U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the Virginia Department of Environmental Quality and

the Hampton Roads Planning District Commission

Groundwater-Quality Data and Regional Trends in the

Virginia Coastal Plain, 1906–2007

Professional Paper 1772
Cover.  Groundwater-quality samples being collected from research observation wells at Fentress, Virginia, during 2006. Truck-mounted 

submersible pumping system is being operated by a U.S. Geological Survey (USGS) hydrologic technician. Second vehicle is 

equipped for sample processing. Well being pumped and two wells to the left are among seven colocated wells completed in 

different aquifers. The wells constitute a groundwater-research station established by the Virginia Department of Environmental 

Quality (DEQ). The research station is part of a network of wells for monitoring saltwater intrusion in the Virginia Coastal Plain, 

from which groundwater samples are periodically collected and analyzed by the USGS in cooperation with DEQ and the 

Hampton Roads Planning District Commission. (photograph by Roger M. Moberg, Hydrologic Technician, USGS)
Groundwater-Quality Data and Regional 

Trends in the Virginia Coastal Plain, 

1906–2007

By E. Randolph McFarland

Prepared in cooperation with the Virginia Department of Environmental Quality and the 

Hampton Roads Planning District Commission

Professional Paper 1772

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, 

natural hazards, and the environment, visit http://www.usgs.gov or call 1-888-ASK-USGS

For an overview of USGS information products, including maps, imagery, and publications, 

visit http://www.usgs.gov/pubprod

To order this and other USGS information products, visit http://store.usgs.gov

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

McFarland, E.R., 2010, Groundwater-quality data and regional trends in the Virginia Coastal Plain, 1906–2007: 

U.S. Geological Survey Professional Paper 1772, 86 p., 14 pls.

(available online at http://pubs.usgs.gov/pp/1772/)

Library of Congress Cataloging-in-Publication Data

McFarland, E. Randolph.

 Groundwater-quality data and regional trends in the Virginia coastal plain,

1906-2007 / by E. Randolph McFarland ; prepared in cooperation with the

Virginia Department of Environmental Quality and the Hampton Roads Planning

District Commission. -- lst ed.

 p. cm. -- (Professional paper ; 1772)

 Includes bibliographical references.

 ISBN 978-1-4113-2680-4

 1. Groundwater--Quality--Virginia--History. 2. Hydrogeology--Virginia--History. 3. Water

chemistry--Virginia--History. I. Virginia. Dept. of Environmental Quality. II. Hampton Roads 

Planning District Commission (Va.) III. Geological Survey (U.S.) IV. Title. V. Series: U.S. 

Geological Survey professional paper ; 1772.

 GB1025.V8M33 2010

 551.4909755--dc22

 2010009310
iii

Contents

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

Introduction.....................................................................................................................................................2

Purpose and Scope ..............................................................................................................................4

Description of the Study Area ............................................................................................................4

Geologic Setting...........................................................................................................................5

Groundwater Conditions.............................................................................................................5

Methods of Investigation.....................................................................................................................7

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

Groundwater-Quality Data ...........................................................................................................................9

Data Quality............................................................................................................................................9

Data Distribution .................................................................................................................................10

Data Use and Limitations...................................................................................................................13

Regional Trends in Groundwater Quality .................................................................................................13

Major Ions............................................................................................................................................14

Spatial Distribution ....................................................................................................................14

Hydrochemical Processes .......................................................................................................15

Saltwater-Transition Zone.................................................................................................................17

Configuration ..............................................................................................................................18

Chloride-Concentration Sections...................................................................................18

Chloride Iso-Concentration Surfaces............................................................................19

Aquifer Chloride Concentrations....................................................................................25

Relations to Groundwater Flow...............................................................................................31

Emplacement of Seawater..............................................................................................32

Freshwater Flushing.........................................................................................................32

Brine and the Inner Chesapeake Bay Impact Crater..................................................33

Movement ...................................................................................................................................34

Rates of Change in Chloride Concentration .................................................................36

Patterns of Change in Chloride Concentration ............................................................42

Intrusion and Upconing ...................................................................................................45

Monitoring..........................................................................................................................46

Selected Constituents........................................................................................................................48

Fluoride........................................................................................................................................48

Iron and Manganese.................................................................................................................51

Nitrate and Ammonium.............................................................................................................54

Considerations for Groundwater Use..............................................................................................58

Hardness .....................................................................................................................................58

Chloride........................................................................................................................................59

Selected Constituents...............................................................................................................60

Summary and Conclusions.........................................................................................................................61

Acknowledgments.......................................................................................................................................62

References Cited..........................................................................................................................................62

Attachment 1. – Chemical-constituent values and related data ......................................CD in pocket

Attachment 2. – Summary of groundwater-quality constituent values ..............................................67
iv

Plates (In envelope)

1–3. Maps showing locations of sampled groundwater wells and boreholes in:

1. The northern part of the Virginia Coastal Plain

2. The southern part of the Virginia Coastal Plain

3. The Virginia Eastern Shore

4–9. Maps showing the major ion composition of groundwater in:

4. The surficial aquifer

5. The Yorktown-Eastover aquifer

6. The Piney Point aquifer

7. The Aquia aquifer

8. The Potomac aquifer

9. Basement bedrock

10–14. Dissolved chloride-concentration sections:

10. DD–DD’ and ED–ED’

11. FD–FD’ and GD–GD’

12. ID–ID’ and JD–JD’

13. DS–DS’ and ES–ES’

14. FS–FS’ and GS–GS’

Figures

1. Map showing groundwater-sample locations and chloride-concentration 

sections in the Virginia Coastal Plain........................................................................................3

2. Generalized hydrogeologic section and directions of groundwater flow...........................6

3. Diagram of stratigraphic relations among hydrogeologic units ...........................................7

4–8. Maps showing approximate altitude and configuration of:

4. The 50 milligram per liter chloride iso-concentration surface...................................20

5. The 250 milligram per liter chloride iso-concentration surface.................................21

6. The 1,000 milligram per liter chloride iso-concentration surface..............................22

7. The 10,000 milligram per liter chloride iso-concentration surface............................23

8. The 19,000 milligram per liter chloride iso-concentration surface............................24

9–13. Maps showing groundwater chloride concentrations in:

9. The Yorktown-Eastover aquifer.......................................................................................26

10. The Piney Point aquifer.....................................................................................................27

11. The Aquia aquifer ..............................................................................................................28

12. The Virginia Beach aquifer ..............................................................................................29

13. The Potomac aquifer.........................................................................................................30

14. Graphs showing rates of change in concentrations of chloride in groundwater 

and yearly averaged rates of groundwater withdrawal ......................................................35

15–19. Maps showing rates of change in concentrations of chloride in groundwater in:

15. The surficial aquifer ..........................................................................................................37

16. The Yorktown-Eastover aquifer.......................................................................................38

17. The Piney Point aquifer.....................................................................................................39

18. The Aquia aquifer ..............................................................................................................40

19. The Potomac aquifer.........................................................................................................41
v

20–21. Graphs showing concentrations of chloride in groundwater and yearly averaged 

rates of groundwater withdrawal in:

20. Well 60C 61 at Portsmouth................................................................................................43

21. Selected wells on the central York-James Peninsula.................................................44

22–26. Maps and generalized hydrogeologic sections showing distributions 

in groundwater of:

22. Fluoride................................................................................................................................49

23. Iron .......................................................................................................................................52

24. Manganese.........................................................................................................................53

25. Nitrate nitrogen..................................................................................................................56

26. Ammonium nitrogen ..........................................................................................................57

Tables

1. Numbers of sampled wells, water-quality samples, 

and chemical-constituent values.............................................................................................11

2. Summary of sequentially collected groundwater samples .................................................36

3. Municipalities operating brackish groundwater-development facilities..........................59
vi

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2

) 259.0 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L)

Flow rate

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

/s)

SI to Inch/Pound

Multiply By To obtain

Volume

liter (L) 0.2642 gallon (gal)

Mass

milligram (mg) 0.00003527 ounce (oz)

Concentration

milligram per liter (mg/L) 0.0001335 ounce per gallon (oz/gal)

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

°F = (1.8 × °C) + 32

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

(NGVD 29).

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).

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

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius 

(μS/cm at 25 °C).

Concentrations of chemical constituents in water are given in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).
Abstract

A newly developed regional perspective of the hydrogeology of the Virginia Coastal Plain incorporates updated 

information on groundwater quality in the area. Local-scale 

groundwater-quality information is provided by a comprehensive dataset compiled from multiple Federal and State agency 

databases. Groundwater-sample chemical-constituent values 

and related data are presented in tables, summaries, location 

maps, and discussions of data quality and limitations.

Spatial trends in groundwater quality and related 

processes at the regional scale are determined from interpretive analyses of the sample data. Major ions that dominate 

the chemical composition of groundwater in the deep Piney 

Point, Aquia, and Potomac aquifers evolve eastward and 

with depth from (1) “hard” water, dominated by calcium and 

magnesium cations and bicarbonate and carbonate anions, to 

(2) “soft” water, dominated by sodium and potassium cations 

and bicarbonate and carbonate anions, and lastly to (3) “salty” 

water, dominated by sodium and potassium cations and 

chloride anions. Chemical weathering of subsurface sediments 

is followed by ion exchange by clay and glauconite, and 

subsequently by mixing with seawater along the saltwatertransition zone. The chemical composition of groundwater in 

the shallower surficial and Yorktown-Eastover aquifers, and in 

basement bedrock along the Fall Zone, is more variable as a 

result of short flow paths between closely located recharge and 

discharge areas and possibly some solutes originating from 

human sources.

The saltwater-transition zone is generally broad and 

landward-dipping, based on groundwater chloride concentrations that increase eastward and with depth. The configuration 

is convoluted across the Chesapeake Bay impact crater, 

however, where it is warped and mounded along zones having 

vertically inverted chloride concentrations that decrease 

with depth. Fresh groundwater has flushed seawater from 

subsurface sediments preferentially around the impact crater 

as a result of broad contrasts between sediment permeabilities. 

Paths of differential flushing are also focused along the 

inverted zones, which follow stratigraphic and structural 

trends southeastward into North Carolina and northeastward 

beneath the chloride mound across the outer impact crater. 

Brine within the inner impact crater has probably remained 

unflushed. Regional movement of the saltwater-transition zone 

takes place over geologic time scales. Localized movement 

has been induced by groundwater withdrawal, mostly along 

shallow parts of the saltwater-transition zone. Short-term 

episodic withdrawals result in repeated cycles of upconing and 

downconing of saltwater, which are superimposed on longerterm lateral saltwater intrusion. Effective monitoring for 

saltwater intrusion needs to address multiple and complexly 

distributed areas of potential intrusion that vary over time.

A broad belt of large groundwater fluoride concentrations 

underlies the city of Suffolk, and thins and tapers northward. 

Fluoride in groundwater probably originates by desorbtion 

from phosphatic sedimentary material. The high fluoride belt 

possibly was formed by initial adsorbtion of fluoride onto 

sediment oxyhydroxides, followed by desorbtion along the 

leading edge of the advancing saltwater-transition zone.

Large groundwater iron and manganese concentrations 

are most common to the west along the Fall Zone, across 

part of the saltwater-transition zone and eastward, and within 

shallow groundwater far to the east. Iron and manganese 

initially produced by mineral dissolution along the Fall Zone 

are adsorbed eastward and with depth by clay and glauconite, 

and subsequently desorbed along the leading edge of the 

advancing saltwater-transition zone. Iron and manganese in 

shallow groundwater far to the east are produced by reaction 

of sediment organic matter with oxyhydroxides.

Large groundwater nitrate and ammonium concentrations 

are mostly limited to shallow depths. Most nitrate and ammonium is recycled near the land surface, but some can also be 

produced by decomposition of sediment organic matter, by ion 

exchange along the leading edge of the advancing saltwatertransition zone, and possibly from site-specific human sources.

Groundwater is a heavily used water-supply resource 

throughout the study area, and its suitability for various uses 

is largely determined by its chemical quality. Whereas soft 

water in deep aquifers generally is suitable for a wide variety 

of uses, hard water can require ion-exchange treatment. Salty 

water in deep aquifers in the eastern part of the study area is 

undergoing increasingly widespread production for drinkingwater supplies. Treatment operations use desalination and 

blending. Saltwater intrusion can potentially be mitigated by 

Groundwater-Quality Data and Regional Trends in the 

Virginia Coastal Plain, 1906–2007

By E. Randolph McFarland
2 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

redirecting locations and operation of wells producing shallow 

freshwater and by conjunctive freshwater/saltwater groundwater development close to and within the saltwater-transition 

zone. Groundwater with large fluoride concentrations is 

blended with water from different sources in the city of Suffolk. An array of methods can treat large groundwater iron and 

manganese concentrations, and associated hydrogen sulfide 

concentrations, depending on the level of treatment needed. 

Large groundwater nitrate concentrations can potentially be 

mitigated by various treatment methods or management of 

groundwater withdrawals.

Introduction

Groundwater in the Atlantic Coastal Plain Physiographic 

Province in eastern Virginia (fig. 1) is a heavily used resource. 

During the late 1800s, the rate of groundwater withdrawal is 

estimated to have been close to zero, but the withdrawal rate 

has increased continuously during the 20th century (Harsh 

and Laczniak, 1990). During 1992, major groundwater users 

regulated by the Virginia Department of Environmental 

Quality (DEQ) reported withdrawal rates totaling approximately 94 million gallons per day (Mgal/d) from Coastal 

Plain aquifers (Hammond and Focazio, 1995); additional 

unregulated withdrawals at that time were unknown. By 2000, 

the combined regulated and estimated unregulated withdrawals totaled 137.4 Mgal/d (Pope and others, 2008).

As a result of long-term withdrawals, groundwater levels 

in the Coastal Plain aquifers have declined by as much as 

200 feet (ft) near large withdrawal centers, and flow gradients 

have been reversed from a previously seaward direction to a 

landward direction (Harsh and Laczniak, 1990), creating the 

potential for saltwater intrusion. Increasing withdrawals are 

projected, which could result in further water-level declines 

and intrusion potential, thereby limiting continued use of the 

groundwater resource.

To manage the groundwater resource, the DEQ regulates 

groundwater withdrawals in the most heavily used parts of 

the Virginia Coastal Plain, designated as Groundwater 

Management Areas, which include areas south of the York 

and Mattaponi Rivers, and the Virginia Eastern Shore. 

Withdrawals greater than 300,000 gallons per month in 

these areas must be approved under the DEQ Groundwater 

Withdrawal Permit Program, which requires groundwater 

users to submit withdrawal-related information that is needed 

to evaluate the potential effects of the withdrawals on the 

groundwater system.

The DEQ relies on a sound scientific understanding of 

Virginia Coastal Plain geology and hydrology to provide a 

valid context within which to make groundwater-management 

decisions. The U.S. Geological Survey (USGS) has been 

advancing knowledge of the geology and hydrology of the 

Virginia Coastal Plain since the beginning of the 20th century. 

A widely recognized description of the hydrogeology of the 

Virginia Coastal Plain resulted from the USGS Regional 

Aquifer-System Analysis (RASA) and related investigations 

completed during the 1980s. A hydrogeologic framework was 

developed by Meng and Harsh (1988) to provide a basis for 

construction of a digital computer model of groundwater flow 

(Harsh and Laczniak, 1990). The RASA framework and model 

were updated and adopted by the DEQ as an aid in managing 

the groundwater resource, by evaluating the potential effects 

of existing and proposed withdrawals (McFarland, 1998). 

Among subsequent followup investigations, the chemical 

quality of groundwater was summarized based on the RASA 

framework (Focazio and others, 1993).

Following the RASA investigation and related efforts, 

additional findings provided a basis for major changes in 

the definition of hydrogeologic conditions, with the most 

significant being discovery of the world’s sixth largest meteorimpact crater beneath the lower Chesapeake Bay (Powars and 

Bruce, 1999; Powars, 2000). Knowledge of the impact crater 

and other recently recognized geologic features resulted in the 

need for a complete redefinition of the groundwater system. 

Additionally, locations and rates of groundwater withdrawals 

increased following the RASA investigation and related 

efforts, which further complicated understanding of the 

flow system. 

In order for resource-management efforts to adequately 

address these developments, a comprehensive effort was 

begun in 2000 by the USGS, in cooperation with the DEQ and 

the Hampton Roads Planning District Commission (HRPDC), 

to develop a new, regional perspective of the hydrogeology 

of the Virginia Coastal Plain that reflects current groundwater 

conditions and incorporates updated knowledge of the geology 

and hydrology of the area. During 2000–2004, a refined 

hydrogeologic framework was formulated that provides an 

enhanced level of detail on the geologic relations, configuration, composition, and hydrologic aspects of a stratified series 

of 19 aquifers, confining units, and confining zones (collectively termed hydrogeologic units) (McFarland and Bruce, 

2006). In turn, these findings were applied to development 

of a revised digital computer model of the groundwater-flow 

system that is consistent with present-day (2008) hydrologic 

conditions and can be used to project future conditions 

(Heywood and Pope, 2009).

The newly revised framework and model address many 

aspects of the presence and movement of groundwater in the 

Virginia Coastal Plain. Knowledge of the chemical quality of 

groundwater is equally critical to sound management of the 

resource, but was not incorporated into the framework and 

model. Concentrations of various chemical constituents should 

be known in order to effectively plan for possible limitations 

and treatment requirements for groundwater use. Groundwater 

quality in the Virginia Coastal Plain, however, is more variable 

than in any other part of the State (Heller, 2008). Moreover, 

groundwater chemical-constituent data are most useful when 

individual sample values are accurately associated with the 

hydrogeologic unit from which each sample was collected. 

The most recent detailed characterization of groundwater 
Introduction 3

Figure 1. Groundwater-sample locations and chloride-concentration sections in the Virginia Coastal Plain. Location of Chesapeake 

Bay impact crater from Powars and Bruce (1999). Further detail shown on plates 1–3.

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

EXPLANATION

Groundwater-sample location

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

DD

ED

FD

GD

ID

JD DS

ES

FS

GS JD’

ID’

GD’

FD’

ED’

DD’

GS’

ES’

DS’

GD GD’ Line of chloride-concentration section

Figure 1. Locations of Chesapeake Bay impact crater (from Powars and Bruce, 1999), groundwater samples, and chloride-concentration

 sections in the Virginia Coastal Plain. (Further details are shown on plates 1–3.)

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

Piedmont

River

FS’

Fall Zone

Piedmont

Fall Zone

LOCATION OF THE STUDY AREA

IN VIRGINIA

Plate 1

Plate 3

Plate 2

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4 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

quality in the Virginia Coastal Plain was based on the RASA 

framework (Focazio and others, 1993), which has since 

been substantively revised to incorporate the impact crater 

and other geologic features. Hence, application of the newly 

revised hydrogeologic framework is required to understand 

the groundwater quality of the Virginia Coastal Plain in the 

context of current knowledge of the aquifer system.

Knowledge of the chemical quality of groundwater 

supports resource-management efforts at two distinct levels. 

First, well drillers and owners, environmental consultants, 

and water-resource managers have short-term, practical 

needs for local-scale information on groundwater quality in 

making case-by-case estimates of constituent concentrations 

for the development and operation of water-supply wells, 

waste-management facilities, and other groundwater related 

activities. Second, informed long-term decisions by planners 

and policy makers are contingent upon a regional understanding of groundwater-quality trends and related processes. 

Toward this end, a perspective that encompasses the entire 

Virginia Coastal Plain as an integrated hydrochemical system 

can be provided by a synthesis of groundwater constituent 

concentrations and related data. Certain constituents represent 

resource-management issues of particular interest and thereby 

warrant in-depth interpretive analysis, such as concentrations 

of chloride associated with areas of brackish groundwater 

along the saltwater-transition zone.

To address these needs, an investigation was undertaken 

during 2006–08 by the USGS, in cooperation with the DEQ 

and the HRPDC, which incorporates updated knowledge of 

groundwater quality into the newly developed regional perspective of the hydrogeology of the Virginia Coastal Plain. An 

expanded and comprehensive set of groundwater constituent 

values and related data was compiled from multiple Federal 

and State agency databases. Associations of samples with 

hydrogeologic units were determined based on the revised 

hydrogeologic framework. Chemical-constituent values and 

related data were summarized and interpreted to serve both 

local and regional scale resource-management needs.

Purpose and Scope

The chemical quality of groundwater in the Virginia 

Coastal Plain is documented herein. Data retrieved from 

multiple Federal and State agency databases represent 

10,165 groundwater samples collected from 1,991 wells 

and boreholes during 1906 to 2007. Analytical results are 

presented for 37 chemical constituents, for which 73,610 individual sample constituent values are reported. Constituent 

values are electronically tabulated, along with corresponding 

well and borehole identification and location information, 

collection date and time, collection-interval altitude, and 

associations among hydrogeologic units. Sample locations and 

hydrogeologic unit associations are also presented on maps.

This report serves as a companion document to a newly 

revised hydrogeologic framework of the Virginia Coastal Plain 

(McFarland and Bruce, 2006; available at http://pubs.usgs.

gov/pp/2006/1731/). The hydrogeologic framework provides 

the basis for hydrogeologic associations among the groundwater samples. Moreover, the two reports are designed to be 

used together, such that groundwater quality can be directly 

integrated with other hydrologic and geologic information 

needed for sound management of the resource.

This report is organized to convey two distinct levels of 

information. The first part of the report provides groundwatersample data to aid in estimating groundwater quality at 

the local scale. Tabulated chemical-constituent values and 

related data, and mapped sample locations and hydrogeologic 

unit associations are presented. The number of wells and 

boreholes, samples, and chemical-constituent analyses among 

hydrogeologic units and descriptive statistics of constituent 

values are summarized. Estimation of local-scale groundwater 

quality is enabled by cross-referencing mapped sample locations with tabulated data. Use of the groundwater-sample data 

is supported by discussions of data quality and limitations.

The second part of the report presents a conceptual 

understanding of spatial trends in groundwater quality and 

related processes at the regional scale. Variations in the 

concentrations of major ions that dominate the chemical 

composition of groundwater are summarized by maps of major 

water-supply aquifers. Hydrochemical processes that control 

groundwater composition are described. In addition, chemical 

constituents of particular resource-management interest are 

individually addressed. Based on spatial variations in chloride 

concentration, the saltwater-transition zone is portrayed 

three-dimensionally using maps and sections. Relations of the 

configuration of the saltwater-transition zone to groundwater 

flow are discussed. A summary of wells that exhibit changes in 

chloride concentration over time also is tabulated, and selected 

examples of chloride-concentration changes that result from 

different groundwater conditions are illustrated. In addition 

to chloride, regional trends for other selected constituents 

are summarized by maps and sections of concentrations of 

fluoride, iron and manganese, and nitrate and ammonium. 

Lastly, considerations for the use of groundwater related to 

various chemical constituents are discussed.

Description of the Study Area

The Virginia Coastal Plain occupies an area of approximately 13,000 square miles (mi2

) between approximately 

latitude 36°30′ and 38°30′ N. and longitude 75°30′ and 

77°30′ W. (fig. 1). The climate is temperate and humid, with 

annual precipitation of more than 40 inches (in.; National 

Weather Service, 1996), and the region is generally heavily 

vegetated. Major urban centers along the western margin 

of the area include Fredericksburg and Richmond. A large 

metropolitan area, collectively referred to as Hampton 

Roads, occupies the southeast and consists of 16 localities, 

including the counties of Gloucester, Isle of Wight, James 

City, Southampton, Surry, and York, and the cities of Franklin, 
Introduction 5

Hampton, Newport News, Norfolk, Poquoson, Portsmouth, 

Williamsburg, Chesapeake, Suffolk, and Virginia Beach. The 

latter three cities are large and comparable to the counties in 

land area, and these cities encompass rural as well as urban 

land uses. The rest of the Virginia Coastal Plain is mostly rural 

and fairly evenly divided between cropland and forest. Small 

towns are widely scattered, many of which serve as county 

seats. Residential development is increasing by conversion of 

farmland in proximity to urban centers and along waterfronts.

The Virginia Coastal Plain is characterized by rolling 

terrain and deeply incised stream valleys in the northwestern 

part, and gently rolling-to-level terrain, broad stream valleys, 

and extensive wetlands in the eastern and southern parts. 

Topography is dominated by valleys of the major rivers, 

including the Potomac, Rappahannock, Piankatank, Mattaponi, Pamunkey, York, Chickahominy, James, Appomattox, 

Blackwater, Nottoway, Meherrin, Nansemond, Elizabeth, and 

North Landing (fig. 1). Lowlands consisting of terraces, flood 

plains, and wetlands occupy valley floors and are flanked 

by broad uplands along basin boundaries. The uplands and 

lowlands are bounded by relict erosional scarps associated 

with the rivers (Johnson and Ramsey, 1987); these scarps 

are obscured in places by the present-day tributary drainage 

pattern. Land-surface altitude ranges from over 300 ft across 

some western uplands to 0 ft along the Atlantic coast.

The major rivers receive flow from dense and extensive 

networks of tributaries that extend across their entire drainage 

basins. These rivers collectively drain to the east and southeast 

into Chesapeake Bay, a large estuary formed by submersion 

of the Susquehanna River Valley as a result of rising sea 

level. Major rivers draining from the west also become 

estuarine upon entering the Coastal Plain. Chesapeake Bay 

separates most of the Virginia Coastal Plain to the west from 

the Virginia Eastern Shore to the east, which makes up the 

southernmost part of the Delmarva Peninsula. The northernmost part of the Virginia Coastal Plain is hydraulically isolated 

by the Potomac River north of Fredericksburg and is excluded 

from this investigation.

Geologic Setting

The Coastal Plain is underlain by a seaward-thickening 

wedge of regionally extensive, generally eastward-dipping 

strata of unconsolidated to partly consolidated sediments of 

Cretaceous, Tertiary, and Quaternary age that unconformably 

overlie a basement of consolidated bedrock (fig. 2). The 

sediment wedge extends from Cape Cod, Massachusetts, 

southward to the Gulf of Mexico and offshore to the Continental Shelf. Sediment thickness in Virginia ranges from 0 ft at its 

western margin to more than 6,000 ft along the Atlantic coast 

(Onuschak, 1972). The sediments were deposited by seaward 

progradation of fluvial plains and deltas along the North 

American continental margin, followed by a series of transgressions and regressions by the Atlantic Ocean in response 

to changes in sea level. A thick sequence of nonmarine strata 

primarily of Cretaceous age is overlain by a much thinner 

sequence of marine strata of Tertiary age, which is in turn 

overlain by a veneer of nearly flat-lying terrace and flood-plain 

deposits primarily of Quaternary age (Meng and Harsh, 1988).

Coastal Plain sediments in Virginia were further affected 

during the Tertiary period by the impact of an asteroid or 

comet near the mouth of the present-day Chesapeake Bay 

(Powars and Bruce, 1999). The buried Chesapeake Bay 

impact crater is greater than 50 miles (mi) in diameter and 

extends across a large part of the southeastern Virginia Coastal 

Plain (fig. 1). The crater was formed within the preexisting 

sediments and contains a unique assemblage of impact-related 

material. Subsequent sediment deposition has buried the crater 

approximately 1,000 ft below the present-day land surface.

The area to the west of the Coastal Plain is the Piedmont 

Physiographic Province (Piedmont) (figs. 1, 2), which is 

characterized by rolling terrain. Residual soils range from 

nearly 0 to 100 ft thick and are underlain by igneous and 

metamorphic bedrock of late Proterozoic and early Paleozoic 

age, along with fault-bounded structural basins containing 

sedimentary and igneous bedrock of Triassic and Jurassic 

age. Shallow alluvial deposits of Quaternary age are localized in stream valleys. The transitional part of the Coastal 

Plain toward the Piedmont is designated as the Fall Zone. 

Numerous falls and rapids are present along streams across 

the Fall Zone, because gradients increase as the streams flow 

generally eastward from resistant bedrock onto more easily 

eroded sediments. From the Fall Zone, the Piedmont bedrock 

dips eastward beneath the sediment wedge to constitute the 

basement that underlies the Coastal Plain. The Fall Zone 

encompasses a belt several miles wide, along which the 

configuration of surface exposures of sediment and bedrock 

is intricate. Streams have eroded through Coastal Plain 

sediments to expose Piedmont bedrock in their valley floors, 

whereas interstream divides are capped by uneroded sediments 

overlying the bedrock (Mixon and others, 1989). 

Groundwater Conditions

Sediments of the Virginia Coastal Plain are represented 

by a hydrogeologic framework of aquifers, confining units, 

and confining zones, collectively termed hydrogeologic units 

(fig. 3) (McFarland and Bruce, 2006). Permeable sediments 

that act as regionally extensive conduits for groundwater flow 

are designated as aquifers, and less permeable sediments that 

partly restrict flow are designated as confining units. Less 

distinct transitional intervals between aquifers and confining 

units are considered confining zones. Parts of some aquifers 

provide a widely used supply of water in Virginia.

A complex history of sediment deposition has produced 

numerous lateral variations in lithology. Consequently, 

the positions of unit margins are divergent, and their areal 

distribution has a complex, overlapping “patchwork” configuration. In particular, some units pinch out westward toward 

the Fall Zone, where the vertical sequence of sediments 

varies widely compared to other parts of the Coastal Plain. In 

addition, major discontinuities among units are present along 
6 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Figure 2. Generalized hydrogeologic section and directions of groundwater flow in the Virginia Coastal Plain. 

VERTICAL SCALE GREATLY EXAGGERATED

WEST EAST

Aquia

aquifer

Yorktown-Eastover aquifer

SUFFOLK SCARP Surficial aquifer

Yorktown confining zone

Nanjemoy-Marlboro confining unit

Potomac confining zone

PIEDMONT

PHYSIOGRAPHIC

PROVINCE COASTAL PLAIN PHYSIOGRAPHIC PROVINCE

Chesapeake Bay

Potomac aquifer

Calvert confining unit

Piney Point aquifer

10 0 20 MILES

20 KILOMETERS 10 0

Fall Zone

ALTITUDE, IN FEET ABOVE OR BELOW NATIONAL GEODETIC VERTICAL DATUM OF 1929

Saltwater

interface

Chickahominy confining unit Exmore Matrix confining unit

Exmore

Clast

confining

unit

CHESAPEAKE BAY IMPACT CRATER

500

0

–500

–1,000–1,500

Potomac

confining

zone

EXPLANATION

AquiferConfining unit or zoneSaproliteBedrock

Direction of groundwater flowFractures

Figure 2. Generalized hydrogeologic section and directions of groundwater flow in the Virginia Coastal Plain.
Introduction 7

the margin of the Chesapeake Bay impact crater (fig. 2). None 

of the hydrogeologic units extends across the entire Virginia 

Coastal Plain.

Hydrogeologic conditions in the Coastal Plain are 

distinct from the Piedmont. In the Coastal Plain, groundwater 

is largely present in pores between the sediment grains. By 

contrast, groundwater in the Piedmont is present mostly in 

fractures in the bedrock and in pores in weathered residuum 

overlying the bedrock.

Groundwater in the Coastal Plain is recharged principally 

by precipitation that infiltrates the land surface and percolates 

to the water table. Most of the unconfined groundwater flows 

relatively short distances and discharges to nearby streams, 

but a small amount flows downward to recharge the deeper 

confined aquifers (fig. 2), primarily along the Fall Zone and 

beneath surface-drainage divides between major river valleys 

(Harsh and Laczniak, 1990). Because aquifers in the Fall Zone 

are shallow and subcrop along major rivers, flow interactions 

with the rivers result from direct hydraulic connections to 

the land surface (McFarland, 1999). The basement bedrock 

imposes a relatively impermeable underlying boundary. 

Localized flow interaction is theorized to occur between 

bedrock fractures and the overlying sediments, but the extent 

and magnitude of such interaction are not well known.

Because of stratification of the Coastal Plain sediments, 

horizontal hydraulic conductivity generally is greater than 

vertical hydraulic conductivity. Hence, flow through the 

confined aquifers primarily is lateral in the down-dip direction 

to the east (fig. 2) and toward large withdrawal centers and 

major discharge areas along large rivers and the Atlantic coast. 

Contrasting density between freshwater and saltwater to the 

east causes the confined groundwater to discharge by upward 

flow across intervening confining units. In addition, hydraulic 

boundaries along the Chesapeake Bay impact crater have been 

theorized to cause a lateral divergence of flow to either side of 

the impact crater (McFarland and Bruce, 2005).

Methods of Investigation

The investigation presented herein was undertaken 

during 2006–08. A newly expanded and comprehensive set 

of groundwater quality and related data was compiled from 

multiple Federal and State agency databases. Chemicalconstituent values and corresponding well and borehole 

identification, location, sample-collection date and time, and 

collection-interval altitude information were retrieved during 

2006 from the Quality of Water Data Base (QWDATA) and 

Groundwater Site Inventory (GWSI) database maintained by 

the USGS. These data were augmented during 2007 primarily 

with data retrieved from the Storage and Retrieval (STORET) 

database maintained by the U.S. Environmental Protection 

Agency (USEPA). Smaller amounts of data were also provided 

by the DEQ, the Virginia Department of Health (VDH), and 

the USGS National Research Program (NRP).

The dataset represents groundwater-quality samples collected from wells and boreholes located in the Virginia Coastal 

Plain and a few miles to the west, along the easternmost part 

of the Piedmont (fig. 1; pls. 1–3). Individual sample values 

were included only for chemical constituents that are present 

naturally and generally represent ambient groundwater composition (although some of these constituents can be artificially 

increased above natural concentrations). Concentration values 

for artificial chemicals that are not present naturally and 

originate solely from human contamination (for example, 

synthetic organic compounds) were excluded. Likewise, any 

samples known to represent treated water were not included. 

Samples also were excluded for which collection-interval 

altitudes could not be determined, primarily those from wells 

having inadequate construction information.

For consistency of the dataset, all groundwater 

samples are identified using the well and borehole 

Figure 3. Simplified composite stratigraphic 

relations among hydrogeologic units of the 

Virginia Coastal Plain (adapted from McFarland 

and Bruce, 2006). Not all hydrogeologic units are 

present at every location. Associations among 

some hydrogeologic units that cross stratigraphic 

boundaries also are not shown.

surficial aquifer

Yorktown confining zone

Yorktown-Eastover aquifer

Saint Marys confining unit

Calvert confining unit

Saint Marys aquifer

Chickahominy confining unit

Exmore Matrix confining unit

Exmore Clast confining unit

Piney Point aquifer

Nanjemoy-Marlboro confining unit

Aquia aquifer

Peedee confining zone

Peedee aquifer

Virginia Beach confining zone

Virginia Beach aquifer

Upper Cenomanian confining unit

Potomac confining zone

Potomac aquifer

Basement bedrock

Figure 3. Simplified composite stratigraphic relations

 among hydrogeologic units of the Virginia Coastal Plain 

 (adapted from McFarland and Bruce, 2006). Not all

 hydrogeologic units are present at every location.

 Associations among some hydrogeologic units that

 cross stratigraphic boundaries are also not shown.
8 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

identification-numbering system used at the USGS Virginia 

Water Science Center, which is based on the USGS 7.5-minute 

topographic quadrangle maps on which the wells and boreholes are located. Different well and borehole identificationnumbering systems, however, are used among the non-USGS 

data sources. Moreover, some wells and boreholes have been 

sampled multiple times. Because the different samples from 

an individual well or borehole can originate from different 

data sources, they can be associated with different well or 

borehole identification numbers. Accordingly, a combination 

of automated data-relational operations and manual screening 

was used to distinguish multiple samples collected from a 

single well or borehole. For wells and boreholes already 

represented in the USGS databases, the existing USGS 

identification number was assigned to any samples originating 

from non-USGS data sources. Conversely, samples determined 

to have been collected from wells and boreholes that were 

not represented in USGS databases were assigned new USGS 

identification numbers.

Groundwater-quality data are most useful when 

chemical-constituent values are accurately associated with the 

hydrogeologic unit from which each sample was collected. 

Accordingly, the newly revised hydrogeologic framework of 

the Virginia Coastal Plain (McFarland and Bruce, 2006) was 

applied so that the groundwater-quality dataset would reflect 

current knowledge of the aquifer system. Each groundwater 

sample was associated with one of the 19 aquifers, confining 

units, or confining zones delineated by the framework. 

A geographic information system (GIS), developed 

as part of the hydrogeologic framework investigation, was 

used to locate each sample three-dimensionally among the 

hydrogeologic units. Approximate configurations of the top 

surfaces of all hydrogeologic units are represented in the GIS 

by raster datasets. Groundwater-quality sample locations 

and related data were added to the GIS as georeferenced 

points. GIS operations were then performed to identify the 

regionally estimated altitudes of hydrogeologic unit top 

surfaces at each sample location. Subsequently, a series of 

data-relational operations was performed to determine from 

which hydrogeologic unit each sample was collected, based 

on the collection-interval altitude. Resulting sample hydrogeologic unit associations were incorporated with the compiled 

chemical-constituent values and related data.

Previous Investigations

Evolving knowledge of groundwater in the Virginia 

Coastal Plain is documented by a large and diverse body of 

literature. Some of the most important historical contributions 

to the geologic and hydrologic aspects are summarized by 

McFarland and Bruce (2006). 

The chemical quality of groundwater in the Virginia 

Coastal Plain has also been addressed by many published 

investigations. Most of these studies were of limited 

geographic extent, and relatively few encompass the entire 

Virginia Coastal Plain. In the past, groundwater quality 

typically was treated as a subtopic among other geologic and 

hydrologic aspects of a particular study. Other publications are 

limited to a specific issue related to groundwater quality, most 

widely chloride and other aspects of groundwater salinity.

Individual works are cited throughout this report to reference information supporting particular discussions. Continued 

development of groundwater resources has entailed many 

additional investigations of a site-specific nature for individual 

water-supply systems, which are not widely published or 

referenced herein. The following review is not intended to be 

wholly inclusive, but to highlight some of the efforts that have 

had regional significance. They are organized here chronologically and by broadness of scope.

Pioneering work on groundwater quality in the Virginia 

Coastal Plain was achieved during the early and middle 

parts of the 20th century. Sanford (1913) made probably the 

earliest description of groundwater quality, which was treated 

as a subtopic on groundwater in the entire Virginia Coastal 

Plain and also in individual counties, described by separate 

chapters. A series of publications by Cederstrom followed, 

again treating groundwater quality as a subtopic in reports on 

groundwater in southeastern Virginia (1939, 1945), the YorkJames Peninsula (1957), and the Middle Peninsula (1968), 

and also including separate chapters on individual counties. 

More indepth assessments of the entire Virginia Coastal 

Plain, however, were also made of chloride concentrations in 

relation to groundwater salinity (Cederstrom, 1943) and of 

hydrochemical processes that control groundwater composition (Cederstrom, 1946a, b).

Some major advances in understanding groundwater 

quality in the Virginia Coastal Plain were made during 

the middle and later parts of the 20th century. A scientific 

milestone was established by Back (1966), who developed the 

concept of hydrochemical facies to describe the sequence by 

which groundwater composition evolves along regional flow 

paths across the entire Northern Atlantic Coastal Plain (Virginia northward through New Jersey). Further understanding 

was gained of groundwater salinity within the Virginia Coastal 

Plain by Larson (1982) and across the entire Northern Atlantic 

Coastal Plain by Meisler (1989). Both of these works present 

spatial distributions of groundwater chloride concentrations to 

depict the configuration of the saltwater-transition zone and to 

theorize processes related to emplacement of saltwater within 

the aquifer system.

A detailed systematic compilation of groundwater-quality 

information from throughout the Virginia Coastal Plain was 

presented by Focazio and others (1993). Descriptions are 

given of selected individual chemical constituents and of 

variations in overall groundwater composition. Summary 

statistics are tabulated for groundwater-sample data from 

five major water-supply aquifers for 16 chemical constituents. 

A series of maps also presents areal concentration distributions 

for nine constituents in each of the aquifers. 

Statewide studies that incorporate the Virginia Coastal 

Plain include Powell and Hamilton (1988), who described 
Groundwater-Quality Data 9

Virginia as part of a nationwide summary of groundwater 

quality. Nelms and others (2003) used trace-level hydrochemical tracers to estimate apparent groundwater ages throughout 

Virginia. Heller (2008) related groundwater quality to 

characteristics of wells located across Virginia.

Several additional groundwater investigations took 

place during the later part of the 20th century that, as in earlier 

efforts, generally treat groundwater quality as a subtopic 

among other geologic and hydrogeologic aspects of particular 

subregional areas, including

• the Northern Neck—Sinnott, 1969; Newton and 

Siudyla, 1979

• the Middle Peninsula—Siudyla and others, 1977

• the York-James Peninsula—Commonwealth of 

Virginia, 1973; Laczniak and Meng, 1988

• southeastern Virginia—Commonwealth of Virginia, 

1974; Siudyla and others, 1981

• the Eastern Shore—Commonwealth of Virginia, 1975; 

Fenema and Newton, 1982; Richardson, 1994; Speiran, 

1996; Sanford and others, 2009a, b

• the James River Basin—Lichtler and Wait, 1974

• Hanover County—Ellison and Masiello, 1979

• Henrico County—Wigglesworth and others, 1984

• James City County—Harsh, 1980

• York County—Richardson and Brockman, 1992

• Dahlgren (King George County)—Harlow and Bell, 

1996; Bell, 1996

• city of Virginia Beach—Smith, 2003

Some of these reports include details of chloride in 

groundwater and(or) other aspects of groundwater salinity. 

Other investigations focused specifically on groundwater 

salinity. Brown and Silvey (1977) evaluated the feasibility 

of underground injection as a means of freshwater storage in 

the brackish part of the aquifer system at Norfolk, Virginia. 

Smith used numerical simulation to estimate the potential for 

saltwater intrusion on the York-James Peninsula (1999) and in 

Virginia Beach (2003). 

The most recent efforts to characterize groundwater 

salinity were undertaken as part of a larger investigation of the 

Chesapeake Bay impact crater. McFarland and Bruce (2005) 

analyzed chemical tracers and other constituents in samples 

of pore water extracted from cores of crater-fill and overlying sediments drilled at three locations along the landward 

margin of the outer crater. Based on these data, along with 

existing groundwater-sample data from several nearby 

wells, the configuration of the saltwater-transition zone was 

described, and its relations to the impact event, groundwater 

flow, sea-level fluctuation, and groundwater withdrawal were 

theorized. Subsequently, Sanford and others (2009b) extracted 

and analyzed additional sediment pore-water samples from 

two coreholes drilled within the inner crater. A large sediment 

thickness was found to contain brine having salinity levels 

greatly exceeding that of seawater, as well as those of groundwater along the outer crater margin and beyond. The earlier 

interpretations of McFarland and Bruce (2005) were expanded 

by Sanford and others (2009b) to include possible origins of 

the brine, water/mineral chemical reactions, and alternative 

relations to groundwater flow.

Groundwater-Quality Data

In order to aid in estimating groundwater quality at 

the local scale, a newly expanded and comprehensive set 

of groundwater quality and related data was compiled from 

multiple Federal and State agency databases. Associations 

of groundwater samples among 19 hydrogeologic units and 

basement bedrock were determined based on a newly revised 

hydrogeologic framework (McFarland and Bruce, 2006). 

Data Quality

A dataset is electronically tabulated (Attachment 1), 

consisting of individual groundwater-sample chemicalconstituent values along with corresponding well and borehole 

identification and location information, collection date and 

time, collection-interval altitudes, and associations among 

hydrogeologic units. Sample locations and hydrogeologic unit 

associations are also presented with detailed base-map information (pls. 1–3). Sample-collection dates range from 1906 

to 2007. Additional aspects of the sample data are described 

below, generally in the order tabulated (Attachment 1).

The well or borehole from which each groundwater 

sample was collected is identified by a USGS identification 

number. Multiple samples collected from a single well or borehole are identified by different collection dates and(or) times. 

Multiple samples collected from a finished well typically 

exhibit identical or similar collection-interval altitudes, as 

well as the same hydrogeologic unit association. By contrast, 

multiple samples have sometimes been collected at varying 

depths in a borehole, generally by zone testing of a borehole, 

or by pore-water extraction from a sediment core. Such 

samples represent widely varying collection-interval altitudes 

and different hydrogeologic units (see “Data Distribution”). 

Well and borehole locations are presented as latitude 

and longitude coordinates, referenced to the North American 

Datum of 1927 (NAD 27), which currently (2009) is the datum 

used on all published USGS 7.5-minute topographic quadrangle maps within the Virginia Coastal Plain. Coordinates of 

a minority of locations have been determined independently 

(not part of this study) using satellite-based global positioning 

systems (GPS) to within fractions of a second. Most locations, 
10 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

however, were likely estimated from topographic maps and 

should generally be regarded as accurate to no more than plus 

or minus (+/–) 5 seconds. Likewise, only the locations of those 

wells sampled by the USGS are known to have been verified 

in the field.

Collection-interval altitudes are referenced to the 

National Geodetic Vertical Datum of 1929 (NGVD 29), as 

calculated from reported interval top and bottom depths below 

the altitudes of land surface at the well and borehole locations. 

Collection intervals of finished wells having multiple screened 

intervals were designated using the top of the uppermost 

screen and the bottom of the lowermost screen. Although a 

minority of location land-surface altitudes have been leveled 

independently to within a hundredth of a foot, most altitudes 

were likely estimated from topographic maps accurate to 

+/– 5 ft. In addition, measured collection-interval depths 

commonly can vary by several feet, depending on whether 

the measurement was taken from true land surface, the top 

of surface casing, or some other datum usually not recorded. 

Hence, collection-interval altitudes generally should be 

regarded as accurate to no more than +/– 10 ft.

Associations of groundwater samples with hydrogeologic 

units are presented directly as they were determined by a 

GIS developed as part of a revised hydrogeologic framework 

investigation (see “Methods of Investigation”). The GIS 

provided an efficient means of associating the large number 

of samples with the hydrogeologic units. The associations are 

based on regionally estimated hydrogeologic unit altitudes, 

however, which can vary from actual altitudes at any specific 

location by several feet or more. Altitudes of hydrogeologic 

units were directly determined at 403 geophysically logged 

sites that serve as control points for structural contour maps 

developed for the framework (McFarland and Bruce, 2006). 

Based on stratigraphic correlation, altitudes were interpolated 

between logs at all other locations, including those of most 

sampled wells and boreholes. As a result, samples having 

collection intervals within a few feet of the actual top or 

bottom of a hydrogeologic unit can potentially be associated 

by the GIS with a nearby adjacent unit. Particularly, some 

samples associated with a confining unit or a confining zone 

may actually have been collected from an adjacent aquifer (see 

“Data Distribution”).

Tabulated groundwater-sample data include values for 

37 chemical constituents. Chemical-constituent values are 

presented essentially as they were reported and were retrieved 

directly from the original data sources. Because of the broad 

range of numerical values, dataset cells are formatted to four 

decimal places to preclude rounding or truncating of values. 

The constituent values represent the results of groundwatersample collection and analysis by numerous individuals and 

diverse organizations spanning a century-long period. Limits 

of analytical precision are not fully documented, but likely 

vary widely for each constituent, both among the different data 

sources and possibly over time. Hence, reported values should 

not be assumed to represent analytical precision limits or other 

measures of accuracy (see “Data Use and Limitations”). In 

addition, only a very few obvious errors among the chemicalconstituent values were corrected or deleted here (for example, 

dates set in the future). Based on visual examination and 

summary statistics of the compiled dataset, a small number of 

apparently erroneous values possibly remain (see “Data Use 

and Limitations”), but information is not adequate to evaluate 

them definitively for correction or deletion.

Data Distribution

No groundwater samples in the compiled dataset 

(Attachment 1) include values for every chemical constituent, 

and most samples have values for only a few constituents. 

Constituent values among individual hydrogeologic units also 

alternately exhibit spatial gaps, lateral and vertical variability 

at local scales, and(or) non-uniform areal trends at the regional 

scale. Likewise, constituent values are not representative of 

the entire regional extent of those hydrogeologic units from 

which samples were collected over a limited area, because the 

chemical composition of groundwater varies regionally.

Because of the large volume of the compiled 

groundwater-quality data, summaries are presented to 

describe their distribution. The data represent 1,991 wells 

and boreholes, from which 10,165 groundwater samples 

were collected (table 1). Results of sample analysis consist of 

73,610 chemical-constituent values. The numbers of wells and 

boreholes, samples, and constituent values vary among the 

hydrogeologic units, primarily in relation to their importance 

to water-resource management. The largest numbers are exhibited by the Potomac aquifer, which provides approximately 

75 percent of groundwater withdrawal in the Virginia Coastal 

Plain (McFarland and Bruce, 2006). Many water-quality 

sampling programs have focused on the Potomac aquifer for 

various evaluations of public drinking-water systems and 

heavy industry. 

Large numbers of wells and boreholes, samples, and 

constituent values also represent the Yorktown-Eastover 

aquifer and the surficial aquifer (table 1). The YorktownEastover aquifer is the second most heavily used groundwater 

source in the Virginia Coastal Plain, providing 13 percent 

of the total withdrawal (McFarland and Bruce, 2006) and 

serving diverse small businesses and public drinking-water 

systems, rural domestic and agricultural uses, and low-density 

residential developments. The surficial aquifer provides only 

4 to 5 percent of the total groundwater withdrawal, but is 

the principal source for many thousands of private domestic 

supply wells at widely dispersed rural locations. Additional 

wells in the surficial aquifer have been sampled as part of 

various investigations of near surface contamination.

Relatively few wells and boreholes, samples, and 

constituent values represent the Piney Point aquifer, Aquia 

aquifer, and basement bedrock (table 1), which are locally 

important groundwater supplies. The Piney Point and Aquia 

aquifers provide approximately 5 percent and 3 percent, 

respectively, of groundwater withdrawal in the Virginia 
Groundwater-Quality Data 11

Coastal Plain (McFarland and Bruce, 2006), whereas use of 

the basement bedrock has not been quantified. Remaining 

aquifers include the Saint Marys, Peedee, and Virginia Beach, 

which are only minor groundwater supplies and exhibit among 

the smallest numbers of wells and boreholes, samples, and 

constituent values.

Samples from eight wells and boreholes were collected 

from multiple hydrogeologic units (table 1). The numbers of 

samples and chemical-constituent values are included among 

those separately listed for each hydrogeologic unit. At some 

of these locations, samples were collected from different 

aquifers during borehole zones tests conducted prior to 

completion of permanent wells. At other locations, samples 

were collected from different aquifers, as well as confining 

units and confining zones, as part of a larger study of the 

Chesapeake Bay impact crater in which pore water was 

extracted from cores of crater-fill and overlying sediments 

(McFarland and Bruce, 2005; Sanford and others, 2009b) 

(see “Previous Investigations”). 

Although the majority of groundwater samples are 

associated with aquifers, some samples are associated with 

a confining unit or a confining zone (table 1), for example 

sediment-core pore water collected within the Chesapeake 

Bay impact crater as described previously and primarily from 

the Saint Marys, Chickahominy, Exmore matrix, and Exmore 

clast confining units. Elsewhere, wells are rarely completed in 

a confining unit or confining zone. Hence, potentially many of 

the samples associated with a confining unit or confining zone 

were actually collected from an adjacent aquifer. Because of 

the limited accuracy of regionally estimated hydrogeologic 

unit altitudes, samples having collection intervals within a 

few feet of the top or bottom of an aquifer can potentially be 

associated with a nearby adjacent confining unit or a confining 

zone (see “Data Quality”). Most notably among these, many 

of the relatively large number of wells and boreholes, samples, 

and constituent values associated with the NanjemoyMarlboro confining unit are likely from the underlying 

Aquia aquifer. In many areas, only gradational differences in 

Table 1. Numbers of sampled wells, water-quality samples, and chemical-constituent values 

among hydrogeologic units of the Virginia Coastal Plain.

Hydrogeologic unit 1 Sampled wells

and boreholes

Water-quality

samples

Chemicalconstituent

values

Surficial aquifer 469 2,231 13,054

Yorktown confining zone 84 298 2,806

Yorktown-Eastover aquifer 444 2,361 18,112

Saint Marys confining unit 30 220 1,644

Saint Marys aquifer 3 19 193

Calvert confining unit 7 26 187

Piney Point aquifer 94 417 2,519

Chickahominy confining unit 7 26 200

Exmore matrix confining unit 1 22 180

Exmore clast confining unit 6 97 456

Nanjemoy-Marlboro confining unit 43 183 1,217

Aquia aquifer 28 62 736

Peedee confining zone 1 7 87

Peedee aquifer 3 13 173

Virginia Beach confining zone 0 0 0

Virginia Beach aquifer 5 13 153

Upper Cenomanian confining unit 7 25 289

Potomac confining zone 28 85 903

Potomac aquifer 618 3,234 27,495

Basement bedrock 105 826 3,206

Multiple hydrogeologic units 2 8 included above included above

Total 1,991 10,165 73,610

1From McFarland and Bruce, 2006.

2Samples collected from individual hydrogeologic units by zone testing or sediment-core pore-water extraction 

are included among hydrogeologic units as listed separately.
12 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

texture and lithologic composition distinguish the NanjemoyMarlboro confining unit from the Aquia aquifer, and both 

hydrogeologic units are relatively thin across much of their 

extents. Similar relations exist between the Calvert confining 

unit and underlying Piney Point aquifer, and between the 

Yorktown, Peedee, Virginia Beach, and Potomac confining 

zones and each of their respective underlying aquifers. 

Locations of sampled wells and boreholes are not 

uniformly distributed (fig. 1; pls. 1–3). The largest concentration of wells and boreholes is on the York-James Peninsula, 

southeastern Virginia, and the Virginia Eastern Shore, 

corresponding to the greatest and most widespread use of 

groundwater. Localized concentrations of wells are also 

present at two large industrial withdrawal centers at Franklin 

and West Point. Smaller groupings are associated with various 

local-scale investigations of groundwater at locations along the 

Fall Zone and in the eastern parts of King George, Hanover, 

and Henrico Counties. The remaining wells and boreholes are 

generally located at widely dispersed rural locations.

Locations of groundwater samples among hydrogeologic 

units are also not uniformly distributed geographically (fig. 1; 

pls. 1–3). The most numerous and widespread samples are 

from the heavily used Potomac aquifer and span most of the 

Virginia Coastal Plain with varying density. Potomac aquifer 

samples are largely absent, however, in the far southeastern 

parts of the Middle and York-James Peninsulas, and most of 

the city of Virginia Beach and the Virginia Eastern Shore. 

Here, the Potomac aquifer is several hundred feet or more 

deep and is largely unused because of increased salinity. 

Correspondingly, most groundwater samples in these areas 

are from the surficial and Yorktown-Eastover aquifers, which 

are comparatively shallow, contain freshwater, and locally 

dominate the groundwater supply. Whereas samples from the 

Yorktown-Eastover aquifer are for the most part only in these 

areas, additional samples from the more widely used surficial 

aquifer span much of the rest of the Virginia Coastal Plain 

with varying density.

Most samples from the Piney Point and Aquia aquifers 

are clustered around the northwestern part of the York-James 

Peninsula centered in eastern New Kent County and western 

James City County, from which additional samples are scattered to the north and northeast across the Middle Peninsula 

and Northern Neck (pls. 1–3). Most samples from basement 

bedrock are along the Fall Zone and in the eastern Piedmont. 

Small numbers of samples from other hydrogeologic units are 

at widely dispersed locations.

Groundwater-sample chemical constituents are organized 

among six groups (Attachment 2), including general 

parameters, major cations, major anions, nutrients, secondary constituents, and radiochemical constituents. General 

parameters, major cations, and major anions are individually 

listed in commonly published order. Nutrients are listed in 

order of general interest to resource management. Secondary 

constituents are listed loosely on a combined basis of hydrochemical associations and general prevalence in groundwater. 

Radiochemical constituents are listed alphabetically. Summary 

statistics for all constituents are tabulated for samples collected from all hydrogeologic units collectively, as well as for 

samples collected from each hydrogeologic unit individually. 

Statistic values include the number of analyses for each 

constituent, followed by constituent value minima, maxima, 

means, and medians. Minima and maxima are cited directly 

from the compiled groundwater-sample dataset (Attachment 1) 

and are subject to the same qualifications (see “Data Quality”). 

Conversely, means and medians were calculated from the 

individual constituent values and are limited to no more than 

three significant figures.

For samples collected from all hydrogeologic units collectively and for samples collected from each hydrogeologic 

unit individually, the largest numbers of analyses are generally 

represented for the general parameters, major cations, and 

major anions (Attachment 2). The most widely analyzed 

general parameter is pH. Whereas the numbers of analyses 

among the major cations are generally evenly divided, 

bicarbonate is more widely analyzed than the other major 

anions for samples from most hydrogeologic units. Generally 

fewer analyses are represented for nutrients and secondary 

constituents. Relatively very few analyses are represented for 

radiochemical constituents, which are wholly lacking from 

several hydrogeologic units. Radon 222, however, is more 

widely analyzed than the other radiochemical constituents, 

mostly from the surficial, Yorktown-Eastover, Piney Point, 

and Potomac aquifers. Among the radiochemical and other 

constituents represented by few analyses, more samples are 

known to have been collected and analyzed from some wells, 

but were excluded from the compiled groundwater-quality 

data because well-construction information was inadequate to 

determine sample collection-interval altitudes (see “Methods 

of Investigation”).

Ranges and central tendencies among the chemicalconstituent values are partly dependent on the locations of 

sampled wells and boreholes, because the chemical composition of groundwater varies regionally, and sampled locations 

are not uniformly distributed across the entire extent of every 

hydrogeologic unit. Minimum values are at or close to zero 

for many constituents (Attachment 2). Analytical detection 

limits likely vary among the different data sources and over 

time (see “Data Quality”). Maxima represent extreme values 

for some constituents, which in some cases are potentially 

suspect of being erroneous (see “Data Use and Limitations”). 

Substantial differences are apparent between mean and 

median values for some constituents, which can result from 

a small number of extreme values. Median values are based 

on a ranking of the samples and, in such cases, are generally 

considered more representative of most of the samples than 

mean values.
Regional Trends in Groundwater Quality 13

Data Use and Limitations

Local-scale knowledge of the chemical quality of 

groundwater is required by well drillers and owners, 

environmental consultants, and water-resource managers 

who have short-term, practical needs in making case-by-case 

estimates of chemical-constituent concentrations for diverse 

groundwater-related activities. Accordingly, the complete 

groundwater-sample dataset is presented here (Attachment 1). 

In order to access local-scale groundwater-quality information, 

mapped sample locations can be readily cross-referenced 

with tabulated groundwater-sample data to estimate specific 

chemical-constituent values.

Valid application of the compiled groundwater-sample 

data requires an understanding of the previously described 

data quality and associated limitations. The data originate 

from diverse sources having long and potentially complex but 

not fully documented histories. Potential data errors range 

from well construction and sample collection, preservation, 

and analysis, to typographic mistakes and other reporting 

problems. (Some samples of treated or otherwise chemically 

altered water possibly are also represented, although known 

cases were excluded during compilation.) Moreover, reported 

chemical-constituent values vary widely among the original 

sources, and do not necessarily reflect analytical precision 

limits. In practical terms, most values should be considered no 

more accurate than three significant figures. Some erroneous 

values also likely remain in the compiled dataset, for example, 

the overall maximum temperature and dissolved-oxygen 

values of 210 degrees Celsius (°C) and 16.1 milligrams 

per liter (mg/L), respectively (Attachment 2), which could 

actually represent more realistic measurements of 21.0 °C and 

1.61 mg/L.

For interpretive analyses presented later in this report, the 

compiled groundwater-quality data were screened to exclude 

inappropriate values. Techniques used included calculations of 

major ion charge balance (see “Major Ions”) and correlation 

between chloride-concentration and specific conductance 

values (see “Saltwater-Transition Zone”). The rationale that 

was applied is described in the relevant report sections.

Users of this report are advised to use judgment in their 

application of individual groundwater-sample data, based 

on the limitations described previously. Sampled well and 

borehole locations should be verified in the field whenever 

possible. Moreover, the likely representativeness of chemicalconstituent values at any given location should be evaluated, 

using sound hydrogeologic experience and common sense. 

Extreme data values are particularly considered as suspect. 

Lastly, sample hydrogeologic unit associations are to be 

viewed with due regard for limitations in the accuracy of the 

hydrogeologic framework GIS. Collection-interval altitudes 

should be examined to evaluate whether any particular 

samples have potentially been collected from a nearby part of 

an adjacent hydrogeologic unit.

Regional Trends in 

Groundwater Quality

Local-scale information on the chemical quality of 

groundwater in the Virginia Coastal Plain is provided in a 

previous section of this report (see “Groundwater-Quality 

Data”). Individual groundwater-sample data can be applied 

in making case-by-case estimates of chemical-constituent 

concentrations. A conceptual understanding of groundwaterquality trends and related processes at the regional scale, 

however, is also required by planners and policy makers in 

making informed long-term resource-management decisions. 

Accordingly, a synthesis of groundwater constituent concentrations is presented herein, to provide a perspective that 

encompasses the entire Virginia Coastal Plain as an integrated 

hydrochemical system.

Among the general parameters (Attachment 2), pH is the 

most widely analyzed property. Other than a small number 

of extremes, most pH values are relatively close to the mean 

and median, both slightly above 7 collectively. Among most 

individual hydrogeologic units, pH is generally maintained 

from approximately 7 to slightly above 8, in association with 

congruent dissolution of calcium-magnesium carbonates 

(see “Hydrochemical Processes”). Two notable exceptions 

are the surficial aquifer and basement bedrock, within which 

pH is closer to 6, possibly reflecting relatively direct recharge 

with precipitation.

Most values for groundwater temperature are close to the 

mean and median (Attachment 2), both in the mid to upper 

teens (degrees Celsius) collectively and among individual 

hydrogeologic units. Subsurface temperatures are effectively 

controlled by the geothermal gradient.

Mean and median values for concentrations of dissolved 

oxygen are both relatively low (Attachment 2), close to or 

less than 1 mg/L collectively and among most individual 

hydrogeologic units. Dissolved oxygen is widely consumed 

by chemical weathering reactions involving iron-bearing 

and associated minerals (see “Iron and Manganese”). Higher 

values for some hydrogeologic units are probably a result 

of collection procedures that allowed sample contact with 

the atmosphere. Within the surficial aquifer, however, actual 

concentrations are possibly close to the approximately 3 mg/L 

indicated, as a result of contact with the atmosphere across the 

water table. 

Some extreme individual values of pH, temperature, and 

dissolved oxygen are possibly the result of sample-collection, 

analysis, and(or) reporting errors (see “Data Use and Limitations”). By contrast, the relatively large ranges exhibited 

by specific conductance, total dissolved solids, and filtered 

residue more likely result from large difference among the 

total ion concentrations of the groundwater samples (see 

“Major Ions”).
14 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Major Ions

Regional trends in groundwater quality are determined 

largely by variations in the concentrations of major ions that 

dominate the chemical composition of groundwater. Although 

groundwater potentially contains many chemical constituents, 

the major ions typically are at the greatest concentrations and 

are considered here to include the cations calcium (Ca2+), 

magnesium (Mg2+), sodium (Na+), and potassium (K+), and the 

anions bicarbonate (HCO3

–

), carbonate (CO3

–

), sulfate (SO4

2–), 

and chloride (Cl–). Summary statistics of compiled major ion 

concentration values are tabulated for groundwater samples 

collected from all hydrogeologic units collectively, as well as 

for samples collected from each hydrogeologic unit individually (Attachment 2), and include the number of analyses for 

each major ion, followed by its minimum, maximum, mean, 

and median.

Accurately determined concentrations of all of the major 

ions are needed for groundwater samples to reliably indicate 

regional trends in groundwater quality. For many of the 

samples, however, concentrations of some or all major ions are 

not included among the compiled chemical-constituent values, 

and the accuracy of those that are included is uncertain. 

Accordingly, samples having complete and accurate major ion 

concentrations were identified by categorizing the samples on 

the basis of charge-balance values calculated for each sample.

Charge balance represents the relative difference in an 

aqueous solution between the amounts of positively charged 

cations and negatively charged anions, in terms of the milliequivalents per liter (meq/L) of each ion, which accounts 

for their different electrical charges and molecular weights. 

Because groundwater is thermodynamically stable and electrically neutral, the total positive electrical charge resulting from 

all cations equals the total negative charge from all anions, and 

the true charge balance is zero percent. Only an approximate 

charge balance, however, was calculated here from the 

compiled chemical-constituent values for each groundwater 

sample. Calculations included only the concentrations of 

major ions and not other chemical constituents that are 

probably at smaller concentrations. In addition, all concentration values are not exact, but rather are limited within various 

ranges of analytical accuracy.

Groundwater samples were categorized by charge balance 

into three groups: (1) those having values within 5 percent of 

zero, (2) those having values between 5 and 10 percent from 

zero, and (3) those having values greater than 10 percent from 

zero. Based on the known limitations described previously, 

the first group of samples was considered to have acceptably 

complete and accurate major ion concentration values. 

Examination of major ion concentrations of the second group 

of samples indicated trends that generally followed those of 

the first group and likely also represent complete and accurate 

values. Thus, the first and second groups were combined 

to include 2,289 samples having complete major ion concentrations from which regional trends in groundwater quality 

can be determined. The third group of samples was 

considered to have incomplete and(or) inaccurate major 

ion concentration values.

Spatial Distribution

Spatial variations in the groundwater quality of the 

Virginia Coastal Plain are exhibited among the primary 

regional water-supply aquifers and basement bedrock 

(pls. 4–9), as reflected by the relative dominance among the 

major ions. Groundwater samples having complete major ion 

analyses were categorized by predominant ion type, based on 

50 percent or greater of a particular cation or anion (or ion 

pair in some cases), which make up the total cations or anions, 

as milliequivalents per liter. Designated cation types are the 

pairs calcium plus magnesium (Ca2++ Mg2+) and sodium 

plus potassium (Na++ K+). Anion types are bicarbonate plus 

carbonate (HCO3

–

+ CO3

–

), sulfate (SO4

2–), and chloride (Cl–

). 

The ion pairs calcium plus magnesium and bicarbonate plus 

carbonate are associated by chemical weathering of subsurface 

sediments, and sodium plus potassium by cation exchange and 

saltwater mixing (see “Hydrochemical Processes”). Samples 

in which no cation or anion (or pair) is 50 percent or greater 

than the total are categorized as a mixed type. The overall 

major ion type for a sample is the combination of the cation 

and anion type, for example (Ca2++ Mg2+)+(HCO3

–

+ CO3

–

). 

Contrasting sample-location colors distinguish the 

major ion type of each groundwater sample among the 

primary regional water-supply aquifers and basement bedrock 

(pls. 4–9). Base map content and scaling are identical to that 

used in a revised hydrogeologic framework of the Virginia 

Coastal Plain (McFarland and Bruce, 2006; available at 

http://pubs.usgs.gov/pp/2006/1731/) to enable direct comparison of groundwater quality to other aspects of the aquifers. 

Some groundwater samples that were associated by the 

hydrogeologic framework GIS with confining units or confining zones were likely actually collected from an adjacent 

aquifer (see “Data Quality”) and are included here with the 

corresponding aquifer as appropriate. Other hydrogeologic 

units include relatively few samples.

Conventional contouring of the concentration of each 

major ion within every hydrogeologic unit is not feasible 

because of spatial data gaps, regionally non-uniform concentration gradients, and local-scale concentration variability 

(either laterally or vertically) within some hydrogeologic units. 

Likewise, presentation of spatial variations in groundwater 

quality by contouring is not practical because of the very large 

number of maps that would result.

Although sample major ion types do not convey specific 

concentrations of individual ions, values for filterable residue 

do reflect the total ion concentrations of the samples. Accordingly, estimated total ion concentration is contoured based 

on values for filterable residue for aquifers that include a 

sufficient number of adequately distributed samples (pls. 6–8), 

with some generalization at locations where vertical variations 
Regional Trends in Groundwater Quality 15

are present. Contours were manually generated to be 

consistent with the sample values but not wholly constrained 

by them. To accommodate abrupt lateral changes in total ion 

concentration, contour intervals range from 100 mg/L toward 

the west to 5,000 mg/L toward the east. For aquifers and 

basement bedrock in which local-scale variations preclude 

contouring, total ion concentrations are portrayed by scaling 

the size of open sample-location symbols proportionately to 

values for filterable residue (pls. 4, 5, 9). The locations of 

some additional samples that lack values for filterable residue 

are shown by single-sized solid symbols. Major ion type, 

together with estimated total ion concentration, provides an 

efficient spatial summary of the compiled major ion concentrations and a generally coherent depiction of spatial variations in 

groundwater quality.

Most of the groundwater samples are categorized as one 

of three major ion types:

1. (Ca2++ Mg2+)+(HCO3

–

+ CO3

–

) type water, 

dominated by calcium and magnesium cations, 

bicarbonate and carbonate anions, and commonly 

referred to as “hard” water,

2. (Na++ K+)+(HCO3

–

+ CO3

–

) type water, dominated 

by sodium and potassium cations, bicarbonate and 

carbonate anions, and commonly referred to as 

“soft” water, and

3. (Na++ K+)+Cl– type water, dominated by sodium 

and potassium cations, chloride anions, and 

commonly referred to as “salty” water.

In addition, some samples of (Na++ K+)+mixed anion type 

water were collected from the Potomac aquifer (pl. 8). Relatively small numbers of additional samples are generalized 

here as “other,” which are categorized among the remaining 

major ion types dominated by other ion combinations or as a 

mixed type in which no cation or anion (or pair) is 50 percent 

or greater than the total.

The degree to which spatial variations among major 

ion type and total ion concentration indicate regional trends 

in groundwater quality differs among the aquifers and 

basement bedrock. No trend at a regional scale is apparent 

from the compiled sample data from the surficial aquifer, 

Yorktown-Eastover aquifer, and basement bedrock (pls. 4, 

5, 9). Relatively few groundwater samples having complete 

major ion analyses were collected from either of these aquifers 

or bedrock, with most samples located within areas of limited 

extent and leaving large expanses with few or no samples. 

Moreover, the chemical composition of groundwater in these 

units is highly variable locally, with samples exhibiting 

contrasting major ions types and total ion concentrations.

In contrast to the preceding aquifers and basement 

bedrock, regional trends in groundwater quality are apparent 

in the Piney Point, Aquia, and Potomac aquifers (pls. 6–8). 

Trends are most clearly exhibited by the Potomac aquifer, 

from which groundwater samples were widely collected 

across the entire Virginia Coastal Plain except for the Virginia 

Eastern Shore. Locations of samples from the Piney Point 

aquifer are limited to the Northern Neck, Middle Peninsula, 

and York-James Peninsula eastward to the southern tip of 

the Virginia Eastern Shore. Locations of samples from the 

Aquia aquifer span most of its extent in Virginia except its 

western margin. 

Total ion concentration in the Piney Point, Aquia, and 

Potomac aquifers increases generally eastward (pls. 6–8). 

Although areas to the west exhibit some local-scale variability, 

a consistent and abrupt eastward increase in total ion concentration in all three aquifers conforms to the outer margin of the 

Chesapeake Bay impact crater. In addition, a spatial sequence 

is apparent in the relative dominance among the major ions. 

(Ca2++ Mg2+)+(HCO3

–

+ CO3

–

) type “hard” water is present 

generally toward the west. (Na++ K+)+ (HCO3

–

+ CO3

–

) type 

“soft” water primarily occupies a broad north-south trending 

belt spanning much of the central part of the Virginia Coastal 

Plain. Toward the east, (Na++ K+)+Cl– type “salty” water 

(along with some samples of (Na++ K+)+mixed anion type 

water in the Potomac aquifer) conforms closely to the 

abruptly increasing total ion concentration along the outer 

crater margin.

Hydrochemical Processes

Diverse processes can potentially affect the chemical 

composition of groundwater in coastal aquifers, including 

mixing, ion exchange, diagenesis, and oxidation-reduction 

reactions (Jones and others, 1998). Water composing almost 

all of the groundwater samples presented herein likely 

originated as precipitation that infiltrated the land surface and 

has undergone processes that controlled its chemical composition as it flowed through the subsurface. One possible exception is a sample located in the southwestern part of the city of 

Suffolk, which is from basement bedrock and composed of 

(Na++ K+)+Cl– type “salty” water of relatively high total ion 

concentration (pl. 9). Bedrock at this location is composed 

of Jurassic- to Triassic-age sedimentary rock. The chemical 

composition of this water possibly was produced by diagenetic 

processes associated with lithification of the bedrock. 

Alternatively, relatively large chloride concentrations have 

been observed in groundwater in bedrock near Fredericksburg 

(Subitzky, 1961), which is likely of (Na+

+ K+)+Cl– type and is 

attributed to the presence of residual seawater (see “SaltwaterTransition Zone”).

Regional trends in the groundwater quality of the 

Virginia Coastal Plain were first recognized about 70 years 

ago (Cederstrom, 1939), from which time an understanding 

has been sought of how hydrochemical processes control 

groundwater composition. The concept of hydrochemical 

facies (Back, 1966) remains the most widely accepted 

fundamental description of the sequence by which groundwater composition evolves along regional flow paths throughout the entire Northern Atlantic Coastal Plain and is generally 

described below.
16 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Water infiltrating the land surface combines with carbon 

dioxide originating from soil organic material to form carbonic 

acid (hydrogen ions and bicarbonate anions):

H2O + CO2  H+ + HCO3

–

 (Drever, 1988).

With downward percolation to the water table, followed by 

flow through the saturated zone, water and hydrogen ions react 

with minerals making up subsurface sediments. These chemical weathering reactions are as highly varied as the many mineral species with which the water can come in contact. One of 

the most widespread reactions, however, is the dissolution of 

calcium-magnesium carbonates:

Ca,MgCO3 + H+  Ca2+, Mg2+ + HCO3

–

(Freeze and Cherry, 1979),

by which the chemical composition of groundwater becomes 

dominated by calcium and magnesium cations and bicarbonate 

anions to produce (Ca2++ Mg2+)+(HCO3

–

+ CO3

–

) type “hard” 

water. This type of reaction is termed congruent dissolution, 

because consumption of the solid mineral results entirely in 

dissolved chemical constituents. As a result of a continued 

supply of carbon dioxide under “open system” conditions 

(Freeze and Cherry, 1979), pH varies over a relatively small 

range, generally between 7 and 8.

Other widely occurring chemical weathering reactions 

include those involving diverse minerals collectively termed 

aluminosilicates. As one example, orthoclase feldspar reacts 

with hydrogen ions and water to produce potassium ions, 

dissolved silica, and the solid clay mineral kaolinite:

2KAlSi3

O8 + 2H+ + 9H2O  2K+ + 4H4SiO4 + Al2Si2O5(OH)4

(Drever, 1988).

This type of reaction is termed incongruent dissolution, 

because consumption of the original solid mineral results in 

both dissolved chemical constituents and a newly formed 

solid mineral. Diverse dissolved constituents can be produced, 

depending on the composition of the original aluminosilicate 

mineral. Such reactions, however, commonly take place at 

relatively slow rates because the produced clay forms coatings 

around the aluminosilicate grains, reducing contact with water. 

Thus, the dissolved constituents are produced in relatively 

small amounts that are less apt to dominate the chemical 

composition of groundwater. Similarly, small concentrations 

of various constituents can also result from other chemical 

weathering reactions (see “Selected Constituents”).

Following recharge at the water table, groundwater in the 

Virginia Coastal Plain continues to flow generally eastward 

through the saturated zone as far as several tens of miles, to 

depths as great as several hundred feet or more, and remains 

in contact with subsurface sediments for as long as several 

tens of thousands of years (Nelms and others, 2003) or more 

(McFarland and Bruce, 2005; Sanford and others, 2009b). Ion 

concentrations increase along regional flow paths, as more 

dissolved ions in groundwater are produced by continued 

chemical weathering. Other processes, however, also act to 

alter the relative dominance among the major ions. In water 

flowing to sufficiently great distances and depths, calcium 

and magnesium cations are adsorbed by the sediments onto 

grains of clay and glauconite, displacing previously adsorbed 

sodium and potassium cations into solution to thereby become 

the dominant cations. Bicarbonate anions remain unaffected. 

Thus, the earlier formed (Ca2++ Mg2+)+(HCO3

–

+ CO3

–

) type 

“hard” water evolves to a (Na++ K+)+(HCO3

–

+ CO3

–

) type 

“soft” water.

Lastly, and farther east toward the coast, groundwater 

mixes along a broad transition zone with seawater that occupies the sediments beneath the Atlantic Ocean. Cations remain 

dominated by sodium and potassium, but the concentration of 

chloride originating from seawater increases markedly 

and supersedes bicarbonate as the dominant anion. Thus, 

(Na++ K+)+ (HCO3

–

+ CO3

–

) type “soft” water evolves once 

more to a (Na++ K+)+Cl– type “salty” water. The total ion 

concentration of groundwater also increases abruptly into 

the brackish range, as a result of mixing with seawater. The 

saltwater-transition zone has a complex configuration resulting 

from changes in groundwater flow over time (see “SaltwaterTransition Zone”). In addition, changes in the major ion 

composition of groundwater along the saltwater-transition 

zone are associated with variations among the concentrations 

of various other constituents (see “Selected Constituents”).

Hydrochemical processes that control groundwater 

quality are strongly reflected by regional trends in major ion 

type and total ion concentration exhibited by the Potomac 

aquifer (pl. 8) and to a lesser extent by the Piney Point and 

Aquia aquifers (pls. 6, 7). The sequence by which groundwater composition evolves along regional flow paths is best 

developed in the Potomac aquifer, which is the thickest and 

deepest among the hydrogeologic units (fig. 2) and through 

which the greatest volume of groundwater flows. The hydrochemical sequence is also best discerned by the numerous 

and widespread groundwater samples collected from the 

Potomac aquifer. 

Recharge to the Potomac aquifer directly from infiltration 

at the land surface takes place primarily along the westernmost 

margin of the aquifer, as indicated by (Ca2++ Mg2+)+(HCO3

–

+ 

CO3

–

) type “hard” water being almost wholly constrained to 

that area (pl. 8). By contrast, the widespread presence of 

(Na++ K+)+ (HCO3

–

+ CO3

–

) type “soft” water in much of 

the aquifer farther east infers that any additional recharge to 

that part of the aquifer has first leaked downward through a 

substantial thickness of sediments of overlying hydrogeologic 

units and thereby exchanged much of the calcium and 

magnesium cations for sodium and potassium cations. The 

complete absence of any groundwater samples of mixed 

composition between these two types of water reflects the 

abrupt shift in the relative concentrations among the cations 

and indicates the efficiency with which cation exchange by the 

sediments alters dominance among the cations. (Two locations 

along the western margin of the Potomac aquifer of samples 
Regional Trends in Groundwater Quality 17

of (Na+K)+Cl type “salty” water and another two locations of 

samples of (Na+K)+mixed anion type water possibly reflect 

human solute sources.)

Still farther east, the presence of (Na++ K+)+Cl– type 

“salty” water in the Potomac aquifer conforms closely 

to a marked increase in total ion concentration along the 

saltwater-transition zone (pl. 8). In addition, mixed composition between this and (Na+

+ K+)+ (HCO3

–

+ CO3

–

) type “soft” 

water to the west is indicated by an appreciable number of 

(Na++ K+)+mixed anion type groundwater samples. Hence, 

in contrast to the abrupt effect of cation exchange to the 

west, freshwater/seawater mixing along the transition zone is 

relatively gradational.

Hydrochemical processes are reflected similarly in the 

Piney Point and Aquia aquifers (pls. 6, 7), although not as 

fully as in the Potomac aquifer. Eastwardly increasing total 

ion concentrations and evolving major ion types are generally 

apparent in both aquifers, although limited in detail and extent 

because of the scarcity of groundwater samples to the west and 

south. In addition, some samples of (Ca+Mg)+(HCO3+CO3) 

type “hard” water from both aquifers are located on the 

York-James Peninsula (farther east than generally found in 

the Potomac aquifer), because of differences in both of the 

aquifers’ sediment composition and depth. Sediments making 

up the Piney Point and Aquia aquifers contain much greater 

amounts of calcium-magnesium carbonates than those of 

the Potomac aquifer, which upon dissolution likely increase 

the relative dominance of calcium and magnesium cations 

in groundwater. Both aquifers are also shallower than the 

Potomac aquifer (fig. 2). Groundwater has flowed a shorter 

distance—and is less chemically evolved—than in the 

Potomac aquifer at the same geographic location.

In contrast to the Piney Point, Aquia, and Potomac 

aquifers, regional groundwater-flow paths do not exist in the 

surficial aquifer, Yorktown-Eastover aquifer, and basement 

bedrock. The surficial aquifer and Yorktown-Eastover aquifer 

are shallowest among the regional water-supply aquifers 

(fig. 2). Likewise, all but one of the samples from bedrock 

are located along the Fall Zone (pl. 9) and are also shallow. 

Shallow groundwater is divided among many localized and 

poorly known flow cells that encompass short flow paths 

between closely located recharge and discharge areas. The 

locations of groundwater samples having complete major ion 

analyses are too widely dispersed to discern spatial variations 

in groundwater quality within individual flow cells.

Shallow groundwater in the surficial aquifer, YorktownEastover aquifer, and basement bedrock also does not flow 

to sufficiently great distances and depths to undergo the 

complete sequence of chemical evolution reflected by the 

regional trends in major ion type and total ion concentration of deeper aquifers. For example, most groundwater 

samples from the Yorktown-Eastover aquifer are of either 

(Ca+Mg)+(HCO3

+CO3) type “hard” water of relatively low 

total ion concentration or (Na+K)+Cl type “salty” water of 

higher total ion concentration (pl. 5). Only a few samples 

of (Na+K)+(HCO3

+CO3) type “soft” water are present. 

“Hard” water is produced by dissolution of large amounts 

of calcium-magnesium carbonates that compose the aquifer 

sediments, but then evolves directly to “salty” water by mixing 

with seawater, either laterally along discharge areas at the 

Atlantic coast, Chesapeake Bay, and tidal streams and rivers, 

or vertically with the regional saltwater-transition zone. Most 

groundwater-flow paths are too short for cation exchange by 

the sediments to effectively “soften” the water.

Lastly, shallow groundwater of major ion types other 

than “hard,” “soft,” or “salty” can result locally from nearby 

human solute sources. Shallow groundwater in many areas 

has entered the subsurface within human timeframes and can 

potentially contain solutes originating from human sources 

(Nelms and others, 2003). Some groundwater samples at 

widely spaced locations from the surficial aquifer, YorktownEastover aquifer, and basement bedrock are categorized as 

“other” (pls. 4, 5, 9) and include (Ca+Mg)+SO4

, (Ca+Mg)+Cl, 

and (Ca+Mg)+mixed anion major ion types. Various aspects 

of closely located samples of shallow groundwater have been 

previously documented at Dahlgren (King George County) 

(Bell, 1996; Harlow and Bell, 1996) and on the Virginia 

Eastern Shore (Speiran, 1996). Solute sources at other sample 

locations potentially include a wide variety of both rural and 

urban activities, but are beyond the scope of the regional 

analysis here to consider in detail.

Saltwater-Transition Zone

The saltwater-transition zone in the Virginia Coastal Plain 

exhibits a marked eastward increase in the total ion concentration of groundwater along the outer margin of the Chesapeake 

Bay impact crater and a coincident evolution of its major ion 

type from a (Na++ K+)+(HCO3

–

+ CO3

–

) type “soft” water to 

a (Na++ K+)+Cl– type “salty” water (pls. 6–8) (see “Major 

Ions”). Chloride supersedes bicarbonate as the dominant 

anion as a result of mixing of groundwater with seawater that 

occupies the sediments beneath the Atlantic Ocean.

Chloride is a chemical constituent of particular resourcemanagement interest in the Virginia Coastal Plain, and its 

concentration is of primary concern in association with the 

saltwater-transition zone. Chloride concentrations below the 

U.S. Environmental Protection Agency (1990) secondary 

maximum contaminant level of 250 mg/L are commonly 

referred to as “fresh.” Concentrations between freshwater 

and that of seawater (19,000 mg/L) (Hem, 1985) are referred 

to as “brackish” and are widespread in large parts of the 

major water-supply aquifers across the eastern part of the 

Virginia Coastal Plain. Concentrations above that of seawater 

constitute “brine.” Chloride concentrations of brackish 

groundwater across the saltwater-transition zone are a major 

issue regarding the use of groundwater (see “Considerations 

for Groundwater Use”).

Spatial variations in groundwater chloride concentration 

are used herein to depict the saltwater-transition zone in a 

manner that most directly relates to resource-management 
18 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

interests. The compiled dataset (Attachment 1) includes 

6,650 chloride concentrations (Attachment 2), of which 

2,289 samples were considered to be acceptably accurate 

based on charge balance of major ions (see “Major Ions”). The 

accuracy of the chloride concentrations of other groundwater 

samples having incomplete analyses of major ions is uncertain 

and cannot be evaluated by charge balance. Paired values of 

chloride concentration and specific conductance, however, can 

be used to evaluate the accuracy of chloride concentration. 

Accordingly, a LOWESS smooth regression (Helsel and 

Hirsch, 2002) of specific conductance versus chloride concentration was performed on 1,683 samples that have values for 

both chloride concentration and specific conductance. Regression residual values were within 3 standard deviations of the 

mean for 1,658 of the samples, which were thereby designated 

as having acceptably accurate chloride concentrations. 

These samples were combined with those accepted based on 

charge balance, for a total of 3,947 chloride concentrations 

used to depict the saltwater-transition zone. Residual values 

of 25 samples were greater than 3 standard deviations of 

the mean and were considered to reflect unreliable chloride 

concentrations that were not used.

Configuration

Initial understanding of the physical principles governing the transition from freshwater to saltwater in coastal 

aquifers has been widely attributed to Ghyben (1888) and 

Herzberg (1901). In a homogeneous unconfined aquifer 

under hydrostatic conditions, freshwater is separated from 

the more dense seawater by a landward sloping interface. 

Hubbert (1940) elaborated that with steady-state outflow 

to the sea, the interface is displaced seaward. Henry (1960) 

described the transition as a dispersive mixing zone rather than 

a sharp boundary, which Pinder and Cooper (1970) further 

characterized with transient movement in a confined aquifer. 

In the Atlantic Coastal Plain from New Jersey through North 

Carolina, Meisler (1989) described a relatively broad transition zone between freshwater and saltwater.

Chloride-Concentration Sections

In characterizing groundwater salinity associated with the 

Chesapeake Bay impact crater, McFarland and Bruce (2005) 

depicted the vertical configuration of part of the saltwatertransition zone by a single hydrogeologic section across the 

lower part of the Middle Peninsula, upon which variations in 

groundwater specific conductance are represented by contours. 

A similar approach has been modified and expanded on for 

this report. Ten chloride-concentration sections, collectively 

spanning most of the Virginia Coastal Plain, depict the 

vertical configuration of the entire saltwater-transition zone by 

representing variations in groundwater chloride concentrations 

with point values and contours (pls. 10–14).

Chloride-concentration section point values alternately 

represent (1) the chloride concentration of a single 

groundwater sample or (2) a generalized groundwater chloride 

concentration, based on multiple samples collected from a 

single well or borehole at different times and(or) from multiple 

closely located wells or boreholes. Some samples are located 

directly on a section, whereas other samples are projected onto 

the section from locations in general proximity to the section.

Chloride-concentration section contours represent lines 

of equal groundwater chloride concentration that interpolate 

across areas between point values and, in some instances, 

extrapolate beyond the area spanned by point values. Contours 

were manually generated to be consistent with the point values 

but not wholly constrained by them. In addition, because the 

sections intersect at multiple locations, contoured features 

that were apparent from some sections were extrapolated onto 

adjacent sections to maintain consistency among the sections. 

To accommodate a general downward acceleration in chloride 

concentration, contour intervals range from 100 mg/L at 

shallow depths to as much as 6,000 mg/L below. 

In order to enable direct comparison of chlorideconcentration variations to other aspects of the aquifers, 

chloride-concentration section letter designations, content, and 

scaling used here are identical to those used for hydrogeologic 

sections in a revised hydrogeologic framework of the Virginia 

Coastal Plain (McFarland and Bruce, 2006; available at 

http://pubs.usgs.gov/pp/2006/1731/). A full description is 

given there of construction of the hydrogeologic sections. The 

chloride-concentration sections have been extended to greater 

depth, to encompass the full span of groundwater-sample 

locations. Sections DD–DD′ and ED–ED′ (pl. 10), FD–FD′ 

and GD–GD′ (pl. 11), and ID–ID′ and JD–JD′ (pl. 12) are 

oriented along the general regional stratigraphic dip of the 

hydrogeologic units. Among these, sections DD–DD′ and 

ED–ED′ (pl. 10) span the Chesapeake Bay impact crater. 

Sections DS–DS′ and ES–ES′ (pl. 13) and FS–FS′ and GS–GS′ 

(pl. 14) are oriented along stratigraphic strike. All of these 

sections span the impact crater, except DS–DS′, which is 

positioned several miles to the west (pl. 13). Seven additional 

hydrogeologic sections, mostly spanning the northern and 

western parts of the Virginia Coastal Plain, were constructed 

for the hydrogeologic framework, but are not included here 

because groundwater samples located in those areas do not 

span sufficient ranges of chloride concentration and depth.

Chloride-concentration sections oriented along dip 

exhibit an eastward and downward trend in increasing 

groundwater chloride concentration, which reflects a broad 

but generally landward-dipping saltwater-transition zone 

between fresh groundwater to the west and saltwater to the 

east (pls. 10–12). The altitude of the 250 mg/L contour line 

separating fresh and brackish groundwater is as deep as 

–1,000 ft in some western areas, but is higher than –100 ft 

toward the east. The 19,000 mg/L contour is equivalent to 

the chloride concentration of seawater and extends westward 

below –2,000 ft before truncating against basement bedrock, 

but to the east is higher at –1,000 ft beneath parts of the 

Virginia Eastern Shore (pl. 10). At greater depths, brine having 

chloride concentrations greater than seawater is beneath the 
Regional Trends in Groundwater Quality 19

Virginia Eastern Shore and the lower Chesapeake Bay, as well 

as the northern part of Virginia Beach (pl. 11).

Chloride-concentration sections oriented along strike 

also exhibit groundwater chloride concentrations that increase 

downward, but that are also positioned at relatively high 

altitudes across the Chesapeake Bay impact crater (pls. 13, 

14). Both the 250 mg/L contour and 19,000 mg/L contour are 

steeply dipping and displaced upward by as much as 1,000 ft 

across the impact crater relative to adjacent areas (section 

GS–GS′, pl. 14). This chloride “mound” extends outside the 

impact crater several miles to the west beneath the York-James 

Peninsula, where the 250 mg/L contour is displaced upward 

by more than 600 ft (section DS–DS′, pl. 13). The chloride 

mound is most elevated within confining units composed of 

low-permeability sediments deposited over the impact crater, 

which thicken and deepen into the impact crater (sections 

DD–DD′ and ED–ED′, pl. 10; section GD–GD′, pl. 11).

In addition to broad regional trends described previously, 

local-scale variations in groundwater chloride concentration 

take the form of concentration inversions. The downward 

increasing concentration trend is locally reversed across some 

vertical intervals having sufficient numbers of adequately 

distributed point values, including within and slightly outside 

of the Chesapeake Bay impact crater (sections DD–DD′ and 

ED–ED′, pl. 10; section ES–ES′, pl. 13), but also well outside 

the impact crater to the south (section JD–JD′, pl. 12; section 

ES–ES′, pl. 13).

Contours depict possible relations of the concentration 

inversions to regional groundwater flow and associated 

chloride-concentration trends. The inversions correlate along 

distinct zones that partly follow stratigraphic dip (sections 

DD–DD′ and ED–ED′, pl. 10; section JD–JD′, pl. 12). Parts of 

some inverted zones are extrapolated up-dip to areas exhibiting irregular altitudes among contrasting chloride concentrations, as well as onto adjacent sections. The inverted zones 

also follow a regional north-south structural trend, represented 

by an eastward plunging ridge in basement bedrock located 

south of the James River, termed the Norfolk arch (section 

ES–ES′, pl. 13). Inverted zones parallel the flanks of the 

Norfolk arch that dip northward into the Salisbury embayment across the Virginia Coastal Plain into Maryland and 

southward into North Carolina. Thus, the three-dimensional 

trend of the inverted zones is the resultant of the dip-oriented 

and flank-oriented components represented on the chlorideconcentration sections. The inverted zones are inferred to 

reflect paths of differential flushing of fresh groundwater (see 

“Freshwater Flushing”).

The different aspects of variations in groundwater 

chloride concentration described previously combine to make 

up the overall configuration of the saltwater-transition zone. 

The generally eastward and downward increasing groundwater 

chloride concentrations are displaced upward to form a mound 

across the Chesapeake Bay impact crater, across which zones 

of inverted chloride concentration are superimposed. Thus, the 

generally landward-dipping saltwater-transition zone 

has a convoluted configuration, which can loosely be 

described as being warped around the impact crater along the 

inverted zones.

Chloride Iso-Concentration Surfaces

In order to gain additional perspectives on the configuration of the saltwater-transition zone, the representations of 

groundwater chloride concentration on chloride-concentration 

sections were expanded by a series of groundwater chloride 

iso-concentration surfaces. Each contour appearing on a 

section represents a three-dimensional trace, resulting from the 

intersection of the vertical plane of the section with a sloping 

surface of equal groundwater chloride concentration that is 

projected through the section. Five of these iso-concentration 

surfaces, corresponding to section contours ranging from 

50 mg/L to 19,000 mg/L, were delineated by correlating 

among the sections (figs. 4–8). The altitude of each isoconcentration surface was identified at various points along the 

sections directly from the section contours. Contours in map 

view were then drawn manually to interpolate altitudes across 

areas between sections. The contours represent lines of equal 

altitude that depict the configuration of each iso-concentration 

surface. Contour intervals are variable, as small as 50 ft at 

high altitudes to reflect subtle features of the shallowest 

parts of the surfaces, and as much as 250 ft at lower altitudes 

where the surfaces are steeply dipping. Section alignments are 

indicated, so that particular features of the iso-concentration 

surfaces can be cross referenced with the sections, to be seen 

in side view.

The extent of each groundwater chloride isoconcentration surface is limited by those of the corresponding 

chloride-concentration section contours and is greatest for 

the relatively shallow 50 mg/L surface (fig. 4) and least for 

the deep 19,000 mg/L surface (fig. 8). The iso-concentration 

surfaces are further limited to depicting broad regional 

groundwater chloride-concentration trends. Subjective 

judgment was required in manually generating contours to 

depict plausible surface configurations, which are consistent 

with the section contours but not wholly constrained by them. 

Moreover, because contouring is constrained to a planer view, 

the surfaces cannot depict vertically inverted groundwater 

chloride-concentration zones. In such areas, contours are 

generalized across the inverted parts of the surfaces.

Groundwater chloride iso-concentration surfaces 

provide a general indication of different groundwater chloride 

concentrations at various altitudes within the Coastal Plain 

sediments, which has practical use for site-specific activities 

such as locating and constructing groundwater wells. Accounting for variations in land-surface altitude, groundwater having 

a particular chloride concentration would be encountered at 

relatively shallow depths at locations where the corresponding 

iso-concentration surface is at a high altitude and at greater 

depths where the surface is at a low altitude. (Evaluation 

of the possibility of encountering inverted chlorideconcentration zones would entail examination of the chlorideconcentration sections.)
20 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Figure 4. Approximate altitude and configuration of the 50 milligrams per liter chloride iso-concentration surface in the Virginia Coastal 

Plain. Surface is delineated on the basis of correlation among chloride-concentration sections (pls. 10–14). Mapped extent of surface is 

limited to that of section contours. Location of Chesapeake Bay impact crater from Powars and Bruce (1999).

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

EXPLANATION

Iso-concentration surface contour—Approximate

 altitude of 50 milligrams per liter chloride

 iso-concentration surface. Contour interval

 is variable. Datum is NGVD 29

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

DD

ED

FD

GD

ID

JD DS

ES

FS

GS JD’

ID’

GD’

FD’

ED’

DD’

GS’

ES’

DS’

GD GD’ Line of chloride-concentration section

Figure 4. Approximate altitude and configuration of the 50 milligrams per liter chloride iso-concentration surface in the Virginia

 Coastal Plain. Surface is delineated based on correlation among chloride-concentration sections (Plates 10–14). Mapped extent

 of surface is limited to that of section contours. Location of Chesapeake Bay impact crater (from Powars and Bruce, 1999).

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

Piedmont

River

FS’

Fall Zone

Piedmont

Fall Zone

LOCATION OF THE STUDY AREA

IN VIRGINIA

–50

–50

0

–100

–750

–750

–500

–250

–100

–50

–250

–50

–500

–100

–250

–750

–500

–1,000

–750

–100

–500

–250

–500
Regional Trends in Groundwater Quality 21

Figure 5. Approximate altitude and configuration of the 250 milligrams per liter chloride iso-concentration surface in the Virginia Coastal 

Plain. Surface is delineated on the basis of correlation among chloride-concentration sections (pls. 10–14). Mapped extent of surface is 

limited to that of section contours. Location of Chesapeake Bay impact crater from Powars and Bruce (1999).

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

EXPLANATION

Iso-concentration surface contour—Approximate

 altitude of 250 milligrams per liter chloride

 iso-concentration surface. Contour interval

 is variable. Datum is NGVD 29

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

DD

ED

FD

GD

ID

JD DS

ES

FS

GS JD’

ID’

GD’

FD’

ED’

DD’

GS’

ES’

DS’

GD GD’ Line of chloride-concentration section

Figure 5. Approximate altitude and configuration of the 250 milligrams per liter chloride iso-concentration surface in the Virginia

 Coastal Plain. Surface is delineated based on correlation among chloride-concentration sections (Plates 10–14). Mapped extent

 of surface is limited to that of section contours. Location of Chesapeake Bay impact crater (from Powars and Bruce, 1999).

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

Piedmont

River

FS’

Fall Zone

Piedmont

Fall Zone

LOCATION OF THE STUDY AREA

IN VIRGINIA

–250

–900

–1,000

–750

–1,000

–1,000

–750

–500

–500

–100

–750

–1,000

–750

–500

–250

–500

–250

–100
22 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Figure 6. Approximate altitude and configuration of the 1,000 milligrams per liter chloride iso-concentration surface in the Virginia 

Coastal Plain. Surface is delineated on the basis of correlation among chloride-concentration sections (pls. 10–14). Mapped extent of 

surface is limited to that of section contours. Location of Chesapeake Bay impact crater from Powars and Bruce (1999).

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

EXPLANATION

Iso-concentration surface contour—Approximate

 altitude of 1,000 milligrams per liter chloride

 iso-concentration surface. Contour interval

 is variable. Datum is NGVD 29

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

DD

ED

FD

GD

ID

JD DS

ES

FS

GS JD’

ID’

GD’

FD’

ED’

DD’

GS’

ES’

DS’

GD GD’ Line of chloride-concentration section

Figure 6. Approximate altitude and configuration of the 1,000 milligrams per liter chloride iso-concentration surface in the Virginia

 Coastal Plain. Surface is delineated based on correlation among chloride-concentration sections (Plates 10–14). Mapped extent

 of surface is limited to that of section contours. Location of Chesapeake Bay impact crater (from Powars and Bruce, 1999).

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

Piedmont

River

FS’

Fall Zone

Piedmont

Fall Zone

LOCATION OF THE STUDY AREA

IN VIRGINIA

–1,100

–1,400

–1,250

–1,000

–150

–250

–1,250

–1,000

–750

–500

–250

–500

–1,100

–1,000

–1,000

–250

–1,000

–750

–500

–300

–1,250
Regional Trends in Groundwater Quality 23

Figure 7. Approximate altitude and configuration of the 10,000 milligrams per liter chloride iso-concentration surface in the Virginia 

Coastal Plain. Surface is delineated on the basis of correlation among chloride-concentration sections (pls. 10–14). Mapped extent of 

surface is limited to that of section contours. Location of Chesapeake Bay impact crater from Powars and Bruce (1999).

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

EXPLANATION

Iso-concentration surface contour—Approximate

 altitude of 10,000 milligrams per liter chloride

 iso-concentration surface. Contour interval

 is variable. Datum is NGVD 29

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

DD

ED

FD

GD

ID

JD DS

ES

FS

GS JD’

ID’

GD’

FD’

ED’

DD’

GS’

ES’

DS’

GD GD’ Line of chloride-concentration section

Figure 7. Approximate altitude and configuration of the 10,000 milligrams per liter chloride iso-concentration surface in the Virginia

 Coastal Plain. Surface is delineated based on correlation among chloride-concentration sections (Plates 10–14). Mapped extent

 of surface is limited to that of section contours. Location of Chesapeake Bay impact crater (from Powars and Bruce, 1999).

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

Piedmont

River

FS’

Fall Zone

Piedmont

Fall Zone

LOCATION OF THE STUDY AREA

IN VIRGINIA

–1,500

–1,250

–1,000

–750

–500

–450

–1,100

–1,000

–1,800

–750

–1,000

–1,900

–1,800

–1,250

–1,500

–1,250

–1,750

–1,750 –500
24 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Figure 8. Approximate altitude and configuration of the 19,000 milligrams per liter chloride iso-concentration surface in the Virginia 

Coastal Plain. Surface is delineated on the basis of correlation among chloride-concentration sections (pls. 10–14). Mapped extent of 

surface is limited to that of section contours. Location of Chesapeake Bay impact crater from Powars and Bruce (1999).

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

EXPLANATION

Iso-concentration surface contour—Approximate

 altitude of 19,000 milligrams per liter chloride

 iso-concentration surface. Contour interval

 is variable. Datum is NGVD 29

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

DD

ED

FD

GD

ID

JD DS

ES

FS

GS JD’

ID’

GD’

FD’

ED’

DD’

GS’

ES’

DS’

GD GD’ Line of chloride-concentration section

Figure 8. Approximate altitude and configuration of the 19,000 milligrams per liter chloride iso-concentration surface in the Virginia

 Coastal Plain. Surface is delineated based on correlation among chloride-concentration sections (Plates 10–14). Mapped extent

 of surface is limited to that of section contours. Location of Chesapeake Bay impact crater (from Powars and Bruce, 1999).

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

Piedmont

River

FS’

Fall Zone

Piedmont

Fall Zone

LOCATION OF THE STUDY AREA

IN VIRGINIA

–1,750

–1,500

–1,250

–1,000

–2,300

–2,250

–2,000

–1,500

–2,000

–1,750

–1,500

–1,250

ES’ DS’
Regional Trends in Groundwater Quality 25

The groundwater chloride iso-concentration surfaces 

also have practical use for analyses of the groundwater-flow 

system (see “Relations to Groundwater Flow”), by collectively 

depicting various features making up the three-dimensional 

configuration of the saltwater-transition zone. As exhibited by 

the relatively shallow 50 mg/L, 250 mg/L, and 1,000 mg/L 

iso-concentration surfaces (figs. 4–6), the earlier described 

chloride mound across the Chesapeake Bay impact crater 

(see “Chloride-Concentration Sections”) exhibits a broad 

dome shape and is centered along the western to southwestern 

side of the outer impact crater. These surfaces also exhibit 

a westward plunging spur ridge projecting from the mound 

beneath the York-James Peninsula. The ridge on the 50 mg/L 

surface extends fully to the Fall Zone (fig. 4), rising in altitude 

to take on a saddle-like shape. Flank areas of the mound and 

ridge on all three surfaces dip evenly northwestward beneath 

the Middle Peninsula and Northern Neck, and southwestward 

beneath Surry and Isle of Wight Counties. Farther south, 

a second, more irregularly shaped ridge plunges westward 

beneath the cities of Suffolk and Chesapeake. This ridge 

is separated from the mound by a southwestward plunging 

trough, trending from the southern tip of the Virginia Eastern 

Shore across the lowermost parts of Chesapeake Bay and the 

James River.

Overall similar configurations are exhibited at greater 

depth by the 10,000 mg/L and 19,000 mg/L iso-concentration 

surfaces (figs. 7, 8), although across smaller extents. The 

chloride mound exhibits a more plug-like shape, however, 

with a level plateau and precipitously dipping flanks and is 

centered farther eastward into the inner crater.

Aquifer Chloride Concentrations

In addition to the groundwater chloride-concentration 

sections and chloride iso-concentration surfaces, further 

perspective on the configuration of the saltwater-transition 

zone is provided by a series of chloride-concentration maps 

of water-supply aquifers (figs. 9–13). Chloride-concentration 

sections depict the configuration of the saltwater-transition 

zone only across discrete vertical profiles of the hydrogeologic 

units (pls. 10–14), and iso-concentration surfaces depict only 

broad regional trends of chloride concentrations at various 

altitudes, independent of their positions within hydrogeologic 

units (figs. 4–8). Some resource-management information 

needs are specific to individual hydrogeologic units, typically 

one or more of the major water-supply aquifers. Accordingly, 

chloride concentrations were delineated within the YorktownEastover, Piney Point, Aquia, Virginia Beach, and Potomac 

aquifers. All of these hydrogeologic units represent major 

regional water-supply resources except for the Virginia Beach 

aquifer, which is of local importance in the city of Suffolk, 

where it can be favored over the deeper Potomac aquifer 

because of its higher groundwater levels (McFarland and 

Bruce, 2006).

Conventional contouring of groundwater chloride concentration is not feasible for some aquifers, because of spatial 

data gaps, regionally non-uniform concentration gradients, 

and local-scale concentration variability (either laterally or 

vertically). Instead, the GIS developed as part of the newly 

revised hydrogeologic framework of the Virginia Coastal Plain 

(McFarland and Bruce, 2006) (see “Methods of Investigation”) was applied to delineate chloride concentrations among 

the aquifers. GIS raster datasets that represent the approximate 

configurations of the top surfaces of all hydrogeologic units 

were used in conjunction with the chloride iso-concentration 

surfaces. Each iso-concentration surface is a sloping surface 

of equal groundwater chloride concentration (see “Chloride 

Iso-Concentration Surfaces”) that projects through the space 

occupied by the various hydrogeologic units.

GIS operations were performed to delineate a series of 

three-dimensional traces that result from intersections of the 

hydrogeologic unit top surfaces with the iso-concentration 

surfaces. Each trace is a projected contour line of equal 

chloride concentration along the top surface of the hydrogeologic unit. Segments of chloride-concentration contours 

were combined from among various hydrogeologic unit 

surfaces to construct continuous contours across both the 

bottom and top surfaces of the water-supply aquifers. The 

chloride-concentration value of each contour corresponds to 

each of the iso-concentration surfaces, resulting in a variable 

contour interval. Likewise, the extents of the contours are 

limited by those of the iso-concentration surfaces. Also like 

the iso-concentration surfaces, the contours represent broad 

regional trends of chloride concentrations that do not include 

all local-scale features, such as vertically inverted groundwater 

chloride-concentration zones.

The Piney Point, Aquia, and Virginia Beach aquifers are 

thin (fig. 2), and vertical variations in chloride concentration 

within these aquifers are small relative to lateral variations. 

The relatively steep dips of the chloride iso-concentration 

surfaces result in little difference between the projected positions and alignments of the top-surface chloride-concentration 

contours and those of the bottom-surface contours. Accordingly, a single set of chloride-concentration contours was 

manually generated for each of these aquifers that closely 

approximates the positions and alignments of both their 

top-surface and bottom-surface contours (figs. 10–12).

In contrast to the Piney Point, Aquia, and Virginia Beach 

aquifers, the Yorktown-Eastover aquifer and especially the 

Potomac aquifer are thick (fig. 2). Vertical variations in 

chloride concentration within these aquifers are comparable in 

magnitude to lateral variations. Chloride iso-concentration surface dips are shallow enough that the projected positions and 

alignments of the top-surface chloride-concentration contours 

differ substantially from those of the bottom-surface contours. 

Accordingly, both the top-surface and bottom-surface 

chloride-concentration contours serve together to delineate 

chloride concentrations within the Yorktown-Eastover and 

Potomac aquifers (figs. 9, 13, respectively). For each aquifer, 

the two sets of projected contours are distinguished by 

different colors. For added clarity, the different areas 

spanned by the two sets of contours are also shaded using 

corresponding colors.
26 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Figure 9. Approximate groundwater chloride concentrations, in milligrams per liter, across the top (red) and bottom (blue) of the YorktownEastover aquifer in the Virginia Coastal Plain. Areas spanned by alternate sets of contours are shaded in corresponding colors. Mapped extents 

of contours are limited to those of chloride iso-concentration surfaces (figs. 3–7). Area outside extent of aquifer is shaded gray. Location of 

Chesapeake Bay impact crater from Powars and Bruce (1999).

EXPLANATION

Chloride-concentration contour—Approximate concentration of 

 chloride in milligrams per liter in groundwater across the top 

 of the Yorktown-Eastover aquifer. Contour interval is variable

Area where the two sets of contours overlap

250

Chloride-concentration contour—Approximate concentration of 

 chloride in milligrams per liter in groundwater across the bottom 

 of the Yorktown-Eastover aquifer. Contour interval is variable

250

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE

BAY

Potomac

River

James

York

River

Rappahannock

River

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

River

LOCATION OF THE STUDY AREA

Piedmont

IN VIRGINIA

Fall Zone

Piedmont

Fall Zone

Figure 9. Approximate concentration in milligrams per liter of chloride in groundwater across the top (red) and bottom (blue) of the Yorktown-Eastover

 aquifer in the Virginia Coastal Plain. Areas spanned by alternate sets of contours are shaded in corresponding colors. Mapped extents of contours

 are limited to those of chloride iso-concentration surfaces (figure 3–7). Area outside extent of aquifer is shaded gray. Location of Chesapeake Bay

 impact crater (from Powars and Bruce 1999).

50

50

50

1,000

250

50

50

250

1,000

1,000
Regional Trends in Groundwater Quality 27

Figure 10. Approximate groundwater chloride concentrations, in milligrams per liter, in the Piney Point aquifer in the Virginia Coastal Plain. 

Mapped extent of contours is limited to that of chloride iso-concentration surfaces (figs. 3–7). Area outside extent of aquifer is shaded gray. 

Location of Chesapeake Bay impact crater from Powars and Bruce (1999).

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

EXPLANATION

Chloride-concentration contour—Approximate

 concentration of chloride in milligrams per liter

 in groundwater in the Piney Point aquifer.

 Contour interval is variable 

50

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

River

LOCATION OF THE STUDY AREA

Piedmont

IN VIRGINIA

Fall Zone

Piedmont

Fall Zone

Figure 10. Approximate concentration in milligrams per liter og chloride in groundwater in the Piney Point aquifer in the Virginia Coastal Plain.

 Mapped extent of contours is limited to that of chloride iso-concentration surfaces (figures 3–7). Area outside extent of aquifer is shaded gray.

 Location of the Chesapeake Bay impact crater is from Powars and Bruce (1999). 

50

250

1,000

19,000

10,000

250 50
28 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Figure 11. Approximate groundwater chloride concentrations, in milligrams per liter, in the Aquia aquifer in the Virginia Coastal Plain. 

Mapped extent of contours is limited to that of chloride iso-concentration surfaces (figs. 3–7). Area outside extent of aquifer is shaded 

gray. Location of Chesapeake Bay impact crater from Powars and Bruce (1999).

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

EXPLANATION

Chloride-concentration contour—Approximate

 concentration of chloride in milligrams per liter

 in groundwater in the Aquia aquifer.

 Contour interval is variable 

50

Figure 11. Approximate concentration in milligrams per liter og chloride in groundwater in the Aquia aquifer in the Virginia Coastal Plain.

 Mapped extent of contours is limited to that of chloride iso-concentration surfaces (figures 3–7). Area outside extent of aquifer is shaded gray.

 Location of the Chesapeake Bay impact crater is from Powars and Bruce (1999). 

250

1,000

50

250

50

10,000

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

River

LOCATION OF THE STUDY AREA

Piedmont

IN VIRGINIA

Fall Zone

Piedmont

Fall Zone

-750
Regional Trends in Groundwater Quality 29

Figure 12. Approximate groundwater chloride concentrations, in milligrams per liter, in the Virginia Beach aquifer in the Virginia Coastal 

Plain. Mapped extent of contours is limited to that of chloride iso-concentration surfaces (figs. 3–7). Area outside extent of aquifer is 

shaded gray. Location of Chesapeake Bay impact crater from Powars and Bruce (1999).

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

EXPLANATION

Chloride-concentration contour—Approximate

 concentration of chloride in milligrams per liter

 in groundwater in the Virginia Beach aquifer.

 Contour interval is variable

50

Figure 12. Approximate concentration in milligrams per liter og chloride in groundwater in the Virginia Beach aquifer in the Virginia Coastal Plain.

 Mapped extent of contours is limited to that of chloride iso-concentration surfaces (figures 3–7). Area outside extent of aquifer is shaded gray.

 Location of the Chesapeake Bay impact crater is from Powars and Bruce (1999). 

50

250

1,000

10,000

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

River

LOCATION OF THE STUDY AREA

Piedmont

IN VIRGINIA

Fall Zone

Piedmont

Fall Zone
30 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Figure 13. Approximate groundwater chloride concentrations, in milligrams per liter, across the top (red) and bottom (blue) of the Potomac 

aquifer in the Virginia Coastal Plain. Areas spanned by alternate sets of contours are shaded in corresponding colors. Mapped extents of 

contours are limited to those of chloride iso-concentration surfaces (figs. 3–7). Area outside extent of aquifer is shaded gray. Location of 

Chesapeake Bay impact crater from Powars and Bruce (1999).

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

EXPLANATION

Chloride-concentration contour—Approximate concentration of 

 chloride in milligrams per liter in groundwater across the top 

 of the Potomac aquifer. Contour interval is variable

Area where the two sets of contours overlap

250

Chloride-concentration contour—Approximate concentration of 

 chloride in milligrams per liter in groundwater across the bottom 

 of the Potomac aquifer. Contour interval is variable

250

Figure 13. Approximate concentration in milligrams per liter of chloride in groundwater across the top (red) and bottom (blue) of the Potomac

 aquifer in the Virginia Coastal Plain. Areas spanned by alternate sets of contours are shaded in corresponding colors. Mapped extents of contours

 are limited to those of chloride iso-concentration surfaces (figure 3–7). Area outside extent of aquifer is shaded gray. Location of Chesapeake Bay

 impact crater (from Powars and Bruce 1999).

50

50

250

1,000

1,000

10,000

19,000

10,000

19,000

50

250

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

River

LOCATION OF THE STUDY AREA

Piedmont

IN VIRGINIA

Fall Zone

Piedmont

Fall Zone
Regional Trends in Groundwater Quality 31

Groundwater chloride concentrations across the entire 

thickness of the Yorktown-Eastover aquifer (fig. 9) are less 

than 50 mg/L beneath the lower Northern Neck, upper Middle 

Peninsula and York-James Peninsula, and areas south of the 

James River across most of the city of Suffolk and westward. 

Conversely, greater chloride concentrations are projected 

beneath the lower Middle Peninsula and York-James Peninsula, areas south of the James River across the southeastern 

city of Suffolk, most of the city of Chesapeake and eastward, 

and the Virginia Eastern Shore. The area across the bottom 

of the aquifer where concentrations exceed 50 mg/L extends 

farther west than the area across the top of the aquifer, 

reflecting the generally landward dip of the saltwater-transition 

zone. Middle parts of the aquifer where concentrations 

exceed 50 mg/L are positioned between these two areas. The 

chloride “mound” (see “Chloride-Concentration Sections” 

and “Chloride Iso-Concentration Surfaces”) is exhibited 

along the western part of the Chesapeake Bay impact crater, 

where concentrations exceed 250 mg/L across part of the 

top of the aquifer and 1,000 mg/L across part of the bottom 

of the aquifer. Concentrations greater than 50 mg/L are also 

projected across the city of Virginia Beach southwestward 

as far as the southeastern city of Suffolk, corresponding to 

the chloride “ridge.” Additionally, beneath Chesapeake Bay 

and the Virginia Eastern Shore, concentrations exceeding 

50 mg/L are projected across the top of the aquifer along two 

narrow belts, where the top of the aquifer has been incised by 

paleochannels (McFarland and Bruce, 2006).

Groundwater chloride concentrations are projected 

similarly among the Piney Point, Aquia, and Virginia Beach 

aquifers (figs. 10–12). Within the extent of each aquifer, 

concentrations are generally less than 50 mg/L beneath the 

lower Northern Neck, upper Middle Peninsula, and areas 

south of the James River across most of the city of Suffolk and 

westward. Greater concentrations are projected beneath the 

lower Middle Peninsula, most of the York-James Peninsula, 

areas south of the James River across the southeastern city of 

Suffolk, most of the city of Chesapeake and eastward, and the 

Virginia Eastern Shore. The chloride mound is exhibited by 

the conformance of large chloride concentrations in the Piney 

Point aquifer with the Chesapeake Bay impact crater, where 

concentrations exceed 19,000 mg/L (seawater concentration) 

within the inner crater beneath the Virginia Eastern Shore. 

(The Aquia aquifer does not span the Virginia Eastern Shore 

and the lowermost parts of the Middle Peninsula and YorkJames Peninsula, and the Virginia Beach aquifer is present 

only from the city of Virginia Beach westward into Southampton County.) Large concentrations in all three aquifers are also 

projected across the city of Virginia Beach southwestward as 

far as the southeastern city of Suffolk, corresponding to the 

chloride ridge. 

Because of the large thickness of the Potomac aquifer 

(fig. 2) and corresponding vertical variations in groundwater 

chloride concentration, projected concentrations differ 

substantially between the top and the bottom of the aquifer 

(fig. 13). Across the top of the Potomac aquifer, concentrations 

are projected similarly to the overlying aquifers, encompassing 

areas as far east as the lower Northern Neck, upper Middle 

Peninsula, and areas south of the James River across most of 

the city of Suffolk with concentrations of less than 50 mg/L. 

Farther east and also similar to overlying aquifers, greater 

concentrations across the top of the Potomac aquifer reflect 

conformance of the chloride mound with the Chesapeake 

Bay impact crater, where concentrations exceed 19,000 mg/L 

(seawater concentration) generally beneath Chesapeake 

Bay, and the chloride ridge across the city of Virginia Beach 

southwestward into the southeastern city of Suffolk.

Groundwater chloride concentrations projected across the 

bottom of the Potomac aquifer are greater than across the top 

of the aquifer and extend farther inland, exceeding 50 mg/L in 

proximity to the Fall Zone (fig. 13). (At an isolated location 

along the Fall Zone in the southwestern corner of Charles City 

County, chloride concentrations exceeding 50 mg/L are also 

projected to the top of the aquifer.) Concentrations across the 

bottom of the aquifer continue eastward to increase uniformly 

to 19,000 mg/L along the western margin of the Chesapeake 

Bay impact crater beneath the lowermost parts of the Middle 

Peninsula and York-James Peninsula, and southward into 

the city of Chesapeake. Thus, the area across the bottom of 

the Potomac aquifer where concentrations exceed 50 mg/L 

extends farther west by roughly 30 mi than the area across 

the top of the aquifer, reflecting the broad landward dip of 

the saltwater-transition zone through the large thickness of 

the aquifer. Middle parts of the aquifer where concentrations 

exceed 50 mg/L are positioned between these two areas. 

Also, unlike the top of the Potomac aquifer, concentrations 

projected across the bottom of the aquifer do not reflect the 

chloride mound across the Chesapeake Bay impact crater, 

but rather exhibit a north-south trending planer configuration 

that results from chloride iso-concentration surfaces being 

truncated against basement bedrock. Although chloride 

concentrations greater than 50 mg/L across the bottom of the 

Potomac aquifer likely span much of Middle Peninsula and 

Northern Neck farther to the north than shown, the extent of 

chloride-concentration contours is constrained by that of the 

corresponding iso-concentration surfaces.

Relations to Groundwater Flow

The saltwater-transition zone is a major regional boundary on the groundwater-flow system. Fresh groundwater 

originating from recharge at the water table leaks downward 

into confined aquifers and, prior to large groundwater withdrawals, flowed downgradient to the east. Upon encountering 

dense saltwater along the coast, groundwater leaks back 

upward to discharge to the Atlantic Ocean, major rivers, and 

Chesapeake Bay. The saltwater-transition zone encompasses 

a volume of sediments within which dispersive mixing takes 

place between fresh and salty water across a density gradient.

Alternate mechanisms have been theorized to explain the 

origin and configuration of the saltwater-transition zone in the 

Virginia Coastal Plain and beyond. More than 60 years ago, 
32 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Cederstrom (1943) described the irregular configuration of the 

saltwater-transition zone as a “saltwater wedge” and theorized 

“differential flushing” of groundwater around a “structural 

depression” in the area of the then-unknown Chesapeake Bay 

impact crater, which left residual seawater retained in the nowrecognized crater-fill sediments. Similarly, Meisler (1989) 

attributed large-scale lateral and vertical salinity variations 

across the entire North Atlantic Coastal Plain to variations in 

flow rates among different parts of the aquifer system and to 

variable sea-level fluctuations across the region. Alternatively, 

Manheim and Horn (1968) and Meisler (1989) cited upward 

diffusion of solutes from the dissolution of basement evaporite 

deposits for parts of the Atlantic Coastal Plain. Increases in 

salinity by membrane filtration (reversed osmotic movement 

of water between particles of clay) was also suggested by 

Larson (1982) for the Virginia Coastal Plain in general and 

by Powars and Bruce (1999) for the Chesapeake Bay impact 

crater specifically, as a result of loading, compaction, and 

dewatering of crater-fill sediments.

McFarland and Bruce (2005) reviewed the preceding 

works in detail and concluded on the basis of their own 

hydrochemical evidence that mixing of fresh groundwater 

and seawater has taken place along the outer margin of the 

Chesapeake Bay impact crater and that small amounts of 

brine that exceed seawater salinity by as much as 35 percent 

could result from a moderate degree of seawater evaporation. 

Subsequently using a similar approach, Sanford and others 

(2009b) concluded that seawater altered by heat-induced 

diagenesis has been retained within the inner crater since 

the time of the impact event and suggested that brine with 

concentrations as much as twice that of seawater was present 

in deep target sediments and(or) basement bedrock prior to the 

impact and were excavated to higher altitudes by the blast.

Emplacement of Seawater

Exclusive of brine (see “Brine and the Inner Chesapeake 

Bay Impact Crater”), both geologic and hydrochemical 

evidence indicates that much of the saltwater now present in 

Virginia Coastal Plain sediments originated from seawater, 

rather than evaporate dissolution or membrane filtration 

(McFarland and Bruce, 2005; Sanford and others, 2009b). 

The Atlantic Coastal Plain is contiguous with the offshore 

Continental Shelf, which together represent an approximately 

160-mi-wide wedge of sediment that mantles the eastern 

margin of North America from the Fall Zone to the Atlantic 

Basin. Seawater has been repeatedly emplaced within 

various volumes of the sediment wedge as a result of partial 

to complete inundation by the Atlantic Ocean over the past 

approximately 65 million years (m.y.). The most recent 

complete inundation took place during the Pliocene Epoch 

approximately 1.8 to 5.3 m.y., when seawater likely occupied 

nearly the entire volume of sediments. Relatively large 

groundwater chloride concentrations have been attributed to 

apparently isolated residual seawater as far west as Fredericksburg (Subitzky, 1961).

Upon emergence of the sediment wedge, eastward and 

downward flowing fresh groundwater began flushing seawater 

from the sediments. Areas remained inundated as far west as 

the eastern city of Suffolk as recently as 115,000 years ago 

during interglacial periods of the Pleistocene Epoch, when sea 

levels were higher than at present by as much as 20 ft (Bradley, 1999). Subsequently, nearly the entire sediment wedge 

emerged over the next approximately 95,000 years. Sea level 

declined as low as 390 ft to 470 ft below present day, during 

the Pleistocene glacial maximum from 18,000 to 21,000 years 

ago (Peltier, 1994; Bradley, 1999). The hydraulic gradient of 

fresh groundwater was two to four times greater than at present, and enhanced flushing emplaced freshwater throughout 

nearly the entire volume of sediment as the saltwater-transition 

zone was displaced eastward to the edge of the Continental 

Shelf (Heywood, 2003). 

Since the Pleistocene glacial maximum, sea level has 

risen to its current position, and seawater has re-inundated the 

approximate eastern half of the sediment wedge, as well as 

the Pleistocene-age valley of the Susquehanna River to form 

Chesapeake Bay, and the valleys of several other major rivers 

to form large estuaries. Modern seawater is now reentering 

the sediments, and the saltwater-transition zone is migrating 

to the west. Eastward flushing by fresh groundwater, however, 

has continued beneath the still emergent part of the sediment 

wedge, but has been progressively truncated by the westward 

migrating saltwater-transition zone.

Freshwater Flushing

The manner in which seawater has been flushed from 

the sediments by fresh groundwater can be inferred from the 

position and configuration of the saltwater-transition zone. 

Although flow is currently redirected toward large withdrawal 

centers, the present-day saltwater-transition zone reflects 

pre-stress conditions because flow velocities are very slow, 

and groundwater has not yet been displaced over regionally 

appreciable distances in response to pumping.

The configuration of the saltwater-transition zone 

indicates that seawater has been flushed from the sediments 

by fresh groundwater in a complex three-dimensional fashion. 

The chloride “mound” that conforms with the Chesapeake 

Bay impact crater (see “Configuration”) infers that flushing 

has been impeded across the impact crater as a result of 

broad contrasts between sediment permeabilities inside and 

outside the impact crater, including confining units composed 

of low-permeability sediments that thicken and deepen into 

the impact crater and(or) hydraulic barriers created by faults 

associated with the impact crater. Distinct zones of preferential 

flow, however, cannot be explicitly delineated solely on the 

basis of sediment composition and hydraulic properties. 

Individual strata within the chaotically mixed crater-fill 

sediments, as well as undisrupted but highly interbedded 

fluvial-deltaic sediments outside the impact crater, exhibit 

extreme compositional contrasts over very short distances 

(for example, as observed in sediment cores), but cannot be 
Regional Trends in Groundwater Quality 33

effectively correlated at a regional scale (McFarland and 

Bruce, 2006). Likewise, faults have been generally theorized 

to act alternately as either conduits or barriers to groundwater 

flow (Caine and others, 1996), but their distribution and 

structure across the impact crater have not been delineated in 

adequate detail to determine their hydraulic effect. Alternately, 

variations in groundwater chloride concentrations reflect the 

effective regional scale result of flushing, which has been 

integrated across smaller-scale hydraulic influences by mixing 

and dispersion.

A lateral divide of the generally eastward flow of fresh 

groundwater is indicated by the spur ridge on the 50 mg/L, 

250 mg/L, and 1,000 mg/L iso-concentration surfaces that 

extends westward of the chloride mound beneath the YorkJames Peninsula (figs. 4–6) (see “Chloride Iso-Concentration 

Surfaces”). Seawater has been preferentially flushed around 

the Chesapeake Bay impact crater, either northeastward 

beneath the Middle Peninsula and Northern Neck or 

southeastward beneath the area south of the James River. Less 

flushing has taken place along the ridge where flow is divided, 

as also indicated by greater total ion concentrations (pl. 8). 

Chloride becomes the dominant anion, however, only east of 

Williamsburg (see “Major Ions”).

Fresh groundwater deepens northeastward into the 

Salisbury embayment, where marked increases in the depth 

of basement bedrock and sediment thickness possibly provide 

the most transmissive path for lateral flow around the impact 

crater (figs. 3, 4). Fresh groundwater also deepens initially 

southeastward beneath Surry and Isle of Wight Counties 

and the city of Suffolk, but not farther beneath the cities of 

Chesapeake and Virginia Beach. Unlike the Salisbury embayment to the north, sediments to the south are relatively thin 

and shallow across the Norfolk arch, possibly precluding an 

effective lateral flow route. As a likely result, southeastward 

flushing has apparently been focused into distinct vertical 

intervals, as indicated by inverted chloride-concentration 

zones (section ES–ES′, pl. 13) (see “Chloride-Concentration 

Sections”). Paths of differential flushing are inferred by the 

inverted zones, which are bounded vertically by stagnant 

intervals within which seawater is relatively unflushed, probably as the result of broad contrasts in sediment permeabilities. 

A vertical flow divide coincides with the Norfolk arch 

(section ES–ES′, pl. 13), from which differential flushing 

has either (1) continued southeastward along one path into 

North Carolina or (2) has been redirected northeastward into 

the Salisbury embayment along two parallel paths that pass 

beneath and undercut the chloride mound along the outer 

margin of the Chesapeake Bay impact crater. These latter two 

paths partly align in proximity to contacts between hydrogeologic units, possibly as a result of broad contrast in permeability between the units. The two paths have also entrained 

differential flushing from north of the spur ridge projecting 

from the chloride mound (section DD–DD′, pl. 10). Hence, 

fresh groundwater originating from both north and south of the 

spur ridge has contributed to differential flushing along these 

paths across the outer crater. The two paths also are positioned 

at a similar altitude and are possibly contiguous with a broad 

and thick groundwater salinity inversion described as a “deep 

freshwater wedge” (Meisler, 1989) that extends from the 

northern part of Chesapeake Bay northeastward across the 

Salisbury embayment, beneath the Delmarva Peninsula to the 

Continental Shelf off New Jersey. Differential flushing along 

the paths beneath the impact-crater chloride mound possibly 

has acted as a feeder to the larger salinity inversion to 

the northeast.

The inverted configuration of the saltwater-transition 

zone is probably a relict feature of Pleistocene-age freshwater 

flushing. Inverted chloride-concentration zones indicate 

unstable conditions, where relatively dense saltwater is not 

in equilibrium hydrodynamically with underlying fresher and 

less dense water. Fresh groundwater is theorized to have been 

overridden by the Atlantic Ocean and Chesapeake Bay as a 

result of rapid sea-level rise and inundation of the Continental 

Shelf (Meisler, 1989). Fresh groundwater now occupying the 

sediments has taken as long as 40,000 years to flow eastward 

to the present-day position of the saltwater-transition zone 

(Nelms and others, 2003). This water had travelled only 

about halfway to its present position, however, by the time 

of the Pleistocene glacial maximum, when flushing by older 

freshwater had displaced the saltwater-transition zone to 

the far edge of the Continental Shelf (see “Emplacement of 

Seawater”). Since then, freshwater has continued eastward, 

while the coastline migrated westward at an approximately 

three times faster rate. Seawater has infiltrated relatively 

shallow sediments beneath the inundated area, but has not 

had adequate time to fully displace underlying freshwater, 

which is effectively stalled at depth. Adjustment toward a 

stable saltwater-transition zone configuration could be made 

by continued seawater emplacement, but likely preceded by 

complex intermediate configurations lasting for centuries 

(Kooi and Groen, 2000). Alternatively, with continued 

climatic warming, sea-level rise, and inundation, seawater will 

continue to override and stall an ever greater volume of fresh 

groundwater, thereby further propagating inversion of the 

saltwater-transition zone.

Brine and the Inner Chesapeake Bay Impact Crater

In contrast to the outer margin of the Chesapeake Bay 

impact crater, the degree of flushing by fresh groundwater 

across the inner crater is less certain. Brine as concentrated 

as twice that of seawater salinity has been observed below 

–2,200 ft in corehole 63G 69 (pl. 3) in the inner crater 

(Sanford and others, 2009b). Vertical chloride-concentration 

variations in brine between –1,000 ft and –2,200 ft possibly 

have resulted from blast-induced deposition of sediment clasts 

having contrasting pore-water chloride concentrations from 

before the impact (Sanford and others, 2009b). Although 

paths of differential flushing correlate along inverted chlorideconcentration zones across the northwestern side of the outer 

crater (section DD–DD′, pl. 10), they are not continuous from 

south to north across the inner crater (section GS–GS′, pl. 14). 
34 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Moreover, the plug-like shape of the chloride mound in the 

inner crater generally infers that little or no flushing has taken 

place, either because of decreased sediment permeabilities 

toward the center of the impact crater or the density of the 

brine creates a barrier to flow. Any flushing across the inner 

crater has most likely been above –2,200 ft and along its 

northwestern side in continuity with flushing into the Salisbury 

embayment that generally has taken place preferentially 

northwest of the impact crater. Below –2,200 ft, brine in the 

inner crater becomes uniformly more concentrated (Sanford 

and others, 2009b), precluding the likelihood of any degree 

of flushing.

Hydrochemical evidence (McFarland and Bruce, 2005; 

Sanford and others, 2009b) and estimates of solute advection 

and diffusion rates (Sanford, 2003) further indicate that 

some saltwater has been retained within the inner part of 

the Chesapeake Bay impact crater prior to the most recent 

inundation by seawater, possibly as long ago as the time of 

the impact event. Moreover, the presence of brine in the inner 

crater cannot be accounted for solely by the emplacement 

of seawater within the sediments. The brine is theorized 

to have essentially maintained its present position during 

repeated inundation and reemergence since the impact, being 

largely bypassed during periods of freshwater flushing, even 

during the Pleistocene glacial maximum when seawater from 

surrounding areas was displaced as far eastward as the edge of 

the Continental Shelf (see “Emplacement of Seawater”). Since 

then, modern seawater now reentering the sediments with the 

current sea-level rise has merged with the brine. The presence 

of the brine at its observed concentrations and apparent extent, 

however, has not yet been fully explained, and its origin 

remains largely conjectural.

Movement

Movement of the saltwater-transition zone over time is of 

resource-management interest in relation to potential increases 

in chloride concentration. An understanding of past changes 

in chloride concentration and their relations to groundwater 

withdrawal is needed to anticipate possible future changes.

Heath (1983) defines “saltwater encroachment” as any 

movement of the saltwater-transition zone toward fresh 

groundwater. “Lateral encroachment” denotes withdrawalinduced horizontal movement and is referred to here by the 

more widely used term “intrusion.” “Upconing” is also defined 

as vertical movement to a pumping well from an underlying 

volume of salty groundwater. Because upconing can result 

from displacement of relatively small volumes of water, it is 

generally regarded as a more prevalent cause of increasing 

concentrations of chloride in groundwater than intrusion.

Barlow and Wild (2002) reviewed movement of the 

saltwater-transition zone throughout the Atlantic Coastal 

Plain that has been documented for more than 100 years 

and which varies widely in form and degree under different 

hydrogeologic conditions. Although in general only small 

parts of aquifers have been affected by saltwater intrusion, 

some more widespread instances have also occurred. One of 

the most thoroughly documented is at Cape May County, New 

Jersey, where a comprehensive, long-term data-collection 

effort provided estimates of intrusion rates ranging from 92 

to 220 feet per year (ft/yr) over various multidecade periods 

during 1940–90 (Lacombe and Carleton, 2002).

In the Virginia Coastal Plain, the potential for widespread saltwater intrusion has been recognized for at least 

35 years (Lichtler and Wait, 1974). Rates of groundwater 

withdrawal in the Virginia Coastal Plain have increased 

during the past century to greater than 140 Mgal/d (fig. 14) 

(J.P. Pope, U.S. Geological Survey, written commun., 

2009). Consequently, regional (multicounty) hydraulic 

gradients have been approximately doubled by widespread 

water-level declines as great as 200 ft (Hammond and 

others, 1994a, b, c) and redirected landward toward large 

withdrawal centers located at varying distances west of the 

saltwater-transition zone.

Both intrusion and upconing in the Virginia Coastal 

Plain have been investigated at varying scales. Among early 

efforts, Cederstrom (1943) described how the onset of a rapid 

withdrawal within or close to salty groundwater can result in 

increasing chloride concentrations (presumably as a result of 

upconing), generally until the pumping well reaches hydraulic 

equilibrium. Lichtler and Wait (1974) asserted that flow 

gradients altered by large long-term withdrawals have created 

the potential for area-wide intrusion, but also acknowledged 

that movement rates are likely very slow. Larson (1982) 

concluded that increases in groundwater chloride concentration are primarily of local extent.

Relatively recent investigations have focused on different 

parts of the Virginia Coastal Plain, based on observed changes 

in chloride concentration in combination with digital simulation of concentration change and movement of the saltwatertransition zone. On the Virginia Eastern Shore, Richardson’s 

(1994) simulation indicated no intrusion within the YorktownEastover aquifer prior to 1989, but predicted slow intrusion 

during a future period of 50 years. Intrusion on the Virginia 

Eastern Shore was also recently investigated by Sanford and 

others (2009a). No area-wide increases in chloride concentration were discerned from groundwater samples collected 

between the early 1980s and 2003, but localized increases 

of as much as 300 mg/L during 2003–2050 were predicted 

by simulation. On the York-James Peninsula, Smith (1999) 

identified a concentration increase in only one well from a 

network of wells, from 2,000 to 2,400 mg/L between 1984 

and 1996. Simulation indicated intrusion within the Potomac 

aquifer, however, at rates of approximately 7 to 115 ft/yr over 

various multidecade periods during 1890–1997. Upconing was 

also simulated to result from a typical hypothetical production 

well and was determined to occur within the volume of the 

aquifer within 100 ft below the well.
Regional Trends in Groundwater Quality 35

Figure 14. Rates of change in concentrations of chloride in groundwater and yearly averaged rates of groundwater 

withdrawal reported by regulated groundwater users. Rates of concentration change are based on samples from 

518 wells used to calculate 1,749 rates and are plotted relative to the midpoint in time of two sequential samples 

having a collection interval of 90 days or greater. Withdrawal data are unavailable beyond 2003.

Figure 14. Rates of change in concentrations of chloride in groundwater, and yearly averaged rates

 of groundwater withdrawal reported by regulated groundwater users. Rates of concentration

 change are based on samples from 518 wells used to calculate 1,749 rates, and are plotted

 relative to the midpoint in time of two sequential samples having a collection interval of 90 days

 or greater. Withdrawal data are unavailable beyond 2003.

Surficial aquifer

Yorktown-Eastover aquifer

Piney Point aquifer

Aquia aquifer

Potomac aquifer

Surficial aquifer

Yorktown-Eastover aquifer

Piney Point aquifer

Aquia aquifer

Potomac aquifer

10,000

1,000

100

10

1

0.1

0.01

–0.01

–0.1

–1

–10

–100

–1,000

–10,000

150

100

50

0

RATE OF CHANGE IN CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER PER YEAR WITHDRAWAL, IN MILLION GALLONS PER DAY

1940 1950 1960 1970 1980 1990 2000 2010

Increase Decrease
36 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Rates of Change in Chloride Concentration

The compiled groundwater-quality data were used to 

determine changes in the concentration of chloride in groundwater over time. Because groundwater samples from separate 

wells located in proximity can exhibit substantial differences 

in chloride concentration, samples collected multiple times 

from individual wells are needed to indicate concentration 

changes at specific geographic locations and depths. Of the 

6,650 samples analyzed for chloride (Attachment 2), 5,565 

represent two or more samples collected sequentially and from 

the same depth in 631 wells.

Chloride concentrations among sequentially collected 

groundwater samples vary widely, ranging from zero to 

24,000 mg/L (table 2). Most concentrations, however, are 

relatively small: 70 percent are 234 mg/L or less, and the 

median concentration is 52 mg/L. The number of samples 

collected from each well also varies widely, ranging from 2 to 

266. Most wells, however, were sampled relatively few times: 

70 percent of the wells were sampled 8 times or less, and the 

median number of samples per well is 4.

The duration of sequential sample collection at each well 

ranges from zero to 62 years, with 70 percent of the wells 

sampled over a period of 10 years or less, and the median 

duration is 5.8 years (table 2). The frequency of sample 

collection at each well also varies widely. Some samples were 

collected on the same day, whereas others are separated by 

as much as 22,480 days (62 years). Most samples, however, 

were collected within relatively short timeframes: 70 percent 

of samples were collected within 141 days or less from the 

previous sample, and the median period between sequential 

pairs of samples is 96 days.

Samples collected from 518 wells were used to calculate 

1,749 rates of change in groundwater chloride concentration, 

as the differences in concentration between each sequential 

pair of groundwater samples collected at each well, divided 

by the periods of time between sample collection. Rates of 

concentration change provide a consistent basis for comparing 

changes in groundwater chloride concentration over time, by 

accounting for the widely varying periods of time between 

the collection of sequential samples. Calculation of rates 

of concentration change included samples (1) designated 

as having acceptably accurate chloride concentrations (see 

“Configuration”), based on major ion charge balance and correlation with specific conductance, and (2) collected sequentially from each well 90 days or more apart, to eliminate 

duplicate samples collected on the same day, concentration 

differences resulting primarily from sample collection and(or) 

analysis error, and unrealistically large rates of concentration 

change that exceed the range of observed concentrations.

Rates of change in groundwater chloride concentration 

range from increases as rapid as 4,603 milligrams per liter per 

year (mg/L/yr) to decreases as rapid as –4,284 mg/L/yr 

(a minus sign denotes a concentration decrease) (table 2). 

Most rates, however, are relatively small: 70 percent have 

magnitudes within plus or minus (+/–) 23 mg/L/yr, and the 

median rate magnitude is +/– 6 mg/L/yr.

Rates of concentration change vary widely and sporadically (fig. 14), both collectively and within major water-supply 

aquifers. Increases in chloride concentration coincide with 

decreases throughout the period of sample collection. Neither 

increases nor decreases appear to become more frequent, or 

to change in magnitude, as groundwater withdrawal increased 

during the period. Thus, the overall change in chloride 

concentration is small, with a mean rate of concentration 

change of –3.4 mg/L/yr. The rates primarily represent changes 

in groundwater chloride concentration during 1970–2006, 

however, with only a few rates calculated for earlier periods. 

Most of the rates are calculated from a large number of 

samples collected during approximately 1970–90 and again 

during 2004–06, with fewer samples collected during the 

intervening period. 

Rates of change in groundwater chloride concentration 

exhibit some contrasts spatially among different parts of 

major water-supply aquifers (figs. 15–19). Across the 

sampled extent of each aquifer, both decreasing rates of 

concentration and increasing rates of moderate magnitude 

(up to 20 mg/L/yr) are generally widespread. More rapid 

rates of increasing chloride concentration, however, appear 

for some aquifers to be constrained to specific areas. Also 

noted, and of particular resource-management interest, are 

locations where an initial chloride concentration below the 

U.S. Environmental Protection Agency (1990) secondary 

maximum contaminant level of 250 mg/L has increased to 

above 250 mg/L.

Table 2. Summary of sequentially collected groundwater samples.

[mg/L, milligram per liter; yr, year; mg/L/yr, milligram per liter per year]

Minimum Maximum 70th percentile Median

Chloride concentration (mg/L) 0 24,000 234 52

Samples per well 2 266 8 4

Duration of sample collection (yr) 0 62 10 5.8

Frequency of sample collection (days) 0 22,480 141 96

Rate of chloride-concentration change 1 (mg/L/yr) –4,284 4,603 +/–23 +/–6

1Based on samples from 518 wells used to calculate 1,749 rates.
Regional Trends in Groundwater Quality 37

Figure 15. Rates of change in concentrations of chloride in groundwater in the surficial aquifer of the Virginia Coastal Plain. Rate values are 

calculated from 186 changes in chloride concentration over periods of 90 days or greater during 1970–2006 in 55 wells. Location of Chesapeake 

Bay impact crater from Powars and Bruce (1999). [mg/L, milligrams per liter; mg/L/yr, milligrams per liter per year]

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

EXPLANATION

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

CD’

Rate of change in concentration of chloride in groundwater

decreases, and increases to 20 mg/L/yr

 greater than 20 to 100 mg/L/yr increases

 greater than 100 to 1,000 mg/L/yr increases

 greater than 1,000 mg/L/yr increases

 change producing concentration greater than

 250 mg/L

 

Figure 15. Rates of change in concentrations of chloride in groundwater in the surficial aquifer of the Virginia Coastal Plain. Rate

 values are calculated from 186 changes in chloride concentration over periods of 90 days or greater during 1970–2006 in 55 wells.

 Location of Chesapeake Bay impact crater from Powars and Bruce (1999).

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

Piedmont

River

Fall Zone

Piedmont

Fall Zone

LOCATION OF THE STUDY AREA

IN VIRGINIA
38 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Figure 16. Rates of change in concentrations of chloride in groundwater in the Yorktown-Eastover aquifer of the Virginia Coastal Plain. Rate 

values are calculated from 634 changes in chloride concentration over periods of 90 days or greater during 1969–2006 in 174 wells. Area outside 

extent of aquifer is shaded gray. Location of Chesapeake Bay impact crater from Powars and Bruce (1999). [mg/L, milligrams per liter; mg/L/yr, 

milligrams per liter per year]

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE

BAY

Potomac

River

James

York

River

Rappahannock

River

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN

SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

River

LOCATION OF THE STUDY AREA

Piedmont

IN VIRGINIA

Fall Zone

Piedmont

Fall Zone

EXPLANATION

Rate of change in concentration of chloride in groundwater

decreases, and increases to 20 mg/L/yr

 greater than 20 to 100 mg/L/yr increases

 greater than 100 to 1,000 mg/L/yr increases

 greater than 1,000 mg/L/yr increases

 change producing concentration greater than

 250 mg/L

 

Figure 16. Rates of change in concentrations of chloride in groundwater in the Yorktown-Eastover aquifer of the Virginia Coastal

 Plain. Rate values are calculated from 634 changes in chloride concentration over periods of 90 days or greater during 1969–2006

 in 174 wells. Area outside extent of aquifer is shaded gray. Location of Chesapeake Bay impact crater from Powars and Bruce (1999).
Regional Trends in Groundwater Quality 39

Figure 17. Rates of change in concentrations of chloride in groundwater in the Piney Point aquifer of the Virginia Coastal Plain. Rate values 

are calculated from 47 changes in chloride concentration over periods of 90 days or greater during 1953–2006 in 23 wells. Area outside extent of 

aquifer is shaded gray. Location of Chesapeake Bay impact crater from Powars and Bruce (1999). [mg/L/yr, milligrams per liter per year]

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

River

LOCATION OF THE STUDY AREA

Piedmont

IN VIRGINIA

Fall Zone

Piedmont

Fall Zone

EXPLANATION

Rate of change in concentration of chloride in groundwater

decreases, and increases to 20 mg/L/yr

 greater than 20 to 100 mg/L/yr increases

 greater than 100 to 1,000 mg/L/yr increases

 

Figure 17. Rates of change in concentrations of chloride in groundwater in the Piney Point aquifer of the Virginia Coastal Plain.

 Rate values are calculated from 47 changes in chloride concentration over periods of 90 days or greater during 1953–2006 in 23

 wells. Area outside extent of aquifer is shaded gray. Location of Chesapeake Bay impact crater from Powars and Bruce (1999).
40 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Figure 18. Rates of change in concentrations of chloride in groundwater in the Aquia aquifer of the Virginia Coastal Plain. Rate values are 

calculated from 39 changes in chloride concentration over periods of 90 days or greater during 1972–2006 in 16 wells. Area outside extent of 

aquifer is shaded gray. Location of Chesapeake Bay impact crater from Powars and Bruce (1999). [mg/L, milligrams per liter; mg/L/yr, milligrams 

per liter per year]

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

River

LOCATION OF THE STUDY AREA

IN VIRGINIA

Piedmont

Fall Zone

Piedmont

Fall Zone

EXPLANATION

Rate of change in concentration of chloride in groundwater

decreases, and increases to 20 mg/L/yr

 greater than 20 to 100 mg/L/yr increases

 greater than 100 to 1,000 mg/L/yr increases

 change producing concentration greater than

 250 mg/L

 

Figure 18. Rates of change in concentrations of chloride in groundwater in the Aquia aquifer of the Virginia Coastal Plain. Rate

 values are calculated from 39 changes in chloride concentration over periods of 90 days or greater during 1972–2006 in 16 wells.

 Area outside extent of aquifer is shaded gray. Location of Chesapeake Bay impact crater from Powars and Bruce (1999).
Regional Trends in Groundwater Quality 41

Figure 19. Rates of change in concentrations of chloride in groundwater in the Potomac aquifer of the Virginia Coastal Plain. Rate values are 

calculated from 807 changes in chloride concentration over periods of 90 days or greater during 1948–2006 in 229 wells. Area outside extent of 

aquifer is shaded gray. Location of Chesapeake Bay impact crater from Powars and Bruce (1999). [mg/L, milligrams per liter; mg/L/yr, milligrams 

per liter per year]

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

1,000

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

River

LOCATION OF THE STUDY AREA

IN VIRGINIA

Piedmont

Fall Zone

Piedmont

Fall Zone

EXPLANATION

Rate of change in concentration of chloride in groundwater

decreases, and increases to 20 mg/L/yr

 greater than 20 to 100 mg/L/yr increases

 greater than 100 to 1,000 mg/L/yr increases

 greater than 1,000 mg/L/yr increases

 change producing concentration greater than

 250 mg/L

 

Figure 19. Rates of change in concentrations of chloride in groundwater in the Potomac aquifer of the Virginia Coastal Plain. Rate

 values are calculated from 807 changes in chloride concentration over periods of 90 days or greater during 1948–2006 in 229 wells.

 Area outside extent of aquifer is shaded gray. Location of Chesapeake Bay impact crater from Powars and Bruce (1999).
42 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

At many locations exhibiting rapid rates of increasing 

chloride concentration, decreasing or moderately increasing 

rates of chloride concentration are also indicated (figs. 15–19). 

These contrasting colocated rates potentially reflect different 

changes in chloride concentration over time among samples 

collected from a single well, or alternatively among samples 

collected from different colocated wells, or some combination. 

Contrasting rates among samples collected from different 

wells potentially reflect different changes in chloride concentration within vertically separated parts of a single aquifer.

In the surficial aquifer, 186 rates of change in chloride 

concentration were calculated on the basis of samples collected during 1970–2006 from 55 wells, located mostly toward 

the coast (fig. 15). Decreasing rates and increasing rates of 

moderate magnitude are indicated across the sampled extent. 

Only a few rapid rates of increase are indicated at relatively 

isolated locations, including two on the Virginia Eastern 

Shore, another in far northern Isle of Wight County, and two 

more in proximity in the city of Virginia Beach. Three of these 

locations also correspond to increases in chloride concentration above 250 mg/L.

In the Yorktown-Eastover aquifer, 634 rates of change in 

chloride concentration were calculated on the basis of samples 

collected during 1969–2006 from 174 wells, located mostly 

toward the coast (fig. 16). Decreasing rates and increasing 

rates of moderate magnitude are indicated across the sampled 

extent. Rapid rates of increase are also widespread, however, 

including several locations spanning much of the Virginia 

Eastern Shore, one location near the mouth of the York River, 

and several locations south of Chesapeake Bay. Several 

of these locations also correspond to increases in chloride 

concentration above 250 mg/L. 

In the Piney Point aquifer, 47 rates of change in chloride 

concentration were calculated on the basis of samples collected during 1953–2006 from 23 wells, located mostly within 

and near James City County on the York-James Peninsula 

(fig. 17). Almost all locations indicate decreasing rates or 

increasing rates of moderate magnitude. Only one location 

in James City County and another near the mouth of the 

York River indicates rapid rates of increase. No increases in 

concentration above 250 mg/L are indicated.

In the Aquia aquifer, 39 rates of change in chloride 

concentration were calculated on the basis of samples collected during 1972–2006 from 16 wells at widely scattered 

locations (fig. 18). Most locations indicate decreasing rates 

or increasing rates of moderate magnitude. Rapid rates of 

increase are indicated at solitary locations near Williamsburg, 

in southeastern city of Suffolk, and in central and far eastern 

city of Chesapeake. Two of these locations also correspond to 

increases in chloride concentration above 250 mg/L.

In the Potomac aquifer, 807 rates of change in chloride 

concentration were calculated on the basis of samples 

collected during 1948–2006 from 229 widely located wells 

(fig. 19). Decreasing rates and increasing rates of moderate 

magnitude are indicated across the sampled extent. Rapid 

rates of increase are also indicated at many locations toward 

the coast, but spanning areas farther inland than the locations 

of rapid rates of increase in shallower aquifers. Locations of 

rapid rates of increase in the Potomac aquifer generally form 

two, broadly fan-shaped areas flanking the outer margin of the 

Chesapeake Bay impact crater: (1) from the upper parts of the 

Middle Peninsula and York-James Peninsula, extending to the 

east and south, and (2) from southern Southampton County, 

extending and broadening to the east and northeast across 

the cities of Suffolk, Chesapeake, Portsmouth, and Norfolk. 

The two areas are separated by a band of locations across Isle 

of Wight County, almost entirely indicating decreasing rates 

or increasing rates of moderate magnitude. A rapid rate of 

increase is also indicated at a solitary outlying location to the 

far northwest in Caroline County. Many of the locations of 

rapid rates of increase also correspond to increases in chloride 

concentration above 250 mg/L.

Patterns of Change in Chloride Concentration

Temporal changes in chloride concentrations in 

individual wells were also examined to determine possible 

relations to groundwater withdrawal. Because most wells have 

been sampled only a few times over a short period and exhibit 

relatively small chloride concentrations, data from wells 

having the longest duration and greatest frequency of sample 

collection, as well as relatively large chloride concentrations, 

were examined. In some instances, other wells are in proximity to those selected, but have either not been extensively 

sampled or are completed in different aquifers having small 

chloride concentrations. Conversely, from the selected wells, 

chloride concentrations of all samples were included for 

examination to provide the most complete record possible 

for each well needed to discern patterns of concentration 

change. Yearly averaged rates of groundwater withdrawal, 

reported through 2003 to the DEQ by regulated groundwater 

users (J.P. Pope, U.S. Geological Survey, written commun., 

2009), were used to determine possible relations to changes in 

chloride concentration.

Although variations among groundwater chloride 

concentrations at each well are unique, three patterns of 

change are generally apparent from examination, which 

are represented by two examples (figs. 20–21). The first 

example is represented by well 60C 61, completed in the 

Yorktown-Eastover aquifer in the city of Portsmouth (pl. 2). 

Groundwater samples from well 60C 61 increased from less 

than 500 mg/L in 1990 to approximately 3,500 mg/L by 

2000 (fig. 20). Chloride concentrations fluctuated moderately 

between sequential samples throughout the period and also 

fluctuated markedly but ephemerally during 1999, with a sharp 

decrease followed by a similar increase. During 2000–2004, 

concentrations generally stabilized with moderate fluctuation 

between 3,000 and 3,500 mg/L.

All reported groundwater withdrawals in proximity to 

well 60C 61 are from the Yorktown-Eastover aquifer, and are 

from several locations across an approximately 2-mi-wide 

area. Their combined withdrawal rate increased during 
Regional Trends in Groundwater Quality 43

1990–93, remained steady during 1994–95, decreased during 

1996–99, and increased again during 2000–2003 but only to 

approximately half of the earlier rate (fig. 20). Groundwater 

chloride concentration in well 60C 61 generally increased 

during the periods of both increasing and steady withdrawal, 

and continued to increase during the period of withdrawal 

decrease. Chloride concentration then stabilized while 

withdrawal was moderately increased.

None of the other examined wells exhibit sustained 

and substantially increasing trends in chloride concentration 

similar to well 60C 61. Instead, either broad patterns of 

gradual, relatively small concentration change or punctuated 

patterns of numerous short-term fluctuations in concentration 

are indicated. Most of the examined wells are solitary and not 

in proximity to another examined well. 

The most complete example of both broad and punctuated patterns of concentration change is represented by nine 

examined wells located in the central part of the York-James 

Peninsula. Groundwater samples having relatively large 

chloride concentrations were frequently collected during 

1984–2006 (fig. 21) from these nine wells located within 1 to 

2 mi of each other (pl. 2). All of the wells are completed in the 

Potomac aquifer and are colocated in groups of two or three at 

four different locations.

A broad pattern of concentration change is represented by 

the colocated wells 58F 50, 58F 51, and 58F 52, from each of 

which seven to eight samples were collected during 1984–99 

(fig. 21). Samples from the deepest among these wells, 58F 50, 

have the greatest chloride concentrations, which gradually 

increased from approximately 2,100 to 2,600 mg/L. Samples 

from the shallower wells 58F 51 and 58F 52 have smaller 

concentrations that vary within less than 100 mg/L.

A punctuated pattern of concentration change is represented by the other six wells, which are colocated in pairs at 

three locations, and from each of which approximately four 

samples per year were collected during 1998–2006 (fig. 21). 

Chloride concentrations fluctuate among these samples, 

some by as much as several 100 mg/L/yr. Samples from the 

Figure 20. Chloride concentrations in groundwater in well 60C 61 at Portsmouth, 

Virginia, and yearly averaged rates of groundwater withdrawal reported by regulated 

groundwater users at nearby locations. Withdrawal data are unavailable beyond 2003.

5,500

5,000

4,500

4,000

3,500

3,000

2,500

2,000

1,500

1,000

500

0

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0

Figure 20. Chloride concentrations in groundwater in well 60C 61 at

 Portsmouth, Virginia, and yearly averaged rates of groundwater

 withdrawal reported by regulated groundwater users at nearby 

 locations. Withdrawl data are unavailble beyond 2003.

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

WITHDRAWAL, IN MILLION

GALLONS PER DAY

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004
44 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Figure 21. Chloride concentrations in groundwater in selected wells on the central York-James Peninsula, Virginia, and yearly 

averaged rates of groundwater withdrawal reported by regulated groundwater users at nearby locations. Colocated wells are 

grouped as shown. Withdrawal data are unavailable beyond 2003. Sample-collection interval altitude datum is National Geodetic 

Vertical Datum of 1929.

2,600

2,400

2,200

2,000

1,800

1,600

1,400

1,200

1,000

800

600

400

5

4

3

2

1

0

1,900

1,700

1,500

1,300

1,100

900

700

2,100

1,900

1,700

1,500

1,300

1,100

900

700

500

300

2,500

2,300

2,100

1,900

1,700

1,500

1,300

Figure 21. Chloride concentrations in groundwater in selected wells on the central York-James Peninsula, Virginia, and

 yearly averaged rates of groundwater withdrawal reported by regulated groundwater users at nearby locations.

 Co-locted wells are grouped as shown. Withdrawal data are unavailable beyond 2003. Sample-collection interval,

 altitude is National Geodetic Vertical Datum 29. CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER WITHDRAWAL, IN MILLION GALLONS PER DAY

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

Increase

Decrease

Increase

Increase

Increase

1984 1985 1986 1987 1988 1990 1991 1989 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Well 58F 50

–1,145 to –1,165 feet

Well 58F 51

–765 to –775 feet

Well 58F 52

–472 to –482 feet

Well 58F 57

–467 to –557 feet

Well 58F 86

–623 to –1,115 feet

Well 58F 88

–475 to –530 feet

Well 58F 89

–987 to –1,101 feet

Well 58F 90

–474 to –565 feet

Well 58F 91

–625 to –1,080 feet
Regional Trends in Groundwater Quality 45

deeper of the two wells at each location have both the greatest 

chloride concentrations and generally the largest fluctuations 

among samples.

Reported groundwater withdrawals in proximity to the 

nine examined wells in the central part of the York-James Peninsula are entirely from the Potomac aquifer and are located 

across an approximately 5-mi-wide area. Their combined 

withdrawal rate was relatively large during 1984–93, but 

decreased markedly during 1994–98 (fig. 21). Withdrawal then 

increased during 1999–2000, decreased markedly during 2001, 

and increased again during 2002–03. Groundwater chloride 

concentration in well 58F 50 increased during the initial 

period of large withdrawal, remained steady while withdrawal 

decreased, and increased again as withdrawal increased. Relations to withdrawal are less apparent from the small variations 

in chloride concentration in the two shallower colocated wells 

58F 51 and 58F 52. Among the other six wells, chloride in the 

relatively deep wells 58F 91 and 58F 86 initially increased 

in concentration as groundwater withdrawal increased during 

1999–2000, but then decreased to earlier concentrations. 

Conversely, a similarly ephemeral decrease in concentration 

occurred in the relatively shallow well 58F 90 during the same 

period. During the second withdrawal increase of 2002–03, 

concentrations in well 58F 86 fluctuated sporadically, while 

only a single sample collected from well 58F 91 in early 2003 

had an increased chloride concentration, and well 58F 90 had 

a similar solitary increase later the same year. Relations to 

withdrawal are less apparent from the concentration fluctuations in wells 58F 87, 58F 88, and 58F 89 that were relatively 

small throughout 1998–2003. Withdrawal data are unavailable 

beyond 2003 with which to compare to later fluctuations 

in concentration.

The broad pattern of change in groundwater chloride 

concentration represented by the colocated wells 58F 50, 

58F 51, and 58F 52 possibly is an artifact of the relatively 

low frequency of sample collection, as long as several years 

between sequential samples (fig. 21). Fluctuating concentrations similar to those in the other six wells in the central part 

of the York-James Peninsula likely would be observed, if 

sample collection was increased to a similar frequency.

Intrusion and Upconing

Based on general considerations, previous investigations, 

and results presented herein, primarily localized movement 

of the saltwater-transition zone resulting from groundwater 

withdrawal is most apparent in the Virginia Coastal Plain. 

Likewise, regional intrusion has probably not occurred. Large 

withdrawals that have steepened and redirected regional 

hydraulic gradients apparently have derived much of the 

water by release from storage and have not been imposed long 

enough to displace water across multicounty areas. Moreover, 

given current understanding of the emplacement of the 

saltwater-transition zone, its overall position and configuration 

have evolved over a geologic time scale of tens of thousands 

of years and, thus, will not establish a hydrodynamically stable 

configuration for centuries into the future (see “Relations to 

Groundwater Flow”). The amount of time needed under present conditions for regional intrusion to take place is uncertain, 

but could potentially be very long by human standards.

Within the preceding context, the compiled groundwaterquality data can provide a perspective on withdrawal-induced 

movement of the saltwater-transition zone. Implicit is that 

a change in groundwater chloride concentration reflects 

movement of the saltwater-transition zone in some form. 

Locations of rapid rates of increase in chloride concentration 

(figs. 15–19) indicate where movement toward a volume 

of relatively fresh groundwater likely has occurred. Where 

these locations have been determined with sufficient density 

and extent in the Yorktown-Eastover and Potomac aquifers 

(figs. 16, 19, respectively), they generally delineate areas 

where movement has been relatively prevalent. These areas 

primarily overlie or flank the chloride mound and ridges (see 

“Chloride Iso-Concentration Surfaces”), where the saltwatertransition zone is shallow and likely to be intercepted by 

production wells. Rapid rates of concentration increase in 

the Potomac aquifer, however, also extend northwestward 

to the upper Middle Peninsula and York-James Peninsula, 

and southwestward to southern Southampton County, where 

the saltwater-transition zone is relatively deep. Chloride 

concentrations can be as great as 1,000 mg/L toward the 

bottom of the Potomac aquifer in these areas (fig. 13), where 

pumping from sufficiently deep wells can cause concentrations 

to increase. 

Lateral intrusion cannot be distinguished from vertical 

upconing based solely on the calculated rates of increase in 

groundwater chloride concentration. Instances of a sustained 

increasing trend, however, are possibly more likely to result 

from intrusion than upconing. As one example in the relatively 

shallow Yorktown-Eastover aquifer, well 60C 61 (fig. 20) is in 

proximity to a brackish water embayment of the James River 

(pl. 2). The aquifer is projected there to be incised along the 

shoreline (McFarland and Bruce, 2006) from which localized 

intrusion of brackish surface water has possibly been induced 

by nearby groundwater withdrawals. As another example in a 

deep part of the Potomac aquifer, gradual increases in chloride 

concentrations in well 58F 50 coincided with periods of 

large or increasing withdrawal (fig. 21). Common to both the 

Yorktown-Eastover and Potomac aquifer examples, chloride 

concentrations stabilized but did not decrease with the 

decrease in withdrawal, suggesting that lateral advancement 

of the saltwater-transition zone slowed or stopped, but was not 

followed by recession to its earlier position. 

In contrast to the sustained increases in groundwater 

chloride concentration resulting from intrusion, upconing possibly is reflected by more ephemeral concentration increases. 

Both increases and decreases in chloride concentration are 

widely indicated at varying rates throughout the period of 

sample collection (fig. 14). Moreover, at many of the locations 

of rapid rates of increase, decreasing rates are also indicated 

(figs. 15–19). In individual wells, chloride concentration has 

commonly fluctuated among sequentially collected samples, 
46 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

including increases such as those in wells 58F 91 and 58F 86 

that were followed by decreases to earlier concentrations 

(fig. 21).

Upconing is at least partly accounted for by the relation 

originally described by Ghyben (1888) and Herzberg 

(1901) as:

ds = hf (rf df / (rs – rf ) ),

where 

ds is depth of saltwater below sea level,

hf is the freshwater head above sea level,

	 rf is density of freshwater, and

	 rs is density of saltwater.

By this relation, and given approximate densities of 

freshwater and seawater, a decline in freshwater head of 1 ft 

resulting from groundwater withdrawal would correspond 

proportionately to a rise in saltwater of 40 ft. Conversely, 

recovery of freshwater head upon decrease or cessation of 

withdrawal would result in a proportional deepening of saltwater. The freshwater in effect “floats” in dynamic equilibrium 

on the more dense saltwater beneath. Actual conditions are 

made more complex, however, by dispersion between the 

volumes of freshwater and saltwater, heterogeneities among 

sediment hydraulic properties, and groundwater being under 

flowing rather than static conditions (Heath, 1983).

In addition to fluctuating groundwater chloride concentrations, groundwater withdrawal at many locations in proximity 

to the saltwater-transition zone has historically been episodic 

(Focazio and Speiran, 1993). Pumping rates typically vary in 

response to high water demands, either seasonally or over longer periods during droughts, which are interrupted by extended 

periods of no withdrawal. Hence, the general prevalence of 

fluctuating chloride concentrations probably reflects numerous 

punctuated instances of withdrawal-induced upconing of salty 

groundwater, followed by its downward recession or “downconing” upon a decrease or cessation of withdrawal. Cycles 

of upconing and downconing can potentially occur repeatedly 

in proximity to a single production well. Although the relative 

prevalence of upconing versus intrusion is unknown, distances 

of vertical displacement are generally much smaller than those 

of horizontal displacement, and upconing is widely regarded 

as the more frequent cause of increasing groundwater chloride 

concentrations (Heath, 1983). Potentially, both intrusion and 

upconing can coincide in various combinations, although the 

overall amount of movement has so far remained localized.

Relations are not entirely apparent between the yearly 

averaged rates of reported groundwater withdrawals and all 

chloride-concentration fluctuations in the examined wells (see 

“Patterns of Change in Chloride Concentration”). Closely 

spaced production wells are commonly operated at different 

times by different water authorities and as a result have 

divergent pumping histories. Different wells can thereby make 

up the averaged rate from one year to the next. Moreover, 

the reported rates do not represent all withdrawals. The DEQ 

requires reporting of groundwater withdrawals only from 

those users of 300,000 gallons or more per month. Users of 

smaller amounts are exempt from reporting requirements, but 

are known anecdotally through local supply-well drillers to be 

numerous and widespread (T.S. Bruce, Virginia Department of 

Environmental Quality, oral commun., 2009). Thus, complex 

hydraulic interactions and associated changes in chloride 

concentration are likely to result from a continually varying 

array of supply wells taken in and out of production and which 

are only partly known.

Monitoring

Increases in groundwater chloride concentration in 

the Virginia Coastal Plain are often ephemeral, probably 

resulting from individual instances of localized upconing, and 

reversible to varying degrees with a decrease or cessation of 

the associated withdrawal (see “Intrusion and Upconing”). A 

greater concern exists for lateral intrusion, which potentially 

represents an essentially permanent repositioning of the 

saltwater-transition zone. Although the timing remains uncertain, intrusion in the Virginia Coastal Plain will eventually 

broaden as long as large groundwater withdrawals continue. 

Accordingly, some form of monitoring of intrusion is regarded 

as necessary to provide advanced detection of the likely threat 

to the water supply. 

Intrusion monitoring throughout the Atlantic Coastal 

Plain has most widely entailed collection and analysis of 

groundwater samples over time from various networks of 

wells. In addition, geophysical methods in boreholes and at 

the land surface are continually being refined and increasingly 

applied, which have advantages in providing broad and 

cost-efficient indications of movement of the saltwatertransition zone. Geophysical methods generally entail various 

indirect measurements, however, and require some form of 

direct observation to interpret the results. Hence, groundwater 

sampling will likely continue to provide the baseline of 

information necessary for intrusion monitoring.

The compiled groundwater-quality data were used to 

indicate where movement of the saltwater-transition zone 

toward a volume of relatively fresh groundwater has likely 

occurred and been relatively frequent, and to determine some 

relations between groundwater withdrawal and different 

forms of movement (see “Intrusion and Upconing”). The data 

are a combined result, however, of numerous well-sampling 

programs undertaken at different times by various entities to 

achieve diverse objectives. Conversely, the sampled wells do 

not collectively make up a single consistent network designed 

and maintained for the express purpose of monitoring intrusion. Most wells have been sampled either once or only a few 

times over a short period and not in the coordinated fashion 

with other wells needed to systematically detect movement 

of the saltwater-transition zone. Likewise, most wells are not 

optimally located and designed for monitoring. 

A minority of samples among the compiled groundwaterquality data were collected as part of a network for intrusion 
Regional Trends in Groundwater Quality 47

monitoring currently (2009) being maintained by the USGS in 

cooperation with HRPDC and the DEQ. A program of annual 

groundwater-sample collection in the Virginia Coastal Plain 

began in 1997. Initially, a network of approximately 90 wells 

was planned to be sampled on a rotating basis of 20 wells per 

year, with the entire network to begin a new cycle of sampling 

on a 5-year interval. The network has since been modified, 

with the addition of newly drilled wells and omission of wells 

damaged, destroyed, or otherwise becoming unavailable 

or deemed unsuitable. Sampling frequency has also been 

reduced, on average by half. 

At present, the intrusion monitoring network includes 

113 wells. Many of the wells are grouped among 24 locations, 

including from 2 to as many as 8 wells each. Groundwater 

chloride concentrations exceed 100 mg/L in 67 of the wells. 

Wells are currently prioritized for sampling based on those 

exhibiting relatively large chloride concentrations and possible 

increasing trends in concentration, although 20 of the wells 

have so far been sampled only once. None of the current 

network wells were constructed solely for intrusion monitoring. Rather, access for sampling has been gained to production 

wells and to observation wells located at or near production 

facilities that were constructed to satisfy various regulatory 

requirements. In addition, research observation wells grouped 

at several locations have been constructed and maintained by 

the DEQ.

Not all network wells are optimally located and 

designed for intrusion monitoring. Production wells typically 

have relatively long and often multiple screened intervals 

through which water from varying depths are mixed, thereby 

integrating and losing distinction among parts of the aquifer 

having contrasting chloride concentrations. Both regulatory 

and research observation wells generally have single, shorter 

screened intervals that effectively isolate a distinct part of the 

aquifer, but conversely have been constructed to serve a broad 

array of information needs, including description of the hydrogeologic framework and water-level monitoring. Optimal well 

locations and designs to address these needs do not necessarily 

coincide with those for intrusion monitoring. In addition, use 

of the observation wells for characterizing any aspects of 

water quality specifically requires development of the wells 

beyond that needed for other purposes. Construction of the 

wells necessarily entails hydraulic rotary drilling techniques 

in deep boreholes, which introduces large volumes of drilling 

fluid that substantially differs in chemical composition from 

groundwater. Sufficient removal of the drilling fluid to enable 

subsequent collection of representative groundwater-quality 

samples is time consuming and costly. 

The current intrusion monitoring network in the Virginia 

Coastal Plain is in an early and relatively formative stage. By 

contrast, detailed descriptions of movement of the saltwatertransition zone in New Jersey were accomplished using a 

network of wells much more densely arrayed across a single 

county and extensively sampled over a 50-yr period (Lacombe 

and Carleton, 2002). A comparable effort will likely be needed 

in the Virginia Coastal Plain to detect intrusion during coming 

years and decades.

Relations between groundwater withdrawal and the 

saltwater-transition zone in the Virginia Coastal Plain present 

a substantial challenge for designing an effective intrusion 

monitoring network. Ideally, intrusion would be anticipated 

across a distinct front ahead of which all withdrawals are 

positioned within the freshwater part of the aquifer system and 

at some distance from the saltwater-transition zone. Observation wells constructed for collection of groundwater-quality 

samples could then be positioned in a straightforward manner 

across the intervening area. By contrast, in the Virginia 

Coastal Plain, large and increasing groundwater withdrawals 

are positioned close to and wholly within the saltwatertransition zone (see “Considerations for Groundwater Use”), 

which has a three-dimensionally complex configuration (see 

“Configuration”). Rather than a distinct front, multiple areas 

of potential intrusion are probably complexly distributed 

and are likewise specific to numerous individual production 

areas. Moreover, many withdrawals are not documented, and 

among those that are, individual supply wells are continually 

being taken in and out of production in response to varying 

demands (see “Intrusion and Upconing”). Numerous shortterm instances of localized upconing are thereby superimposed 

upon any longer-term intrusion trend, but cannot always 

be clearly distinguished or related to withdrawal based on 

changes in groundwater chloride concentration.

The effectiveness of intrusion monitoring in the Virginia 

Coastal Plain could be improved by collection of groundwaterquality samples from a network of wells designed specifically 

to address the issues discussed previously. Although sampling 

of wells at and near groundwater production facilities needs to 

be maintained, additional observation wells suitably positioned 

away from production areas are probably more critical toward 

providing advanced detection of intrusion. Appropriately 

designing an intrusion monitoring network would depend on 

a detailed hydrologic analysis that is beyond the scope of the 

current study, but would entail identifying multiple, complexly 

distributed areas of potential intrusion with the objective of 

determining locations at which observation wells would most 

likely intercept intrusion. Aspects of the saltwater-transition 

zone could be addressed using newly gained details presented 

herein (see “Configuration”). Also needed, however, would be 

as complete an account as possible of groundwater withdrawal 

locations and amounts or rates. Because of the episodic nature 

of withdrawal among production facilities, observation well 

locations need to be arrayed with sufficient extent and density 

to detect variations in intrusion resulting from changes in 

withdrawal among different facilities over time and also with 

regard to locations of projected future withdrawals. A largescale drilling program would then be needed for observation 

well construction and would specifically include extensive 

development of the wells prior to collection of groundwaterquality samples.

Collection of groundwater-quality samples from 

observation wells that compose the intrusion monitoring 
48 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

network needs to be frequent enough to distinguish shortterm fluctuations in chloride concentrations associated with 

upconing from sustained long-term concentration increases 

that indicate potential lateral intrusion. Although detection 

of intrusion is of greater concern than upconing, short-term 

fluctuations in chloride concentration can potentially obscure 

long-term concentration trends. Periods as long as several 

years between sequential samples produce broad patterns 

of change in chloride concentration, which cannot represent 

concentration fluctuations that are evident from samples 

collected as frequently as four times per year (see “Patterns of 

Change in Chloride Concentration”). As a result, infrequent 

sampling cannot provide the context within which to tell 

whether a concentration difference between two sequential 

samples is part of a long-term trend or alternatively is only one 

among a series of short-term fluctuations. An analysis of the 

durations of episodic withdrawals could potentially determine 

the appropriate frequency of sample collection from the 

monitoring network.

Lastly, intrusion could potentially take from years to 

decades of monitoring to detect. For example, substantial 

rates of intrusion determined in Cape May County, New 

Jersey, average approximately 170 ft/yr (Lacombe and 

Carleton, 2002). Put another way, however, 30 years of 

monitoring was required to detect 1 mi of movement of the 

saltwater-transition zone.

Selected Constituents

Concentrations of 37 chemical constituents in groundwater are tabulated (Attachment 1) and summarized (Attachment 2). Many constituents can be present at generally smaller 

concentrations than the major ions that dominate the chemical 

composition of groundwater (see “Major Ions”). Some of 

these constituents are of particular resource-management 

interest, but not all of them are adequately represented among 

the tabulated concentration values to determine regional 

trends (see “Data Distribution”). Fluoride, iron, manganese, 

nitrate, and ammonium were selected for interpretive analysis, 

based both on having relevance to resource management 

and a sufficient number of adequately distributed samples. 

Concentration values for other constituents were deemed of 

less interest and(or) inadequate for effective analysis.

Concentrations of fluoride, iron, manganese, nitrate, 

and ammonium are presented in map and sectional views to 

convey a conceptual understanding of their distribution and 

related processes at the regional scale (figs. 22–26). Locations 

of samples collected from all hydrogeologic units are shown in 

a single map view for each constituent. Similarly, in sectional 

view, all sample locations are projected onto a single vertical 

plane along lines of longitude, which approximate geologic 

strike. Hydrogeologic-unit associations among samples in 

sectional view are generalized from McFarland and Bruce 

(2006) by approximate boundaries between basement bedrock, 

fluvial-deltaic sediments of the Potomac aquifer, and overlying 

hydrogeologic units dominated by marine sediments. (The 

location of the Chesapeake Bay impact crater within the 

Potomac aquifer is omitted for clarity.) Areas of contrasting 

groundwater major ion type and the saltwater-transition zone 

are also approximated. The maps and sections portray broad 

regional trends among concentrations of the selected constituents, within which local variations can be present.

All groundwater samples analyzed for the selected constituents were included from the compiled data, not just those 

included in other data analyses based on calculations of major 

ion charge balance (see “Major Ions”) and correlation between 

chloride-concentration and specific conductance values (see 

“Saltwater-Transition Zone”). Fluoride, iron, manganese, 

nitrate, and ammonium concentrations of individual samples 

are separately tabulated (Attachment 1) and are subject to the 

same qualifications as other sample data (see “Data Quality”). 

Fluoride

Fluoride anions (F–) are the dominant form of fluorine 

dissolved in water. The enforceable drinking-water standard 

for fluoride is 4 mg/L (U.S. Environmental Protection Agency, 

1990), based on consumption of water having larger concentrations leading to bone disease. In addition, to protect against 

dental fluorosis, a secondary standard is 2 mg/L, above which 

brown staining and pitting of teeth can result. Conversely, 

smaller fluoride concentrations promote dental health.

Large fluoride concentrations pose a considerable 

limitation on groundwater use in parts of the Virginia Coastal 

Plain. Widespread fluorosis among children has been recognized in the area south of the James River as long as 70 years 

ago (Cederstrom, 1939). Recent drinking-water production 

has included blending of groundwater having large fluoride 

concentrations with water having small concentrations of 

fluoride (see “Considerations for Groundwater Use”). 

Tabulated groundwater-sample data include 3,845 

fluoride-concentration values (Attachment 1). Mean and 

median values (Attachment 2) are below both primary and 

secondary drinking-water standards. Map and sectional views 

(fig. 22) indicate a large fluoride concentration of 18 mg/L in 

a sample from a well open to basement bedrock along the Fall 

Zone in central Henrico County, possibly as a result of the 

particular bedrock mineralogy there. Additional large concentrations ranging from 13 to 30 mg/L are present in 11 vertically aligned pore-water samples extracted from sediments at 

the center of the Chesapeake Bay impact crater. The thermal 

history of the impact crater has uniquely altered groundwater 

chemistry at that location (Sanford and others, 2009b), which 

is likely distinct from the rest of the Virginia Coastal Plain and 

remains an area of active research (see “Brine and the Inner 

Chesapeake Bay Impact Crater”).

Most of the large fluoride concentrations are present 

in 671 groundwater samples having between 2 and 4 mg/L, 

216 samples having between 4 and 10 mg/L, and one sample 

having 18 mg/L. Most of these samples are from a broad belt 
Regional Trends in Groundwater Quality 49

Figure 22. Distribution of 

fluoride in groundwater in the 

Virginia Coastal Plain in (A) map 

view and (B) generalized 

sectional view. Positions of 

groundwater major-ion types 

are approximate. Subsurface 

materials are generalized from 

McFarland and Bruce (2006). 

Location of the Chesapeake Bay 

impact crater is from Powars 

and Bruce (1999). Crater-fill 

sediment is not shown in 

sectional view. Altitude datum 

is National Geodetic Vertical 

Datum of 1929. [mg/L, milligrams 

per liter]

EXPLANATION

Concentration of fluoride in groundwater

 up to 2 mg/L

 greater than 2 mg/L to 4 mg/L

 greater than 4 mg/L to 10 mg/L

 greater than 10 mg/L

500

0

–500

–1,000

–1,500

–2,000

–2,500

ALTITUDE, IN FEET

West East

Vertical scale greatly exaggerated

Hard

Marine sediments

Phosphatic material

Chesapeake Bay impact crater

Hard

Soft Salty

Salty

Saltwatertransition

Basement zone

bedrock

Potomac aquifer

fluvial-deltaic

sediments

Land surface

Figure 22. Distribution of fluoride in groundwater in the Virginia Coastal Plain, in map view (A) and

 generalized sectional view (B). Positions of groundwater major ion types are approximate.

Subsurface materials are generalized from McFarland and Bruce (2006)Location of the Chesapeake

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R. R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN

SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000

0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

Piedmont

River

Fall Zone

Piedmont

Fall Zone

LOCATION OF THE STUDY AREA

IN VIRGINIA

A

B
50 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

(fig. 22A) that spans southeastern Virginia from the city of 

Franklin eastward across the city of Suffolk and the western 

city of Chesapeake, and that narrows northward parallel 

to the outer margin of the Chesapeake Bay impact crater 

across eastern Isle of Wight and Surry Counties, the central 

York-James Peninsula, and the lower Middle Peninsula and 

Northern Neck.

Vertically, the large concentrations across the high fluoride belt are near or within the uppermost part of the Potomac 

aquifer at elevations ranging from –300 to –700 ft (fig. 22B), 

with the greatest thickness of large concentrations to the south 

of the James River. Groundwater fluoride concentrations in 

shallower sediments are generally less than 2 mg/L. Within the 

Potomac aquifer, concentrations also are generally less than 

2 mg/L on either side of the high fluoride belt and likewise at 

greater depth beneath the belt. Thus in three dimensions, the 

belt consists of large concentrations clustered into a roughly 

lens-shaped mass that is centered approximately 500 ft 

beneath the city of Suffolk and that thins and tapers northward.

The origin of the high fluoride belt has not been determined with certainty. The fluoride concentration of seawater 

is approximately 1.3 mg/L (Hem, 1985) and cannot directly 

account for the larger concentrations of fluoride in groundwater. Alternatively, fluorine is present in solid form within 

some marine sediments as phosphatic material composed of 

fluorapatite (Ca5

(PO4)

3

F), which makes up fossil bones and 

teeth deposited as remains of marine animals (McFarland 

and Bruce, 2006). Fluorapatite releases dissolved fluoride to 

groundwater by ion exchange with hydroxyl (OH–) to form 

hydroxylapatite (Ca5

(PO4)

3

OH). Adsorbtion of hydroxyl 

results from an increase in its concentration in groundwater, 

thereby displacing adsorbed fluoride into groundwater and 

increasing its concentration. Dissolved hydroxyl concentration 

also varies directly with pH and bicarbonate concentration. 

Thus, increases in pH and bicarbonate concentration also 

correspond to desorbtion of fluoride.

Fluorapatite-bearing phosphatic material in the South 

Carolina Coastal Plain has been cited as the source for 

groundwater fluoride concentrations as large as 5.5 mg/L 

(Zack, 1980). In the Virginia Coastal Plain, however, spatial 

relations are not consistent with phosphatic material being a 

direct source of the high fluoride belt. Groundwater fluoride 

concentrations in the phosphatic material are generally less 

than 2 mg/L (fig. 22B), whereas underlying fluvial-deltaic 

sediments of the Potomac aquifer in which the high fluoride 

belt is positioned lack phosphatic material. The high fluoride 

belt appears to be essentially isolated within the upper part of 

the Potomac aquifer, separate from the phosphatic material, 

and lacking any plume or sheet of high fluoride groundwater 

that would reflect transport of fluoride between the two 

locations. Despite phosphatic material being the most 

obvious source of groundwater fluoride, hydroxyl concentrations there are apparently adequate to desorb only enough 

fluoride from fluorapatite to produce relatively low dissolved 

fluoride concentrations.

Other fluorite-bearing minerals have been suggested as 

possible sources of groundwater fluoride, including tourmaline 

and topaz (Sinnott and Whetstone, 1962), but if present are 

only trace components of the Potomac aquifer sediments. 

More abundant sediment components have also been cited, 

including mica and clay (Cederstrom, 1945; Sinnott and 

Whetstone, 1962; Maciuika, 1992). Incongruent dissolution 

of various mica minerals can potentially release a diverse 

variety of solutes to groundwater, including fluoride, but no 

direct relation has been established between mica and large 

groundwater fluoride concentrations. Moreover, clay minerals 

modify the composition of groundwater primarily by exchange 

of cations and do not affect fluoride anions. Oxyhydroxides of 

iron and manganese, however, can exchange both cations and 

anions (Drever, 1988), and are also abundant within Potomac 

aquifer sediments as amorphous colloidal particles and 

coatings on sediment grains (McFarland and Bruce, 2006) (see 

“Iron and Manganese”). Adsorbtion and desorbtion of fluoride 

onto oxyhydroxides can potentially take place alternately with 

hydroxyl as well as other anions in a manner similar to that 

described previously for fluorapatite.

In addition to its source, other aspects of the high 

fluoride belt are possibly related to its presence. Groundwater fluoride concentration has been directly related to 

(Na+K)+(HCO3

+CO3) “soft” type groundwater (Cederstrom, 

1945; Sinnott and Whetstone, 1962; Maciuika, 1992) and to 

bicarbonate concentration (Zack, 1980). In addition, the high 

fluoride belt aligns with the outer margin of the saltwatertransition zone (fig. 22B), where (Na+K)+(HCO3

+CO3) 

“soft” type groundwater transitions eastward and with depth 

to (Na+K)+Cl “salty” type groundwater (see “Major Ions”). 

Groundwater pH and bicarbonate concentrations are relatively 

high, and chloride concentrations increase from approximately 

20 mg/L to as much as 1,000 mg/L. Farther eastward and to 

greater depth beyond the high fluoride belt, chloride concentrations continue to increase, but fluoride concentrations 

generally decline to below 2 mg/L. 

Based on the preceding information, a hypothesis is 

proposed herein of how the high fluoride belt was possibly 

formed as a consequence of migration of the saltwatertransition zone. Fresh groundwater has flowed from the 

marine sediments eastward and downward into the underlying 

Potomac aquifer for the past approximately 1.8 m.y., following 

emergence from the most recent region-wide inundation by 

the Atlantic Ocean (see “Saltwater-Transition Zone”). During 

the Pleistocene glacial maximum from 18,000 to 21,000 years 

ago, fresh groundwater flowed nearly to the edge of the 

Continental Shelf, under a hydraulic gradient two to four times 

greater than at present. North-to-south trending belts of hard, 

soft, and salty type groundwater, and their corresponding total 

ion concentrations (see “Major Ions”), were displaced eastward proportionately. Fluoride was released from phosphatic 

material to groundwater across large areas at small concentrations of 2 mg/L or less that are generally widespread. Upon 

transport into the Potomac aquifer, however, some portion 

of the dissolved fluoride was adsorbed onto oxyhydroxides 
Regional Trends in Groundwater Quality 51

within the Potomac aquifer sediments. During the prolonged 

period of groundwater flow and transport, the oxyhydroxides 

acted in a filter-like manner to accumulate a large quantity 

of adsorbed fluoride potentially across a wide span of the 

Potomac aquifer as far east as the edge of the Continental 

Shelf.

With the subsequent rise in sea level, the Atlantic 

Ocean has reinundated the Continental Shelf, and the 

saltwater-transition zone has migrated westward to its present 

position. The belts of hard, soft, and salty type groundwater, 

and their corresponding total ion concentrations, have been 

displaced to the west proportionately. Under these conditions, 

oxyhydroxides within the Potomac aquifer sediments have 

become a source of fluoride, which is being desorbed back 

into solution by adsorbtion of hydroxyl, chloride, and(or) other 

anions, where soft (Na+K)+(HCO3+CO3) type groundwater 

transitions eastward and with depth to salty (Na+K)+Cl type 

groundwater. Thus, much of the fluoride in groundwater 

probably originated from phosphatic material, but was 

adsorbed onto oxyhydroxides within the Potomac aquifer 

sediments and subsequently desorbed along the advancing 

saltwater-transition zone to result in the current position of the 

high fluoride belt.

Considering the possible relation of the high fluoride belt 

to the saltwater-transition zone, their present-day configurations are not static. Areas farther east likely first developed 

large groundwater fluoride concentrations, straddling the 

leading edge of the advancing saltwater-transition zone. Low 

fluoride seawater has since reentered the sediments in these 

areas, as sea level has risen to its current position. Thus, the 

high fluoride belt is a relatively transient feature associated 

with westward migration of the saltwater-transition zone.

Iron and Manganese

Iron and manganese are closely associated in groundwater and are primarily in the form of cations having a charge of 

2 (Fe2+ and Mn2+, respectively), although smaller amounts of 

differently charged cations, ion complexes, and other forms 

can also be present (Hem, 1985). Secondary drinking-water 

standards (U.S. Environmental Protection Agency, 1990) are 

0.3 mg/L for total iron and 0.05 mg/L for total manganese, 

based primarily on aesthetic considerations. Water exceeding 

the standards can commonly have a rusty or brown to black 

color and metallic taste, and can produce sediment and reddish 

to orange or black staining. Conversely, both iron and manganese are essential elements for plant and animal life. 

Iron and manganese are pervasive in groundwater 

throughout the Virginia Coastal Plain with highly variable 

concentrations at the local scale. Wells producing water with 

relatively small concentrations can often be in proximity to 

other wells exhibiting substantially larger and problematic 

concentrations. Removal of iron and(or) manganese is one of 

the most common necessary treatments of water from supply 

wells (see “Considerations for Groundwater Use”). 

Tabulated groundwater-sample data include 3,554 ironconcentration values and 2,581 manganese-concentration 

values (Attachment 1). (Note that tabulated values are in 

micrograms per liter (μg/L), equivalent to 0.001 mg/L. Iron 

and manganese drinking-water standards are equivalent 

to 300 μg/L and 50 μg/L, respectively.) Median iron and 

manganese concentrations (Attachment 2) are both below 

their drinking-water standards, but their mean concentrations 

are above. Large differences between the mean and median 

concentrations reflect small numbers of extreme large values. 

Tabulated groundwater-sample iron- and manganeseconcentration values are summarized in map and sectional 

views (figs. 23, 24, respectively), and exhibit similar trends. 

Although some large concentrations are present throughout the 

Virginia Coastal Plain, a spatial sequence is apparent in relative differences among the concentrations. To the west along 

the Fall Zone, iron and manganese concentrations exceeding 

drinking-water standards are relatively common (figs. 23A, 

24A). Eastward, concentrations at or below the drinking-water 

standards dominate a broad north-south trending belt spanning 

much of the central part of the Virginia Coastal Plain. The two 

areas generally correspond to those of (Ca2++ Mg2+)+(HCO3

–

+ 

CO3

–

) type “hard” water toward the west and (Na++ K+)+ 

(HCO3

–

+ CO3

–

) type “soft” water toward the east (see “Major 

Ions”). Still farther east, iron and manganese concentrations 

increase widely to include many of the largest concentrations 

and generally correspond to the area of (Na++ K+)+Cl– type 

“salty” water. 

Further aspects of the spatial sequence in groundwater 

iron and manganese concentrations are exhibited vertically 

(figs. 23B, 24B). As in map view, concentrations furthest 

updip to the west commonly exceed drinking-water standards 

where hard water is dominant, and eastward are more widely 

at or below the drinking-water standards where soft water 

is dominant. Farther east, however, two vertically separated 

trends are apparent. Within the Potomac aquifer and lower part 

of the marine sediments, iron concentrations widely exceed 

the drinking-water standard across the saltwater-transition 

zone and beyond, where soft water evolves to salty water. 

Manganese concentrations follow a similar trend, but only 

widely exceed the drinking-water standard farther east beyond 

the saltwater-transition zone. 

Distinct from these findings, many additional iron 

and manganese concentrations exceed their drinking-water 

standards within the shallow part of the marine sediments 

(figs. 23B, 24B), where hard water remains dominant far to 

the east. As seen in map view (figs. 22A, 23A), these large 

iron and manganese concentrations at shallow depths occupy 

a broad belt spanning the lower parts of the Middle Peninsula 

and York-James Peninsula northeastward across the Virginia 

Eastern Shore and southeastward across the cities of Chesapeake and Virginia Beach. 

The origin of iron and manganese in groundwater is 

complex in detail. Both constituents can potentially take part 

in a broad array of chemical reactions with various sediment 

components. Speciation of iron and manganese among diverse 
52 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Figure 23. Distribution of iron 

in groundwater in the Virginia 

Coastal Plain in (A) map view 

and (B) generalized sectional 

view. Positions of groundwater 

major-ion types are approximate. 

Subsurface materials are 

generalized from McFarland and 

Bruce (2006). Location of the 

Chesapeake Bay impact crater 

is from Powars and Bruce (1999). 

Crater-fill sediment is not shown 

in sectional view. Altitude datum 

is National Geodetic Vertical 

Datum of 1929. [mg/L, milligrams 

per liter]

EXPLANATION

Concentration of iron in groundwater

 up to 0.3 mg/L

 greater than 0.3 mg/L to 1 mg/L

 greater than 1 mg/L to 10 mg/L

 greater than 10 mg/L

West East

Vertical scale greatly exaggerated

Land surface

Marine sediments

Chesapeake Bay impact crater

Salty

Salty

Saltwatertransition

Basement zone

bedrock

Potomac aquifer

fluvial-deltaic

sediments

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

500

0

–500

–1,000

–1,500

–2,000

–2,500

ALTITUDE, IN FEET

Figure 23. Distribution of iron in groundwater in the Virginia Coastal Plain, in map view (A) and

 generalized sectional view (B). Positions of groundwater major ion types are approximate.

 Subsurface materials are generalized from McFarland and Bruce (2006). Location of the

Hard

Hard

Soft

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R. R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN

SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000

0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

Piedmont

River

Fall Zone

Piedmont

Fall Zone

LOCATION OF THE STUDY AREA

IN VIRGINIA

A

B
Regional Trends in Groundwater Quality 53

Figure 24. Distribution of 

manganese in groundwater 

in the Virginia Coastal Plain in 

(A) map view and (B) generalized 

sectional view. Positions of 

groundwater major-ion types 

are approximate. Subsurface 

materials are generalized from 

McFarland and Bruce (2006). 

Location of the Chesapeake Bay 

impact crater is from Powars and 

Bruce (1999). Crater-fill sediment 

is not shown in sectional view. 

Altitude datum is National 

Geodetic Vertical Datum of 1929. 

[mg/L, milligrams per liter]

EXPLANATION

Concentration of manganese in groundwater

 up to 0.05 mg/L

 greater than 0.05 mg/L to 0.2 mg/L

 greater than 0.2 mg/L to 2 mg/L

 greater than 2 mg/L

West East

Vertical scale greatly exaggerated

Land surface

Marine sediments

Chesapeake Bay impact crater

Salty

Salty

Saltwatertransition

Basement zone

bedrock

Potomac aquifer

fluvial-deltaic

sediments

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

500

0

–500

–1,000

–1,500

–2,000

–2,500

ALTITUDE, IN FEET

Figure 24. Distribution of manganese in groundwater in the Virginia Coastal Plain, in map view

 (A) andgeneralized sectional view (B). Positions of groundwater major ion types are approximate.

 Subsurface materials are generalized from McFarland and Bruce (2006). Location of the

Hard

Hard

Soft

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R. R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000

0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

Piedmont

River

Fall Zone

Piedmont

Fall Zone

LOCATION OF THE STUDY AREA

IN VIRGINIA

A

B
54 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

dissolved and solid-phase forms are functions of both pH and 

oxidation-reduction, or “redox,” potential (Eh). Whereas pH is 

a measure of the activity of protons (hydrogen ions) transferred in acid-base reactions, Eh is a measure of the activity 

of electrons transferred in redox reactions. In practical terms, 

the Eh of groundwater is partly reflected by the presence or 

absence of dissolved oxygen. Iron and manganese will exist in 

more positively charged forms under oxidizing conditions and 

in less positively charged forms under less oxidizing (reducing) conditions. Moreover, oxidizing conditions generally 

correspond to relatively small concentrations of dissolved iron 

and manganese and reducing conditions to larger concentrations. Large concentrations of iron in groundwater in the 

Maryland Coastal Plain were shown to result from reducing 

conditions (Back and Barnes, 1965). Transformation reactions 

among various dissolved and solid-phase forms of both iron 

and manganese can also involve complex organic compounds 

and can be catalyzed by microorganisms (see “Considerations 

for Groundwater Use”). Further details of iron and manganese 

in water are subjects of a broad field of hydrochemical 

research, which is beyond the scope of this report. 

Processes affecting groundwater iron and manganese in 

the Virginia Coastal Plain can be broadly generalized. In updip 

areas to the west, iron and manganese probably originate 

from the dissolution of various minerals. Clumps, nodules, 

and other massive forms of pyrite (FeS2

) are commonly 

associated with fossil plant debris and other organic material 

within some sediments of the Potomac aquifer. More finely 

disseminated pyrite is also present in some overlying sediments. Dissolved iron originating from dissolution of pyrite 

is also often associated with large concentrations of hydrogen 

sulfide in groundwater (see “Considerations for Groundwater 

Use”). In addition, basement bedrock is composed of a 

diverse assemblage of igneous and metamorphic minerals, 

many of which potentially contain both iron and manganese. 

Although mineral dissolution produces some dissolved iron 

and manganese, additional amounts are precipitated back into 

the sediments where dissolved oxygen is present as amorphous 

oxyhydroxides consisting of colloidal particles and coatings 

on sediment grains. 

Concentrations of iron and manganese in groundwater 

decrease eastward and downward, where the major ion 

composition of groundwater evolves from hard to soft as a 

result of ion exchange (see “Major Ions”). Similar to calcium 

and magnesium, iron and manganese cations are adsorbed by 

clay and glauconite within the sediments to release previously 

adsorbed sodium and potassium cations into solution. Replenishment of dissolved iron and manganese by further dissolution of pyrite and(or) other minerals possibly is hindered by 

lack of a sustained supply of dissolved oxygen at depth. 

Farther east, concentrations of iron and manganese in 

groundwater increase across and beyond the saltwatertransition zone, where the major ion composition of 

groundwater evolves from soft to salty as a result of mixing 

with seawater. The increases cannot be directly accounted for 

by emplacement of seawater, in which iron and manganese 

concentrations are small (approximately 3 μg/L, and 2 μg/L, 

respectively) (Hem, 1985). Instead, iron and manganese concentrations are increased by ion exchange, which is reversed 

from that taking place to the west. Sodium cations in large 

concentrations associated with seawater are adsorbed onto 

clay and glauconite to release previously adsorbed iron and 

manganese cations back into solution. In addition, iron and 

manganese cations are possibly exchanged for sodium cations 

by oxyhydroxides in a similar manner to fluoride anions for 

hydroxyl and chloride anions (see “Fluoride”). Unlike fluoride, 

however, large iron and manganese concentrations extend 

eastward beyond the saltwater-transition zone, indicating that 

their desorbtion persists as seawater is emplaced within 

the sediments.

Other processes take place in shallow, dominantly hard 

groundwater across the broad eastward belt of large iron and 

manganese concentrations. These sediments compose mostly 

the surficial and Yorktown-Eastover aquifers, consisting of 

sands and clays with large amounts of fossil shell material, 

but also substantial quantities of organic matter as fossil plant 

debris and peat. Organic matter reacts with iron and manganese oxyhydroxides to produce dissolved iron and manganese, 

generalized as (Freeze and Cherry, 1979):

CH2

O + 4 Fe(OH)3 + 7 H+  4 Fe2+ + HCO3

–

 + 10 H2O, and

CH2

O + 2 MnO2 + 3 H+  2 Mn2+ + HCO3

–

 + 2 H2O.

The organic matter is also a potential source of ammonium 

(see “Nitrate and Ammonium”) and hydrogen sulfide (see 

“Considerations for Groundwater Use”).

Nitrate and Ammonium

Dissolved nitrogen takes several forms, including the 

anions nitrate (NO3

–

) and nitrite (NO2

–

), the cation ammonium 

(NH4

+

), the gases nitrogen (N2) and nitrous oxide (N2O), and 

diverse organic compounds. Primary drinking-water standards 

(U.S. Environmental Protection Agency, 1990) are 10 mg/L 

of nitrogen in the form of nitrate, and 1 mg/L in the form of 

nitrite, based on consumption of water having larger concentrations leading to acute methemoglobinemia (a blood disease) 

in infants and long-term diuresis and hemorrhaging of the 

spleen in adults. Although other forms of nitrogen in drinking 

water are unregulated, some can potentially be transformed 

into nitrate. In addition to effects on drinking water, transport 

of large amounts of nitrogen by both groundwater and streams 

has been cited as a major cause of eutrophication of Chesapeake Bay (Phillips, 2007). Conversely, ecologically balanced 

amounts of nitrogen are essential for plant and animal life. 

Some forms of dissolved nitrogen are pervasive in 

groundwater throughout the Virginia Coastal Plain, with 

concentrations generally variable at the local scale. Wells 

producing water with relatively small concentrations can often 

be in proximity to other wells exhibiting substantially larger 
Regional Trends in Groundwater Quality 55

concentrations. Removal of nitrate and other forms 

of dissolved nitrogen are potential necessary treatments 

of water from some supply wells (see “Considerations for 

Groundwater Use”). 

Tabulated groundwater-sample data include 2,711 nitrateconcentration values and 2,293 ammonium-concentration 

values (Attachment 1). Concentrations are expressed as 

milligrams per liter of nitrogen to allow direct comparison 

among the two forms. No values are included for other forms 

of dissolved nitrogen. Nitrate-concentration mean and median 

values are both below the drinking-water standard by one 

and two orders of magnitude, respectively (Attachment 2). 

Ammonium-concentration maximum, mean, and median 

values are all about an order of magnitude greater than their 

corresponding nitrate-concentration values. 

Groundwater nitrate- and ammonium-concentration values are summarized in map and sectional views (figs. 25, 26). 

For both constituents, concentrations at or below 1 mg/L are 

present throughout the Virginia Coastal Plain. The locations of 

larger nitrate concentrations are generally scattered (fig. 25A), 

including those of 33 samples that exceed the 10 mg/L 

drinking-water standard. By contrast, some larger ammonium 

concentrations are more clustered in one area around the lower 

part of the Northern Neck and in a second area around the city 

of Virginia Beach (fig. 26A).

Vertically, groundwater nitrate and ammonium concentrations at or below 1 mg/L are also widespread (figs. 25B, 26B). 

Nitrate concentrations greater than 1 mg/L span most of the 

area of shallow (Ca2++ Mg2+)+(HCO3

–

+ CO3

–

) type “hard” 

water, although a few are also present at greater depth across 

the saltwater-transition zone. Ammonium concentrations 

follow similar trends, although those greater than 1 mg/L are 

generally at greater depths. Within the area of shallow hard 

water far to the east, ammonium concentrations greater than 

1 mg/L generally underlie nitrate concentrations greater than 

1 mg/L. Some deeper ammonium concentrations are also 

substantially greater than nitrate concentrations across and 

beyond the saltwater-transition zone.

Processes affecting nitrogen in groundwater are complex 

in detail. Speciation of nitrogen among the various forms are 

functions of both pH and oxidation-reduction, or “redox,” 

potential (Eh), and chemical transformation reactions are often 

mediated by microorganisms. In broad terms, groundwater 

nitrate generally originates from sources at or near the land 

surface (Freeze and Cherry, 1979). Decomposition of organic 

nitrogen from the remains of plants and animals initially 

produces ammonium. Where oxygen is available, ammonium 

is converted primarily to nitrate (nitrite is temporarily produced as an unstable intermediate form). Where vegetation is 

active, nitrate is largely taken back up by plants (Drever, 1988) 

to effectively recycle nitrogen near the land surface.

Ammonium and nitrate that is not recycled near the land 

surface can be transported in groundwater to greater depths. 

Excess amounts of both constituents in groundwater can 

originate from artificial sources, such as fertilizer-application 

and livestock-production areas, and buried sewer lines 

and septic systems. In addition, organic matter that was 

deposited within sediments at depth is a potential natural 

source. Ammonium can be removed from groundwater by 

ion exchange in a manner similar to other cations (see “Major 

Ions” and “Iron and Manganese”). Nitrate generally is more 

readily transported by groundwater flow, as long as oxygen is 

available, although some nitrate can potentially be adsorbed 

by iron and manganese oxyhydroxides. In addition, where 

oxygen becomes unavailable, nitrate can be partly converted 

by microbially mediated denitrification to nitrogen and(or) 

nitrous oxide gases, which are chemically stable and nonreactive, pose no detriment to drinking water, and can be removed 

from groundwater by vaporization.

The widespread presence of groundwater nitrate and 

ammonium concentrations at or below 1 mg/L is consistent 

with much of the nitrogen being recycled near the land surface 

throughout most of the Virginia Coastal Plain. In addition, 

nitrate transported to greater depth possibly undergoes 

denitrification, where dissolved-oxygen concentrations are 

generally small within confined aquifers and confining units 

and zones (Attachment 2).

Nitrate and ammonium concentrations greater than 

1 mg/L in shallow groundwater indicate additional amounts 

not taken up by plants, which in many cases likely originate 

from human sources. Shallow groundwater in many areas has 

entered the subsurface within human timeframes (Nelms and 

others, 2003), and human sources possibly have locally altered 

the major ion type of shallow groundwater at widely spaced 

locations (see “Major Ions”). Removal of nitrate by denitrification is precluded by the relatively large dissolved-oxygen 

concentrations within the surficial aquifer (Attachment 2). 

Site-specific human sources of nitrate and(or) ammonium 

potentially include a wide variety of both rural and urban 

activities, but are beyond the scope of the regional analysis 

herein to consider in detail.

Some groundwater nitrate and ammonium concentrations 

greater than 1 mg/L possibly result from other processes. 

In shallow hard water far to the east, both constituents are 

possibly produced by decomposition of organic matter, which 

is present in substantial quantities as fossil plant debris and 

peat within sediments of the surficial and Yorktown-Eastover 

aquifers. Ammonium is more prevalent than nitrate at depths 

below which oxygen is available. The organic matter possibly 

also reacts with iron and manganese oxyhydroxides to produce 

dissolved iron and manganese (see “Iron and Manganese”).

Across and beyond the saltwater-transition zone, 

groundwater nitrate and ammonium concentrations greater 

than 1 mg/L possibly result from ion exchange. The concentrations cannot be directly accounted for by emplacement of 

seawater, in which dissolved nitrogen is primarily in the form 

of nitrate at concentrations less than 1 mg/L (not including 

nitrogen gas) (Hem, 1985). Instead, sodium cations in large 

concentrations associated with seawater are adsorbed onto 

clay and glauconite, and also possibly onto iron and manganese oxyhydroxides to release previously adsorbed ammonium 

cations back into solution. Similarly to iron and manganese 
56 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Figure 25. Distribution of 

nitrate nitrogen in groundwater 

in the Virginia Coastal Plain in 

(A) map view and (B) generalized 

sectional view. Positions of 

groundwater major-ion types 

are approximate. Subsurface 

materials are generalized from 

McFarland and Bruce (2006). 

Location of the Chesapeake Bay 

impact crater is from Powars and 

Bruce (1999). Crater-fill sediment 

is not shown in sectional view. 

Altitude datum is National 

Geodetic Vertical Datum of 1929. 

[mg/L, milligrams per liter]

EXPLANATION

up to 1 mg/L 

greater than 1 mg/L to 10 mg/L

greater than 10 mg/L

Concentration of nitrate nitrogen

in groundwater

West East

Vertical scale greatly exaggerated

Land surface

Marine sediments

Chesapeake Bay impact crater

Salty

Salty

Saltwatertransition

Basement zone

bedrock

Potomac aquifer

fluvial-deltaic

sediments

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

500

0

–500

–1,000

–1,500

–2,000

–2,500

ALTITUDE, IN FEET

Figure 25. Distribution of nitrate nitrogen in groundwater in the Virginia Coastal Plain, in map view

 (A) andgeneralized sectional view (B). Positions of groundwater major ion types are approximate.

 Subsurface materials are generalized from McFarland and Bruce (2006). Location of the

Hard

Hard

Soft

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R. R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000

0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

Piedmont

River

Fall Zone

Piedmont

Fall Zone

LOCATION OF THE STUDY AREA

IN VIRGINIA

A

B
Regional Trends in Groundwater Quality 57

Figure 26. Distribution 

of ammonium nitrogen in 

groundwater in the Virginia 

Coastal Plain in (A) map view 

and (B) generalized sectional 

view. Positions of groundwater 

major-ion types are approximate. 

Subsurface materials are 

generalized from McFarland and 

Bruce (2006). Location of the 

Chesapeake Bay impact crater 

is from Powars and Bruce (1999). 

Crater-fill sediment is not shown 

in sectional view. Altitude datum 

is National Geodetic Vertical 

Datum of 1929. [mg/L, milligrams 

per liter]

EXPLANATION

up to 1 mg/L 

greater than 1 mg/L to 10 mg/L

greater than 10 mg/L to 100 mg/L

greater than 100 mg/L

Concentration of ammonium nitrogen

in groundwater

West East

Vertical scale greatly exaggerated

Land surface

Marine sediments

Chesapeake Bay impact crater

Salty

Salty

Saltwatertransition

zone

Basement

bedrock

Potomac aquifer

fluvial-deltaic

sediments

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

500

0

–500

–1,000

–1,500

–2,000

–2,500

ALTITUDE, IN FEET

Figure 26. Distribution of ammonium nitrogen in groundwater in the Virginia Coastal Plain, in map view

 (A) andgeneralized sectional view (B). Positions of groundwater major ion types are approximate.

 Subsurface materials are generalized from McFarland and Bruce (2006). Location of the

Hard

Hard

Soft

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R. R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000

0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

Piedmont

River

Fall Zone

Piedmont

Fall Zone

LOCATION OF THE STUDY AREA

IN VIRGINIA

A

B
58 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

(see “Iron and Manganese”), large ammonium concentrations 

extend eastward beyond the saltwater-transition zone, indicating that their desorbtion persists as seawater is emplaced 

within the sediments. In addition, nitrate anions are possibly 

exchanged by oxyhydroxides for hydroxyl and chloride anions 

in a similar manner to fluoride anions (see “Fluoride”). 

Considerations for Groundwater Use

The suitability of groundwater for various uses is largely 

determined by its chemical quality. Concentrations of some 

chemical constituents can potentially impose limitations and 

treatment requirements for a particular use. Hence, a complete 

understanding of groundwater quality in the Virginia Coastal 

Plain includes knowledge of relations to its intended uses. 

During 2000, groundwater use in the Virginia Coastal 

Plain was 137.4 Mgal/d (Pope and others, 2008), of which 

nearly half was used to supply major industries and diverse 

commercial enterprises. Urban municipalities withdrew nearly 

another quarter, to provide public drinking water in developed 

areas. More than another quarter of the water was withdrawn 

from small, privately owned domestic supply wells that are 

widely dispersed across large rural areas. Irrigation and other 

uses made up withdrawals equivalent to less than 3 percent of 

the total.

General relations between groundwater quality and use 

in the Virginia Coastal Plain are summarized on the basis 

of regional trends and related processes presented in earlier 

sections of this report. Discussion is limited to only some 

chemical constituents that are generally present naturally 

and does not address constituents that originate from human 

contamination. Water-treatment technology and groundwater 

remediation are broad and complex fields of engineering and 

environmental science. Major water-production facilities 

providing industrial, commercial, and public supplies commonly employ large and elaborate processing operations to 

achieve the required chemical quality for a particular use. In 

addition, the quality of water in the environment and produced 

for drinking involves diverse regulatory and public-policy 

issues. A full discussion of these topics is beyond the scope of 

this report. 

Various groundwater-quality issues affect diverse uses, 

ranging among individual, privately owned domestic wells, 

small community or commercial water systems, and major 

water-production facilities. In addition to chemical treatment, 

sound planning and other resource-management strategies can 

potentially avoid the use of groundwater having an unsuitable 

quality for an intended use.

Hardness

The primary influence on the suitability of groundwater for various uses is exerted by the major ions that 

dominate its chemical composition (see “Major Ions”). 

(Na+

+ K+)+(HCO3

–

+ CO3

–

) “soft” type water in deep aquifers 

beneath the central part of the Virginia Coastal Plain is 

generally suitable for a wide variety of uses. By contrast, 

(Ca2++ Mg2+)+(HCO3

–

+ CO3

–

) “hard” type water in parts of 

these aquifers to the west, and at some locations in shallower 

aquifers and basement bedrock along the Fall Zone, can 

potentially require treatment for some uses. 

The concept of water having properties described as 

either hard or soft has been traced as far back as Hippocrates 

(460–377 B.C.) (Hem, 1985). Distinct from the categorization 

used herein, an array of alternative definitions of hard water 

exists (Freeze and Cherry, 1979; Hem, 1985; Drever, 1988; 

Wilson and others, 1999a). Various semiquantitative measures 

are commonly reported, most widely in terms of equivalent 

concentration of calcium carbonate, but are inconsistently 

determined and generally not useful in providing a precise 

hydrochemical description. Many reported hardness values are 

actually calculated by different methods from the separately 

analyzed concentrations of individual chemical constituents 

(Hem, 1985). Similarly, from the chemical-constituent 

concentration values tabulated in this report (Attachment 1), 

hardness values can potentially be calculated by a variety 

of schemes.

Hardness is the most commonly reported water problem 

in the United States, and by some estimates, 85 percent of 

water nationwide is considered hard (Wilson and others, 

1999a). Hard water is not associated with negative health 

effects, however, and is not regulated by drinking-water 

standards (U.S. Environmental Protection Agency, 1990). 

Problematic aspects of hard water include reaction with soap 

and detergent to form insoluble compounds and precipitation 

of solid deposits (Wilson and others, 1999a). The effectiveness 

of cleansing agents is hindered when combined in hard water 

mostly with calcium and magnesium cations to produce a 

waxy slime, scum, or other forms of residue. In addition, 

scaly deposits of calcium carbonate and other precipitates can 

accumulate with deleterious effects in plumbing, appliances, 

and other devices and systems through which hard water 

is conveyed. 

The most common form of treatment for hard water is 

softening by ion exchange (Wilson and others, 1999a). Artificial water softening is analogous to the softening from ion 

exchange by subsurface sediments that takes place naturally in 

deep aquifers beneath the central part of the Virginia Coastal 

Plain (see “Major Ions”). Water-softening systems are made 

up of columns of small, densely packed synthetic resin beads, 

through which hard water is transmitted to adsorb calcium 

and magnesium cations in exchange for sodium cations. 

Water softening is commonly used by major water-production 

facilities, but smaller systems are also designed for individual 

domestic wells and small community or commercial water 

supplies. Appropriate system design depends on the required 

water-production capacity and other features, such as the 

desired method of column regeneration (manual versus 

automated). Required system maintenance entails periodic 

back-flushing with a concentrated sodium chloride solution 

followed by freshwater to regenerate the ion-exchange column 
Regional Trends in Groundwater Quality 59

by desorbing calcium and magnesium cations with a renewed 

supply of sodium cations. 

Although generally considered superior to hard water for 

most uses, soft water possesses some limitations. For privately 

owned softening systems, the concentration of sodium 

produced should be evaluated by users for whom sodium 

consumption is a health concern. Some systems can use 

potassium instead of sodium, whereas others include a bypass 

feature to supply untreated water for drinking and cooking. In 

addition, as an aesthetic consideration, bathing with soft water 

(either treated or naturally present) can produce a slippery feel 

on skin. Alternative explanations are that (1) the nonreactivity 

of soft water with soap possibly hinders its ability to remove 

soap from the surface of skin or (2) when hard water is used, 

small amounts of insoluble residue possibly absorb oil from 

the skin to produce a greater drying sensation.

Chloride

The most substantial limitations and treatment requirements for groundwater use in the Virginia Coastal Plain likely 

are posed by (Na++ K+)+Cl– “salty” type water having large 

total ion concentrations in the eastern parts of deep aquifers 

(see “Major Ions”) and at some locations in shallower aquifers. Localized changes in groundwater chloride concentration, 

including some exceeding the secondary maximum contaminant level of 250 mg/L (U.S. Environmental Protection 

Agency, 1990), are associated with possible movement of salty 

groundwater in response to withdrawals close to and within 

the saltwater-transition zone (see “Movement”).

Salty groundwater in parts of the Virginia Coastal Plain 

is currently being increasingly developed as a water source. 

Brackish groundwater is withdrawn and treated as a means 

of addressing mounting water demands in rapidly growing 

metropolitan areas underlain by the saltwater-transition zone. 

As of 2007, five municipalities operate large-scale brackish 

groundwater development (BGD) facilities (table 3) as parts 

of their public drinking-water systems. Rates of withdrawal 

among the individual BGD plants during 2005 ranged 

from 0.66 Mgal/d to 3.54 Mgal/d and collectively totaled 

10.58 Mgal/d. Each municipality has received permits from 

the DEQ, however, authorizing greater withdrawal rates 

that total 30 Mgal/d. BGD operation is expected to increase 

during the coming several decades. In addition, desalinization is known anecdotally through local well drillers to be 

increasingly used among small water-supply operations, such 

as centralized drinking-water systems serving residential 

subdivisions (T.S. Bruce, Virginia Department of Environmental Quality, oral commun., 2009). With desalination generally 

improving in efficiency and decreasing in cost, parts of 

aquifers containing salty water once thought to be avoided are 

increasingly viewed as favorable for development.

In addition to the direct withdrawal of salty groundwater 

by desalination facilities, other groundwater uses in the 

Virginia Coastal Plain are potentially affected by salty 

groundwater. Among these, essentially the entire water supply 

on the Virginia Eastern Shore is obtained from groundwater, 

because fresh surface water generally is unavailable (Sanford 

and others, 2009a). Additionally, in the city of Virginia Beach, 

numerous withdrawals are made from residential privately 

owned wells, many for seasonal uses such as watering lawns 

and gardens (Smith, 2005), because use of public water 

is restricted to consumptive purposes. In both areas, fresh 

groundwater is available only from the shallow and laterally 

constrained surficial and Yorktown-Eastover aquifers, beneath 

which salty groundwater occupies deeper hydrogeologic units. 

Treatment and other resource-management options 

to address issues associated with salty groundwater in the 

Virginia Coastal Plain are varied. Municipal BGD operations 

are large and complex, and have only relatively recently been 

implemented in response to growing demands for public 

drinking water. Each BGD plant uses methods of desalination 

that include reverse osmosis and electrodialysis reversal, 

but also blends water from multiple wells having different 

chloride concentrations. Smaller distillation and reverse 

osmosis systems for privately owned wells and small community or commercial water supplies have historically been 

viewed as inefficient and costly (Parrott and others, 1999), but 

are improving and being increasingly used.

In addition to treating salty groundwater directly withdrawn, other measures can potentially minimize its movement. 

Where withdrawals are primarily of shallow freshwater, such 

as the Virginia Eastern Shore and city of Virginia Beach, 

groundwater development could be redirected by locating and 

operating production wells to maintain chloride concentrations 

below levels requiring treatment. In other areas where many 

production wells are already positioned close to and within the 

saltwater-transition zone, withdrawals of fresh groundwater 

could potentially be offset by withdrawals of salty groundwater to balance gradients across the saltwater-transition zone 

and thereby preclude saltwater intrusion. Areas of potential 

intrusion, however, are probably numerous, complexly 

distributed, and variable over time (see “Movement”). Hence, 

Table 3. Municipalities operating brackish groundwater 

development (BGD) facilities in the Virginia Coastal Plain. 1

[Mgal/d, million gallons per day]

Municipality

Permitted 

withdrawal

rate (Mgal/d) 2

Withdrawal rate

during 2005

City of Chesapeake 5 3.54

Gloucester County 8 0.66

James City County 6 1.50

City of Newport News 7 2.17

City of Suffolk 4 2.71

Total 30 10.58

1 J.P. Pope, U.S. Geological Survey, written commun., 2009.

2As of November 2007.
60 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

a program of conjunctive freshwater/saltwater groundwater 

development would require detailed hydrologic analysis of 

potential effects of proposed withdrawals, comprehensive 

coordination among the numerous and diverse water suppliers 

operating production wells, and an effective, long-term 

saltwater-intrusion monitoring network (see “Monitoring”).

Selected Constituents

In addition to the major ions, some other chemical 

constituents can influence the suitability of groundwater in the 

Virginia Coastal Plain for various uses. Among these, fluoride 

is present in groundwater at concentrations exceeding primary 

and secondary drinking-water standards (4 mg/L and 2 mg/L, 

respectively) (U.S. Environmental Protection Agency, 1990), 

along a broad belt beneath the city of Suffolk that thins and 

tapers northward (see “Fluoride”). Because fluoride is chemically similar to chloride, it can be removed from groundwater 

using distillation and reverse osmosis systems similar to 

those used to remove chloride, but with the same shortcomings. Removal by ion exchange has also been suggested, 

alternatively using tricalcium phosphate, magnesium oxide, 

or magnesium hydroxide (Sinnott and Whetstone, 1962), 

hydroxylapatite (Zack, 1980), or “bone char” (Maciuika, 

1992), a type of charcoal produced from animal bones. 

In addition to treatment for removal of fluoride, blending 

of water from different sources can lower fluoride concentrations to beneficial levels (Sinnott and Whetstone, 1962). BGD 

operations in the city of Suffolk during 2007 (table 3) included 

blending of groundwater having large fluoride concentrations 

with water from a different well, from which essentially all 

fluoride had been removed as a result of desalination by electrodialysis reversal (J.P. Pope, U.S. Geological Survey, written 

commun., 2009). Deep groundwater in the city of Suffolk that 

underlies the high fluoride belt is also salty, requiring treatment for large chloride concentrations. At shallower depths, 

however, the Saint Marys aquifer can potentially provide a 

limited source of groundwater having smaller concentrations 

of both fluoride and chloride (McFarland and Bruce, 2006). 

The Saint Marys aquifer has so far only been very lightly used 

by a small number of private domestic wells.

Concentrations of iron and manganese in groundwater 

that exceed secondary drinking-water standards (0.3 mg/L 

and 0.05 mg/L, respectively) (U.S. Environmental Protection 

Agency, 1990) are at scattered locations throughout the 

Virginia Coastal Plain, but are more common to the west 

along the Fall Zone, across the saltwater-transition zone 

and eastward within the Potomac aquifer and lower part of 

the marine sediments, and within the shallow part of the 

marine sediments far to the east (see “Iron and Manganese”). 

Large concentrations of dissolved hydrogen sulfide gas also 

commonly coincide with those of iron and manganese (Ross 

and others, 2002) and can originate from dissolution of pyrite 

and decomposition of organic matter. Hydrogen sulfide is not 

associated with negative health effects and is not regulated 

by drinking-water standards (U.S. Environmental Protection 

Agency, 1990), but can produce a strong objectionable odor 

and taste, and can induce corrosion in plumbing. Only 30 

analyses of hydrogen sulfide are included among the tabulated 

groundwater-sample data (Attachments 1, 2), which are 

inadequate to determine regional trends.

Removal of iron, manganese, and hydrogen sulfide are 

common necessary treatments of water from supply wells. 

One or more of these treatments are commonly used by major 

water-production facilities, but smaller systems are also 

designed for individual domestic wells and small community 

or commercial water supplies. Appropriate system design 

depends on the required water-production capacity and 

other considerations, including the level of treatment needed 

(Wilson and others, 1999b; Ross and others, 2002). Phosphate 

compounds chemically suspend iron and manganese at 

concentrations up to 3 mg/L, but must be added while the iron 

and manganese are still dissolved and become ineffective at 

elevated temperatures (such as boiling). Water softening used 

to treat hard water (see “Hardness”) can also remove iron 

and manganese at concentrations up to 5 mg/L, but only by 

systems designed to do so. Hydrogen sulfide at concentrations 

less than 1 mg/L can be removed by activated carbon filters. 

Larger concentrations of iron, manganese, and hydrogen 

sulfide generally require some form of oxidation combined 

with filtering, either by chemical solutions, greensand, or 

aeration. These methods require varying degrees of pre- and 

post-treatment, and potentially intensive maintenance. 

Concentrations of nitrate in groundwater that exceed 

the drinking-water standard of 10 mg/L (U.S. Environmental 

Protection Agency, 1990) are at widely scattered locations 

throughout the Virginia Coastal Plain, but mostly are limited 

to shallow depths (see “Nitrate and Ammonium”). Relatively 

large concentrations of ammonium are also present at somewhat greater depths and can transform to produce additional 

nitrate. Both constituents can be produced in some places 

by decomposition of sediment organic matter, but elsewhere 

possibly originate from site-specific human sources. 

Removal of nitrate is a potential necessary treatment 

of water primarily from relatively shallow wells, which 

typically provide individual domestic and small community 

or commercial water supplies. Treatment options include 

distillation and reverse osmosis systems (Parrott and others, 

2002) similar to those used to remove chloride, but with the 

same shortcomings. Alternatively, ion exchange can be used 

in a manner analogous to water softening, but configured to 

exchange nitrate anions for chloride anions. Such systems 

can require post-treatment to reduce corrosiveness, however, 

and their efficiency can be reduced by the presence of other 

anions, such as sulfate.

In addition to treatment for removal of nitrate, use of a 

different source can potentially provide groundwater having 

acceptably small concentrations. Wells producing water with 

relatively large nitrate or ammonium concentrations can often 

be in proximity to other wells exhibiting substantially smaller 

concentrations. In some instances where a specific source 

of nitrate and(or) ammonium is likely, a new well located 
Summary and Conclusions 61

and constructed to avoid the source is a possible long-term 

solution. Blending of water from a different source can also 

produce acceptable nitrate and(or) ammonium concentrations. 

Likewise, well siting and construction in areas undergoing 

new development can be undertaken to avoid previously 

existing and likely future sources.

Summary and Conclusions

Groundwater in the Virginia Coastal Plain is heavily 

used, and information on its chemical quality is critical to 

sound resource management. An investigation was undertaken 

during 2006–08 by the U.S. Geological Survey, in cooperation 

with the Virginia Department of Environmental Quality 

and the Hampton Roads Planning District Commission, to 

incorporate updated knowledge of groundwater quality into a 

newly developed regional perspective of the hydrogeology of 

the Virginia Coastal Plain. Results of the study include 

the following:

1. Data retrieved from multiple Federal and State 

agency databases represent 10,165 groundwater 

samples collected from 1,991 wells and boreholes 

during 1906 to 2007. Analytical results for 37 chemical constituents include 73,610 individual sample 

constituent values.

2. Chemical-constituent values and related data enable 

well drillers and owners, environmental consultants, 

and water-resource managers to address needs for 

local-scale information on groundwater quality. Use 

of the groundwater-sample data is supported by 

tables, summaries, location maps, and discussions of 

data quality and limitations. 

3. A series of interpretive analyses of the sample data 

enable planners and policy makers to reach informed 

water-resource decisions, based on a conceptual 

understanding of spatial trends in groundwater quality and related processes at the regional scale. Maps, 

sections, and discussion describe concentrations 

of major ions, the configuration of the saltwatertransition zone, and selected other constituents of 

particular interest to resource management.

4. Based on concentrations of major ions that dominate 

the chemical composition of groundwater, most 

groundwater samples collected from water-supply 

aquifers are categorized as either

a. “hard” water, dominated by calcium and 

magnesium cations and bicarbonate and 

carbonate anions, 

b. “soft” water, dominated by sodium and 

potassium cations and bicarbonate and carbonate 

anions, or 

c. “salty” water, dominated by sodium and 

potassium cations and chloride anions.

The chemical composition of groundwater in the 

deep Piney Point, Aquia, and Potomac aquifers 

evolves eastward and with depth, from hard to soft to 

salty. Chemical weathering of subsurface sediments 

is followed by ion exchange by clay and glauconite, 

and subsequently by mixing with seawater along the 

saltwater-transition zone. The chemical composition of groundwater in the shallower surficial and 

Yorktown-Eastover aquifers, and in basement 

bedrock along the Fall Zone, is more variable as a 

result of short flow paths between closely located 

recharge and discharge areas and possibly some 

solutes originating from human sources.

5. Groundwater chloride concentrations generally 

increase eastward and with depth, but are also 

displaced upward across the Chesapeake Bay impact 

crater and inverted across distinct downwarddecreasing vertical intervals. Although generally 

landward-dipping, the saltwater-transition zone is 

also convoluted where warped and mounded around 

the impact crater along the inverted zones. Fresh 

groundwater has flushed seawater from subsurface 

sediments preferentially around the impact crater 

as a result of broad contrasts between sediment 

permeabilities. Paths of differential flushing are 

also focused along the inverted zones, which follow 

stratigraphic and structural trends southeastward 

into North Carolina and northeastward beneath the 

chloride mound across the outer impact crater. Brine 

within the inner impact crater has probably remained 

unflushed, but its origin has also not yet been fully 

explained. Regional movement of the saltwatertransition zone takes place over geologic time 

scales. Localized movement has been induced by 

groundwater withdrawal, mostly along shallow parts 

of the saltwater-transition zone. Short-term episodic 

withdrawals result in repeated cycles of upconing 

and downconing, which are superimposed upon 

longer-term lateral intrusion. Effective monitoring 

for saltwater intrusion needs to address multiple and 

complexly distributed areas of potential intrusion 

that vary over time. 

6. Groundwater fluoride concentrations exceed the 

drinking-water standard along a broad belt beneath 

the city of Suffolk that thins and tapers northward. 

Widespread small concentrations of fluoride 

probably originate by desorbtion from phosphatic 

sedimentary material. The high fluoride belt was 

possibly formed by initial adsorbtion of fluoride onto 

sediment oxyhydroxides, followed by desorbtion 

along the leading edge of the advancing saltwatertransition zone.
62 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

7. Groundwater iron and manganese concentrations 

exceed drinking-water standards at scattered locations, but are more common to the west along the 

Fall Zone, across part of the saltwater-transition zone 

and eastward, and within shallow groundwater far to 

the east. Iron and manganese initially produced by 

mineral dissolution along the Fall Zone are adsorbed 

eastward and with depth by clay and glauconite, 

and subsequently desorbed along the leading edge 

of the advancing saltwater-transition zone. Iron and 

manganese in shallow groundwater far to the east 

are produced by reaction of sediment organic matter 

with oxyhydroxides. 

8. Groundwater nitrate concentrations exceed the 

drinking-water standard at widely scattered locations, but are mostly limited to shallow depths. 

Ammonium is also present in large concentrations at 

some locations and can transform to produce additional nitrate. Although most nitrate and ammonium 

is naturally recycled near the land surface, some can 

be produced by decomposition of sediment organic 

matter, ion exchange along the leading edge of the 

advancing saltwater-transition zone, and possibly 

from site-specific human sources.

9. The chemical quality of groundwater determines its 

suitability for diverse uses. Soft water in deep aquifers is generally suitable for a wide variety of uses. 

Hard water at other locations can require treatment, 

most commonly by ion exchange. Salty water in the 

eastern parts of deep aquifers is undergoing increasingly widespread production for drinking-water 

supplies. Treatment operations use desalination and 

blending. Saltwater intrusion can potentially be 

mitigated by redirecting locations and operation of 

wells producing shallow freshwater and by conjunctive freshwater/saltwater groundwater development 

close to and within the saltwater-transition zone. In 

areas of large fluoride concentrations, blending of 

water from different sources is used as an alternative 

to removal. Large iron and manganese concentrations and associated hydrogen sulfide concentrations 

can potentially be mitigated by one or more method 

from an array of treatment methods, depending on 

the level of treatment needed. Large nitrate concentrations can potentially be mitigated by various 

treatment methods or withdrawal management.

Acknowledgments

This study was supported by the Virginia Department 

of Environmental Quality (DEQ) and the Hampton Roads 

Planning District Commission (HRPDC). Special thanks for 

program planning go to Scott Kudlas, Terry Wagner, Scott 

Bruce, and Mary Ann Massie of DEQ, and to John Carlock 

and Whitney Katchmark of HRPDC. 

Groundwater-quality data compilation was greatly 

assisted by database retrieval, organization, and transfer 

provided by Brad White, Todd Beach, and Beverly Quinlan at 

DEQ. Pore-water chemistry data from the inner Chesapeake 

Bay impact crater was provided by Ward Sanford of the 

U.S. Geological Survey (USGS) National Research Program.

Thanks also are extended to the many drillers and owners 

of water-supply wells, who have provided well data from 

across the Virginia Coastal Plain.

The quality of this report was aided greatly by Otto S. 

Zapecza of the USGS New Jersey Water Science Center, 

John S. Clarke of the USGS Georgia Water Science Center, 

and Rebecca J. Deckard, Kay E. Hedrick, and Jeffrey L. 

Corbett of the USGS Enterprise Publishing Network.

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64 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

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Attachment 2 67

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

All hydrogeologic units

General parameters

pH (standard units) 5,653 0.1 13.55 7.2 7.5

temperature (degrees Celsius) 2,592 2.0 210 16.2 15.6

dissolved oxygen (mg/L) 1,822 0 16.1 2.0 .6

specific conductance (μS/cm) 5,016 8.0 178,000 1,380 399

total dissolved solids (mg/L) 183 210 487,000 39,100 5,400

filtered residue (mg/L) 3,073 12.0 470,000 2,130 351

Major cations (all mg/L)

calcium 3,954 0 160,000 143 15.0

magnesium 3,936 .0006 1,110 21.9 3.9

sodium 4,176 .02 203,000 844 97.0

potassium 3,878 .044 581.6 15.4 7.7

Major anions (all mg/L)

bicarbonate 7,450 0 377,000 604 97.6

carbonate 164 0 45,000 1,190 11.0

sulfate 3,905 0 32,100 117 13.0

chloride 6,650 0 55,560 587 37.0

Nutrients (all mg/L)

nitrate nitrogen 2,711 0 30.25 0.61 0.05

ammonium nitrogen 2,293 0 685 9.3 .2

orthophosphate 2,323 0 1,390 2.8 .1

total dissolved phosphorous 1,865 .001 88 .32 .10

Secondary constituents

silica (mg/L) 1,066 0.4 38,400 302 19.0

aluminum (μg/L) 146 0 10,000 193 30.0

iron (μg/L) 3,554 0 1,730,000 2,020 200

manganese (μg/L) 2,581 0 22,000 91.4 20.0

arsenic (μg/L) 177 0 24 1.8 1.0

bromide (mg/L) 398 0 242 23.2 1.87

fluoride (mg/L) 3,845 0 30.0069 1.5 .62

iodide (mg/L) 61 0 9.27 1.01 .66

hydrogen sulfide (mg/L) 30 0 .51 .04 0

Radiochemical constituents

gross α: uranium (μg/L) 5 0.4 1.4 0.98 1.0

gross α: radium + uranium – radon (pCi/L) 7 .3 35.2 8.6 1.0

gross α: radium – uranium – radon (pCi/L) 2 .3 .5 .4 .4

gross β (pCi/L) 9 2.5 12.4 5.2 3.7

gross β: cesium (pCi/L) 7 .6 12 3.8 2.6

gross β: strontium (pCi/L) 7 .5 9.4 3.0 2.3

radon 222 (pCi/L) 57 50 2,970 390 280

radium 226 (pCi/L) 3 0 .5 .2 .2

radium 228 (pCi/L) 9 0 1.4 .46 .40

uranium (μg/L) 3 .06 .30 .15 .09
68 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Surficial aquifer

General parameters

pH (standard units) 1,342 3.5 12.4 5.9 5.9

temperature (degrees Celsius) 1,098 3 27.6 15.2 15.0

dissolved oxygen (mg/L) 934 0 16.1 3.3 2.5

specific conductance (μS/cm) 1,774 15 28,600 898 310

total dissolved solids (mg/L) 0 — — — —

filtered residue (mg/L) 503 12 22,800 586 252

Major cations (all mg/L)

calcium 469 0.18 300 32.7 25

magnesium 466 .03 850 15.2 5.6

sodium 462 .3 6,400 86.8 13

potassium 458 .1 200 5.4 2

Major anions (all mg/L)

bicarbonate 807 0 3,952.8 164 19.5

carbonate 1 12 12 12 12

sulfate 500 .1 2,000 52.2 26.3

chloride 686 1 13,000 173 27

Nutrients (all mg/L)

nitrate nitrogen 482 0.01 30.25 2.6 0.2

ammonium nitrogen 639 .002 685 23.9 .1

orthophosphate 456 0 11 .2 .03

total dissolved phosphorous 385 .001 3 .1 .1

Secondary constituents

silica (mg/L) 195 3.3 76 17 14

aluminum (μg/L) 89 1 1,700 139 30

iron (μg/L) 435 3 140,000 3,580 500

manganese (μg/L) 416 2 22,000 215 50

arsenic (μg/L) 32 0 10 1.6 1.0

bromide (mg/L) 47 0 11 .3 .04

fluoride (mg/L) 346 0 3.1 .2 .1

iodide (mg/L) 1 1.1 1.1 1.1 1.1

hydrogen sulfide (mg/L) 0 — — — —

Radiochemical constituents

gross α: uranium (μg/L) 5 0.4 1.4 1 1

gross α: radium + uranium – radon (pCi/L) 0 — — — —

gross α: radium – uranium – radon (pCi/L) 0 — — — —

gross β (pCi/L) 0 — — — —

gross β: cesium (pCi/L) 7 .6 12 3.8 2.6

gross β: strontium (pCi/L) 7 .5 9.4 3.0 2.3

radon 222 (pCi/L) 10 50 2,970 488 220

radium 226 (pCi/L) 0 — — — —

radium 228 (pCi/L) 0 — — — —

uranium (μg/L) 2 .06 .09 .07 .07
Attachment 2 69

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Yorktown confining unit

General parameters

pH (standard units) 198 4 9.5 7.1 7.0

temperature (degrees Celsius) 43 12.2 20.2 16.7 16.4

dissolved oxygen (mg/L) 30 0 7.2 1.3 .2

specific conductance (μS/cm) 170 17 7,810 789 390

total dissolved solids (mg/L) 0 — — — —

filtered residue (mg/L) 164 78 5,468 413 267

Major cations (all mg/L)

calcium 144 1.8 670 48.8 37.0

magnesium 143 .1 130 9.6 6.8

sodium 150 1.0 540 39.6 20.5

potassium 144 .4 44 4.5 2.9

Major anions (all mg/L)

bicarbonate 280 0 17,043.4 232 95.2

carbonate 0 — — — —

sulfate 149 .06 235.8 24.6 9.0

chloride 251 2.0 2,890 87.6 37.0

Nutrients (all mg/L)

nitrate nitrogen 140 0.01 13 0.41 0.05

ammonium nitrogen 141 .01 370 23.0 .30

orthophosphate 110 .006 .52 .07 .06

total dissolved phosphorous 113 .002 25.2 .36 .10

Secondary constituents

silica (mg/L) 22 10.8 56.4 34.9 35.9

aluminum (μg/L) 1 20 20 20 20

iron (μg/L) 137 10 1,730,000 17,200 800

manganese (μg/L) 114 4.6 14,600 312 80

arsenic (μg/L) 18 0 24 2.4 1.0

bromide (mg/L) 17 .01 2.6 .31 .09

fluoride (mg/L) 124 .02 1.7 .17 .10

iodide (mg/L) 1 .163 .163 .163 .163

hydrogen sulfide (mg/L) 0 — — — —

Radiochemical constituents

gross α: uranium (μg/L) 0 — — — —

gross α: radium + uranium – radon (pCi/L) 0 — — — —

gross α: radium – uranium – radon (pCi/L) 0 — — — —

gross β (pCi/L) 0 — — — —

gross β: cesium (pCi/L) 0 — — — —

gross β: strontium (pCi/L) 0 — — — —

radon 222 (pCi/L) 2 190 740 465 465

radium 226 (pCi/L) 0 — — — —

radium 228 (pCi/L) 0 — — — —

uranium (μg/L) 0 — — — —
70 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Yorktown-Eastover aquifer

General parameters

pH (standard units) 1,149 0.1 9.7 7.7 7.6

temperature (degrees Celsius) 275 5.0 26.0 16.3 16.0

dissolved oxygen (mg/L) 84 0 6.9 .5 .2

specific conductance (μS/cm) 877 8.0 67,895 1,110 489

total dissolved solids (mg/L) 2 339 314,000 157,000 157,000

filtered residue (mg/L) 729 43 301,000 985 289

Major cations (all mg/L)

calcium 1,107 0.2 160,000 188 36.0

magnesium 1,093 .01 890 17.5 9.8

sodium 1,164 .02 45,700 146 38.0

potassium 1,090 .2 86 12.7 8.4

Major anions (all mg/L)

bicarbonate 2,252 0 212,000 207 34.2

carbonate 9 1 8 2.4 1

sulfate 1,013 0 5,760 24.6 5.6

chloride 1,832 0 55,560 236 34.5

Nutrients (all mg/L)

nitrate nitrogen 721 0 8 0.2 0.05

ammonium nitrogen 611 .01 170 1.5 .5

orthophosphate 626 0 40 .3 .1

total dissolved phosphorous 559 .1 88 .4 .1

Secondary constituents

silica (mg/L) 159 3 19,300 146 20.4

aluminum (μg/L) 5 0 100 28.0 10.0

iron (μg/L) 1,036 0 48,000 1,580 405

manganese (μg/L) 712 0 1,200 66.9 30.0

arsenic (μg/L) 26 0 12 2.9 1.0

bromide (mg/L) 43 0 11.3 1.5 .3

fluoride (mg/L) 926 0 10 .3 .2

iodide (mg/L) 2 .009 1.5 .7 .7

hydrogen sulfide (mg/L) 1 0 0 0 0

Radiochemical constituents

gross α: uranium (μg/L) 0 — — — —

gross α: radium + uranium – radon (pCi/L) 0 — — — —

gross α: radium – uranium – radon (pCi/L) 1 0.3 0.3 0.3 0.3

gross β (pCi/L) 1 5.2 5.2 5.2 5.2

gross β: cesium (pCi/L) 0 — — — —

gross β: strontium (pCi/L) 0 — — — —

radon 222 (pCi/L) 7 280 710 370 310

radium 226 (pCi/L) 0 — — — —

radium 228 (pCi/L) 1 .7 .7 .7 .7

uranium (μg/L) 0 — — — —
Attachment 2 71

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Saint Marys confining unit

General parameters

pH (standard units) 109 4.5 9.9 7.8 7.8

temperature (degrees Celsius) 21 13.33 21.0 16.5 16.2

dissolved oxygen (mg/L) 8 .1 7.1 2.0 .4

specific conductance (μS/cm) 73 70 18,400 3,150 570

total dissolved solids (mg/L) 17 584 29,874 9,080 5,930

filtered residue (mg/L) 55 136 7,320 810 245

Major cations (all mg/L)

calcium 96 1.0 778.5 88.0 30.2

magnesium 99 .1 897.2 86.2 23.1

sodium 107 5.0 8,770 812 279

potassium 98 1.9 458.9 48.1 21.6

Major anions (all mg/L)

bicarbonate 206 0 960 155 126

carbonate 2 2.0 2.0 2.0 2.0

sulfate 103 .05 6,010 313 13.0

chloride 166 1.0 14,900 1,130 290

Nutrients (all mg/L)

nitrate nitrogen 52 0 3.4 0.2 0.05

ammonium nitrogen 47 .1 10.0 2.1 .7

orthophosphate 46 .01 .9 .08 .04

total dissolved phosphorous 42 .1 .4 .1 .1

Secondary constituents

silica (mg/L) 12 11.0 37.5 20.8 19.9

aluminum (μg/L) 1 20 20 20 20

iron (μg/L) 104 10 23,200 1,130 188

manganese (μg/L) 59 .004 380 40.9 10.0

arsenic (μg/L) 5 1.0 3.0 2.0 2.0

bromide (mg/L) 20 4.0 57.60 19.0 14.5

fluoride (mg/L) 90 .06 30.0 1.9 .3

iodide (mg/L) 4 .17 .47 .31 .30

hydrogen sulfide (mg/L) 1 0 0 0 0

Radiochemical constituents

gross α: uranium (μg/L) 0 — — — —

gross α: radium + uranium – radon (pCi/L) 0 — — — —

gross α: radium – uranium – radon (pCi/L) 0 — — — —

gross β (pCi/L) 0 — — — —

gross β: cesium (pCi/L) 0 — — — —

gross β: strontium (pCi/L) 0 — — — —

radon 222 (pCi/L) 1 430 430 430 430

radium 226 (pCi/L) 0 — — — —

radium 228 (pCi/L) 0 — — — —

uranium (μg/L) 0 — — — —
72 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Saint Marys aquifer

General parameters

pH (standard units) 14 7.1 9.5 8.2 8.2

temperature (degrees Celsius) 6 17.0 18.9 18.1 18.1

dissolved oxygen (mg/L) 2 .3 1.9 1.1 1.1

specific conductance (μS/cm) 11 122.6 13,100 4,440 2,900

total dissolved solids (mg/L) 0 — — — —

filtered residue (mg/L) 9 293 7,430 3,820 1,780

Major cations (all mg/L)

calcium 11 5.9 90.0 26.0 15.0

magnesium 11 4.7 70.6 23.8 8.6

sodium 11 77.0 2,700 887 423

potassium 11 15.0 65.5 30.4 24.6

Major anions (all mg/L)

bicarbonate 14 0 1,106 309 182

carbonate 0 — — — —

sulfate 12 .9 87.0 30.6 19.6

chloride 13 1.91 5,570 1,740 550

Nutrients (all mg/L)

nitrate nitrogen 7 0.04 0.5 0.2 0.05

ammonium nitrogen 9 .1 8.0 2.8 2.1

orthophosphate 5 .02 .41 .2 .06

total dissolved phosphorous 7 .1 1.0 .4 .1

Secondary constituents

silica (mg/L) 5 11.0 41.2 25.1 21.0

aluminum (μg/L) 1 20.0 20.0 20.0 20.0

iron (μg/L) 11 11.0 8,000 3,000 1,100

manganese (μg/L) 9 1.6 100 36.2 30.0

arsenic (μg/L) 0 — — — —

bromide (mg/L) 3 .02 13.7 4.6 .03

fluoride (mg/L) 11 .32 2.37 1.5 2.0

iodide (mg/L) 0 — — — —

hydrogen sulfide (mg/L) 0 — — — —

Radiochemical constituents

gross α: uranium (μg/L) 0 — — — —

gross α: radium + uranium – radon (pCi/L) 0 — — — —

gross α: radium – uranium – radon (pCi/L) 0 — — — —

gross β (pCi/L) 0 — — — —

gross β: cesium (pCi/L) 0 — — — —

gross β: strontium (pCi/L) 0 — — — —

radon 222 (pCi/L) 0 — — — —

radium 226 (pCi/L) 0 — — — —

radium 228 (pCi/L) 0 — — — —

uranium (μg/L) 0 — — — —
Attachment 2 73

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Calvert confining unit

General parameters

pH (standard units) 14 5.7 8.2 6.9 6.9

temperature (degrees Celsius) 3 13.3 13.8 13.6 13.6

dissolved oxygen (mg/L) 2 .6 8.2 4.4 4.4

specific conductance (μS/cm) 7 141 20,700 3,640 290

total dissolved solids (mg/L) 6 34,072 39,134 35,600 35,000

filtered residue (mg/L) 4 30 271 152 154

Major cations (all mg/L)

calcium 13 2 2,330 874 1,210

magnesium 13 .6 953.4 468 798

sodium 12 4.3 11,600 6,110 9,990

potassium 12 .5 581.6 251 340

Major anions (all mg/L)

bicarbonate 18 0 185.44 112 146

carbonate 1 6.0 6.0 6.0 6.0

sulfate 13 1.0 4,820 2,000 1,880

chloride 17 1.0 20,600 8,120 304

Nutrients (all mg/L)

nitrate nitrogen 1 0.16 0.16 0.16 0.16

ammonium nitrogen 3 .1 .1 .1 .1

orthophosphate 2 .05 1.4 .72 .72

total dissolved phosphorous 3 .1 1.5 .6 .2

Secondary constituents

silica (mg/L) 8 15.2 32.0 20.9 20.5

aluminum (μg/L) 1 20.0 20.0 20.0 20.0

iron (μg/L) 6 40.0 30,000 5,230 125

manganese (μg/L) 5 40.0 400 149 47.0

arsenic (μg/L) 1 1.0 1.0 1.0 1.0

bromide (mg/L) 8 18.5 80.8 65.3 70.1

fluoride (mg/L) 13 .06 24.6 9.4 .87

iodide (mg/L) 1 1.0 1.0 1.0 1.0

hydrogen sulfide (mg/L) 0 — — — —

Radiochemical constituents

gross α: uranium (μg/L) 0 — — — —

gross α: radium + uranium – radon (pCi/L) 0 — — — —

gross α: radium – uranium – radon (pCi/L) 0 — — — —

gross β (pCi/L) 0 — — — —

gross β: cesium (pCi/L) 0 — — — —

gross β: strontium (pCi/L) 0 — — — —

radon 222 (pCi/L) 0 — — — —

radium 226 (pCi/L) 0 — — — —

radium 228 (pCi/L) 0 — — — —

uranium (μg/L) 0 — — — —
74 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Piney Point aquifer

General parameters

pH (standard units) 179 4.7 9.4 7.8 8.0

temperature (degrees Celsius) 66 5.0 24.8 16.9 17.0

dissolved oxygen (mg/L) 20 0 6.5 .6 .2

specific conductance (μS/cm) 96 45.0 20,400 1,290 331

total dissolved solids (mg/L) 0 — — — —

filtered residue (mg/L) 107 30.0 9,010 643 226

Major cations (all mg/L)

calcium 143 0.8 100 22.4 15.0

magnesium 143 .32 110 5.2 2.7

sodium 138 2.4 3,100 191 41.5

potassium 135 .73 83.0 10.8 7.9

Major anions (all mg/L)

bicarbonate 295 0 1,450 137 135

carbonate 30 1.0 166 26.6 13.0

sulfate 151 .8 470 21.0 6.9

chloride 351 1.0 7,120 118 4.2

Nutrients (all mg/L)

nitrate nitrogen 88 0 3.7 0.2 0.05

ammonium nitrogen 21 .10 4.3 .68 .11

orthophosphate 49 0 1.9 .07 .03

total dissolved phosphorous 17 .10 2.0 .24 .10

Secondary constituents

silica (mg/L) 64 3.4 59.0 32.8 33.5

aluminum (μg/L) 3 20.0 600 270 200

iron (μg/L) 108 9.0 10,400 417 100

manganese (μg/L) 62 1.7 240 32.3 10.0

arsenic (μg/L) 8 1.0 5.0 2.0 1.0

bromide (mg/L) 15 .01 28.5 5.0 .03

fluoride (mg/L) 220 .07 4.11 .76 .56

iodide (mg/L) 3 .725 1.02 .86 .84

hydrogen sulfide (mg/L) 0 — — — —

Radiochemical constituents

gross α: uranium (μg/L) 0 — — — —

gross α: radium + uranium – radon (pCi/L) 0 — — — —

gross α: radium – uranium – radon (pCi/L) 0 — — — —

gross β (pCi/L) 0 — — — —

gross β: cesium (pCi/L) 0 — — — —

gross β: strontium (pCi/L) 0 — — — —

radon 222 (pCi/L) 7 150 1,400 580 320

radium 226 (pCi/L) 0 — — — —

radium 228 (pCi/L) 0 — — — —

uranium (μg/L) 0 — — — —
Attachment 2 75

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Chickahominy confining unit

General parameters

pH (standard units) 14 5.6 9.2 7.8 8.0

temperature (degrees Celsius) 7 16.0 21.0 17.9 18.1

dissolved oxygen (mg/L) 2 .1 .2 .1 .1

specific conductance (μS/cm) 13 480 38,500 8,130 2,160

total dissolved solids (mg/L) 4 38,688 44,742 42,600 43,500

filtered residue (mg/L) 7 290 1,540 1,040 1.010

Major cations (all mg/L)

calcium 13 3.0 2,200 634 5.0

magnesium 13 .12 1,060 316 2.6

sodium 13 80.0 12,500 4,080 530

potassium 13 8.8 353.4 84.8 14.0

Major anions (all mg/L)

bicarbonate 12 0 888.16 373 326

carbonate 1 12 12 12 12

sulfate 12 .6 4,990 1,390 48.9

chloride 19 5.0 24,700 6,300 520

Nutrients (all mg/L)

nitrate nitrogen 5 0.01 0.24 0.06 0.02

ammonium nitrogen 2 .49 .60 .54 .54

orthophosphate 4 0 .37 .15 .11

total dissolved phosphorous 2 .1 .7 .4 .4

Secondary constituents

silica (mg/L) 7 12.0 37.7 26.9 28.6

aluminum (μg/L) 0 — — — —

iron (μg/L) 8 40.0 1,000 323 202

manganese (μg/L) 6 5.0 240 57.0 15.0

arsenic (μg/L) 0 — — — —

bromide (mg/L) 9 .32 129 52.6 45.5

fluoride (mg/L) 10 .50 13.0 3.3 2.4

iodide (mg/L) 3 .13 1.66 .88 .84

hydrogen sulfide (mg/L) 1 .51 .51 .51 .51

Radiochemical constituents

gross α: uranium (μg/L) 0 — — — —

gross α: radium + uranium – radon (pCi/L) 0 — — — —

gross α: radium – uranium – radon (pCi/L) 0 — — — —

gross β (pCi/L) 0 — — — —

gross β: cesium (pCi/L) 0 — — — —

gross β: strontium (pCi/L) 0 — — — —

radon 222 (pCi/L) 0 — — — —

radium 226 (pCi/L) 0 — — — —

radium 228 (pCi/L) 0 — — — —

uranium (μg/L) 0 — — — —
76 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Exmore matrix confining unit

General parameters

pH (standard units) 17 4.0 8.66 7.1 7.3

temperature (degrees Celsius) 1 20.0 20.0 20.0 20.0

dissolved oxygen (mg/L) 0 — — — —

specific conductance (μS/cm) 5 5,840 42,200 21,400 19,200

total dissolved solids (mg/L) 11 33,453 50,110 43,700 45,500

filtered residue (mg/L) 1 3,400 3,400 3,400 3,400

Major cations (all mg/L)

calcium 16 26.8 2,480 2,000 2,140

magnesium 16 11.5 1,100 722 802

sodium 16 1,250 15,300 12,700 13,800

potassium 16 29.8 480 289 307

Major anions (all mg/L)

bicarbonate 17 24.0 1,260 150 82.0

carbonate 1 3.0 3.0 3.0 3.0

sulfate 16 175 5,230 2,580 2,540

chloride 19 1,230 28,400 21,600 24,900

Nutrients (all mg/L)

nitrate nitrogen 0 — — — —

ammonium nitrogen 0 — — — —

orthophosphate 0 — — — —

total dissolved phosphorous 0 — — — —

Secondary constituents

silica (mg/L) 1 47.8 47.8 47.8 47.8

aluminum (μg/L) 0 — — — —

iron (μg/L) 1 7,140 7,140 7,140 7,140

manganese (μg/L) 1 89.4 89.4 89.4 89.4

arsenic (μg/L) 0 — — — —

bromide (mg/L) 20 4.91 242 86.0 97.0

fluoride (mg/L) 1 .3 .3 .3 .3

iodide (mg/L) 4 .634 2.51 1.13 .688

hydrogen sulfide (mg/L) 0 — — — —

Radiochemical constituents

gross α: uranium (μg/L) 0 — — — —

gross α: radium + uranium – radon (pCi/L) 0 — — — —

gross α: radium – uranium – radon (pCi/L) 0 — — — —

gross β (pCi/L) 0 — — — —

gross β: cesium (pCi/L) 0 — — — —

gross β: strontium (pCi/L) 0 — — — —

radon 222 (pCi/L) 0 — — — —

radium 226 (pCi/L) 0 — — — —

radium 228 (pCi/L) 0 — — — —

uranium (μg/L) 0 — — — —
Attachment 2 77

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Exmore clast confining unit

General parameters

pH (standard units) 33 5.4 8.3 7.0 7.1

temperature (degrees Celsius) 6 16.0 24.0 19.6 19.8

dissolved oxygen (mg/L) 3 .1 .2 .2 .2

specific conductance (μS/cm) 17 2,380 61,200 25,800 17,100

total dissolved solids (mg/L) 20 28,387 63,581 51,700 54,900

filtered residue (mg/L) 4 1,330 40,000 12,700 4,810

Major cations (all mg/L)

calcium 28 5.4 6,890 3,580 3,840

magnesium 28 .70 1,020 445 410

sodium 28 529 17,500 12,000 13,200

potassium 28 12.0 280 140 125

Major anions (all mg/L)

bicarbonate 79 0 2,190 75.6 0

carbonate 1 4.0 4.0 4.0 4.0

sulfate 24 63.0 2,660 1,260 1,390

chloride 87 241 37,900 9,300 381

Nutrients (all mg/L)

nitrate nitrogen 1 0 0 0 0

ammonium nitrogen 2 2.7 31.0 16.8 16.8

orthophosphate 1 .49 .49 .49 .49

total dissolved phosphorous 0 — — — —

Secondary constituents

silica (mg/L) 7 16.0 34.2 24.8 24.0

aluminum (μg/L) 0 — — — —

iron (μg/L) 6 3,380 89,100 43,700 49,700

manganese (μg/L) 6 30.8 2,160 831 818

arsenic (μg/L) 0 — — — —

bromide (mg/L) 32 4.96 152 93.3 102

fluoride (mg/L) 5 .2 2.3 1.1 .7

iodide (mg/L) 9 .023 9.27 1.72 .70

hydrogen sulfide (mg/L) 1 .05 .05 .05 .05

Radiochemical constituents

gross α: uranium (μg/L) 0 — — — —

gross α: radium + uranium – radon (pCi/L) 0 — — — —

gross α: radium – uranium – radon (pCi/L) 0 — — — —

gross β (pCi/L) 0 — — — —

gross β: cesium (pCi/L) 0 — — — —

gross β: strontium (pCi/L) 0 — — — —

radon 222 (pCi/L) 0 — — — —

radium 226 (pCi/L) 0 — — — —

radium 228 (pCi/L) 0 — — — —

uranium (μg/L) 0 — — — —
78 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Nanjemoy-Marlboro confining unit

General parameters

pH (standard units) 85 3.8 9.72 6.9 7.5

temperature (degrees Celsius) 53 5.0 25.5 15.9 16.0

dissolved oxygen (mg/L) 41 .1 8.9 4.3 6.0

specific conductance (μS/cm) 68 36.0 3,300 513 270

total dissolved solids (mg/L) 4 380 252,000 63,300 394

filtered residue (mg/L) 43 139 219,000 5,530 276

Major cations (all mg/L)

calcium 65 0.54 24,000 386 14.2

magnesium 64 .1 59.0 5.7 3.6

sodium 65 12.0 76,300 1,280 61.0

potassium 63 3.9 32.0 11.2 9.8

Major anions (all mg/L)

bicarbonate 128 0 175,000 1,480 0

carbonate 8 4.0 14,000 1,770 10.5

sulfate 66 0 510 20.5 7.5

chloride 144 1.0 7,630 84.3 4.8

Nutrients (all mg/L)

nitrate nitrogen 33 0 7.36 1.1 0.09

ammonium nitrogen 26 .002 200 8.1 .1

orthophosphate 29 0 60 2.2 .04

total dissolved phosphorous 17 .007 .2 .07 .1

Secondary constituents

silica (mg/L) 31 11.0 31,000 1,040 42.0

aluminum (μg/L) 11 0 530 68.2 10.0

iron (μg/L) 46 0 14,000 760 40.5

manganese (μg/L) 27 0 240 51.6 10.0

arsenic (μg/L) 6 0 3.0 1.3 1.0

bromide (mg/L) 7 .02 1.68 .46 .04

fluoride (mg/L) 81 .1 2.70 .77 .57

iodide (mg/L) 0 — — — —

hydrogen sulfide (mg/L) 0 — — — —

Radiochemical constituents

gross α: uranium (μg/L) 0 — — — —

gross α: radium + uranium – radon (pCi/L) 0 — — — —

gross α: radium – uranium – radon (pCi/L) 1 0.5 0.5 0.5 0.5

gross β (pCi/L) 1 8.6 8.6 8.6 8.6

gross β: cesium (pCi/L) 0 — — — —

gross β: strontium (pCi/L) 0 — — — —

radon 222 (pCi/L) 3 160 670 370 280

radium 226 (pCi/L) 0 — — — —

radium 228 (pCi/L) 1 .6 .6 .6 .6

uranium (μg/L) 0 — — — —
Attachment 2 79

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Aquia aquifer

General parameters

pH (standard units) 44 7.0 12.5 8.3 8.1

temperature (degrees Celsius) 33 8.3 23.8 17.1 17.7

dissolved oxygen (mg/L) 21 0 5.9 1.1 .3

specific conductance (μS/cm) 46 285 9,700 1,550 600

total dissolved solids (mg/L) 2 208,000 423,000 315,000 315,000

filtered residue (mg/L) 35 199 399,000 17,800 428

Major cations (all mg/L)

calcium 43 0 12,000 287 3.8

magnesium 42 .07 34.0 4.9 1.5

sodium 43 11.0 197,000 10,500 150

potassium 40 2.0 230 21.2 10.6

Major anions (all mg/L)

bicarbonate 43 0 336,000 11,400 381

carbonate 13 4.0 45,000 4,290 14.0

sulfate 43 2.0 13,900 487 11.6

chloride 46 0 2,700 248 7.3

Nutrients (all mg/L)

nitrate nitrogen 21 0 0.25 0.07 0.05

ammonium nitrogen 18 0 60.0 3.7 .09

orthophosphate 27 0 180 10.3 .12

total dissolved phosphorous 12 .012 .30 .07 .04

Secondary constituents

silica (mg/L) 33 0.8 31,400 1,420 16.9

aluminum (μg/L) 6 0 600 147 50.0

iron (μg/L) 35 0 2,300 252 89.0

manganese (μg/L) 21 0 100 16.4 10.0

arsenic (μg/L) 4 0 5.0 1.5 .5

bromide (mg/L) 11 .03 9.7 2.3 .06

fluoride (mg/L) 44 .29 4.4 1.8 1.8

iodide (mg/L) 0 — — — —

hydrogen sulfide (mg/L) 4 0 .02 .005 0

Radiochemical constituents

gross α: uranium (μg/L) 0 — — — —

gross α: radium + uranium – radon (pCi/L) 2 0.3 1.0 0.65 0.65

gross α: radium – uranium – radon (pCi/L) 0 — — — —

gross β (pCi/L) 2 2.6 5.8 4.2 4.2

gross β: cesium (pCi/L) 0 — — — —

gross β: strontium (pCi/L) 0 — — — —

radon 222 (pCi/L) 0 — — — —

radium 226 (pCi/L) 0 — — — —

radium 228 (pCi/L) 2 0 .50 .25 .25

uranium (μg/L) 0 — — — —
80 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Peedee confining zone

General parameters

pH (standard units) 6 7.8 10.0 8.4 8.2

temperature (degrees Celsius) 2 17.5 18.4 17.9 17.9

dissolved oxygen (mg/L) 2 .1 .3 .2 .2

specific conductance (μS/cm) 5 4,140 4,400 4,300 4,300

total dissolved solids (mg/L) 0 — — — —

filtered residue (mg/L) 3 2,226 2,490 2,360 2,370

Major cations (all mg/L)

calcium 5 7.7 12.0 9.3 8.0

magnesium 5 8.2 10.7 9.8 10.0

sodium 5 70 940 673 780

potassium 5 27.0 31.2 28.6 28.0

Major anions (all mg/L)

bicarbonate 7 0 1,121 666 916

carbonate 1 7.0 7.0 7.0 7.0

sulfate 5 68.2 175.7 135 149

chloride 5 32.0 876 696 860

Nutrients (all mg/L)

nitrate nitrogen 3 0.05 0.10 0.07 0.05

ammonium nitrogen 3 1.5 1.7 1.6 1.7

orthophosphate 3 .05 .20 .13 .13

total dissolved phosphorous 3 .20 .20 .20 .20

Secondary constituents

silica (mg/L) 2 10.7 12.0 11.3 11.3

aluminum (μg/L) 0 — — — —

iron (μg/L) 5 61.0 7,300 2,060 140

manganese (μg/L) 5 5.0 50.0 16.1 10.0

arsenic (μg/L) 0 — — — —

bromide (mg/L) 2 3.2 4.82 4.0 4.0

fluoride (mg/L) 4 2.5 2.8 2.7 2.7

iodide (mg/L) 0 — — — —

hydrogen sulfide (mg/L) 1 .224 .224 .224 .224

Radiochemical constituents

gross α: uranium (μg/L) 0 — — — —

gross α: radium + uranium – radon (pCi/L) 0 — — — —

gross α: radium – uranium – radon (pCi/L) 0 — — — —

gross β (pCi/L) 0 — — — —

gross β: cesium (pCi/L) 0 — — — —

gross β: strontium (pCi/L) 0 — — — —

radon 222 (pCi/L) 0 — — — —

radium 226 (pCi/L) 0 — — — —

radium 228 (pCi/L) 0 — — — —

uranium (μg/L) 0 — — — —
Attachment 2 81

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Peedee aquifer

General parameters

pH (standard units) 12 7.5 8.5 7.9 7.9

temperature (degrees Celsius) 6 16.5 20.8 19.2 19.9

dissolved oxygen (mg/L) 4 0 .2 .07 .05

specific conductance (μS/cm) 12 5,581 80,000 13,900 9,070

total dissolved solids (mg/L) 0 — — — —

filtered residue (mg/L) 9 3,628 5,217 4,700 5,120

Major cations (all mg/L)

calcium 10 18.9 93.0 30.7 24.5

magnesium 10 19.4 360 61.3 31.0

sodium 10 1,500 1,980 1,720 1,700

potassium 10 39.0 57.0 48.8 50.0

Major anions (all mg/L)

bicarbonate 11 0 914 696 855

carbonate 0 — — — —

sulfate 10 119 281 186 170

chloride 11 1,290 2,820 2,180 2,500

Nutrients (all mg/L)

nitrate nitrogen 4 0.05 0.05 0.05 0.05

ammonium nitrogen 6 2.3 5.9 4.1 4.6

orthophosphate 4 .01 .21 .06 .01

total dissolved phosphorous 3 .1 .2 .1 .1

Secondary constituents

silica (mg/L) 6 9.94 16.8 14.0 15.0

aluminum (μg/L) 2 20 30 25 25

iron (μg/L) 10 130 16,000 2,180 298

manganese (μg/L) 7 10.0 160 34.9 10.0

arsenic (μg/L) 0 — — — —

bromide (mg/L) 4 3.9 9.9 7.7 8.6

fluoride (mg/L) 12 1.20 2.25 1.53 1.46

iodide (mg/L) 0 — — — —

hydrogen sulfide (mg/L) 0 — — — —

Radiochemical constituents

gross α: uranium (μg/L) 0 — — — —

gross α: radium + uranium – radon (pCi/L) 0 — — — —

gross α: radium – uranium – radon (pCi/L) 0 — — — —

gross β (pCi/L) 0 — — — —

gross β: cesium (pCi/L) 0 — — — —

gross β: strontium (pCi/L) 0 — — — —

radon 222 (pCi/L) 0 — — — —

radium 226 (pCi/L) 0 — — — —

radium 228 (pCi/L) 0 — — — —

uranium (μg/L) 0 — — — —
82 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Virginia Beach aquifer

General parameters

pH (standard units) 10 7.9 8.8 8.2 8.1

temperature (degrees Celsius) 7 14.5 19.3 17.7 18.5

dissolved oxygen (mg/L) 3 .1 .4 .3 .3

specific conductance (μS/cm) 11 640 4,200 1,890 1,570

total dissolved solids (mg/L) 0 — — — —

filtered residue (mg/L) 9 359 2,150 933 888

Major cations (all mg/L)

calcium 9 1.11 10.0 2.8 1.6

magnesium 9 1.1 20.0 3.8 1.8

sodium 10 150 790 400 338

potassium 10 .3 15.0 11.8 13.0

Major anions (all mg/L)

bicarbonate 11 0 783 521 705

carbonate 4 23 36 28 27

sulfate 10 2.0 110 39 38

chloride 11 1.2 960 230 110

Nutrients (all mg/L)

nitrate nitrogen 5 0 0.56 0.17 0.09

ammonium nitrogen 2 .22 .82 .52 .52

orthophosphate 4 .09 .83 .45 .43

total dissolved phosphorous 1 .1 .1 .1 .1

Secondary constituents

silica (mg/L) 7 1.9 10 6.6 9.0

aluminum (μg/L) 0 — — — —

iron (μg/L) 4 28.0 270 127 106

manganese (μg/L) 3 2.0 10 5.0 3.0

arsenic (μg/L) 0 — — — —

bromide (mg/L) 2 .42 .73 .57 .57

fluoride (mg/L) 10 1.0 6.4 3.9 4.2

iodide (mg/L) 0 — — — —

hydrogen sulfide (mg/L) 0 — — — —

Radiochemical constituents

gross α: uranium (μg/L) 0 — — — —

gross α: radium + uranium – radon (pCi/L) 0 — — — —

gross α: radium – uranium – radon (pCi/L) 0 — — — —

gross β (pCi/L) 0 — — — —

gross β: cesium (pCi/L) 0 — — — —

gross β: strontium (pCi/L) 0 — — — —

radon 222 (pCi/L) 1 200 200 200 200

radium 226 (pCi/L) 0 — — — —

radium 228 (pCi/L) 0 — — — —

uranium (μg/L) 0 — — — —
Attachment 2 83

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Upper Cenomanian confining unit

General parameters

pH (standard units) 20 6.0 8.7 8.1 8.1

temperature (degrees Celsius) 5 18.5 21.11 19.8 20.0

dissolved oxygen (mg/L) 2 0 1.4 .7 .7

specific conductance (μS/cm) 18 595 6,060 4,470 5,660

total dissolved solids (mg/L) 0 — — — —

filtered residue (mg/L) 12 334 3,367 2,570 3,160

Major cations (all mg/L)

calcium 18 1.4 22.0 10.9 14.0

magnesium 17 .4 15 8.1 11

sodium 17 9.0 1,180 507 360

potassium 18 .8 35 19 22

Major anions (all mg/L)

bicarbonate 23 0 964 498 545

carbonate 0 — — — —

sulfate 15 8.6 167 67 26

chloride 17 3.0 1,600 737 310

Nutrients (all mg/L)

nitrate nitrogen 13 0.05 0.33 0.07 0.05

ammonium nitrogen 13 .5 2.5 1.9 2.1

orthophosphate 13 .02 .25 .08 .05

total dissolved phosphorous 12 .1 .2 .1 .1

Secondary constituents

silica (mg/L) 4 11 13 12 12

aluminum (μg/L) 2 20 50 35 35

iron (μg/L) 16 70 13,000 2,490 700

manganese (μg/L) 15 10 240 39 20

arsenic (μg/L) 1 4.0 4.0 4.0 4.0

bromide (mg/L) 1 6.12 6.12 6.12 6.12

fluoride (mg/L) 17 1.29 6.0 2.3 2.1

iodide (mg/L) 0 — — — —

hydrogen sulfide (mg/L) 0 — — — —

Radiochemical constituents

gross α: uranium (μg/L) 0 — — — —

gross α: radium + uranium – radon (pCi/L) 0 — — — —

gross α: radium – uranium – radon (pCi/L) 0 — — — —

gross β (pCi/L) 0 — — — —

gross β: cesium (pCi/L) 0 — — — —

gross β: strontium (pCi/L) 0 — — — —

radon 222 (pCi/L) 0 — — — —

radium 226 (pCi/L) 0 — — — —

radium 228 (pCi/L) 0 — — — —

uranium (μg/L) 0 — — — —
84 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Potomac confining zone

General parameters

pH (standard units) 68 5.1 9.5 7.8 8.1

temperature (degrees Celsius) 13 15.6 24.2 19.2 18.0

dissolved oxygen (mg/L) 3 .1 .9 .4 .2

specific conductance (μS/cm) 35 192 6,900 1,450 750

total dissolved solids (mg/L) 0 — — — —

filtered residue (mg/L) 52 65.0 3,575 653 408

Major cations (all mg/L)

calcium 57 1.0 520 23.7 9.0

magnesium 57 .1 20.0 4.3 2.5

sodium 55 4.0 1,400 231 110

potassium 53 1.1 35.0 8.7 8.0

Major anions (all mg/L)

bicarbonate 80 0 1,901 261 158

carbonate 3 1.0 4.0 2.3 2.0

sulfate 50 .7 114.4 19.1 12.8

chloride 61 .1 2,380 162 13.4

Nutrients (all mg/L)

nitrate nitrogen 46 0 4.9 0.24 0.05

ammonium nitrogen 31 .1 3.7 .44 .10

orthophosphate 35 0 1.8 .34 .29

total dissolved phosphorous 27 .1 2.1 .33 .30

Secondary constituents

silica (mg/L) 14 11 56 24 15

aluminum (μg/L) 3 10 100 43 20

iron (μg/L) 49 30 26,400 1,100 200

manganese (μg/L) 46 5.5 470 43 20

arsenic (μg/L) 4 1.0 1.0 1.0 1.0

bromide (mg/L) 2 .04 5.9 3.0 3.0

fluoride (mg/L) 58 .1 5.7 1.7 1.3

iodide (mg/L) 0 — — — —

hydrogen sulfide (mg/L) 1 0 0 0 0

Radiochemical constituents

gross α: uranium (μg/L) 0 — — — —

gross α: radium + uranium – radon (pCi/L) 0 — — — —

gross α: radium – uranium – radon (pCi/L) 0 — — — —

gross β (pCi/L) 0 — — — —

gross β: cesium (pCi/L) 0 — — — —

gross β: strontium (pCi/L) 0 — — — —

radon 222 (pCi/L) 0 — — — —

radium 226 (pCi/L) 0 — — — —

radium 228 (pCi/L) 0 — — — —

uranium (μg/L) 0 — — — —
Attachment 2 85

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Potomac aquifer

General parameters

pH (standard units) 1,741 1.6 13.55 8.0 8.0

temperature (degrees Celsius) 396 2.0 210 19.9 19.0

dissolved oxygen (mg/L) 130 0 3.5 .3 .2

specific conductance (μS/cm) 1,192 9.0 63,800 2,020 760

total dissolved solids (mg/L) 107 210 487,000 32,100 1,200

filtered residue (mg/L) 1,291 51.0 470,000 3,300 492

Major cations (all mg/L)

calcium 1,638 0 44,700 47.4 2.7

magnesium 1,639 .0006 854 6.5 1.0

sodium 1,807 .0809 203,000 1,000 198

potassium 1,612 .044 124 11.6 8.0

Major anions (all mg/L)

bicarbonate 3,085 0 377,000 975 195

carbonate 86 0 28,000 1,430 10.0

sulfate 1,649 .2 32,100 128 20.6

chloride 2,850 0 26,900 438 96.0

Nutrients (all mg/L)

nitrate nitrogen 1,030 0 13.5 0.12 0.05

ammonium nitrogen 656 0 180 1.8 .10

orthophosphate 839 0 1,390 6.9 .31

total dissolved phosphorous 605 .01 3.3 .38 .30

Secondary constituents

silica (mg/L) 468 0.4 38,400 451 19.0

aluminum (μg/L) 21 0 10,000 610 100

iron (μg/L) 1,498 0 42,800 592 100

manganese (μg/L) 1,032 0 3,580 42.1 10.0

arsenic (μg/L) 60 0 7.2 1.3 1.0

bromide (mg/L) 142 0 100 10.5 1.9

fluoride (mg/L) 1,826 0 18.36 2.44 2.28

iodide (mg/L) 33 0 3.7 .95 .56

hydrogen sulfide (mg/L) 20 0 .125 .014 0

Radiochemical constituents

gross α: uranium (μg/L) 0 — — — —

gross α: radium + uranium – radon (pCi/L) 5 0.3 35.2 11.8 3.5

gross α: radium – uranium – radon (pCi/L) 0 — — — —

gross β (pCi/L) 5 2.5 12.4 4.9 3.1

gross β: cesium (pCi/L) 0 — — — —

gross β: strontium (pCi/L) 0 — — — —

radon 222 (pCi/L) 23 170 490 260 240

radium 226 (pCi/L) 3 0 .5 .2 .2

radium 228 (pCi/L) 5 0 1.4 .5 .3

uranium (μg/L) 1 .3 .3 .3 .3
86 Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007

Attachment 2.  Summary of groundwater-quality constituent values for hydrogeologic units of the Virginia Coastal Plain.—Continued

[minimum and maximum values are cited from Attachment 1; mg/L, milligrams per liter; μS/cm, microseimens per centimeter; μg/L, micrograms per liter; 

α, alpha; β, beta; pCi/L, picocuries per liter; —, no data]

Constituent Analyses Minimum Maximum Mean Median

Basement bedrock

General parameters

pH (standard units) 598 3.68 12.2 6.4 6.2

temperature (degrees Celsius) 551 8.0 26.0 15.0 15.0

dissolved oxygen (mg/L) 531 0 8.3 .5 .2

specific conductance (μS/cm) 586 22.0 178,000 495 122

total dissolved solids (mg/L) 10 24,315 64,422 46,800 48,500

filtered residue (mg/L) 36 29.0 3,870 332 147

Major cations (all mg/L)

calcium 69 1.0 8,350 810 10.0

magnesium 68 .2 383 35.8 3.3

sodium 63 2.4 15,773 1,980 15.0

potassium 62 .84 509 28.7 3.2

Major anions (all mg/L)

bicarbonate 82 0 322 84.4 51.7

carbonate 3 2.0 230 82 14

sulfate 64 0 2,110 247 8.6

chloride 64 1.0 38,300 4,310 4.4

Nutrients (all mg/L)

nitrate nitrogen 59 0 20 0.46 0.05

ammonium nitrogen 63 .002 15.5 2.4 .10

orthophosphate 70 0 .66 .10 .08

total dissolved phosphorous 57 .001 .92 .13 .10

Secondary constituents

silica (mg/L) 21 0.7 39 22 20

aluminum (μg/L) 0 — — — —

iron (μg/L) 39 10.0 46,600 1,620 100

manganese (μg/L) 35 10.0 2,120 123 20.0

arsenic (μg/L) 12 1 8 2 1

bromide (mg/L) 13 .01 241 105 130

fluoride (mg/L) 47 .06 18 .78 .20

iodide (mg/L) 0 — — — —

hydrogen sulfide (mg/L) 0 — — — —

Radiochemical constituents

gross α: uranium (μg/L) 0 — — — —

gross α: radium + uranium – radon (pCi/L) 0 — — — —

gross α: radium – uranium – radon (pCi/L) 0 — — — —

gross β (pCi/L) 0 — — — —

gross β: cesium (pCi/L) 0 — — — —

gross β: strontium (pCi/L) 0 — — — —

radon 222 (pCi/L) 3 360 850 670 790

radium 226 (pCi/L) 0 — — — —

radium 228 (pCi/L) 0 — — — —

uranium (μg/L) 0 — — — —
Prepared by:

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Raleigh, NC 27607

For additional information regarding this publication, contact:

Director

USGS Virginia Water Science Center

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phone: 1-804-261-2600

email: dc_va@usgs.gov

Or visit the Virginia Water Science Center Web site at:

http://va.water.usgs.gov
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McFarland—Groundwater-Quality Data and Regional Trends in the Virginia Coastal Plain, 1906–2007—Professional Paper 1772