U.S. Department of the Interior 

U.S. Geological Survey

U.S. ENVIRONMENTAL PROTECTION AGENCY REGION 1 

WASTE MANAGEMENT DIVISION

Geophysical Surveys near the 

Charles George Municipal Landfill and 

Flint Pond, Tyngsboro, Massachusetts

By Joseph D. Ayotte, Thomas J. Mack, and Craig M. Johnston

Open-File Report 99-231

Pembroke, New Hampshire 

1999
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does 

not constitute endorsement by the U.S. Geological Survey.

For additional information write to:

District Chief

U.S. Geological Survey

New Hampshire/Vermont District

361 Commerce Way

Pembroke, NH 03275-3718

or through our website at 

http://nh.water.usgs.gov

Copies of this report can be purchased 

from:

U.S. Geological Survey 

Information Services 

Box 25286 

Federal Center 

Denver, CO 80225
CONTENTS

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

Introduction ................................................................................................................................................................^ 1

Purpose and Scope....................................................................................................................................................... 1

Description of the Study Area ..................................................................................................................................... 1

Ground-Penetrating-Radar Surveys....................................................................................................................................... 3

Survey Methods................................................... 3

Interpretation of Surveys ............................................................................................................................................. 3

Terrain-Conductivity Survey................................................................................................................................................. 5

Survey Methods........................................................................................................................................................... 5

Interpretation of Surveys ............................................................................................................................................. 5

Summary..................................................................................................................................................................^ 8

References Cited.................................................................................................................................................................... 8

FIGURES

1-2. Maps showing:

1. Location of the study area in Tyngsboro, Massachusetts................................................................................... 2

2. Locations of geophysical surveys near Charles George Landfill and in Flint Pond.......................................... 4

3. Figure showing selected ground-penetrating-radar profiles ....................................................................................... 6

4. Map showing terrain-conductivity survey sites .......................................................................................................... 7

CONVERSION FACTORS AND ABBREVIATIONS

Multiply By To obtain

foot (ft) 0.3048 meter 

mile (mi) 1.609 kilometer

Temperature in degrees Fahrenheit (°F) can be converted 

to degrees Celsius (°C) as follows: °C = 5/9 (°F - 32).

Sea Level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 

(NGVD of 1929) a geodetic datum derived from a general adjustment of the first-order level nets 

of both the United States and Canada, formerly called Sea Level Datum of 1929.

OTHER ABBREVIATIONS USED IN REPORT:

jo,S/m microsiemens per meter at 25° Celsius

ft/ns foot per nanosecond

MHz megahertz

Contents III
Geophysical Surveys near the

Charles George Municipal Landfill and in

Flint Pond, Tyngsboro, Massachusetts

By Joseph D. Ayotte, Thomas J. Mack, anc/Craig M. Johnston

Abstract

Two geophysical techniques were used to 

provide a preliminary lithologic and waterquality characterization of Flint Pond near the 

Charles George Municipal Landfill in Tyngsboro, 

Massachusetts. Ground-penetrating-radar surveys 

were conducted on the pond, and on some nearby 

roads, to provide lithologic information. Electromagnetic-terrain conductivity surveys were 

conducted on the pond surface to provide a 

preliminary assessment of water quality.

Water depths were less than 5 feet in the 

study area. The pond bottom consisted of finegrained sediment that generally ranged from less 

than 1 to about 4 feet thick. Stratified-drift and 

lake-bottom sediments are estimated to be less 

than 10 feet thick throughout most of the pond 

area. At the southern end of the pond, an area of 

stratified drift is about 35 feet thick. Groundpenetrating-radar data indicate that overburden is 

thinner at the northern end of the pond than at the 

southern end.

A large terrain-conductivity anomaly of up 

to 74 microsiemens per meter was identified at the 

northern end of the pond. The magnitude and 

location of the anomaly suggest that it is not 

likely to be related to the nearby landfill. Terrain 

conductivities elsewhere in the study area were 

generally low, less than 10 microsiemens per 

meter, and did not indicate the presence of a 

detectable electrically conductive contaminant 

plume from the landfill.

INTRODUCTION

Flint Pond in Tyngsboro, Massachusetts, is 

1/4 mi downgradient of the Charles George Municipal 

Landfill (fig. 1). The pond is a recreational resource 

for the area and is bordered by homes and seasonal 

camps. The stratified-drift aquifer beneath the 

southern end of the pond is used for water supply. The 

extent and potential migration of contamination from 

the landfill towards the pond is a concern for local 

residents and those using the pond for recreational 

activities. The U.S. Geological Survey (USGS), in 

cooperation with the U.S. Environmental Protection 

Agency (USEPA), conducted preliminary geophysical 

surveys during February 1997 beneath the pond, and at 

some selected sites near the pond and landfill, to 

assess underlying lithologic conditions and to provide 

an indicator of ground-water quality.

Purpose and Scope

The purpose of this report is to present the 

results of ground-penetrating radar surveys, used to 

determine aquifer lithology; and terrain-conductivity 

surveys, used to provide an indicator of ground-water 

quality in the aquifer beneath Flint Pond. The study 

was limited to Flint Pond and areas immediately 

adjacent to Flint Pond and the Charles George 

Municipal Landfill.

Description of the Study Area

The study area (fig. 1) consists of Flint Pond, 

including the surrounding shoreline, and selected areas 

adjacent to Flint Pond and the municipal landfill. 

Most of the study area is overlain by glacial till, 

consisting of an unsorted silty sand. An area of

Abstract 1
71°27' 71°26'

42°41'

42°40'

-^-rirvcvw T ' n T -i V / i

--^IfiM '^ i

Base from U.S. Geological Survey 

Digital raster graph, Lowell, Mass.-N.H., 

7.5x15 minute, 1:25,000,1987

1,000-meter Universal Transverse Mercator grid, zone 19

1,000 

_I__

2,000

I

3.000

200 400 600

^^ I

800 1,000 METERS

4,000 FEET

__I

0 20 40 KILOMETERS

Figure 1. Location of the study area in Tyngsboro, Massachusetts.

Geophysical Surveys near the Charles George Municipal Landfill and in Flint Pond, Tyngsboro, Massachusetts
stratified sand and gravel more than 100 ft thick lies 

between the landfill and the pond (TRC Companies, 

Inc., 1996). Ice-contact deposits are exposed at the 

west shore of the pond between the pond and the 

highway, and outwash sediments are present at the 

southeastern shore of the pond, near a private 

academy.

GROUND-PENETRATING-RADAR 

SURVEYS

Ground-penetrating-radar (GPR) surveys were 

conducted on Flint Pond and adjacent areas to provide 

information on the thickness of glacial stratified drift. 

The GPR surveys were conducted at the following 

locations and are identified by profile file number on 

fig. 2:

1. The wetland at the north end of Flint Pond 

(F372, F373, F374);

2. The entire shoreline of Flint Pond (file F375, F379, 

F380, F381, F382, F383, F384);

3. Kendall Road, east past Flint Road (files F6, F7, 

F8);

4. Dirt road west of the private academy to Route 3 

(files F3,F4,F11,F12);

5. Dunstable Road, west side of the Charles George 

Municipal Landfill (files F9, F10); and

6. Route 3, east side of Charles George Municipal 

Landfill (file Fl 4).

Survey Methods

GPR surveys were conducted according to 

methods described by Beres and Haeni (1991). The 

GPR-survey system transmits radio-frequency electromagnetic pulses into the ground and receives energy 

reflected back from subsurface reflectors. Reflectors 

can be any subsurface contact between geologic 

materials with different physical and electrical properties, such as the interface between lithologic units or 

layers within a unit. The surveys were conducted with 

bi-static 100 MHz center-frequency transmitting and 

receiving antennas that were towed over frozen pond 

surfaces and adjacent to selected roads. The profiles 

can be examined visually to provide indications of 

lithologic properties. Interpretation of GPR profiles is 

improved by comparison with lithologic logs.

Beres and Haeni (1991) provide an interpretive 

guide for various types of typical reflector patterns for 

unconsolidated deposits. Parallel reflectors indicate 

the presence of laminated fine-grained sediments, such 

as lake-bottom sediments observed in this study. 

Complex, subparallel, and chaotic reflectors generally 

indicate coarse-grained sediments. Inverted V-shaped 

reflectors are indicative of point reflectors that could 

be from cobbles or boulders in till. Near-surface 

inverted V-shaped reflectors often indicate objects 

such as buried pipelines, culverts, or conduits.

Transmission velocities of the radar signal were 

used to interpret depth to a reflector. The approximate 

velocities of electromagnetic waves are 0.1 ft/ns in 

water, 0.3 ft/ns in organic soils (silt and peat), 0.2 ft/ns 

in saturated sand, and 0.4 ft/ns in unsaturated sand 

(Beres and Haeni, 1991). The velocity of electromagnetic waves in ice is approximately 0.5 ft/ns. In 

general, the ice thickness was about 1 ft, and thus did 

not significantly affect the interpretation of depths to 

subsurface reflectors. Interpretation of GPR profiles, 

which includes water, organic materials, and saturated 

sediment, requires the use of multiple depth scales. 

For any radar frequency, the primary factor limiting 

depth of penetration is the electrical conductivity of 

the subsurface materials (Beres and Haeni, 1991); 

however, high frequencies are attenuated faster than 

low frequencies. Observations of GPR profiles in New 

Hampshire (Ayotte and others, 1999) indicate that 

electromagnetic-radar waves penetrate organic 

sediments with little attenuation. Electrically conductive materials, such as the clay minerals commonly 

found in tills, significantly limit radar-signal penetration. Lithologic information obtained from drilling 

logs was used to confirm interpretations of GPR 

profiles.

Interpretation of Surveys

GPR profiles in the study area indicate shallow, 

unconsolidated deposits underlying most of the areas 

investigated. The following discussion of depths to 

reflectors is based on estimates of GPR velocities in 

typical glacial sediments. Stratified-drift and lakebottom sediments are estimated to be less than 10 ft 

thick beneath most of the pond, particularly along the 

perimeter, and water depths were generally less than 

5 ft. One exception is a buried channel at the south 

end of the pond, where stratified-drift deposits about

GROUND-PENETRATING-RADAR SURVEYS 3
71°27' 71°26'

42=41'

42°40'

Base from U.S. Geological Survey 

Digital raster graph, Lowell, Mass.-N.H., 

7.5x15 minute, 1:25,000, 1987

1,000

I

2,000

I

3,000

I__

I I I I 

400 600 800 1,000 METERS

4,000 FEET

I

200

1,000-meter Universal Transverse Mercator grid, zone 19

EXPLANATION

Ground-penetrating radar survey profile line and profile file number 

Terrain conductivity (EM) data point

F6

Figure 2. Locations of geophysical surveys near Charles George Landfill and in Flint Pond, Tyngsboro, Massachusetts.

Geophysical Surveys near the Charles George Municipal Landfill and in Flint Pond, Tyngsboro, Massachusetts
35 ft thick were found (fig. 3c-c'). The pond bottom 

consisted of fine-grained sediment that is less than 1 ft 

to about 4 ft thick. At the northern end of the pond, the 

buried channel is less prominent, but is about 15 ft 

thick (fig. 3a-a'). The radar profile shows a change in 

the reflector response, which could represent the pond 

bottom and stratified-drift deposits. Similarly, in the 

middle of the pond, a change in reflector response is 

seen in figure 3b-b', which could represent the bottom 

of stratified-drift deposits. This surface in fig. 3b-b' is 

deeper than in profile a-a' and is approximately 20 to 

25 ft thick. No drilling information is available to 

confirm the GPR results. GPR data indicate that a 

buried channel could extend the entire length of the 

pond.

Electromagnetic-wave penetration was limited 

because the overburden was relatively thin. GPR 

profiles elsewhere in the study area either showed 

little-to-no stratified-drift thickness or were otherwise 

not informative and are not presented. These uninterpretable records were in the southern end of the pond 

(files F4, Fl 1 and F12), and information collected 

from the profiles was inconclusive and not presented 

here because of the chaotic nature of coarse-grained 

sediments and depths to bedrock, or other stratigraphic 

changes that exceeded the GPR signal penetration.

TERRAIN-CONDUCTIVITY SURVEY

An electromagnetic-terrain conductivity survey 

was conducted to identify a possible ground-watercontaminant plume in glacial sediments beneath Flint 

Pond. Terrain-conductivity surveys were done on 

Flint Pond and at selected locations on the shoreline. 

The surveys consisted of point measurements 100 ft 

apart, and were located using a high-resolution global 

positioning system (GPS).

Survey Methods

A Geonics EM31 was used to measure electromagnetic-terrain conductivity in February 1997, at 

points along lines on the pond and at selected locations 

on the shore (fig. 2). The EM31 has an effective 

exploration depth of about 20 ft. The apparent terrain 

conductivity includes the electromagnetic effects of 

the sediments and the associated pore water. The 

glacial sediments (sand and gravel) show little

variation at the site; however, the extent of glacial till, 

which affects terrain conductivity in the study area, is 

not known.

Ground water associated with contaminant 

plumes from landfills can have a high specific conductance, (exceeding 1,000 (iS/cm), and can result in 

apparent terrain conductivities of 10 to 20 (iS/m. 

Uncontaminated ground water in nearby southern 

New Hampshire typically has a much lower specific 

conductance, about 200 (iS/m (Toppin, 1987). In 

uncontaminated sand- and gravel-aquifer materials, 

terrain conductivities are typically less than 5 (iS/m. 

The conductivity of wetland and lake-bottom 

sediments in areas of New Hampshire (Ayotte and 

others, 1999) are relatively low (less than 4 (iS/m) in 

areas without known contamination. Variation in the 

apparent terrain conductivity at this site results from 

variations in the depth of the water column in the lake, 

the thickness of stratigraphic units, and the groundwater quality.

Interpretation of Surveys

Terrain conductivities ranged from 1.0 to more 

than 70 (iS/m. Low-terrain conductivities, less than 

6 (iS/m, were measured along most of the southern 

and eastern parts of Flint Pond, indicating generally 

low conductivity ground water (fig. 4). The lowest 

readings (1.0 (iS/m) were obtained on a trail to the 

private academy where the water table is more than 

10 ft deep and the sediments consist of coarse-grained 

sands.

Three terrain-conductivity measurements were 

between 7.4 and 8.8 |iS/m near the brook that crosses 

Route 3 (fig. 4) and flows to the pond. This area is 

topographically down slope of the municipal landfill 

and the most likely area to be affected by a contaminant plume. It is not known if these points represent 

an anomalous conductivity from anthropogenic 

sources (such as culture or trash) or if this represents 

the effects of a contaminant plume; however, the three 

measurements cover an area less than 200 ft wide and 

are bounded by low-terrain conductivities. These data 

indicate an isolated, or possibly cultural source for this 

small area of slightly elevated terrain conductivity 

rather than a landfill leachate plume.

Slightly higher readings (6 to 15 (iS/m) were 

obtained along the western shoreline of Flint Pond 

(fig. 4). A terrain conductivity of 15 (iS/m was

TERRAIN-CONDUCTIVITY SURVEY 5
00

Q

fc 

O 

U

w

oo 

O

H

I

200

400

600

800

600

800

0

200

400

600

800

25 50 75 100 FEET

25 METERS

300 FEET 

METERS

Ice surface

300 FEET

25 SO 75 METERS

EXPLANATION 

- - Approximate bottom of stratified drift

Figure 3. Selected ground-penetrating-radar profiles in Flint Pond, Tyngsboro, Massachusetts.

6 Geophysical Surveys near the Charles George Municipal Landfill and in Flint Pond, Tyngsboro, Massachusetts
71°26'30" 71°26' 71°25'30"

42°40'30"

42°40'

Base from U.S. Geological Survey

Digital raster graph, Lowed, Mass.-N.H.

7.5x15 minute, 1:25,000 scale, 1987

1,000-meter Universal Transverse Mercator grid; zone 19

1,000 2,000 FEET

200 400 600 METERS

EXPLANATION

[J Terrain conductivity (EM) data.

Lighter gray shades are areas of higher terrain electrical conductivity. 

Darker gray shades are areas of lower terrain electrical conductivity

3.2 EM data point and value in millisiemens per meter (mS/m)

Figure 4. Terrain-conductivity survey sites in Flint Pond, Tyngsboro, Massachusetts.

TERRAIN-CONDUCTIVITY SURVEY 7
measured at the middle of the western shoreline of 

Flint Pond centered in a zone of slightly elevated 

(greater than 10 jlS/m) terrain conductivity. The 

landform immediately west of this area appears to be a 

glacial esker, which should contain relatively coarsegrained, low-terrain-conductivity sediments. It is not 

known if the origin of this slightly elevated conductivity area is related to an area of high-terrain conductivity at the northern end of the pond.

A large area of high-terrain conductivities (up to 

74 jiS/m) was found at the northern end of Flint Pond 

(fig. 4) where a brook discharges to a nearby wetland. 

Terrain conductivities in this range are exceptionally 

high, much higher than the conductivity typically 

observed for fine-grained sediments. In addition, 

similar wetland sediments have low-terrain conductivities. Values greater than about 30 |J,S/m indicate high 

concentrations of dissolved solids. Such high concentrations are most likely associated with a high ionic 

solution such as a contaminant plume from salt 

storage.

SUMMARY

Ground-penetrating-radar (GPR) surveys at 

Flint Pond, in Tyngsboro, Massachusetts, indicate that 

water depths were less than 5 ft, and that the lake 

bottom consists of fine-grained sediment that range in 

thickness from less than 1 to about 4 ft. Estimated 

thicknesses of stratified-drift and lake-bottom 

sediments are generally less than 10 ft thick

throughout most of the pond area. Results of the GPR 

surveys indicate that stratified-drift deposits are about 

35 ft thick in the southern part and about 15 ft thick in 

the northern part of the pond.

A terrain-conductivity survey detected a large, 

high-conductivity anomaly (up to 74 jiS/m) at the 

northern end of the pond. Based on the magnitude and 

location of the anomaly, it is not likely to be related to 

the contaminant plume from a nearby landfill. Terrain 

conductivities elsewhere in the study area were 

generally low, less than 10 jlS/m, and did not indicate 

the presence of a detectable electrically conductive 

contaminant plume from the landfill.

REFERENCES CITED

Ayotte, J.D., Mack, T.J., and Johnston, C.M., 1999,

Geophysical surveys of Country Pond and adjacent 

wetland, and implications for contaminant-plume 

monitoring, Kingston, New Hampshire: U.S. Geological Survey Open-File Report 99-51, 20 p.

Beres, Milan, Jr., and Haeni, F.P., 1991, Application of 

ground-penetrating-radar methods in hydrogeologic 

studies: Ground Water, v. 29, no. 3, p. 375-386.

Toppin, K.W., 1987, Hydrogeology of stratified-drift 

aquifers and water quality in the Nashua Regional 

Planning Commission Area, south-central New 

Hampshire: U.S. Geological Survey Water-Resources 

Investigations Report 86-4358, 45 p.

TRC Companies, Inc., 1996, Technical support long-term 

groundwater monitoring Charles George Reclamation 

Trust Landfill, Tyngsboro, Mass.: Draft Report, 

October 1996.

8 Geophysical Surveys near the Charles George Municipal Landfill and in Flint Pond, Tyngsboro, Massachusetts
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