Hydrogeologic Framework of the Shallow

Ground-Water System in the Cox Hall Creek Basin, 

Cape May County, New Jersey

by Pierre J. Lacombe and Otto S. Zapecza 

In cooperation with the

Cape May County Board of Chosen Freeholders and the

New Jersey Department of Environmental Protection

Open-File Report 2006-1044

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2006

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reproduce any copyrighted materials contained within this report.
iii

Contents

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

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

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

Previous Investigations........................................................................................................................4

Methods of Investigation..............................................................................................................................4

Ground-Penetrating-Radar Surveys..................................................................................................4

Interpretation of Natural Gamma-Ray Geophysical Well Log.......................................................6

Review of Well Drillers’ Logs ..............................................................................................................6

Hydrogeologic Framework...........................................................................................................................7

Holly Beach Water-Bearing Zone......................................................................................................7

Confining Unit Overlying the Estuarine Sand Aquifer.....................................................................9

Estuarine Sand Aquifer........................................................................................................................9

Effect of Clay Lenses and the Confining Layer on Potential Vulnerability of Wells to 

Contamination..........................................................................................................................9

Summary..........................................................................................................................................................9

Acknowledgments.......................................................................................................................................10

References Cited..........................................................................................................................................10

Figures

1-3. Maps showing—

 1. Location of Cox Hall Creek Basin, Lower Township, Cape May County, 

New Jersey...........................................................................................................................2

2. Cox Hall Creek Basin, Cox Hall Creek, Mickels Run, and the Cox Hall Creek tide 

gate, Lower Township, Cape May County, New Jersey................................................3

3. Ground-penetrating-radar traverses, lines of hydrogeologic section, and 28

privately owned domestic wells in the Cox Hall Creek Basin, Lower Township, 

Cape May County, New Jersey.........................................................................................5

4. Photograph showing a ground-penetrating-radar (GPR) unit...............................................6

5. Section A-A’ showing the hydrogeologic framework along Clubhouse Road and

Bay Drive, location and screened interval of selected wells, and the natural

gamma-ray log for well 9-210 in the Cox Hall Creek Basin, Lower Township, Cape

May County, New Jersey ...........................................................................................................7

6. Section B-B’ showing the hydrogeologic framework along Avalon Road, location

and screened interval of selected wells, and the natural gamma-ray log for well

9-210 in the Cox Hall Creek Basin, Lower Township, Cape May County, New Jersey......8
iv

Table

1. New Jersey Department of Environmental Protection (NJDEP) well permit numbers

for 28 selected privately owned domestic wells in the Cox Hall Creek area, Cape

May County, New Jersey. ...........................................................................................................9

CONVERSION FACTORS AND VERTICAL DATUMS

Multiply By To obtain

Length

inch (in.) 2.54 centimeter

foot (ft) 0.3 meter

mile (mi) 1.61 kilometer

Area

square mile (mi2) 2.59 square kilometer

Frequency

megahertz (MHz) 0.000001 hertz

Mean sea level, the vertical datum used for the purposes of this report, is a tidal 

datum that pertains to local mean sea level; it is not a fixed datum and is not 

equivalent to the National Geodetic Vertical Datum of 1929 (NGVD 29).
Hydrogeologic Framework of the Shallow 

Ground-Water System in the Cox Hall Creek Basin, 

Cape May County, New Jersey

by Pierre J. Lacombe and Otto S. Zapecza

Abstract

Cape May County is investigating the feasibility of 

restoring the lowermost reach of Cox Hall Creek to its former 

state as a tidal saltwater wetland; however, the potential for 

contamination of the shallow ground-water system, which provides water to hundreds of nearby privately owned domestic 

wells, with saltwater from the restored wetland is of particular 

concern. To evaluate the potential effectiveness and risks of 

restoring the saltwater wetlands, the County needs information 

about the hydrogeologic framework in the area, and about the 

potential vulnerability of the domestic wells to contamination.

The shallow ground-water system in the Cox Hall Creek 

area consists of unconsolidated Holocene and Pleistocene 

deposits. The Holly Beach water-bearing zone, the unconfined 

(water-table) aquifer, is about 35 feet thick and contains a 

2- to 4-foot-thick clay lens about 10 feet below land surface; a 

lower, more discontinuous clay lens about 30 to 35 feet below 

land surface ranges up to 5 feet in thickness. A 75-foot-thick 

confining unit separates the Holly Beach water-bearing zone 

from the underlying estuarine sand aquifer.

The clay lenses in the Holly Beach water-bearing zone 

likely retard the movement of contaminants from septic tanks, 

lawns, and other surficial sources, protecting wells that tap the 

lower, sandy part of the aquifer. The clay lenses also may protect these wells from salty surface water if withdrawals from 

the Holly Beach water-bearing zone are not increased substantially. Deeper wells that tap the estuarine sand aquifer are 

more effectively protected from saltwater from surface sources 

because of the presence of the overlying confining unit.

Introduction

Cox Hall Creek Basin, a small watershed in the southwestern part of Lower Township in Cape May County, New 

Jersey (fig. 1), encompasses about 3.36 mi2 (fig. 2) and is 

relatively flat, with land-surface elevations ranging from -2 

to 21 ft. The primary streams in the basin, Cox Hall Creek 

and its tributary, Mickels Run, flow west and discharge to the 

Delaware Bay. Before about 1930, wetlands at the confluence 

of Cox Hall Creek and Mickels Run were affected by tides 

and, therefore, were salty. The saltwater wetlands flooded 

twice daily with each high tide that flowed in from the Delaware Bay. Construction of a dam and tide gate (an opening 

through which water flows freely in one direction but which 

closes automatically and prevents the water from flowing in 

the other direction) at the mouth of Cox Hall Creek effectively 

restricted the tidal movement of saltwater to this estuarine 

basin, resulting in the creation of a complex of freshwater wetlands. The quality of water in these wetlands has been altered 

by contaminants from point and nonpoint sources, including 

discharge from a sewage-treatment plant into Cox Hall Creek 

during 1969-90; 14 stormwater outfalls; septic systems; and 

runoff from residential, commercial, and agricultural areas. 

As a result of three interventions, the former saltwater 

wetland has become a freshwater marsh that contains fecal 

coliform bacteria at concentrations that can exceed the New 

Jersey maximum contaminant level. The interventions are 

(1) construction of the dam, which prevented tidal flushing; 

(2) intensive residential development in the village of Town 

Bank beginning in the 1950s, which increased the volume of, 

and contributed nonpoint-source contaminants to, stormwater 

runoff; and (3) the discharge of processed wastewater from 

the sewage-treatment plant for more than 20 years. On many 

occasions, excessive concentrations of fecal coliform bacteria 

have caused beaches near the mouth of Cox Hall Creek to 

be declared unsafe for swimming. Phragmites, a non-native 

species of common freshwater reedgrass, has choked wetlands 

channels, caused drainage problems, and become a fire hazard 

to adjacent residences.

To improve drainage and the quality of both surface water 

and ecological habitats, and to reduce fire potential, Cape May 

County is investigating the potential of restoring the lower 

reach of Cox Hall Creek to its former state as a tidal saltwater 
 Hydrogeology, Cox Hall Creek Basin, Cay May County, New Jersey

Maurice River

Tuckahoe River

Great Egg Harbor Bay

Belleplain

State Forest

Cape May

Court House

Cape

DELAWARE BAY

ATLANTIC OCEAN

Atlantic County

Cumberland County

Upper

Township

Woodbine

Dennis

Township

Middle Township

Lower Township

Cape May City

West Cape May

Cape May Point

Ocean City

Sea Isle City

Avalon

Stone Harbor

North

Wildwood

West 

Wildwood

Wildwood

City Wildwood

Crest

Base from U.S. Geological Survey digital data, 1:100,000, 1983.

Universal Transverse Mercator Projection, Zone 18

75

39

15'

74 52'30" 74 45' 74 37'30"

0 1 2 3 4 MILES

0 1 2 3 4 KILOMETERS

39

07'

30"

39

Reeds

Beach

Villas

Town Bank

Cape May County

Rio

Grande

Cape May 

County

Airport

May Canal

North

Cape May

Wildwood mainland 

well field

Steelmantown

Head of River

Swainton

Great Cedar

Dennisville

Marmora

GARDEN

STATE

PARKWAY

Beesley

Point

Jobs

Point

Stone Harbor 

Manor

Strathmere

Cox Hall

Creek Basin

PENNSYLVANIA

NEW YORK

DELAWARE

DEL AWARE BAY

RA RITA N BAY

A T L A N T I C O C E A N

0

0

10 MILES

10 KILOMETERS

D e la wa re R ive r

i

Trenton

75 74

41

40

39

STUDY AREA

Philadelphia

CAPE

MAY

Figure 1. Location of Cox Hall Creek Basin, Lower Township, Cape May County, New Jersey.
Cox Hall Creek

Mickels Run

Tide gate

EXPLANATION

0 1,000 2,000 3,000 4,000 FEET

0 250 500 750 1,000 METERS

Base from U.S. Geological Survey

digital infrared orthophoto quads, 1995

74°57' 74°55'

39°

38°

59'

74°56'

Boundary of Cox Hall Creek Basin

Boundary of area shown in figure 3

BREAKWATER

ROAD

BAY

S

H

O

R

E

ROAD

CHARLE

KECHEMECHE

SHAWMOUNT

TABERNACLE ROAD

Figure 2. Cox Hall Creek Basin, Cox Hall Creek, Mickels Run, and the Cox Hall Creek tide gate, Lower Township, Cape May County, 

New Jersey.

Introduction   
wetland. However, the potential for contamination of the shallow ground-water system, the source of water to hundreds of 

nearby privately owned domestic wells, with saltwater from 

the restored wetland is of particular concern.

To evaluate the potential effectiveness and risks of restoring the saltwater wetlands, the County needs information 

about the hydrogeologic framework in the area, and about the 

potential vulnerability of the domestic wells to contamination 

from the surface. To address these information needs, the U.S. 

Geological Survey (USGS), in cooperation with the Cape May 

County Board of Chosen Freeholders and the N.J. Department 

of Environmental Protection (NJDEP), conducted a study in 

fall 2002 to characterize the framework of the shallow groundwater system in the wetlands area of the Cox Hall Creek 

Basin. The characterization was made on the basis of interpretations of ground-penetrating-radar (GPR) surveys and a 

borehole geophysical log, and analyses of available lithologic 

logs of sediments encountered during well drilling in conjunction with a previously published interpretation of the regional 

hydrogeology. Hydrogeologic sections developed from these 

data were combined with available well-construction information for the domestic wells to assess the wells’ potential 

vulnerability to contamination from the restored saltwater 

wetlands. The geophysical methods used and the types of data 

collected can be applied to the study of the many wetlands 

adjacent to the Delaware Bay and other coastal areas that are 

similarly protected from tides.

Purpose and Scope

This report presents the results of an investigation of the 

hydrogeologic framework of the shallow ground-water system 

in the vicinity of the Cox Hall Creek wetlands adjacent to the 

village of Town Bank in Lower Township, Cape May County, 

New Jersey. The use of ground-penetrating radar and borehole 

geophysical methods in conjunction with well drillers’ logs to 

delineate the framework is described. Hydrogeologic sections developed from the geophysical and lithologic data are 

combined with records of well locations and screen placement 

to characterize the effect of confining layers on the potential 

vulnerability of wells in the Cox Hall Creek area to contamination from the surface. 

Previous Investigations

Comprehensive regional studies of the hydrogeologic 

framework and ground-water resources of Cape May County, 

New Jersey, by the USGS include those by Gill (1962) and 

Lacombe and Carleton (2002). Spitz (1996, 1998) investigated 

the hydrologic feasibility of water-supply-development alternatives in Cape May County and simulated ground-water flow 

and saltwater encroachment in the shallow aquifer system. 

Surficial geologic maps of southern New Jersey by Newell 

and others (2000) include a description of surficial sediments 

in the Cox Hall Creek area. 

Methods of Investigation

Ground-penetrating-radar (GPR) surveys were conducted 

along four traverses adjacent to the Cox Hall Creek wetlands 

in the village of Town Bank (fig. 3). A natural gamma-ray log 

was run in USGS observation well 9-210 on the west side of 

the wetland area, and well-construction records, including well 

drillers’ logs, for the 28 privately owned domestic wells for 

which such records were available, were reviewed.

Ground-Penetrating-Radar Surveys

A GPR survey is a method of determining shallow subsurface lithology and hydrogeology along continuous transects 

without drilling wells or excavating trenches. A radar signal is 

emitted from a transmitting antenna and the reflected signal is 

received at a receiving antenna. The radar signals are reflected 

by subsurface features such as the water table and sand and 

gravel strata, and are attenuated by clay strata and saltwater. 

The data from the radar signals are recorded as digital files and 

on paper strip charts as hundreds of wiggle traces. The data are 

interpreted by comparing them with available information on 

known geologic and hydrologic features and characteristics, 

information from well logs, and water levels. 

On October 8, 2002, the USGS conducted a GPR survey 

using a Mala Geoscience GPR system. This system consists 

of 100-megahertz (MHz) unshielded transmitting and receiving antennas, electronics, a personal (laptop) computer, and 

rechargeable batteries (fig. 4). The transmitting antenna was 

fixed at a 1-meter separation from the receiving antenna, and 

all associated electronics and batteries were attached to the 

antenna network. A field person carried the antenna network 

along a designated traverse, about 2 in. above land surface. 

Data were collected at a normal walking pace of about 200 

to 250 ft/min. Signals were transferred through a cable to 

the data-processing electronics and recording equipment in a 

backpack and a personal computer in a frontpack carried by a 

second person. The settings for the GPR system were maintained at the manufacturer’s default settings. Field personnel who carried the data-processing electronics and personal 

computer noted the locations of major features, such as road 

intersections, forest and field edges, wells, and other obvious 

physical features, along each traverse. 

GPR data were collected along public roads along the 

west side of the lower Cox Hall Creek wetlands, including part 

or all of the following residential streets: Shore Drive, Mallow Road, Clubhouse Road, Fernwood Road, Norcross Road, 

Brookdale Road, Lawnside Road, Delair Road, Oxford Road, 

Race Track Road, and a dirt utility path on the west side of the 

Cox Hall Creek wetlands, as well as across the Ponder Lodge 

golf course north of the Cox Hall Creek wetlands (fig. 3). GPR 

data were used to determine the lateral continuity of lithologic 

units indicated on drillers’ logs of wells in the area. Particular 

attention was given to the depths at which radar signals were 

attenuated by clay layers in the subsurface. The GPR data 

   Hydrogeology, Cox Hall Creek Basin, Cay May County, New Jersey
Cox Hall Creek

Base from Cape May County lot and block digital files, 2003

Mallow

Baywyn

Hollywood

Forest

Fern

Windward

Mulberry

Robin

Springtime

Dune

Beachplum

Bay Drive

Shore Drive

Bybrook Drive

Pinewood

Fernwood

Avalon

Brookdale

Cedarbrook

Delair

Elwood

Folson

Racetrack

Adelphia

Delford

Englewood

Fairview

Clubhouse

Lawnside

Wayside

Glencreek

Town Bank

Oxford

Norcross

A

A'

B

B'

0 500 1,000 1,500 2,000 FEET

0 100 200 300 400 500 METERS

9-210

39°

38°

59'

30"

74°57'30" 74°57' 74°56'30"

Golf Course Central

Golf Course East

Golf Course West

Culvert

Beach Drive

Shore Drive

9-212

P O N D E R L O D G E

G O L F C O U R S E 

37-372

37-344

37-473

37-352

37-390

37-345

37-400

37-1353

37-1727

37-1403

37-1364

37-1526

37-1498

37-02132

37-02405

37-04071

37-04309

37-03901

37-04172

37-04047 37-02230

37-03251

 37-1331

37-03445

37-04465

37-02950

37-04016

 37-03582

EXPLANATION

COX HALL CREEK BASIN BOUNDARY

GROUND-PENETRATING-RADAR 

TRAVERSE

LINE OF HYDROGEOLOGIC SECTION--

Sections shown in figures 5 and 6

Domestic well and N.J. Department of

Environmental Protection permit 

number

U.S. Geological Survey observation well 

and number

37-352

9-210

B B'

Dirt path

Wetlands

Figure 3. Ground-penetrating-radar traverses, lines of hydrogeologic section, and 28 privately owned domestic wells in the Cox 

Hall Creek Basin, Lower Township, Cape May County, New Jersey.

Methods of Investigation   
Laptop Computer

Transmitting Antenna

Receiving Antenna

Figure 4. Ground-penetrating-radar (GPR) unit. (Transmitting and receiving antennas are the two black units lying on the beach. 

The gray boxes on top of the antennas are battery packs. The wood rack holds the GPR antennas a specified distance apart. A 

personal (laptop) computer is carried on the front of the operator and a small electronic box is carried on the back of the operator. 

Typically, a second field person carries the antenna unit about 2 inches above land surface.)

were enhanced with lithologic data from the natural gammaray log of well 9-210, near the mouth of Cox Hall Creek, and 

drillers’ logs for 28 privately owned domestic wells within 

600 ft of the GPR traverses (fig. 3). These data were used to 

prepare hydrogeologic sections A-A’ (south to north; fig. 5) 

and B-B’ (west to east; fig. 6) through the residential community west of the Cox Hall Creek wetlands. These sections 

also show the vertical profiles of the privately owned domestic 

wells for which hydrogeologic data were available. 

Interpretation of Natural Gamma-Ray 

Geophysical Well Log

A natural gamma-ray geophysical log was made for 

USGS observation well 9-210 (fig. 3). Data were recorded 

using a Mt. Sopris MGXII geophysical logger. Data were 

collected at a rate of 20 ft/min and are stored in the USGS 

GEOLOG database at the USGS New Jersey Water Science 

Center in West Trenton, New Jersey. The vertical or y-axis 

on the log represents depth below land surface in feet. The 

horizontal or x-axis on the log represents the relative natural 

gamma-ray emissions in counts per second (see trace of log 

in figures 5 and 6). Strata with low natural gamma-ray values, 

such as sand and gravel, are interpreted as water-bearing units. 

Strata with high natural gamma-ray values are interpreted as 

clay or confining units.

Review of Well Drillers’ Logs

In New Jersey, State-licensed well drillers must file 

a well record with NJDEP upon completion of a well. On 

each well record, the driller must indicate the property where 

the well is located, the purpose of the well, well-construction information, the depth of the interval that is open to the 

aquifer, the yield, the results of any hydraulic tests that were 

conducted, the pumping equipment that was installed, and the 

lithology of the strata that were penetrated while the borehole 

was being drilled. Each well record is stored on microfilm and 

in a computer database at the NJDEP Office of Well Records 

in Trenton, New Jersey. Well records for privately owned 

   Hydrogeology, Cox Hall Creek Basin, Cay May County, New Jersey
domestic wells used in this study are identified by their respective NJDEP well permit numbers (fig. 3 and table 1).

Hydrogeologic Framework

The descriptions of hydrogeologic units in this report are 

based on the regional hydrostratigraphic framework of Cape 

May County delineated by Lacombe and Carleton (2002). The 

hydrogeologic framework of the study area was characterized by combining this information with the interpretation of 

data from the GPR survey, natural gamma-ray logging, well 

drillers’ logs, and well-construction records collected for this 

study. The shallow ground-water system in the Cox Hall Creek 

area consists of (from the land surface downward): unconsolidated Holocene and Pleistocene deposits that make up the 

unconfined Holly Beach water-bearing zone, a confining unit 

overlying the estuarine sand aquifer, and the estuarine sand 

aquifer itself. 

Holly Beach Water-Bearing Zone

The Holly Beach water-bearing zone is the uppermost 

(unconfined) aquifer in Cape May County. In the Cox Hall 

Creek area, the aquifer generally consists of the upper part of 

the Cape May Formation and is composed predominantly of 

gravel interbedded with coarse- to fine-grained sand, although 

silt, clay, and organic muck can predominate in wetland areas. 

-170

-160

-150

-140

-130

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

20

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000

DISTANCE, IN FEET

ALTITUDE, IN FEET

37-3901

37-0344

37-4309

37-4071

37-2705

37-4172

37-3582

37-4047

37-2230

37-3251 37-1353

37-0390

37-4465

37-2950

37-4016

37-0345

37-0400

37-1498

9-212

9-210 37-1526

37-0473

37-1727

37-1331

37-1403

37-0352

37-3445

37-1364

Land surface

Holly Beach

water-bearing zone

Clay

lens

Confining unit

Estuarine sand aquifer

Confining unit

SOUTH NORTH

Well screen

A A'

-180

Discontinuous

clay lens

GAMMA-RAY LOG

EXPLANATION

RADIATION INCREASES

FEET

-170

-160

-150

-140

-130

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

20

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000

DISTANCE, IN FEET

ALTITUDE, IN FEET

37-3901

37-0344

37-4309

37-4071

37-2705

37-4172

37-3582

37-4047

37-2230

37-3251 37-1353

37-0390

37-4465

37-2950

37-4016

37-0345

37-0400

37-1498

9-212

9-210 37-1526

37-0473

37-1727

37-1331

37-1403

37-0352

37-3445

37-1364

Land surface

Holly Beach

water-bearing zone

Clay

lens

Confining unit

Estuarine sand aquifer

Confining unit

SOUTH NORTH

Well screen

A A'

-180

Discontinuous

clay lens

GAMMA-RAY LOG

EXPLANATION

RADIATION INCREASES

FEET

Figure 5. Section A-A’ showing the hydrogeologic framework along Clubhouse Road and Bay Drive, location and screened 

interval of selected wells, and the natural gamma-ray log for well 9-210 in the Cox Hall Creek Basin, Lower Township, Cape May 

County, New Jersey. (Line of section is shown in figure 3. Datum is mean sea level.) 

Hydrogeologic Framework 
Clay and silt lenses within the Holly Beach water-bearing 

zone can confine ground water locally. The aquifer contains 

freshwater in interior Cape May County, freshwater and saltwater beneath the saltwater wetlands and barrier islands, and 

saltwater beneath the Atlantic Ocean and the Delaware Bay.

In the Cox Hall Creek area, the water table is at the land 

surface in the wetlands. Records for wells installed from the 

mid-1980s to the mid-1990s show that static water levels at 

the time of well completion generally were less than 10 ft 

below land surface in residential areas of Town Bank. This 

information is consistent with the water-level altitudes in 1991 

reported by Lacombe and Carleton (2002, fig. 35) and with 

results of the GPR surveys conducted as part of this study.

According to Lacombe and Carleton (2002, fig. 14), the 

altitude of the base of the Holly Beach water-bearing zone in 

the Cox Hall Creek area is about 50 ft below mean sea level. 

Hydrogeologic sections A-A’ and B-B’ (figs. 5 and 6, respectively) show that the base of the aquifer ranges from about -40 

to -55 ft in the area, and that the aquifer is about 50 ft thick. 

Results of the GPR surveys indicate the presence of two clay 

lenses within the Holly Beach water-bearing zone in the study 

area. An upper clay lens about 2 to 4 ft thick underlies most of 

the area, and a lower, more discontinuous clay lens about 30 

to 35 ft below land surface is up to 5 ft thick. The top of the 

regional confining unit that underlies the Holly Beach water0

10

0

ALTITUDE, IN FEET

20

-30

-40

-50

-60

-70

-80

-90

-100

-110

-120

-130

-140

-150

-160

-170

-180

-10

-20

500 1,000 1,500 2,000 2,500 3,000 3,500 4,000

DISTANCE, IN FEET

B B'

WEST EAST

FEET

0

10

0

ALTITUDE, IN FEET

20

-30

-40

-50

-60

-70

-80

-90

-100

-110

-120

-130

-140

-150

-160

-170

-180

-10

-20

500 1,000 1,500 2,000 2,500 3,000 3,500 4,000

DISTANCE, IN FEET

B B'

WEST EAST

FEET

Confining unit

Estuarine sand aquifer

Land surface

Clay lens

Holly Beach

water-bearing

zone

Discontinuous clay lens

Confining unit

Delaware

Bay

37-0390 37-2950 37-4016

37-0345

37-4465

9-212

37-0400

Well screen

GAMMA-RAY LOG

RADIATION INCREASES

EXPLANATION

Confining unit

Estuarine sand aquifer

Land surface

Clay lens

Holly Beach

water-bearing

zone

Discontinuous clay lens

Confining unit

Delaware

Bay

37-0390 37-2950 37-4016

37-0345

37-4465

9-212

37-0400

Well screen

GAMMA-RAY LOG

RADIATION INCREASES

EXPLANATION

Figure 6. Section B-B’ showing the hydrogeologic framework along Avalon Road, location and screened interval of selected wells, 

and the natural gamma-ray log for well 9-210 in the Cox Hall Creek Basin, Lower Township, Cape May County, New Jersey. (Line of 

section is shown in figure 3. Datum is mean sea level.) 

 Hydrogeology, Cox Hall Creek Basin, Cay May County, New Jersey
bearing zone generally was the maximum depth of penetration 

of the ground-penetrating radar. 

Confining Unit Overlying the Estuarine Sand 

Aquifer

The confining unit that overlies the estuarine sand aquifer 

is composed predominantly of fine-grained clay and silt. 

Lacombe and Carleton (2002, fig. 15) show that this confining 

unit generally is more than 75 ft thick in the Cox Hall Creek 

area. This information is consistent with the geophysical log 

and drillers’ logs collected for this study (figs. 5 and 6). The 

confining unit is laterally continuous throughout most of the 

peninsular part of Cape May County.

Estuarine Sand Aquifer

The estuarine sand aquifer is the shallowest confined 

aquifer in the southern part of Cape May County. The extent 

of the aquifer is defined by the extent of the confining unit 

that overlies it. The aquifer consists of gravel and coarse- to 

fine-grained sand of the lower part of the Cape May Formation and the upper part of the Cohansey Formation. According 

to Lacombe and Carleton (2002, figs. 16 and 17), the top of 

the aquifer is approximately 120 ft below mean sea level and 

the aquifer is about 50 ft thick in the Cox Hall Creek area. 

This information is consistent with altitude and thickness data 

for the estuarine sand aquifer based on the geophysical log and 

drillers’ logs collected for this study.

Effect of Clay Lenses and the Confining 

Layer on Potential Vulnerability of Wells to 

Contamination

Sections A-A’ (fig. 5) and B-B’ (fig. 6) show the locations 

and screened intervals of 28 privately owned domestic wells 

in the Cox Hall Creek area determined from well-construction 

data recorded on drillers’ well records available for this study. 

Most of the shallow wells are completed in the lower, sandy 

part of the Holly Beach water-bearing zone adjacent to the 

Cox Hall Creek wetlands.

The clay lenses in the Holly Beach water-bearing zone 

likely slow the movement of contaminants from septic tanks, 

lawns, and other surficial sources of contamination, mitigating their effect on the quality of water in the domestic wells 

that tap the lower part of the aquifer. By retarding the flow of 

ground water, the clay lenses maximize the time available for 

biodegradation and adsorption of contaminants to occur. The 

clay lenses also may protect these wells from salty surface 

water if withdrawals from the Holly Beach water-bearing zone 

are maintained at or near current levels. If a well with a continuous screen were installed through the upper, more continuous clay lens in the Holly Beach water-bearing zone, and if 

the well were near the newly formed saltwater wetland or the 

Delaware Bay, then that well could become contaminated with 

saltwater, and possibly anthropogenic contaminants as well. 

In addition, if new wells were installed in the Holly Beach 

water-bearing zone west of the Cox Hall Creek wetlands and, 

consequently, ground-water withdrawals were increased substantially, water in wells near the saltwater wetlands and the 

Delaware Bay could become salty or contaminated. Deeper 

wells that draw water from the confined estuarine sand aquifer 

are more effectively protected from saltwater and other contaminants present at the surface because of the presence of the 

overlying regional confining unit.

Summary

Residential development in the Cox Hall Creek Basin, 

Lower Township, Cape May County, New Jersey, and construction of a dam and tide gate at the mouth of Cox Hall 

Creek have caused the formerly tidally influenced saltwater 

wetlands in the basin to become fresh, and have resulted in 

increased flooding, degraded water quality, and an increased 

risk of fire. Plans to restore the lower reach of Cox Hall Creek 

to its former state as a tidal saltwater wetland have raised 

concerns over the potential for contamination of the shallow 

ground-water system with saltwater from the surface. The 

shallow ground-water system in the basin is the source of 

water to hundreds of privately owned domestic wells. 

To address the above concerns, the U.S. Geological Survey, in cooperation with the Cape May County Board of Chosen Freeholders and the N.J. Department of Environmental 

Protection, conducted a study in fall 2002 to characterize the 

Table 1. New Jersey Department of Environmental 

Protection (NJDEP) well permit numbers for 28 selected 

privately owned domestic wells in the Cox Hall Creek area, 

Cape May County, New Jersey.

Well permit number Well permit number

37-1364 37-1353

37-03445 37-03251

37-352 37-02230

37-01331 37-03582

37-1403 37-04047

37-1727 37-04172

37-473 37-03901

37-1526 37-04309

37-1498 37-04071

37-400 37-02405

37-345 37-344

37-04016 37-372

37-02950 37-02132

37-04465

37-390

Summary 
hydrogeologic framework of the shallow ground-water system 

in the wetlands area of the basin through the interpretation of 

ground-penetrating-radar surveys, a borehole geophysical log, 

and available lithologic (drillers’) logs in conjunction with a 

previously published interpretation of the regional hydrogeology. Hydrogeologic sections developed from these data were 

combined with available well-construction information for the 

domestic wells to assess the wells’ potential vulnerability to 

contamination from restored saltwater wetlands.

The shallow ground-water system in the Cox Hall Creek 

area consists of unconsolidated Holocene and Pleistocene 

deposits. The Holly Beach water-bearing zone, the unconfined 

aquifer, is about 35 ft thick and contains a 2- to 4-ft-thick clay 

lens about 10 ft below land surface, and a lower, more discontinuous clay lens about 5 ft thick. A 75-ft-thick confining 

unit separates the Holly Beach water-bearing zone from the 

underlying estuarine sand aquifer.

The clay lenses in the Holly Beach water-bearing zone 

likely retard the movement of contaminants from the surface 

and protect wells that are screened in the lower, sandy part of 

the aquifer. The clay lenses also may protect these wells from 

saltwater from the restored wetlands provided that withdrawals from the Holly Beach water-bearing zone are not increased 

substantially. Shallow wells with continuous screens installed 

through the clay lenses near the restored saltwater wetlands or 

the Delaware Bay potentially would be vulnerable to contamination. Wells that tap the estuarine sand aquifer are more 

effectively protected from saltwater from surface sources 

because of the presence of the overlying confining unit.

Acknowledgments

The author thanks Peter Joesten of the USGS for his 

assistance in the field during data collection. In addition, Mr. 

Joesten provided a technical review of the data presentation 

and interpretation. The author also thanks Brian O’Connor 

and John Vona of the Cape May County Planning Department 

for assistance in operating the GPR instruments and recording 

location information in the field.

References Cited

Gill, H.E., 1962, Ground-water resources of Cape May 

County, N.J.--Saltwater invasion of principal aquifers: New 

Jersey Department of Conservation and Economic Development Special Report 18, 171 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.

Newell, W.L., Powars, D.S., Owens, J.P., Stanford, S.D., 

and Stone, B.D., 2000, Surficial geologic map of central and southern New Jersey: U.S. Geological Survey 

Miscellaneous Investigations Series Map I-2540-D, 3 pl. 

and pamphlet.

Spitz, F.J., 1996, Hydrologic feasibility of water-supply-development alternatives in Cape May County, New Jersey: U.S. 

Geological Survey Water-Resources Investigations Report 

96-4041, 42 p.

Spitz, F.J., 1998, Analysis of ground-water flow and saltwater 

encroachment in the shallow aquifer system of Cape May 

County, New Jersey: U.S. Geological Survey Water-Supply 

Paper 2490, 51 p.

10 Hydrogeology, Cox Hall Creek Basin, Cay May County, New Jersey