U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2015–5117

Prepared in cooperation with the Virginia Department of Environmental Quality

A Conceptual Framework and Monitoring Strategy for 

Movement of Saltwater in the Coastal Plain Aquifer System 

of Virginia
Cover. Left, observation well 62C 5 SOW 093 completed in the unconfined surficial aquifer in the city of Virginia Beach, Virginia. Well depth 

is 65 feet, and depth to water has averaged 5 feet over the past 10 years. A groundwater chemical quality sample collected on July 30, 

2013, had a chloride concentration of 58.7 milligrams per liter. The well is included in the U.S. Geological Survey real-time groundwaterlevel network available at http://waterdata.usgs.gov/nwis/dv?referred_module=sw&site_no=364504076031301. Additional information on 

observation well 62C 5 SOW 093 is available at http://groundwaterwatch.usgs.gov/AWLSites.asp?S=364504076031301. Photograph by 

Roger M. Moberg, U.S. Geological Survey.

Right, collection of a groundwater chemical quality sample on March 15, 2001, from observation well 61C 1 completed in the confined 

Potomac aquifer in the city of Norfolk, Virginia. The wellhead is finished below the level of the parking lot. Well depth is 970 feet, and depth to 

water has declined from about 15 to 85 feet over the past 47 years. From the time of this sample to another sample collected on July 29, 2010, 

groundwater chloride concentration increased from 1,150 to 1,210 milligrams per liter. More information on observation well 61C 1 is available 

at http://groundwaterwatch.usgs.gov/AWLSites.asp?S=365223076122101. Photograph by Karl M. Dydak, U.S. Geological Survey.
A Conceptual Framework and Monitoring 

Strategy for Movement of Saltwater in the 

Coastal Plain Aquifer System of Virginia

By E. Randolph McFarland

Prepared in cooperation with the Virginia Department of Environmental Quality

Scientific Investigations Report 2015–5117

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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

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U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

McFarland, E.R., 2015, A conceptual framework and monitoring strategy for movement of saltwater in the Coastal 

Plain aquifer system of Virginia: U.S. Geological Survey Scientific Investigations Report 2015–5117, 30 p., 1 pl., 

http://dx.doi.org/10.3133/sir20155117.

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

ISBN 978-1-4113-3970-5
iii

Contents

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

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

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

Description of the Study Area ............................................................................................................3

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

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

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

Conceptual Framework of Saltwater in the Coastal Plain Aquifer System of Virginia ....................10

The Saltwater-Transition Zone .........................................................................................................10

Configuration ..............................................................................................................................10

Formation.....................................................................................................................................10

Saltwater Movement..........................................................................................................................12

Upconing and Lateral Intrusion...............................................................................................12

Historical Changes in Groundwater Chloride Concentration.............................................13

Chloride-Concentration Trends ...............................................................................................13

Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System

of Virginia.........................................................................................................................................14

Proximity of Groundwater-Production Wells to Saltwater..........................................................14

Observation Wells...............................................................................................................................16

Locations ....................................................................................................................................17

Construction and Development ..............................................................................................18

Sample Collection .....................................................................................................................19

Sampling Frequency..................................................................................................................20

Sample Analysis.........................................................................................................................20

Limitations and Constraints...............................................................................................................21

Classification of Production Wells..........................................................................................21

Construction of Observation Wells ........................................................................................22

Summary and Conclusions.........................................................................................................................24

References Cited..........................................................................................................................................26
iv

Figures

1. Map showing locations of groundwater-production wells, projected 

locations of lateral-intrusion observation wells, and the configuration of

the 250-milligrams-per-liter chloride iso-concentration surface .........................................4

2. Generalized hydrogeologic section and directions of predevelopment 

groundwater flow in the Virginia Coastal Plain .......................................................................6

3. Map showing simulated hydraulic head during 2003 and groundwater

total ion concentration in the Potomac aquifer.......................................................................8

4. Diagram showing projection of lateral intrusion observation-well location

from groundwater-production well............................................................................................9

5. Diagrams showing movement of saltwater............................................................................11

6. Diagram showing generalized temporal relations among groundwater

chloride concentrations, water levels, and withdrawals ....................................................14

7. Diagram showing generalized spatial relations of groundwater-production

wells to chloride iso-concentration surfaces........................................................................15

8. Diagram showing generalized spatial relations among groundwaterproduction wells, observation wells, and the 250-milligrams-per-liter 

chloride iso-concentration surface.........................................................................................17

9. Diagram showing pump placement and responses of observation wells

to purging .....................................................................................................................................19

10. Diagram showing adjustments to observation-well depths relative to 

positions within the aquifer system.........................................................................................23

11. Diagram showing generalized spatial relations of observation wells to

multiple neighboring groundwater-production wells...........................................................24

Digital Storage Media

[CD–ROM in pocket attached to back cover. Also available for downloading from

http://dx.doi.org/10.3133/sir20155117.]

Attachment 1. Excel file containing groundwater-production wells, vertical positions, 

 and lateral distances and directions relative to chloride iso-concentration 

 surfaces, and projected locations of lateral-intrusion observation wells

Plate

[In pocket attached to back cover. Also available for downloading from http://dx.doi.org/

10.3133/sir20155117.]

1. Locations of groundwater-production wells, projected locations of lateral intrusion 

observation wells, and the configuration of the 250-milligrams-per-liter chloride 

iso-concentration surface in the Coastal Plain of Virginia

Table

1. Groundwater-production wells, vertical positions and lateral distances and 

directions relative to chloride iso-concentration surfaces, and projected 

locations of lateral intrusion observation wells in the Coastal Plain of Virginia..............28
v

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

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) 

Flow rate

feet per year (ft/yr) 0.3048 meters per year (m/yr)

gallons per month (gal/mo) 3.785 liters per month (L/mo)

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

Supplemental Information

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

Datums

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. 

Abbreviations

GIS geographic information system 

LOWESS Locally Weighted Scatterplot Smoothing

RASA Regional Aquifer-System Analysis

SMCL secondary maximum contaminant level

USGS U.S. Geological Survey

VA DEQ Virginia Department of Environmental Quality 
vi

Acknowledgments

This study was supported by the Virginia Department of Environmental Quality (VA DEQ). 

Special thanks for program planning go to Scott Kudlas, Craig Nichol, Scott Bruce, and 

Mary Ann Massie of VA DEQ. 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 

scientific integrity of this report was aided greatly from reviews by Bruce Campbell of the 

U.S. Geological Survey (USGS) South Carolina Water Science Center, and Emmanuel Charles of 

the USGS New Jersey Water Science Center. Editorial and graphical quality of the report was 

aided greatly by Rebekah Davis and Caryl Wipperfurth of the USGS Science Publishing Network.

This project has been funded in part by the U.S. Environmental Protection Agency (EPA) under 

assistance agreement BG–98392505–1 to the VA DEQ. The contents of this document do not 

necessarily reflect the views and policies of the EPA, nor does the EPA endorse trade names or 

recommend the use of commercial products mentioned in this document.
A Conceptual Framework and Monitoring Strategy for 

Movement of Saltwater in the Coastal Plain Aquifer System 

of Virginia

By E. Randolph McFarland

Abstract

A conceptual framework synthesizes previous studies to 

provide an understanding of conditions, processes, and relations of saltwater to groundwater withdrawal in the Virginia 

Coastal Plain aquifer system. A strategy for monitoring 

saltwater movement is based on spatial relations between the 

saltwater-transition zone and 612 groundwater-production 

wells that were regulated during 2013 by the Virginia 

Department of Environmental Quality. The vertical position and 

lateral distance and direction of the bottom of each production 

well’s screened interval was calculated relative to previously 

published groundwater chloride iso-concentration surfaces. 

Spatial analysis identified 81 production wells completed in the 

Yorktown-Eastover and Potomac aquifers that are positioned 

in closest proximity to the 250-milligrams-per-liter chloride 

surface, and from which chloride concentrations are most likely 

to increase above the U.S. Environmental Protection Agency’s 

250-milligrams-per-liter secondary maximum-contaminant 

level. Observation wells are specified to distinguish vertical 

upconing from lateral intrusion among individual production 

wells. To monitor upconing, an observation well is to be 

collocated with each production well and completed at about 

the altitude of the 250-milligrams-per-liter chloride isoconcentration surface. To monitor lateral intrusion, a potential 

location of an observation well is projected from the bottom of 

each production well’s screened interval, in the lateral direction to the underlying chloride surface to a distance of 1 mile.

Monitoring potential withdrawal-induced movement of 

saltwater in the Virginia Coastal Plain aquifer system is needed 

to detect increases in chloride concentration before groundwater-production wells become contaminated. An investigation 

was undertaken during 2014 by the U.S. Geological Survey in 

cooperation with the Virginia Department of Environmental 

Quality, to provide a sound scientific understanding of 

saltwater movement and guidance to implement a monitoring 

program. Previous studies have theorized that the saltwater 

originated primarily from seawater repeatedly emplaced within 

aquifer sediments during the past about 65 million years. 

Subsequent flushing by fresh groundwater has been impeded 

across sediments filling the Chesapeake Bay impact crater. The 

resulting saltwater-transition zone has been mapped to exhibit 

a warped and steeply mounded dome shape about centered on 

the impact crater, and flanked by a nearly level and shallow 

plateau shape to the southeast. Groundwater chloride concentrations have historically fluctuated during periods of weeks 

to months, probably as a result of localized vertical upconing 

beneath individual production wells. Lateral intrusion takes 

several decades or more to horizontally displace groundwater 

across distances of about 1 mile toward production wells. 

Upconing is relatively immediate, but reversible, whereas 

lateral intrusion under the regionally landward hydraulic 

gradient may slowly, but permanently reposition the saltwatertransition zone. Upconing coupled with lateral intrusion is 

theorized to produce composite chloride-concentration trends 

that vary widely over time in response to changing water 

demands, and evolve dynamically from hydraulic interactions 

among multiple neighboring production wells.

Some aspects of observation-well construction and 

sampling are of particular importance to monitoring 

saltwater movement in the Virginia Coastal Plain aquifer 

system. Observation wells should feature screened intervals 

generally of no more than 10 feet that isolate distinct parts 

of the aquifer, and be thoroughly developed for removal 

of drilling fluid and introduced water. Presample purging 

should fully displace stratified saltwater in the well casing 

upward to the pump. Stable flow should be maintained as 

field parameters are measured and sample containers are 

filled with filtered water isolated from the atmosphere and 

unaffected by surface temperature. Groundwater samples 

from both upconing and lateral-intrusion observation wells 

should initially be collected four times per year when wells 

are newly established, but can be more optimally timed with 

withdrawal once responses in chloride concentrations can be 
2 A Conceptual Framework and Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia

reliably predicted. Concentrations of major ions (1) determine 

the dominant chemical composition of groundwater at each 

well, (2) establish the relative position of the well within the 

saltwater-transition zone, and (3) provide data quality control 

by calculation of sample charge balance. For these reasons, 

samples initially collected for the first year from newly 

established observation wells should be analyzed for calcium, 

magnesium, sodium, and potassium cations and chloride, 

bicarbonate, carbonate, sulfate, fluoride, and bromide anions. 

Inflection-point titration for alkalinity should be completed 

in the field. Analysis of chloride and field parameters may be 

adequate on a long-term basis once the dominant chemical 

composition at each well is established. Specific conductance 

may also provide a surrogate for chloride concentration 

depending on regulatory policy.

The saltwater-movement monitoring strategy is 

limited and constrained. Relative monitoring needs among 

groundwater-production wells, and construction of observation 

wells, depend on the accuracy of previously mapped groundwater chloride iso-concentration surfaces. Production wells in 

similar proximity to saltwater can differ in aquifer hydraulic 

conductivity, rates of withdrawal, and screened-interval 

lengths. Only production wells making withdrawals reported 

to the Virginia Department of Environmental Quality have 

been accounted for; undocumented production wells can result 

in spurious changes in groundwater chloride concentration. 

Upconing observation wells should be as close as possible to 

corresponding production wells, so long as production wells 

are not damaged by borehole deviation. Projected locations of 

some lateral-intrusion observation wells may be precluded and 

require adjustment. Depths of upconing and lateral-intrusion 

observation wells may also require adjustment to be within the 

same aquifer as their corresponding production wells. Existing 

unused wells can be adapted as observation wells if differences from specified locations and construction are kept to a 

minimum and are accounted for. Where multiple production 

wells are in proximity, a modified monitoring approach may 

be needed to determine their net effect on changes in chloride 

concentration, and may require more than one lateral-intrusion 

observation well depending on the vertical positions of 

production-well screened intervals.

Introduction

Groundwater in the Coastal Plain Physiographic Province 

of eastern Virginia (fig. 1) is a heavily used resource. The rate 

of groundwater withdrawal is estimated to have been close to 

zero during the late 1800s, but increased continuously during 

the 20th century. By 2003, withdrawal rates from Coastal Plain 

aquifers in Virginia totaled approximately 117 million gallons 

per day (Mgal/d) (Heywood and Pope, 2009). As a result, 

groundwater levels have declined by as much as 150 feet (ft) 

below sea level near large withdrawal centers, and sediment 

compaction has produced widespread land subsidence and 

exacerbated effects of sea-level rise (Pope and Burbey, 2004; 

Eggleston and Pope, 2013). Flow gradients have also been 

regionally reversed from a previously seaward direction to 

a landward direction, creating the potential for movement 

of saltwater through the aquifer system toward production 

wells. Withdrawals are likely to increase, which could result 

in further water-level decline, land subsidence, and potential 

saltwater contamination.

To manage the groundwater resource, the Virginia 

Department of Environmental Quality (VA DEQ) regulates 

groundwater withdrawals throughout the Virginia Coastal 

Plain. Withdrawals greater than 300,000 gallons per 

month must be approved under the VA DEQ Groundwater 

Withdrawal Permit Program (Code of Virginia, Title 62.1, 

Chapter 25). Groundwater users are required to submit 

information needed to evaluate the effects of their withdrawals 

on the aquifer system.

The VA DEQ relies on a sound scientific understanding 

of Virginia Coastal Plain geology and hydrology 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 (Meng 

and Harsh, 1988; Harsh and Laczniak, 1990). Subsequent 

investigations included discovery of the largest known 

meteor-impact crater in the United States buried beneath the 

lower Chesapeake Bay (Powars and Bruce, 1999; Powars, 

2000). With knowledge of the impact crater, a complete 

redefinition of the aquifer system provides the most current 

(2015) and comprehensive understanding of groundwater 

conditions (McFarland and Bruce, 2006; Heywood and Pope, 

2009; McFarland, 2010 and 2013; Eggleston and Pope, 2013). 

For more than three decades, results of USGS investigations 

have been extensively applied by the VA DEQ to evaluate the 

effects of groundwater withdrawals.

In addition to regulating groundwater withdrawals, 

the VA DEQ regulates the chemical quality of groundwater. 

Hence, effects of withdrawal-induced movement of saltwater 

on concentrations of chloride within parts of the Virginia 

Coastal Plain aquifer system are of concern. Drinking water 

must not exceed a chloride-concentration secondary maximum 

contaminant level (SMCL) of 250 milligrams per liter (mg/L)

(U.S. Environmental Protection Agency, 1990). Some 

concentrations in groundwater, however, are as much as that 

of seawater (19,000 mg/L) and greater (McFarland, 2010). 

The potential for limitations on groundwater use, and (or) 

requirements to treat and substantially reduce chloride concentration, is therefore widespread. Saltwater contamination at 

some locations has been indicated by increases in chloride 

concentrations over time (McFarland, 2010). In addition, large 

amounts of brackish groundwater are being withdrawn at 

several primary production facilities, where costly desalination 

treatment is necessary before use.
Introduction 3

Fresh groundwater having a chloride concentration 

below the 250-mg/L SMCL is separated from saltwater along 

a saltwater-transition zone. In a simple coastal groundwater 

system, the saltwater-transition zone is positioned at land 

surface along the coastline and slopes downward and landward 

evenly beneath the surface (Barlow, 2003). By contrast, the 

saltwater-transition zone in the Virginia Coastal Plain has 

been mapped as far as 30 miles (mi) inland across the upper to 

middle parts of the aquifer system (McFarland, 2010), and is 

warped and steeply mounded in a roughly concentric fashion 

over and around sediments that fill the Chesapeake Bay 

impact crater (fig. 1, pl. 1). Landward of the impact crater, the 

saltwater-transition zone has been further delineated at a low 

angle across much of the deep part of the aquifer system as far 

as 70 mi inland (McFarland, 2010).

In relation to the saltwater-transition zone, groundwater 

withdrawal in the Virginia Coastal Plain is also complexly 

distributed. Production wells that were regulated by the 

VA DEQ during 2013 widely span the area (fig. 1, pl. 1, 

attachment 1). Because of the position and unusual configuration of the saltwater-transition zone, the production wells are 

oriented to it at highly variable distances and directions. Well 

depths also span a broad range of vertical distances from the 

saltwater-transition zone. Saltwater moves toward production 

wells both horizontally by lateral intrusion and vertically by 

upconing. Wells to the west are laterally distant from saltwater, but upconing can still potentially contaminate wells of 

sufficient depth. Upconing is a more immediate but generally 

reversible cause of contamination, whereas lateral intrusion 

potentially results in a permanent reposition of the saltwatertransition zone. The two processes cannot be distinguished, 

however, from a single increase in chloride concentration.

The VA DEQ is currently (2015) developing a program 

to monitor the movement of saltwater and detect increases in 

chloride concentration before groundwater-production wells 

exceed the 250-mg/L SMCL. In a simple coastal groundwater 

system, lateral movement of saltwater takes place across a 

distinct front toward withdrawals of freshwater inland, and 

can be detected simply by periodically determining concentrations of chloride in groundwater across the intervening 

area. By contrast, no readily distinguishable front exists in 

the Virginia Coastal Plain across which saltwater movement 

toward production wells can be anticipated. Instead, proximity 

to saltwater, and the potential immediacy and permanence of 

contamination by vertical upconing and lateral intrusion, varies 

widely among individual production wells. The VA DEQ is 

therefore tailoring requirements among individual withdrawalpermit holders to monitor increases in groundwater chloride 

concentration that may exceed the 250-mg/L SMCL. 

To address the complexity of movement of saltwater in 

the Virginia Coastal Plain, an investigation was undertaken 

during 2014 by the U.S. Geological Survey (USGS) in 

cooperation with the VA DEQ, to develop a conceptual 

framework and monitoring strategy for saltwater movement. 

Conditions, processes, and relations of saltwater to groundwater withdrawal were synthesized from previous studies. 

Spatial relations between the saltwater-transition zone and 

regulated groundwater-production wells were also analyzed, 

and locations and other considerations for observation wells to 

monitor saltwater movement were determined.

Purpose and Scope

A conceptual framework and monitoring strategy is 

presented for movement of saltwater in the Virginia Coastal 

Plain aquifer system. The framework and strategy were 

developed to provide the VA DEQ with (1) a sound scientific 

understanding of the presence and movement of saltwater, and 

(2) operational guidance to implement a monitoring program. 

The conceptual framework consists of generalized descriptions 

synthesized from previous studies of conditions, processes, 

and relations of saltwater to groundwater withdrawal. The 

monitoring strategy is based on spatial relations between the 

saltwater-transition zone and 612 production wells that were 

regulated by the VA DEQ during 2013. Analysis identifies 

81 production wells completed in the Yorktown-Eastover 

and Potomac aquifers from which chloride concentrations 

are most likely to increase above the 250-mg/L SMCL, and 

locations and depths of observation wells to distinguish 

vertical upconing from lateral intrusion among individual 

production wells. Aspects of observation-well construction 

and sampling are described that are of particular importance to 

monitoring saltwater movement in the Virginia Coastal Plain 

aquifer system. Limitations and constraints on monitoring are 

also discussed that must be recognized for effective strategy 

implementation.

Description of the Study Area

The Virginia Coastal Plain is about 13,000 square miles 

(mi2) in size (fig. 1). Primary urban centers include the cities 

of Fredericksburg, Virginia, and Richmond, Va., along the 

western margin, and several cities and counties to the east 

and south. The remainder of the Virginia Coastal Plain is 

mostly rural and fairly evenly divided between cropland and 

forest. Land-surface altitude ranges from more than 300 ft 

across some western uplands to 0 ft along the Atlantic coast. 

Rolling terrain and deeply incised stream valleys are present 

to the northwest and gently rolling-to-level terrain, broad 

stream valleys, and extensive wetlands to the east and south. 

Primary rivers include the Potomac, Rappahannock, York, 

and James Rivers (fig. 1), and 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 and become estuarine 

upon entering the Virginia Coastal Plain. Distinct landmasses 

defined by the estuarine rivers include, from north to south, 

the Northern Neck, Middle Peninsula, York-James Peninsula, 

and Southeastern Virginia (fig. 1). Chesapeake Bay separates 

these parts of the Virginia Coastal Plain to the west from the 

Virginia Eastern Shore to the east.
SUSSEX

CAROLINE

HANOVER

DINWIDDIE

Suffolk

SURRY

ESSEX

SOUTHAMPTON

CHESTERFIELD

SPOTSYLVANIA

HENRICO

Chesapeake

KING & QUEEN

YORK

ISLE OF WIGHT

GREENSVILLE

NEW KENT

KING WILLIAM

RICHMOND

PRINCE

GEORGE

GLOUCESTER

WESTMORELAND

NORTHUMBERLAND

CHARLES CITY

KING GEORGE

JAMES

 CITY

MIDDLESEX

LANCASTER

Norfolk

NORTHAMPTON

MATHEWS

Richmond

Hampton Newport News

Petersburg Poquoson

Hopewell

Franklin

Colonial

Heights

Virginia

Beach

Emporia

Portsmouth

Williamsburg

Fredericksburg

–500

–250

–250

–250

–750

–1,000

–100

–900

–500

–1,000

–750

–750

–100

–1,000

77°W 76°W

38°N

37°N

Area of plate 1

F A L L

Z O N E

Chesapeake Bay

impact crater

Base modified from U.S. Geological Survey

State of Virginia, 1:500,000, 1973

ATLANTIC

OCEAN

CHESAPEAKE

BAY

NORTH CAROLINA

VIRGINIA

MARYLAND

Potomac River

James River

York River

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

River

VIRGINIA

EASTERN

SHORE

SOUTHEASTERN VIRGINIA

West

Point

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

STAFFORD

Rappahannock

l l l l l l l l l l l l l l

4 A Conceptual Framework and Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia
EXPLANATION

Water-quality contour—Shows altitude of 250-milligrams-per-liter 

 (mg/L) chloride iso-concentration surface. Hachures indicate

 depression. Contour intervals 150 and 250 feet. Datum is NGVD 29

Groundwater-production well screened-interval bottom position 

 relative to chloride iso-concentration surfaces

>50 feet above and >1 lateral mile from 250-mg/L surface

>50 feet above and ≤1 lateral mile from 250-mg/L surface

>50 feet below 250-mg/L surface

Below 1,000-mg/L surface

≤50 feet above 250-mg/L surface

Yorktown-Eastover aquifer

Potomac aquifer

≤50 feet below 250-mg/L surface

Yorktown-Eastover aquifer

Potomac aquifer

Piedmont

Study area

Atlantic Ocean

Chesapeake Bay

Fall Zone

Coastal

Plain

VIRGINIA

–100

Introduction 5

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, Paleogene, Neogene, 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. 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. Fluvial strata primarily of Cretaceous age 

are overlain by marine strata of Paleogene and Neogene age, 

which are overlain in turn by terrace and floodplain deposits 

primarily of Quaternary age.

Coastal Plain sediments in Virginia were affected during 

the Paleogene 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 mi in diameter and extends across a large 

part of the southeastern Virginia Coastal Plain (fig. 1). The 

crater formed within the preexisting sediments and contains 

unique impact-related materials as deep as basement bedrock. 

Subsequent deposition has buried crater-fill sediments about 

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

The Piedmont Physiographic Province (Piedmont) lies to 

the west of the Coastal Plain (figs. 1 and 2) and is 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. The transitional part of the Coastal Plain 

adjacent to the Piedmont is designated as the Fall Zone, a 

belt several miles wide across which streams have eroded 

through Coastal Plain sediments to expose Piedmont bedrock 

in their valley floors (Mixon and others, 1989). From the 

Fall Zone, the Piedmont bedrock slopes eastward beneath the 

sediment wedge to constitute the basement that underlies the 

Coastal Plain. 

Groundwater Conditions

Sediments of the Virginia Coastal Plain are represented 

by a hydrogeologic framework (fig. 2) (McFarland and Bruce, 

2006). Groundwater is present in pores between sediment 

grains. 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 termed confining 

zones. Parts of some aquifers provide a widely used supply 

of water in Virginia.

Figure 1 (pages 4 and 5). Locations of groundwaterproduction wells, projected locations of lateralintrusion observation wells, and the configuration of 

the 250-milligrams-per-liter chloride iso-concentration 

surface. Locations of groundwater-production wells 

regulated by the Virginia Department of Environmental 

Quality during 2013. Well symbols are color referenced 

to figures 7 and 8, and attachment 1. Symbol pointers 

indicate projected locations of lateral intrusion observation 

wells. Approximate altitude and configuration of the 

250-milligrams-per-liter chloride iso-concentration 

surface modified from McFarland (2010). Location of 

Chesapeake Bay impact crater from Powars and Bruce 

(1999). Further detail shown on plate 1.
6 A Conceptual Framework and Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia

0

VERTICAL SCALE GREATLY EXAGGERATEDDATUM IS NATIONAL GEODETIC VERTICAL DATUM OF 1929

WEST EAST

Aquia

aquifer

Yorktown-Eastover aquifer

Suffolk Scarp Surficial aquifer

Yorktown confining zone

Nanjemoy-Marlboro confining unit

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

Chickahominy confining unit Exmore matrix confining unit

Exmore

clast

confining

unit

Chesapeake Bay impact crater

500

–500

–1,000–1,500

FEET

Outcrops

Potomac confining zone

Sal wt ater-transit oi

n

oz

en

EXPLANATION

Aquifer

Confining unit or zone

Saprolite

Bedrock

Direction of groundwater flowFracture

Figure 2. Generalized hydrogeologic section and directions of predevelopment groundwater flow in the Virginia Coastal Plain (modified from McFarlandand Bruce, 2006). The Saint Marys, Peedee, and Virginia Beach aquifers are of limited extent and are not shown.
Introduction 7

Twenty hydrogeologic units are currently (2015) recognized (McFarland and Bruce, 2006), including in downward 

order the following:

• Surficial aquifer,

• Yorktown confining zone,

• Yorktown-Eastover aquifer,

• Saint Marys confining unit,

• Saint Marys aquifer,

• Calvert confining unit,

• Piney Point aquifer,

• Chickahominy confining unit,

• Exmore matrix confining unit,

• Exmore clast confining unit,

• 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, and

• bedrock.

The positions of hydrogeologic-unit margins do not coincide, 

and their areal distribution has a complex, overlapping “patchwork” configuration. Some units pinch out westward toward 

the Fall Zone, and major discontinuities are present along the 

margin of the Chesapeake Bay impact crater (fig. 2).

Groundwater in the Virginia Coastal Plain is recharged by 

precipitation that infiltrates the land surface and percolates to 

the water table. Unconfined groundwater flows short distances 

and discharges to nearby streams, but a small amount leaks 

downward to recharge the deeper confined aquifers (fig. 2) 

primarily along the Fall Zone and beneath surface-drainage 

divides between primary river valleys. Bedrock imposes a 

relatively impermeable underlying boundary. Flow through 

the confined aquifers is primarily lateral in the down-dip 

direction to the east toward large withdrawal centers and 

primary 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 leakage across intervening confining units. In addition, 

hydraulic boundaries along the Chesapeake Bay impact crater 

cause a lateral divergence of flow to either side of the impact 

crater (McFarland, 2010).

Groundwater withdrawal in the Virginia Coastal Plain 

increased continuously during the past century and has totaled 

nearly 100 Mgal/d for the past several decades (Heywood 

and Pope, 2009). The largest individual withdrawals supply 

industrial facilities at the cities of Franklin, Va., and West 

Point, Va. (fig. 1, pl. 1), where all but the deepest part of the 

aquifer system contains freshwater. Farther east and closer to 

saltwater, many additional large withdrawals supply public 

drinking water and diverse other uses, including several 

brackish groundwater development facilities that directly withdraw and treat water containing from several hundred to more 

than 1,000 mg/L of chloride. Most of these large withdrawals 

are regulated by the VA DEQ and are well documented. An 

estimated 200,000 small, unregulated, and mostly undocumented withdrawals also widely span the Virginia Coastal 

Plain and supply primarily individual domestic use (Pope and 

others, 2008).

Regional water-level cones of depression as deep as 

150 ft below sea level are centered on the two industrialwithdrawal locations about 60 mi inland (fig. 3). As a result, 

the hydraulic gradient across the Virginia Coastal Plain has 

been regionally redirected landward and about doubled. Waterlevel declines from additional large withdrawals farther east 

have an additive effect on the aquifer system and contribute to 

the landward hydraulic gradient. Numerous but small individual domestic withdrawals probably result in only localized 

water-level declines. Region-wide rates of water-level decline 

vary between about 1 to 2 feet per year. Based on results of 

previous studies (Nelms and others, 2003; Heywood and Pope, 

2009; McFarland, 2010 and 2013), probably several decades 

would be needed for water levels to stabilize in equilibrium 

with current (2015) withdrawals. In addition, much of the 

withdrawn water has been removed from aquifer storage rather 

than displaced across regionally appreciable distances. As a 

result, sediment compaction has produced widespread land 

subsidence and exacerbated effects of sea-level rise (Pope and 

Burbey, 2004; Eggleston and Pope, 2013).

Methods of Investigation

Withdrawal rates and allied information were compiled 

on 612 groundwater-production wells in the Virginia 

Coastal Plain and regulated by the VA DEQ during 2013 

(attachment 1). To facilitate integration with the VA DEQ 

Groundwater Withdrawal Permit Program, production 

wells are identified by VA DEQ numbers. Maximum 

permitted withdrawal rates were obtained from the VA DEQ 

groundwater-withdrawal permitting database. Reported rates 

of actual withdrawals during 2013 were obtained from the 

groundwater-withdrawal permitting database and the VA DEQ 

Virginia Water Use Data System. Not all wells have both 

permitted and reported withdrawal rates. Withdrawal-rate data 

were cross-referenced to production-well location and screeninterval altitude data previously compiled for model simulations of groundwater flow. Aquifers in which the production 

wells are completed were determined from the hydrogeologic 

framework of McFarland and Bruce (2006) following the 

methodology described by McFarland (2010). Withdrawals 

are made by one or more of the production wells from all 

currently (2015) recognized aquifers of the Virginia Coastal 
8 A Conceptual Framework and Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia

SUSSEX

CAROLINE

HANOVER

DINWIDDIE

Suffolk

SURRY

ESSEX

SOUTHAMPTON

CHESTERFIELD

SPOTSYLVANIA

HENRICO

Chesapeake

KING & QUEEN

YORK

ISLE OF 

WIGHT

GREENSVILLE

NEW KENT

KING WILLIAM

RICHMOND

PRINCE GEORGE

GLOUCESTER

CHARLES CITY

JAMES

CITY

MIDDLESEX

LANCASTER

Norfolk

NORTHAMPTON

MATHEWS

Richmond

Hampton

Newport News

Petersburg

Poquoson

Hopewell

Franklin

Colonial

Heights

Fredericksburg

Virginia

Beach Emporia

Portsmouth

Williamsburg

l l l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l l l l l l l l l

l l l l l l l l l l

l l l l l l l l l l l l l l l l

l l l l l l l l l

0

–25

–25

25

–50

–50

–25

–50

0

–75

–75

–75

75

50

–100

100

125

–125

–150

–150

100

75

125

0

–100

50

25

50

75

–25

–50

100

–125

200

500

1,000 10,000

20,000

500

1,000

200

77°W 76°W

38°N

37°N

Chesapeake Bay

impact crater

Base modified from U.S. Geological Survey

State of Virginia, 1:500,000, 1973

ATLANTIC

OCEAN

CHESAPEAKE

BAY

NORTH CAROLINA

VIRGINIA

MARYLAND

Potomac River

James River

York River

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

River

VIRGINIA

EASTERN

SHORE

SOUTHEASTERN VIRGINIA

AQUIFER MARGIN

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

STAFFORD

Rappahannock

EXPLANATION

Simulated hydraulic head contour—Shows 

 altitude of hydraulic head in Potomac aquifer.

 Hachures indicate depression. Contour 

 interval 25 feet. Datum is NGVD 29

Line of groundwater total ion concentration—

 Interval is variable, in milligrams per liter

25

WESTMORELAND

200

NORTHUMBERLAND

KING GEORGE

West

Point

Figure 3. Simulated hydraulic head during 2003 and groundwater total ion concentration in the Potomac aquifer. 

Simulated hydraulic head contours modified from Heywood and Pope (2009). Total ion concentration contours 

modified from McFarland (2010). Location of Chesapeake Bay impact crater from Powars and Bruce (1999).
Introduction 9

Plain hydrogeologic framework (McFarland and Bruce, 2006), 

except the Saint Marys and Peedee aquifers, which are not 

developed for water supplies.

Proximity of all the production wells to saltwater was 

determined by a series of geographic information system 

(GIS) spatial analyses using groundwater 250-mg/L and 

1,000-mg/L chloride iso-concentration surfaces previously 

mapped by McFarland (2010). Approximate configurations 

of the chloride surfaces are represented in the GIS by raster 

datasets and surface-altitude contour lines. The shapes of the 

two surfaces are about parallel but the 1,000-mg/L surface 

is positioned deeper. Production-well locations and related 

data were added to the GIS as georeferenced points. For these 

analyses, the bottom of the screened interval was considered 

to be the open part of each well in closest proximity to the 

chloride surfaces.

Operations were initially used in the GIS to identify the 

regionally estimated altitudes of the 250-mg/L and 1,000-mg/L 

chloride surfaces at each production-well location (attachment 1). The vertical position of the bottom of each well’s 

screened interval was subsequently calculated relative to 

the chloride-surface altitudes. Positions below the chloride 

surfaces were assigned positive values, which designate the 

well as having penetrated the surfaces. Positions above the 

chloride surfaces were assigned negative values to designate 

no penetration. No data were generated for wells beyond the 

mapped extents of the chloride surfaces, for which cells are 

left blank in attachment 1. Production wells are categorized 

based on their vertical positions relative to the chloride 

surfaces, and are color referenced to figures 1, 7, and 8, and 

plate 1. Of the total 612 production wells, 81 wells in the 

Yorktown-Eastover and Potomac aquifers are positioned most 

closely to the 250-mg/L chloride iso-concentration surface. 

Within each category, production wells are listed in order 

of priority for monitoring based on vertical proximity to the 

chloride surfaces (attachment 1).

Lateral distances and directions of production wells to 

underlying chloride surfaces also were determined (attachment 1). Lateral distances and directions for wells above 

the 250-mg/L surface are relative to the 250-mg/L surface, 

and for wells below the 250-mg/L surface are relative to the 

1,000-mg/L surface. The wells were classified based on the 

altitude of the bottom of each well’s screened interval in 

closest vertical proximity to each chloride-surface altitude 

contour line. GIS operations were then used to calculate lateral 

distance and direction from the bottom of each well to the 

corresponding contour line. Direction is specified as degrees 

from east (fig. 4). With east being zero degrees, directions 

oriented counterclockwise from east are designated by positive 

values, and clockwise from east by negative values. GIS 

operations cannot calculate lateral distances and directions 

for some wells having screened-interval bottoms positioned 

above the maximum regional altitude of the 250-mg/L 

chloride surface. Instead, for wells within 50 ft vertically of 

Production well

Observation well

EAST

LongitudeP

LatitudeP

LongitudeO=

LongitudeP

+(R×cosine(θ)/(111,320×cosine(LatitudeP))

LatitudeO=

LatitudeP+(R×sine(θ)/110,540)

Positive

Negative

θ

Increasing salinity

R (meters)

Figure 4. Projection of lateral intrusion observation-well location from groundwater-production well. Based on lateral 

directions to chloride surfaces, the observation well is positioned in the most proximal direction to saltwater (at the angle θ

from east) at a distance R of 1 mile (1,609 meters) from the production well. LatitudeO and longitudeO of the observation 

well is calculated from latitudeP and longitudeP of the production well by formulas that apply trigonometric functions to the 

directions to saltwater. Formula coefficients account for variable distances spanned per unit of latitude and longitude that 

result from the curvature of the Earth’s surface.
10 A Conceptual Framework and Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia

the 250-mg/L chloride surface, lateral directions to the surface 

were manually estimated. Lateral distances and directions also 

were not calculated for wells below the 1,000-mg/L surface.

Lastly, locations for observation wells to monitor lateral 

saltwater intrusion were projected from the 81 production 

wells in the Yorktown-Eastover and Potomac aquifers 

that are positioned most closely to the 250-mg/L chloride 

iso-concentration surface (attachment 1). Observation-well 

locations are based on lateral directions to the 250-mg/L and 

1,000-mg/L chloride surfaces. Latitudes and longitudes of 

the observation wells were calculated at a distance of 1 mi 

from the latitudes and longitudes of the production wells by 

formulas that apply trigonometric functions to the directions 

to chloride surfaces (fig. 4). Formula coefficients account for 

variable distances spanned per unit of latitude and longitude 

that result from the curvature of the Earth’s surface.

Conceptual Framework of Saltwater 

in the Coastal Plain Aquifer System 

of Virginia

Withdrawal-induced movement of saltwater in the 

Virginia Coastal Plain aquifer system is complex (see 

“Introduction” section). An adequate understanding of the 

presence and movement of saltwater is therefore fundamental 

to effective monitoring. Generalized descriptions of conditions, processes, and relations of saltwater to groundwater 

withdrawal are synthesized below from previous studies.

 The most current (2015) and comprehensive hydrochemical description of the Virginia Coastal Plain aquifer 

system is presented by McFarland (2010). Water having 

chloride concentrations below the 250-mg/L SMCL is 

commonly referred to as “fresh” and is present throughout 

much of the aquifer system. As exemplified by the Potomac 

aquifer (fig. 3), groundwater exhibits a marked eastward 

increase in total ion concentration along the western outer 

margin of the Chesapeake Bay impact crater. Here, chloride 

supersedes bicarbonate as the dominant anion in groundwater. 

Chloride concentrations between freshwater and that of 

seawater (19,000 mg/L) (Hem, 1985) are referred to as 

“brackish,” and are present in the eastern part of the aquifer 

system. Offshore, seawater occupies sediments beneath 

the Atlantic Ocean. Chloride concentrations above that 

of seawater constitute “brine” and have been observed in 

some sediments that fill the Chesapeake Bay impact crater 

(McFarland and Bruce, 2005; McFarland, 2010; Sanford 

and others, 2009, 2013).

The Saltwater-Transition Zone

In coastal groundwater systems, fresh groundwater is 

separated from underlying and denser saltwater by a downward and landward sloping boundary termed the saltwatertransition zone (Ghyben, 1888; Herzberg, 1901; Henry, 1960; 

Barlow, 2003). The saltwater-transition zone encompasses a 

volume of sediments within which dispersive mixing takes 

place across a density gradient between freshwater and 

saltwater (fig. 5).

The saltwater-transition zone is a primary regional 

boundary on groundwater flow in the Virginia Coastal Plain 

(fig. 2). 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 reaching dense saltwater along the coast, 

groundwater leaks back upward to discharge to the Atlantic 

Ocean, primary rivers, and Chesapeake Bay.

Configuration

The configuration of the saltwater-transition zone in the 

Virginia Coastal Plain is complex. A series of sub-parallel 

groundwater chloride iso-concentration surfaces—ranging 

from 50 mg/L to 19,000 mg/L—were mapped from chloride 

concentrations in 6,650 widely spaced samples of groundwater 

collected during 1906–2007 (McFarland, 2010). The chloride 

surfaces collectively represent the saltwater-transition zone, 

and indicate that groundwater chloride concentrations broadly 

increase eastward and downward.

As exemplified by the 250-mg/L chloride iso-concentration surface (fig. 1, pl. 1), the saltwater-transition zone 

in Virginia consists of two distinct components. Across the 

east-central part of the Virginia Coastal Plain, the saltwatertransition zone exhibits a warped and steeply mounded 

dome shape centered on the western to southwestern side 

of the Chesapeake Bay impact crater. A westward plunging 

spur ridge projects from the mound beneath the York-James 

Peninsula. Flank areas of the mound and ridge evenly 

slope northwestward beneath the Middle Peninsula and 

Northern Neck, and southwestward beneath Surry and Isle of 

Wight Counties.

Secondly, across the southeastern Virginia Coastal Plain, 

the saltwater-transition zone exhibits a nearly level plateau 

shape at shallow depth beneath the city of Virginia Beach, 

Va., and eastern city of Chesapeake, Va. (fig. 1, pl. 1). The 

landward margin of the plateau is irregularly shaped and 

slopes steeply downward to the west beneath the western 

city of Chesapeake, Va., and eastern city of Suffolk, Va. The 

plateau is also separated from the mound to the north by a 

southwestward plunging trough, trending from the southern 

tip of the Virginia Eastern Shore across the lowermost 

part of Chesapeake Bay and the cities of Norfolk, Va., and 

Portsmouth, Va.

Formation

Geologic and hydrochemical evidence indicates that 

much of the saltwater in the Virginia Coastal Plain aquifer 

system originated from seawater (McFarland and Bruce, 2005; 

McFarland, 2010) rather than earlier cited saltwater sources 

such as evaporate dissolution (Manheim and Horn, 1968; 
Conceptual Framework of Saltwater in the Coastal Plain Aquifer System of Virginia 11

Production

well

Production

well

Intermittent

localized

decline

Intermittent

localized

decline

Regional hydraulic head

Regional hydraulic head

Intermittent

vertical

upconing

Intermittent

vertical

upconing

Sustained

lateral intrusion

Sustained

lateral intrusion

Aquifer Overlying sediments Aquifer Overlying sediments

Freshwater

Screened interval

Screened interval

Dispersive

mixing

Dispersive

mixing

A

Freshwater

B

Saltwater

Saltwater

Regionwide

decline

NOT TO SCALE

Meisler, 1989) or membrane filtration (Larson, 1981; Powars 

and Bruce, 1999). In addition, brine as much as twice the 

concentration of seawater has been found thousands of feet 

deep within the center of the Chesapeake Bay impact crater. 

The brine has been alternately attributed to impact-induced 

hydrothermal boiling (Sanford, 2003, 2005), exhumation of 

pre-impact sediments and (or) basement bedrock (Sanford and 

others, 2009), or partly land-locked Cretaceous-age basins 

(Sanford and others, 2013).

Seawater has been repeatedly emplaced within the aquifer 

sediments during cycles of submersion by the Atlantic Ocean 

during the past about 65 million years (McFarland, 2010). 

During emergent periods, eastward and downward flowing 

fresh groundwater flushed seawater from the sediments. Sea 

level had declined to the edge of the Continental Shelf during 

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

ago (Peltier, 1994; Bradley, 1999), when fresh groundwater 

likely flushed seawater from most of the sediments. Flushing 

Figure 5. Movement of saltwater. A, Fresh groundwater is separated from underlying dense saltwater 

by the landward sloping saltwater-transition zone. Dispersive mixing takes place across a density 

gradient. Sustained lateral intrusion takes place with horizontal displacement across the regional 

hydraulic gradient. Intermittent upconing results directly beneath the pumping well from a density 

adjustment to localized water-level decline. B, Saltwater eventually shifts laterally landward during 

several decades. Upconing continues from farther into the saltwater-transition zone.
12 A Conceptual Framework and Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia

was impeded, however, in low hydraulic conductivity 

sediments filling the Chesapeake Bay impact crater where 

seawater has been retained. Sea level has since risen to its 

current position and modern seawater is reentering submerged 

sediments. Flushing by fresh groundwater also continues, 

however, through sediments that are still emerged.

Before discovery of the Chesapeake Bay impact crater, 

the unusual configuration of the saltwater-transition zone in 

Virginia was recognized more than 100 years ago (Sanford, 

1913). In addition, differential flushing of saltwater by fresh 

groundwater around a structural depression was theorized 

more than 70 years ago (Cederstrom, 1943). As exemplified 

by the 250-mg/L chloride iso-concentration surface (fig. 1, 

pl. 1), generally eastward flow of fresh groundwater is divided 

by the spur ridge that extends westward of the chloride mound 

beneath the York-James Peninsula. Seawater has probably 

been preferentially flushed around the Chesapeake Bay impact 

crater, either northeastward beneath the Middle Peninsula 

and Northern Neck or southeastward beneath Southeastern 

Virginia. Additionally, at several locations the broadly 

downward increasing chloride-concentration trend is locally 

reversed across distinct vertical intervals (McFarland, 2010, 

pls. 10, 12, and 13). These inverted-concentration intervals 

have been correlated to infer continuous zones through which 

seawater has been flushed. Absence of any inverted-concentration intervals at the center of the impact crater (Sanford and 

others, 2009, 2013) indicates that flushing has not penetrated 

the center within which brine is restricted.

Saltwater Movement

Groundwater withdrawal has redirected the hydraulic 

gradient landward across the Virginia Coastal Plain (see 

“Groundwater Conditions” section). The resulting potential for 

region-wide saltwater movement toward withdrawal centers 

has been recognized for more than 40 years (Lichtler and Wait, 

1974). The region-wide saltwater-transition zone, however, has 

remained in about the same position for more than 100 years 

(Sanford, 1913). Fresh groundwater has also been determined 

to reside in the sediments for tens of thousands of years (Nelms 

and others, 2003). Based on these considerations, withdrawn 

groundwater is apparently released primarily from aquifer 

storage rather than displaced across regionally appreciable 

distances. As a result, the position and configuration of the 

saltwater-transition zone still reflects pre-pumping coastward 

flushing of saltwater by fresh groundwater, and region-wide 

reconfiguration in response to groundwater withdrawal could 

possibly take centuries (McFarland, 2010).

Despite the apparent lack of region-wide movement of 

the saltwater-transition zone, changes in groundwater chloride 

concentration have been observed among individual wells 

(McFarland, 2010). Chloride concentrations have also been 

observed to increase in some newly pumped wells until their 

water levels stabilize (Cederstrom, 1943).

Upconing and Lateral Intrusion

Withdrawal-induced horizontal movement of saltwater 

is referred to herein as “lateral intrusion”, and is distinct 

from upward vertical movement referred to as “upconing” 

(fig. 5). Lateral intrusion takes place with displacement of 

groundwater across a horizontal distance toward a pumping 

well. By contrast, upconing results from a density adjustment 

to a relatively small volume of water across a shorter vertical 

distance centered directly beneath the pumping well.

Upconing is generally regarded as the more common 

cause of changes in groundwater chloride concentration than 

lateral intrusion (Heath, 1987). Fresh groundwater in effect 

“floats” in dynamic equilibrium on more dense saltwater 

beneath (fig. 5). A 1-ft decline in freshwater head theoretically 

results in a localized rise in saltwater of 40 ft (Ghyben, 1888; 

Herzberg, 1901) and recovery of the freshwater head results 

in a proportional deepening of saltwater. Dispersion, sediment 

heterogeneities, and flowing conditions, however, complicate 

this relation (Heath, 1987). Deepening of saltwater can lag 

behind recovery of freshwater head by weeks to months.

A general prevalence of fluctuating groundwater chloride 

concentrations in the Virginia Coastal Plain (see ”Historical 

Changes in Groundwater Chloride Concentration” section) has 

been attributed to numerous instances of intermittent localized 

upconing that reverse with a decrease or cessation of withdrawal (McFarland, 2010). Many of the large groundwater 

withdrawals in the Virginia Coastal Plain are intermittent 

in response to cyclical water demands. Water-level declines 

and recoveries at production wells possibly result in localized chloride-concentration increases and decreases within 

weeks to months of changes in pumping. Where multiple 

neighboring production wells are operated at different times, 

complex hydraulic interactions can potentially cause concentration fluctuations to evolve dynamically (see “Chloride 

Concentration Trends” section).

Lateral intrusion probably occurs concomitantly with 

upconing in the Virginia Coastal Plain but more slowly. A 

production well’s water-level decline may result in localized 

upconing, but is also superimposed on the regional hydraulic 

gradient (fig. 5A). A potential for lateral intrusion is created 

partly by the individual well, but also by the additive effect 

of all groundwater withdrawals throughout the aquifer 

system and contributing to the regional hydraulic gradient. 

Detailed long-term monitoring in the Coastal Plain of New 

Jersey during 1940–90 indicated that lateral intrusion across 

distances of about 1 mi would take from 24 to 60 years 

(Lacombe and Carleton, 2002). In the Virginia Coastal 

Plain, based on groundwater ages (Nelms and others, 2003), 

corresponding lateral flow velocities (McFarland, 2013), 

and the current (2015) regional hydraulic gradient, lateral 

intrusion of 1 mi probably takes several decades or more. As 

saltwater slowly shifts laterally landward (fig. 5B), upconing 

to the production well may take place from farther within the 

saltwater-transition zone. As a result, localized groundwater 
Conceptual Framework of Saltwater in the Coastal Plain Aquifer System of Virginia 13

chloride concentration fluctuations produced by upconing at 

many production wells are possibly superimposed on a slower, 

but more widespread and sustained concentration increase 

produced by lateral intrusion.

As long as the regional hydraulic gradient is maintained 

in a landward direction, the potential for lateral intrusion will 

persist. Lateral intrusion may be slow by human standards 

and have a less immediate effect than upconing. In contrast 

to the reversibility of upconing, however, lateral intrusion 

potentially results in a permanent reposition of the saltwatertransition zone.

Historical Changes in Groundwater 

Chloride Concentration

A historical record of 6,650 groundwater chemicalquality samples collected during 1906-2007 formed the basis 

for regional delineation of the saltwater-transition zone in 

Virginia (McFarland, 2010). The samples were collected at 

different times by various entities to achieve diverse objectives. As a result, most wells were not optimally located, 

constructed, or sampled for monitoring the movement of 

saltwater. Nonetheless, changes in groundwater chloride 

concentration were observed among a minority of individual 

wells, indicating at least localized saltwater movement in some 

form. Increases in concentration generally coincided with 

decreases in concentration, however, and neither increases 

nor decreases became larger or more frequent as groundwater 

withdrawal steadily increased during the past century. Neither 

was any consistent landward trend of increasing concentrations observed.

Only 10 wells have been identified among the historical 

data that were sampled an adequate number of times to 

determine site-specific trends in groundwater chloride concentration during periods of several years or more (McFarland, 

2010). Only one shallow well had a substantial sustained 

increase in concentration (from less than 500 mg/L to about 

3,500 mg/L during a 10-year period), which was attributed 

to localized lateral intrusion of brackish river water from the 

nearby tidal James River. Elsewhere, another deeper well had 

a small gradual concentration increase (from about 2,100 mg/L 

to 2,700 mg/L during a 20-year period), which was attributed 

to regional lateral intrusion of the saltwater-transition zone. 

The other 8 wells had either nearly constant concentrations 

varying by less than 100 mg/L, or intermittent fluctuations 

as much as several hundred mg/L that were attributed to 

localized upconing.

Chloride-Concentration Trends

On the basis of the processes and historical conditions 

documented by previous studies described above, possible 

general trends in groundwater chloride concentration near 

and within the saltwater-transition zone are theorized below. 

In proximity to active groundwater-production wells, 

vertical upconing coupled with lateral intrusion can result 

in a composite chloride-concentration trend (fig. 6). During 

periodic cycles of upconing, concentrations can increase from 

and decrease to a baseline concentration reflecting the regional 

position of the saltwater-transition zone. Based on historical 

groundwater chloride concentrations, some upconing cycles 

have lasted from several months to 2 years and spanned 

concentrations as much as several hundred mg/L (McFarland, 

2010). Lateral intrusion can also gradually increase the 

baseline concentration, but probably during a period of years 

to decades and by a small amount relative to concentration 

ranges spanned by upconing cycles.

Magnitudes, timing, and durations of upconing cycles 

are probably controlled by corresponding withdrawals and 

resulting water-level fluctuations, and may vary widely 

in response to changing water demands (fig. 6). Chloride 

concentrations during dormant periods without withdrawal 

may be unaffected by upconing and remain at the baseline 

level, varying only slightly as a result of lateral intrusion, 

differences in sampling conditions, or both. Conversely, 

prolonged withdrawal and water-level decline may maintain 

increased concentrations, and closely timed periods of 

withdrawal may interrupt full water-level recovery and prevent 

return of concentrations to baseline values that requires 

weeks to months.

Where a production well does not hydraulically interact 

directly with other production wells, fluctuations in groundwater chloride concentration may correspond directly with 

withdrawal and water-level decline at the single well. At many 

locations in the Virginia Coastal Plain, however, multiple 

production wells are in proximity, and some production 

facilities operate collocated wells. A complex local history of 

hydraulic interaction can potentially result from continually 

varying arrays of neighboring production wells taken in and 

out of service over time. Concentration fluctuations in some 

areas may be a dynamically evolving composite produced by 

the timing of pumping among multiple neighboring wells. 

Some neighboring production wells may also lack complete 

information or be wholly unknown. As a result, cause-andeffect relations between concentration fluctuations and 

withdrawals among multiple neighboring production wells 

can be difficult to discern.

In contrast to the above, groundwater chloride concentrations can be relatively constant under other conditions. 

Locations outside the immediate influence of any individual 

withdrawals may be unaffected by upconing, and chloride 

concentrations possibly vary only slightly around the baseline 

level reflecting the position of the saltwater-transition zone. 

Alternately, at locations where withdrawal is kept constant 

during multiple years, persistent upconing may be maintained 

without any intervening recovery or dormant periods.
14 A Conceptual Framework and Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia

Years

Chloride concentration

Lateral intrusion baseline 

Upconing

cycle 

Partial

return

Withdrawal

Dormant period 

Prolonged

withdrawal 

False trend 

Infrequent

sample 

Frequent

sample 

Water level

Partial

recovery 

Regional decline

Local fluctuation 

Brief

cessation 

Increasing Increasing

Monitoring Strategy for Movement of 

Saltwater in the Coastal Plain Aquifer 

System of Virginia

A strategy to monitor the movement of saltwater in the 

Virginia Coastal Plain aquifer system was developed from 

an understanding of conditions, processes, and relations 

of saltwater to groundwater withdrawal (see “Conceptual 

Framework” section). On the basis of spatial relations between 

the saltwater-transition zone and 612 regulated groundwaterproduction wells, 81 production wells were identified from 

which chloride concentrations are most likely to increase 

above the 250-mg/L SMCL. Potential locations of observation 

wells to monitor saltwater movement were determined to 

distinguish vertical upconing from lateral intrusion among 

individual production wells. For effective strategy implementation, observation-well construction and development, 

Figure 6. Generalized temporal 

relations among groundwater 

chloride concentrations, water 

levels, and withdrawals. Units 

are arbitrary. Vertical upconing 

is coupled with lateral intrusion 

to result in a composite chlorideconcentration trend. During 

upconing cycles, concentrations 

increase from and decrease to a 

baseline concentration reflecting 

the regional position of the 

saltwater-transition zone. Lateral 

intrusion gradually increases 

the baseline concentration. 

Upconing cycles are controlled 

by withdrawals and water-level 

fluctuations. During dormant 

periods, chloride concentrations 

can remain at the baseline 

level. Conversely, prolonged 

withdrawal and water-level 

decline can maintain increased 

concentrations. Closely timed 

withdrawals can interrupt 

water-level recovery and return 

of concentrations to baseline 

values. Frequent samples (black 

dots) distinguish upconing 

from lateral intrusion, whereas 

infrequent samples (white dots) 

can produce a false chlorideconcentration trend.

and groundwater-sample collection technique, frequency, 

and analysis must be tailored to monitoring the movement 

of saltwater in the Virginia Coastal Plain aquifer system. 

Limitations and constraints on monitoring saltwater movement 

must also be recognized.

Proximity of Groundwater-Production Wells

to Saltwater

To provide a basis for monitoring saltwater movement, 

spatial relations between the saltwater-transition zone and 

regulated groundwater-production wells were determined. 

GIS analyses were completed on 612 production wells for 

which the VA DEQ provided actual and maximum permitted 

withdrawal rates for 2013, and well location and screeninterval altitude data (see “Methods of Investigation” section). 

Withdrawals are made by one or more of the production wells 

from all currently (2015) recognized aquifers of the Virginia 
Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia 15

Coastal Plain hydrogeologic framework (McFarland and 

Bruce, 2006) except the Saint Marys and Peedee aquifers, 

which are not developed for water supplies. The vertical 

position of the bottom of each well’s screened interval, and 

its lateral distance and direction, were determined relative to 

previously mapped groundwater 250-mg/L and 1,000-mg/L 

chloride iso-concentration surfaces (McFarland, 2010). 

Screened-interval bottom vertical positions, and lateral 

distances and directions to the chloride surfaces are included 

with compiled production-well withdrawal rates and allied 

information (attachment 1).

In order to identify production wells from which chloride 

concentrations are most likely to increase above the 250-mg/L 

SMCL, the production wells are categorized relative to the 

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

Categories are based on vertical and lateral distances of the 

bottoms of well screened intervals to the chloride surfaces 

(figs. 1 and 7, pl. 1). Also as a means to prioritize relative 

needs for monitoring, the wells are listed within each category 

in order of vertical proximity to the 250-mg/L chloride surface 

(attachment 1).

Screened-interval bottoms of 495 of the 612 production 

wells (colored blue, figs. 1 and 7, pl. 1) are either more than 

50 ft above and 1 mi laterally from the 250-mg/L chloride 

surface, or are beyond the mapped extent of the surface. These 

wells are screened in several different aquifers and located 

across most of the Virginia Coastal Plain. Within the accuracy 

of the chloride surfaces (see “Limitations and Constraints” 

section), they are considered to be farthest from saltwater and 

least likely to increase above the 250-mg/L chloride SMCL.

Other production wells that are closer to or below the 

250-mg/L chloride iso-concentration surface are more likely to 

increase above the 250-mg/L SMCL. Among these, screenedinterval bottoms of 5 wells are more than 50 ft above but 

NOT TO SCALE

A

B

F

About

100 FEET About 5 MILES

NORTH

1,000-milligrams-per-liter

chloride iso-concentration surface

250-milligrams-per-liter

chloride iso-concentration surface

Production

wells

C G E D

Figure 7. Generalized spatial relations of groundwater-production wells to chloride iso-concentration surfaces. 

Wells are color referenced to figures 1 and 8, plate 1, and Attachment 1. A, Lateral directions to a chloride 

surface vary among well locations. B, Lateral distances to a chloride surface at a single location varies with well 

depth. C, Where a chloride surface is steeply inclined, a well can be more than 50 feet above but less than 1 mile 

laterally from the surface. D and E, The lateral direction to saltwater is relative to the 1,000-milligrams-per-liter 

chloride surface for wells below the 250-milligrams-per-liter chloride surface. F, Lateral directions to saltwater 

were manually estimated for wells that are shallower than the highest altitude of the chloride surface. G, Lateral 

distances and directions to saltwater were not determined for wells below the 1,000-milligrams-per-liter surface.
16 A Conceptual Framework and Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia

less than 1 mi laterally from the 250-mg/L chloride surface 

(colored green, figs. 1 and 7, pl. 1). These wells are all in the 

Potomac aquifer and arrayed along the flanks of the mounded 

part of the saltwater-transition zone where the 250-mg/L 

surface is steep. Screened-interval bottoms of 54 additional 

wells (colored yellow) are within 50 ft above the 250-mg/L 

surface. These wells include 45 in the Yorktown-Eastover 

aquifer (square symbols), which are mostly positioned across 

the plateau part of the saltwater-transition zone far to the 

southeast, along with one well over the salinity mound at the 

southern tip of Mathews County, Va. The other 9 wells are 

in the Potomac aquifer (circle symbols), and are positioned 

either at the westward margin of the plateau or along the 

flanks of the mound. Chloride concentrations in wells within 

50 ft above or 1 mi laterally from the 250-mg/L surface may 

be close to the 250-mg/L SMCL and likely to increase above 

it depending on (1) the accuracy of the chloride surfaces, 

(2) aquifer hydraulic conductivities, (3) well withdrawal rates, 

and (4) the lengths and vertical positions of well screened 

intervals intercepting overlying fresh groundwater (see 

“Limitations and Constraints” section).

Screened-interval bottoms of 22 production wells are 

within 50 ft below the 250-mg/L chloride iso-concentration 

surface (colored orange, figs. 1 and 7, pl. 1). These wells 

include 12 in the Yorktown-Eastover aquifer (square symbols) 

positioned across the salinity plateau, and 10 in the Potomac 

aquifer (circle symbols) positioned either at the westward 

margin of the plateau or along the flanks of the salinity 

mound. Chloride concentrations in these wells may be close 

to the 250-mg/L-SMCL or have already marginally exceeded 

it depending on (1) the accuracy of the chloride surfaces, 

(2) aquifer hydraulic conductivities, (3) well withdrawal rates, 

and (4) the lengths and vertical positions of screened intervals 

intercepting overlying fresh groundwater (see “Limitations 

and Constraints” section).

Screened-interval bottoms of another 24 production wells 

are more than 50 ft below the 250-mg/L chloride surface but 

are above the 1,000-mg/L surface (colored red, figs. 1 and 7, 

pl. 1). These wells are all in the Potomac aquifer and positioned either at the westward margin of the salinity plateau or 

along the flanks of the salinity mound. Chloride concentrations 

in these wells probably have already exceeded the 250-mg/L 

SMCL, depending on (1) the accuracy of the chloride surfaces, 

(2) aquifer hydraulic conductivities, (3) well withdrawal rates, 

and (4) the lengths and vertical positions of screened intervals 

intercepting overlying fresh groundwater (see “Limitations 

and Constraints” section).

Lastly, screened-interval bottoms of 12 production wells 

are below the 1,000-mg/L surface at widely scattered locations 

in James City County, Va. and the cities of Newport News, 

Va., Portsmouth, Va., and Chesapeake, Va. (colored black, 

figs. 1 and 7, pl. 1). These wells are all in the Potomac aquifer 

and constitute parts of primary groundwater-production 

facilities that withdraw and treat brackish groundwater from 

far within the saltwater-transition zone.

Several spatial relations are apparent between production wells and the chloride surfaces. Wells arrayed along the 

flanks of the salinity mound can have widely varying lateral 

directions to saltwater (fig. 7A). Lateral and vertical proximity 

to chloride surfaces at any location can differ substantially 

depending on well depth (fig. 7B). Where chloride surfaces 

are steep, lateral distance can be relatively short compared 

to vertical distance (fig. 7C). The lateral direction for wells 

positioned below the 250-mg/L chloride surface is relative 

to the underlying 1,000-mg/L chloride surface (figs. 7D and 

7E). Where the 250-mg/L chloride surface is at a low angle, 

lateral directions were manually estimated for wells with 

screen-interval bottoms positioned above the highest regional 

altitude of the chloride surface (fig. 7F) (see “Methods of 

Investigation” section).

Observation Wells

Potential locations of observation wells to monitor saltwater movement were determined. Based on spatial analyses 

of 612 regulated groundwater-production wells (see “Proximity 

of Groundwater-Production Wells to Saltwater” section), 

81 production wells in the Yorktown-Eastover and Potomac 

aquifers were identified as having screened-interval bottoms 

positioned most closely to the 250-mg/L chloride isoconcentration surface (table 1, in back of report). Chloride 

concentrations in these production wells are the most likely 

to increase above the 250-mg/L SMCL depending on (1) the 

accuracy of the chloride surfaces, (2) aquifer hydraulic 

conductivities, (3) well withdrawal rates, and (4) the lengths 

and vertical positions of screened intervals intercepting 

overlying fresh groundwater (see “Limitations and Constraints” 

section). As a means to prioritize relative needs for monitoring, 

the production wells are listed in order of vertical proximity to 

the 250-mg/L chloride surface. Conditions at and near some 

production wells may constrain the construction of observation 

wells (see “Limitations and Constraints” section).

Of the 81 production wells, 5 have screened-interval 

bottoms positioned more than 50 ft above but less than 

1 mi laterally from the 250-mg/L chloride surface (colored 

green, figs. 1 and 7, pl. 1). Screened-interval bottoms of 

an additional 54 wells are within 50 ft above the 250-mg/L 

chloride surface (colored yellow), and 22 wells are within 

50 ft below the 250-mg/L chloride surface (colored orange). 

Of the 81 production wells, 57 are completed in the YorktownEastover aquifer (square symbols) and 24 in the Potomac 

aquifer (circle symbols). Locations of the production wells 

span the mounded part of the saltwater-transition zone across 

the east-central Virginia Coastal Plain, and the plateau part of 

the saltwater-transition zone far to the southeast.

Potential locations for observation wells to monitor 

saltwater movement were not determined for 495 production 

wells either beyond the mapped extent of the 250-mg/L 

chloride surface or having screened-interval bottoms more 

than 50 ft above and 1 mi laterally from the surface (colored 
Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia 17

blue, figs. 1 and 7, pl. 1). Chloride concentrations in these 

wells are least likely to increase above the 250-mg/L SMCL. 

Neither were observation-well locations determined for 

24 production wells having screened-interval bottoms more 

than 50 ft below the 250-mg/L chloride surface (colored red) 

or 12 production wells positioned below the 1,000-mg/L 

chloride surface (colored black) that probably already exceed 

the 250-mg/L SMCL.

Locations 

Potential locations were determined for observation 

wells to monitor both vertical upconing and lateral intrusion 

of saltwater among individual groundwater-production wells 

(see “Saltwater Movement” section). To monitor upconing, 

an observation well is to be collocated with each of the 

81 production wells and completed at about the altitude of the 

250-mg/L chloride iso-concentration surface (fig. 8). Altitudes 

of chloride surfaces at the production wells are included with 

compiled production-well withdrawal rates and allied information (table 1, attachment 1). Care should be used during 

drilling so that boreholes do not vertically deviate to intercept 

and damage nearby production wells (see “Construction of 

Observation Wells” section).

To monitor lateral intrusion, locations for observation 

wells were projected from production wells based on lateral 

directions to underlying chloride surfaces (see “Methods 

of Investigation” section). Latitudes and longitudes were 

calculated at a distance of 1 mi from the production wells, and 

are included with the compiled production-well withdrawal 

rates and allied information (table 1, attachment 1).

The locations of lateral-intrusion observation wells 

are projected in the nearest directions to saltwater from the 

production wells, within the accuracy of the chloride isoconcentration surfaces (see “Limitations and Constraints” 

section). Because a localized water-level decline created 

by each production well is superimposed on the regional 

hydraulic gradient (fig. 5), lateral intrusion to the production 

well is most likely along the projected direction.

The specified distance of 1 mi from production wells to 

lateral-intrusion observation wells is based on (1) an adequate 

separation from the production wells to avoid fluctuations in 

groundwater chloride concentrations resulting from upconing 

and (2) an appropriate time horizon for resource management. 

Documented aquifer tests in the Virginia Coastal Plain that 

included observation wells more than a few hundred feet from 

pumping wells showed no water-level response (McFarland, 

2013). The effect of upconing at production wells is therefore 

presumed to be less than1 mi. The distance traversed by lateral 

intrusion during a period of several decades also is about 

1 mi, based on groundwater ages (Nelms and others, 2003), 

corresponding lateral flow velocities (McFarland, 2013), the 

current (2015) regional hydraulic gradient in the Virginia 

Production

well

Collocated

vertical upconing

observation well

Projected

lateral intrusion

observation well

1 MILE

Screened

interval

Within 50 FEET

250-milligrams-per-liter chloride iso-concentration surface Within 50 FEET

1 MILE

1 MILE

More than

50 FEET

NOT TO SCALE

Figure 8. Generalized spatial relations among groundwater-production wells, observation wells, and the 250-milligrams-per-liter 

chloride iso-concentration surface. Production wells are color referenced to figures 1 and 7, plate 1, and attachment 1. Locations 

are specified for observation wells to monitor both vertical upconing and lateral intrusion among 81 individual groundwaterproduction wells. To monitor upconing, an observation well is collocated with each production well and completed at the altitude 

of the 250-milligrams-per-liter chloride iso-concentration surface. To monitor lateral intrusion, a location for an observation well is 

projected at a distance of 1 mile from each production well, based on lateral directions to underlying chloride surfaces. Chloride 

iso-concentration surface altitudes, and lateral intrusion production-well latitudes and longitudes, are included with productionwell withdrawal rates and allied information (table 1 and attachment 1).
18 A Conceptual Framework and Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia

Coastal Plain, and similar rates of lateral intrusion estimated in 

the New Jersey Coastal Plain (Lacombe and Carleton, 2002). 

For production wells having screened-interval bottoms 

positioned above the 250-mg/L chloride surface (colored green 

and yellow, figs. 7A–C and F), lateral-intrusion observation 

well locations are projected in the direction to the 250-mg/L 

surface. Conversely, for production wells having screenedinterval bottoms positioned below the 250-mg/L chloride 

surface (colored orange, fig. 7D), locations are projected in 

the directions to the underlying 1,000-mg/L chloride surface. 

Because chloride surfaces are sub-parallel, however, directions 

to the two chloride surfaces from an individual production 

well are generally similar.

Construction and Development 

A large body of literature documents numerous considerations to be taken into account in the construction and use 

of observation wells. Considerable knowledge and experience 

also is required to successfully implement complex procedures 

involved in well drilling and collection of groundwater 

chemical-quality samples. The reader is directed to Claassen 

(1982), Lapham and others (1997), and Wilde (1999), among 

the many sources available for a complete discussion. Only 

selected aspects that are of particular importance to monitoring 

saltwater movement in the Virginia Coastal Plain aquifer 

system are summarized below.

During drilling of any observation wells in proximity to 

preexisting wells, care should be used so that boreholes do not 

vertically deviate to the extent that they would intercept and 

damage the preexisting wells. Upon completion, observationwell screened intervals should effectively isolate the parts 

of the aquifer either (1) for upconing observation wells, at 

the altitude where chloride concentrations approximating 

250 mg/L are expected, or (2) for lateral-intrusion observation 

wells, at the same altitude as the bottom of the corresponding 

production well’s screened interval (fig. 8). Observation 

wells should therefore feature single, short screened intervals 

of no more than about 10 ft. Solid casing positioned below 

the screened interval—termed “cleanout” or “tailpipe” and 

commonly used with production wells—should not be used 

with observation wells because it would preclude effective 

purging before collection of groundwater chemical-quality 

samples. The well filter pack also should not extend any 

farther above the screened interval than needed to ensure that 

no grout enters the well. Particular care should be used that 

the annular space above the filter pack be uniformly sealed, 

grouted, or both to prevent overlying water of potentially 

different composition from entering the well.

Drilling of the observation wells will likely cause 

contamination of the aquifer. Because drilling disrupts the 

original structure of aquifer materials, well development 

(air lifting, pumping, backwashing, surging, or a combination of these) is generally used to restore the aquifer at and 

near screened intervals to its original condition. Because 

most aquifer sediments in the Virginia Coastal Plain are 

unconsolidated, hydraulic rotary drilling is most widely used. 

In addition to disruption of sediment structure, hydraulic 

rotary drilling typically introduces large volumes of drilling 

fluid to the borehole that differ in chemical composition from 

groundwater. Alternatively, rotosonic coring can minimize 

sediment disruption, but depending on borehole conditions can 

require introduction of large volumes of water, which may also 

differ in composition from groundwater. By either method, 

groundwater can be substantially altered by contamination 

with drilling fluids or introduced water, and also by contact 

with well construction materials (Claassen, 1982). Unlike 

production wells, however, continuous heavy pumping is 

not usually imposed on observation wells. Development of 

observation wells intended for collection of groundwater 

chemical-quality samples is therefore more critical than for 

production wells, or for observation wells intended solely for 

water-level monitoring or aquifer testing. The need cannot be 

overstated for thorough removal of drilling fluid or introduced 

water to enable collection of groundwater samples that are 

representative of the aquifer.

During hydraulic rotary drilling of the observation 

wells, the composition of drilling fluid should be controlled 

with regard to subsequent effects on well development. Fluid 

composition is controlled during drilling by the addition of 

bentonite and other substances to water circulating through 

the borehole and drilling system. Fluid viscosity is typically 

maintained high enough to stabilize the borehole and otherwise 

mechanically facilitate the drilling process. Although high 

viscosity minimizes fluid loss through the borehole wall, 

viscosity that is too high can heavily clog aquifer sediments 

(Claassen, 1982). As a result, thorough fluid removal during 

well development can become difficult to nearly impossible. 

Conversely, a less viscous fluid may penetrate the aquifer 

farther but can be more easily removed. Considerable knowledge and experience is needed to maintain drilling fluid at the 

optimal viscosity for both minimal sediment penetration during 

drilling and thorough removal during well development. By 

contrast, rotosonic coring is more advantageous than hydraulic 

rotary drilling because only water is introduced to the borehole 

and can be removed more easily than drilling fluid.

The most effective method of development and its 

required duration are likely to vary greatly among the 

observation wells. A record should be kept during drilling 

of the volume of drilling fluid loss or water introduction. 

Because of mixing and dispersion in the aquifer, and contact 

of groundwater with well construction materials, removal of a 

larger volume during well development will likely be needed 

in many cases. Various development techniques and measures 

of completeness are presented by Claassen (1982), Lapham 

and others (1997), and Wilde (1999). A record of pumping 

rates and durations along with monitoring of field parameters 

including turbidity, pH, dissolved oxygen, specific conductance, and temperature, are considered basic documentation. 

Stabilization of field parameters to values representative of the 

aquifer is widely regarded as necessary to achieve adequate 

well development.
Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia 19

Sample Collection 

Following adequate development, groundwater chemicalquality samples can be collected from the observation wells. 

As with well drilling and development, a large body of 

literature—including Claassen (1982), Lapham and others 

(1997), and Wilde (1999)—documents complex procedures 

and numerous considerations to successfully implement 

sample collection. Only selected aspects of particular 

importance to monitoring saltwater movement in the Virginia 

Coastal Plain aquifer system are summarized here.

Because groundwater can be substantially altered by 

contact with well construction materials, purging of the observation wells is needed immediately before each instance of 

sample collection. Stabilization of field parameters (turbidity, 

pH, dissolved oxygen, specific conductance, and temperature) 

after removal of at least 3 well volumes is widely regarded as 

necessary to achieve adequate purging. Performance of the 

well during development—particularly pumping rates and 

associate water-level drawdown—can be used to estimate 

required pumping rates and durations for purging (Lapham 

and others, 1997). A record of field parameters, pumping rate 

and duration, pre-pumping static water level, and stabilized 

pumping water level is considered basic documentation.

Because of the proximity to saltwater, purging of the 

observation wells should incorporate additional measures. 

Downward increasing salinity gradients have been widely 

observed by geophysical logging in unpumped observation 

wells containing saltwater in the Coastal Plain of Virginia. 

Stagnant water filling the well casing is sufficiently dense to 

stratify over time under the effect of gravity. The upper part of 

the casing contains relatively fresh water that is not representative of water in the aquifer and must be removed before 

sample collection. The position of the sampling pump should 

therefore be lowered with the declining water level during 

purging (fig. 9, left). Pump position should be maintained as 

closely as possible directly beneath the water level to ensure 

that water entering the screened interval from below displaces 

all stagnant water upward to the pump before sample collection. Incorrectly setting the pump to a position substantially 

below the stabilized water level throughout purging (fig. 9, 

right) will leave an overlying column of undisplaced stagnant 

water that can mix with water entering the pump during 

collection and alter sample composition.

Additional measures are necessary during purging of the 

observation wells, and subsequently as samples are collected, 

to ensure that the samples are representative of the aquifer. 

Groundwater resides in the confined aquifers of the Virginia 

Coastal Plain for tens of thousands of years (Nelms and 

others, 2003) and commonly contains little or no dissolved 

oxygen. Groundwater composition has resulted from chemical 

Correct

Screened

interval

Incorrect

Static

water

Pump level

Pump

Before

purging

Purging

complete

Before

purging

Purging

complete

Static

water

level

Stabilized

pumping

water level 

Direction

of flow

in well

Stabilized

pumping

water level 

Direction

of flow

in well

Aquifer Overlying sediments

Freshwater

Saltwater

NOT TO SCALE

Figure 9. Pump placement and responses of observation wells to purging. Unpumped observation 

wells in proximity to saltwater widely contain stagnant water that has stratified over time. Fresh water is 

not representative of water in the aquifer and must be removed prior to sample collection. (Left) During 

purging, the sampling pump should be lowered with the declining water level so that water entering the 

screened interval displaces all stagnant water upward to the pump. (Right) Setting the pump below the 

stabilized water level throughout purging will leave overlying undisplaced stagnant water that can mix 

with and alter sample composition.
20 A Conceptual Framework and Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia

equilibria established largely in the absence of oxygen, but 

can be substantially altered by exposure to atmospheric 

oxygen. Pumping equipment should therefore be configured 

to measure field parameters in water kept in isolation from the 

atmosphere, such as using a flow-through chamber (Wilde, 

1999). Chemical equilibria can also be altered by changes in 

temperature. Care should be used to not subject pump hoses 

and other equipment to conditions that would greatly affect 

water temperature, such as exposure to direct sunlight or 

other sources of heat.

Upon completion of purging the observation wells, a 

final set of field-parameter measurements should be recorded 

as groundwater chemical-quality samples are immediately 

collected. Sample containers should be filled in continuum 

with purging while stable flow is maintained (Wilde, 1999). 

Any interruption or change in flow rate can potentially alter 

hydraulic conditions between the aquifer and observation 

well. Water-level recovery or additional decline can induce 

turbulence; increase dissolved oxygen, turbidity, or both; alter 

other field parameters; and otherwise affect sample composition. Optimally, pump hoses can be configured to include a side 

discharge from which sample water can be easily obtained as 

conditions are kept uniform. Sample water should be filtered so 

that only dissolved species are analyzed. A membrane having 

a 0.45-micron pore size is generally adequate for analysis of 

major ions (see “Sample Analysis” section). In-line capsule 

filters avoid any additional handling of sample water during 

collection. Sample water should receive minimal atmospheric 

contact while filling containers and promptly using preservation that follow laboratory protocols. Because carbonate 

species are among those constituents that can be altered by 

atmospheric contact and temperature changes, alkalinity should 

be promptly determined in the field as part of sample collection. Inflection-point titration should be completed because 

fixed endpoint titration can lead to errors (Hem, 1985).

In addition to routine samples as described above, collection of quality-control samples should be considered. Samplecollection and analysis procedures can be evaluated using 

various replicate, spike, or blank samples, or a combination of 

these, but is beyond the scope presented here. Approaches to 

designing a sampling quality-control program are discussed 

by Wilde (1999).

Sampling Frequency

In the broadest terms, groundwater chemical-quality 

samples need to be collected from the observation wells 

frequently enough to accurately characterize the range and 

variability of chloride concentration that results from saltwater 

movement. In proximity to active groundwater-production 

wells, vertical upconing coupled with lateral intrusion can 

result in a composite chloride-concentration trend (see 

“Chloride-Concentration Trends” section). More specifically, 

therefore, sampling frequency needs to enable upconing to be 

distinguished from lateral intrusion for the relative immediacy 

and permanence of saltwater contamination to be assessed (see 

“Upconing and Lateral Intrusion” section).

Optimally efficient sampling of observation wells can 

in theory be timed with withdrawal from their corresponding 

production wells, if responses in chloride concentrations can 

be reliably predicted. Seasonal upconing cycles would require 

relatively frequent sample collection, whereas fewer samples 

may be needed during longer continuously active or dormant 

periods (fig. 6). Relations between observation wells and their 

corresponding production wells cannot be known, however, 

without an established record of cause-and-effect behavior. 

In many areas, sampling frequency also needs to account for 

concentration fluctuations that evolve dynamically in response 

to multiple neighboring production wells that are operated 

at different times, but which may not be entirely known (see 

“Chloride-Concentration Trends” section).

In light of the above—and the seasonal nature of many 

groundwater withdrawals in response to cyclical water 

demands—a sample-collection frequency of four times per 

year should be considered initially for newly established 

observation wells. Upconing cycles can thereby probably be 

identified in most cases. Quarterly sampling would initially 

be used at both upconing and lateral-intrusion observation 

wells. Upconing wells are to be collocated purposefully with 

corresponding production wells so as to exhibit effects of 

upconing (see “Locations” section). Although locations of 

lateral intrusion wells are projected to avoid upconing effects 

from corresponding production wells, other neighboring 

production wells can potentially impose effects that are 

not initially known. Complete records of withdrawals from 

corresponding production wells and neighboring production 

wells are critical to enable cause-and-effect relations with 

chloride-concentration fluctuations to be discerned.

Where upconing is active, sampling too infrequently 

would likely not distinguish individual upconing cycles. 

Depending on the timing, infrequent sampling also has the 

potential to produce a false chloride-concentration trend 

(fig. 6). Adjustments to the quarterly frequency can potentially 

be considered, however, to optimize sampling efficiency 

among individual observation wells. If a strong record of 

cause-and-effect behavior can be established, responses in 

chloride concentrations may be reliably predicted and thereby 

enable sampling to be timed with withdrawal. Future prospects 

for any additional withdrawals that could alter conditions from 

the historical trend must also be considered.

Sample Analysis

Concentrations of chloride are of primary concern with 

regard to movement of saltwater in the Virginia Coastal Plain 

aquifer system (see “Introduction” section). Compared to the 

SMCL of 250 mg/L, some chloride concentrations in groundwater are as much as that of seawater (19,000 mg/L) and greater 

(McFarland, 2010). The potential for chloride contamination 

because of saltwater movement is therefore substantial.
Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia 21

A relation of chloride concentration to specific conductance may also potentially provide a basis for adjustments to 

sample analyses. A Locally Weighted Scatterplot Smoothing 

(LOWESS) regression (Helsel and Hirsch, 2002) indicated 

such a relation among 1,658 historical groundwater samples 

collected in the Virginia Coastal Plain (McFarland, 2010). 

Because specific conductance is a function of total ion 

concentration, and saltwater is dominated by sodium cations 

and chloride anions, specific conductance is directly proportional to chloride concentration. If relations between chloride 

concentration and specific conductance among individual 

observation wells can be demonstrated, specific conductance 

can potentially be considered as a surrogate for chloride 

concentration to monitor saltwater movement at each well 

during a period of years to decades. 

As an alternative to sample collection, specific 

conductance can be continuously monitored in appropriately 

instrumented wells. Determining changes in groundwater 

salinity on this basis, however, is dependent on the hydraulic 

connection between the aquifer and unpumped well. Any trend 

in specific conductance in the well can potentially be obscured 

by stratification of stagnant water (see ”Sample Collection” 

section). As another nonsampling alternative, time-domain 

electromagnetic-induction well logging has been successfully 

used to determine changes in groundwater salinity (Prinos, 

2014). Induction logging has the advantage of detecting 

changes in the bulk conductivity of the aquifer across a 

diameter of as much as several feet centered on the well. Wells 

must be cased only with non-conductive materials, however, 

and logged frequently enough to distinguish upconing cycles. 

In addition to any conductance-based form of monitoring, 

chloride concentrations may still be required by regulatory 

policy to demonstrate adherence to the chloride SMCL.

Limitations and Constraints

Several factors limit and constrain monitoring the 

movement of saltwater in the Virginia Coastal Plain aquifer 

system and must be recognized for effective implementation 

of the strategy presented here. Assessing relative needs for 

monitoring among groundwater-production wells is dependent 

upon the accuracy of groundwater chloride iso-concentration 

surfaces along with variations among aquifer hydraulic 

conductivities, and well withdrawal rates and screenedinterval lengths. Monitoring is further limited by incomplete 

withdrawal information. Construction of observation wells 

is also dependent upon chloride-surface accuracy, and can be 

constrained by the lateral extent of upconing, physical impediments, aquifer depths, and neighboring production wells.

Classification of Production Wells

Out of 612 groundwater-production wells, 81 wells 

are identified from which chloride concentrations are most 

likely to increase above the 250-mg/L chloride SMCL. The 

Chloride is among several ionic species that dominate the 

chemical composition of groundwater in the Virginia Coastal 

Plain. Major cations include calcium, magnesium, sodium, and

potassium. Major anions in addition to chloride include 

bicarbonate, carbonate, and sulfate; and in some areas also 

fluoride and bromide. As exemplified by the Potomac aquifer, 

primarily sodium and bicarbonate are present at small 

concentrations in the western and central parts of the aquifer 

system (McFarland 2010). Eastward across the saltwatertransition zone, however, total ion concentration increases 

markedly (fig. 3) and chloride supersedes bicarbonate as the 

dominant anion.

In light of the above, analysis of all major cations and 

anions should be considered for groundwater samples initially 

collected from newly established observation wells. The 

dominant chemical composition of groundwater at each well 

must be determined to establish its relative position within 

the saltwater-transition zone. Concentrations of all major 

ions also provide a valuable means of data quality control. 

Calculation of sample charge balance is widely used to 

evaluate the analytical accuracy of major-ion concentrations 

(Hem, 1985). For all major ions to be accurately accounted 

for, the sum of cation concentrations should be about equal to 

that of anion concentrations, both in units of milliequivalents 

per liter (meq/L) to account for different electrical charges and 

molecular weights among ions. The percent relative difference 

between cations and anions is calculated as:

 sum of cations (meq/L)– sum of anions (meq/L) ×100 sum of cations (meq/L)+sum of anions (meq/L)

A margin of error of within plus or minus 5 percent is 

generally considered acceptable. Values of greater magnitude 

indicate that either one or more concentration values are 

erroneous, or that one or more unanalyzed ionic species are 

present at substantial concentrations. Several measures are 

necessary during sample collection for major-ion concentrations to result in a correct charge balance and reflect 

chemical equilibria that are representative of the aquifer 

(see “Sample Collection” section).

Assuming that newly established observation wells 

will be sampled initially on a quarterly basis (see “Sampling 

Frequency” section), analysis of all major ions should be 

considered for the first year of sample collection. Adjustments 

to analyses of samples collected from individual wells 

can potentially be considered once the dominant chemical 

composition at each well is established. Chloride is considered 

to be a conservative constituent that generally does not 

undergo chemical reactions in groundwater, which would 

alter its concentration over time. Instead, changes in chloride 

concentration are likely to result primarily from movement of 

saltwater rather than alteration of dominant chemical composition. Analysis of chloride—along with field parameters needed 

to ensure proper sample collection (see “Sample Collection” 

section)—may therefore be adequate to monitor saltwater 

movement during a period of years to decades.
22 A Conceptual Framework and Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia

production wells are classified based on vertical and lateral 

distances to previously mapped (McFarland, 2010) groundwater chloride iso-concentration surfaces (see “Proximity 

of Groundwater-Production Wells to Saltwater” section). 

Potential locations and depths of observation wells are also 

based on depths and directions to the chloride surfaces (see 

”Locations” section). Monitoring is therefore dependent upon 

the accuracy of the chloride surfaces.

The chloride surfaces are based on 6,650 groundwatersample chloride concentrations that originate from diverse 

historical data sources during 1906 –2007 (McFarland, 

2010). Potential data errors include well construction; sample 

collection, preservation, and analysis; typographic mistakes; 

and other reporting problems. The chloride surfaces are also 

interpolated across areas between sample locations and, 

in some instances, extrapolated beyond the area spanned 

by samples. The surfaces were manually generated to be 

consistent with the sample chloride concentrations, but are not 

wholly constrained by them. A general indication is therefore 

represented of different groundwater chloride concentrations 

at various altitudes within the Coastal Plain sediments.

As a means to prioritize relative needs for monitoring, 

production wells are listed in order of vertical proximity to 

the 250-mg/L chloride surface (table 1, attachment 1). Some 

production wells in similar proximity, however, can differ in 

other respects. Among these, aquifer hydraulic conductivities 

and well rates of withdrawal vary widely. A production well in 

a highly conductive aquifer, having a large withdrawal rate, or 

both may be considered a higher priority for monitoring than 

a production well in the same proximity to saltwater, but in a 

less conductive aquifer or having a smaller withdrawal rate.

In addition, some wells in similar proximity to saltwater 

can produce water having substantially contrasting chloride 

concentrations. Vertical and lateral distances of production 

wells to chloride surfaces are referenced to the bottoms 

of screened intervals, which are considered to be the open 

part of each well in closest proximity to saltwater. Chloride 

concentrations in groundwater intercepted by production 

wells, however, are also dependent on well screened-interval 

lengths. Screened intervals intersect varying thicknesses of 

the aquifer, across which groundwater can be relatively fresh 

compared to that at the bottoms of screens. Wells with short 

screened intervals intercept isolated parts of the aquifer having 

relatively uniform chloride concentrations. By contrast, wells 

with long or multiple screened intervals intercept greater 

thicknesses of the aquifer across which groundwater of 

varying chloride concentrations enters the wells at different 

altitudes and is mixed. Proportions of mixing between water of 

contrasting composition can also vary over time as withdrawal 

rates vary (Yager and Heywood, 2014).

Not all groundwater-production wells have been 

accounted for here. Thousands of production wells are 

widely distributed throughout the Virginia Coastal Plain 

aquifer system, and are continually evolving as new wells 

are constructed and existing wells are taken into and out of 

service. Most production wells provide small individual water 

supplies, and less than 1 percent of groundwater withdrawals 

are large enough to be subject to requirements for reporting 

to the VA DEQ. Among these, some lack complete information on withdrawal rates, location, and well construction. 

Additional large withdrawals are also likely being made that 

are wholly unknown, as has been indicated at some locations 

by spurious fluctuations in groundwater levels. Although the 

VA DEQ directs considerable resources at maintaining records 

of the largest withdrawals, fully accounting for every production well is probably unrealistic.

Considering the above, monitoring saltwater movement 

can be anticipated to result in some spurious changes in 

groundwater chloride concentration that do not seem attributable to known withdrawals. Efforts should be made to identify 

unknown production wells in proximity that could account for 

the concentration changes. Complete records of withdrawals 

among neighboring production wells are critical to enable 

cause-and-effect relations with chloride-concentration fluctuations to be discerned. In some cases, however, an element of 

doubt may necessarily persist.

Construction of Observation Wells 

Conditions at and near some groundwater-production 

wells potentially constrain the construction of corresponding 

observation wells. Conditions at each location will likely vary 

and require adapting to on a case-by-case basis, and are not 

addressed individually here. Not every circumstance can be 

fully anticipated, but many may be approached by following 

the general guidelines below.

Upconing observation wells should be as close as 

possible to corresponding production wells. The lateral extent 

of upconing is unknown, but probably variable. Upconing is 

theorized, however, to be centered on each production well 

during withdrawal. Upconing observation wells need therefore 

to also be centered to isolate chloride-concentration fluctuations resulting from the corresponding production well and 

avoid the effects of hydraulic interactions with neighboring 

production wells. Care should also be used during drilling, 

however, so that boreholes do not vertically deviate to the 

extent that they would intercept and damage the nearby 

production wells.

Construction of lateral-intrusion observation wells 

may also be precluded at some of their projected locations. 

Observation-well locations are projected across open water 

from parts of the city of Virginia Beach, Va., and Lancaster 

and Mathews Counties, Va. Offshore drilling may be impractical or cost-prohibitive. Elsewhere, other water bodies and 

preexisting structures or activities, neighboring production 

wells, restricted access, or other impediments may make 

some locations infeasible. Projected locations are therefore 

best regarded as a general guide that should be anticipated 

as requiring adjustment. Keeping in mind the basis for the 

projected location directions and distance (see “Locations” 
Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia 23

section), adjustment to either distance, direction, or both may 

be needed.

In addition to their locations, the depths of some 

upconing or lateral-intrusion observation wells may require 

adjustment relative to their positions within the aquifer system. 

Specified depths for observation wells (see “Construction 

and Development” section) will in most cases be positioned 

in the same aquifers as their corresponding production wells, 

requiring no adjustment. Chloride iso-concentration surfaces, 

however, pass through both aquifers and confining units. 

Because of variations in aquifer dip, specified depths for some 

observation wells may be determined during drilling to be 

marginally above or below the aquifer of their corresponding 

production wells (fig. 10). In such cases, observation-well 

depths should be adjusted to be within the same aquifer as 

corresponding production wells, but remaining as close to the 

originally specified depths as possible.

In place of new construction, existing unused wells can 

potentially be considered to adapt as observation wells for 

monitoring saltwater movement. The basis for location and 

construction of observation wells presented here, however, 

should be kept in mind in assessing the suitability of existing 

wells for monitoring. Any differences of existing wells from 

this basis should also be kept to a minimum, and would 

require accounting for in interpreting how changes in chloride 

concentrations reflect saltwater movement.

To the extent possible, observation wells should be located 

and constructed to identify changes in groundwater chloride 

concentration directly associated with their corresponding 

individual production wells. Some production wells can be in 

proximity or collocated, however, including at several known 

locations in the counties of James City and Lancaster, Va., and 

the cities of Chesapeake, Portsmouth, and Virginia Beach, Va. 

(fig. 1, pl. 1). Upconing resulting from multiple neighboring 

production wells is theorized to produce a dynamically 

evolving composite of chloride-concentration fluctuations 

that does not correspond with the withdrawal from any single 

production well (see “Saltwater Movement” section). An additive effect on lateral intrusion can also potentially take place at 

the relatively remote distance of 1 mi without corresponding 

directly to withdrawal from any one of the wells. Neither 

upconing nor lateral-intrusion observation wells may be able 

to distinguish chloride-concentration changes resulting from 

individual production wells at these locations.

Given the above, a modified approach may be needed 

in some cases to construct observation wells with the objective of determining the localized net effect of neighboring 

production wells on changes in chloride concentration. A 

single upconing monitoring well would be collocated among 

the production wells and completed at about the altitude of 

the 250-mg/L chloride iso-concentration surface (fig. 11, left 

side). A lateral-intrusion observation well would also be about 

centrally located among locations projected from the production wells. If screened-interval bottoms of all production wells 

are positioned at similar altitudes, a single lateral-intrusion 

observation well can be completed at the same altitude. If the 

bottoms of the production wells have substantially varying 

altitudes, however, more than one lateral-intrusion observation 

well may need to be completed at the appropriate altitudes 

(fig. 11, right side). This modified approach should also 

account for (1) which of the production wells are active, likely 

to be active (versus those going unused on an indefinite basis), 

or both; and (2) any additional neighboring production wells 

close enough to affect chloride concentrations.

Lateral intrusion

observation well

Projected depth Adjustment

Adjustment

NOT TO SCALE

Production

well

Confining unit

Aquifer

Aquifer

Confining unit

Collocated

vertical upconing

observation well

Screened

interval

250-milligrams-per-liter chloride iso-concentration surface

Figure 10. Adjustments to observation-well depths relative to positions within the aquifer system. Chloride 

iso-concentration surfaces pass through both aquifers and confining units. Depths for observation wells that 

are above or below the aquifer of their corresponding production wells should be adjusted to be within the 

same aquifer, but remaining as close to the originally specified depths as possible.
24 A Conceptual Framework and Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia

Summary and Conclusions

Groundwater withdrawal has created the potential for 

movement of saltwater in the Virginia Coastal Plain aquifer 

system. Monitoring saltwater movement is needed to detect 

increases in chloride concentration before groundwaterproduction wells become contaminated. The saltwater-transition zone and the distribution of groundwater withdrawals 

are complex, which poses a substantial challenge for effective 

monitoring of saltwater movement. To assess the immediacy 

and permanence of contamination, the manner of saltwater 

movement must also be determined by distinguishing vertical 

upconing from lateral intrusion. An investigation was therefore 

undertaken during 2014 by the U.S. Geological Survey in 

cooperation with the Virginia Department of Environmental 

Quality (VA DEQ), to develop a conceptual framework and 

monitoring strategy for saltwater movement in the Virginia 

Coastal Plain aquifer system. The framework and strategy 

provides a sound scientific understanding of saltwater movement and guidance to implement a monitoring program.

A conceptual framework synthesizes previous studies to 

provide an understanding of conditions, processes, and relations of saltwater to groundwater withdrawal. Hydrochemical 

evidence indicates that saltwater in the aquifer system originated primarily from seawater repeatedly emplaced within 

aquifer sediments during the past about 65 million years, and 

was subsequently flushed by eastward and downward flowing 

fresh groundwater. Flushing has been impeded, however, in 

low hydraulic conductivity sediments filling the Chesapeake 

Bay impact crater, where seawater has been retained. The 

resulting saltwater-transition zone exhibits a warped and 

steeply mounded dome shape about centered on the impact 

crater that is flanked by a nearly level and shallow plateau 

shape to the southeast. Despite that groundwater withdrawal 

has redirected the hydraulic gradient landward, the saltwatertransition zone has not moved because withdrawn water is 

released from aquifer storage rather than displaced across 

regionally appreciable distances.

Historical groundwater chemical-quality samples exhibit 

fluctuating chloride concentrations. Numerous instances 

of intermittent localized upconing beneath individual 

groundwater-production wells are theorized to reverse within 

weeks to months of a decrease or cessation of withdrawal. By 

contrast, lateral intrusion in the form of horizontal displacement of groundwater toward a production well probably takes 

several decades or more to cross distances of about 1 mile. 

Upconing is relatively immediate, but reversible, whereas 

lateral intrusion will continue under the regionally landward 

hydraulic gradient to slowly, but permanently reposition the 

saltwater-transition zone.

Upconing coupled with lateral intrusion is theorized to 

produce a composite chloride-concentration trend. Upconing 

cycles result from withdrawals that vary widely over time 

in response to changing water demands. Concentration 

fluctuations of as much as several hundred milligrams per liter 

(mg/L) increase from and decrease to a baseline concentration, 

which because of lateral intrusion increases by a smaller 

amount during a period of years to decades. A dynamically 

evolving composite of concentration fluctuations can result 

from complex local hydraulic interactions among neighboring 

Lateral intrusion

observation wells

NOT TO SCALE

Neighboring

production

wells

Collocated

vertical upconing

observation well

Screened

intervals Projected depth

Projected depth

250-milligrams-per-liter chloride iso-concentration surface

Figure 11. Generalized spatial relations of observation wells to multiple neighboring groundwater-production 

wells. Observation wells may be unable to distinguish chloride-concentration changes resulting from individual 

production wells, but can be constructed to determine their localized net effect. (Left) A single upconing monitoring 

well is collocated among the production wells and completed at the approximate altitude of the 250-milligramsper-liter chloride iso-concentration surface. (Right) Lateral intrusion observation wells are projected from the 

production wells. A single lateral intrusion observation well can be completed at the same altitude as screenedinterval bottoms of production wells positioned at similar altitudes. Additional lateral intrusion observation wells 

may be needed for production wells having screened-interval bottoms with substantially varying altitudes.
Summary and Conclusions 25

production wells, some of which may be unknown. 

Conversely, groundwater chloride concentrations can be nearly 

constant where unaffected by upconing, or where withdrawal 

is kept constant during periods of years to decades.

A strategy for monitoring saltwater movement in the 

Virginia Coastal Plain aquifer system is based on spatial relations 

between the saltwater-transition zone and 612 groundwaterproduction wells that were regulated by the VA DEQ during 

2013. The vertical position and lateral distance and direction 

of the bottom of each production well’s screened interval 

was calculated relative to previously mapped 250-mg/L and 

1,000-mg/L groundwater chloride iso-concentration surfaces. 

Spatial analysis identified 81 production wells completed in the 

Yorktown-Eastover and Potomac aquifers that are positioned 

in closest proximity to the 250-mg/L chloride surface, and 

from which chloride concentrations are most likely to increase 

above the U.S. Environmental Protection Agency’s 250-mg/L 

secondary maximum contaminant level. Observation wells are 

specified to distinguish vertical upconing from lateral intrusion 

among individual production wells. To monitor upconing, an 

observation well is to be collocated with each production well 

and completed at about the altitude of the 250-mg/L chloride isoconcentration surface. To monitor lateral intrusion, a potential 

location of an observation well is projected from the bottom of 

each production well’s screened interval, in the lateral direction 

to the underlying chloride surface to a distance of 1 mi.

The following aspects of observation-well construction and 

sampling are of particular importance to monitoring saltwater 

movement in the Virginia Coastal Plain aquifer system:

1. Observation wells should feature screened intervals 

generally of no more than 10 feet that isolate distinct 

parts of the aquifer.

2. Observation wells should be thoroughly developed to 

remove drilling fluid or introduced water.

3. Before each collection of groundwater chemical-quality 

samples, observation wells should be purged by lowering 

the position of the sampling pump as closely as possible 

directly beneath the declining water level so as to fully 

displace all stratified saltwater in the well casing upward 

to the pump.

4. In order to maintain chemical equilibria that are representative of the aquifer, field parameters should be 

measured during purging from water kept in isolation 

from the atmosphere and conditions that would greatly 

affect water temperature.

5. Sample containers should be filled with filtered water 

having minimal atmospheric contact while stable flow is 

maintained without interruption or change in flow rate.

6. Inflection-point titration for alkalinity should be 

completed in the field.

7. In order to distinguish individual upconing cycles and 

not potentially produce a false chloride-concentration 

trend, a sample-collection frequency of four times per 

year should be considered initially for newly established 

upconing and lateral-intrusion observation wells.

8. Once responses in chloride concentrations to changes in 

production-well withdrawal can be reliably predicted, 

observation-well sampling can be more optimally timed 

with production-well withdrawal.

9. Analysis of major ions—including calcium, magnesium, 

sodium, and potassium cations and chloride, bicarbonate, carbonate, sulfate, fluoride, and bromide anions—

should be considered for groundwater samples initially 

collected for the first year from newly established observation wells. Concentrations of major ions (1) determine 

the dominant chemical composition of groundwater at 

each well, (2) establish its relative position within the 

saltwater-transition zone, and (3) provide data-quality 

control by calculation of sample charge balance.

10. Once the dominant chemical composition at each well 

is established, analysis of chloride along with field 

parameters may be adequate during periods from years 

to decades.

11. Specific conductance can potentially be considered as 

a surrogate for chloride concentration depending on 

regulatory policy.

The following factors limit and constrain monitoring 

saltwater movement in the Virginia Coastal Plain aquifer 

system, and must be recognized for effective implementation 

of the monitoring strategy:

1. Relative needs for monitoring among groundwaterproduction wells and construction of observation 

wells are dependent upon the accuracy of groundwater 

chloride iso-concentration surfaces.

2. Production wells in similar proximity to saltwater can 

differ in aquifer hydraulic conductivities, and well rates 

of withdrawal and screened-interval lengths.

3. Only production wells making withdrawals large 

enough to be subject to reporting requirements to the 

VA DEQ, and having complete information, have been 

accounted for.

4. Undocumented production wells are numerous and 

widespread, and can result in spurious changes in 

groundwater chloride concentration.

5. Upconing observation wells should be as close as 

possible to corresponding production wells so long as 

boreholes do not vertically deviate during drilling to 

intercept and damage the production wells.

6. Construction of some lateral-intrusion observation 

wells may be precluded by various impediments at their 

projected locations and require adjustment to either the 

projected distance, direction, or both.
26 A Conceptual Framework and Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia

7. Specified depths of some upconing or lateral-intrusion 

observation wells may be marginally above or below 

the aquifer of their corresponding production wells, 

and require adjustment to be within the same aquifer.

8. Existing unused wells can be potentially adapted as 

observation wells for monitoring saltwater movement 

so long as differences from specified observation-well 

locations and construction are kept to a minimum, and 

are accounted for in interpreting changes in chloride 

concentrations.

9. Where multiple production wells are in proximity 

or collocated, neither upconing nor lateral-intrusion 

observation wells may be able to distinguish chloride 

concentration changes resulting from individual 

production wells. A modified approach can be used, 

however, to determine the net effect of neighboring 

production wells on changes in chloride concentration. 

More than one lateral-intrusion observation well may 

be required depending on whether production-well 

screened intervals are similarly positioned.

References Cited

Barlow, P.M., 2003, Ground water in freshwater-saltwater 

environments of the Atlantic Coast: U.S. Geological Survey 

Circular 1262, 113 p.

Bradley, R.S., 1999, Paleoclimatology: Burlington, Mass., 

Harcourt/Academic Press, 613 p.

Cederstrom, D.J., 1943, Chloride in ground water in the 

Coastal Plain of Virginia: Virginia Geological Survey 

Bulletin 58, 36 p.

Claassen, H.C., 1982, Guidelines and techniques for 

obtaining water samples that accurately represent the water 

chemistry of an aquifer: U.S. Geological Survey Open-File 

Report 82–1024, 49 p.

Code of Virginia, Title 62.1, Chapter 25, The ground water 

management act of 1992.

Eggleston, Jack, and Pope, Jason, 2013, Land subsidence 

and relative sea-level rise in the southern Chesapeake Bay 

region: U.S. Geological Survey Circular 1392, 30 p. 

[Also available at http://dx.doi.org/10.3133/cir1392.]

Ghyben, W.B., 1888, Nota in verband met de voorgenomen 

putboring nabij Amsterdam: Tijdschrift van Let Koninklijk 

Inst. Van Ing.

Harsh, J.F., and Laczniak, R.J., 1990, Conceptualization and 

analysis of ground-water flow system in the Coastal Plain of 

Virginia and adjacent parts of Maryland and North Carolina: 

U.S. Geological Survey Professional Paper 1404F, 100 p.

Heath, R.C., 1987, Basic ground-water hydrology: U.S. Geological Survey Professional Paper 2220, 84 p.

Helsel, D.R., and Hirsch, R.M., 2002, Statistical methods in 

water resources: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 4, chap. A3, 510 p.

Hem, J.D., 1985, Study and interpretation of the chemical 

characteristics of natural water: U.S. Geological Survey 

Water-Supply Paper 2254, 263 p.

Henry, H.R., 1960, Salt water intrusion into coastal aquifers: 

International Association of Scientific Hydrology, Publication 52, p. 478–487.

Herzberg, Alexander, 1901, Die Wasserversorgung einiger

Nordseebader: J. Gasbeleucht, Wasserversorg, 44, 

p. 815–819.

Heywood, C.E., and Pope, J.P., 2009, Simulation of 

groundwater flow in the Coastal Plain aquifer system of 

Virginia: U.S. Geological Survey Scientific Investigations 

Report 2009–5039, 115 p.

Lacombe, P.J., and Carleton, G.B., 2002, Hydrogeologic 

framework, availability of water supplies, and saltwater 

intrusion, Cape May County, New Jersey: U.S. Geological 

Survey Water-Resources Investigations Report 01–4246, 

151 p.

Lapham, W.W., Wilde, F.D., and Koterba, M.T., 1997, Guidelines and standard procedures for studies of ground-water 

quality: selection and installation of wells, and supporting 

documentation: U.S. Geological Survey Water-Resources 

investigations Report 96–4233, 110 p.

Larson, J.D., 1981, Distribution of saltwater in the Coastal 

Plain aquifers of Virginia: U.S. Geological Survey OpenFile Report 81–1013, 25 p.

Lichtler, W.F., and Wait, R.L., 1974, Summary of the groundwater resources of the James River basin, Virginia: U.S. 

Geological Survey Open-File Report 74–139, 54 p.

Manheim, F.T., and Horn, M.K., 1968, Composition of deeper 

subsurface waters along the Atlantic continental margin: 

Southeastern Geology, v. 9, no. 4, p. 215–236.

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.
References Cited 27

McFarland, E.R., 2013, Sediment distribution and hydrologic 

conditions of the Potomac aquifer in Virginia and parts 

of Maryland and North Carolina: U.S. Geological Survey 

Scientific Investigations Report 2013–5116, 67 p.

McFarland, E.R., and Bruce, T.S., 2005, Distribution, origin, 

and resource-management implications of ground-water 

salinity along the western margin of the Chesapeake Bay 

impact structure in eastern Virginia, in Horton, J.W., Jr., 

Powars, D.S., and Gohn, G.S., eds., Studies of the Chesapeake Bay impact structure—The USGS-NASA Langely 

corehole, Hampton, Virginia, and related coreholes and 

geophysical surveys: U.S. Geological Survey Professional 

Paper 1688, chap. K, 32 p.

McFarland, E.R., and Bruce, T.S., 2006, The Virginia Coastal 

Plain hydrogeologic framework: U.S. Geological Survey 

Professional Paper 1731, 118 p.

Meisler, Harold, 1989, The occurrence and geochemistry of 

salty ground water in the northern Atlantic Coastal Plain: 

U.S. Geological Survey Professional Paper 1404-D, 51 p.

Meng, A.A., III, and Harsh, J.F., 1988, Hydrogeologic framework of the Virginia Coastal Plain: U.S. Geological Survey 

Professional Paper 1404–C, 82 p.

Mixon, R.B., Berquist, C.R., Jr., Newell, W.L., Johnson, G.H.,

Powars, D.S., Schindler, J.S., and Rader, E.K., 1989, 

Geologic map and generalized cross sections of the Coastal 

Plain and adjacent parts of the Piedmont, Virginia: U.S. 

Geological Survey Miscellaneous Investigations Series 

Map I–2033, scale 1:250,000.

Nelms, D.L., Harlow, G.E., Jr., Plummer, L.N., and 

Busenberg, Eurybiades, 2003, Aquifer susceptibility in 

Virginia, 1998–2000: U.S. Geological Survey WaterResources Investigations Report 03–4278, 58 p.

Peltier, W.R., 1994, Ice age paleotopography: Science, v. 265, 

no. 5169, p. 195–201.

Pope, J.P., and Burbey, T.J., 2004, Multiple-aquifer characterization from single borehole extensometer records: Groundwater, v. 42, no. 1, p. 45–58.

Pope, J.P., McFarland, E.R., and Banks, R.B., 2008, Private 

domestic-well characteristics and the distribution of 

domestic withdrawals among aquifers in the Virginia 

Coastal Plain: U.S. Geological Survey Scientific 

Investigations Report 2007–5250, 47 p.

Powars, D.S., 2000, The effects of the Chesapeake Bay 

impact crater on the geological framework and correlation 

of hydrogeologic units of southeastern Virginia, south of 

the James River: U.S. Geological Survey Professional 

Paper 1622, 53 p., 1 pl.

Powars, D.S., and Bruce, T.S., 1999, The effects of the 

Chesapeake Bay impact crater on the geological framework 

and correlation of hydrogeologic units of the lower 

York-James Peninsula, Virginia: U.S. Geological Survey 

Professional Paper 1612, 82 p., 9 pls.

Prinos, S.T., 2014, Using state-of-the-art technology to 

evaluate saltwater intrusion in the Biscayne aquifer of 

Miami-Dade County, Florida: U.S. Geological Survey 

Fact Sheet 2014–3050, 6 p. [Also available at 

http://dx.doi.org/10.3133/fs20143050.]

Sanford, Samuel, 1913, The underground water resources of 

the Coastal Plain Province of Virginia: Virginia Geological 

Survey Bulletin 5, 361 p.

Sanford, W. E., 2003, Heat flow and brine evolution following 

the Chesapeake Bay Bolide Impact. Journal of Geochemical 

Exploration, v. 78–79, p. 243–247.

Sanford, W. E., 2005, Simulation of the hydrothermal response 

to the Chesapeake Bay bolide impact, Geofluids, v. 5, 

p. 185–201.

Sanford, W.E., Voytek, M.A., Powars, D.S., Jones, B.F., 

Cozzarelli, I.M., Cockell, C.S., and Eganhouse, R.P., 2009, 

Pore-water chemistry from the ICDP-USGS corehole in the 

Chesapeake Bay impact structure—Implications for paleohydrology, microbial habitat, and water resources: Geological Society of America Special Paper 458, p. 867–890.

Sanford, W.E, Doughten, M.W., Coplen, T.B., Hunt, A.G., 

and Bullen, T.D., 2013, Evidence for high salinity of Early 

Cretaceous sea water from the Chesapeake Bay crater: 

Nature, v. 503, p. 252–256.

U.S. Environmental Protection Agency, 1990, Secondary 

maximum-contaminant levels (section 143.3 of part 143, 

National secondary drinking water regulations): U.S. Code 

of Federal Regulations, Title 40, Parts 100 to 149, revised 

as of July 1, 1990, p. 674.

Wilde, F.D., 1999, National field manual for the collection

of water-quality data, collection of water samples:

U.S. Geological Survey Techniques of Water-Resources 

Investigations, book 9, chapter A4, 103 p.

Yager, R. M., and Heywood, C.E., 2014, Simulation of the 

effects of seasonally varying pumping on intraborehole 

flow and the vulnerability of public-supply wells to 

contamination: Groundwater, v. 52, no. 1, p. 40–52.
28 A Conceptual Framework and Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia Table 1. Groundwater-production wells, vertical positions and lateral distances and directions relative to chloride iso-concentration surfaces, and projected locations of lateral intrusion observation wells in the Coastal Plain of Virginia.—Continued [Altitude refers to the distance in feet above or below the National Geodetic Vertical Datum of 1929. Lateral direction is in degrees from east, and lateral directions oriented counterclockwise have positive values and clockwise have negative values. Latitude and longitude are in decimal degrees referenced to the North American Datum of 1927. VA DEQ, Virginia Department of Environmental Quality; ft, foot; Mgal/d, million gallons per day; mg/L, milligram per liter; m, meter; GWPDB, Groundwater Withdrawal Permit Database; VWUDS, Virginia Water Use Data System; —, no data] Monitoring priority VA DEQ number plate 1 Altitudes (ft) Aquifer Groundwater withdrawal rates (Mgal/d) Reported withdrawal data source Chloride iso- concentration sur- face altitudes Screened-interval bottom Location of lateral-intrusion observation well Land surface Screened- interval top Screened- interval bottom Permitted Reported 250 mg/L 1,000 mg/L Vertical position (ft) Lateral distance (m) Lateral direction Latitude Longitude Well screened-interval bottom positioned within 50 feet above the 250-mg/L chloride iso-concentration surface (indicated in yellow on figures 1, 7, and 8, plate 1, and attachment 1)

1 228-00335 25 –79 –99 Yorktown-Eastover 0.01187 0 GWPDB –100 –250 –– 180 36.8953 –76.06312 228-00407 12 –88 –98 Yorktown-Eastover 0.00022 0 GWPDB –100 –250 –2 –– 15 36.774 –76.0093 234-00277 21 –84 –94 Yorktown-Eastover 0.01859 0 GWPDB –100 –671 –6 3,668 –27.77 36.6415 –76.20954 234-00272 0 –82 –92 Yorktown-Eastover 0.0058 0.01236 GWPDB –100 –761 –8 –– 15 36.7279 –76.24925 199-00116 72 –338 –368 Potomac 0.00959 0 GWPDB –378 –670 –9 4,722 31.21 37.2476 –76.62596 228-00334 24 –70 –90 Yorktown-Eastover 0.01187 0 GWPDB –100 –250 –10 –– 180 36.8961 –76.06347 234-00213 6 –77 –89 Yorktown-Eastover 0.0058 0.00011 GWPDB –100 –757 –11 –– 15 36.7282 –76.24818 234-00001 6 –81 –86 Yorktown-Eastover 0.0058 0.00537 GWPDB –100 –749 –14 –– 15 36.7277 –76.24879 161-00200 40 –395 –830 Potomac 0.935 0 GWPDB –845 –1,153 –15 3,067 –22.88 36.8046 –76.6025

10 228-00440 5 –93 –100 Yorktown-Eastover

— 0.00026 VWUDS –115 –250 –15 –– 15 36.7425 –75.9537

11 228-00444 8 –72 –82 Yorktown-Eastover 0.01448 0 GWPDB –100 –250 –18 –– 15 36.8774 –75.9704

12 228-00394 8 –124 –134 Yorktown-Eastover 0.01748 0.00466 GWPDB –153 –250 –19 –– 15 36.8413 –75.988913 228-00470 9 –61 –81 Yorktown-Eastover 0.03133 0 GWPDB –100 –250 –19 –– 0 36.7644 –76.084914 228-00469 9 –61 –81 Yorktown-Eastover 0.03133 0 GWPDB –100 –250 –19 –– 0 36.7648 –76.08515 228-00468 9 –61 –81 Yorktown-Eastover 0.03133 0 GWPDB –100 –250 –19 –– 0 36.7651 –76.084816 146-00329 39 –758 –949 Potomac 0.12625 0.42513 GWPDB –969 –1,396 –20 –– –30 36.9572 –76.593517 228-00336 24 –100 –120 Yorktown-Eastover 0.01187 0 GWPDB –100 –250 –20 –– 180 36.895 –76.065318 228-00442 12 –68 –78 Yorktown-Eastover 0.01448 0.01364 GWPDB –100 –250 –22 –– 15 36.8689 –75.970519 228-00391 15 –37 –77 Yorktown-Eastover 0.03151 0.01664 GWPDB –100 –250 –23 –– 15 36.8596 –75.979320 228-00443 21 –56 –76 Yorktown-Eastover 0.01448 0.0144 GWPDB –100 –250 –24 –– 15 36.8735 –75.975221 228-00441 16 –54 –74 Yorktown-Eastover 0.01448 0.00936 GWPDB –100 –250 –26 –– 15 36.8702 –75.975322 146-00281 45 –755 –940 Potomac 0.64307 0.3921 GWPDB –968 –1,395 –28 –– –30 36.9569 –76.593523 228-00453 19 –61 –71 Yorktown-Eastover 0.01448 0 GWPDB –100 –250 –29 –– 15 36.8699 –75.964924 147-00009 17 –443 –503 Potomac 0.02521 0.00387 GWPDB –532 –784 –29 811 66.04 37.1972 –76.609625 234-00242 15 –55 –70 Yorktown-Eastover 0.01859 0 GWPDB –100 –652 –30 –– –30 36.63 –76.207726 234-00243 15 –55 –70 Yorktown-Eastover 0.01859 0 GWPDB –100 –651 –30 –– –30 36.6302 –76.207527 228-00417 16 –54 –69 Yorktown-Eastover 0.02237 0 GWPDB –100 –250 –31 –– 0 36.7603 –76.0522
Table 1 29

Table 1. Groundwater-production wells, vertical positions and lateral distances and directions relative to chloride iso-concentration surfaces, and projected locations of lateral intrusion observation wells in the Coastal Plain of Virginia.—Continued[Altitude refers to the distance in feet above or below the National Geodetic Vertical Datum of 1929. Lateral direction is in degrees from east, and lateral directions oriented counterclockwise have positive values and clockwise have negative values. Latitude and longitude are in decimal degrees referenced to the North American Datum of 1927. VA DEQ, Virginia Department of Environmental Quality; ft, foot; Mgal/d, million gallons per day; mg/L, milligram per liter; m, meter; GWPDB, Groundwater Withdrawal Permit Database; VWUDS, Virginia Water Use Data System; —, no data]

Monitoring 

priority

VA DEQ 

number 

plate 1

Altitudes

(ft)

Aquifer

Groundwater

withdrawal rates

(Mgal/d) Reported withdrawal 

data source

Chloride isoconcentration surface altitudes Screened-interval bottom Location of lateral-intrusion observation well

Land

surface

Screenedinterval

top

Screenedinterval

bottom

Permitted Reported 250 mg/L 1,000 mg/L

Vertical

position

(ft)

Lateral 

distance

(m)

Lateral 

direction Latitude Longitude

Well screened-interval bottom positioned within 50 feet above the 250-mg/L chloride iso-concentration surface (indicated in yellow on figures 1, 7, and 8, plate 1, and attachment 1)—Continued

28 228-00414 14 –52 –67 Yorktown-Eastover 0.02237 0 GWPDB –100 –250 –33 –– 0 36.7536 –76.053629 234-00269 19 –61 –81 Yorktown-Eastover — 0.01613 GWPDB –115 –646 –34 1,449 –30.18 36.5613 –76.23730 234-00281 19 –61 –81 Yorktown-Eastover — 0.0069 GWPDB –115 –648 –34 1,501 –30.18 36.5616 –76.237531 234-00270 19 –61 –81 Yorktown-Eastover — 0.00787 GWPDB –115 –648 –34 1,486 –30.18 36.5614 –76.237432 228-00392 6 –46 –66 Yorktown-Eastover 0.03151 0.01166 GWPDB –100 –250 –34 –– 15 36.8615 –75.97633 161-00371 40 –580 –730 Potomac 1.09208 2.19581 GWPDB –764 –1,130 –34 1,051 –53.33 36.8005 –76.576134 228-00435 13 –55 –65 Yorktown-Eastover 0.02237 0 GWPDB –100 –250 –35 –– 0 36.7617 –76.058135 228-00436 13 –55 –65 Yorktown-Eastover 0.02237 0 GWPDB –100 –250 –35 –– 0 36.7622 –76.058636 234-00246 20 –50 –65 Yorktown-Eastover 0.01859 0 GWPDB –100 –673 –35 –– –30 36.6327 –76.211137 234-00247 20 –50 –65 Yorktown-Eastover 0.01859 0 GWPDB –100 –691 –35 –– –30 36.6344 –76.214438 234-00248 20 –50 –65 Yorktown-Eastover 0.01859 0 GWPDB –100 –691 –35 –– –30 36.6344 –76.214439 234-00249 20 –50 –65 Yorktown-Eastover 0.01859 0 GWPDB –100 –670 –35 –– –30 36.6408 –76.209740 234-00250 20 –50 –65 Yorktown-Eastover 0.01859 0 GWPDB –100 –670 –35 –– –30 36.6408 –76.209741 234-00251 20 –50 –65 Yorktown-Eastover 0.01859 0 GWPDB –100 –681 –35 –– –30 36.6416 –76.211942 234-00252 20 –50 –65 Yorktown-Eastover 0.01859 0 GWPDB –100 –682 –35 –– –30 36.6419 –76.212243 234-00253 20 –50 –65 Yorktown-Eastover 0.01859 0 GWPDB –100 –683 –35 –– –30 36.6419 –76.212544 234-00254 20 –50 –65 Yorktown-Eastover 0.01859 0 GWPDB –100 –683 –35 –– –30 36.6419 –76.212545 228-00438 18 –54 –64 Yorktown-Eastover 0.01433 0.00317 GWPDB –100 –250 –36 –– –40 36.8129 –76.13246 234-00255 20 –50 –65 Yorktown-Eastover 0.01859 0 GWPDB –103 –743 –38 –– –30 36.6397 –76.225247 228-00415 16 –44 –59 Yorktown-Eastover 0.02237 0 GWPDB –100 –250 –41 –– 0 36.7564 –76.058648 228-00437 17 –48 –58 Yorktown-Eastover 0.01433 0.00388 GWPDB –100 –250 –42 –– –40 36.814 –76.125649 161-00203 24 –358 –878 Potomac 0.935 0 GWPDB –922 –1,168 –44 8,360 –45.08 36.793 –76.643150 228-00416 16 –45 –55 Yorktown-Eastover 0.02237 0 GWPDB –100 –250 –45 –– 0 36.7614 –76.056451 151-00093 41 –552 –671 Potomac — 0.03136 VWUDS –716 –1,250 –45 –– –80 37.6334 –76.394652 228-00409 15 –45 –65 Yorktown-Eastover 0.00329 0.00165 GWPDB –112 –250 –47 –– 15 36.8024 –76.010653 147-00235 40 –372 –422 Potomac 0.09315 0.0022 GWPDB –470 –750 –48 –– 20 37.2589 –76.672954 157-00014 0 –40 –59 Yorktown-Eastover — 0.00261 VWUDS –108 –150 –50 –– –95 37.2924 –76.2766
30 A Conceptual Framework and Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia Table 1. Groundwater-production wells, vertical positions and lateral distances and directions relative to chloride iso-concentration surfaces, and projected locations of lateral intrusion observation wells in the Coastal Plain of Virginia.—Continued [Altitude refers to the distance in feet above or below the National Geodetic Vertical Datum of 1929. Lateral direction is in degrees from east, and lateral directions oriented counterclockwise have positive values and clockwise have negative values. Latitude and longitude are in decimal degrees referenced to the North American Datum of 1927. VA DEQ, Virginia Department of Environmental Quality; ft, foot; Mgal/d, million gallons per day; mg/L, milligram per liter; m, meter; GWPDB, Groundwater Withdrawal Permit Database; VWUDS, Virginia Water Use Data System; —, no data] Monitoring priority VA DEQ number plate 1 Altitudes (ft) Aquifer Groundwater withdrawal rates (Mgal/d) Reported withdrawal data source Chloride iso- concentration sur- face altitudes Screened-interval bottom Location of lateral-intrusion observation well Land surface Screened- interval top Screened- interval bottom Permitted Reported 250 mg/L 1,000 mg/L Vertical position (ft) Lateral distance (m) Lateral direction Latitude Longitude Well screened-interval bottom positioned within 50 feet below the 250 mg/L chloride iso-concentration surface (indicated in orange on figures 1, 7, and 8, plate 1, and attachment 1)

1 147-00262 112 –464 –684 Potomac 0.42024 0.11532 GWPDB –676 –991 8 7,350 –62.67 37.3505 –76.76122 220-00003 10 –632 –704 Potomac 0.21 0 GWPDB –695 –1,050 9 11,849 –39.59 36.8538 –76.32973 228-00338 10 –20 –110 Yorktown-Eastover 0.01187 0 GWPDB –100 –250 10 –– 180 36.8956 –76.06724 228-00333 23 –55 –111 Yorktown-Eastover 0.01187 0 GWPDB –100 –250 11 –– 180 36.8958 –76.06455 147-00117 80 –371 –420 Potomac 0.06438 0.08836 GWPDB –406 –692 14 4,713 8.92 37.2348 –76.62986 161-00201 52 –383 –898 Potomac 0.935 0 GWPDB –883 –1,154 15 –– –20 36.7972 –76.6157 220-00009 4 –632 –704 Potomac 0.21 0.01715 GWPDB –686 –1,047 18 11,581 –38.52 36.8509 –76.32868 228-00337 22 –8 –118 Yorktown-Eastover 0.01187 0.04713 GWPDB –100 –250 18 –– 180 36.8958 –76.06599 151-00067 41 –661 –737 Potomac

— 0.00038 VWUDS –716 –1,250 21 9,693 –47.59 37.637 –76.3855

10 228-00474 7 –113 –123 Yorktown-Eastover 0.00329 0.00329 GWPDB –100 –250 23 0 0 36.8208 –76.0711

11 228-00472 6 –114 –124 Yorktown-Eastover 0.00329 0.00013 GWPDB –100 –250 24 –– 0 36.8194 –76.0691

12 228-00475 5 –115 –125 Yorktown-Eastover 0.00329 0.00001 GWPDB –100 –250 25 –– 0 36.82 –76.069813 228-00473 4 –117 –127 Yorktown-Eastover 0.00329 0.00005 GWPDB –100 –250 27 –– 0 36.82 –76.070614 199-00115 61 –369 –399 Potomac 0.00959 0 GWPDB –370 –669 29 4,020 –1.21 37.242 –76.622615 234-00041 9 –101 –131 Yorktown-Eastover 0.11142 0 GWPDB –100 –469 31 –– 0 36.6125 –76.163716 234-00133 12 –106 –136 Yorktown-Eastover 0.00712 0.00002 GWPDB –100 –487 36 –– 0 36.6143 –76.173517 234-00239 9 –106 –136 Yorktown-Eastover 0.11142 0.10669 GWPDB –100 –465 36 –– 0 36.6118 –76.16218 234-00203 10 –110 –140 Yorktown-Eastover 0.11142 0.16348 GWPDB –100 –472 40 –– 0 36.6133 –76.165319 234-00240 12 –124 –144 Yorktown-Eastover

— 0.00167 GWPDB –100 –494 44 –– 0 36.6128 –76.178

20 216-00035 50 –475 –530 Potomac 0.69105 0.47055 GWPDB –482 –725 48 3,363 40.73 37.1872 –76.574521 147-00219 85 –343 –439 Potomac 0.06438 0 GWPDB –390 –658 49 3,858 6.03 37.2371 –76.620622 220-00041 6 –659 –769 Potomac 0.52 0.0015 GWPDB –719 –1,061 50 13,046 –39.36 36.8602 –76.3405

Well screened-interval bottom positioned more than 50 feet above and within 1 lateral mile of the 250 mg/L chloride iso-concentration surface

(indicated in green on figures 1, 7, and 8, plate 1, and attachment 1)

1 216-00003 28 –446 –460 Potomac 0.0235 0 GWPDB –538 –856 –78 755 31.97 37.1538 –76.55632 151-00041 15 –620 –693 Potomac

— 0.02543 VWUDS –774 –1,250 –80 1,134 –69.62 37.6491 –76.4342

3 151-00042 11 –611 –684 Potomac

— 0.00004 VWUDS –766 –1,250 –81 740 –69.62 37.6427 –76.4425

4 147-00260 106 –470 –660 Potomac 0.19703 0.04144 GWPDB –758 –1,036 –98 640 –83.23 37.3722 –76.79955 216-00071 28 –380 –400 Potomac 0.0235 0 GWPDB –567 –856 –167 1,491 41.81 37.1762 –76.5918
Manuscript approved August 21, 2015

Prepared by the USGS Science Publishing Network

Edited by Rebekah Davis, Rolla PSC

Illustrations and layout by Caryl J. Wipperfurth, 

Raleigh PSC

For more information concerning this report, contact:

 Director

 USGS Virginia Water Science Center

 1730 East Parham Road

 Richmond, VA 23228

 (804) 261-2600

 (804) 261-2657 fax

Or visit the Virginia Water Science Center Web site:

http://va.water.usgs.gov/
McFarland—A Conceptual Framework and Monitoring Strategy for Movement of Saltwater in the Coastal Plain Aquifer System of Virginia—Scientific Investigations Report 2015–5117

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20155117