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Scientific Investigations Report 2011–5158

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

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

1U.S. Environmental Protection Agency, Region 1

Thumbnail of and link to report PDF (5.75 MB)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 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. The electrically resistive river water and sediment in this study area were conducive to the penetration of the ground-penetrating radar and step-frequency 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 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 with similar mineralogy 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.

Revised June 19, 2012

First posted October 21, 2011

For additional information contact:
Director
U.S. Geological Survey
New Hampshire-Vermont Water Science Center
361 Commerce Way
Pembroke, NH 03275
(603)226-7800

http://nh.water.usgs.gov

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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.



Contents

Abstract

Introduction

Site Background

Previous Investigations

Purpose and Scope

Study Area Description

Geologic Setting

River Flow and Channel Features

Methods of Data Collection and Analysis

Ground-Penetrating Radar

Step-Frequency Electromagnetic Induction

Sediment Sampling and Direct-Current Resistivity

Interpretation of Riverbed Conductivity

Estimation of Riverbed Properties by Reach

Characteristics of Riverbed Sediment

Wheeler Bay, Reference Reach

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

Reach AR–4, Upstream from the Smith Dam

Reach AR–5, Upstream from the Cross Power Dam

Reach AR–6, Between Cross Power and Cascade Dams

Reach AR–7, Upstream from the Brown Dam

Reach AR–8, Between the Brown and Gorham Dams

Reach AR–9, Between the Gorham and Shelburne Dams

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

Summary and Conclusions

References Cited


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