storage released 

from the confined aquifers for the entire NACP aquifer system 

was about 490,000 Mgal/d; and nearly 70 percent of that 

storage release occurred in the Virginia and North Carolina 

geographic areas (fig. 24F–G).

Figure 24. Distribution of cumulative aquifer storage change 

by hydrogeologic unit for the historical period (1900–1985), 

period of emphasis (1986–2013), and a future period (2014–2043)

for A, the Northern Atlantic Coastal Plain aquifer system and 

regional aquifers in the B, New York, C, New Jersey, D, Delmarva, 

E, Maryland, F, Virginia, and G, North Carolina geographic areas. 

Geographic areas are shown in figure 12.—Continued
52 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

The storage released from the confining units during 

the historical period appears to be distributed more evenly 

from the New Jersey to the Virginia geographic areas, with 

the Virginia area (fig. 24F ) accounting for about 30 percent 

of the nearly 344,000 Mgal/d of storage release from the 

confining units for the entire NACP aquifer system (fig. 24A). 

Although the storage release from the confining units in the 

Virginia geographic area accounted for about 30 percent of 

the total storage released from the confining units for the 

entire NACP aquifer system, it only accounted for 43 percent 

of the total storage release in this geographic area (fig. 24F ); 

by comparison, the 81,600 Mgal/d of storage loss from the 

confining units in the Delmarva geographic area accounted 

for about 72 percent of the total storage released in this area 

(fig. 24D). The amount of storage released from confining 

units in these geographic areas is directly related to the 

distribution of pumping from confined aquifers and the 

presence of overlying confining units.

The differences in the amounts of water released from 

storage by each hydrologic units group among the geographic 

areas are directly related to the distribution of pumping in those 

geographic areas. Nearly all the groundwater withdrawals 

in the New York geographic area are from the Magothy 

aquifer (fig. 15B); although the Magothy aquifer is considered 

confined, it is in direct hydrologic connection with the overlying surficial aquifer (locally referred to as the upper glacial 

aquifer, fig. 6D). Because of the direct connection between the 

Magothy aquifer and the overlying surficial aquifer, pumping 

in the Magothy results in water-level changes in the surficial 

and a corresponding loss of storage from the surficial aquifer. 

Conversely, most of the groundwater withdrawals in the 

Virginia geographic area are in the confined Potomac aquifer 

system (fig. 15F), and therefore, nearly all the water released 

from storage in this geographic area is from the confined 

aquifers and the intervening confining units (fig. 6A). This 

release from storage from the confined aquifers results in large 

water-level decreases in these aquifers (fig. 25), particularly 

in the Potomac-Patapsco aquifer in southern Virginia and 

northeastern North Carolina. The effect of pumping from the 

Potomac-Patapsco appears to have little effect on storage in 

the surficial aquifer in this area (fig. 24F–G ).

Although pumping rates generally increased throughout 

the historic period, there have been notable large-scale 

decreases in groundwater withdrawals at different points 

in time in response to contamination concerns, changes in 

industrial water-use practices, or improved water conservation 

measures. For example, there were large reductions in groundwater withdrawals in the late 1930s on western Long Island 

(New York geographic area) in response to saltwater intrusion 

concerns (Buxton and Shernoff, 1999). This resulted in a shift 

to a reservoir system in upstate New York to meet the water 

demands for western Long Island and a rise in groundwater 

levels over time as pumping was curtailed. A comparison 

of predevelopment water levels with current [2013] water 

levels would show only a decrease in water levels in the 

western Long Island area (fig. 25) for the Potomac-Patapsco 

aquifer (locally referred to as the Lloyd aquifer); whereas, a 

comparison of water levels from the end of the historical period 

[1985] to the end of the period of emphasis [2013] shows water 

levels continuing to rise in western Long Island (fig. 26).

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). Groundwater withdrawals in 

this area generally were reduced by as much as 30 Mgal/d by 

the late 1980s, and the nearby Manasquan Reservoir was used 

to meet water-supply needs. By 2003, reductions in pumping 

resulted in water-level increases in the Potomac-Patapsco 

aquifer (locally referred to as the Middle Potomac-RaritanMagothy aquifer) by more than 80 ft near large pumping 

centers in northern Monmouth County, N.J., and as much as 

10 ft at Sandy Hook, N.J. (fig. 26).

Groundwater levels continued to decrease in the southern 

portion of the NACP aquifer system at the southernmost 

extent of the aquifer in North Carolina throughout the period 

of emphasis in spite of groundwater withdrawals remaining 

constant throughout the period (fig. 26). The large water-level 

decreases along the southern boundary of the study area in 

North Carolina may be exacerbated by the representation of 

the model boundary in this area, as described in detail in the 

model documentation in Masterson and others (2016). As a 

result, the water budgets and water-level changes reported 

for North Carolina in this analysis should be considered to 

be affected by this modeling limitation. An analysis of the 

North Carolina portion of the NACP aquifer system was 

provided in the Southern Atlantic Coastal Plain assessment 

(Campbell and Coes, 2010).

Comparison of Current [2013] and 

Future [2043] Conditions

Two conditions were considered as possible future 

conditions (as described in the “Changes in Hydrologic 

Budgets Over Time” section). These conditions included 

first continuing the current [2013] pumping and recharge 

stresses for an additional 30 years since about half of the 

total groundwater withdrawn from the NACP aquifer system 

occurred in the 30 years prior to 2013. The analysis for a 

future [2043] condition was conducted to determine the effect 

of continued release of water from storage on water levels in 

the aquifer system.

A comparison of the changes in water levels in the 

Potomac-Patapsco aquifer from current [2013] to future [2043] 

conditions indicates a large range in the aquifer response from 

Long Island to southern Virginia. These results show that a 

continuation of the current pumping and recharge stresses 

results in a leveling off of water levels in the deep system on 

Long Island compared with the deep system in Virginia where 

simulated water levels continue to decrease after an additional 

30 years of constant pumping (fig. 27).
Simulation of the Hydrologic System 53

Study area

EXPLANATION

Potomac-Patapsco aquifer

Water-level change, in feet

≤0

>0 to 5

>5 to 10

>10 to 20

>20 to 40

>40 to 60

>60 to 80

>80 to 100

>100 to 150

>150

0 50 100 MILES

0 50 100 KILOMETERS

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

Albers Equal Area Conic projection

Standard parallels 34°30' N and 41°30' N

Central meridian 75°30' W 

North American Datum of 1983

Monmouth

County

Sandy Hook

78° 75° 72°

39°

36°

Chesapeake Bay

Delaware Bay

Long Island Sound

NEW CT

YORK

NEW

JERSEY

PENNSYLVANIA

MARYLAND

DC DELAWARE

VIRGINIA

NORTH CAROLINA

ATLANTIC OCEAN

Pamlico Sound

Albemarle 

Sound

pp_masterson_nacp_fig25_new

WV

Figure 25. Change in water levels in response to groundwater withdrawals from 1900 to 1985 in the Potomac-Patapsco 

aquifer of the Northern Atlantic Coastal Plain aquifer system. Positive values indicate water levels that are lower in 1985 

than in 1900. ≤, less than or equal to; >, greater than.
54 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

Study area

EXPLANATION

Potomac-Patapsco aquifer

Water-level change, in feet

–90 to –20

>–20 to –10

>–10 to 0

>0 to 10

>10 to 20

>20 to 30

>30 to 40

>40 to 50

>50 to 60

>60 to 110

0 100 KILOMETERS

0 50

50

100 MILES

Monmouth

County

Sandy Hook

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

78° 75° 72°

39°

36°

Chesapeake Bay

Delaware Bay

Long Island Sound

NEW CT

YORK

NEW

JERSEY

PENNSYLVANIA

MARYLAND

DC DELAWARE

VIRGINIA

NORTH CAROLINA

ATLANTIC OCEAN

Pamlico Sound

Albemarle Sound

pp_masterson_nacp_fig26_new

WV

Figure 26. Change in water levels in response to groundwater withdrawals from 1986 to 2013 in the PotomacPatapsco aquifer of the Northern Atlantic Coastal Plain aquifer system. Positive values indicate water levels that are 

lower in 2013 than in 1986. >, greater than.
Simulation of the Hydrologic System 55

Study area

EXPLANATION

Potomac-Patapsco aquifer

Water-level change, in feet

≤0.1

>0.1 to 5

>5 to 10

>10 to 15

>15 to 20

>20 to 25

>25 to 30

≥30

0 100 KILOMETERS

0 50

50

100 MILES

Monmouth

County

Sandy Hook

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

Albers Equal Area Conic projection

Standard parallels 34°30' N and 41°30' N

Central meridian 75°30' W 

North American Datum of 1983

78° 75° 72°

39°

36°

Chesapeake Bay

Delaware Bay

Long Island Sound

NEW CT

YORK

NEW

JERSEY

PENNSYLVANIA

MARYLAND

DC

VIRGINIA

NORTH CAROLINA

ATLANTIC OCEAN

Pamlico Sound

Albemarle Sound

DELAWARE

WV

pp_masterson_nacp_fig27_new

Figure 27. Change in water levels in projected response to groundwater withdrawals from 2013 to 2043 in the 

Potomac-Patapsco aquifer of the Northern Atlantic Coastal Plain aquifer system. Positive values indicate water 

levels that are lower in 2043 than in 2013. ≤, less than or equal to; ≥, greater than or equal to.
56 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

The simulated water-level change on Long Island from 

the current [2013] to the future [2043] condition is nearly zero 

for most the island, whereas water levels in southern Virginia 

continue to decrease by as much as 30 ft (fig. 27). These 

responses are consistent with the previously discussed changes 

in hydrologic budgets for these geographic areas. For the 

New York geographic area, the change in storage from current 

[2013] to future [2043] conditions is almost zero (fig. 20B), 

and therefore, the change in water levels over that time also is 

zero as the groundwater system approaches equilibrium with 

respect to the current [2013] pumping and recharge stresses 

(fig. 27). However, in the Virginia geographic area, the amount 

of water released from storage from 2014 to 2043 is about 

25 percent of the total change in the hydrologic budget and 

is similar to the amount of water released from storage for 

the period of emphasis (fig. 20F). This continued response 

suggests that the flow system in this area is still equilibrating 

to the current [2013] pumping stress and that water levels will 

continue to decrease with the continued release of water from 

storage until the system reaches a new equilibrium.

The second analysis that was conducted to examine how 

the groundwater system may respond in the future included 

the removal of all wastewater return flow. The analysis of 

the change in hydrologic budget components across the 

NACP aquifer system indicated that the New York geographic 

area appeared to be most affected by this change in recharge 

to the groundwater system. The simulated removal of wastewater return flow from about 340,000 septic systems in the 

unsewered parts of central and eastern Long Island resulted 

in a reduction of groundwater discharge of about 17 Mgal/d 

to streams and about 57 Mgal/d to coastal waters in the 

New York geographic area (fig. 21).

The removal of wastewater return flow had the greatest 

effect on water levels in the areas that received large volumes 

of this additional recharge. The hydrologic response to 

the removal wastewater return flow is well documented in 

Nassau County on western Long Island where the transition 

began in the mid-1950s from onsite domestic septic systems 

to more centralized wastewater treatment facilities (Buxton 

and Smolensky, 1999) in response to increased concerns 

about nonpoint source contamination of the local groundwater 

system. 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 substantial decreases 

in water levels (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 groundwater-fed streams. The most notable example of this 

hydrologic response is East Meadow Brook in southwestern 

Nassau County, where flows have decreased by as much as 

70 percent from predevelopment conditions (Scorca, 1997).

The results of the analysis in this report indicate that water 

levels would decrease by more than 10 ft in parts of northwestern Suffolk County, N.Y. (fig. 28), where there is a high 

concentration of onsite septic systems that provide an additional 

source of recharge to the less permeable glacial moraine sediments in north-central Long Island. These water-level decreases 

are in response to the 74-Mgal/d reduction in wastewater return 

flow that also resulted in reductions in groundwater discharge 

to streams and coastal waters of 17 and 57 Mgal/d, respectively 

(fig. 21). Therefore, the transition from onsite domestic septic 

systems to large, centralized wastewater treatment facilities may 

mitigate water-quality concerns for surface receiving waters, 

but this improvement may create concerns for water quantity to 

small surface-water bodies that receive substantial groundwater 

flow associated with the additional recharge from wastewater.

Changes in the Boundary Between the Fresh 

and Saline Groundwater Flow Systems

The total input of freshwater to the groundwater flow 

system from aquifer recharge in the NACP aquifer system is 

balanced almost entirely by groundwater discharge to streams, 

freshwater tidal estuaries, and saline coastal waters. Previously 

in this report, total coastal discharge included discharge to 

freshwater tidal estuaries and to saline coastal waters and 

accounted for about 60 percent of the total outflow from the 

groundwater system for current [2013] conditions (fig. 17). 

For this analysis of the effects of pumping on the boundary 

between fresh and saline groundwater flow systems, only the 

change in discharge to saline coastal waters is considered.

Reduction in Groundwater Discharge to 

Saline Coastal Waters

Groundwater withdrawals of about 1,330 Mgal/d 

created a reduction of more than 500 Mgal/d of freshwater 

discharging to saline coastal waters (fig. 29), which was 

almost 40 percent of the total groundwater system response 

to groundwater pumping for current [2013] conditions. The 

geographic distribution of this reduction in coastal discharge 

indicates that nearly all of the reduction calculated for the 

NACP aquifer system occurred in the New York geographic 

area during the beginning of the historical period. By the end 

of the period of emphasis; however, the reduction in coastal 

discharge in New York accounted for less than 60 percent 

of the total reduction in coastal discharge across the entire 

NACP aquifer system (fig. 29). This distribution in the coastal 

discharge reduction compares well with the distribution of 

pumping over time by geographic area (fig. 15). Pumping in 

the New York geographic area comprised most of the total 

withdrawals throughout the NACP aquifer system during 

the historical period and as pumping increased in the other 

geographic areas, the withdrawals in New York geographic 

area decreased as a percentage of the total withdrawals in the 

NACP aquifer system; a similar response was observed in the 

decrease in coastal discharge by geographic area (fig. 29).
Simulation of the Hydrologic System 57

40°30'

74°

41°

73° 72°

ATLANTIC OCEAN

NEW

YORK

CONNECTICUT

NEW

JERSEY

SUFFOLK

NASSAU COUNTY

COUNTY QUEENS

COUNTY

KINGS

COUNTY

Long Island Sound

0 20

20

40 MILES

0 40 KILOMETERS

EXPLANATION

Water-level decrease, 

in feet

≤0.1

>0.1 to 0.5

>0.5 to 1

>1 to 2

>2 to 3

>3 to 4

>4 to 5

>5 to 8

>8 to 10

>10 

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

Albers Equal Area Conic projection

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

North American Datum of 1983

pp_masterson_nacp_fig28_new

North Carolina

Maryland

Virginia

Delmarva

New York

New Jersey

EXPLANATION

1900

1920

1939

1945

1952

1957

1964

1967

1972

1977

1980

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2013

2018

2023

2028

2033

2038

2043

2048

2053

2058

Year

Change in groundwater discharge to coast,

in million gallons per day

300

200

100

0

−100

400

500

600

700

Historical period Period of emphasis Future period

Geographic area

Figure 28. Change in water levels in the surficial aquifer on Long Island, New York, of the Northern Atlantic 

Coastal Plain aquifer system with the removal of wastewater return flow for current [2013] conditions. ≤, less 

than or equal to; >, greater than.

Figure 29. Change in coastal groundwater discharge in the Northern Atlantic 

Coastal Plain aquifer system in response to groundwater pumping for the 

historical period (1900–1985), the period of emphasis (1986–2013), and a future 

period (2014–2058) by geographic area. Positive value indicates a reduction 

from the nonpumping condition. Geographic areas are shown in figure 12.
58 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

Landward Movement of Saline Groundwater

The total change in discharge to saline coastal waters 

includes both a reduction in groundwater discharge and the 

landward movement of saline groundwater into the freshwater 

system. This lateral, landward movement of saline groundwater is depicted schematically in figure 30 by the blue arrows 

indicating the expansion of brackish water into the freshwater 

flow system as the saline and fresh groundwater systems are 

mixed. The amount of lateral movement of saline groundwater is assessed in this report as a change in the simulated 

inflow from the model boundary between fresh and saline 

groundwater, not the physical change in location of this model 

boundary. A more rigorous method of assessing this hydrologic response would be to simulate the physical movement of 

this boundary; however, an analysis such as this was beyond 

the scope of this regional groundwater availability assessment. 

The modeling methodology used to conduct the analysis in 

this report is described in Masterson and others (2016).

An assessment of the potential for an increase in the 

landward movement of saline groundwater into the surficial 

and confined aquifer systems can be made for current [2013] 

conditions (fig. 31A, C ). About 46 Mgal/d of saline groundwater moved into the surficial aquifer system, comprising 

about 9 percent of the total reduction in coastal discharge in the 

shallow surficial aquifer system (fig. 32A). Nearly 90 percent 

of this inflow of saline groundwater into the surficial aquifer 

system was in the Delmarva, New York, New Jersey, and 

North Carolina geographic areas, with smaller amounts in 

the Virginia and Maryland geographic areas (fig. 31A). This 

distribution of inflows can be correlated to the distribution of 

groundwater withdrawals by aquifer across the geographic 

areas (fig. 15). The Potomac-Patapsco aquifer is the major 

source of pumped water in Virginia and Maryland, and because 

this aquifer is very deep and confined along the coast in these 

geographic areas (fig. 6A–B), pumping from this aquifer has 

little effect on shallow groundwater discharge to the coast.

Lateral movement of saline groundwater into the confined 

system was calculated to be 26 Mgal/d (fig. 31C), which 

is about half of the 46 Mgal/d that was calculated for the 

surficial aquifer system (fig. 31A); almost 60 percent of the 

total lateral movement was in the New York and New Jersey 

geographic areas. The only geographic area in which the 

inflow of saline groundwater is greater to the confined system 

than to the surficial system is in the Virginia geographic area 

where inflow into the confined system is 6 Mgal/d, almost 

three times more than the 2.2 Mgal/d of inflow in the surficial 

aquifer system. This response in the Virginia geographic area is 

attributed to the distribution of pumping where most of the large 

withdrawals in Virginia are in the Potomac-Patapsco aquifer 

(figs. 15 and 16) and the possible effects of the low-permeability 

sediments of the Chesapeake Bay impact crater (fig. 6A).

The total change in coastal outflow from the confined 

system is about 31 Mgal/d with about 85 percent (about 

26 Mgal/d) being from the inflow of saline groundwater and the 

remaining 15 percent (about 5 Mgal/d) derived from a reduction 

of groundwater discharge from the confined system to the shallow 

coastal waters along the coastal margin (fig. 32C). This reduction 

of coastal outflow from the confined system is primarily in the 

New York and New Jersey geographic areas where the shallow 

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

Saline groundwater

Zone of mixing

Figure 30. The effects of 

pumping on groundwater 

flow in the Northern Atlantic 

Coastal Plain aquifer system. 

Modified from Barlow (2003).
Simulation of the Hydrologic System 59

confined aquifers are not completely separated from the overlying 

surficial aquifer by an intervening confining unit (fig. 6C–D).

A future [2043] condition in which the current [2013] 

pumping and recharge rates were continued for an additional 

30 years was simulated with the numerical model to assess if 

the rates of change in groundwater discharge to the coast will 

remain the same after 30 years. The results of this modeling 

analysis indicate that the change in discharge to the shallow 

coastal waters in the surficial aquifer system appear to be 

approximately in equilibrium with the current [2013] pumping 

stress. The total change in groundwater discharge is about 

504 Mgal/d for the future [2043] condition, the same as for the 

current [2013] condition. The total rate of lateral movement of 

saline groundwater in the surficial aquifer system increased from 

46 to 47 Mgal/d from current [2013] to future [2043] conditions, 

indicating only a negligible change in the rate (fig. 31A–B).

A comparison of the changes in coastal discharge from 

the confined system between current [2013] and future [2043] 

conditions indicate that the confined system also appears to 

be in equilibrium with the current [2013] pumping stresses. 

Although, the rates of change in groundwater discharge to 

the coast appear to be similar between current [2013] and 

future [2043] conditions, it is important to note that this is 

at the regional or geographic area scale and that these small 

changes can have substantial effects at the local or wellfield 

scale. For example, although the inflow of saline groundwater 

in the confined system only increased by about 9 percent from 

current [2013] to future [2043] conditions about 50 percent of 

that increase was in the Virginia geographic area (fig. 31C–D), 

indicating that concerns of saltwater intrusion may still exist 

at the local scale even as the regional system approaches 

equilibrium with respect to the current [2013] pumping stress.

North Carolina

Maryland

Virginia

Delmarva

New York

New Jersey

EXPLANATION

Geographic area

9.6

3.1

2.2

11.0

9.8

10.4

A. Coastal groundwater inflow to 

 surficial aquifer system in 2013

7.6

2.6

7.2

3.5 7.8

D. Coastal groundwater inflow to

 confined system in 2043

9.2

3.4

2.6

11.5

10.1

10.1

B. Coastal groundwater inflow to

 surficial aquifer system in 2043

7.7

2.1

6.0

3.1 7.4

7.4

C. Coastal groundwater inflow to

 confined system in 2013

Rate, in million gallons per day

10.1

2.2

6.1

3.5

9.2

C. Change in coastal outflow from 

 confined system in 2013

42.1

4.2

89.0

47.5

26.3

295.1

A. Change in coastal outflow from 

 surficial aquifer system in 2013

41.3

5.8

91.4

48.5

27.5

289.9

B. Change in coastal outflow from 

 surficial aquifer system in 2043

10.0

2.8

7.3

3.9

9.6

D. Change in coastal outflow from 

 confined system in 2043

North Carolina

Maryland

Virginia

Delmarva

New York

New Jersey

EXPLANATION

Geographic area

9.2 Rate, in million gallons per day

Figure 31. Distribution of change in coastal groundwater discharge, 

in million gallons per day, as inflow from saline groundwater in 

response to pumping for the surficial aquifer system in A, 2013 and 

B, 2043, and the confined aquifer system in C, 2013 and D, 2043. 

Geographic areas are shown in figure 12.

Figure 32. Distribution of change in coastal groundwater discharge,

in million gallons per day, as a net reduction in total groundwater 

discharge to the coast for in response to pumping for the surficial 

aquifer system in A, 2013 and B, 2043, and the confined aquifer system

in C, 2013 and D, 2043. Geographic areas are shown in figure 12.
60 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

Simulation of Effects of Climate Change

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 in 

the northern Atlantic Coastal Plain province. Most climate 

models (Coulson and others, 2010) forecast warmer future 

temperatures with more variable and less predictable precipitation patterns. Therefore, predictions of potential changes in 

the amount of aquifer recharge are very uncertain. However, 

increases in temperatures potentially may have several effects, 

including increasingly more severe drought conditions that 

could act as drivers to increase summertime water demand. 

Increased evapotranspiration coupled with more highly variable precipitation patterns during the growing season months 

will likely prompt agricultural producers to rely more heavily 

on irrigation from groundwater sources to minimize the effects 

of droughts. 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 powerplants (Legesse and others, 2003).

Coastal communities in the northern Atlantic Coastal 

Plain province also are faced with the potential consequences 

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 in southern Virginia 

(National Oceanic and Atmospheric Administration, 2012). Sealevel rise throughout the region averages about 3.5 mm/yr, a rate

higher than the global average (Sallenger and others, 2012).

Two analyses were conducted using the numerical 

model developed for this investigation to assess the effects 

of climate change stressors on the water resources of the 

NACP aquifer system. The first modeling analysis explored 

the potential effects of a severe drought combined with 

a revised estimate of irrigation withdrawals. The second 

modeling analysis assessed the effects of an increase in the 

current [2013] sea-level position on water levels, streamflow, 

and coastal discharge.

Effects of Drought Conditions on 

Water Resources

The prolonged regional drought from 1960 to 1967 

produced the largest recorded annual departures from average 

stream discharge (James and others, 1991) and is generally 

considered the drought of record for the northern Atlantic 

Coastal Plain province (Fisher and Smart, 1999; Polsky and 

others, 2000). The SWB model (Westenbroek and others, 

2012) was used to calculate recharge for this 8-year drought 

using the historical temperature and precipitation data from 

1960 to 1967. Because the actual irrigation withdrawals for 

the 1960s were not available, this model also was used to 

estimate the increased crop water demand for this drought 

period that then was used to determine a corresponding 

increase in irrigation pumping. The increase in water demand 

then was applied to the current [2013] simulated irrigation 

locations to determine the hydrologic response of a drought of 

the magnitude of the 1960 –1967 drought for current [2013] 

conditions. The resulting response included lower water levels 

(that is, release of water from storage), reduced streamflow, 

and reduced discharge to the coast (fig. 33).

The average total reduction in recharge during the 8-year 

simulated drought period was about 11,000 Mgal/d, a reduction of about 60 percent of the current [2013] rate of about 

18,000 Mgal/d or almost 9 inches per year less recharge over 

the entire study area. This reduction of freshwater inflow is 

27

27

50

23

26

66

8

27

40

33 22

61

17 29

50

21

36

34

30 22

52

26

Increase in release

 from storage

Percentage of total

Decrease in discharge 

 to streams

Decrease in discharge

 to coast

EXPLANATION

Northern Atlantic Coastal Plain 

aquifer system, total:

11,000 million gallons per day

Virginia

geographic area,

1,900 million gallons 

per day

North Carolina

geographic area,

1,900 million gallons 

per day

New York 

geographic area,

1,300 million gallons 

per day

New Jersey

geographic area,

2,200 million gallons 

per day

Delmarva

geographic area,

2,800 million gallons 

per day

Maryland

geographic area, 

900 million gallons 

per day

Figure 33. Distribution of changes in hydrologic budget 

components, in percent, in response to a simulated 8-year drought 

for pumping conditions in 2013 across the Northern Atlantic 

Coastal Plain aquifer system. Change values shown are total 

simulated reductions in groundwater discharge to coastal waters 

and streams and increases in water released from storage. 

Geographic areas are shown in figure 12. 
Simulation of Effects of Climate Change 61

about an order magnitude more than the total groundwater 

withdrawals for current [2013] conditions. The primary 

system response to this reduction in recharge is a reduction 

in groundwater discharge to coastal waters (fig. 33). Half 

of the reduction in recharge across the entire NACP aquifer 

system was as groundwater that would have discharged to 

coastal waters; about 23 percent to streams, and the remaining 

27 percent would have gone into aquifer storage.

The relative distribution of the system response to 

drought across the various geographic areas shows that 

reductions in coastal discharge accounts for almost 66 percent 

of the response in the New York geographic area, whereas in 

the Virginia area reductions in coastal discharge only account 

for about a third of the total response in that area (fig. 33). By 

further comparison, the reduction in streamflow accounts for 

one-third of the system response in the New Jersey geographic 

area, but only 8 percent in the New York geographic area, 

which can be accounted for by the small number of streams 

present the New York geographic area. Water lost from 

aquifer storage is about one-quarter (27 percent) of the system 

response across the entire NACP aquifer system, but locally is 

as high as 36 percent in the Virginia geographic area.

Almost 25 percent of the 11,000 Mgal/d of the total 

decrease in recharge across the NACP aquifer system was in 

the Delmarva geographic area (fig. 34). The average drought 

recharge calculated by the SWB model (Westenbroek and 

others, 2012) for the Delmarva geographic area for this 

report was substantially lower than the average recharge 

from 1980 to 2010 throughout most of the Delmarva Peninsula, 

including the entire State of Delaware (fig. 35). The departure 

from the average annual recharge in this area was as much as 

12 to 16 inches below average, or almost a 50-percent reduction in average recharge from 1980 to 2010.

Droughts are of particular concern in the Delmarva 

geographic area because of the high density of agricultural 

industry in the area (fig. 7). Agricultural withdrawals account 

for more than half of the groundwater use in the counties that 

comprise the Delmarva geographic area (Maupin and others, 

2014) and these counties have some of the highest rates of 

irrigation withdrawals per square mile east of the Mississippi 

River (Alley and others, 2013). In order to fully understand 

the potential effects of droughts on agriculture in this area, 

it is important to understand the reductions in precipitation 

during the growing season months of April through October.

Precipitation during this 8-year drought period was 

below average for many of the growing season months in the 

Delmarva geographic area (fig. 36). Increases in agricultural 

water use are most associated with droughts because the 

need for crop irrigation is dependent on antecedent climatic 

conditions. Therefore, as precipitation decreases during 

the growing season months (April–October), water use for 

irrigation must increase to meet water demand crops. The 

actual increase in irrigation water use during the 1960s 

drought was not recorded during that time; therefore, it is 

not possible to assess the effects that the actual increased 

irrigation withdrawals had on the aquifer system. However, 

based on the calculations from the SWB model, that 

increase was estimated to be about 110 Mgal/d across the 

entire NACP aquifer system. In the case of the Delmarva 

geographic area, withdrawals for irrigation would have 

increased by almost 87 Mgal/d, which is about 77 percent 

of the total increase for the entire NACP aquifer system 

during this drought period (fig. 34B).

Groundwater levels are affected by droughts both from 

the decrease in recharging precipitation and from increased 

pumping to meet irrigation needs. Water levels in the surficial 

aquifer would be most affected by drought in the Delmarva 

geographic area because the unconfined, surficial aquifer is 

the most susceptible of all the aquifers in the NACP aquifer 

system to changes in recharge and nearly 75 percent of the 

increase in water withdrawn for irrigation in the Delmarva 

geographic area is from the surficial aquifer (fig. 37).

Figure 34. Distribution of the 

change as a percentage of the 

total in A, reduction in recharge 

and B, increase in irrigation 

withdrawals in response to a 

simulated 8-year drought for 

current [2013] conditions by 

geographic area across the 

Northern Atlantic Coastal Plain 

aquifer system. Geographic 

areas are shown in figure 12.

9

77

7

7

12

20

9 25

17

17

Total withdrawals: 110 million

gallons per day

Total recharge: 11,000 million

gallons per day

A. Reduction in recharge, 

 as percentage of total

B. Increase in irrigation withdrawals, 

 as percentage of total

North Carolina

Maryland

Virginia

Delmarva

New York

New Jersey

EXPLANATION

Geographic area

77 Percentage

 of total
62 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

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

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 to 2

>2 to 4

>4 to 6

>6 to 8

>8 to 10

>10 to 12

>12 to 16

>16 to 20

>20 to 30

>30 to 52

EXPLANATION

Difference between average 

recharge and 8-year drought 

recharge, in inches per year

Study area

pp_masterson_nacp_fig35_new

0 25 50 MILES

0 25 50 KILOMETERS

39°

38°

37°

76° 75° 74°

NEW

JERSEY

PENNSYLVANIA

MARYLAND

DELAWARE

Chesapeake Bay

Delaware Bay

ATLANTIC OCEAN

Figure 35. Difference between average recharge (1980–2010) and average drought recharge (1960–1967) in inches 

per year calculated by the Soil Water Balance model (Westenbroek and others, 2012) for the Delmarva geographic 

area of the Northern Atlantic Coastal Plain aquifer system. Geographic areas are shown in figure 12. >, greater than.
Simulation of Effects of Climate Change 63

Month and year

1960 1961 1962 1963 1964 1965 1966 1967

Precipitation, in inches

Drought (1960–1967)

Average (1980–2010)

EXPLANATION

April

May

June

July August

September

October

April

May

June

July August

September

October

April

May

June

July August

September

October

April

May

June

July August

September

October

April

May

June

July August

September

October

April

May

June

July August

September

October

April

May

June

July August

September

October

April

May

June

July August

September

October

12

10

8

6

4

2

0

pp_masterson_nacp_fig36_new

Other confined

Aquia

Surficial 

Aquifer

Rate, in million

 gallons per day

Piney Point

EXPLANATION

2.5

64.7

64.7

10.2

9.9

Figure 36. Distribution of monthly precipitation in inches for the growing season months (April to October) for the drought of 

1960–1967 and average conditions from 1980 to 2010 for the Delmarva geographic area of the Northern Atlantic Coastal Plain 

aquifer system. Geographic areas are shown in figure 12.

Figure 37. Increase in withdrawals 

for irrigation, in million gallons per day, 

in response to drought conditions in 

the Delmarva geographic area of the 

Northern Atlantic Coastal Plain aquifer 

system. Geographic areas are shown in 

figure 12.
64 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

The numerical model was used to predict the response 

of the aquifer system to the drought based on the decrease 

in aquifer recharge as calculated in the SWB model and the 

associated increase in irrigation water use. Groundwaterlevel decreases of 20 to 40 ft were calculated in the 

surficial aquifer for areas away from the coast and streams 

(fig. 38A). These water-level decreases can be attributed 

both to reductions in recharge and in increases in irrigation 

withdrawals needed to meet increased water demand for 

crops. The effects attributed solely to increased withdrawals 

can be determined by using the numerical model to simulate 

drought recharge conditions with nondrought withdrawals and 

then comparing the water-level change when the irrigation 

withdrawals were increased for drought conditions. An 

analysis of the additional water-level decreases in the surficial 

aquifer system attributed solely to increased withdrawals 

shows that, in the center of the Delmarva Peninsula where 

irrigation withdrawals are most concentrated, as much as 

20 percent of the model-calculated decreases are the result of 

increased pumping, but the reduction in water levels in the 

unconfined, surficial aquifer is mostly the result of decreased 

recharge (fig. 38B).

Study area

EXPLANATION

Water-level decrease, in feet

>0 to 5

>5 to 10

>10 to 15

0 >15 to 20

>20 to 40

>40 to 60

>60

EXPLANATION

Percentage of decrease

0 to 1

>1 to 2

>2 to 3

>3 to 5

>5 to 10

>10 to 15

>15 to 20

>20 to 25

>25 to 50

>50 to 100 

Study area

A B

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

Albers Equal-Area Conic projection

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

North American Datum of 1983

NEW

JERSEY PENNSYLVANIA

MARYLAND

DELAWARE

DELAWARE

Chesapeake Bay

Delaware Bay

ATLANTIC 

OCEAN

ATLANTIC 

OCEAN

76°

39°

75° 74°

38°

37°

NEW

JERSEY PENNSYLVANIA

MARYLAND

Chesapeake Bay

Delaware Bay

76° 75° 74°

39°

38°

37°

0 25 50 MILES

0 25 50 KILOMETERS

Figure 38. Simulated water-level response of the surficial aquifer in the Delmarva geographic area of the Northern Atlantic 

Coastal Plain aquifer system to drought conditions as A, decrease in water levels and B, percentage of water-level decreases 

attributed to increased withdrawals for irrigation. >, greater than.
Simulation of Effects of Climate Change 65

The Piney Point aquifer (fig. 6B) is the second most 

utilized aquifer in the Delmarva geographic area and 

it accounts for about 10 percent of the total irrigation 

withdrawals in the area (fig. 37). This aquifer is confined 

throughout its entire extent in the Delmarva geographic area 

and, therefore, is less susceptible to short-term changes in 

recharge than the unconfined surficial aquifer. The maximum 

drought water-level decrease of about 25 ft occurs in northern 

Delaware near the western extent of the Piney Point aquifer 

(fig. 39A). Because of the lack of hydrologic connection to 

recharge in the surficial aquifer, however, the water-level 

decreases in the Piney Point aquifer, unlike in the surficial 

aquifer, can be almost entirely (about 95 percent) attributed 

to increases in groundwater withdrawals (fig. 39B).

The results of the analysis in this report indicate that the 

effects of a long-term drought in the Delmarva geographic 

area depend on whether or not an aquifer is confined. In the 

unconfined, surficial aquifer, nearly all the water-level decreases 

can be attributed to a reduction in recharge, but in the confined, 

Piney Point aquifer, nearly all the water-level decreases are 

the result of increased irrigation withdrawals. These results 

suggest that water-levels may not be susceptible to the effects of 

droughts in the confined aquifers of the NACP aquifer system 

not used for irrigation, unlike in the unconfined surficial aquifer.

0 to 1

>1 to 5

>5 to 10

>10 to 15

>15 to 20

>20 to 25

>25 to 30

>30 to 35

>35 to 50

>50

EXPLANATION

Water-level decrease, in feet

Study area

Piney Point aquifer

0 to 20

>20 to 30

>30 to 40

>40 to 50

>50 to 60

>60 to 70

>70 to 80

>80 to 90

>90 to 95

>95 to 100

EXPLANATION

Percentage of decrease

Study area

Piney Point aquifer

A B

0 25 50 MILES

0 25 50 KILOMETERS

NEW

JERSEY

PENNSYLVANIA

MARYLAND

DELAWARE

Chesapeake Bay

Delaware Bay

76° 75° 74°

39°

38°

37°

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

Albers Equal-Area Conic projection

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

North American Datum of 1983

NEW

JERSEY

PENNSYLVANIA

MARYLAND

DELAWARE

Chesapeake Bay

Delaware Bay

ATLANTIC OCEAN

ATLANTIC OCEAN

76°

39°

75° 74°

38°

37°

Figure 39. Simulated water-level response of Piney Point aquifer to drought conditions as A, decreases in water levels and 

B, percentage of water-level decreases attributed to increased withdrawals for irrigation. >, greater than.
66 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

Effects of Sea-Level Rise on the Groundwater 

Flow System

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 coastal United States 

(Sallenger and others, 2012). In the lower Chesapeake Bay 

area, the anomalous rates of observed sea-level rise are 

attributed in part to the eustatic sea-level rise coupled with the 

response to glaciohydroisostatic adjustment from loading and 

unloading of the continental ice and water masses from the 

last glacial maximum (DeJong and others, 2015) and with land 

subsidence from the compaction of confining units caused by 

groundwater pumping (Pope and Burbey, 2004; Eggleston and 

Pope, 2013). These concerns are heightened because projected 

rates of sea-level rise may result in an increase of 3 to 5 ft 

in mean sea level during the next 100 years in parts of the 

northern Atlantic Coastal Plain province (Intergovernmental 

Panel on Climate Change, 2008).

Previous studies have shown that a change in sea-level 

position can affect groundwater flow in shallow, coastal aquifer 

systems, causing changes in groundwater levels and surfacewater discharge and increased potential for saltwater intrusion 

(Masterson and Garabedian, 2007; Werner and Simmons, 2009; 

Langevin and Zygnerski, 2013). The effects from changes to 

the flow system include groundwater inundation of subterranean infrastructure such as basements, septic systems and 

subway tunnels (Flood and Cahoon, 2011; Rotzoll and Fletcher, 

2012; Walter and others, 2016). The effects of changes to 

the flow system also have ecohydrological implications on 

groundwater dependent species in ecologically sensitive areas 

along the coast (Masterson and others, 2013a).

The potential effects of sea-level rise on groundwater 

flow in the NACP aquifer system were assessed by uniformly 

increasing the sea-level position by 3 ft for a 100-year simulation period and comparing the model results with a simulation 

in which the model boundary remained constant at the current 

[2013] sea-level position. The greatest effects of sea-level 

rise in the NACP aquifer system are changes in the height of 

the water table (fig. 40) and the redistribution of groundwater 

discharge to surface waters (fig. 41).

Groundwater levels rose from 0 to 3 ft in response to a 

3-ft simulated change in sea-level position, with the largest 

response occurring along the shoreline and away from 

nontidal streams (fig. 40). About 37 percent (or 10,000 m2 ) of 

the area of the northern Atlantic Coastal Plain physiographic 

province may experience about a 0.5-ft or more increase 

in water levels with the 3-ft increase in sea-level position, 

whereas about 18 percent (almost 5,000 m2) of land of the 

northern Atlantic Coastal Plain physiographic province may 

experience a 2-ft or more increase in water levels with the 

3-ft increase in sea-level position.

The New York and Delmarva geographic areas show the 

greatest water-level response to sea-level rise throughout the 

northern Atlantic Coastal Plain province. The response in these 

areas can be attributed to the fact that Long Island is completely 

surrounded by saline surface water and the Delmarva Peninsula 

is almost completely surrounded by saline surface water so the 

response on the peninsula is similar to that of an island. When 

the height of the saline surface water (or sea-level position) is 

increased, the water table will respond in unison except in the 

vicinity of nontidal rivers. The stage in tidal rivers was assumed 

to rise uniformly by the same rate of rise as the saline surface 

waters; therefore, extending the influence of the sea-level rise 

increased water level farther inland from the coast.

Increases in the height of the water table can be of 

particular concern in areas where the depth to water (vadose 

zone) is thin because of the risk of groundwater inundation of 

subterranean structures such as building basements, domestic 

septic systems, and subway systems such in those on western 

Long Island. 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. 1). 

High rates of withdrawals resulted in large drawdowns of 

the water table in the underlying aquifers in these areas. The 

development and completion of sanitary sewer systems in 

the mid-1930s 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 decreased by as much as 45 ft 

from the levels in 1903, and by 1951, water levels in southern 

Queens County had decreased by as much as 35 ft from the 

levels in1903 (Buxton and Shernoff, 1999).

The decreases 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 production 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, and the rising water levels in 

some areas ultimately began to flood subterranean structures 

that had been built when the water table had been 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 

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). The additional increase in water-table height 

from sea-level rise only will exacerbate this condition over 

time and require additional, costly remediation of the excess 

groundwater flooding.
Simulation of Effects of Climate Change 67

masterson_nacp_fig40_new

72° 75° 78°

39°

36°

Study area

EXPLANATION

Water-level change, in feet 

0 to 0.1

>0.1 to 1

>1 to 2

>2 to 3

Stream

Tidal river

Chesapeake Bay

Delaware Bay

Long Island Sound

NEW CT

YORK

NEW

JERSEY

PENNSYLVANIA

MARYLAND

DC

DELAWARE

VIRGINIA

NORTH CAROLINA

ATLANTIC OCEAN

Pamlico Sound

Albemarle Sound

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

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 50 100 MILES

0 50 100 KILOMETERS

Surface-water cells in model

WV

Figure 40. Model-simulated increase in water levels in the surficial aquifer in response to a 3-foot increase in the 

current [2013] sea-level position across the Northern Atlantic Coastal Plain aquifer system. >, greater than.
68 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

Another effect of sea-level rise on the groundwater 

flow system is the redistribution of groundwater discharge to 

surface waters. As water levels rise in response to sea-level 

rise, the hydraulic gradients to streams increase thus increasing 

groundwater discharge to streams. The additional groundwater 

flow to streams is water that otherwise would have discharged 

directly to coastal waters, but instead is diverted to streams 

and ultimately reaches the coast as surface-water discharge.

The total increase in groundwater discharge to streams 

(and corresponding decrease in coastal discharge) in the 

NACP aquifer system is about 115 Mgal/d (about 177 cubic 

feet per second [ft3/s]; fig. 41A). This increase is about 2 percent 

of the almost 6,000 Mgal/d (9,270 ft3/s) total streamflow in the 

NACP aquifer system in 2013 (fig. 41B). The distribution of 

this increase in groundwater discharge to streams varies across 

the geographic areas, with a low of about 6 percent in the 

Maryland area to a high of about 35 percent in the Delmarva 

area. Groundwater discharge to streams in the Delmarva area 

is calculated to increase by about 40 Mgal/d (60 ft3/s, or about 

5 percent) from the streamflow under current [2013] conditions 

of about 820 Mgal/d (1,280 ft3/s). By comparison, the 

New York geographic area shows a small increase in streamflow 

(about 14 Mgal/d, or 22 ft3/s), yet this change represents about 

a 12 percent increase from the streamflow under current [2013] 

conditions (about 140 Mgal/d, or 211 ft3/s).

Although the increases in groundwater discharge to 

streams (and corresponding decreases in coastal discharge) 

calculated for these geographic areas represent only a small 

increase from above current [2013] conditions, locally, this 

redistribution of groundwater discharge from the coast to 

streams can alter the delivery of freshwater input to coastal 

receiving waters and have ecohydrological implications 

on the sensitive ecosystems which rely on a balance of 

groundwater discharge and surface-water flow (Masterson 

and others, 2013a).

Use of Numerical Models to Inform 

Groundwater Monitoring Networks

Groundwater-level monitoring networks provide resource 

managers the ability to examine the current status and longterm trends in the hydrologic system. The USGS maintains 

a network of wells to monitor the effects of droughts and 

other climate variability on groundwater levels as part of the 

USGS Climate Response Network (http://groundwaterwatch.

usgs.gov/Net/OGWNetwork.asp?ncd=crn). This network 

was designed such that water levels measured in the selected 

monitoring wells should primarily reflect climatic variability 

and not human influences; however, determining the extent 

to which water levels are responding to changes in climate 

compared to anthropogenic stressors can be very difficult 

without the aid of a numerical model.

Numerical models are often developed to predict aquifer 

system response to anthropogenic and environmental stressors 

and therefore can be very valuable tools to design monitor 

networks established to observe and characterize the aquifer 

system response to such stressors. The groundwater model 

for the NACP aquifer system was used to demonstrate the 

extent to which climate variability can affect the deep confined 

Potomac-Patapsco aquifer. The Potomac-Patapsco aquifer is 

one of the deepest and yet one of the most utilized aquifers 

in the NACP aquifer system (figs. 6 and 16). The top of the 

aquifer may be as deep as 5,000 ft below sea level along 

the coast, but it crops out or subcrops at several hundred 

feet above sea level along the Fall Zone at the westernmost 

extent of the NACP aquifer system (Pope and others, 2016). 

It is along the Fall Zone where the overlying confining units 

become discontinuous and this portion of the PotomacPatapsco aquifer may be locally unconfined and susceptible 

to climatic effects on water levels.

masterson_nacp_fig41_new

1,200

(20%)

140

(2%)

1,690

(28%)

460

(8%)

1,680

(28%) 820

(14%)

15

(13%)

7

(6%)

26

(22%)

40

(35%)

13

(12%)

14

(12%)

460

(8%)

115 million gallons per day 5,990 million gallons per day

A B

Rate, in million 

 gallons per day

North Carolina

Maryland

Virginia

Delmarva

New York

New Jersey

EXPLANATION

Geographic area

Percentage of total

Figure 41. Distribution of simulated A, increase in groundwater discharge to streams in response to sea-level rise 

and B, net groundwater discharge to streams in 2013 across the Northern Atlantic Coastal Plain aquifer system. 

Geographic areas are shown in figure 12.
Challenges for Future Groundwater Availability Assessments—Lessons Learned 69

A numerical analysis was conducted to determine the 

extent to which the Potomac-Patapsco aquifer may be susceptible to climatic variability and thus can be used to inform 

monitoring network placement. The average recharge rate for 

the 1986–2008 was simulated in this analysis for the entire 

23-year period and the model-calculated water levels (heads) 

then were compared with heads in which the recharge rate was 

varied annually for the same period. Because the groundwater 

withdrawals were the same in both cases, the calculated head 

differences between these two simulations can be attributed 

solely to the change in recharge.

The variance in heads calculated for this 23-year period 

was used to assess which areas in the Potomac-Patapsco 

aquifer have the potential to be susceptible to climatic 

variability (fig. 42). The regions with the greatest calculated 

head variance are along the Fall Zone in Maryland, Delaware, 

and New Jersey. Head changes calculated for the PotomacPatapsco on Long Island (locally referred to as the Lloyd 

aquifer) also are affected by changes in aquifer recharge 

and may be the result of the leaky confining unit (Raritan) 

separating the Lloyd from the overlying unconfined (Magothy) 

aquifer. Monitoring wells selected in these areas would have 

greater potential to measure water levels that reflect climatic 

variability than in areas with lower head variance (shown in 

green) away from the Fall Zone.

Challenges for Future Groundwater 

Availability Assessments—

Lessons Learned

Regional assessments of groundwater availability of 

major aquifers across the Nation, such as the one described 

in this report for the NACP aquifer system, benefit greatly 

from the information and knowledge developed from previous 

investigations at both the regional and local scales. The 

regional assessment from the 1980s (Trapp and Meisler, 1992) 

and the numerous subregional investigations since the last 

regional assessment provided the foundational knowledge 

on water use, hydrogeologic framework, and hydrologic 

boundaries in the NACP aquifer system; however, many 

challenges still exist as advancement to this knowledge base 

continues into the future. Some of the main challenges include 

the quality and availability of hydrologic data, model discretization of space and time, and the balance between regional 

and local expectations.

The successful execution of all quantitative assessments, 

whether regional or local, depends in large part on the quality 

and availability of basic hydrologic data in the study area. The 

primary hydrologic data types include water use, monitoring, 

and hydrogeologic information. Issues with these data often 

are magnified for regional studies given the scale of these 

efforts and the time needed to synthesize and compile the 

necessary information.

The availability of accurate and complete groundwater 

withdrawal data is needed for a comprehensive assessment 

of groundwater availability because the response of the 

aquifer system to changing stresses can only be properly 

evaluated if those stresses are correctly quantified and applied. 

Therefore, withdrawals from wells must be reported and 

recorded correctly and completely, along with information on 

well screen depths and aquifers they intersect. Information 

on groundwater withdrawal and use initially available at the 

regional scale for this study was from existing databases 

supported by NWIS (U.S. Geological Survey, 2011). This 

database includes detailed withdrawal information aggregated to the county and watershed scale that is adequate 

to support compilations like those provided every 5 years 

by the USGS National Water-Use Information Program 

(http://water.usgs.gov/watuse/wunwup.html). However, a database of site-specific withdrawal information with the spatial 

and attribute detail necessary to properly represent stresses 

in a numerical model is not available. Such information was 

compiled specifically for this study from various sources, with 

variable levels of quality and completeness. For areas or times 

for which such information was unreliable or unattainable, 

the accuracy of the numerical simulations of the groundwater 

system was affected accordingly.

Similar concerns can be raised for the monitoring or 

observation data of groundwater levels and streamflows. These 

data also are stored in the NWIS database and the accuracy 

of the measurements and the reported spatial and vertical 

locations of these observations are critical to correctly understanding how the physical system is responding to hydrologic 

stresses, which is information critical to determining how well 

the numerical representation of the system matches reality. 

Therefore, the importance of basic data of high quality on 

water use and monitoring cannot be overstated for water 

availability studies.

Hydrogeologic information (such as aquifer-test results, 

lithologic borings, and geophysical logs and the hydrostratigraphic surfaces that are developed from this information) also 

is necessary to understanding the distribution and movement 

of groundwater flow in the subsurface. The analysis of the 

hydrologic conditions in the NACP aquifer system relied 

heavily on the hydrostratigraphic extents and elevations 

that were developed initially as part of the previous regional 

assessment and subsequently updated locally by more 

site-specific studies. This information is often difficult and 

expensive to obtain but is key to developing a conceptual 

understanding of the aquifer framework that is then applied to 

the numerical model used in groundwater availability assessments. Missing or incomplete hydrogeologic information can 

increase uncertainty in the accuracy of the study results.

Discretizing space and time also can be a challenge for 

regional groundwater availability studies. Advancements in 

groundwater modeling software and computing capabilities 

have allowed for the synthesis of the available hydrologic 

information into a much finer discretized and detailed numerical representation of the groundwater flow system than was 
70 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

Study area

EXPLANATION

Potomac-Patapsco aquifer

Variance in water-level 

change, in feet squared

0 to 0.1

>0.1 to 0.5

>0.5 to 1.0

>1.0 to 2.0

>2.0 to 3.0

>3.0 to 4.0

>4.0 to 5.0

>5.0 to 20.6

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

Albers Equal-Area Conic projection

Standard parallels 34°30' N and 41°30' N

Central meridian 75°30' W 

North American Datum of 1983

78° 72°

39°

36°

Chesapeake Bay

Delaware Bay

Long Island Sound

NEW CT

YORK

NEW

JERSEY

PENNSYLVANIA

MARYLAND

DC DELAWARE

VIRGINIA

NORTH CAROLINA

ATLANTIC OCEAN

Pamlico Sound

Albemarle 

Sound

0 25 50 MILES

0 25 50 KILOMETERS

masterson_nacp_fig42_new

WV

75°

Figure 42. Variance of water-level change from the difference between annually varying and average annual 

recharge from 1986 to 2008 for the Potomac-Patapsco aquifer of the Northern Atlantic Coastal Plain aquifer system. 

>, greater than.
Acknowledgments 71

possible during the original RASA study for the NACP aquifer 

system. However, limitations inherent in regional assessments 

still pose challenges for these studies; for instance, even if the 

basic hydrologic data are of good quality and readily available 

and much is known of the detailed hydrogeologic framework, 

this information needs to be represented numerically in a 

groundwater flow model. The numerical representation of this 

hydrologic information is completely dependent upon how 

the aquifer system and stresses are subdivided (or discretized) 

in space and time, and, is dependent in large part on what 

software and computing capabilities are available at the time 

of the analysis. Lumping hydraulic parameters in space and 

pumping stresses and time-varying observational data in time 

can limit the capabilities of the tools developed to simulate 

how the groundwater flow system responds to changes in 

hydrologic stresses and can make accurate representation of 

the actual flow system more difficult, particularly at the local 

scale. Therefore, development of the tools to conduct water 

availability analyses needs to consider the scale of the problem 

to be addressed and the limitations associated with scale.

An additional challenge of groundwater availability 

studies is balancing regional and local expectations. For 

instance, regional tools that span multiple States are not the 

appropriate choice to assess the potential effect of a particular 

pumping well on a nearby surface-water body and, conversely, 

a detailed, site-specific numerical model developed to localscale groundwater and surface-water interactions does not 

provide guidance on the response of groundwater resources 

to global changes in climate on a regional scale. However, 

often times it is the site-specific issues that are of greatest 

interest locally, making it more challenging for the reader to 

understand the benefits of large, regional-scale groundwater 

availability assessments and for the authors to associate the 

benefits of large, regional-scale groundwater availability 

studies to the local-level concerns.

Regional-scale studies, such as the one described in this 

report, are not designed to address local-scale issues. The 

modeling tools developed to address local-scale issues often 

times have problems with artificial boundaries used in models 

with limited aerial extent, which may have unacceptable 

influence on results. Therefore, regional-scale analyses can 

provide the hydrogeologic framework and boundaries that the 

local-scale efforts can build upon.

In the case of the NACP aquifer system, the water-level 

response in the deep confined Potomac aquifer group to 

large-scale multistate pumping extends well beyond individual 

State lines. The numerical model developed for the analysis 

of regional groundwater flow in the NACP aquifer system 

provided the tool necessary to fully characterize the extent of 

this water-level decrease and quantify the change in system-wide 

hydrologic budget components and can serve as the foundation 

for small, fine-scale models needed to fully assess the effects 

and ramifications of these regional stresses on a local level.

All models, whether regional or local in scale, are at 

best oversimplifications of complex natural systems but 

can provide useful information in light of their limitations. 

Therefore, it is important to understand how these tools can be 

used to further the science and improve the understanding of 

the groundwater availability for the Nation.

Acknowledgments

The authors wish to thank the following individuals for 

their assistance and guidance throughout this investigation:

Kevin Dennehy (retired), Tom Reilly (emeritus), Arlen Harbaugh

(retired), Rod Sheets, Chris Langevin, and Joe Hughes of the 

USGS. The authors also would like to thank Jack Eggleston, 

Brian Clark, and Tom Mack of the USGS for providing 

technical reviews for this report.
72 Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System

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Manuscript approved May 31, 2016

Prepared by the USGS Science Publishing Network

Edited by Anna Glover and Kay P. Naugle

Pembroke and Reston Publishing Service Centers

Illustrations by James E. Banton

Lafayette Publishing Service Center

Layout by Caryl J. Wipperfurth

Reston Publishing Service Center

For more information concerning this report, 

please contact:

Water Availability and Use Science Program

U.S. Geological Survey

150 National Center

12201 Sunrise Valley Drive

Reston, VA 20192

http://water.usgs.gov/wausp/
Masterson and others—Assessment of Groundwater Availability in the Northern Atlantic Coastal Plain Aquifer System—Professional Paper 1829

ISSN 1044-9612 (print)

ISSN 2330-7102 (online)

http://dx.doi.org/10.3133/pp1829