U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5079

USGS Climate and Land Use Change Research and Development Program

Well Network Installation and Hydrogeologic Data 

Collection, Assateague Island National Seashore, 

Worcester County, Maryland, 2010
Cover. View looking south at West Ocean City, Maryland, Assateague Island, and Sinepuxent Bay. Photograph by Joanna Woerner, 

IAN Image Library (www.ian.umces.edu/imagelibrary/).

Borehole geophysical logging in observation wells near Kilometer Marker 3 

(KM3), Assateague Island National Seashore, Worcester County, Maryland. 

[Photograph by John Masterson, U.S. Geological Survey.]
Well Network Installation and 

Hydrogeologic Data Collection, 

Assateague Island National Seashore, 

Worcester County, Maryland, 2010

By William S.L. Banks, John P. Masterson, and Carole D. Johnson

Scientific Investigations Report 2012–5079

U.S. Department of the Interior

U.S. Geological Survey

USGS Climate and Land Use Change Research and Development Program
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Suggested citation:

Banks, W.S.L., Masterson, J.P., and Johnson, C.D., 2012, Well network installation and hydrogeologic data collection, 

Assateague Island National Seashore, Worcester County, Maryland, 2010: U.S. Geological Survey Scientific 

Investigations Report 2012–5079, 20 p.

ISBN 978-1-4113-3413-7
iii

Contents

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

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

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

Well Network Installation.............................................................................................................................3

Hydrogeologic Data Collection....................................................................................................................9

Water Levels..........................................................................................................................................9

Borehole Geophysical Logging ........................................................................................................12

Profile at Km3..............................................................................................................................12

Profile at Km9..............................................................................................................................13

Profile at Km14............................................................................................................................13

Profile at Km23............................................................................................................................13

Profile at Km28............................................................................................................................14

Consideration for Additional Investigation..............................................................................................14

Summary........................................................................................................................................................20

Acknowledgments.......................................................................................................................................20

References Cited..........................................................................................................................................20
iv

Figures

1–2. Maps showing—

1. Location of Assateague Island National Seashore study area, 

Worcester County, Maryland.............................................................................................2

2. Well locations on Assateague Island National Seashore, 

Worcester County, Maryland.............................................................................................3

3. Photograph showing truck-mounted hollow-stem auger drill rig used 

for monitoring well installation on Assateague Island National Seashore, 

Worcester County, Maryland......................................................................................................7

4–5. Graphs showing—

4. Continuous water-level data, intermittent manual measurements, 

tide altitude data, and total daily precipitation data for the shallow 

monitoring wells at Km9A, Km23A, Km23D, and Km28A from 

August 12 to September 28, 2010, Assateague Island National 

Seashore, Maryland..........................................................................................................10

5. Continuous water-level data, intermittent manual measurements, 

tide altitude data, and total daily precipitation data for the deeper 

monitoring wells at Km9A, Km23A, Km23D, Km28A, and Km28B 

from August 12 to September 28, 2010, Assateague Island National 

Seashore, Maryland..........................................................................................................11

6a–6e. Diagrams showing—

6a. Natural gamma and electromagnetic induction logs collected at 

transect Km3, Assateague Island National Seashore, Maryland .............................15

6b. Natural gamma and electromagnetic induction logs collected at 

transect Km9, Assateague Island National Seashore, Maryland .............................16

6c. Natural gamma and electromagnetic induction logs collected at 

transect Km14, Assateague Island National Seashore, Maryland ...........................17

6d. Natural gamma and electromagnetic induction logs collected at 

transect Km23, Assateague Island National Seashore, Maryland ...........................18

6e. Natural gamma and electromagnetic induction logs collected at 

transect Km28, Assateague Island National Seashore, Maryland ...........................19

Table

1. Location, physical characteristics, and water-level information for 26 

wells, Assateague Island National Seashore, Maryland, 2010.............................................4
v

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047.0 square meter (m2)

square mile (mi2) 2.590 square kilometer (km2)

Volume

gallon (gal) 3.785 liter (L)

Flow rate

inch per year (in/yr) 2.54 centimeter per year (cm/yr)

gallon per day (gal/d) 3.785 liter per day (L/d)

Pressure

pound per square inch (lb/in2) 6.895 kilopascal (kPa)

Density

pound per cubic foot (lb/ft3) 0.01602 gram per cubic centimeter (g/cm3)

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

 °C = (°F - 32) / 1.8

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

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

Conversion Factors and Datums

Well Network Installation and Hydrogeologic Data 

Collection, Assateague Island National Seashore, 

Worcester County, Maryland, 2010

By William S.L. Banks, John P. Masterson, and Carole D. Johnson

Abstract

The U.S. Geological Survey, as part of its Climate and 

Land Use Change Research and Development Program, is 

conducting a multi-year investigation to assess potential 

impacts on the natural resources of Assateague Island National 

Seashore, Maryland that may result from changes in the 

hydrologic system in response to projected sea-level rise. As 

part of this effort, 26 monitoring wells were installed in pairs 

along five east-west trending transects. Each of the five transects has between two and four pairs of wells, consisting of 

a shallow well and a deeper well. The shallow well typically 

was installed several feet below the water table—usually in 

freshwater about 10 feet below land surface (ft bls)—to measure water-level changes in the shallow groundwater system. 

The deeper well was installed below the anticipated depth to 

the freshwater-saltwater interface—usually in saltwater about 

45 to 55 ft bls—for the purpose of borehole geophysical logging to characterize local differences in lithology and salinity 

and to monitor tidal influences on groundwater.

Four of the 13 shallow wells and 5 of the 13 deeper 

wells were instrumented with water-level recorders that collected water-level data at 15-minute intervals from August 

12 through September 28, 2010. Data collected from these 

instrumented wells were compared with tide data collected 

north of Assateague Island at the Ocean City Inlet tide gage, 

and precipitation data collected by National Park Service staff 

on Assateague Island. These data indicate that precipitation 

events coupled with changes in ambient sea level had the 

largest effect on groundwater levels in all monitoring wells 

near the Atlantic Ocean and Chincoteague and Sinepuxent 

Bays, whereas precipitation events alone had the greatest 

impact on shallow groundwater levels near the center of the 

island. Daily and bi-monthly tidal cycles appeared to have 

minimal influence on groundwater levels throughout the island 

and the water-level changes that were observed appeared to 

vary among well sites, indicating that changes in lithology 

and salinity also may affect the response of water levels in 

the shallow and deeper groundwater systems throughout the 

island.

Borehole geophysical logs were collected at each of 

the 13 deeper wells along the 5 transects. Electromagnetic 

induction logs were collected to identify changes in lithology; 

determine the approximate location of the freshwater-saltwater 

interface; and characterize the distribution of fresh and brackish water in the shallow aquifer, and the geometry of the fresh 

groundwater lens beneath the island. Natural gamma logs were 

collected to provide information on the geologic framework 

of the island including the presence and thickness of finergrained deposits found in the subsurface throughout the island 

during previous investigations. 

Results of this investigation show the need for collection 

of continuous water-level data in both the shallow and deeper 

parts of the flow system and electromagnetic induction and 

natural gamma geophysical logging data to better understand 

the response of this groundwater system to changes in 

precipitation and tidal forcing. Hydrologic data collected as 

part of this investigation will serve as the foundation for the 

development of numerical flow models to assess the potential 

effects of climate change on the coastal groundwater system of 

Assateague Island.

Introduction

Assateague Island lies along the Atlantic coast of 

Maryland and Virginia (fig. 1). It is an undeveloped barrier 

island complex backed by wetland and marsh systems and 

consists of large stretches of dunes separated by low-lying 

areas that often are submerged or overwashed during storm 

surges. This barrier island complex is characterized by low 

topographic relief with a mean altitude of about 6.6 feet (ft) 
2 Well Network Installation and Hydrogeologic Data Collection, Assateague Island National Seashore, Maryland, 2010

and maximum dune altitudes of about 33 ft above sea level. 

The impact of climate change and sea-level rise on this barrier 

island complex is expected to include changes in erosion rates, 

island morphology, marsh health, and groundwater flow. These 

changes will affect use of the island by both humans and 

wildlife; specifically, the island serves as an important breeding ground and habitat for endangered species such as Piping 

Plovers. In response to these concerns, the U.S. Geological 

Survey (USGS) is working with State and other Federal agencies to improve the understanding of climate change on coastal 

ecosystems, including the effects on groundwater flow, to better develop capabilities for predicting potential climate-change 

effects and to provide the necessary tools for adaptation and 

mitigation of potentially adverse impacts.

The study area is underlain by unconsolidated sediments 

in the Atlantic Coastal Plain that form a wedge of sands, silts, 

and clays over 7,000 ft thick. These sediments compose a 

series of confined and unconfined aquifers with flow paths 

ranging from tens of feet to hundreds of miles collectively 

known as the Atlantic Coastal Plain (Owens and Denny, 1979). 

The groundwater hydrology of Assateague Island is controlled 

largely by the landforms of the island (Krantz, 2009). The confined aquifer system in the study area is generally overlain by 

younger sediments ranging in age from Tertiary to Quaternary. 

These sediments were deposited in a variety of environments 

including alluvial, tidal marsh, back barrier, and marine marginal, and form an unconfined, surficial aquifer. 

The water table in the shallow, unconfined aquifer system 

generally follows the local topography and water levels can 

range from land surface to as deep as 10 ft below land surface 

(ft bls); however, the water table often is less than 3 ft bls 

(Dillow and others, 2002). The only surface-water features 

on the island are a limited number of groundwater-fed ponds, 

which represent surface-water expressions of the water table. 

As a result, the groundwater hydrology, including the distribution of fresh and brackish water in the shallow aquifer, and 

the geometry of the fresh groundwater lens beneath the island, 

affects the distribution of plant communities and habitat for 

a diverse community of invertebrate and vertebrate wildlife 

(Krantz, 2009). 

The areas within Assateague Island that are considered to 

be most vulnerable to sea-level rise are those with the highest 

occurrence of overwash and the highest rates of shoreline 

change (Pendleton and others, 2004). However, in addition to increased erosion and surface inundation from rising 

sea level, the groundwater flow system can be significantly 

affected by increased water-table altitude, subsurface flooding 

of low-lying areas, and the potential for saltwater intrusion 

(Masterson and Garabedian, 2007). Understanding how 

sea-level rise may affect groundwater hydrology in shallow, 

unconfined coastal systems such as Assateague Island may be 

vital in assessing the potential impacts of sea-level rise on the 

sustainability of Federally listed endangered species such as 

Piping Plovers. This information is not currently available to 

resource managers and decision makers at Assateague Island 

National Seashore (ASIS).

Figure 1. Location of Assateague Island National Seashore 

study area, Worcester County, Maryland.
Well Network Installation 3

Purpose and Scope 

This report describes the study design, drilling methods, 

and data-collection techniques used to install and monitor a 

network of 26 groundwater wells for the purpose of assessing 

the potential impacts of sea-level rise on the natural resources 

of ASIS (fig. 2). The report also provides a description of the 

continuous water-level data collected at nine monitoring wells 

from early August through September 2010 and geophysical 

logs collected from 13 deeper monitoring wells in October 

2010.

Well Network Installation

A network of 13 monitoring well pairs was installed to 

monitor groundwater levels and determine the current depth 

of the interface between freshwater and saltwater in the shallow groundwater system that underlies ASIS (fig. 2). All of 

the drilling was completed by the USGS Eastern Region’s 

National Earth Surface Processes Geologic Mapping drilling 

team from August 1 through August 13, 2010. Well construction information and selected water-level data are provided in 

table 1. Latitude and longitude data for each well pair were 

Figure 2. Well locations on Assateague Island National Seashore, Worcester County, Maryland.
4 Well Network Installation and Hydrogeologic Data Collection, Assateague Island National Seashore, Maryland, 2010 Table 1. Location, physical characteristics, and water-level information for 26 wells, Assateague Island National Seashore, Maryland, 2010. [The U.S. Geological Survey (USGS) well identification number is a 15-digit number. The first 6 digits are the latitude, the next 7 digits are the longitude, and the last 2 digits refer to the sequence number for identifying one or more wells at a particlular latitude and longitude; (° ′ ′′), degrees, minutes, seconds; ft, feet; NAVD 88, North American Vertical Datum of 1988; --, no data; conductance range, in microsie- mens per centimeter (μS/cm), refer to figure 6, A = 0–1,300, B = 1,301–28,000, C = 28,001–60,000, D = greater than 60,000] Field identifier USGS well identification number Local well number Construc- tion date Latitude (° ’ ”) Longitude (° ’ ”) Conductance range at time of geophysical logging, Oct. 2010 Altitude of top of well casing (ft NAVD 88) Altitude of top of screen (ft NAVD 88) Altitude of bottom of screen (ft NAVD 88) Vertical accuracy of wells surveyed (±ft) Km3A-D 381804075063401 WO Cg 104 8/11/2010 38 18 04.8 75 06 34.2

B 7.76 -41.4 -46.4 --

Km3A-S 381804075063402 WO Cg 103 8/11/2010 38 18 04.8 75 06 34.2

B 7.69 -2.5 -7.5 0.10

Km3B-D 381805075063501 WO Cg 102 8/12/2010 38 18 05.4 75 06 35.5

B 12.90 -42.0 -47.0 --

Km3B-S 381805075063502 WO Cg 101 8/12/2010 38 18 05.4 75 06 35.5

B 12.74 -1.8 -6.8 --

Km9A-D 381450075075401 WO Dg 27 8/10/2010 38 14 50.6 75 07 54.0

D 5.56 -14.2 -19.2 --

Km9A-S 381450075075402 WO Dg 28 8/10/2010 38 14 50.6 75 07 54.0

B 5.58 -4.1 -9.1 --

Km9B-D 381452075075901 WO Dg 29 8/10/2010 38 14 52.2 75 07 58.9

C 7.96 -41.5 -46.5 0.13

Km9B-S 381452075075902 WO Dg 30 8/9/2010 38 14 52.0 75 07 59.0

A 8.30 -5.9 -10.9 --

Km9C-D 381452075080101 WO Dg 31 8/9/2010 38 14 52.9 75 08 01.7

C 9.46 -40.1 -45.1 --

Km9C-S 381452075080102 WO Dg 32 8/9/2010 38 14 52.9 75 08 01.7

A 9.31 -0.7 -5.7 --

Km14A-D 381155075091801 WO Dg 33 8/11/2010 38 11 55.6 75 09 18.8

C 6.56 -42.2 -47.2 0.12

Km14A-S 381155075091802 WO Dg 34 8/7/2010 38 11 55.6 75 09 18.8

A 6.50 -2.1 -7.1 --

Km14B-D 381157075092901 WO Dg 35 8/8/2010 38 11 57.1 75 09 29.1

D 4.45 -45.5 -50.5 --

Km14B-S 381157075092902 WO Dg 36 8/8/2010 38 11 57.1 75 09 29.1

A 4.38 -2.7 -7.7 --

Km23A-D 380731075105601 WO Ef 28 8/6/2010 38 07 31.4 75 10 56.2

D 6.88 -42.6 -47.6 --

Km23A-S 380731075105602 WO Ef 29 8/6/2010 38 07 31.4 75 10 56.2

C 6.92 -3.3 -8.3 --

Km23B-D 380730075110401 WO Ef 30 8/5/2010 38 07 30.5 75 11 04.3

D 5.88 -43.9 -48.9 --

Km23B-S 380730075110402 WO Ef 31 8/5/2010 38 07 30.5 75 11 04.3

B 4.77 -3.0 -8.0 0.05

Km23C-D 380731075111101 WO Ef 32 8/5/2010 38 07 31.0 75 11 11.5

D 4.67 -45.4 -50.4 --

KM23C-S 380731075111102 WO Ef 33 8/5/2010 38 07 31.6 75 11 11.3

B 4.40 -2.8 -7.8 0.07

Km23D-D 380734075111301 WO Ef 34 8/5/2010 38 07 34.1 75 11 13.8

D 4.38 -43.4 -48.4 --

Km23D-S 380734075111302 WO Ef 35 8/5/2010 38 07 34.1 75 11 13.8

B 5.27 -3.2 -8.2 --

Km28A-D 380457075122301 WO Ff 11 8/6/2010 38 04 57.7 75 12 23.8

B 5.99 -39.4 -44.4 --

Km28A-S 380457075122302 WO Ff 12 8/6/2010 38 04 57.6 75 12 24.0

A 5.01 -3.9 -8.9 0.16

Km28B-D 380512075125401 WO Ef 36 8/6/2010 38 05 12.4 75 12 54.4

D 4.47 -43.7 -48.7 --

Km28B-S 380512075125402 WO Ef 37 8/6/2010 38 05 12.4 75 12 54.4

A 3.92 -4.7 -9.7 --
Well Network Installation 

5

Table 1. Location, physical characteristics, and water-level information for 26 wells, Assateague Island National Seashore, Maryland, 2010.—Continued[The U.S. Geological Survey (USGS) well identification number is a 15-digit number. The first 6 digits are the latitude, the next 7 digits are the longitude, and the last 2 digits refer to the sequence number for identifying one or more wells at a particlular latitude and longitude; (° ′ ′′), degrees, minutes, seconds; ft, feet; NAVD 88, North American Vertical Datum of 1988; --, no data; conductance range, in microsiemens per centimeter (μS/cm), refer to figure 6, A = 0–1,300, B = 1,301–28,000, C = 28,001–60,000, D = greater than 60,000]

Field 

identifier

USGS 

well 

identification 

number 

Water-level date

Depth of 

water below 

measuring 

point 

(ft)

Waterlevel time 

(Eastern 

Standard 

Time)

Water-level 

altitude 

(ft NAVD 88)

Water-level date

Depth of 

water below 

measuring 

point 

(ft)

Waterlevel time 

(Eastern 

Standard 

Time)

Water-level 

altitude 

(ft NAVD 88)

Km3A-D 381804075063401 8/13/2010 7.46 819 0.30 -- -- -- --

Km3A-S 381804075063402 8/13/2010 4.84 817 2.85 -- -- -- --

Km3B-D 381805075063501 8/13/2010 12.63 836 0.27 -- -- -- --

Km3B-S 381805075063502 8/13/2010 10.14 838 2.60 -- -- -- --

Km9A-D 381450075075401 8/13/2010 5.33 1016 0.23 9/4/2010 4.21 1017 1.35

Km9A-S 381450075075402 8/13/2010 5.10 955 0.48 9/4/2010 3.96 1111 1.62

Km9B-D 381452075075901 8/13/2010 7.46 747 0.50 9/4/2010 6.77 1144 1.19

Km9B-S 381452075075902 8/13/2010 5.30 749 3.00 9/4/2010 5.65 1143 2.65

Km9C-D 381452075080101 8/13/2010 5.13 745 4.33 9/4/2010 4.37 1150 5.09

Km9C-S 381452075080102 8/13/2010 4.13 744 5.18 9/4/2010 4.33 1151 4.98

Km14A-D 381155075091801 -- -- -- -- 9/4/2010 5.20 1332 1.36

Km14A-S 381155075091802 -- -- -- -- 9/4/2010 3.51 1330 2.99

Km14B-D 381157075092901 -- -- -- -- 9/4/2010 3.69 1314 0.76

Km14B-S 381157075092902 -- -- -- -- 9/4/2010 3.27 1313 1.11

Km23A-D 380731075105601 8/12/2010 5.76 1439 1.12 9/20/2010 5.80 1105 1.08

Km23A-S 380731075105602 8/12/2010 4.49 1416 2.43 9/20/2010 3.94 1132 2.98

Km23B-D 380730075110401 -- -- -- -- 9/20/2010 4.61 1156 1.27

Km23B-S 380730075110402 -- -- -- -- 9/20/2010 3.09 1155 1.68

Km23C-D 380731075111101 -- -- -- -- 9/20/2010 3.62 1201 1.05

KM23C-S 380731075111102 -- -- -- -- 9/20/2010 3.60 1200 0.80

Km23D-D 380734075111301 8/12/2010 3.71 1304 0.67 9/20/2010 3.57 1229 0.81

Km23D-S 380734075111302 8/12/2010 4.65 1252 0.62 9/20/2010 4.68 1221 0.59

Km28A-D 380457075122301 8/12/2010 5.19 1147 0.80 9/20/2010 4.87 945 1.12

Km28A-S 380457075122302 8/12/2010 4.57 1133 0.62 9/20/2010 4.04 928 0.97

Km28B-D 380512075125401 8/12/2010 4.25 1048 0.22 9/20/2010 4.03 1002 0.44

Km28B-S 380512075125402 8/12/2010 5.12 1110 -1.20 9/20/2010 4.30 955 -0.38
6 Well Network Installation and Hydrogeologic Data Collection, Assateague Island National Seashore, Maryland, 2010 Table 1. Location, physical characteristics, and water-level information for 26 wells, Assateague Island National Seashore, Maryland, 2010.—Continued [The U.S. Geological Survey (USGS) well identification number is a 15-digit number. The first 6 digits are the latitude, the next 7 digits are the longitude, and the last 2 digits refer to the sequence number for identifying one or more wells at a particlular latitude and longitude; (° ′ ′′), degrees, minutes, seconds; ft, feet; NAVD 88, North American Vertical Datum of 1988; --, no data; conductance range, in microsie- mens per centimeter (μS/cm), refer to figure 6, A = 0–1,300, B = 1,301–28,000, C = 28,001–60,000, D = greater than 60,000] Field identifier USGS well identification number Water-level date Depth of water below measuring point (ft) Water-level time (Eastern Standard Time) Water-level altitude (ft NAVD 88) Availability and duration of continuous water-level data Km3A-D 381804075063401 -- -- -- -- -- Km3A-S 381804075063402 -- -- -- -- -- Km3B-D 381805075063501 -- -- -- -- -- Km3B-S 381805075063502 -- -- -- -- -- Km9A-D 381450075075401 9/28/2010 5.84 745 -0.28 Aug. 14 to Sept. 28, 2010 Km9A-S 381450075075402 9/28/2010 5.60 804 -0.02 Aug. 14 to Sept. 28, 2010 Km9B-D 381452075075901 9/28/2010 7.37 819 0.59 -- Km9B-S 381452075075902 9/28/2010 5.55 821 2.75 -- Km9C-D 381452075080101 9/28/2010 5.01 825 4.45 -- Km9C-S 381452075080102 9/28/2010 4.18 820 5.13 -- Km14A-D 381155075091801 -- -- -- -- -- Km14A-S 381155075091802 -- -- -- -- -- Km14B-D 381157075092901 -- -- -- -- -- Km14B-S 381157075092902 -- -- -- -- -- Km23A-D 380731075105601 9/28/2010 5.68 1022 1.20 Aug. 13 to Sept. 28, 2010 Km23A-S 380731075105602 9/28/2010 3.77 1012 3.15 Aug. 13 to Sept. 28, 2010 Km23B-D 380730075110401 9/28/2010 4.54 1041 1.34 -- Km23B-S 380730075110402 9/28/2010 2.61 1040 2.16 -- Km23C-D 380731075111101 9/28/2010 3.51 1046 1.16 -- KM23C-S 380731075111102 9/28/2010 3.01 1044 1.39 -- Km23D-D 380734075111301 9/28/2010 3.36 1057 1.02 Aug. 13 to Sept. 28, 2010 Km23D-S 380734075111302 9/28/2010 4.11 1055 1.16 Aug. 13 to Sept. 24, 2010 Km28A-D 380457075122301 9/28/2010 4.48 917 1.51 Aug. 13 to Sept. 28, 2010 Km28A-S 380457075122302 9/28/2010 3.17 914 1.84 Aug. 13 to Sept. 28, 2010 Km28B-D 380512075125401 9/28/2010 3.64 929 0.83 Aug. 13 to Sept. 28, 2010 Km28B-S 380512075125402 -- -- -- -- --
Well Network Installation 7

acquired using a Garmin eTrex GPS receiver with horizontal 

accuracy of 20 ft, and are referenced to the North American 

Datum of 1983 (NAD 83). Prior to drilling, all sites were field 

checked and approved by National Park Service (NPS) personnel to ensure that these sites were compliant with all Park 

permitting regulations. 

Altitudes for selected wells in each transect were surveyed using a Trimble 5800 Real Time Kinematic Rover GPS 

receiver and were referenced to the North American Vertical 

Datum of 1988 (NAVD 88). Uncertainty in altitude determination for these wells ranged from 0.05 to 0.13 ft. The altitudes 

of all other well sites in each of the five transects were determined with a digital level using the surveyed well as a reference. Wells were surveyed to an accuracy of 0.01 ft. (table 1).

 Boreholes were drilled to the desired depth using 

5.625-inch (in.)-outer diameter, 2.25-in.-inner diameter, 

hollow-stem augers. Because only a small part of the island 

has paved roads, all of the wells, with the exception of those 

drilled along the Kilometer Marker 14 (Km14) transect, were 

installed with a truck-mounted CME-55 hollow-stem auger 

drill rig (fig. 3). This pickup-mounted drill rig allowed for 

greater site accessibility, but restricted the total drilling depth 

to less than 55 ft at most locations. Because the area in the 

vicinity of the Km14 transect was paved, a truck-mounted 

CME-75 hollow-stem auger drill rig was used at the two well 

pairs along this transect.

Prior to drilling, the drill bit was sealed using a temporary polyvinyl chloride (PVC) plug to prevent aquifer material from filling the auger cavity. Wells were constructed 

using flush-threaded, 2-in.-diameter, schedule-80 PVC casing 

and factory-cut, 0.010-in. slot-size, 5-ft PVC screens. Once 

drilled to depth, well casings and screens were pre-assembled 

and lowered in place inside the auger annulus prior to auger 

removal. The well casing and screen assembly were used to 

dislodge the PVC plug allowing formation material to collapse 

around the screen and casing and hold the well in place.

Well pairs were installed along five east-west trending 

transects with each transect including between two and four 

well pairs consisting of a shallow well and a deeper well 

(fig. 2). Each of the shallow wells typically was screened several feet below the water table – usually in freshwater about 10 

ft bls. Each of the deeper wells was screened below the anticipated depth to the freshwater-saltwater interface—usually in 

saltwater about 45 to 55 ft bls (Krantz, 2009) (table 1). 

Two nomenclature systems are used to describe the 

transect locations and well names. Wells were labeled in 

accordance with standard naming procedures described by the 

Maryland Department of the Environment and are based on 

an alphanumeric system where the first two characters (letters, 

both capitalized), indicate the county (in this case, WO for 

Worcester). The next two characters (the first capitalized, 

the second lower case) correspond to a 5-minute county grid 

Figure 3. Truck-mounted hollow-stem auger drill rig used for monitoring well installation on 

Assateague Island National Seashore, Worcester County, Maryland. [Photograph by William Banks, 

U.S. Geological Survey, Aug. 8, 2010.]
8 Well Network Installation and Hydrogeologic Data Collection, Assateague Island National Seashore, Maryland, 2010

block. The last two characters (numbers) represent a sequence 

within the grid block. The second system complies with the 

local NPS naming conventions on Assateague Island and is 

based on the Kilometer Markers starting with kilometer 0 at 

Ocean City Inlet at the north end of the island and increasing 

to the south, ending at the Maryland-Virginia border. Using 

the second system, wells were identified first by Kilometer 

Marker number, then by letter sequence beginning with “A,” 

representing the easternmost well and ending with the westernmost well (“B,” “C,” or “D,” depending on transect). The 

final character denotes depth and is either a “D” for deeper or 

an “S” for shallow (table 1).

Hydraulic connection between the newly installed wells 

and the surrounding aquifer material was achieved by using 

either compressed air or a small-volume electric pump to 

remove the majority of aquifer material that may have entered 

the well during installation, and to allow for the free flow of 

water between the surrounding aquifer and the well; a process 

typically referred to as “well development.” Wells located 

along the Km14 transect were developed using compressed 

air. This method required that a volume of air under pressure 

(usually 100 pounds per square inch) be injected near the 

bottom of the well, causing a corresponding volume of water 

and sand to be ejected from the well head. Once evacuated, the 

well was allowed to recover for up to 5 minutes and the process was then repeated. The well development process was run 

as many times as necessary until little or no sand was ejected 

from the well. 

For wells located along the remaining transects, a lowvolume, battery-powered pump was lowered to the bottom 

of the well screen and used to evacuate all of the water and 

sand present in the well. The well was then allowed to recover 

for up to 15 minutes and the process was repeated. In some 

cases, it was necessary to repeat this process for several days. 

Wells that were developed using the pump method were more 

likely to retain some sand material in the well despite repeated 

attempts at well development. 

The selection of transect and well locations was based 

on several factors including island geomorphology, consideration of potential impact on sensitive or endangered plants and 

animals, and drill-rig access limitations. The four southernmost transect locations (Km9, Km14, Km23, and Km28) were 

selected to complement previous geophysical and hydrologic 

data-collection efforts (Krantz, 2009). 

The northernmost transect (Km3) just south of the Ocean 

City Inlet is in an area where the island is approximately 1,100 

ft wide (fig. 2). This low-lying part of the island is subject to 

frequent and extensive overwash events, but it is also interspersed with sand dunes up to 15 ft in height and serves as a 

critical location for Piping Plover nesting habitat (Jack Kumer, 

National Park Service, oral commun., 2010). Two well pairs 

were installed in an east-west transect near the center of the 

island along an assumed path of groundwater flow orthogonal 

to the long axis of the island. The “B” well pair was located 

near a dune crest approximately 500 ft from Sinepuxent Bay, 

whereas the “A” well pair was located in a potential overwash 

zone approximately 500 ft from the Atlantic Ocean. The 

altitude of the bottom of both shallow wells is about 7 ft below 

sea level (NAVD 88), whereas the bottom of both deeper wells 

is about 47 ft below sea level (NAVD 88) (table 1). 

Three well pairs were installed along the transect at Km9, 

where the island is about 1,800 ft wide. The “C” (westernmost) pair was installed about 1,030 ft from the ocean and 

about 760 ft from Chincoteague Bay. The “B” (middle) pair 

was installed about 800 ft from the Atlantic Ocean and about 

1,000 ft from Chincoteague Bay. The “A” (easternmost) pair 

was installed less than 400 ft from the Atlantic Ocean, and 

about 1,400 ft from Chincoteague Bay. The eastern side of the 

island in the area where well pair Km9A is located consists of 

open beach and is subject to periodic overwash events. The 

center of the island near the location of well pair B consists 

of low-relief dunes and inland beach grasses, and the western 

side of the island near the location of well pair C is in an area 

dominated by estuarine wetlands. 

 The bottoms of the shallow wells in the C and B pairs 

are screened approximately 6 and 11 ft, respectively, below 

sea level (NAVD 88) whereas the bottoms of the deeper wells 

in the C and B pairs are screened about 45 ft below sea level 

(NAVD 88). Given the proximity of well pair A to the ocean 

(less than 400 ft), the wells were installed so that only 10 ft 

separates the bottom of the screens in the deeper and shallow 

wells—about 19 and 9 ft, respectively, below sea level (NAVD 

88) (table 1). 

The Km14 transect is located where the island is approximately 3,000 ft wide and in an area that is used for camping 

and other recreational activities. The wells in the A pair are 

located in the campground west of a manmade dune, approximately 500 ft from the Atlantic Ocean (fig. 1). Well pair B is 

located in a wooded area near the center of the island, about 

1,500 ft from both Chincoteague Bay and the Atlantic Ocean 

(fig. 1). The bottom altitudes for the deeper wells in each well 

pair are between about 47 and 51 ft below sea level (NAVD 

88), whereas the bottom altitudes for both of the shallow wells 

are about 7 ft below sea level (NAVD 88) (table 1).

The Km23 transect is located in the oldest and most 

stable part of the island, known as the island core (Krantz, 

2009). Four well pairs were installed in this transect across the 

3,400-ft width of the island, along an existing east-west oriented access road. The altitudes of the bottom of the screened 

intervals for the four deeper wells range from approximately 

47 to 51 ft below sea level (NAVD 88). The altitudes of the 

bottom of the screened intervals of the shallow wells were all 

approximately 8 ft below sea level (NAVD 88). 

The Km28 transect consists of two well pairs located 

along an existing access road in the widest part (nearly 

7,100 ft) of the island core. The altitude of the bottom of the 

screened interval of the deeper well of the B (westernmost) 

well pair is about 49 ft below sea level (NAVD 88). The 

deeper well in the A well pair is about 44 ft below sea level 

(NAVD 88). The altitudes of the bottom of the shallow well 

screens in the two well pairs are approximately 9 ft below sea 

level (NAVD 88) (table 1).
Hydrogeologic Data Collection 9

Hydrogeologic Data Collection

Hydrogeologic data collected during this study included 

water levels and borehole geophysical logs. Periodic manual 

measurements were made at all 26 of the monitoring wells; 

9 of these wells (4 shallow and 5 deeper) were instrumented 

for continuous water-level data recording during a 7-week 

period between August 12 and September 28, 2010. Borehole 

geophysical logs were collected from the 13 deeper wells to 

characterize changes in lithology and salinity in the shallow 

groundwater flow system.

Water Levels

Water levels were measured at least two or three times in 

each monitoring well between August 12 and September 28, 

2010. The number of measurements was dependent upon site 

access due to changing weather and beach conditions (table 1). 

In order to measure the altitude of water levels relative to a 

fixed datum, a permanent measuring point referenced to sea 

level (NAVD 88) was established on the top of each well casing. The height above land surface for each measuring point is 

listed in table 1. Water-level altitudes in all of the observation 

wells were then calculated by subtracting the depth to water 

from the altitude of the permanent measuring point. These 

measurements were then recorded to the nearest 0.01 ft 

(table 1).

 Continuous water-level measurements are useful for 

determining the potential influences of changes in precipitation and tides on water levels both in terms of the magnitude 

and timing of any potential response. Nine of the 26 wells 

were instrumented with vented pressure transducers (In-Situ 

Troll 500s and In-Situ Aqua Troll 200s) to record water levels 

continuously at 15-minute intervals during the 7-week period. 

Water levels were not corrected to account for density differences due to saltwater, and all transducers were calibrated 

prior to deployment to a water density of 0.999 grams per 

cubic centimeter (g/cm3) (table 1). 

The four instrumented shallow wells included the A 

series wells at Km9, Km23, and Km28 and the D series wells 

at Km23. The five instrumented deeper wells included the 

same well series as the instrumented shallow wells and also 

the deeper well at Km28B. These wells were selected because 

of their proximity to the ocean and, in the case of the wells at 

Km23D, to Chincoteague Bay, to determine how tides may 

affect water levels in these areas. Tides around Assateague 

Island exhibit semi-diurnal or daily cycles (two high and two 

low tides daily) and lunar cycles that vary bi-weekly and 

exhibit a relative maxima (spring tide) and relative minima 

(neap tide). Intermittent manual water-level measurements 

also were collected from these wells to verify the accuracy of 

the data recorders. 

Water levels collected at each of the continuously measured monitoring wells were plotted along with tide data from 

the National Oceanic and Atmospheric Administration’s Ocean 

City Inlet, Md. tide gage (station 8570283) and precipitation 

data from a weather station operated by NPS personnel at 

Assateague Island National Park headquarters, 1 mile west of 

the island (Brian Sturgis, National Park Service, written commun., 2010), in order to illustrate the factors that may affect 

the change in water levels over time in both the shallow and 

deeper flow systems. An analysis of these data indicates that 

tidal fluctuations of as much as 3 ft appear to have a minor 

influence on inland water levels as shown by the small perturbations (water-level changes less than 0.1 ft) in the hydrographs for both the shallow and deeper wells (figs. 4 and 5). 

This influence did not appear to differ across the island from 

the ocean to the bay as shown by the similar lack of waterlevel response at well pair Km28A (fig. 4).

The largest effect on water levels in all of the shallow 

wells where continuous data were collected appeared to be 

the periodic precipitation events that occurred throughout 

the measurement period. Increases in water levels in these 

wells were directly related to the amount of precipitation 

that occurred during this period. The largest precipitation 

event occurred on August 18, 2010 and resulted in waterlevel increases of about 0.7 ft in response to the nearly 1.8 in. 

of precipitation. However, the largest water-level increases 

observed during the measurement period were observed in the 

shallow wells at both the Km9A and Km23A sites (1.24 and 

1.44 ft, respectively) and occurred on September 3, 2010 after 

a precipitation event of only 0.34 in. These anomalously high 

water-level increases were most likely the result of overland 

flooding produced by a storm surge that allowed for rapid 

infiltration down the well annulus, causing a spike in water 

levels at these two sites—a response that was not observed at 

the other well transects. 

Because the data-collection period was about 7 weeks, 

it was also possible to partly assess potential longer-term 

astronomical and meteorological effects on the groundwater 

system. The sinusoidal change in tidal elevation over the measurement period represents a change in the ambient sea-level 

position that was most likely the result of building onshore 

flow generated by changes in the prevailing wind field from 

passing weather systems. Daily tidal amplitude did not vary 

over this time period and indicated that spring and neap tide 

influences are negligible. 

The plots of the continuous water levels in the shallow 

wells do not show effects from these longer-term astronomical 

and meteorological conditions/events compared to the short 

duration precipitation and daily tidal events. However, the 

water levels in the deeper wells did appear to be influenced 

by changes in ambient sea level over the measurement period. 

This effect was most notable from September 18–22, 2010, 

when a rise in water levels was observed with no corresponding precipitation event. This water-level rise appeared to coincide with an increase in the ambient sea level that was most 

likely the result of building onshore flow, which is consistent 

with water-level changes observed in a hydrogeologically 

similar groundwater system on Fire Island, N.Y. (Schubert, 

2010). 
10 Well Network Installation and Hydrogeologic Data Collection, Assateague Island National Seashore, Maryland, 2010

Figure 4. Continuous water-level data, intermittent manual measurements, tide altitude data, and total daily 

precipitation data for the shallow monitoring wells at Km9A, Km23A, Km23D, and Km28A from August 12 to 

September 28, 2010, Assateague Island National Seashore, Maryland. [Tide data from National Oceanic and 

Atmospheric Administration, 2010; precipitation data from Brian Sturgis, National Park Service, written commun., 

2010.]
Hydrogeologic Data Collection 11

Figure 5. Continuous water-level data, intermittent manual measurements, tide altitude data, and total daily 

precipitation data for the deeper monitoring wells at Km9A, Km23A, Km23D, Km28A, and Km28B from August 12 

to September 28, 2010, Assateague Island National Seashore, Maryland. [Tide data from National Oceanic and 

Atmospheric Administration, 2010; precipitation data from Brian Sturgis, National Park Service, written commun., 

2010.]
12 Well Network Installation and Hydrogeologic Data Collection, Assateague Island National Seashore, Maryland, 2010

Although the water levels in the deeper wells all appeared 

to be influenced by changes in the ambient sea level, and to a 

lesser extent, precipitation events, the extent to which water 

levels were affected varied among the sites, indicating that 

changes in lithology and salinity also may affect groundwater 

flow across the island and between the shallow and deeper 

parts of the flow system. An example of the differences in the 

flow systems of the shallow wells and deeper wells is most 

apparent at the Km23A well pair, where the shallow well 

is mainly influenced by precipitation and periodic overland 

flooding, such as the event that occurred on September 4, 

2010; the deeper well showed a minor response to precipitation, but appears to have been more affected by changes in 

the ambient sea level (figs. 4 and 5). In contrast, there appears 

to be no difference in the response of water levels between 

the shallow and deeper wells at Km9A (fig. 5), which may 

be caused by the small vertical distance (10 ft) between these 

well screens compared to the 40-ft difference at Km23A 

(table 1).

Borehole Geophysical Logging 

Natural gamma and electromagnetic induction (EMI) logs 

were collected at each of the 13 deeper wells installed along 

the 5 transects to help characterize changes in the aquifer 

material and the transition between fresh and saltwater in the 

shallow groundwater flow system. Gamma logging measures 

the natural-gamma radioactivity of the material surrounding 

the borehole (Keys, 1990). The most significant, naturally 

occurring sources of gamma radiation are potassium-40 and 

daughter products of uranium and thorium decay series. 

Gamma emissions can commonly be correlated with sediment type in unconsolidated materials such as those found at 

ASIS. Potassium-40 is abundant in some feldspar and mica, 

and uranium and thorium can be concentrated by geochemical 

processes. Gamma logs are typically recorded in counts per 

second at 0.1-ft intervals. 

Deviations in the gamma log indicate changes in lithology or the presence of altered zones or mineralization. The 

vertical resolution of the gamma probe is generally 1 to 2 ft, 

and the probe is able to detect gamma radiation through plastic 

and steel casing. Because the gamma log does not have a 

unique lithologic response, interpretation must be correlated 

with other information such as drilling logs and descriptions 

of geologic cores and cuttings where available as well as 

other geophysical logs. In general, clays and silts have higher 

gamma counts than sands and gravels. 

EMI logging records the electrical conductivity of the 

sediment and the fluids in the sediment surrounding the 

borehole (Williams and others, 1993). Changes in electrical 

conductivity are caused by variations in porosity, borehole 

diameter, the concentration of dissolved constituents in the 

pore fluid, and the amount of metallic minerals. The EMI 

logging tool is most sensitive to the aquifer material and pore 

water approximately 1 ft away from the probe, and the tool 

has a vertical resolution of approximately 2 ft. In boreholes 

with diameters of 6 in. or less, the conductivity of the borehole 

fluids has a negligible effect on the induction log response. 

The full-tool response is about 10 ft in thickness, which means 

that it requires a 10-ft-thick zone to get a true conductivity 

measurement of the formation. If there is a narrow layer 

(less than 10 ft thick) and it is sufficiently conductive, it may 

be detected, but the actual conductivity of that layer will be 

greater than what is measured with the tool. The tool response 

of thin conductive zones (less than 1.6 ft thick) is expected to 

be less than half of the zone’s true conductivity (Taylor and 

others, 1989). For this investigation, EMI conductivity was 

recorded in millisiemens per meter (mS/m) at 0.1-ft intervals. 

The tool was calibrated to 465 mS/m and to air (0 mS/m) at 

the time of logging. The EMI tool was allowed to thermally 

equilibrate prior to calibration and logging. 

The EMI log was used to delineate changes in the aquifer 

materials and (or) changes in electrical properties of pore 

water (for instance, to identify freshwater or saltwater). In 

freshwater, clays and silts generally have higher electrical 

conductivity than the EMI conductivity of sands and gravels. 

However, if the sand or gravel unit is saturated with a highly 

conductive fluid, such as brackish or saltwater, then the EMI 

conductivity of the coarse material can have a higher EMI 

conductivity value than that of the fine materials. Because of 

this complexity, the gamma and EMI logs were interpreted 

together to determine whether increased EMI conductivity 

is likely caused by the presence of clay and silt or by electrically conductive water. In addition, specific conductance (SC), 

temperature, and water-level values were collected in both the 

shallow and deeper wells to help interpret the borehole logs 

and identify the source of the conductivity. Since the wells 

had been developed prior to the borehole logging, the water 

in the wells was thought to be generally representative of the 

screened interval, which was in the lowermost 5 ft of the boreholes. To confirm this theory/assumption, SC was measured in 

several places over the length of the screened interval to verify 

that the water in the well was not stratified. 

A comparison of the natural gamma and EMI logs collected at the 13 deeper wells in the 5 transects provides the 

information necessary to characterize changes in aquifer 

material and determine both the extent of the shallow freshwater lens and the presence of a deeper freshwater flow system 

underlying Assateague Island. All logs were converted from 

depth below top of casing to altitude for easier comparison. 

Natural gamma and EMI data can be used to better understand 

the water-level changes observed in the shallower and deeper 

parts of the groundwater flow system over time (figs. 6a-e). 

Profile at Km3

The well transect at Km3 is shown in the form of a 

profile with 3B on the west (left) and 3A on the east (right) 

(fig. 6a and fig. 2). The SC measurement collected at the time 

of logging in Km3B-S was freshwater (1,340 microsiemens 

per centimeter at 25 degrees Celsius or μS/cm), and the SC 
Hydrogeologic Data Collection 13

in Km3B-D was mildly conductive (10,400 μS/cm). In both 

wells Km3B and Km3A, the EMI logs showed an increase in 

EMI conductivity at an altitude of 0 ft and another increase 

at an altitude at about -25 ft, where there was also an increase 

in gamma counts. The increase in gamma is interpreted as a 

change in lithology from sand to finer sand, silt, or clay. The 

gamma counts were fairly uniform from -25 ft to the bottom of 

the boreholes. However, in the bottom of the boreholes, EMI 

conductivity declined from an altitude of about -35 ft to the 

bottom of the holes, indicating that this change in conductivity 

is water-related. Although the EMI and gamma patterns were 

fairly similar in wells Km3B and Km3A, the SC measurements were different. The SC in the shallow well at Km3A-S 

was moderately conductive at 25,300 μS/cm (more conductive than 3B-S), and the water in Km3A-D was fresh (1,410 

μS/cm). The gamma logs were fairly consistent between wells 

Km3B and Km3A and may indicate the presence of a fairly 

continuous low-permeability layer. These results, along with 

anecdotal evidence that this location is prone to overwash 

events, are consistent with the infiltration of saltwater downward into an aquifer consisting of sand that overlies a more 

electrically conductive formation with fine sands and silts with 

conductive water, that in turn, overlies freshwater.

Profile at Km9

The profile at Km9 is shown in figure 6b with Km9C 

on the west (left) and Km9A on the east (right) (fig. 6b). The 

SC in the well pair at 9C indicated highly conductive water 

(56,700 μS/cm) in the shallow well and freshwater (1,100 

μS/cm) in the deeper well. The SC in the well pair at 9B had 

a moderately high SC (36,100 μS/cm) in the shallow well and 

freshwater (810 μS/cm) in the deeper well. The well pair at 9A 

had moderately conductive water in the shallow well (18,200 

μS/cm) and very high SC (65,000 μS/cm) in the deeper well, 

which is not that much deeper than the shallow well at 9B. 

Collectively, these SC measurements indicate increased SC in 

the shallow to mid-range depths and freshwater at depth. 

In the uppermost sections of the Km9 wells, the gamma 

counts were fairly low, which is consistent with the presence 

of medium- to coarse-grained sand, and the EMI conductivity 

showed minor increases at altitudes of +2, -5, and -4 ft in 

wells Km9C, Km9B, and Km9A, respectively. In each of the 

well pair locations, the increase in EMI conductivity occurred 

at altitudes higher than the increase in gamma emissions, 

indicating that the increase in EMI conductivity is fluidrelated, which is consistent with the SC values in the wells. 

The gamma logs for all three wells along this profile are fairly 

similar and show increases in gamma counts at altitudes of 

about -8, -12, and -15 ft (in wells 9C, 9B, and 9A, respectively). This increase in gamma counts is likely caused by the 

presence of fine sand, silt, and (or) clay. 

There are segments in the gamma logs that have patterns 

that can be traced in the logs of other nearby boreholes. One 

segment shows a decrease in gamma counts in well Km9C at 

an altitude of -22 ft, and is similar in shape and magnitude to a 

decrease in gamma counts in well Km9B at -24 ft. This zone, 

which is likely a layer of relatively coarser material (sand), 

also coincides with the gradual decline in EMI conductivity in 

wells Km9C and Km3B. Although the gamma counts increase 

slightly below the coarse material, the EMI conductivity 

values remain low in both wells, which is consistent with the 

presence of freshwater at depth. Collectively, these results 

indicate the presence of freshwater in the uppermost sand that 

overlies saltwater in the fine-grained sands or silts that extend 

to about -20 to -25 ft. Below an altitude of -24 ft, the materials 

are likely sand and silts with freshwater. A comparison of the 

EMI and gamma logs collected in wells Km 9C, 9B, and 9A 

shows that they are fairly similar, indicating that the lithology 

is somewhat continuous across the well transect. 

Profile at Km14

The profile at Km14 is shown in figure 6c with Km 14B 

on the west (left) and Km14A on the east (right) (fig. 6c). The 

SC collected in the well pair at Km14B at the time of logging 

showed freshwater (630 μS/cm) over highly conductive 

saltwater (60,300 μS/cm) in the deeper well. The SC collected 

in the Km14A well pair showed freshwater (305 μS/cm) over 

highly conductive saltwater (50,300 μS/cm) in the deeper 

well. The EMI logs show gradual increases in conductivity 

starting at an altitude of about -10 ft in wells Km14B and Km 

14A. The gamma logs in these boreholes show fairly abrupt 

increases in gamma emissions at an altitude of about -12 ft, 

indicating the presence of fine materials. The gamma log in 

well Km14A also shows a 3-ft zone at an altitude of about -37 

to -40 ft with a slightly lower gamma count and a corresponding break in slope and reduction in EMI conductivity, which 

was likely caused by a sandy layer. A similar decline in EMI 

conductivity is observed at the same depth in well Km14B, but 

with a less defined gamma signature. 

In general, the gamma and EMI logs along the Km14 

profile are fairly consistent, and layers that can be observed 

in both may be horizontal. Although the gamma and EMI 

logs indicate a change in the lithology from relatively coarser 

material at the surface to finer material [fine sand, silt, and 

(or) clay] at depth, the SC data indicate these changes are also 

water-related, as there is freshwater in the shallow wells over 

saltwater in the deeper wells. This distribution of materials and 

fluid conductivities is consistent with a freshwater lens derived 

from precipitation overlying salty groundwater. 

Profile at Km23

The well transect at Km23 is shown in figure 6d with 

Km23D on the west (left) and Km23A on the east (right) 

(fig. 6d). The SC collected at the time of logging in well 

Km23D showed freshwater (less than 1,420 μS/cm) in the 

shallow well. The shallow wells at Km23C, B, and A had 

slightly to moderately elevated SC (from 4,300 to 31,700 

μS/cm) indicating infiltration of salty surface water, which 
14 Well Network Installation and Hydrogeologic Data Collection, Assateague Island National Seashore, Maryland, 2010

may be caused by periodic overwash events from storm 

surges. In the deeper wells (at altitudes of about -45 ft), all 

four wells had saltwater (greater than 64,000 μS/cm) at depth 

(fig. 6d). An increase in the EMI conductivity coincides with 

an increase in gamma counts, which together are consistent 

with a change in lithology from sand to fine materials (fine 

sand, silt, clay) at about 20 to 23 ft bls. 

The EMI log in well Km23A was different from those 

in the other wells in the shallowest part of the aquifer. This 

log showed an increase in EMI conductivity from -2 to -7 ft 

altitude, which was consistent with a high SC value observed 

in wells Km23A-S. One possible explanation could be that 

brackish water was introduced through the well annulus. A 

mechanism for this process would be overwash from a nearby 

pond of unknown conductivity. It is unlikely (but not impossible) that this site experienced overwash from ocean water as 

the site is about 800 ft from the Atlantic Ocean. The EMI and 

gamma logs in well KM23A below this shallow conductive 

zone were fairly similar to those from the other wells along 

the profile. In the Km23D, C, B, and A profiles, the gamma 

logs show an increase in the gamma counts at an altitude of 

-22 to -27 ft. This increase in gamma counts is interpreted as 

an increase in fine-grained materials. Other minor decreases 

in gamma counts from about -35 to -40 ft may indicate the 

presence of a coarser or sandy layer similar to that observed 

in wells Km23C, B, and A. The EMI logs in wells Km23D, C, 

and B decrease at about -38 ft. The similarities in the EMI and 

gamma logs indicate that the layers are fairly continuous and 

level. 

Collectively, the logs indicate that along the transect at 

Km23, there is either sand and freshwater or sand and brackish water at shallow depths, overlying finer-grained materials 

with highly conductive water from altitudes starting at about 

-22 to -25 ft and extending to the bottom of the boreholes at 

an altitude of about -48 ft. There was no freshwater found at 

depth in these boreholes that might indicate the presence of a 

regional groundwater flow system. Surface geophysics suitable for deeper imaging or a deeper well would be needed to 

estimate or measure the conductivity below the depths of these 

boreholes. 

Profile at Km28

The well transect at Km28 is shown in figure 6e with 

Km28B on the west (left) and Km28A on the east (right) 

(fig. 6e). The SC collected at the time of logging in Km28B 

indicated freshwater (775 μS/cm) over saltwater (greater than 

100,000 μS/cm). This is consistent with findings by Krantz 

(2009) that indicate the groundwater is highly dynamic, 

ranging from freshwater to saltwater with respect to episodic 

overwash events (Krantz, 2009). The SC in the well pair in 

Km28A indicates freshwater (185 μS/cm) over moderately 

conductive (25,500 μS/cm) water. The EMI log in Km28B 

shows a fairly gradual increase in the conductivity starting 

from an altitude of -15 ft and increasing to the bottom of the 

borehole. The gamma log shows a minor increase in counts at 

an altitude of about -16 ft, indicating a change in the aquifer 

materials from coarser to finer. The increase in EMI conductivity in Km28A is coincident with an abrupt increase in the 

gamma counts at an altitude of about -24 ft. The EMI conductivity decreases at the same location where the gamma 

decreases at altitudes of about -35 to -39 ft, indicating the 

presence of a medium- to coarse-grained sand layer. Below 

this sand (or relatively coarser) layer, the gamma count 

increases and the EMI conductivity continues to decline; however, the EMI log from the deeper well at Km28A indicates 

there is conductive water at depth. Collectively, the logs in this 

profile indicate that there is a freshwater sand layer overlying 

fine materials and saltwater at the depths of these boreholes. 

Deeper drilling or deep-imaging surface geophysical logging 

at this location would be necessary in order to determine 

the presence of the regional freshwater groundwater flow 

system possibly observed in the profiles in the northern part of 

Assateague Island. 

Consideration for Additional 

Investigation

The results of this investigation show a need for further 

study. A comparison of the natural gamma and EMI logs collected at the 13 deeper wells in all 5 transects, along with measurements of SC, could provide information that is useful in 

characterizing changes in aquifer material and determining the 

extent of the shallow freshwater lens underlying Assateague 

Island as characterized in this investigation. Installation of a 

deeper borehole or deep-imaging surface geophysical methods 

would be needed to identify the presence of a deeper freshwater system and to support more in-depth lithologic interpretations. Results of this investigation show the need for the 

collection of continuous water-level data in both the shallow 

and deeper parts of the flow system, and analysis of EMI and 

natural gamma geophysical logging data to better understand 

the response of this groundwater system to changes in precipitation and tidal forcing. Hydrologic data collected as part of 

this investigation will serve as the foundation for the development of numerical flow models to assess the potential effects 

of climate change on groundwater flow in the shallow aquifer 

system of Assateague Island. 
Consideration for Additional Investigation 15

Km3B 

Top of casing

Top of casing

1,340

10,400

10

 5

 0

-5

-10

-15

-20

-25

-30

-35

-40

-45

-50

25,300

1,410

Approximate altitude, in feet (North American Vertical Datum of 1988)

Km3A 

 EMI 

Conductivity

0 1,000

Gamma

0 cps 100

 Specific 

Conductance

0 100,000

mS/m

μS/cm

 EMI 

Conductivity

0 1,000

Gamma

0 cps 100

 Specific 

Conductance

0 100,000

mS/m

μS/cm

Screened 

interval

Screened 

interval

Screened 

interval

Screened 

interval

Figure 6a. Natural gamma and electromagnetic induction logs collected at transect Km3, Assateague Island National 

Seashore, Maryland [Electromagnetic induction (EMI) conductivity was collected in millisiemens per meter (mS/m); natural 

gamma in counts per second (cps); specific conductance in microsiemens per centimeter (μS/cm) at 25 degrees Celsius with 

the numeric value shown next to the blue scale bar; black and white striped box indicates the location of the screened interval].
16 Well Network Installation and Hydrogeologic Data Collection, Assateague Island National Seashore, Maryland, 2010

10

 5

 0

-5

-10

-15

-20

-25

-30

-35

-40

-45

-50

Top of casing

56,700

1,100

18,200

65,000

Top of casing

36,100

810

Top of casing

Screened 

interval

Screened 

interval

Screened 

interval

Screened 

interval

Screened 

interval

Screened 

interval

Km9B Km9A 

 EMI 

Conductivity

0 1,000

Gamma

0 cps 100

 Specific 

Conductance

0 100,000

mS/m

μS/cm

 EMI 

Conductivity

0 1,000

Gamma

0 cps 100

 Specific 

Conductance

0 100,000

mS/m

μS/cm

Km9C

 EMI 

Conductivity

0 1,000

Gamma

0 cps 100

 Specific 

Conductance

0 100,000

mS/m

μS/cm

Approximate altitude, in feet (North American Vertical Datum of 1988)

Figure 6b. Natural gamma and electromagnetic induction logs collected at transect Km9, Assateague Island National 

Seashore, Maryland [Electromagnetic induction (EMI) conductivity was collected in millisiemens per meter (mS/m); natural 

gamma in counts per second (cps); specific conductance in microsiemens per centimeter (μS/cm) at 25 degrees Celsius 

with the numeric value shown next to the blue scale bar; black and white striped box indicates the location of the screened 

interval].
Consideration for Additional Investigation 17

Figure 6c. Natural gamma and electromagnetic induction logs collected at transect Km14, Assateague Island National 

Seashore, Maryland [Electromagnetic induction (EMI) conductivity was collected in millisiemens per meter (mS/m); natural 

gamma in counts per second (cps); specific conductance in microsiemens per centimeter (μS/cm) at 25 degrees Celsius with the 

numeric value shown next to the blue scale bar; black and white striped box indicates the location of the screened interval].

 5

 0

-5

-10

-15

-20

-25

-30

-35

-40

-45

-50

Top of casing

Top of casing

632

60,300

305

50,300

Screened 

interval

Screened 

interval

Screened 

interval

Km14B Km14A 

 EMI 

Conductivity

0 1,000

Gamma

0 cps 100

 Specific 

Conductance

0 100,000

mS/m

μS/cm

 EMI 

Conductivity

0 1,000

Gamma

0 cps 100

 Specific 

Conductance

0 100,000

mS/m

μS/cm

Approximate altitude, in feet (North American Vertical Datum of 1988)
18 Well Network Installation and Hydrogeologic Data Collection, Assateague Island National Seashore, Maryland, 2010

 5

 0

-5

-10

-15

-20

-25

-30

-35

-40

-45

-50

1,420 10,300 4,305 31,700

95,550

78,500 67,600

64,000

Top of casing

Screened 

Screened interval

Screened interval

interval

Screened 

interval

Screened 

interval

Screened 

interval

Screened 

interval

Screened 

interval

Top of casing Top of casing Top of casing

Km23B Km23A 

 EMI 

Conductivity

1,000

Gamma

0 cps 100

 Specific 

Conductance

0 100,000

mS/m

μS/cm

Km23C

0

 EMI 

Conductivity

1,000

Gamma

0 cps 100

 Specific 

Conductance

0 100,000

mS/m

μS/cm

0

 EMI 

Conductivity

1,000

Gamma

0 cps 100

 Specific 

Conductance

0 100,000

mS/m

μS/cm

0

 EMI 

Conductivity

1,000

Gamma

0 cps 100

 Specific 

Conductance

0 100,000

mS/m

μS/cm

0

Km23D

Approximate altitude, in feet (North American Vertical Datum of 1988)

Figure 6d. Natural gamma and electromagnetic induction logs collected at transect Km23, Assateague Island National Seashore, 

Maryland [Electromagnetic induction (EMI) conductivity was collected in millisiemens per meter (mS/m); natural gamma in counts per 

second (cps); specific conductance in microsiemens per centimeter (μS/cm) at 25 degrees Celsius with the numeric value shown next 

to the blue scale bar; black and white striped box indicates the location of the screened interval].
Consideration for Additional Investigation 19

Figure 6e. Natural gamma and electromagnetic induction logs collected at transect Km28, Assateague Island National Seashore, 

Maryland [Electromagnetic induction (EMI) conductivity was collected in millisiemens per meter (mS/m); natural gamma in counts 

per second (cps); specific conductance in microsiemens per centimeter (μS/cm) at 25 degrees Celsius with the numeric value 

shown next to the blue scale bar; black and white striped box indicates the location of the screened interval].

 5

 0

-5

-10

-15

-20

-25

-30

-35

-40

-45

-50

185

25,500

775

>100,000

Top of casing

Screened 

interval

Screened 

interval

Screened 

interval

Screened 

interval

Top of casing

Km28B Km28A 

 EMI 

Conductivity

0 1,000

Gamma

0 cps 100

 Specific 

Conductance

0 100,000

mS/m

μS/cm

 EMI 

Conductivity

0 1,000

Gamma

0 cps 100

 Specific 

Conductance

0 100,000

mS/m

μS/cm

Approximate altitude, in feet (North American Vertical Datum of 1988)
20 Well Network Installation and Hydrogeologic Data Collection, Assateague Island National Seashore, Maryland, 2010

Summary

The U.S. Geological Survey, through the Climate and 

Land Use Change Research and Development Program, 

conducted a multi-year investigation to assess potential 

impacts on the natural resources of Assateague Island National 

Seashore (ASIS), Maryland that may result from changes in 

the hydrologic system in response to projected sea-level rise. 

A network of 13 well pairs was installed to monitor shallow 

groundwater at ASIS. Well installation was completed between 

August 1 and August 13, 2010. Well location information, 

depths, and selected water-level data are provided, as well as 

latitude, longitude, and measuring-point altitude data for each 

well. Altitude data were acquired using a surveying tool linked 

to a global navigation satellite system and referenced to the 

North American Vertical Datum of 1988; uncertainty ranged 

from 0.05 to 0.13 feet. The altitude of all other wells in each 

transect was calculated relative to the well altitudes surveyed 

with the differential GPS technique. 

 Nine of the 26 wells were instrumented with water-level 

recording devices that collected continuous data at 15-minute intervals for a 7-week period in August and September 

2010. Data from these wells were plotted along with tide data 

collected north of Assateague Island at the Ocean City Inlet 

tide gage and precipitation data collected by National Park 

Service staff at Assateague Island. These data indicate that 

precipitation events have the greatest effect on water levels in 

the shallow groundwater flow system whereas changes in the 

ambient sea level appear to have the greatest effect on water 

levels in the deeper wells. The water levels in both the shallow 

and deeper wells did not appear to be greatly influenced by the 

changing tides. 

Acknowledgments

The authors would like to acknowledge Ms. Courtney 

Schupp, Mr. Jack Kumer, and Mr. Brian Sturgis of the 

National Park Service (NPS) for their scientific and logistical 

support. The authors would also like to acknowledge Dr. 

David Krantz of the University of Toledo for his work in 

advancing the understanding of the hydrogeomorphology at 

Assateague Island National Seashore. The authors extend 

thanks and appreciation to Mr. Brandon Fleming, Mr. Luke 

Myers, and Mr. Eric White of the USGS for their assistance in 

the collection of geophysical and hydrologic data. 

References Cited 

Dillow, J.J.A., Banks, W.S.L, and Smigaj, M.J., 2002, 

Groundwater quality and discharge to Chincoteague and 

Sinepuxent Bays adjacent to Assateague Island National 

Seashore, Maryland: U.S. Geological Survey WaterResources Investigations Report 02–4029, 42 p. (also 

available online at http://pubs.usgs.gov/wri/wri024029/

wri02-4029.pdf ).

Keys, W.S., 1990, Borehole geophysics applied to groundwater investigations: U.S. Geological Survey Techniques 

of Water-Resources Investigations, book 2, chap. E-2, 

149 p. (also available online at http://pubs.usgs.gov/twri/

twri2-e2/).

Krantz, D.E., 2009, A hydrogeomorphic map of Assateague 

Island National Seashore, Maryland and Virginia, accessed 

January 11, 2012 at http://www.eeescience.utoledo.edu/

Faculty/Krantz/download_files/NPS_Report.Assateague_

Hydrogeomorphology.pdf. 

Masterson, J.P., and Garabedian, S.P., 2007, Effects of 

sea-level rise on ground water flow in a coastal aquifer 

system: Ground Water, v. 45, no. 2, p. 209–217, DOI: 

10.1111/j.1745-6584.2006.00279.x.

National Oceanic and Atmospheric Administration, 2010, 

Center for Oceanographic Products and Services, tides and 

currents information, accessed December 5, 2010 at http://

tidesandcurrents.noaa.gov/index.shtml. 

Owens, J.P., and Denny, C.S., 1979, Upper Cenozoic deposits 

of the central Delmarva Peninsula: U.S. Geological Survey 

Professional Paper, 1067–A, 28 p.

Pendleton, E.A., Williams, S.J., and Thieler, E.R., 2004, 

Coastal vulnerability assessment of Assateague Island 

National Seashore (ASIS) to sea-level rise: U.S. Geological 

Survey Open-File Report 2004–1020, 20 p. (also available 

online at http://pubs.usgs.gov/of/2004/1020/).

Schubert, C.E., 2010, Analysis of the shallow groundwater 

flow system at Fire Island National Seashore, Suffolk 

County, New York: U.S. Geological Survey Scientific 

Investigations Report 2009–5259, 106 p. (also available 

online at http://pubs.usgs.gov/sir/2009/5259/).

Taylor, K.C., Hess, J.W., and Massela, A. 1989, Field 

evaluation of a slim-hole induction tool: Ground Water 

Monitoring Review, v. 9, no. 1, p. 100–104, DOI: 10.1111/

j.1745-6592.1989.tb01125.x.

Williams, J.H., Lapham, W.W., and Barringer, T.H., 1993, 

Application of electromagnetic logging to contamination 

investigations in glacial sand-and-gravel aquifers: Ground 

Water Monitoring and Remediation, v. 13, no. 3, p. 129–

138, DOI: 10.1111/j.1745-6592.1993.tb00082.x.
Prepared by USGS West Trenton Publishing Service Center.

Edited by Valerie M. Gaine.

Graphics and layout by Timothy W. Auer.

For additional information, contact:

Director, MD-DE-DC Water Science Center

U.S. Geological Survey

5522 Research Park Drive

Baltimore, MD 21228

or visit our Web site at:

http://md.water.usgs.gov
Banks, W.S.L., and others—Well Network Installation and Hydrogeologic Data Collection, Assateague Island National Seashore, Worcester County, Maryland, 2010—Scientific Investigations Report 2012–5079

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