Prepared in cooperation with the U.S. Environmental Protection Agency

Geophysical Bed Sediment Characterization of the 

Androscoggin River from the Former Chlor-Alkali 

Facility Superfund Site, Berlin, New Hampshire, to the 

State Border with Maine, August 2009

Scientific Investigations Report 2011–5158

U.S. Department of the Interior

U.S. Geological Survey
Cover. Front: Geophysical equipment loaded in canoes. View looking downstream to the south from the right bank of the 

Androscoggin River downstream of the Cascade Dam, Gorham, N.H. 

Back: View looking upstream on the Androscoggin River, upstream of the Shelburne Dam, Shelburne, N.H.
Geophysical Bed Sediment 

Characterization of the Androscoggin 

River from the Former Chlor-Alkali Facility 

Superfund Site, Berlin, New Hampshire, to 

the State Border with Maine, August 2009

By James R. Degnan, Andrew P. Teeple, Craig M. Johnston, 

Mark C. Marvin-DiPasquale and Darryl Luce1

Prepared in cooperation with the U.S. Environmental Protection Agency

Scientific Investigations Report 2011–5158

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Degnan, J.R., Teeple, A.P., Johnston, C.M., Marvin-DiPasquale, M.C., and Luce, Darryl, 2011, Geophysical bed 

sediment characterization of the Androscoggin River from the former Chlor-Alkali Facility Superfund Site, 

Berlin, New Hampshire, to the state border with Maine, August 2009: U.S. Geological Survey Scientific 

Investigations Report 2011–5158, 27 p., at http://pubs.usgs.gov/sir/2011/5158.
iii

Acknowledgments 

Site reconnaissance by Tom Mack was essential in the planning and execution of the 

geophysical surveys. Data collection was possible with field assistance from Brandon Fleming, 

Phillip Harte and Charles Grant, U.S. Geological Survey (USGS). Equipment and technical advice 

provided by Carole Johnson, and Wade Kress, USGS, made high-quality data collection possible. 

Advice from Troy Brosten and John Lane, USGS, helped in the understanding of electromagnetic 

survey results and data processing. 

Pore-water and sediment samples collected by Ann Chalmers and Thor Smith, USGS, 

contributed to the understanding of the survey results. Geographical Information System support 

from Eric Jameson and Laura Hayes, USGS, was helpful for map generation. Critical reviews by 

Glenn Hodgkins, Robert Flynn and Peter Joesten, USGS, are greatly appreciated.

Technical advice from Brian Jones, Geophysical Survey Systems Incorporated, was helpful for 

georeferencing ground-penetrating radar data on the water. Private land owner access provided 

by Richard Demers made data collection possible in a reach with no public access.
This page has been left blank intentionally.
v

Contents

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

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

Site Background....................................................................................................................................2

Previous Investigations........................................................................................................................2

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

Study Area Description........................................................................................................................4

Geologic Setting...........................................................................................................................4

River Flow and Channel Features .............................................................................................4

Methods of Data Collection and Analysis ................................................................................................5

Ground-Penetrating Radar..................................................................................................................6

Step-Frequency Electromagnetic Induction ....................................................................................6

Sediment Sampling and Direct-Current Resistivity.........................................................................8

Interpretation of Riverbed Conductivity..........................................................................................10

Estimation of Riverbed Properties by Reach..................................................................................10

Characteristics of Riverbed Sediment ....................................................................................................11

Wheeler Bay, Reference Reach ......................................................................................................12

Reach AR–3, Between the Former Chlor-Alkali Facility Superfund Site and the 

Riverside Dam........................................................................................................................12

Reach AR–4, Upstream from the Smith Dam ................................................................................13

Reach AR–5, Upstream from the Cross Power Dam.....................................................................13

Reach AR–6, Between Cross Power and Cascade Dams ...........................................................16

Reach AR–7, Upstream from the Brown Dam................................................................................16

Reach AR–8, Between the Brown and Gorham Dams .................................................................20

Reach AR–9, Between the Gorham and Shelburne Dams...........................................................20

Reach AR–10, Downstream from the Shelburne Dam to the New Hampshire, 

Maine Border.........................................................................................................................23

Summary and Conclusions.........................................................................................................................25

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

Figures

1. Map showing location of the study area on the Androscoggin River in Berlin,

Dummer, Gorham and Shelburne, New Hampshire, and locations of the surveyed

reaches, and former Chlor-Alkali Facility Superfund Site on the Androscoggin River,

New Hampshire ............................................................................................................................3

2. Graph showing generalized slope and location of dams on the Androscoggin River 

from the former Chlor-Alkali Facility to the New Hampshire–Maine State border...........5

3. Example of a cross section showing ground-penetrating radar profile and

interpretation in reach AR–7 upstream from the Brown Dam, Androscoggin River,

Gorham, New Hampshire ...........................................................................................................7

4. Example of a cross section showing inverted electromagnetic induction profile

in reach AR–9 between the Gorham and Shelburne Dams, Androscoggin River,

Shelburne, New Hampshire .......................................................................................................8
vi

5. Graph showing the relation between specific-conductance measurements from

riverbed pore-water and resistivity values measured in the laboratory from co-located

riverbed-sediment samples with intact pore water..............................................................10

6. Map and cross section showing the results of geophysical surveys of

riverbed-sediment characteristics in reach AR–3 from the former Chlor-Alkali

Facility Superfund Site to the Riverside Dam, Androscoggin River, Berlin, 

New Hampshire ..........................................................................................................................14

7. Map and cross section showing the results of geophysical surveys of

riverbed-sediment characteristics in reach AR–4 upstream from the Brodie Smith

Dam, Androscoggin River, Berlin, New Hampshire ..............................................................15

8. Map showing the results of geophysical surveys of riverbed-sediment

characteristics in reach AR–5 upstream from the Cross Power Dam, Androscoggin

River, Berlin, New Hampshire...................................................................................................17

 9. Map showing the results of geophysical surveys of riverbed-sediment

characteristics in reach AR–6 between the Cross Power and Cascade Dams,

Androscoggin River, Gorham, New Hampshire.....................................................................18

10. Map showing the results of geophysical surveys of riverbed-sediment

characteristics in reach AR–7 upstream from the Brown Dam, Androscoggin River,

Gorham, New Hampshire ..........................................................................................................19

11. Map showing the results of geophysical surveys of riverbed-sediment

characteristics in reach AR–8 between the Brown and Gorham Dams,

Androscoggin River, Gorham, New Hampshire.....................................................................21

12. Map showing the results of geophysical surveys of riverbed-sediment

characteristics in reach AR–9 between the Gorham and Shelburne Dams,

Androscoggin River, Shelburne, New Hampshire.................................................................22

13. Map showing the results of geophysical surveys of riverbed-sediment

characteristics in reach AR–10 downstream from the Shelburne Dam to the New

Hampshire–Maine State border, Androscoggin River, Shelburne, New Hampshire ......24

Tables

1. Surface, pore-water and sediment conductivity, direct-current resistivity test box

results and sediment grain size .................................................................................................9

2. Skin depth and depth of investigation values determined graphically for selected

electromagnetic frequencies ...................................................................................................11

3. Estimates of electromagnetic conductivity summarized by reach and correlation

with water depth.........................................................................................................................11

4. Percentage of fine sediment summarized by reach.............................................................12
vii

Conversion Factors and Datum

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

yard (yd) 0.9144 meter (m)

Area

square mile (mi2

) 259.0 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

Hydraulic gradient

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

In this report, the words “right” and “left” refer to directions that would be reported by an 

observer facing downstream. 

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.

Specific conductance (SC) is given in millisiemens per meter (mS/m) at 25 degrees Celsius 

(mS/m at 25°C).

List of Acronyms

CEP continuous electromagnetic profiling

DC Direct current 

FDEM multifrequency electromagnetic

GIS geographical information system 

GPR ground-penetrating radar 

GPS global positioning system

NHD National Hydrography Dataset

NAIP National Agriculture Imagery Program 

PAH polynuclear aromatic hydrocarbons 

PPS push-point sampler

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey
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Geophysical Bed Sediment Characterization of the 

Androscoggin River from the Former Chlor-Alkali Facility 

Superfund Site, Berlin, New Hampshire, to the State 

Border with Maine, August 2009

By James R. Degnan, Andrew P. Teeple, Craig M. Johnston, Mark C. Marvin-DiPasquale and Darryl Luce1

Abstract 

The former Chlor-Alkali Facility in Berlin, New 

Hampshire, was listed on the U.S. Environmental Protection 

Agency National Priorities List in 2005 as a Superfund 

site. The Chlor-Alkali Facility lies on the east bank of the 

Androscoggin River. Elemental mercury currently discharges 

from that bank into the Androscoggin River. The nature, 

extent, and the speciation of mercury and the production of 

methyl mercury contamination in the adjacent Androscoggin 

River is the subject of continuing investigations. The 

U.S. Geological Survey, in cooperation with Region I of the 

U.S. Environmental Protection Agency, used geophysical 

methods to determine the distribution, thickness, and physical 

properties of sediments in the Androscoggin River channel at 

a small area of an upstream reference reach and downstream 

from the site to the New Hampshire–Maine State border. 

Separate reaches of the Androscoggin River in the 

study area were surveyed with surface geophysical methods 

including ground-penetrating radar and step-frequency 

electromagnetics. Results were processed to assess sediment 

characteristics including grain size, electrical conductivity, 

and pore-water specific conductance. Specific conductance 

measured during surface- and pore-water sampling was used 

to help interpret the results of the geophysical surveys. The 

electrical resistivity of sediment samples was measured in the 

laboratory with intact pore water for comparison with survey 

results. In some instances, anthropogenic features and land 

uses, such as roads and power lines affected the detection of 

riverbed properties using geophysical methods; when this 

occurred, the data were removed. Through combining results, 

detailed riverbed sediment characterizations were made. 

Results from ground-penetrating radar surveys were 

used to image and measure the depth to the riverbed, depth to 

buried riverbeds, riverbed thickness and to interpret materialtype variations in terms of relative grain size. Fifty two 

percent of the riverbed in the study area was covered with 

gravel and finer sediments. The electrically resistive river 

water and sediment in this study area were conducive to the 

penetration of the ground-penetrating radar and step-frequency 1

U.S. Environmental Protection Agency, Region 1.

electromagnetic signals and allowed for effective sediment 

characterization by geophysical methods. 

The reach between the former Chlor-Alkali Facility 

and the Riverside Dam, had small areas of fine sediment 

(estimated 11 percent of riverbed area), found on the 

upstream left bank and the downstream right bank, with 

an electromagnetic conductivity (31.4 millisiemens per 

meter (mS/m) maximum) that was higher than the upstream 

reference reach. The greatest electromagnetic conductivity 

(195 mS/m), pore-water specific conductance (324 mS/m) and 

lab measured sediment conductivity of (76.8 mS/m, measured 

with a direct-current resistivity test box) in the study were 

measured approximately 1 mile (mi) downstream of the site 

from a sandbar on the left bank. Reaches adjacent to and 

within 2 mi downstream from the site, reaches had elevated 

electromagnetic conductivity despite having lower estimated 

percentages of riverbed area covered in sediment (11, 25, 

and 61 percent, respectively) than the reference reach (97), 

typically finer grained sediment will be more conductive. 

The Shelburne Reservoir is approximately 8 mi downstream 

from the site had the second greatest pore-water specific 

conductance measured, 45.8 mS/m. Many of the locations 

with the largest step-frequency electromagnetic values have 

not been sampled for pore water and sediment. 

Introduction 

This study was conducted by the U.S. Geological 

Survey (USGS), in cooperation with the U.S. Environmental 

Protection Agency (USEPA), to further understand the 

riverbed sediment and potential contaminant distribution 

downstream from the former Chlor-Alkali Facility Superfund 

Site (site) in Berlin, New Hampshire. The results of the study 

will be used for remedial activities and for development of 

an investigation and feasibility study plan for this site. The 

site was associated with a pulp and paper mill on the bank 
2 Geophysical Bed Sediment Characterization of the Androscoggin River, New Hampshire, August 2009

of the Androscoggin River (fig. 1). The Chlor-Alkali Facility 

used electrolytic cells for chlorine gas production for the 

papermaking industry. Mercury was released and seeped into 

the soil, till and underlying fractured bedrock as a result of site 

activities (U.S. Environmental Protection Agency, 2011) and 

may represent a risk to human health and the environment. 

An understanding of the extent of fine sediment and 

mercury contamination in the Androscoggin River is needed 

to determine the potential effects on the environment and to 

provide information for efficient remediation activities in the 

future. Currently (2011), the extent of mercury contamination 

in the Androscoggin River is unknown.

Eggleston (2009) indicated that resuspension of mercurycontaminated sediment can account for a large percentage 

of the annual downstream mercury load in a point-source, 

mercury-impaired watershed of the Shenandoah River in 

Virginia. An understanding of sediment distribution in 

the Androscoggin River downstream from the site would 

identify potential zones of contaminant deposition. Elemental 

mercury, such as that emanating from the site, can be 

transported with fine-grained, organic carbon-rich, riverbed 

sediments. Deposition of these sediments pose a concern 

because they provide an optimal environment for mercury 

methylation (Marvin-DiPasquale, Lutz, and others, 2009, 

Marvin-DiPasquale, Agee, and others, 2011). Methylation 

is the conversion of elemental mercury to the organic form 

(methyl mercury) through microbial activity. Methylmercury 

is the toxic form of mercury and more mobile within the food 

chain. This conversion enables the bioaccumulation of methyl 

mercury in fish. 

Surface geophysical surveys such as ground-penetrating 

radar (GPR) and multifrequency electromagnetic (FDEM) 

surveys are effective techniques for determining the extent and 

nature of riverbed sediments. Sheets and Dumouchelle (2009) 

indicated the FDEM surveys (also known as continuous 

electromagnetic profiling (CEP)) were effective in mapping 

riverbed sediments in a 27 kilometer (km) reach of the Great 

Miami River in Ohio. Geophysical surveys in freshwater lakes 

and rivers have been used in New England to assess sediment 

characteristics for contaminant mapping (Ayotte and others, 

1999), dam removal investigations (Dudley, 1999), aquatic 

habitat studies (Argue and others, 2007), and bridge scour 

assessments (Olimpio, 2000). 

Site Background

From 1899 to 1965, a chlor-alkali facility located on the 

east bank of the Androscoggin River, was used to produce 

chlorine gas for the papermaking industry in Berlin, N.H. 

(fig. 1). The site was associated with a pulp and paper mill, 

and a sawmill. Chlorine was produced at the site using 

electrolytic diaphragm and potentially mercury cells. Chlorine 

was primarily produced to supply the papermaking industry 

for paper beaching. Mercuric chloride may also have been 

produced on site. The sawmill included a wood preserving 

operation from 1888 to 1930, which used mercuric chloride 

in a process known as “Kyanization” to preserve the wood 

(Gove, 1986).

In the 1990s, elemental mercury in the forms of a silvercolored liquid and vapor was observed in and near bedrock 

fractures along the left riverbank, immediately adjacent to the 

site and in river sediment. This prompted the New Hampshire 

Department of Environmental Services to initiate site 

investigations. Since the late 1990s, efforts have been made 

to contain mercury at the site and eliminate the seepage of 

contaminated groundwater to the river (Margaret A. Bastien, 

New Hampshire Department of Environmental Services, 

written commun., 2003). The former Chlor-Alkali Facility 

Superfund Site was placed on the USEPA National Priorities 

List in 2005 (U.S. Environmental Protection Agency, 2011).

Remedial efforts at the site included: (1) removal and 

demolition of buildings associated with the chlor-alkali 

facility, (2) installation of a subsurface bentonite-soil slurry 

(barrier) wall on the site perimeter that is connected to the 

bedrock surface, (3) installation of a synthetic cap over the 

site, to prevent precipitation infiltration, and (4) pressure 

grouting bedrock fractures along the riverbank. The intent 

of these remedial actions was to eliminate groundwater flow 

through the site’s overburden and reduce this driving force for 

contaminant migration. Despite earlier actions to address the 

source of contamination, mercury continues to seep into the 

Androscoggin River at bedrock fractures at the edge of the 

site. Between 1999 and 2006, approximately 135 pounds (lbs) 

of elemental mercury and sediment containing mercury 

have been removed from the river and riverbank (U.S. 

Environmental Protection Agency, 2011). 

Previous Investigations

Investigations at the former Chlor-Alkali Facility 

Superfund Site have revealed elevated mercury, lead, arsenic, 

polynuclear aromatic hydrocarbons (PAHs), organo-chlorine 

chemicals (dioxin, dibenzofurans), and other toxic metals 

in groundwater and soils beneath the site. Mercury and 

lead, exceeding State regulations, were identified in soil and 

groundwater at overburden and bedrock borings, and wells 

installed at the site (Tighe and Bond, Inc., 2001). Sediments 

and surface water were sampled in the Androscoggin River 

upstream from the site and downstream to approximately the 

Maine State border (fig. 1) in the summers of 2006, 2007, 

2009 and 2010 (U.S. Environmental Protection Agency, 2011). 

Elevated specific conductance (SC) in groundwater has 

been found in wells and adjacent soil samples with dissolved 

phase and adsorbed mercury at the site. Despite containment 

and removal efforts, elemental mercury continued to appear 

in depressions at bedrock fractures along the riverbank at 

the site. Degnan and others (2005) assessed the preliminary 

hydrogeology of the site, which was followed up with 

additional targeted sampling and hydrogeologic analysis 

(Weston Solutions, 2005). 
Introduction 

3 Degnan_fig 01

Base from 1:24,000-scale U.S. Geological Survey topographic maps provided byEnvironmental Systems Research Institute (ESRI) ArcGis Online Standard Map Services

16

2

0 1 2 MILES 0.5

0 1 2 KILOMETERS 0.5

1.5

1.5

71°01'30" 06' 71°10'30"

44°28'30"

44°23'

Survey lineLine of sectionWheeler Bay

EXPLANATION

A A' 

Former Chlor-Alkali Facility Superfund Site Former Chlor-Alkali Facility Superfund Site

Area enlarged on figure 13 Area enlarged on figure 13

Area enlarged on figure 12

(section B-B’ on figure 4)

Area enlarged on figure 12

(section B-B’ on figure 4)

Area enlarged

on figure 11 on figure 11

Area enlarged

on figure 10

(section A-A’

on figure 3)

Area enlarged

on figure 10

(section A-A’

on figure 3)

Area enlarged

on figure 9 on figure 9

Area enlarged

on figure 8 on figure 8

Area enlarged

on figure 7 on figure 7

Area enlarged

on figure 6 on figure 6

B'

B

A'

A

A'

MAINE

NEW HAMPHIRE

0 30 60 MILES

0 30 60 KILOMETERS

Area enlarged

CANADA

UNITED STATES

NEW

HAMPSHIRE 

MAINE

MASSACHUSETTS

VERMONT

NEW YORK Concord

Portland

Montpelier

Manchester

Burlington

Dummer

71° 72° 73°

45°44°43° ATLANTIC OCEAN

, Gorham and Shelburne, New Hampshire, and locations of the 

, New Hampshire.

Location of the study area on the Androscoggin River in Berlin, Dummer

-Alkali Facility Superfund Site on the Androscoggin River

Figure 1.surveyed reaches, and former Chlor
4 Geophysical Bed Sediment Characterization of the Androscoggin River, New Hampshire, August 2009

Purpose and Scope 

The purpose of this report is to describe riverbed 

sediment and pore-water conductivity distribution through the 

presentation of the results of geophysical surveys. Processed 

data from geophysical surveys were used to characterize and 

interpret properties of riverbed sediment presented in maps 

and cross sections. Surveys were conducted in navigable 

reaches along 18 miles (mi) of the Androscoggin River from 

the former Chlor-Alkali Facility Superfund Site downstream to 

the Maine State border and upstream at Wheeler Bay (fig. 1), 

during August 2009. Results of distribution, depth, relative 

grain size and electrical conductivity of streambed sediments 

are presented. 

Study Area Description

The Androscoggin River at the Maine State border 

drains water from an approximately 1,500 square miles (mi2

) 

watershed. The drainage area is covered by 87 percent 

forest, 0.3 percent urban, 6.2 percent surface water, 3 percent 

wetlands, 2 percent barren, and 1 percent agriculture (U.S. 

Environmental Protection Agency, 2006). The drainage area 

upstream from the former Chlor-Alkali Facility Superfund Site 

is approximately 1,340 mi2

. In the town of Berlin, erosionresistant bedrock forms a channel with near continuous 

cascades (Federal Emergency Management Agency, 1981), 

whereas downstream from the site, in northern Gorham, the 

channel has a moderate gradient incised into glacial fluvial 

sediment. In the town of Shelburne the channel is slightly 

braided, anabranching (channel splitting, forming islands) 

with localized meandering sections (fig. 1). Flow, sediment 

transport, and channel migration are affected by the bedrock 

geology, glacial and fluvial sediments, and manmade channel 

controls in the Androscoggin River Valley.

Geologic Setting

Bedrock geology, including rock type and structure, serve 

to control channel locations, slope and fluvial characteristics. 

The Androscoggin River channel at the former Chlor-Alkali 

Facility Superfund Site and downstream for 2 mi in Berlin is 

underlain by metamorphosed biotite-quartz monzonite of the 

Oliverian Plutonic Suite (Ordovician) (Billings and Billings, 

1975; Lyons and others, 1997). Degnan and others (2005) 

mapped the rock types on the river bank and river bed at the 

site as predominantly gneiss with lesser amounts of chlorite 

schist or pegmatite, with fracture patterns that were parallel to 

and were reflected in the trend of the river channel. Gneisses 

and amphibolites of the Ammonoosuc Volcanics (Ordovician) 

lie downstream from the site in the channel near the Cascade 

and Brown Dams (Billings and Billings, 1975; Lyons and 

others, 1997). The Ironbound Mountain Formation (Devonian) 

(grey phyllite and metasandstone) underlies the river between 

the Brown Dam and the Gorham Dam (Lyons and others, 

1997). Downstream from the Gorham Dam, the river channel 

is in a deep sediment-filled valley where two-mica granite 

(Devonian) locally intrudes into the older metasedimentary 

rocks of the Littleton (Early Devonian) and Madrid 

Formations (Upper Silurian (?)) (Billings and Billings, 1975; 

Lyons and others, 1997). Schists of the Littleton, Rangeley, 

Madrid and Smalls Falls Formations (Lyons and others, 1997) 

have been eroded and filled with glacial outwash deep beneath 

the Androscoggin River from downstream of the Shelburne 

Dam to the Maine State border (fig. 2).

Overburden geologic materials just upstream from the 

former Chlor-Alkali Facility Superfund Site include stratified 

sand, gravel, and silt alluvium deposited by glacial outwash. 

Overburden in the vicinity of the site and downstream to the 

backwater of the Brown Dam (Cotton, 1975, Gerath, 1978, 

Olimpio and Mullaney, 1997) was generally less than 20 feet 

(ft) thick and consists of thin deposits of glacial till (an 

unsorted mixture of clay, silt, sand, cobbles, and boulders). 

Glacial till covers bedrock on the valley floor and walls 

where slopes are gentle, but is absent along the Androscoggin 

River at the site and for 2 mi downstream from the site (U.S. 

Environmental Protection Agency, 2011). 

Alluvial fan deposits, consisting of sand, gravel, and silt 

are found on the left bank (east) of the river upstream from 

the Cascade and Brown Dams and on the right bank near 

the Gorham Dam and Shelburne Reservoir. Stratified sand, 

gravel, and silt alluvium is the dominant deposit beneath the 

river channel from the backwater behind the Brown Dam to 

the New Hampshire–Maine State border. Ice-contact deposits 

of sand and gravel are found downstream from the Shelburne 

Dam in the river channel in the form of eskers, channel 

fillings, kames, and kame terraces. Undifferentiated glacial 

drift, consisting mostly of till, was found along the left bank of 

the river in Gorham and Shelburne (Gerath, 1978; Gerath and 

others, 1985).

River Flow and Channel Features

Flow in the Androscoggin River is regulated by eight 

hydroelectric dams in the study area; flows are controlled to 

respond to power demands, floods, and structure maintenance. 

The mean annual flow measured at USGS gage 01054000 

in Gorham, N.H., is 2,110 cubic feet per second (ft3

/s). The 

month of May has the highest average annual flow, 4,210 ft3

/s 

and August has the lowest, 1,960 ft3

/s. 

Dams and lakes in the headwaters of the Androscoggin 

River provide storage for a substantial amount of runoff 

and reduce flood peaks (Federal Emergency Management 

Agency, 1994). Though hydroelectric dams in Berlin, 

Gorham, and Shelburne (fig. 2) control flow and sediment 

transport during normal flows, due to minimal storage volume 

they have little effect on controlling flood flows (Federal 

Emergency Management Agency, 1981). The average channel 

slope in the study area from the site to the Maine State 

line is 26.1 feet per mile (ft/mi). The slope is much greater 

(100 ft/mi, fig. 2) between the site and the Cascade Dam 
Methods of Data Collection and Analysis 5

Degnan_fig 02

Generalized slope of the Androscoggin River

0 2 4 6 8 10 12 14 16 18 20

Distance from former Chlor-Alkali Facility Superfund Site, in miles

600

700

800

900

1,000

1,100

Elevation above North American

 Vertical Datum of 1988, in feet

Berlin, New Hampshire

Gorham, New Hampshire

Shelburne, New Hampshire

SAWMILL DAM

RIVERSIDE DAM

BRODIE SMITH DAM

CROSS POWER DAM

CASCADE DAM

BROWN DAM

GORHAM DAM

SHELBURNE DAM

NEW HAMPSHIRE–MAINE State border

Figure 2. Generalized slope and location of dams on the Androscoggin River from the former 

Chlor-Alkali Facility to the New Hampshire–Maine State border.

(fig. 2) and greatly increases the river’s capacity to produce 

hydroelectric power, and transport sediment and contaminants 

in this reach. 

Most of the dams in the study area make use of the head 

drop available at the dam site to generate power. The Riverside 

and Brodie Smith Dams in Berlin (fig. 2) divert water out 

of the river channel and into a penstock (in this case, a large 

pipe) to increase heads on the turbines that are located farther 

downstream. Steeper parts of the channel downstream from 

the Riverside and Brodie Smith Dams receive limited flow 

during average flows, because of the penstock diversion, but 

carry large flood flows. Sediment may accumulate in deeper 

pools in these sections of river; however, these areas are 

not navigable because of steep channel slopes and were not 

surveyed in this investigation. The arrangement of dams and 

penstocks, from the Saw Mill Dam at the former Chlor-Alkali 

Facility Superfund Site to the Berlin–Gorham town line, 

creates areas of backwater where sediment can accumulate 

upstream from the Riverside, Brodie Smith, Cross Power, and 

Cascade Dams (fig. 2). 

The Androscoggin River is incised, boulder filled, and 

average flows form rapids downstream of the Cascade, Brown, 

and Gorham Dams in Gorham for about 2, 1.5, and 1.5 mi, 

respectively. Pooled reaches between these dams affect the 

gradient or the backwater from the next downstream dam. 

Downstream from the Brown Dam to the Shelburne Reservoir, 

the channel gradient decreases and grades into a slightly 

braided sediment-filled channel with anabranching sections. 

The river runs unobstructed downstream from the Shelburne 

Dam in Shelburne and has braided and meandering sections 

for approximately 6.5 mi to the New Hampshire–Maine 

State border. A significant feature that may alter sediment 

distribution is ice-scour during the winter. At Riverside Dam, 

approximately 1,000 ft south of the site, large amounts of ice 

have been observed abrading the bottom of the river and being 

transported over the dam.

Methods of Data Collection 

and Analysis 

GPR and FDEM geophysical survey methods were used 

to investigate sediment that has been deposited on the bed of 

the Androscoggin River from Wheeler Bay downstream to 

the New Hampshire–Maine State border. Direct-current (DC) 

electrical resistivity (the inverse of conductivity) laboratory 

sediment sample tests, with samples from locations with 

known pore-water conductivity, were analyzed to understand 

variations in FDEM conductivity measured in the field with 

FDEM surveys. Thermoplastic canoes with minimal metal 

parts, were used for GPR and FDEM surveys in navigable 

sections of the river.

The study was conducted in two types of river 

environments–pooled backwater (areas behind dams) and riffle 

and rapids from the former Chlor-Alkali Facility Superfund 

Site in Berlin, N.H., downstream to the Shelburne Reservoir 

(fig. 1). The lower velocity pooled parts of reaches are 

more likely to contain finer-grained sediments that are more 

conducive to mercury methylation (Marvin-DiPasquale, Lutz, 

and others, 2009; Marvin-DiPasquale, Agee and others2011). 
6 Geophysical Bed Sediment Characterization of the Androscoggin River, New Hampshire, August 2009

Flow velocity during the study period prevented 

collection of straight cross-section surveys perpendicular to 

flow in several of the narrow, steep gradient reaches. Survey 

techniques were modified to work with high flows: GPR 

and FDEM data were collected by survey crews traveling 

upstream, on the side of the river with lower flow velocity and 

downstream with the thalweg flow on the high velocity side 

and center. After processing, cross sections were extracted 

from the survey data. Geophysical data were georeferenced 

using a global positioning system (GPS) with submeter 

horizontal positioning accuracy (Trimble Navigation 

Limited, 1998). 

Ground-Penetrating Radar

GPR methods have widely been used to map the extent 

and type of geologic material beneath water (Beres and 

Haeni, 1991; Haeni, 1996; Olimpio, 2000). GPR results 

provide an image of the riverbed, general sediment grain 

size, and depth of buried riverbed surfaces. GPR surveys 

in this study made use of a transmitting and receiving 

300 MHz (megahertz) frequency antennas. Electromagnetic 

radar waves were generated to image the subsurface. 

The radar-wave propagation is affected by differences in 

electromagnetic properties of the bed sediment including 

dielectric permittivity, electrical conductivity, and magnetic 

susceptibility because of differences in river and sediment 

water conductance, and sediment type (Keary and Brooks, 

1991). The penetration of GPR signals is limited where the 

radar-wave reflection is scattered because of diffractions from 

large cobbles and boulders on the riverbed. 

At locations where conditions are conducive to 

continuous and successful data collection, GPR provides 

a rapid means of providing detailed insight into riverbedsediment conditions. Concurrent and linked GPS data 

collection is necessary for georeferencing data surveyed 

from the water surface. GPR surveys were performed in a 

continuous data-collection mode, which required a relatively 

constant data collection speed. Display gain was adjusted by 

multiplying data by a constant to increase or decrease signal 

amplitude to provide an improved image. An example of 

data and interpretation from this method is shown in figure 3. 

Adjustments were made during the survey to ensure adequate 

data collection, and were applied later during processing in 

Radan software (Geophysical Survey Systems, Inc., 2008) 

during interpretation. Patterns in the GPR record were used 

to distinguish between fine and course material using the 

methods of Beres and Haeni (1991). Interpreted locations of 

bed material and depth were exported with GPS coordinates 

for analysis in a geographical information system (GIS). 

Radar-wave velocities will vary with study area properties. 

Published velocity estimates were used to calculate depth from 

reflected radar-wave travel time (Beres and Haeni, 1991) for 

this study.

Step-Frequency Electromagnetic Induction

The FDEM method applied in this setting measured 

the combined electrical conductivity of sediments, surface 

and pore water, which is affected by depth, grain size, and 

mineral composition. FDEM surveys were used to indirectly 

measure the electrical conductivity of the riverbed with 

induced electromagnetic signals (Zohdy and others, 1974). 

The FDEM technique also may indicate the presence of 

electrically conductive contaminants such as dissolved major 

ions including sodium, chloride, and calcium, or elemental 

mercury, if concentrations are sufficient. If elemental mercury 

were present in sufficient quantity, its conductivity or low 

resistivity (98 × 10^-8 ohm meters (Giancoli, 1989)) could 

create a large FDEM response. FDEM was used successfully 

to identify a contaminant plume beneath a pond in southern 

New Hampshire (Ayotte and others, 1999). Results can be 

processed and inverted for interpretation of geology and 

water pore-water conductivity with depth. The Geophex 

GEM-2 Plus used in this study, is a portable multi-frequency 

electromagnetic sensor. Data from surveys with the GEM-2 

Plus can be used to calculate the bulk apparent subsurface 

electrical conductivity and magnetic susceptibility (Geophex, 

Ltd., 2007). 

Before and after each survey, FDEM data collection 

was tied into a common base station established for each 

of the 10 reaches in the study area to provide static data for 

comparison with the FDEM survey line data. Base station 

data are used to correct for FDEM drift (shift in instrument 

response with time). The survey and base station data were 

processed to remove irregularities in the data caused by excess 

FDEM noise. All data were linearly shifted, using the base 

station responses to correct for instrument drift that could 

occur during the survey period (13 days). Collection and data 

processing techniques in this study are similar to those used in 

Abraham and others (2006). 

Data were collected in step mode (rapidly switching 

frequencies to apply maximum power to each), 10 frequencies 

were chosen for this study (570; 990; 1,770; 3,090; 5,490; 

9,690; 17,070; 30,090; 53,010; and 93,450 Hz (hertz)). 

Lower frequencies provide deeper depth of investigation 

(Geophex, Ltd., 2007; Huang and Won, 2000; Won, 

Keiswetter, and others, 1996; Won, Choi, and Im, 2006). 

A presurvey environmental noise test was used to select 

frequencies that would minimize the effect of natural or 

anthropogenic electromagnetic noise in the area. Powertransmission lines were present in the immediate area of some 

reaches; the 60 Hz frequency associated with power lines was 

monitored throughout the survey and removed in subsequent 

data filtering. 

Depth of water, conductivity of bed materials and 

background and local anthropogenic (such as power lines) 

noise will affect the success of an FDEM survey for a given 

frequency in a given reach. As a result, the quality of the 
Methods of Data Collection and Analysis 

7

Depth below water surface

Time, in nanoseconds at time of survey, in meters (feet)

0

50

100150200250

0 (0)

1 (3.3)2 (6.6)3 (9.8)

4 (13.1)5 (16.4)

300

A A'

EXPLANATION

Gravel or sandBoulder or cobbleBuried bed

(44.4151541, -71.1911918) (44.4152290, -71.1911691) (44.4153068, -71.1911553) (44.4153840, -71.1911546) (44.4154599, -71.1911575) (44.4155392, -71.1911629) (44.4157173, -71.1911404) (44.4156221, -71.1911565) (44.4158072, -71.1911675)

Location, in latitude and longitude decimal degrees

, Gorham, Example of a cross section showing ground-penetrating radar profile and interpretation in reach AR–7 upstream from the Brown Dam, Androscoggin River

 (location shown in figure 1). 

Figure 3.New Hampshire
8 Geophysical Bed Sediment Characterization of the Androscoggin River, New Hampshire, August 2009

data varies by frequency and reach. A consistent subset 

of frequencies was needed in order to compare data from 

reach to reach on the same scale. Plots of FDEM data by 

frequency for all survey lines were analyzed to pick three 

common frequencies with the lowest noise across the study 

area. A subset of the most stable frequencies (9,690; 17,070; 

and 30,090 Hz) was processed to create a data set of total 

conductivity using WinGEM version 3, 0, 0, 14 (Geophex, 

Ltd., 2007). FDEM data were trimmed in the vicinity of 

bridges and power lines because of the interference caused by 

the metal structures and electromagnetic fields. 

To help understand conductivity variations with depth, 

the most stable five frequencies for a given survey were 

used along selected survey paths for inverse modeling of the 

data for select portions using WinGEM version 3, 0, 0, 14 

(Geophex, Ltd., 2007) software to associate resistivity values 

with depth (fig. 4). Results of the inversion are given in terms 

of resistivity (inverse of conductivity). Resistivity values 

measured from bed sediments (about 200 ohm meters) and 

surface-water SC (SC) values (converted to about 300 ohm 

meters) were used to construct a 2-layered starting model. 

GPR riverbed depth interpretations overlaid on the FDEM 

inversion indicate a correlation with more resistive river water 

(green layer on top) and less resistive (blue layer) sediment 

through approximately 70 percent of the cross section (fig. 4).

At very low frequencies, electromagnetic induction 

response is due more to the magnetic properties of the 

subsurface than electrical properties, and the FDEM 

survey magnetic susceptibility responses are similar to a 

magnetometer survey (Won and Huang, 2004; Won and 

Keiswetter, 1997). Magnetic susceptibility was calculated 

using the raw in-phase component of the lowest frequencies 

used in the surveys (570; 990; 1,770; 3,090 Hz). When a 

magnetic response occurred, generally all four frequencies 

gave a similar response, though the lower frequencies indicate 

magnetic material more often than the higher frequencies. 

Magnetic susceptibility responses (lower frequency FDEM) 

were plotted on maps in reaches near the site in Berlin, N.H., 

to search for metal debris that may affect FDEM responses. 

During low water metal is observed in the river in the form of 

rusted nuts bolts and other debries. Mercury in an elemental 

liquid state is electrically conductive, but is not magnetic and 

would not create a magnetic susceptibility response. Magnetic 

susceptibility responses were not observed in reaches 

near downtown Gorham, but were observed in Shelburne 

downstream from the Gorham Dam in a rural setting. Different 

responses in Shelburne may be a result of variations in 

bedrock geology beneath the river (two mica granite) and are 

not associated with elevated electromagnetic conductivity. 

Determining bedrock properties and the mineralogy of 

sediments is beyond the scope of this report. 

Sediment Sampling and DirectCurrent Resistivity

SC measured during surface-water, pore-water and 

sediment sampling (with subsequent grain size analysis, 

table 1) collected as part of a parallel investigation 

(Jeffrey Deacon, U.S. Geological Survey, written commun., 

2009) were used to help understand and interpret the results 

of the geophysical surveys. The DC resistivity of sediment 

samples was measured in the laboratory with pore water intact 

for comparison with FDEM results. 

Environmental samples were collected during lower 

flow conditions in September 2009 and August 2010 using 

USGS surface-water sampling protocols (Wilde and others, 

1999). Pore-water samples were collected with the use of a 

push-point sampler (PPS) according to protocols described in 

Zimmerman and others (2005). A PPS is designed to sample 

pore water with minimal disturbance to the sediment matrix. 

SC of sampled water was used to determine if discontinuity 

between surface water and pore water is achieved 

during sampling. 

Figure 4. Example of a cross section showing inverted electromagnetic induction profile in reach AR–9 between the 

Gorham and Shelburne Dams, Androscoggin River, Shelburne, New Hampshire (location shown in figure 1). Solid black line 

represents riverbed measured with ground-penetrating radar. 

0 50

(164)

100

(328)

150

(492)

200

(656)

250

(820)

300

(984)

350

(1148)

400

(1312)

Distance, in meters (feet)

0 (0)

-5 (-16.4)

-10 (-32.8)

-15(-49.2)

-20 (-65.6)

Depth from water

-25 (-82.0)

surface at time

of survey, in meters (feet)

100 300 500 600 700

Resistivity, in Ohm meters

EXPLANATION

B B'
Methods of Data Collection and Analysis 9

Table 1. Surface, pore-water and sediment conductivity, direct-current resistivity test box results and sediment grain size. 

[ID, identifier; SW, surface water; SC, specific conductance; mS/m, millisiemens per meter; PW, pore water; sediment conductivity, measured in directcurrent resistivity test box; *, samples collected with core tube; EM, electromagnetic induction estimated in sample location; distance to EM survey in 

feet, distance to EM survey line in feet from sample location, 3-meter sample position accuracy; --, sample more than 65 feet from a survey line]

Reach name Sample 

ID

Sample 

date

SW SC 

(mS/m)

PW SC 

(mS/m)

Sediment 

conductivity 

(mS/m)

Estimated 

EM 

(mS/m)

Distance to 

EM survey 

(feet)

Percent silt 

and clay

Wheeler Bay AR–2_1 9/16/2009 2.7 11.2 8.9 -- -- 69

Wheeler Bay AR–2_2 9/16/2009 2.9 16.4 5.6 -- -- 77

Wheeler Bay AR–2_3 9/16/2009 2.4 8.2 7.1 -- -- 62

Wheeler Bay AR–2_4 8/23/2010 2.5 19.0 13.3* -- -- 74

Wheeler Bay AR–2_5 8/23/2010 2.7 8.3 8.2* -- -- 63

Brodie Smith Dam AR–4_1 8/27/2010 4.7 279.0 67.4* 73.2 38.1 27

Brodie Smith Dam AR–4_2 8/27/2010 5.4 324.0 76.8* 72.8 27.2 59

Cross Power Dam AR–5_1 8/27/2010 3.2 13.2 8.6* -- -- 60

Cross Power Dam AR–5_2 8/27/2010 3.3 16.6 6.8* 3.8 46.9 25

Cascade Dam AR–6_1 9/18/2009 3.0 24.5 5.4 2.2 32.5 28

Cascade Dam AR–6_2 8/26/2010 3.2 41.4 7.8* 2.2 29.2 45

Cascade Dam AR–6_3 8/26/2010 3.4 16.5 12.7* 6.7 63.0 50

Brown Dam AR–7_1 8/26/2010 3.7 5.2 11.3* 6.5 54.1 56

Brown Dam AR–7_2 8/26/2010 3.3 12.7 6.4* -- -- 74

Gorham Dam AR–8_1 9/18/2009 3.6 14.8 3.5 1.2 33.1 51

Gorham Dam AR–8_2 9/18/2009 3.4 15.7 3.6 4.5 0.0 37

Gorham Dam AR–8_3 9/18/2009 3.4 10.8 2.5 -- -- 80

Gorham Dam AR–8_4 8/24/2010 3.4 6.1 3.4* -- -- 18

Gorham Dam AR–8_5 8/24/2010 3.4 9.4 5.8* 4.7 17.1 59

Shelburne Dam AR–9_1 9/17/2009 3.5 11.6 3.1 -- -- 14

Shelburne Dam AR–9_2 9/17/2009 3.5 45.8 3.9 -- -- 3

Shelburne Dam AR–9_3 9/17/2009 3.5 9.3 3.1 2.5 3.6 29

Shelburne Dam AR–9_4 9/17/2009 3.8 15.8 3.9 3.3 28.9 28

Shelburne Dam AR–9_5 8/24/2010 3.6 4.3 4.0* 2.0 49.2 33

Shelburne Dam AR–9_6 8/25/2010 3.6 6.7 16.2* 2.9 27.6 33

Shelburne Dam AR–9_7 8/25/2010 3.7 4.8 3.4* -- -- 38
10 Geophysical Bed Sediment Characterization of the Androscoggin River, New Hampshire, August 2009

Streambed sediment samples were collected within a 

few meters of pore-water sampling locations. Samples were 

extracted using a hand-held coring device or ponar (grab 

sampling device) to a maximum depth of 4 inches (10 cm) in 

2009 according to USGS protocols (Shelton and Capel, 1994), 

and with a hand-held coring device to a maximum depth of 

4 inches (10 cm) in 2010 according to Lutz and others (2008). 

The resistivity of 26 sediment samples were measured 

with DC laboratory resistivity measurements following 

procedures outlined by Advanced Geosciences, Inc., 

(written commun., 2009). To measure the DC resistivity of 

sediment samples in the lab, DC is induced in the sample by 

two current electrodes, and the voltage is measured at two 

potential electrodes. Apparent resistivity is calculated from the 

resistance value and geometric factors corresponding to the 

sample box geometry. These data allow for the correlation of 

known riverbed material with a specific resistivity value and 

pore-water SC. 

DC resistivity values were converted to conductivity 

for comparison with FDEM and pore-water SC results. 

Clay and silt have more conductive responses than sand and 

gravel although the SC of pore water can affect the results of 

FDEM surveys. 

Interpretation of Riverbed Conductivity

Riverbed conductivity can be interpreted qualitatively 

from the processed results of FDEM surveys. The low ionic 

strength, low SC, and shallow depth (average 5.2 ft, (1.6 m)) 

of the Androscoggin River results in little contribution of 

the river water to the total FDEM conductivity value in 

the study area. Sediment conductivity in the study area is 

largely determined by riverbed-sediment type and porewater SC, though sediment and pore-water sampling points 

were separated by as much as two meters and this should be 

considered when comparing the measurements. In general 

greater pore-water conductivity corresponds to greater 

sediment conductivity, but sediment conductivity will be 

affected by mineralogy and increase with decreasing grain 

size. Greater or varying pore-water conductivity also can mask 

the effects of grain size and mineralogy. 

Average water and bed sediment conductivity (measured 

with DC resistivity in the lab) from 2009 sampling were used 

to determine skin depth (Won, 1980), which is the total depth 

of penetration of the FDEM field. Depth of investigation 

(Haung, 2005) was determined graphically for the three 

frequencies selected for the study-area-wide analysis. The 

depth of investigation is always less than the skin depth and 

is determined using the contrast between the host and target 

(water and sediment in this study). Correlation of depth 

measured from GPR and conductivity from FDEM surveys 

were analyzed to assess whether water depth was driving the 

FDEM response. 

The effects of discharging groundwater on the riverbed 

pore-water quality were not known in this study area, but 

may be a factor in some locations near land-use features, such 

as roads. Groundwater conductance likely is considerably 

greater (hundreds of millisiemens per meter (mS/m)) near 

major roads because of the application of road salt (Harte and 

Trowbridge, 2010).

Estimation of Riverbed Properties by Reach

GPR and FDEM survey results were interpolated 

between survey lines to produce continuous grids of estimated 

GPR bed material type, bed thickness, water depth, and FDEM 

total conductivity. Magnetic susceptibility data from FDEM 

surveys was plotted with FDEM grid data to identify areas 

where conductive anomalies likely are associated with ferrous 

objects in the river (metal debris) as opposed to pore water or 

sediment related anomalies. Maps were produced for reaches 

adjacent to and downstream of the site to display sediment and 

conductivity distribution. Summary values for the upstream 

reference reach, Wheeler Bay are included in this report for 

comparison with other reaches, but it was not shown in a map 

since only a small portion of the reach was surveyed.

The TOPOGRID program (Environmental Systems 

Research Institute, 1999) was used in interpolation of GPR 

and FDEM survey results because the routine preserves data 

values and minimum and maximum values are not exceeded 

in the interpolated output grids. The interpolation extent 

was constrained using modified Androscoggin River area 

features from the high-resolution (1:24,000 scale) National 

Hydrography Dataset (NHD) (Simley and Carswell, 2009). 

1 10 100

Pore-water specific conductance, in millisiemens per meter

1

10

100

Sediment conductivity,

in millisiemens per meter

EXPLANATION

Sample

2010

Date

0.018

P-value

0.632

Rho

Sediment and pore-water conductivity correlation

Figure 5. The relation between specific-conductance 

measurements from riverbed pore-water and resistivity values 

measured in the laboratory from co-located riverbed-sediment 

samples with intact pore water.
Characteristics of Riverbed Sediment 11

The polygon area features from NHD were edited to better 

represent the riverbank boundary depicted on high-resolution 

aerial photographs from the National Agriculture Imagery 

Program (NAIP) and to correspond with parts of reaches and 

anabranch channels that were surveyed. 

GPR record interpretations and selection of velocity by 

material were standardized across the study area to provide 

georeferenced results in terms of water depth, bed material 

type, and bed thickness. GPR records were interpreted based 

on the methods presented in Beres and Haeni (1991), five 

materials that give a unique reflection pattern were selected 

for interpretation: (1) water, (2) silt and or sand, (3) sand and 

or gravel, (4) boulder and cobble or both, and (5) bedrock. 

Maps of estimated bed sediment were produced with silt, sand, 

and gravel grouped into one category and cobble, boulder, 

and bedrock in another. Results from GPR interpolations 

were extracted to produce cross sections of bed material and 

sediment thickness in selected locations. 

Characteristics of Riverbed Sediment 

In this report, the words “right” and “left” refer to 

directions that would be reported by an observer facing 

downstream. GPR and FDEM survey track lines (figs. 6–13) 

indicate where measurements were made and passed filtering 

quality control procedures, therefore, an estimated value 

should be considered with less confidence with increasing 

distance from a survey line.

Average water specific conductance (3.4 mS/m) and 

bed sediment (21.9 mS/m) conductivity from 2009 sampling 

were used to determine FDEM skin depth, which is the total 

depth of penetration of the field. Depth of investigation was 

determined graphically for the three frequencies selected for 

the study-area-wide analysis (table 2). Conductivity by reach 

was summarized to assess bulk differences in conductivity of 

the riverbed (table 3). Relatively consistent measurements of 

surface-water SC and a test of correlation between measured 

water depth and conductivity resulted in a widely-varying 

correlation coefficient (table 3). This correlation result adds 

confidence that the FDEM response was due to different 

riverbed material and pore-water SC as well as water depth, 

but not driven by water depth alone. 

The effects of discharging groundwater on the riverbed 

pore-water quality were not known in this study area, but 

the median groundwater SC in Androscoggin and upper 

Connecticut River Basins in New Hampshire is 15.0 mS/m 

(Olimpio and Mullaney, 1997). This is greater than the median 

surface-water SC of 3.4 mS/m measured in this study, both are 

relatively low. 

Cultural features, particularly bridges and power lines, 

affected the interpretation of geophysical surveys in some 

parts of the study area. Parts of reaches with FDEM surveys 

which could not be interpreted (black line, figs. 6–13) also 

may have areas of greater riverbed conductivity. For example, 

sediment sample AR–6–2, collected close to the river bank, 

had an elevated pore-water conductivity and lab conductivity 

(table 1). The sandbar that this sample was located on was not 

surveyed with FDEM and this large conductive value does not 

appear in the mapped data (fig. 9). FDEM data were removed 

from parts of reaches AR–4, AR–8, AR–9 because they were 

affected by power-line interference. Data also were removed 

in the vicinity of metal bridges that cross the river and affected 

FDEM data in parts of reaches AR–3, AR–5, AR–6, and 

AR–10. 

Wheeler Bay, Reference Reach 

Wheeler Bay (fig. 1) is located in reach AR–2 in a rural 

setting in the town of Dummer, N.H., approximately 11 mi 

upstream from the former Chlor-Alkali Facility Superfund 

Site. The Androscoggin River flows to the southeast through 

this reach. The Pontook hydroelectric dam (outside of 

study area) is located upstream from this reach. This reach 

is unaffected by site-related contaminants or the effects of 

urbanization, and is a reference reach for this study and 

ongoing biological, sediment, and biogeochemical studies 

of the Androscoggin River (U.S. Environmental Protection 

Agency, 2011). Geophysical surveys of this reach were 

completed to assess equipment responses in an environment 

unaffected by the site-related contaminants. 

Table 2. Skin depth and depth of investigation values determined graphically for selected electromagnetic frequencies.

[Hz, hertz]

Frequency Hz

Water skin depth 

in meters (feet )

Sediment skin depth 

in meters feet)

Depth of investigation in water 

in meters feet)

Depth of investigation in sediment 

in meters (feet)

Won, 1980 Huang, 2005

9,690 100 (328) 50 (164) 10.1 (33.1) 8.0 (26.2)

17,070 75 (246) 40 (131) 9.9 (32.5) 7.0 (23.0)

30,090 50 (164) 20 (66) 8.0 (26.2) 4.9 (16.1)
12 Geophysical Bed Sediment Characterization of the Androscoggin River, New Hampshire, August 2009

Table 3. Estimates of electromagnetic conductivity summarized by reach and correlation with water depth.

Reach name Reach code

Conductivity,

 in millisiemens per meter Water depth and 

conductivity correlation Average Minimum Maximum

Wheeler Bay AR–2 2.6 0.1 9.4 0.21

Upstream of the Riverside Dam AR–3 3.1 0.6 31.4 -0.31

Upstream of the Smith Dam AR–4 10.1 0.9 194.9 -0.49

Upstream of the Power Dam AR–5 4.2 1.7 10.2 -0.48

Power Dam to Cascade Dam AR–6 2.7 0.8 7.1 -0.05

Cascade Dam to Brown Dam AR–7 2.2 0.6 8.9 -0.03

Brown Dam to Gorham Dam AR–8 0.9 0 7.1 -0.04

Gorham Dam to Shelburne Dam AR–9 2.5 0 12.6 -0.36

Downstream of the Shelburne Dam AR–10 1.7 0.9 3.5 0.56

The part of the reach that was surveyed in this study 

is about 1,000 ft long and varies in width between 300 and 

1,000 ft. The wide channel in this reach is located at a bend 

in the river. River water is mostly pooled, and had an average 

depth of 4 ft and a maximum depth of about 15 ft during 

the time of the survey. Fine sediments have accumulated 

because of a wide riverbed area, lower streamflow velocity 

and the channel gradient in Wheeler Bay. Some boulders 

were observed on the riverbed on the right side of the channel 

where flow velocity is higher. 

Grain size analysis of five sediment samples from 

Wheeler Bay (table 1) indicates between 62 and 77 percent 

of silt and clay, this is higher than most of the samples in 

the study. Silt and clay will increase the FDEM conductivity 

response, and processed GPR results indicate that this is the 

largest riverbed area covered with sediment (97.3 percent; 

gravel and finer) of all the reaches surveyed (table 4). FDEM 

values had an average of 2.6, a minimum of 0.1 and a 

maximum of 9.4 mS/m (table 3). Despite the larger percentage 

of fine sediment in samples (table 1) and interpreted from 

GPR (table 4), FDEM conductivity measurements from the 

bay were less than those from reaches within a few miles 

downstream from the site. 

Reach AR–3, Between the Former Chlor-Alkali 

Facility Superfund Site and the Riverside Dam

Reach AR–3 (fig. 6) in the town of Berlin, N.H. is 

located between the Sawmill Dam, upstream to the north, and 

the Riverside Dam downstream to the south. This reach is 

adjacent to the former Chlor-Alkali Facility Superfund Site 

located on the left bank (east). This reach may be affected with 

site-related contaminants and also may be affected by other 

contaminants related to urbanization. The surrounding land 

has a long and varied history of industrial use. When the dam 

flood gates are shut, as they were during the surveys, most of 

the flow leaves the turbine building at the Sawmill Dam on 

the right bank and flows towards the left bank at the railroad 

bridge and back to the right bank at the Riverside Dam turbine 

intake. Minimal sediment accumulates in these areas. 

The surveyed part of reach AR–3 is about 1,400 ft long 

and was between 200 and 250 ft wide. The narrow straight 

channel in this reach had an average depth of 6.8 ft and a 

maximum depth of about 15.8 ft during the time of the survey. 

This reach has limited areas for fine sediments to accumulate 

because of high streamflow velocity. Bedrock, with some 

boulders and scrap metal, is observed on the riverbed and bank 

along the left bank from the Sawmill Dam to about 650 ft 

downstream. Some fine sediment has accumulated among 

boulders along the left bank just downstream from the bedrock 

and near areas of visible groundwater seeps from the riverbank 

and iron dissolution onto the riverbed. 

Reach AR–3 has the greatest estimated percent of 

bedrock and boulder riverbed material interpreted and 

extrapolated from GPR results (89 percent, table 4) of all 

the reaches surveyed. Fine sediment found on the upstream 

left bank and the downstream right bank have elevated 

electromagnetic conductivity that is interpreted to be from 

elevated bed pore-water SC (fig. 6). FDEM values had 

an average of 3.1, a minimum of 0.6 and a maximum of 

31.4 mS/m (table 3). The maximum FDEM values from this 

reach were elevated above those of the reference reach , 

despite the limited fine material. This indicates that the river 

may be affected by contaminants or the effects of urbanization 

(road salt). FDEM data affected by a steel railroad bridge 

were removed, which is illustrated by the absence of the black 

FDEM survey track upstream from cross-section D–D' (fig. 6). 

FDEM magnetic susceptibility results indicate potential 

magnetic material near the two areas of fine sediment and at 

the upstream extent from the surveyed reach. This response 

likely is a result of metal debris on the riverbed.
Characteristics of Riverbed Sediment 13

Table 4. Percentage of fine sediment summarized by reach.

Estimates based on gridding of ground-penetrating radar interpretations

Reach name Reach code

Area of gravel or finer sediment, Percent of reach area

Gravel or finer 

sediment

Cobble, boulder 

and or bedrock in square meters in square yards

Wheeler Bay AR–2 933,256 1,116,954 97 3

Upstream from the Riverside Dam AR–3 29,894 35,778 11 89

Upstream from the Smith Dam AR–4 85,109 101,862 25 75

Upstream from the Power Dam AR–5 354,077 423,772 61 39

Power Dam to Cascade Dam AR–6 342,268 409,639 51 49

Cascade Dam to Brown Dam AR–7 1,345,012 1,609,759 42 58

Brown Dam to Gorham Dam AR–8 432,820 518,015 20 80

Gorham Dam to Shelburne Dam AR–9 5,794,177 6,934,680 77 23

Downstream from the Shelburne Dam AR–10 6,915,104 8,276,245 86 15

Total 16,231,717 19,426,703 52 48

and indicate conductive pore water; metal within cobble 

and boulders near the left bank is indicated in the FDEM 

magnetometer response. Interpretation and extrapolation of 

GPR results was used to indicate approximately 25 percent 

of the riverbed area is covered by gravel or finer material 

(table 4); fine material is as much as 3 ft thick on top of an 

older buried bed (fig. 7B). Pore-water SC measured in this 

reach was the greatest in the study area, 279 and 324 mS/m 

(table 1) at sediment sample locations AR–4_1 and AR–4_2 

(fig. 7). This reach has the largest FDEM values in the study 

with an average of 10.1, a minimum of 0.9 and a maximum of 

195 mS/m (table 3). Zones of elevated FDEM conductivity are 

measured on a sandbar deposit on the downstream left side of 

the reach. Sediment samples collected near a FDEM survey 

line and measured in the laboratory had values of 67.4 and 

76.8 mS/m, similar to estimated values (73.2 and 72.8 mS/m; 

table 1). FDEM values at this reach were elevated above 

those of the reference reach because of elevated pore-water 

SC within fine grained material. This indicates that the river 

may be affected by contaminants or the effects of urbanization 

(road salt). FDEM data were affected by power-line noise 

downstream (south) of cross-section E–E’ and were removed. 

FDEM data removal is illustrated by the absence of the black 

FDEM survey track where GPR survey tracks are present 

(fig. 7).

Reach AR–5, Upstream from the Cross 

Power Dam

Reach AR–5 is located upstream of the Cross Power 

Dam which controls the water level and flow in this reach. 

The Androscoggin River flows to the south through this reach, 

which is adjacent to a public park on the upstream left bank 

Reach AR–4, Upstream from the Smith Dam 

Reach AR–4 is located downstream of the Riverside 

Dam and upstream from the Smith Dam and Mason St. in 

Berlin, N.H. (fig. 7). Most of the land abutting this reach is 

urban or industrial, although there is a community park on 

the downstream left bank side. The Androscoggin River flows 

to the southwest through this reach. Most of the flow during 

these surveys was along the right bank, from the Riverside 

Dam turbine building outflow to the Smith Dam penstock 

intake on the right bank, downstream side from the Mason 

Street Bridge. Even though this reach is wide, the flow 

velocity was too high along the right side during the surveys 

to collect cross sections. As an alternative to cross sections, a 

counter clockwise survey (upstream on slow side, downstream 

on fast side) was used to collect data in the backwater from the 

dam. Five marker buoys were placed at approximately 20 foot 

intervals from the left bank (where current was at a minimum) 

to the center as points of reference for the survey navigation. 

This surveyed reach was about 1,000 ft long and was 

between 150 to 550 ft wide. The channel in this reach had 

an average depth of 6.4 ft and a maximum depth of about 

16.5 ft, during the time of the survey. Fine sediments have 

accumulated in many areas on the left side of the reach 

because of lower stream flow velocity and channel gradient. 

Cobbles, boulders and bedrock are found along the right bank 

and the upstream part of the left bank. Bedrock outcrop on the 

riverbed was confirmed in September 2009, when this reach 

was drained for maintenance, and the bed was exposed above 

water. A large silt and sand bar deposit with some cobbles and 

boulders were found on the downstream left bank. 

The largest FDEM conductivity values measured in 

this study occur on the bed on the left side of the channel 
14 Geophysical Bed Sediment Characterization of the Androscoggin River, New Hampshire, August 2009

Degnan_fig 06

SAWMILL DAM

RIVERSIDE

DAM

C

C'

D D'

Former Chlor-Alkali Facility

Superfund Site

0 125 250 375 500 FEET

0 62.5 125 METERS

N

Base modified from United States Department of Agriculture

National Agriculture Imagery Program, 2008

Digital orthophoto quarter-quadrangle

Universal Transverse Mercator projection, zone 19

North American Datum of 1983

Estimated elecromagnetic conductivity,

in millisiemens per meter

15

0

Estimated bed material (ground-penetrating

radar)

Cobble, boulder, or bedrock

Silt, sand, or gravel

Electromagetic survey track

Ground-penetrating radar survey track

Line of section

Magnetic susceptibility, in parts per thousand

0.01 to 2.00

2.01 to 5.00

5.01 to 10.00

10.01 to 20.00

20.01 to 35.00

!

!

!

!

!

EXPLANATION

C C'

D D'

C C'

EXPLANATION

Cobble, boulder, or bedrock

Silt, sand, or gravel

Buried bed

Water surface

0 50 100 150 200 250

0 50 100 150 200

Distance, in feet

Depth below water surface

at time of survey, in feet

0

5

10

0

5

10

15

VERTICAL EXAGGERATION x5

Depth below water surface

at time of survey, in feet

Distance, in feet

B

A

Figure 6. The results of geophysical surveys of riverbed-sediment characteristics in reach AR–3 from the former ChlorAlkali Facility Superfund Site to the Riverside Dam, Androscoggin River, Berlin, New Hampshire.
Characteristics of Riverbed Sediment 15

Degnan_fig 07

BRODIE SMITH DAM

0 125 250 375 500 FEET

0 62.5 125 METERS

Base modified from United States Department of Agriculture

National Agriculture Imagery Program, 2008

Digital orthophoto quarter-quadrangle

Universal Transverse Mercator projection, zone 19

North American Datum of 1983

N

Estimated elecromagnetic conductivity,

in millisiemens per meter

50

Greater than 15

15

0

Estimated bed material (ground-penetrating

radar)

Cobble, boulder, or bedrock

Silt, sand, or gravel

Electromagetic survey track

Ground-penetrating radar survey track

Line of section

Magnetic susceptibility, in parts per thousand

0.01 to 2.00

2.01 to 5.00

5.01 to 10.00

10.01 to 20.00

20.01 to 35.00

2010 Sediment sample and identifier

!

!

!

!

!

EXPLANATION

E E'

AR–4_1

E

E'

AR–4_2

AR–4_1

EXPLANATION

Cobble, boulder, or bedrock

Silt, sand, or gravel

Buried bed

Water surface

VERTICAL EXAGGERATION x5

E E'

0 50 100 150 200 250 300 350 400 450 500 550 570

Distance, in feet

0

5

10

12

Depth below water

surface at time of

survey, in feet

B

A

Figure 7. The results of geophysical surveys of riverbed-sediment characteristics in reach AR–4 upstream from the Brodie 

Smith Dam, Androscoggin River, Berlin, New Hampshire.
16 Geophysical Bed Sediment Characterization of the Androscoggin River, New Hampshire, August 2009

and urban land on the right bank including State Route 16. 

During the survey period, the strongest currents flowed along 

the right bank downstream from the hydropower discharge. 

During high streamflows, additional flow enters the reach 

from upstream the left bank. This flow is conveyed through a 

bedrock gorge with a steep gradient (100 ft/mi, fig. 2) from the 

Brodie Smith Dam. 

This surveyed reach is about 2,000 ft long and between 

150 and 400 ft wide, and the depth is 7.7 ft on average (more 

than 16.6 ft near the James Cleveland Bridge). Bedrock 

outcrops along much of the left and right bank upstream from 

the James Cleveland Bridge on Unity Street and was found 

on the left side of the channel downstream. Sandbars form 

along the upstream left bank, downstream left bank, and 

just downstream from the right bank abutment on the James 

Cleveland Bridge. This reach is wider and likely contains 

more sediment that was not measured downstream from the 

Cross Power Dam warning barrels.

Interpretation and extrapolation of GPR results indicate 

approximately 61 percent of the riverbed area in reach AR–5, 

upstream from the Cross Power Dam is covered by gravel or 

finer material (table 4). This reach has larger FDEM values 

than the Wheeler Bay reference reach with an average of 4.2, 

a minimum of 1.7 and a maximum of 10.2 mS/m (table 3). 

This indicates that the river may be affected by contaminants 

or the effects of urbanization (road salt). FDEM data were 

affected by the steel James Cleveland Bridge and were 

removed. FDEM data removal is illustrated by the absence of 

FDEM survey track lines where the GPR survey track lines 

occur (fig. 8). Fine sediments with conductive pore water are 

interpreted just upstream from the Cross Power Dam warning 

barrels. The greatest conductivity in the surveyed reach is at 

the downstream end, on the left bank (fig. 8). Fine sediment 

with conductive pore water accumulated at the upstream end 

from the reach in a bar along the left bank. At this point the 

natural steep gradient bedrock channel joins the hydropower 

penstock outflow entering the backwater from the Cross 

Power Dam. This sediment likely was suspended by high 

flows though this channel and the channel upstream near the 

site between the Riverside Dam and the Brodie Smith Dam. 

Sediment accumulated in this location because of the break 

in slope and bend in the channel. Road salt from the adjacent 

road may contribute to the FDEM response and large FDEM 

values near the downstream right bank.

Reach AR–6, Between Cross Power and 

Cascade Dams

Reach AR–6 is located between the Cross Power Dam 

upstream and the Cascade Dam downstream (fig. 9). The 

Androscoggin River flows to the south through this reach, 

which is bordered by urban residential use land, a railroad and 

State Route 16 on the left bank. The area surveyed upstream 

from the dam warning barrels is within the city of Berlin. The 

Cascade Dam is within the town of Gorham, slightly south of 

the town line with Berlin (fig. 1). The strongest current during 

the surveys was from the turbine outflow at the Cross Power 

Dam along the left bank, even though water was flowing over 

the dam. Upstream and just downstream from the railroad 

bridge, most of the flow in the channel is along the left bank. 

Flow is along the right bank closer to the Cascade Dam and 

warning barrels. Survey lines were set up in a clockwise 

pattern to take advantage of the flow traveling downstream 

with the stronger current along the left bank, and upstream on 

the side with the weaker current along the right bank.

This surveyed reach is about 2,000 ft long, and 250 to 

350 ft wide, with a railroad bridge crossing the upstream 

segment. The channel in this reach had an average depth of 

7.2 ft and a maximum depth of about 16.7 ft during the time 

of the survey. The Cross Power Dam is built on bedrock and 

large cobbles and boulders are seen at the toe of the dam. 

Because of the widening of the channel at the downstream end 

near the Cascade Dam warning barrels, there is considerable 

sediment accumulation just outside of the surveyed area on the 

left bank including a 600 ft long grass covered channel bar.

Interpretation and extrapolation of GPR results indicate 

approximately 51 percent of the riverbed area is covered by 

gravel or finer material (table 4). Even though the percentage 

of fine material (increases conductivity) is nearly one-half of 

that of the Wheeler Bay reference reach, the overall FDEM 

values are similar with an average of 2.7, a minimum of 0.8 

and a maximum of 7.1 mS/m (table 3). The highest estimated 

FDEM values in this reach are likely a result of elevated porewater conductivity as is indicated in pore-water measurements 

and DC resistivity test box measurement of sediment sample 

AR–6_2 (table 1). This sample was 8.9 meters from the 

FDEM survey line and does not appear to have an effect 

on the FDEM results. Fine sediments with greater FDEM 

conductivity are located on the downstream left bank side 

of the reach (fig. 9). FDEM data were affected by the steel 

railroad bridge and were removed. FDEM data removal is 

illustrated by the absence of the FDEM survey where the GPR 

survey tracks are downstream from Cross Power Dam and 

sediment samples AR–6_1 and 2 (fig. 9). Sediment sample 

AR–6_3 had an elevated pore-water conductivity and lab 

conductivity (table 1). This sample was collected close to the 

river bank on a sandbar outside the surveyed area. Therefore, 

the conductive feature does not appear in the mapped data 

(fig. 9).

Reach AR–7, Upstream from the Brown Dam

Reach AR–7 is located less than 6 mi downstream from 

the former Chlor-Alkali Facility Superfund Site and upstream 

from the Brown Dam (fig. 10). The intersection of State 

Routes 2 and 16 in Gorham (fig. 1) is downstream on the right 

bank side. The Androscoggin River flows to the south through 

this reach. The land on the left bank of this reach is a forested 

mountain; the land on right bank is urban, with business and 

residential use.
Characteristics of Riverbed Sediment 17

Degnan_fig 08

 

 

UNITY STREET

JAMES CLEVELAND BRIDGE

AR–5_1

AR–5_2

0 125 250 375 500 FEET

0 62.5 125 METERS

N

Base modified from United States Department of Agriculture

National Agriculture Imagery Program, 2008

Digital orthophoto quarter-quadrangle

Universal Transverse Mercator projection, zone 19

North American Datum of 1983

Estimated elecromagnetic conductivity,

in millisiemens per meter

15

0

Estimated bed material (ground-penetrating

radar)

Cobble, boulder, or bedrock

Silt, sand, or gravel

Electromagetic survey track

Ground-penetrating radar survey track

2010 Sediment sample and identifier

EXPLANATION

AR–5_1

CROSS POWER DAM

16

Figure 8. Results of geophysical surveys of riverbed-sediment characteristics in reach AR–5 upstream from the Cross 

Power Dam, Androscoggin River, Berlin, New Hampshire.
18 Geophysical Bed Sediment Characterization of the Androscoggin River, New Hampshire, August 2009

Degnan_fig 09

Estimated elecromagnetic conductivity,

in millisiemens per meter

15

0

Estimated bed material (ground-penetrating

radar)

Cobble, boulder, or bedrock

Silt, sand, or gravel

Electromagetic survey track

Ground-penetrating radar survey track

2009 Sediment sample and identifier

2010 Sediment sample and identifier

EXPLANATION

AR–6_1

AR–6_2

Base modified from United States Department of Agriculture

National Agriculture Imagery Program, 2008

Digital orthophoto quarter-quadrangle

Universal Transverse Mercator projection, zone 19

North American Datum of 1983

N

AR–6_1

AR–6_2

AR–6_3

0 125 250 375 500 FEET

0 62.5 125 METERS

CASCADE DAM

CROSS POWER DAM

16

Figure 9. Results of geophysical surveys of riverbed-sediment characteristics in reach AR–6 between the Cross 

Power and Cascade Dams, Androscoggin River, Gorham, New Hampshire.
Characteristics of Riverbed Sediment 19

Degnan_fig 10

AR–7_1

AR–7_2

Base modified from United States Department of Agriculture

National Agriculture Imagery Program, 2008

Digital orthophoto quarter-quadrangle

Universal Transverse Mercator projection, zone 19

North American Datum of 1983

0 750 1,500 2,250 3,000 FEET

0 375 750 METERS

N

Estimated elecromagnetic conductivity,

in millisiemens per meter

15

0

Estimated bed material (ground-penetrating

radar)

Cobble, boulder, or bedrock

Silt, sand, or gravel

Electromagetic survey track

Ground-penetrating radar survey track

2010 Sediment sample and identifier

EXPLANATION

AR–7_1

BROWN DAM

16

Figure 10. Results of geophysical surveys of riverbed-sediment characteristics in reach AR–7 upstream from the Brown 

Dam, Androscoggin River, Gorham, New Hampshire.
20 Geophysical Bed Sediment Characterization of the Androscoggin River, New Hampshire, August 2009

The area of the reach surveyed is about 2 mi long and 

between 230 and 600 ft wide. The channel had an average 

depth of 4.6 ft and a maximum depth of about 13.1 ft during 

the time of the survey. Upstream, the channel is incised and 

has a low sinuosity and large boulders on the riverbed. The 

gradient of the channel in the surveyed reach is steep (10 ft/mi, 

fig. 2) in the upstream two thirds and it has the longest 

continuous riffle of all reaches studied. The wide channel at 

the downstream end of this reach had many areas with the 

potential for fine sediments to accumulate because of lower 

stream flow velocity and channel gradient. 

Interpretation and extrapolation of GPR results indicate 

that approximately 42 percent of the riverbed area is covered 

by gravel or finer sediment (table 4). Gravel and finer sediment 

was estimated to be on the riverbed across most of the channel 

from the Brown Dam warning barrels to about 2,200 ft 

upstream along the right bank and to 3,700 ft upstream from 

the barrels along the left bank side (fig. 10). Fine sediment 

identified with GPR, and conductive pore water identified by 

FDEM, was estimated to be between the warning barrels and 

1,000 ft upstream along the right bank side. This reach has an 

estimated area of fine sediments more than three times larger 

than any of the reaches upstream (table 4). Even though the 

percentage of fine material (increases conductivity) is less 

than one-half of that estimated for the Wheeler Bay reference 

reach, the FDEM values are similar with an average of 2.2, 

a minimum of 0.6 and a maximum of 8.9 mS/m (table 3). 

Estimated FDEM values in this reach likely are elevated by 

conductive pore water. Large FDEM values on the furthest 

downstream right bank part of the reach may be affected by 

road salt runoff from the adjacent road.

Reach AR–8, Between the Brown and 

Gorham Dams

Reach AR–8 begins approximately 6 mi downstream 

from the former Chlor-Alkali Facility Superfund Site, 

downstream from the Brown Dam. The town of Gorham is 

on the right bank with urban and residential land use; the left 

bank abuts a forested mountain with the remnants of a lead 

mine near Mascot Pond (fig. 1) and a power-line cut. The 

Androscoggin River generally flows to the southeast through 

this reach. The river valley and channel orientation changes 

from roughly a north–south trend upstream, to an east–west 

trend between the Brown and Gorham Dams (fig. 11). 

The surveyed part of this reach is about 2 mi long and 

is narrow, approximately 150 ft wide, for most of the length. 

The channel is wider at the downstream end (350 ft) at a bend 

in the channel near the warning barrels for the Gorham Dam. 

The channel in this reach had an average depth of 2.9 ft and 

a maximum depth of about 16.2 ft (at the downstream end 

at the bend and bedrock on the left bank) during the time 

of the survey. Boulders and rapids at the upstream segment 

grade into riffle and gravel and finer sediment, with bedrock 

exposure at the downstream segment along the left bank near 

the power-line cut and near the bend in the channel upstream 

from the Gorham Dam. Halfway down the reach, the river is 

anabranching, creating several channels. Sediment properties 

were estimated only for the channel surveyed and were not 

extrapolated to other sections of anabranching channels. 

This reach has the lowest estimated percent of fine bed 

sediment (table 4) in the study, 20 percent, with the exception 

of the steeper gradient reach AR–3 at the former ChlorAlkali Facility Superfund Site. The average and minimum 

estimated FDEM conductivity were the smallest in the study 

(table 3), indicating less conductive pore water. This reach has 

lower FDEM values than the Wheeler Bay reference reach 

with an average of 0.9, a minimum of less than 0.1 and a 

maximum of 7.1 mS/m (table 3). The estimated bed sediment 

conductivity near State Route 16 (fig. 1) along the right 

bank at the upstream end from this reach is slightly elevated 

even though the material is cobble and boulders, this may be 

because of road salt affected groundwater. At the downstream 

end of this reach the river bends 90 degrees at a large bedrock 

outcrop just upstream from the dam. Fine sediments have 

accumulated on the left bank, north of the outcrop and 

have an elevated conductivity (fig. 11) that may be due to 

elevated SC in the pore water. Sediment samples collected 

within 5 meters of an FDEM survey line and measured in 

a laboratory DC resistivity test box had values of 3.6 and 

5.8 mS/m that were close in value to an estimated FDEM of 

4.5- and 4.7-mS/m, respectively (table 1). Fine sediments with 

a lower conductivity have accumulated on the right side of 

the channel. FDEM data were affected by power-line noise 

at the upstream end and downstream (south) in the locations 

of power lines crossing the river and were removed. FDEM 

data removal is illustrated by the absence of the FDEM survey 

track in the location of the GPR survey track (fig. 11).

Reach AR–9, Between the Gorham and 

Shelburne Dams

Reach AR–9 begins slightly more than 8 mi downstream 

from the former Chlor-Alkali Facility Superfund Site and is 

located between the Gorham Dam upstream and the Shelburne 

Dam at the downstream end of the reach. The Androscoggin 

River flows to the east and northeast through this reach; 

through forested mountain areas. The Gorham Dam is located 

about .5 mi upstream from the Shelburne town line (fig. 1) 

and is keyed into bedrock outcropping on the left bank. The 

Androscoggin River upstream of the Shelburne Dam, at the 

downstream end of the surveyed reach (fig. 12), is the largest 

body of pooled water in the study area. 

This reach is almost 3 mi long and varies between 200 

and 1,500 ft wide. The area of the channel surveyed is the 

second largest in the study (table 4). The first one-half of this 

reach is incised and contains boulders with rapids grading into 

braided deposits of gravel with riffles. A large volume of fine 

sediment has been deposited in the middle of the reservoir in 

a delta-like formation where it is wide, has a low gradient, and 
Characteristics of Riverbed Sediment 21

Degnan_fig 11

AR–8_1 AR–8_1

AR–8_2_

AR–8_3

AR–8_4

AR–8_5

Base modified from United States Department of AgricultureNational Agriculture Imagery Program, 2008Digital orthophoto quarter-quadrangleUniversal Transverse Mercator projection, zone 19North American Datum of 1983

BROWN DAM

GORHAM DAM

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

0 250 500 METERS

N

Estimated elecromagnetic conductivity,in millisiemens per meter150Estimated bed material (ground-penetratingradar)Cobble, boulder, or bedrockSilt, sand, or gravelElectromagetic survey trackGround-penetrating radar survey track2010 Sediment sample and identifier2009 Sediment sample and identifier

EXPLANATION

AR–8_4AR–8_3

16

2

, Gorham, New Hampshire

Results of geophysical surveys of riverbed-sediment characteristics in reach AR–8 between the Brown and Gorham Dams, 

.

Figure 11.Androscoggin River
22 Geophysical Bed Sediment Characterization of the Androscoggin River, New Hampshire, August 2009 Estimated elecromagnetic conductivity, in millisiemens per meter 15

0Estimated bed material (ground-penetratingradar)Cobble, boulder, or bedrockSilt, sand, or gravelElectromagetic survey trackGround-penetrating radar survey track2010 Sediment sample and identifier2009 Sediment sample and identifier

EXPLANATION

AR–9_4AR–9_3

AR–9_4

AR–9_3

AR–9_6

AR–9_5

AR–9_7 AR–9_7

AR–9_2 AR–9_2

AR–9_1 AR–9_1

Base modified from United States Department of AgricultureNational Agriculture Imagery Program, 2008Digital orthophoto quarter-quadrangleUniversal Transverse Mercator projection, zone 19North American Datum of 1983

N

0 1,250 2,500 FEET

0 625 METERS 312.5

SHELBURNE DAM

GORHAM DAM

coggin Results of geophysical surveys of riverbed-sediment characteristics in reach AR–9 between the Gorham and Shelburne Dams, Andros Figure 12.

River, Shelburne, New Hampshire.
Characteristics of Riverbed Sediment 23

flow velocity decreases. This sediment is on top of pre-dam 

braided/anabranching channel deposits. The river channel had 

an average depth of 5.7 ft and a maximum of 16.6 ft during the 

time of the survey. Extensive bedrock outcrop observed at the 

downstream base of the Shelburne Dam likely extends to the 

upstream side in the riverbed (fig. 12). 

More than 77 percent of the area of the bed surveyed in 

this reach is estimated to be gravel or finer material (table 4). 

Deeper channels convey most of the flow along the left bank, 

but another deep channel is on the right side (fig. 4). Fine 

sediment with conductive pore water is found at a large area 

in the center of the water body (fig. 12). With the exception 

of the reach behind the Brodie Smith Dam, this reach has 

the second greatest pore-water SC measured, 45.8 mS/m 

(table 1). This reach has a similar average (2.5 mS/m), but 

larger maximum FDEM values (maximum of 12.6 mS/m) than 

the Wheeler Bay reference reach (table 3). This reach had the 

third greatest maximum FDEM conductivity value measured 

in this study (table 3). A sediment sample collected within 5 

meters of an FDEM survey line had a laboratory conductance 

of 3.1 mS/m similar to the estimated FDEM of 2.5 mS/m 

(table 1). FDEM data were affected by power-line noise at the 

downstream end of reach AR–9 in the locations of power lines 

that run parallel to the railroad tracks on the southeast side of 

the Shelburne Reservoir between sediment samples AR–9–7 

and AR–9–5 and were removed (fig. 12). 

At the Gorham Dam, at the upstream end of this reach, 

the river intersects an area of intrusions of two-mica granite 

(Devonian) into the older metasedimentary rocks of the 

Littleton (Early Devonian) and Madrid Formations (Upper 

Silurian (?)) (Billings and Billings, 1975; Lyons and others, 

1997). The contacts are typically approximate because of 

the thick glacial drift (Billings and Billings, 1975) and can 

be found throughout most of this reach. Abundant magnetic 

susceptibility anomalies (not shown) in this reach may be 

because of this contact, representing a change in minerals 

in the bedrock, and the presence of bedrock near or at the 

streambed surface. 

Reach AR–10, Downstream from the Shelburne 

Dam to the New Hampshire–Maine Border

Reach AR–10, in Shelburne, N.H., begins approximately 

11 mi downstream from the former Chlor-Alkali Facility 

Superfund Site at the Shelburne Dam and extends to the 

New Hampshire–Maine State border. The Androscoggin 

River generally flows to the east and southeast through this 

reach. The valley and flood plains widen in this reach and are 

forested and agricultural (fig. 13). The first mile downstream 

from the Shelburne Dam has riffle and rapids grading into fast 

moving water. 

Reach AR–10 is more than 6 mi long, the longest 

reach surveyed in the study and is braided and meandering 

with some anabranching. This reach varies between 150 

and 400 ft wide. The channel had an average depth of 2.5 ft 

and a maximum depth of about 14 ft during the time of the 

survey. There are many areas for fine sediments to accumulate 

because of lower stream flow velocity and channel gradient. 

The river bed is made up of boulders and bedrock within a 

mile downstream from the Shelburne Dam and grades into 

gravel silt and sand further downstream (fig. 13). Glacial 

fluvial gravels, sands, silts and clay can be seen in riverbank 

cuts, outcrops of bedrock are seen on the right bank at the 

downstream end of the reach.

Interpretation and extrapolation of GPR results indicate 

that bed material is estimated to be gravel or finer for 

86 percent of the area surveyed (table 4). This reach has 

smaller FDEM values than the Wheeler Bay reference reach 

with an average of 1.7, a minimum of 0.9 and a maximum of 

3.5 mS/m (table 3). Glacial fluvial silts and clays, observed 

in cut banks, may elevate the minimum estimated FDEM 

conductivity. The estimated FDEM conductivity in this reach 

has the smallest maximum in the study area and the second 

smallest average (table 3) indicating lower SC pore water in 

the bed sediments. FDEM data were affected by power-line 

noise approximately 1.5 mi downstream and interference from 

a metal bridge at approximately 3 mi downstream and the 

data were removed. FDEM data removal is illustrated by the 

absence of the black FDEM survey track in the location of the 

grey GPR survey track (fig. 13).
24 Geophysical Bed Sediment Characterization of the Androscoggin River, New Hampshire, August 2009 Estimated elecromagnetic conductivity, in millisiemens per meter 15

0Estimated bed material (ground-penetratingradar)Cobble, boulder, or bedrockSilt, sand, or gravelElectromagetic survey trackGround-penetrating radar survey track

EXPLANATION

N

Base modified from United States Department of AgricultureNational Agriculture Imagery Program, 2008Digital orthophoto quarter-quadrangleUniversal Transverse Mercator projection, zone 19North American Datum of 1983

SHELBURNE DAM

NEW HAMPSHIRE–MAINE

STATE BORDER

0 1,250 2,500 3,750 5,000 FEET

0 625 1,250 METERS

, Shelburne, New Hampshire , Androscoggin River

Results of geophysical surveys of riverbed-sediment characteristics in reach AR–10 downstream from the Shelburne Dam to the New Figure 13.

Hampshire–Maine State border
Summary and Conclusions 25

Summary and Conclusions

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

with Region I of the U.S. Environmental Protection Agency 

(USEPA), used geophysical methods to survey riverbed 

sediments in the Androscoggin River downstream from 

the former Chlor-Alkali Facility Superfund Site in Berlin, 

New Hampshire. Results were processed and presented in 

tables, maps, and cross sections to assess sediment electrical 

conductivity, pore-water specific conductance (SC), and 

potential contaminant distribution in riverbed sediments. 

The site was listed on the USEPA National Priorities List in 

2005 and has been the subject of continuing investigations 

to determine the nature and extent of dissolved phase and 

adsorbed mercury and methyl mercury contamination. 

The river reaches surveyed in this study ranged from 

pooled water conditions to fast moving water flow conditions 

that varied with channel geometry, dam operation, and runoff. 

Results of ground-penetrating radar (GPR) and multifrequency 

electromagnetic (FDEM) surveys were used to estimate the 

extent and nature of riverbed sediments. In general, wider, 

less steep gradient reaches have more fine sediment, and 

were measured with additional surveys in this study, whereas 

narrow steep gradient reaches had less sediment. SC measured 

during surface-water, pore-water, and sediment sampling 

(with subsequent grain size analysis), collected as part of 

a parallel USGS investigation, were used in processing 

and interpretation of surface geophysical surveys. The 

electrical resistivity of sediment samples was measured in the 

laboratory with pore water intact for comparison with FDEM 

survey results.

Geophysical surveys of the reference reach, Wheeler 

Bay, (upstream end of reach AR–2), were completed to assess 

equipment responses in an environment unaffected by site 

related contaminants. The reference reach, Wheeler Bay, had 

the largest percentage (97) of fine-grained (gravel and finer) 

sediment covered riverbed area. The electromagnetic (FDEM) 

values of this reach were an average of 2.6, a minimum of 

0.1, and a maximum of 9.4 millisiemens per meter (mS/m) 

(table 3).

Reaches AR–3, AR–4, and AR–5 had estimated FDEM 

conductivities elevated above the reference reach despite the 

lower percent of sediment covered riverbed areas (11, 25, and 

61 percent, respectively). Reach AR–3, between the former 

Chlor-Alkali Facility and the Riverside Dam, had small areas 

of fine sediment, found on the upstream left bank and the 

downstream right bank, with an elevated FDEM conductivity 

(31.4 mS/m maximum). The larger FDEM conductivity 

(fig. 6) was likely because of elevated riverbed pore-water SC. 

Reaches AR–4 and AR–5 were located downstream from steep 

gradient (100 ft /mi) bedrock gorges that convey high flows 

from the reach adjacent to the site to pooled areas behind 

dams. Reach AR–4, upstream from the Brodie Smith Dam had 

the largest pore-water SC, FDEM and lab-measured sediment 

conductivity values measured in the study. Pore-water SC 

measured in this reach was 279 and 324 mS/m (table 1) at 

sediment sample locations AR–4_1 and AR–4_2 (fig. 7) on 

a sandbar near the left bank. These sediment samples had 

laboratory-measured sediment conductivity values of 67.4 and 

76.8 mS/m, similar to nearby estimated FDEM values of 73.2 

and 72.8 mS/m (table 1). The largest conductivity measured 

with FDEM in reach AR–5 was 10.2 mS/m on the downstream 

left side (fig. 8). This reach is wide and likely contains 

additional conductive sediment that could not be measured by 

geophysical surveys in this study, downstream from the Cross 

Power Dam warning barrels.

Reach AR–6 and AR–7 had FDEM values similar to 

the reference reach, despite having a smaller percentage of 

riverbed areas covered with sediment (51 and 42 percent). 

Elevated FDEM values, on the downstream left bank of reach 

AR–6 and the downstream right bank of reach AR–7 (figs. 9 

and 10), indicates that fine-grained and conductive sediment 

was deposited in these areas. Sediment downstream from the 

dam warning barrels in these reaches was not surveyed in 

this study.

Reach AR–8 and AR–10 had smaller FDEM values than 

the reference reach. Reach AR–8 had a lower percentage 

(20 percent) of riverbed area covered with sediment than all of 

the other reaches, with the exception of AR–3. Reach AR–10 

had the largest area and a larger percentage (86 percent) of 

riverbed area covered with sediment than all of the other 

reaches in the study with the exception of the reference reach 

AR–2 (table 4).

Reach AR–9 between the Gorham and Shelburne Dams 

contains the largest body of pooled water in the study area, 

the Shelburne Reservoir. The first one-half of the reach has 

cobble, boulder and bedrock bed material; although more than 

77 percent of the area of the riverbed surveyed in reach AR–9 

is estimated to be covered by gravel or finer material (table 

4). The sediment in reach AR–9 had a maximum estimated 

FDEM conductivity of 12.6 mS/m (table 3), greater than 

all of the other reaches except for AR–3 and AR–4 (nearby 

and within a mile downstream from the former Chlor-Alkali 

Facility Superfund Site). In addition to large FDEM values, 

this reach had the second greatest pore-water SC measured, 

45.8 mS/m (table 1).

Through combining results and analysis from groundpenetrating radar (GPR) and FDEM surveys, with sediment 

pore water and laboratory measured conductivity, detailed 

riverbed-sediment characterizations were made. Results from 

GPR surveys were used to image and measure the depth to 

the riverbed, depth to buried riverbeds, riverbed thickness and 

to interpret material-type variations in terms of relative grain 

size. Fifty two percent of the riverbed in the study area was 

covered with gravel and finer sediments. GPR surveys are 

affected by contrasts in the electrical properties of water and 

sediment. The electrically resistive river water and sediment 

in this study area were conducive to the penetration of the 

GPR and FDEM signals and allowed for effective sediment 

characterization by geophysical methods. 
26 Geophysical Bed Sediment Characterization of the Androscoggin River, New Hampshire, August 2009

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Degnan and others—Geophysical Bed Sediment Characterization of the Androscoggin River, New Hampshire, August 2009—Scientific Investigations Report 2011–5158