Prepared in cooperation with the 

New Hampshire Department of Environmental Services and in collaboration with the 

U.S. Environmental Protection Agency

Hydrogeologic Framework, Arsenic Distribution, 

and Groundwater Geochemistry of the GlacialSediment Aquifer at the Auburn Road Landfill 

Superfund Site, Londonderry, New Hampshire

Scientific Investigations Report 2013–5123

U.S. Department of the Interior

U.S. Geological Survey
Front-cover art: Three-dimensional visualization of electrical geophysical results (upper left) with view toward the southeast, and 

conceptual model of hydrogeologic framework (lower right). 

Back-cover art: Photographs of a direct-current resistivity survey at Whispering Pines Pond. 

From left to right, Andrew Teeple, Byron Mah (1964–2012), and Roberto Cruz.
Hydrogeologic Framework, Arsenic 

Distribution, and Groundwater 

Geochemistry of the Glacial-Sediment 

Aquifer at the Auburn Road Landfill 

Superfund Site, Londonderry, 

New Hampshire

By James R. Degnan and Philip T. Harte

Prepared in cooperation with the

New Hampshire Department of Environmental Services and in collaboration with 

the U.S. Environmental Protection Agency

Scientific Investigations Report 2013–5123

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

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as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Degnan, J.R., and Harte, P.T., 2013, Hydrogeologic framework, arsenic distribution, and groundwater geochemistry of 

the glacial-sediment aquifer at the Auburn Road landfill superfund site, Londonderry, New Hampshire: U.S. Geological 

Survey Scientific Investigations Report 2013–5123, 58 p., http://pubs.usgs.gov/sir/2013/5123/.
iii

Acknowledgments

Guidance during development of this project and background information from Byron Mah, 

(1964–2012) of the U.S. Environmental Protection Agency (USEPA), was essential for project 

planning and progress. He visited the field site to help with reconnaissance and to oversee 

geophysical surveys. Byron’s dedication to the site was shown through his questions, site visits, 

relationships with land owners, facilitation of site reuse, and enthusiasm for project meetings. 

He brought people together to develop a more thorough understanding of the processes at 

work at the site to produce a clean and fair outcome. Jean Choi, also of the USEPA, provided 

information and assisted with geophysical surveys and piezometer-installation field work at 

the site. Constructive reviews from Marcel Belaval, Mike Jazinski, Daryl Luce, and Richard Hull 

of the USEPA were helpful in creating a more relevant and useful product. Sean Combs and 

Andrew Fuller from Weston Solutions provided data that helped with this study. Permission 

and logistical support from landowners who allowed access to their land, made data collection 

possible. The authors would like to thank all who have assisted with and supported this project. 

Site reconnaissance and project-planning help from Tom Andrews and Kenneth Richards, 

New Hampshire Department of Environmental Services, helped to keep the study moving 

forward. Help with water-quality and geophysical data collection and formatting by volunteer 

Blaine Ayotte is appreciated.

Site reconnaissance and project-planning help were provided by Joe Ayotte and Tom Mack 

of the U.S. Geological Survey (USGS). Kelsey Ann Regan of the USGS helped in the field and 

with data mining. Field-data collection was possible with assistance from Roberto Cruz, Jason 

Sorenson, and Kelly McMahon of the USGS. Equipment, help in the field, and technical advice 

provided by Andrew Teeple, Alton Anderson, and Eric White of the USGS made high-quality data 

collection possible. Jason Payne of the USGS helped with electromagnetic-data processing. 

Critical report reviews by Richard Moore, Charles Walker, and Andrew Teeple of the USGS are 

greatly appreciated.
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v

Contents

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

Introduction.....................................................................................................................................................2

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

Hydrogeologic Setting .........................................................................................................................2

Arsenic in Groundwater and Contaminants at Landfills ................................................................4

Previous Investigations........................................................................................................................6

Purpose and Scope ..............................................................................................................................8

Methods of Data Collection and Analysis .................................................................................................8

Geophysics and Hydrogeologic Framework ....................................................................................8

Geochemical-Data Evaluation Methods...........................................................................................9

Hydrogeologic Framework and Groundwater Geochemistry ..............................................................10

Hydrogeologic Framework................................................................................................................10

Geochemical Characterization.........................................................................................................15

Persistence of Arsenic.......................................................................................................................18

Data Gaps.............................................................................................................................................24

Aquifer and Landfill-Matrix Solids ..........................................................................................24

Indicators of Leachate and Groundwater Redox .................................................................24

Surface-Water/Groundwater Interactions............................................................................25

Hydrogeologic Framework of the Bedrock Aquifer.............................................................25

Hydrogeologic Framework of the Glacial-Sediment Aquifer .............................................25

Groundwater Flow and Three-Dimensional Geochemical Model.....................................26

Summary and Conclusions.........................................................................................................................26

References Cited..........................................................................................................................................27

Appendix 1. Summary of Results of Geophysical-Data Processing and Borehole 

Geophysical Log Analysis.............................................................................................................35

Appendix 2. Three-Dimensional Diagram Showing Locations of Wells, Bedrock-Surface 

Altitudes from Seismic Refraction Surveys, and Results from Electromagnetic and 

Direct-Current Resistivity Geophysical Surveys.......................................................................41

Appendix 3. Preliminary Bedrock-Surface Map .................................................................................43

Appendix 4. Geochemical Data Used for Redox-Zone Classification, Spring 2008.......................45

Appendix 5. Preliminary Spatial Distribution of Redox Zones, Water Table, and Flow 

Directions in the Glacial-Sediment Aquifer, Spring 2008.........................................................47

Appendix 6. Reactive-Transport Model ................................................................................................51

Appendix 7. Piezometer Installation and Results of Analysis of Groundwater Samples, 2012 ...55

Figures

1. Map showing the location of surface features, wells, and geophysical-survey lines 

discussed in this report, Auburn Road Landfill Superfund Site, Londonderry, 

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

2. Diagram showing the conceptual model of the hydrogeology and groundwater flow 

at the Auburn Road Landfill Superfund Site, Londonderry, New Hampshire .....................5
vi

3. Diagram showing A, aerial view of the location of line 10 of the direct-current 

resistivity survey and wells that are projected on the cross section and B, cross 

section showing direct-current resistivity results, July 2011, the altitude of the 

bedrock surface interpreted from these results, and lithologic logs in wells along 

line 10 in the former gravel pit, Auburn Road Landfill Superfund Site, Londonderry, 

New Hampshire ..........................................................................................................................12

4. Diagram showing A, aerial view of the location of line 12 of the direct-current 

resistivity survey and wells that are projected on the cross section and B, cross 

section showing direct-current resistivity results, July 2011, the altitude of the 

bedrock surface interpreted from these results, and lithologic logs in wells along 

line 12, Auburn Road Landfill Superfund Site, Londonderry, New Hampshire.................13

5. Diagram showing A, aerial view of the location of line 2 of the direct-current 

resistivity survey and wells that are projected on the cross section and B, cross 

section showing direct-current resistivity results, February 2011, the altitude of the 

bedrock surface interpreted from these results, and lithologic logs in wells along 

line 2 beneath Whispering Pines Pond, Auburn Road Landfill Superfund Site, 

Londonderry, New Hampshire..................................................................................................14

6. Graphs showing the specific conductance of groundwater as a function of the 

concentrations of five constituents in water from wells in bedrock and overburden, 

spring 2008, Auburn Road Landfill Superfund Site, Londonderry, New Hampshire ........16

7. Graphs showing the concentration of arsenic as a function of landfill-leachate 

indicators in water from wells in bedrock and overburden, spring 2008, Auburn Road 

Landfill Superfund Site, Londonderry, New Hampshire.......................................................17

8. Graphs showing the concentration of arsenic as a function of redox indicators in 

water from wells in bedrock and overburden, spring 2008, Auburn Road Landfill 

Superfund Site, Londonderry, New Hampshire.....................................................................18

9. Diagram showing A, aerial view of the location of the cross section and wells that 

are projected on the cross section and B, cross section showing redox conditions 

and concentrations of arsenic in groundwater along a flow path from the solid-waste 

landfill to Whispering Pines Pond, spring 2008, Auburn Road Landfill Superfund Site, 

Londonderry, New Hampshire..................................................................................................19

10. Diagram showing A, aerial view of the location of the cross section and wells that are 

projected on the cross section and B, cross section showing redox conditions and 

concentrations of arsenic in groundwater along a flow path from the towndump landfill to Cohas Brook, spring 2008, Auburn Road Landfill Superfund Site, 

Londonderry, New Hampshire..................................................................................................20

11. Graphs showing concentrations of arsenic in groundwater by year and well A,

between landfills, B, in the gravel pit, and C, beneath Whispering Pines Pond, 

Auburn Road Landfill Superfund Site, Londonderry, New Hampshire ..............................21

12. Bar graphs showing results of sensitivity analyses of the A, relative effect of 

parameter adjustment on the dimensionless arsenic concentration, and B, the 

percent difference between the simulated times to achieve the standard for 

dissolved arsenic at Whispering Pines Pond, Auburn Road Landfill Superfund Site, 

Londonderry, New Hampshire..................................................................................................23

Table

1. Hydrogeologic and groundwater-quality data from previous studies, Auburn Road 

Landfill Superfund Site, Londonderry, New Hampshire.........................................................7
vii

Conversion Factors, Datum, and Abbreviations

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.004047 square kilometer (km2)

Flow rate

foot per day (ft/d) 0.3048 meter per day (m/d)

Transmissivity

foot squared per day (ft2/d) 0.09290 meter squared per day (m2/d)

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD of 1929).

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

25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Electrical resistivity is measured in ohm meters.

Abbreviations

DC direct current

DO dissolved oxygen

EM electromagnetic-induction

FDEM frequency-domain electromagnetic

GIS geographic information system

GPR ground-penetrating radar

GPS global positioning system

MCL maximum contaminant level

NHDES New Hampshire Department of Environmental Services

ROD record of decision

SC specific conductance

TOC total organic carbon

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VOC volatile organic compound
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Hydrogeologic Framework, Arsenic Distribution, and 

Groundwater Geochemistry of the Glacial-Sediment 

Aquifer at the Auburn Road Landfill Superfund Site, 

Londonderry, New Hampshire

By James R. Degnan and Philip T. Harte

Abstract

Leachate continues to be generated from landfills at the 

Auburn Road Landfill Superfund Site in Londonderry, New 

Hampshire. Impermeable caps on the three landfills at the 

site inhibit direct infiltration of precipitation; however, high 

water-table conditions allow groundwater to interact with 

landfill materials from below, creating leachate and ultimately 

reducing conditions in downgradient groundwater. Reducing 

conditions can facilitate arsenic transport by allowing it to stay 

in solution or by liberating arsenic adsorbed to surfaces and 

from geologic sources, such as glacial sediments and bedrock. 

The site occupies a 180-acre parcel of land containing 

streams, ponds, wetlands, and former gravel pits located in 

glacial sediment. Four areas, totaling 14 acres, including three 

landfills and one septage lagoon, were used for waste disposal. 

The site was closed in 1980 after volatile organic compounds 

associated with industrial waste dumping were detected. The 

site was added to the U.S. Environmental Protection Agency 

National Priority List in 1982, and the landfills were capped 

in 1996. Although volatile organic compound concentrations 

in groundwater have declined substantially, some measurable concentrations remain. Temporally variable and persistent elevated arsenic concentrations have been measured in 

groundwater affected by the landfill leachate. 

Microbial consumption of carbon found in leachate is a 

driver of reducing conditions that liberate arsenic at the site. 

In addition to sources of carbon in landfill leachate, wetland 

areas throughout the site also could contribute carbon to 

groundwater, but it is currently unknown if any of the wetland 

areas have downward or reversing gradients that could allow 

the infiltration of surface water to groundwater. Red-stained 

sediments and water indicate iron-rich groundwater discharge 

to surface water and are also associated with elevated concentrations of arsenic in sediment and groundwater. Ironrich groundwater seeps have been observed in the wetland, 

streams, and pond downgradient of the landfills. Piezometers 

were installed in some of these locations to confirm groundwater discharge, measure vertical-flow gradients, and to provide a 

way to sample the discharging groundwater.

Understanding the movement of leachate in groundwater is complicated by the presence of preferential flow paths 

through aquifer materials with differing hydraulic properties; 

these preferential flow paths can affect rates of recharge, geochemical conditions, and contaminant fluxes. In areas adjacent 

to the three capped landfills, infiltration of precipitation containing oxygenated water through permeable deltaic sediments 

in the former gravel pit area causes increases in dissolved 

oxygen concentrations and decreases in arsenic concentrations. Layered deltaic sediments produce anisotropic hydraulic 

characteristics and zones of high hydraulic conductivity. The 

glacial-sediment aquifer also includes glaciolacustrine sediments that have low permeability and limit infiltration.

Discharge of leachate-affected groundwater may be 

limited in areas of organic muck on the bottom of Whispering Pines Pond because the muck may act as a semiconfining 

layer. Geophysical survey results were used to identify several 

areas with continuous beds of muck and an underlying highresistivity layer on top of a layer of low resistivity that may 

represent leachate-affected groundwater. The high-resistivity 

layer is likely groundwater associated with oxygenated 

recharge, which would cause arsenic to adsorb onto aquifer 

sediments and reduce concentrations of dissolved arsenic 

in groundwater.

Surface and borehole geophysical data collected in 

2011 were used to identify potentially high-permeability or 

contaminated zones in the aquifer (preferential flowpaths) as 

well as low-permeability zones that may promote contamination through back diffusion. Some groundwater in parts of 

the glacial-sediment aquifer where the leachate plumes were 

present had low electrical resistivity, low dissolved oxygen, 

and high concentrations of arsenic. Low-resistivity zones in 

the underlying bedrock were associated with fractures that 

also may contain leachate. Although surveying the fractured 

bedrock was not a specific objective of this study, the results 
2 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

suggest that such a survey would help to determine if leachate 

and associated concentrations of arsenic are migrating downward into the fractured-bedrock-aquifer system.

An uncalibrated, one-dimensional, reactive-transport 

model was used to assess several conditions that affect arsenic 

mobility. The results indicate that reductive dissolution and 

desorption from glacial sediments control dissolved arsenic 

concentrations. Parameter sensitivity analysis was used to 

identify key data that are needed in order to accurately assess 

the time required for arsenic concentrations to fall to levels 

below the maximum contaminant level at the site. Quantifying 

this time will require accurate characterization of carbon, 

sediment-surface sorption sites, and groundwater fluxes at 

the site.

Introduction

The Auburn Road Landfill Superfund Site in 

Londonderry, New Hampshire is a U.S. Environmental 

Protection Agency (USEPA) National Priority List site with 

long-term elevated concentrations of arsenic in groundwater. 

Arsenic concentrations in many wells on and downgradient 

of the site are above the USEPA maximum contaminant level 

(MCL) for drinking water (10 micrograms per liter (μg/L); 

Weston Solutions, 2008). The New Hampshire Department 

of Environmental Services (NHDES) and the USEPA are 

concerned with understanding the factors controlling arsenic 

mobility, transport, and persistence at the Auburn Road 

Landfill Superfund Site and surrounding area.

Site Background

The Auburn Road Landfill Superfund site is situated 

on a 180-acre parcel of land, of which 4 locations totaling 

14 acres had been used for disposal (fig. 1). The State of 

New Hampshire discovered hazardous waste at the site in 

1979, and the landfills were closed in 1980. In 1982, the site 

was added to the USEPA National Priority List. Twenty-two 

hundred and sixteen waste drums were removed from the 

landfills from 1986 to 1988. Within a 1-mile (mi) radius of 

the landfills, 570 homes serviced by private bedrock wells 

and a private-community supply well (finished in the glacialsediment aquifer) were transferred to the local public-water 

supply in 1987 because of the threat of contamination by 

volatile organic compounds (VOCs). The original record 

of decision (ROD) called for landfill capping and pumping 

and treating of groundwater, but because of the decline in 

VOC concentrations, a revised ROD called for the initiation 

of monitored natural attenuation as an interim remedy (U.S. 

Environmental Protection Agency, 1997).

Wastes were initially distributed in four locations 

throughout the site, but were consolidated to three during 

site remediation efforts. The three landfill sites that remain 

include the solid-waste landfill, the tire-pile landfill, and 

the town-dump landfill (fig. 1). The town-dump landfill in 

the northwest part of the site is on the southern shore of 

Whispering Pines Pond (fig. 1). Wetlands were constructed as 

a replacement for those disturbed during landfill operations 

and capping (U.S. Environmental Protection Agency, 2002) 

and are upgradient and northwest of the tire-pile landfill. 

Waste from a former septage lagoon was consolidated into the 

solid-waste landfill to the southeast. Consolidation of wastes, 

modification of drainages, and construction of impermeable 

caps were completed in 1996. 

Despite the consolidation of the septage lagoon into the 

solid-waste landfill, the capping of the three landfills, and the 

installation of drainage to reduce the generation of landfill 

leachate, several contaminants persist in the groundwater. The 

caps are intended to reduce precipitation infiltration into landfill waste, but groundwater continues to flow under the landfills and interacts with wastes in the landfills. Although concentrations of VOCs have declined, concentrations of arsenic 

remain high. For example, elevated concentrations of arsenic 

(greater than 10 μg/L) in groundwater have been reported in 

samples from monitoring wells in the glacial-sediment aquifer 

downgradient of the landfills (U.S. Environmental Protection 

Agency, 2007 and 2012).

Hydrogeologic Setting

The site is located within a complex hydrogeologic setting that is characterized by glaciofluvial and glaciolacustrine 

sediments associated Pleistocene glacial lake environments, 

the remnants of which are common features in New England. 

Gephart (1985) mapped surficial geologic units for the site 

and surmised that Pleistocene sediments were associated 

with three glacial lakes (Wilson, Derry, and Cohas Brook). 

When the retreating ice margin stood at or near what is now 

the tire-pile landfill and then later at Whispering Pines Pond, 

sand and gravel were deposited and graded first into Glacial 

Lake Derry and then Glacial Lake Wilson, which drained to 

the south and southwest of the site (Gephart, 1985). Based on 

topographic contours, glacial-lake levels for these two lakes 

were about 50 and 30 feet higher in elevation higher than the 

final glacial lake, Glacial Lake Cohas Brook, which drained 

to the west into Glacial Lake Merrimack. The well-sorted 

layering of bedding in the glaciofluvial sediments could cause 

a directional bias in hydraulic conductivity (anisotropy). 

Distal portions of the deltas (away from meltwater source) in 

the glacial-sediment aquifer would have had more uniformly 

dipping foreset beds. Nearer the former ice margins, however, 

preferential flow paths are highly variable and are controlled 

by the orientation of meltwater channels feeding the delta, 

which would cause spatial variability in hydraulic conductivity 

(heterogeneity).

Glaciofluvial sediment at the site is at least twice as permeable as the till (Sevee and Maher Engineering, 1992). The 

glacial sediments at the site are anisotropic because of the layering associated with their deposition, and heterogeneous, with 
Introduction 3

5

11

15

20

2

1

12

3

10

4.2

MW-3 0 3B

Ground Penetrating Radar

Direct Current Resistivity

Whispering

Pines Pond

Town-dump 

landfill

Tire-pile landfill

Solid-waste landfill

Former 

septage 

lagoon

Former 

gravel pit

Former 

gravel pit

Former 

gravel pit

Cohas Brook

Well, and identifier

Auburn Road

MW-109A

MW-109B

CB-1

MW-213

MW-108B

MW-108A

MW-108AN NUS-3 US-2-2

MW-106B

MW-106A

GZ10-3R MW-201

MW-202

GZ-9-4R

NUS-1-2

MW-204 MW-B

NUS-9-1

WPP-3

MW-303A

WPP-2

WPP-1 MW-102B

MW-102A

MW-304A

MW-304B

MW-104A MW-104B

C-2 GZ-6-3R

GZ-6-2R

MW-302A MW-302B

MW-301A

GZ-1-2R

GZ-1-3R

MW-212B

MW-212A

C-1

PZ-218

NUS-12

MW-1B MW-1B MW-1A

C-3

EXPLANATION

Flint Hill fault

71°21'15" 71°21'

42°56'

42°56'15"

0 500 1,000 FEET

0 100 200 300 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

Study site

MAINE

MASSACHUSETTS

VERMONT

NEW HAMPSHIRE

Londonderry

Concord

Coos

Grafton

Carroll

Merrimack

Cheshire

Hillsborough

Sullivan

Rockingham

Belknap

Strafford

Lake

Winnipesaukee

Merrimack 

River

Figure 1. The location of surface features, wells, and geophysical-survey lines discussed in this report, Auburn Road Landfill 

Superfund Site, Londonderry, New Hampshire.
4 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

hydraulic conductivities as determined from aquifer-tests ranging from 0.074 to 140 feet per day (ft/d), and a mean value 

of 18 ft/d (Sevee and Maher Engineering, 1992). Stekl and 

Flanagan (1992) mapped two high-transmissivity zones (up 

to 4,000 square feet per day (ft2/d)) on and near the site—one 

at the northern edge of the site extending under Cohas Brook 

and the other just downstream beneath and to the northwest of 

Cohas Brook.

Glaciofluvial and glaciolacustrine sediments overlie 

thin discontinuous till and bedrock. Complex bedrock surface topography (fig. 2) affects groundwater flow within the 

glacial-sediment aquifer. A bedrock-surface map produced by 

Sevee and Maher Engineering (1992) shows a north-northeast 

trending trough that extends from up gradient (south) of the 

tire-pile landfill to the east side of Whispering Pines Pond. 

This trough may coincide with the extension of one of several 

lineaments mapped by Ferguson and others (1997) at and 

around the site. Lineaments—linear features that may be 

related to fracture zones—indicate location of potential bedrock fractures that could transmit groundwater at and beyond 

the site. Other bedrock-surface troughs include a northwesttrending trough starting beneath Cohas Brook and two 

northeast-trending troughs downgradient of the former septage 

lagoons and the tire-pile landfill.

The bedrock under the site is mapped as the Berwick 

Formation, unnamed member, and contains more calc-silicate 

minerals (15 percent) than other members of the same formation (5 percent) (Lyons and others, 1997). This formation has 

been associated with regionally high concentrations of naturally occurring arsenic in groundwater that can affect drinking 

water wells drilled into the formation (Montgomery, 2003; 

Ayotte, 2003). 

Bedrock features that define the bedrock-surface topography affect the geometry of the unconsolidated sediments. The 

Flint Hill Fault has an undifferentiated direction of motion, 

though an upper fault block is identified to the northwest 

(Lyons and others, 1997). This fault trends to the northnortheast through Whispering Pines Pond (fig. 1) and is 

associated with silicified zones and steeply inclined bedrock topography (Sriramadas, 1966, Freedman, 1950). 

Though this feature is mostly buried under glacial sediment 

at the site, it is an important control on the geometry of the 

glacial-sediment aquifer. 

Bedrock and fracture maps to the south of the site further 

define variations in the Berwick Formation, which, if present 

at the site, may affect concentrations of arsenic in the bedrock 

aquifer (Walsh and Clark, 1999). Discrete fracture zones may 

transmit groundwater and contaminants away from the site. 

Bedrock fracture zones may play an important role in contaminant transport and the transport of uncontaminated groundwater into the capped landfills. The hydraulic conductivity 

of the bedrock aquifer, as defined by hydraulic testing of part 

of the present monitoring well network is low— 4.9×10-2 to 

7.9×10-1 ft/d (Sevee and Maher Engineering, 1992). Similarly, 

low bedrock-aquifer yields indicating low transmissivity in 

the Berwick Formation were identified in a regional yield 

model (Moore and others, 2002), although fracture-correlated 

lineaments were shown to be associated with some permeable 

zones in the bedrock.

Groundwater and surface water generally flow to the 

north and north-northwest from the site towards Cohas Brook 

(Sevee and Maher Engineering, 1993b), but the direction of 

shallow groundwater-flow paths may deviate considerably 

from this pattern—at times flowing south in response to local 

topography before reversing and flowing north deeper in the 

flow system. Cohas Brook, to the north of the site (fig. 1), 

flows west towards the Merrimack River. Groundwater-level 

data (Weston Solutions, 2009) show upward and downward 

gradients in adjacent wells completed in the bedrock and 

overburden, and data from some of the well pairs have gradient reversals. Immediately south of the solid-waste landfill, 

the topographic divide serves as a groundwater-flow divide. It 

is unclear whether flow from the replacement wetlands may 

contribute to higher heads or flow through the waste-site areas.

Arsenic in Groundwater and Contaminants at 

Landfills

Arsenic is known to occur naturally in the bedrock, 

sediment, and groundwater in the region where the landfill is 

located (Ayotte and others, 2003; Moore, 2004, Montgomery 

and others, 2003; Robinson and Ayotte, 2006). Arsenic 

concentrations in water samples collected in 30 percent of 

bedrock wells installed in the Berwick Formation, unnamed 

member (Lyons and others, 1997), exceeded the MCL 

(Montgomery and others, 2003). In the glacial-sediment 

aquifer, however, Ayotte and others (2003) found that 

arsenic concentrations in samples from only 3 percent of 

wells exceeded the MCL of 10 μg/L—largely because the 

geochemical conditions that inhibit arsenic solubility.

Previous studies indicate that arsenic solubility and 

transport can be affected by landfill leachate, groundwater 

hydraulics, and aquifer and groundwater geochemistry. 

Understanding how these factors operate at this site is 

necessary to evaluate the potential effects of the site on 

the environment and to provide for the design of efficient 

remediation strategies. Potential sources of arsenic at this site 

include the landfill materials (although evidence that arsenic 

compounds are in the waste is not currently available) and the 

geologic materials that form the bedrock and glacial-sediment 

aquifers. Arsenic from anthropogenic and geologic sources 

can be mobilized downgradient of waste sites by the mixing 

of landfill leachate with natural groundwater. This process can 

create reducing conditions that can enhance (1) the dissolution 

of oxides containing solid-phase arsenic or (2) desorption of 

arsenic from metal hydroxides (Whitlock and Kelly, 2010; 

Hounslow, 1980). 

Arsenic mobilization from aquifer sediments as a result 

of interaction with landfill leachate has been documented in 

Maine and New Hampshire (Stollenwerk and Colman, 2004; 

deLemos and others, 2006), and has been shown to be related 
Introduction 

5

Solid-waste landfill

Tire-pile landfill

Increased infiltration 

Removed by mining

Glacial Lake

Wilson deposits Elevation of glacial Lake Cohas deposits

WhisperingPines Pond

Cohas

Brook

SOUTH

CONCEPTUAL SCHEMATIC ONLY, NOT TO SCALE

NORTH

Recharge

EXPLANATION

High hydraulic conductivity units Low hydraulic conductivity units

Ice-channel filling, esker, coarse grainedKame delta and outwash deposits, medium to coarse grainedMined kame delta deposits, medium to coarse grainedSurface waterLandfill with impermeable capInterpreted groundwater-flow direction

Till, poorly sorted, fine grainedOrganic muck, fine grainedBedrock

Lake-bottom or bottomset kame delta deposits, fine grainedDiscontinuous horizontal bedsDiscontinuous foreset bedsFractures

Figure 2. The conceptual model of the hydrogeology and groundwater flow at the Auburn Road Landfill Superfund Site, Londonderry, New Hampshire.
6 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

to long-term persistence of elevated concentrations of arsenic. 

For example, reducing conditions created by landfill leachate 

enhance mobilization of arsenic and iron from iron hydroxides 

in marine clay at the Coakley Superfund Site, North 

Hampton and Greenland, N.H. (deLemos and others, 2006). 

Concentrations of constituents such as barium, calcium, and 

ammonium-nitrogen can be used in tracing landfill leachate in 

groundwater as it discharges to surface water as seeps (Ford 

and others, 2011).

Previous investigations of landfills in glacial-sediment 

aquifers by Nielsen and others (1995) and Mack (1995) identified complex layered sediments and leachate-degraded water 

quality beneath and downgradient of sites. The Norman Landfill, which overlies unconsolidated sediments near Norman, 

Oklahoma, was found to be sensitive to hydrologic changes, 

where the upper boundary of the leachate plume was most sensitive to recharge events and seasonal water-table fluctuations 

(Cozzarelli and others, 2011). For example, concentrations 

and locations of contaminant discharge to a wetland area at the 

Norman Landfill varied in response to hydrologic conditions 

(Lorah and others, 2009). When the water table was low, the 

plume fringe (where concentrations of contamination were 

low) intersected and discharged to the center of the wetland. 

By contrast, when the water table was high, discharge to the 

wetland was over a broad area, and contaminant concentrations in discharge to the center of the wetland were high. At 

several wetland and groundwater-discharge locations at the 

Auburn Road Landfill Superfund Site, hydrologic conditions 

may affect contaminant transport.

Reactive-transport models for other landfill-affected 

arsenic-contamination sites have been used to estimate the 

time required for arsenic concentrations to decline to levels 

below the MCL. Information from these sites, such as the Saco 

Landfill Superfund Site in Maine (Stollenwerk and Colman, 

2004), has been used to help identify processes that can affect 

arsenic mobility at the Auburn Landfill Superfund Site.

Selection of methods for and the potential success of 

remediation efforts at arsenic-contaminated landfill sites 

depends on the amount of arsenic that needs to be removed 

from the aquifer materials in order to achieve cleanup goals. 

Low-permeability glacial sediments, which are associated with 

back diffusion and slow flushing rates of groundwater along 

with a complex underlying bedrock fracture system, makes 

the selection of remedial methods challenging. Quantifying groundwater flow based on a sound conceptual model is 

helpful in the development of a successful remediation design. 

Laboratory experiments show that chemical treatments with 

oxalic acid will mobilize arsenic at sites with pump-and-treat 

systems in unconsolidated aquifers, such as the Vineland 

Chemical Company site in New Jersey, which can lead to 

more efficient remediation (Wovkulich and others, 2010, 

2012). Geochemical modeling and laboratory experiments 

show that enhanced sulfate reduction could reduce arsenic 

concentrations by immobilizing arsenic within the aquifer at 

the Winthrop Landfill in Maine (Keimowitz and others, 2011).

Previous Investigations

In response to initial reports of hazardous waste at the 

site, remedial site investigations by the NUS Corporation 

were completed to define the preliminary hydrogeology and 

contaminant distribution at the site (U.S. Environmental 

Protection Agency, 1986). Seismic surveys were done in 1985 

and 1991 to identify the water table, glaciofluvial sediment, 

and till and bedrock interfaces. Electromagnetic surveys were 

used to locate the electrically conductive leachate plume 

(Sevee and Maher Engineering, 1992). Groundwater observed 

in wells to be in contact with waste before and after capping 

of the landfill (Sevee and Maher Engineering, 1993a, Weston 

Solutions, 2006) created a nearly constant source of leachate.

A detailed predesign investigation for the remediation 

of groundwater included a hydrogeologic and geochemical 

characterization completed by Sevee and Maher Engineering 

(1992) with glacial sediment and bedrock hydraulicconductivity tests, groundwater and surface-water-level 

monitoring, and groundwater modeling. The investigation 

used microwells to develop a three-dimensional picture of 

geochemical conditions, which served to define the location 

of the arsenic plume. A report on the monitoring results was 

produced as required by the amended ROD to support the 

monitored natural-attenuation remedy (Sevee and Maher 

Engineering, 2000). White and Sevee (1999) found that 

elevated concentrations of arsenic co-occur with elevated 

concentrations of iron, manganese, alkalinity, ammonia, and 

organic carbon at the site. Although arsenic concentrations 

have decreased slightly in water from wells directly 

adjacent to landfills, concentrations have remained elevated 

at downgradient wells. Monitoring of the site by Weston 

Solutions began in 2000 (Weston Solutions, 2001 annually 

through 2010). A summary of site references and available 

data is provided in table 1.

Tests of soil for total arsenic, along with landfill-leachate 

experiments with soils in the area of the Auburn Road Landfill 

Superfund Site, have shown that arsenic occurs in the aquifer 

sediments and can dissolve into groundwater under chemical 

reduction-oxidation (redox) conditions described as “reducing” (Sevee and Maher Engineering, 1993b; HydroQual, 

Inc., 2009). The results indicate that concentrations of arsenic 

above the MCL could be partly derived from natural sources at 

the site, but arsenic could be mobilized by reducing conditions 

associated with the landfill leachate. Arsenic-containing compounds in the waste materials could also be a source of arsenic 

concentrations in groundwater. A wide range of geochemical 

conditions at the site and the presence of both arsenic (III) and 

arsenic (V) indicate multiple processes of mobilization. Redox 

potential is a major factor related to arsenic solubility and 

is partially controlled by the amount of total organic carbon 

(TOC) available in the system. If the amount of landfill leachate (including TOC) is decreasing, or if the redox is affected 

by an increase in the flow rate of oxygenated groundwater 

through the aquifer, dissolved arsenic could adsorb onto sediments, and concentrations in groundwater would decrease.
Introduction 

7

Table 1. Hydrogeologic and groundwater-quality data from previous studies, Auburn Road Landfill Superfund Site, Londonderry, New Hampshire.[Full citations are in the References Cited section of this report; TOC, total organic carbon]

Year Report type Source Arsenic speciation TOC Ferrous iron Iron speciation Ions Methane Sulfate Sulfide Metals Selected isotopes

1992 Predesign investigation Sevee and Maher x x x1993 Predesign investigation Sevee and Maher x x x2006 Annual report Weston x Nitrate only x x2007 Annual report Weston x x2008 Annual report and analytical 

data reports

Weston Accutest Laboratories 

of New England

x x x x x x

2012 This report, piezometers only U.S. Geological Survey x x x x x x x x
8 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

Although not specifically identified, anthropogenic 

sources of arsenic from materials disposed of in the landfills 

may include plating wastes, industrial solvents, rat poison, 

and lead-arsenic pesticides (Richard Hull, U.S. Environmental 

Protection Agency, written commun., 2012). Arsenic has also 

been associated with gypsum wallboard material at other sites 

(Al-Abed and Jegadeesan, U.S. Environmental Protection 

Agency, 2006) and is a potential source at this site. Geologic 

or sorbed in-situ (geologic or anthropogenic) sources of 

arsenic and geochemical processes driven by landfill leachate 

were modeled by Sevee and Maher Engineering (1993b); the 

modeling included arsenic release by reductive dissolution of 

iron and manganese hydroxides. 

Geochemical and reactive-transport models coupled 

to groundwater-flow models have not been developed for 

the site, although they offer many advantages over current 

models, such as the ability to simulate redox conditions—an 

important driver of arsenic mobilization—simultaneously with 

quantification of groundwater flow. A solute-transport model 

was developed in response to a request in a 5-year site review 

(United States Environmental Protection Agency, 2007), and 

simulations by this model calculated the time required for the 

arsenic concentrations in groundwater to fall below 10 μg/L. 

The predictions indicate that after 50 years, arsenic concentrations will remain above the MCL in groundwater at the site. 

For this estimate, the arsenic plume was modeled as a solute 

that originated from landfill leachate rather than from dissolution of or desorption from iron oxides (HydroQual, Inc., 2009; 

U.S. Environmental Protection Agency, 2007). The results of 

the multiple investigations at this site, however, indicate that 

reducing conditions created by the waste are likely to be one 

of the major potential causes of arsenic concentrations above 

the MCL.

Purpose and Scope

This report describes the results of a U.S. Geological 

Survey (USGS) study, in cooperation with the NHDES 

and in collaboration with the USEPA, of the Auburn Road 

Landfill Superfund Site in Londonderry, N.H. The study 

objectives were to characterize the hydrogeologic framework 

of the glacial-sediment aquifer, assess the distribution of 

concentrations of arsenic in groundwater, identify arsenic 

transport processes, and to evaluate potential geochemical 

conditions and geochemical reactions controlling arsenic 

concentrations at the site. Results from surface-geophysical 

surveys and borehole logging were used to characterize the 

hydrogeology as part of this study and were interpreted in the 

context of existing data to further refine the hydrogeologic 

framework. A secondary objective was to identify gaps in the 

understanding of the flow system and processes that affect 

arsenic concentrations in the groundwater.

Methods of Data Collection and 

Analysis

Groundwater geochemistry data from multiple previous 

studies were analyzed to determine the geochemical evolution of groundwater along flow paths, including indicators of 

leachate presence, characterization of redox conditions, and 

concentrations of arsenic and volatile organic compounds in 

groundwater. The hydrogeologic framework was characterized 

by surface- and borehole-geophysical surveys in areas downgradient of and surrounding the landfills, including surfacewater bodies. Geologic data from maps, boring logs, and 

geophysical surveys (collected as parts of previous studies) 

were used with new data to further refine the characterization 

of the site. Information regarding geology and chemistry that 

had been collected for more than 30 years at the time of this 

study was used to characterize the hydrogeologic framework 

of the site. 

Determining the spatial distribution of geochemical 

conditions throughout this site is necessary in order 

to understand the factors that affect the high arsenic 

concentrations in groundwater. Key factors that may control 

the concentration of arsenic in groundwater at the site include 

(1) landfill sources of arsenic, (2) zones of leachate-affected 

groundwater, (3) zones of reducing groundwater, and (4) zones 

of oxygenated aquifer recharge. In some cases, groundwater 

may exhibit mixed redox conditions because oxygenated 

recharge from precipitation moves downward into leachateaffected groundwater as the groundwater flows from the 

landfill areas to discharge points along ponds and streams. 

This can affect the position of the boundary of the arsenic 

plume spatially and temporally.

Preferential flow paths—zones of high hydraulic conductivity—can move large amounts of leachate-affected groundwater to discharge points and also provide rapid flushing of 

leachate. Zones of low hydraulic conductivity, in contrast, 

can result in slow movement of contaminants and provide 

a steady release of those contaminants over long periods of 

time. Locating preferential groundwater-flow paths for landfill 

leachate and identifying reduction and oxidation zones are 

important for understanding arsenic mobility at the site.

Geophysics and Hydrogeologic Framework

The hydrogeologic framework of the glacial-sediment 

aquifer at the site was characterized by using data from 

new and previous geophysical surveys and well logs (fig. 1; 

appendix 1). The results allowed identification of locations 

in the aquifer where groundwater contained leachate and 

flow paths were associated with zones of high hydraulic 

conductivity (preferential flow paths). Leachate at landfill 

sites often results in plumes of high specific conductance that 

can be mapped in groundwater with electrical geophysical 

techniques. Mapping the conductive plume also gives general 
Methods of Data Collection and Analysis 9

insight into the leachate distribution and hydrogeology at 

landfill sites (Johnson and others, 2002). Locations with 

thick saturated unconsolidated material are important in the 

transport process because they transmit large volumes of 

water for a given hydraulic conductivity. The glacial-sediment 

aquifer may contain high and low hydraulic-conductivity 

materials. The former promotes rapid groundwater flow, which 

can dilute contaminants. The latter results in slow-moving 

groundwater, which reduces the potential for dilution and may 

result in back-diffusion of contaminants into groundwater 

in more permeable zones. Geophysical-survey results and 

observations of the glacial geology at the site were interpreted 

on the basis of the systematic active glacial-retreat model 

presented by Koteff and Pessl (1981).

The results of surface-geophysical surveys and boreholegeophysical logging from this study were used together with 

previously collected data to enhance the interpretation of the 

hydrogeologic framework. The locations of geophysical surveys from this study were determined with a global positioning system (GPS) for geospatial referencing in a geographic 

information system (GIS). This integrated analysis of new 

geophysical surveys with previously collected geophysical 

data provided the basis for a refined hydrogeologic characterization. Additionally, seismic refraction data (Sevee and 

Maher Engineering, 1992) and geologic logs from drilling 

reports were used to interpret the hydrogeology.

Ground-penetrating radar (GPR) and frequency-domain 

electromagnetic (FDEM) geophysical-survey methods were 

used to investigate the shallow hydrogeology of the glacialsediment aquifer. GPR was used to image the water table, 

pond bottom, sediment bedding, and type of sediment beneath 

the land and water surface, according to methods described in 

Beres and Haeni (1991) and Haeni (1996). 

Direct-current (DC) electrical-resistivity surveys provided detailed data about the base of the glacial-sediment 

aquifer. The data were used to calibrate FDEM surveys, which 

were used to indirectly measure the bulk electrical conductivity of the subsurface with induced electromagnetic signals 

(Zohdy and others, 1974). Interpretations of results in this 

report are focused on glacial-sediment-aquifer properties, but 

because of the depths of the surveys, some information about 

the fracture network in the bedrock aquifer was also obtained.

Electrical measurements made with FDEM and DC 

resistivity under ambient groundwater-quality conditions have 

demonstrated that silt and clay produce a lower resistivity 

response than sand, which produces a lower resistivity than 

gravel (Baines and others, 2002). Saturated sand or gravel 

typically has lower resistivity than unsaturated sand or gravel 

(Zohdy and others, 1974; Kearey and Brooks, 1991). If the 

pore water of a sand or gravel is altered by low-resistivity 

groundwater from anthropogenic or natural sources, the sand 

or gravel may have low resistivity values that are similar to 

those of fine-grained aquifer materials (Urish, 1983). 

Neutron-porosity, natural gamma-radiation (gamma), 

and electromagnetic-induction (EM) borehole geophysical logs (Keys, 1990) were collected at selected wells. Logs 

were completed in polyvinyl-chloride-cased bedrock-aquifer 

and glacial-sediment aquifer wells and one bedrock-aquifer 

well cased with steel. Neutron-porosity borehole logs were 

collected to determine aquifer porosity with depth. Naturalgamma-radiation logs were collected to differentiate finegrained from coarse-grained sediment; lithology and leachateaffected zones can be delineated by combining interpretations 

from natural-gamma-radiation logs with those from EM 

borehole logs (Mack, 1993). Geophysical logs were compared 

to well-construction data to assess the effectiveness of the 

screen location in the aquifer.

Geochemical-Data Evaluation Methods

Data from several reports (Sevee and Maher Engineering, 1992, 1993, and 2000; Weston Solutions, 2006; 2007; 

2008; 2009, b; 2010), (table 1) were combined to reveal 

relations among geochemical data, such as redox indicators, 

specific conductance (SC), and changes in the values of these 

parameters with time. Geochemical data were examined to 

identify factors that control, affect, or are correlated with 

arsenic occurrence and mobility. Spearman correlation (Helsel 

and Hirsch, 1992) was used to identify relations between 

parameters suspected to affect SC, concentrations of landfill 

leachate, and redox indicators. In particular, groundwater SC, 

which measures the ability of the water to conduct electrical 

current and is related to the ionic concentration in the water 

(Hem, 1992), was related to specific groundwater-constituent 

concentrations and facilitated the interpretation of the electrical response from geophysical surveys. Redox processes were 

classified into simplified groups by inference from waterquality indicators, such as concentrations of dissolved oxygen, manganese, iron, and sulfate (Jurgens and others, 2009; 

McMahon and Chapelle, 2008). Geochemical data (Weston 

Solutions, written commun., 2012) from water samples collected as a part of a 2008 sampling round (Weston Solutions, 

2009) were used to determine the apparent redox state of the 

groundwater. Zones of reducing groundwater were identified 

by using this simplified redox classification.

The results of redox-state classifications were plotted on 

cross sections to show the general spatial patterns of the redox 

zones within the hydrogeologic framework. Groundwater was 

classified as anoxic if the dissolved oxygen (DO) concentration measured in the field was less than 1 milligram per liter 

(mg/L). Jurgens and others (2009) classified groundwater as 

anoxic if the DO concentration was less than 0.5 mg/L, but the 

threshold was raised up to 1.0 mg/L for this study to account 

for any potential inconsistency in measurement methods.

Several arsenic reactions and potential transport rates 

were investigated by using a simple one-dimensional, 

uncalibrated, reactive-transport geochemical model 

(PHREEQC software, Parkhurst and Appelo, 1999). The 

model was used to simulate the saturated sand and gravel part 

of the glacial-sediment aquifer. The till and bedrock were 

not explicitly simulated; however, mixing and diffusion of 
10 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

anoxic groundwater from the till was accounted for in some 

simulations. The reactive component of the model was used to 

simulate in-situ arsenic sorption-desorption on oxyhydroxide 

coatings by surface-complexation processes. The transport 

component of the model was used to simulate steady-state, 

one-dimensional, uniform groundwater flow along the 

centerline of the arsenic plume as mapped in 2007 and 2008 

(Weston Solutions, 2008, 2009). The model was discretized 

into 82 cells with a uniform length of 10 m for a total length 

of 820 m. The model was not calibrated to field conditions; 

however, the initial simulated arsenic concentrations were 

adjusted to equal the average of the arsenic concentrations 

measured in 2008.

The model was set up to estimate the concentration of 

dissolved arsenic in groundwater for a hypothetical scenario 

in which leachate-contaminated groundwater with high TOC 

concentrations and reducing conditions was exchanged or 

flushed by oxic, ambient groundwater with low TOC concentrations. The hypothetical scenario was based on the assumption of an instantaneous termination of leachate generation and 

is relevant to conditions such as future termination of leachate 

generation. The model did not explicitly account for other 

processes that may affect arsenic mobility, such as mineral and 

gas exchange. The model does not account for the complexity of the three-dimensional flow system but does help to 

identify processes and parameters that affect dissolved arsenic 

concentrations and to describe the additional data needed to 

adequately characterize the site and assess the time required 

for remediation to bring the groundwater into compliance with 

current groundwater standards. Because additional geochemical and hydrogeologic data are needed to more quantitatively 

estimate attenuation times, the uncalibrated one-dimensional 

model simulations focused on the evaluation of parameter 

sensitivity with respect to rates of arsenic attenuation.

Model input included aqueous geochemical data, 

surface-complexation data, and estimates of one-dimensional 

transport rates. Geochemical data for groundwater were 

selected from available data from the site, but had to be 

supplemented by data from other sites including the Saco 

Landfill study (Nielsen and others, 1995), and regional 

groundwater-chemistry data (Flanagan and Stekl, 1990). 

Data regarding surface sites (metal oxide coatings on aquifer 

sediments) for determining surface-complexation reactions 

(adsorption-desorption from sediments) and exchange rates 

for aqueous-solid reactions were largely derived from the 

Saco Landfill study (Nielsen and others, 1995; Stollenwerk 

and Colman, 2004) because these data were unavailable for 

this study site. The arsenic concentrations from soil leaching 

experiments from the Auburn Road Landfill and Saco Landfill 

sites are nearly equal, suggesting some similarity between 

the sites (HydroQual, Inc., 2009, table 3–1). Site estimates 

of groundwater velocities were based on work by Sevee and 

Maher Engineering (1993a).

Hydrogeologic Framework and 

Groundwater Geochemistry

The hydrogeologic framework of the glacial-sediment 

aquifer at the Auburn Road Superfund Site was analyzed by 

using data from other studies and geophysical-survey results 

from this study. Some of the sediments were identified as 

being associated with zones of high hydraulic conductivity 

around the three capped landfills (the former septage lagoon 

was consolidated into the solid-waste landfill) and areas downgradient of the landfills. Spatial relationships between high 

and low hydraulic conductivity sediments also were examined 

to better determine their effect on leachate mobility from the 

three landfill areas. Zones of reducing groundwater at the site 

were delineated with data collected in the spring of 2008 and 

are presented in cross sections, and the associated reductive 

dissolution process was simulated and characterized by using 

the one-dimensional reactive-transport model.

Hydrogeologic Framework

The hydrogeologic framework of the glacial-sediment 

aquifer at the site was characterized by using surface- and 

borehole-geophysical-survey data and stratigraphic logs 

(appendix 1, table 1–2). Preferential flow paths associated 

with zones of high hydraulic conductivity between the three 

capped landfills and downgradient areas were identified from 

the data. Spatial relationships between high and low hydraulic 

conductivity zones affect the movement of leachate, groundwater redox conditions, and the potential for long-term diffusion of leachate, including dissolved organic carbon (DOC) 

in groundwater, from fine-grained aquifer sediments into 

coarse-grained sediments. The glacial depositional environment determines the distribution and patterns of sedimentation 

(Koteff and Pessl, 1981).

Three former hills at the site (mined for sand and gravel) 

that are shown on the U.S. Geological Survey topographic 

map (U.S. Geological Survey, 1950) appear to be kame deltas, 

which are glaciofluvial ice-contact features (Hambrey, 1994) 

that are coarse grained and have high hydraulic conductivity. 

The former hills are associated by elevation and location 

with former glacial lakes mapped by Gephart (1985). All but 

the northernmost portion of one hill (adjacent and south of 

Whispering Pines Pond) were removed by a former aggregatemining operation. Dipping foreset beds were identified from 

GPR survey results within these features along the western 

portion of line 5 (fig. 1). Adjacent glaciolacustrine sediments 

(with well-sorted layers) and till (poorly sorted) contain 

fine-grained and less permeable sediments, which inhibit 

groundwater flow.

Waste materials are as much as 20 ft thick at the site 

(Weston Solutions, 2012). Water-level data (Weston Solutions, 

2012) from the current monitoring network indicate that 

groundwater is in contact with as much as 5 ft of waste 
Hydrogeologic Framework and Groundwater Geochemistry 11

materials at the town-dump landfill (fig. 1). Groundwater 

was shown to be in contact with waste materials at all three 

landfills on the basis of waste-altitude and water-level 

data (Sevee and Maher Engineering, 1993a). Electrically 

conductive groundwater (possibly indicating high leachate 

concentrations) was detected in the glacial-sediment aquifer 

and may also be in some bedrock fractures (figs. 2–5). 

Groundwater interacting with landfill materials creates plumes 

of leachate that appear as low-resistivity zones in the DC 

resistivity and EM surveys (figs. 2–5). By using Archie’s Law 

to convert pore-water SC to bulk resistivity, values ranging 

from 280 to 140 ohm meters were calculated for sand with 

pore-water SC of 250 to 500 microsiemens per centimeter 

at 25 degrees Celsius (μS/cm at 25 °C). These calculated 

bulk-resistivity values are consistent with values measured 

in this glacial-sediment aquifer near wells with high arsenic 

concentrations in groundwater. 

Seismic-refraction data (Sevee and Maher Engineering, 

1992), geologic logs from drilling reports, and the data collected for this study were used to determine the hydrogeologic 

framework. Low resistivity (less than 300 ohm meters) measured beneath and downgradient of each of the three landfills 

was interpreted to indicate electrically conductive landfill 

leachate (figs. 3–5). High-resistivity values (greater than 

1,500 ohm meters) in the unconsolidated sediments below 

the water table were interpreted to indicate pristine groundwater that may represent background conditions or recently 

recharged, oxygenated groundwater. Zones of high-resistivity 

(low-conductivity) groundwater were identified near the surface between the former gravel pit and the southern upgradient 

edge of Whispering Pines Pond on the eastern edge of DC 

resistivity line 1 (fig. 1). 

Along the center of the sediment-filled valley at the site, 

the bedrock surface deepens to the north-northeast, subparallel to the Flint Hill Fault (fig. 1, Lyons and others, 1997). 

The sediments become thicker along the trend of this feature, 

reaching a maximum thickness of 70 ft beneath Whispering 

Pines Pond. Geophysical-survey results also indicate prominent bedrock fractures that likely contain electrically conductive landfill leachate. Although this study was not designed to 

characterize the location and orientation of bedrock fractures, 

the locations of some of these features were visible and interpreted in the processed data (figs. 3–5).

The solid-waste landfill is on a drainage divide; groundwater in the glacial-sediment aquifer flows east toward a 

swamp and an unnamed brook and northwest towards the 

tire-pile landfill. The northwest-flowing groundwater from 

the solid-waste landfill enters a trough in the bedrock that was 

filled with approximately 25 ft of saturated glacial sediment 

(Sevee and Maher Engineering, 1992). Some groundwater in 

glaciolacustrine (low-hydraulic conductivity) sediments on the 

western side of the solid-waste landfill combines with groundwater flow from the tire-pile landfill. Groundwater gradients in 

the spring of 2008 in this area were downward, and flow may 

also have been entering the bedrock aquifer (fig. 9).

The combined groundwater flow passing by the solidwaste and tire-pile landfills flowed north through fine-grained 

till and glaciolacustrine sediments before entering the coarsegrained deltaic sediments beneath the former gravel pit. Finegrained sediments inhibit flow, whereas coarse-grained sediments provide pathways for preferential flow. These textures 

are related to glacial depositional features at the site. Geophysical data can be used to identify these features so that preferential pathways can be mapped systematically. The hydraulic 

conductivity of the glacial-sediment aquifer increases to the 

north towards the center of the former gravel pit and north of 

Whispering Pines Pond (fig. 1, appendix 1, table 1–2), causing 

groundwater to flow more rapidly. 

Fine-grained glaciolacustrine sediment at monitoring 

well MW-104B, at the southern edge of the former gravel pit 

and north of the tire-pile landfill, has the lowest overburden 

hydraulic conductivity measured at the site (table 2). 

Glaciolacustrine sediment is indicated here and at the 

southeast end of line 10 near the surface above the leachate 

plume as a moderate-resistivity anomaly in the DC-resistivity 

results (fig. 3). The gamma log from this well indicates 

layered, fine-grained glaciolacustrine sediment, which is 

consistent with mapped surficial geologic features (Gephart, 

1985). This area is currently vegetated, and water tends to 

pond during precipitation events, further indicating finegrained sediment near the surface.

The remaining portion of the excavated hill at the north 

end of the former gravel pit is a drainage divide between 

the swamps and the stream to the east and the leachateaffected groundwater plume to the west. It is not known 

whether groundwater contaminated with leachate is beneath 

the wetland to the east of the site, but EM results indicated 

that resistivity values in this area are slightly lower than 

background values, suggesting that the groundwater could 

contain leachate. 

Low-resistivity sediment west of MW-302BR is likely 

due to the presence of high-conductivity groundwater. Results 

of DC resistivity surveys indicate that a bedrock trough filled 

with sediments (and likely leachate-affected groundwater) 

appears to be immediately west of well MW-302BR (figs. 1, 

4). To the west of this trough and downgradient of the towndump landfill (beneath Whispering Pines Pond), the bedrock 

surface is relatively shallow (30 ft or less). 

Well MW-302BR intersected and is screened within a 

10-ft-thick high-hydraulic conductivity cobble and boulder 

layer (Sevee and Maher Engineering, 1993b) that may represent a coarse-grained channel filling (such as an esker). This 

is the only detection of this type of material documented in 

a drilling log thus far in studies of this site, but if the material is a continuous feature, its extent would be an important 

control on groundwater flow. GPR records from line 5 indicate 

a feature that may be cobbles and boulders, but drilling would 

be needed to confirm this interpretation. 

The sediment is thickest beneath the eastern half of 

Whispering Pines Pond (fig. 5). Low-resistivity (electrically 

conductive) groundwater is interpreted to be in the sediment 
12 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

NAD83, Scale 1:3,530 Base photo source: 2005 Natural color 1-foot pixel files, NH DOT

325600 E

325700 E

48300 N

48500 N

C-2 C-3

GZ-6-2R

GZ-6-3R

MW-102A

MW-302A

MW-302BR

MW-303A

MW-303B

40

50

60

70

80

90

C-2 C-3

C-2

GZ-6-3R GZ-6-2R

Competent

bedrock 

Competent

bedrock Fractured

bedrock 

Fractured

bedrock 

Town

dump 

Leachate-affected

groundwater 

Line of section

Altitude, in meters

VERTICALLY EXAGGERATED

NORTHWEST SOUTHEAST

440

720

880

1,060

1,360

1,790

2,450

3,630

5,740

9,400

17,500

30,200

Resistivity, in ohm meters

160

EXPLANATION

Outwash and(or) till

Bedrock

Land surface

Approximate bedrock 

surface

Well and identifier 200 FEET

0

0 100

20 40 60 METERS

150

300

250

200

100

Altitude, in feet above National Geodetic Vertical Datum of 1929 

B

A

Figure 3. A, Aerial view of the location of line 10 of the direct-current resistivity survey and wells that are projected on the cross 

section and B, cross section showing direct-current resistivity results, July 2011, the altitude of the bedrock surface interpreted from 

these results, and lithologic logs in wells along line 10 in the former gravel pit, Auburn Road Landfill Superfund Site, Londonderry, New 

Hampshire.
Hydrogeologic Framework and Groundwater Geochemistry 13

NAD83, Scale 1:3,530 Base photo source: 2005 Natural color 1-foot pixel files, NH DOT

325400 E

325800 E

48500 N

48600 N

C-2

C-3

GZ-6-2R GZ-6-3R

GZ-9-4R

MW-102A

MW-106A

MW-106B

MW-108A

MW-201

MW-202

MW-302A

MW-303A MW-303B MW-302BR

NUS-2-2

NUS-3

80

30

40

50

60

70

90

100

MW-201

MW-302A MW-302BR

MW-303A

MW-303B

WEST EAST

Fracturedbedrock Fracturedbedrock 

Competent

bedrock 

Competent

bedrock 

Competent

bedrock 

Competent

bedrock 

Town

dump 

Whispering

Pines Pond 

Leachate-affected 

groundwater 

MW-201

Line of section

VERTICALLY EXAGGERATED

Altitude, in meters

200 FEET

0

0 100

20 40 60 METERS

440

720

880

1,060

1,360

1,790

2,450

3,630

5,740

9,400

17,500

30,200

Resistivity, in ohm meters

160

Outwash and(or) till

Bedrock

Land surface

Approximate bedrock 

surface

Well and identifier

EXPLANATION

150

300

250

200

100

Altitude, in feet above National Geodetic Vertical Datum of 1929

B

A

(Wells are projected onto line of section.) 

Figure 4. A, Aerial view of the location of line 12 of the direct-current resistivity survey and wells that are projected on the cross 

section and B, cross section showing direct-current resistivity results, July 2011, the altitude of the bedrock surface interpreted from 

these results, and lithologic logs in wells along line 12, Auburn Road Landfill Superfund Site, Londonderry, New Hampshire.
14 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

NAD83, Scale 1:3,530 Base photo source: 2005 Natural color 1-foot pixel files, NH DOT

GZ-9-4R

MW-106A MW-106B

MW-201

MW-202

MW-204

MW-B NUS-2-2 NUS NUS-1-2 -3 NUS-9-1

20

30

40

50

60

70

80

MW-201 MW GZ-9-4R -202 MW-B NUS-1-2 NUS-2-2 NUS-3 NUS-9-1

WEST EAST

Fractured

bedrock 

Fractured

bedrock 

Competent

bedrock 

Competent

bedrock 

Whispering

Pines Pond 

MW-201

Line of section

VERTICALLY EXAGGERATED

Altitude, in meters

325500 E

325700 E

48500 N

48600 N

440

720

880

1,060

1,360

1,790

2,450

3,630

5,740

9,400

17,500

30,200

Resistivity, in ohm meters

160

EXPLANATION

Outwash and(or) till

Bedrock

Land surface

Approximate bedrock 

surface

Well and identifier 200 FEET

0

0 100

20 40 60 METERS

150

250

200

100

Altitude, in feet above National Geodetic Vertical Datum of 1929 

B

A

Figure 5. A, Aerial view of the location of line 2 of the direct-current resistivity survey and wells that are projected on the cross section 

and B, cross section showing direct-current resistivity results, February 2011, the altitude of the bedrock surface interpreted from these 

results, and lithologic logs in wells along line 2 beneath Whispering Pines Pond, Auburn Road Landfill Superfund Site, Londonderry, New 

Hampshire.
Hydrogeologic Framework and Groundwater Geochemistry 15

beneath most of the pond. A prominent red to brown color in 

the center of the pond may be a groundwater-discharge zone 

with a plume of iron oxide (reducing groundwater discharges 

to the pond where it mixes with oxygen and forms an iron 

precipitate, which is suspended in the water). The plume can 

be seen on air photos, and it directly overlies a shallow lowresistivity anomaly that extends to the bottom of the bedrock 

trough (fig. 5). GPR results from line 20 (fig. 1) indicate a 

depression in the pond-bed sediments penetrating through 

organic muck into coarse-grained sediments beneath the 

plume. There is an accumulation of organic muck within the 

depression, but it is thin in the middle and likely allows water 

to discharge or recharge the aquifer. 

Iron oxide was observed on sediments where groundwater discharges at the edge of the pond and into Cohas 

Brook. Groundwater also discharges from the toe of the 

town-dump landfill into the pond and between wells MW-201 

and MW-202 on the south shore of the pond (fig. 1). Water 

levels in piezometers WPP-1 and WPP-2 installed at the toe 

of the town-dump landfill and WPP-3 indicated groundwater 

discharge into Whispering Pines Pond, and CB-1 into Cohas 

Brook, at the time of measurement. It is not known whether 

flow-gradient reversals occurred at the piezometer locations, 

but reversals are indicated by historical water-level data 

(Weston Solutions, 2009) from glacial-sediment and bedrockaquifer well pairs at the site. 

Low-permeability organic muck at the bottom of 

Whispering Pines Pond may inhibit groundwater discharge 

into the pond. Similarly, this muck layer would inhibit 

recharge from the pond to the aquifer if the pond-water 

level were to become higher than the groundwater level. 

Groundwater that may contain leachate is observed to 

discharge to the pond where the muck may be thin or absent. 

High-resistivity groundwater (from recent recharge) on 

top of low-resistivity groundwater was identified beneath 

several areas of the pond that have continuous beds of muck 

indicating no or low rates of groundwater discharge (GPR 

lines 11 and 15; DC resistivity lines 1, 3, and 4.2; fig. 1). 

The GPR record is partially attenuated in areas where lowresistivity groundwater is near the pond bottom.

The bedrock surface is higher in altitude downgradient 

of the pond, restricting flow in the glacial-sediment aquifer. 

A northeast-trending buried bedrock ridge or ridges identified through seismic refraction surveys (Sevee and Maher 

Engineering, 1992) and the logs from recently drilled well 

MW-213 (Weston Solutions, 2010) underlie the trailer park to 

the north of Whispering Pines Pond. This ridge is co-located 

and subparallel with the Flint Hill Fault (fig. 1, Lyons and others, 1997) and represents a silicified, erosion-resistant zone in 

the bedrock surface.

The analysis of borehole-geophysical logs helps assess 

the effectiveness of the well network in representing the 

range of hydrogeologic conditions at the site. Logs were 

interpreted in relation to their screened intervals to determine 

how effectively the current monitoring network represents the 

contamination throughout the site. The logs also were used to 

interpret geochemical conditions, the distribution of arsenic, 

and the causes of temporal changes in arsenic concentrations. 

Like water-quality results, some borehole EM-log results vary 

over time, indicating changes in water quality. The locations of 

wells in relation to their screened intervals can also be used to 

delineate water-quality changes over time. Many of the wells 

completed in the glacial-sediment aquifer are screened over 

some or all of the most electrically conductive intervals identified in EM logs (appendix 1, table 2). Well C-2 is a bedrock 

well (without a corresponding well completed in the glacialsediment aquifer) that passes through a conductive EM anomaly between depths of 22 and 36 ft in the glacial sediment. EM 

results from bedrock well MW-202 indicate that there are two 

conductive anomalies, including an anomaly in the glacialsediment aquifer between depths of 53 and 59 ft, that are not 

within the screened intervals of any of the wells. Anomalies 

detected at well depths not intersected by well screens suggest 

that the current monitoring network may not be suitable for 

characterizing the current contaminant distribution.

Geochemical Characterization

The leachate-affected groundwater, associated with 

high concentrations of arsenic at the site, contains organic 

carbon and other constituents that are mobilized as a result 

of groundwater interacting with landfill materials. Temporal 

transport of leachate through heterogeneous formations with 

varying permeability can produce differential flow and create 

back-diffusion. These processes are prominent at boundaries 

between coarse-grained and fine-grained sediments, but have 

not been evaluated at the site. Elevated concentrations of TOC 

were associated with leachate-affected reducing groundwater 

downgradient of the landfills (Weston Solutions, 2009). The 

contribution of TOC by wetland sediments to groundwater, if 

any, is unknown. It is possible that, during some hydrologic 

conditions, surface water might recharge groundwater, causing 

wetland-generated TOC to move into the groundwater and 

reducing conditions to develop.

As a result of leachate generation from the landfills and 

the microbial consumption of organic carbon—a process 

that consumes DO in the groundwater—a plume of anoxic 

groundwater has formed. The leachate plume includes zones 

of varying levels of redox, from mildly reducing to strongly 

reducing, which, in turn, have varying effects on arsenic 

mobilization. Most of the arsenic plume is within the reducing 

leachate plume. Accordingly, dissolved arsenic in groundwater at the site is mostly in the form of arsenic (III) (Weston 

Solutions, 2008), which is the reduced form. Arsenic (V) also 

has been reported as much as 23 percent in well MW-104B. 

Well MW-104B is in fine-grained low-hydraulic-conductivity 

glaciolacustrine sediments and is downgradient of the tire-pile 

landfill. Fine-grained materials also can be a sink for leachatecontaminated groundwater and, through the process of diffusion, may contain groundwater with high concentrations of 

TOC and arsenic. 
16 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

Much of the groundwater in the glacial-sediment aquifer 

at the site contains concentrations of arsenic that exceed the 

USEPA MCL for arsenic in public drinking water (Weston 

Solutions, 2009). Arsenic concentrations are higher in groundwater in the glacial-sediment aquifer than in the bedrock. Fifty 

percent of samples in the glacial sediment exceeded the MCL 

for arsenic in 2008. Regionally, the MCL for arsenic in water 

samples from 3 percent of the wells in the glacial-sediment 

aquifer has been exceeded (Ayotte and others, 2003), although 

it is unclear whether percentages might be higher locally. 

Elevated SC levels (fig. 6), which were inferred by geophysical surveys during this project (2011), are likely related 

to high concentrations of calcium, chloride, sodium, iron, and 

alkalinity (McCleskey and others, 2012). In addition to these 

elements, low concentrations of iodide and bromide are also 

typically associated with landfill leachate. Road-salt-affected 

groundwater (almost exclusively sodium and chloride) was 

identified adjacent to Auburn Road as a separate plume from 

the landfill-leachate plume, based on the absence of iodide and 

bromide (HydroQual, Inc., 2009). Arsenic concentrations in 

the groundwater of the glacial-sediment aquifer correlate with 

indicators of leachate, such as specific conductance (fig. 7). 

Bromide and iodide concentrations varied spatially in 

groundwater at the site and can indicate different sources of 

leachate (landfills). Bromide and iodide were associated with 

the tire-pile landfill but bromide was more common near 

the town dump, whereas iodide was more common near the 

solid-waste landfill. Elevated concentrations of bromide were 

measured adjacent to the town-dump and tire-pile landfills. 

Elevated concentrations of chloride were measured adjacent 

to the town-dump landfill and along Auburn Road. Concentrations of barium and calcium, in addition to potassium, sulfate, 

isotopes of carbon, bromide, iodide, and chloride concentrations, may have potential for identifying leachate derived from 

individual landfills.

Calcium

0 30 60 90

Specific conductance, in microsiemens per centimeter

0

100

200

300

400

500

600

Chloride

0 20 60 100

Sodium

0 20 40 60 90

Iron

0 20 40

Alkalinity

0 50 150 250

0

100

200

300

400

500

Concentration, in milligrams per liter

Bedrock Overburden

Well type

Rho = 0.357

p 0.159 

Rho = 0.419

p 0.094 

Rho = -0.147

p 0.573 

Rho = 0.614

p 0.009 

Rho 0.289

p 0.260 

Rho = 0.613

p 0.002

Rho = 0.405

p 0.050 

 Rho = 0.445

 p 0.029

Rho = 0.340

p 0.104

Rho = 0.557

p 0.005 

as CaCO3

Figure 6. The specific conductance of groundwater as a function of the concentrations of five constituents 

in water from wells in bedrock and overburden, spring 2008, Auburn Road Landfill Superfund Site, 

Londonderry, New Hampshire.
Hydrogeologic Framework and Groundwater Geochemistry 17

Redox indicators typical of reducing conditions correlated with arsenic concentrations (fig. 8), suggesting that 

reducing conditions were keeping arsenic in solution. Conversely, sorption of arsenic to aquifer materials under high DO 

conditions is an important mechanism that limits the mobility 

of arsenic in groundwater. Manganese reduction, which coliberates manganese and arsenic, may be related to the positive 

correlation of manganese and arsenic concentrations (fig. 8). 

A zone of reducing, methanogenic groundwater was 

inferred using a simplified redox-classification method that 

makes use of commonly available data (McMahon and 

Chapelle, 2008) and was mapped from the tire-pile and solidwaste landfills along the bedrock surface to Whispering Pines 

Pond. At many arsenic-contaminated sites, dissolved arsenic concentrations are highest in zones of strongly reducing 

groundwater, such as those indicated by the presence of methane. For example, Harte and others (2012) found a relation 

between high arsenic concentrations and methane at a nearby 

waste site in the town of Raymond, N.H. 

The outer zones of the plume at the Auburn Road Landfill 

Site consist of iron- and sulfate-reducing water (fig. 9), which 

extends from the town-dump landfill under Whispering Pines 

Pond toward Cohas Brook. Manganese-reducing groundwater was identified on the edges of this plume (fig. 10), which 

appears to flow along the bedrock surface and through fractured bedrock before discharging at the edge of Cohas Brook 

(fig. 10). Arsenic has been identified as sorbed to iron oxyhydroxides on the bank of Cohas Brook (Gan and others, 2006; 

Yu and others, 2010).

Rho = 0.631

p 0.007

Rho = 0.473

p 0.055

Rho = -0.099

p 0.706

Rho = 0.478

p 0.052

Rho = 0.606

p 0.010

Rho = 0.486

p 0.050

Rho = 0.317

p 0.131

Rho = 0.672

p 0.000

Rho = 0.591

p 0.002

Rho = 0.560

p 0.005

Rho = 0.621

p 0.001

Rho = 0.487

p 0.016

Bedrock Overburden

Well type

Ammonia

0 1 2 3 4 5 6

Arsenic concentration, in micrograms per liter

0

50

100

150

200

250

300

350

Alkalinity

0 100 200 250 0 30 60 90

Calcium Bromide

0 50 100 150

Iodide

0 20 40 60 0 200 400

0

50

100

150

200

250

300

350

in milligrams per liter in micrograms per liter in microsiemens

per centimeter

Specific 

conductance, Concentration, as CaCO3 as N

Figure 7. The concentration of arsenic as a function of landfill-leachate indicators in water from wells in 

bedrock and overburden, spring 2008, Auburn Road Landfill Superfund Site, Londonderry, New Hampshire.
18 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

Concentration, in milligrams per liter

Rho = -0.392

p 0.120

Rho = 0.096

p 0.713

Rho = 0.590

p 0.013

Rho = 0.513

p 0.035

Rho = -0.512

p 0.036

Rho = 0.679

p 0.000

Rho = -0.270

p 0.202

Rho = 0.873

p 0.000

Rho = -0.413

p 0.045

Rho = -0.538

p 0.008

Bedrock Overburden

Well type

0 2 4 6 8 10

Arsenic concentration, in micrograms per liter

0

50

100

150

200

250

300

350

Nitrate as N

0 1 2 3 4 5 6 0 2 4 5 1 3

Iron

0 10 20 30 40 50 

Sulfate

0 10 20 30 40 50

0

50

100

150

200

250

300

350

Dissolved oxygen Manganese

Figure 8. The concentration of arsenic as a function of redox indicators in water from wells in bedrock and 

overburden, spring 2008, Auburn Road Landfill Superfund Site, Londonderry, New Hampshire.

Persistence of Arsenic

The persistence of arsenic is a function of physical 

and chemical processes at the site, such as (1) groundwater 

interaction with waste materials, (2) generation of leachate, 

(3) production of a reducing plume of groundwater, and (4) 

flushing with oxygenated precipitation recharge. Groundwater 

has continued to interact with landfill materials, generating 

leachate and arsenic concentrations in groundwater 

downgradient of the site since the landfills were capped 

(fig. 11). Precipitation infiltration, capping of landfills, the 

lowering of Whispering Pines Pond, and beaver activity 

raising pond-water levels all have an effect on water levels, 

rates of groundwater flow, leachate generation, redox state, 

and arsenic dissolution and transport. Furthermore, variability 

in the wetting and drying of the landfill materials could cause 

temporal changes in arsenic concentrations downgradient of 

the landfills and needs to be considered when the temporal 

results from an individual monitoring well are analyzed. 

Wells screened at the edge of the plume and in materials with high permeability have arsenic concentrations that 

vary substantially compared to wells within the interior of the 

plume. Concentrations of arsenic in well GZ-6-2R, which is 

screened in the top part of the high SC plume in the sand layer 

of the former gravel pit downgradient of the tire-pile landfill, 

vary more over time (fig. 11B) than those in well GZ-6-3R, 

which is screened in glacial till within the vertical center of the 

plume. Concentrations of arsenic in well GZ-6-2R declined 

since 1994, when the landfill was capped, until 2002 and have 

remained steady with some nondetections through 2010. Similar variations occurred in well MW-302BR. 
Hydrogeologic Framework and Groundwater Geochemistry 19

C-1

C-2

C-3

GZ-1-3R GZ-6-2R

GZ-6-3R

GZ-9-4R MW-102A

MW-104A

MW-202

MW-204

MW-212A MW-302A

MW-302BR

MW-B

NUS-1-2

NUS-9-1

Whispering

Pines Pond 

Town

dump 

Solid-waste landfill

Tire

pile 

C-1

C-2

C-3

GZ-1-3R

GZ-6-2R

GZ-6-3R

MW-102A, GZ-9-4R

MW-102B

MW-104A

MW-202

MW MW-204 -212A

MW-302A,

MW-302BR

MW-B

NUS-1-2

NUS-9-1

236

29

115

MW-104B

50

205

169

206

0

55

0

0

 0

20mixing with oxygenated 

groundwater

325600 E

325700 E

48000 N

48500 N

Line of section

Altitude, in meters

60

50

70

80

90

VERTICALLY EXAGGERATED

SOUTH NORTH

mixing with oxygenated 

groundwater

EXPLANATION

Outwash and(or) till

Bedrock

Methanogenesis

Iron and(or) sulfate reducing zone

CB-1

90

Well and identifier

Arsenic concentration, in micrograms per liter

Land surface

Potentiometric surface of overburden

Bedrock surface

Groundwater-flow direction

NAD83, Scale 1:3,530 Base photo source: 2005 Natural color 1-foot pixel files, NH DOT

400 FEET

0

0 200

40 80 120 METERS

B

A

300

250

200

Altitude, in feet above National Geodetic Vertical Datum of 1929 

Figure 9. A, Aerial view of the location of the cross section and wells that are projected on the cross section and B, cross section 

showing redox conditions and concentrations of arsenic in groundwater along a flow path from the solid-waste landfill to Whispering 

Pines Pond, spring 2008, Auburn Road Landfill Superfund Site, Londonderry, New Hampshire.
20 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

NAD83, Scale 1:3,530 Base photo source: 2005 Natural color 1-foot pixel files, NH DOT

325500 E

325600 E

48400 N

48700 N

MW-102A

MW-106A

MW-106B

MW-108A

MW-108B

MW-201

MW-303A

MW-303B

NUS-2-2

NUS-3

60

70

80

90

MW-102A

MW-106A

MW-106B

MW-108A

MW-108B

MW-303A MW-201

MW-303B

NUS-2-2

Altitude, in meters

VERTICALLY EXAGGERATED

SOUTH NORTH

Whispering

Pines Pond

Town

Dump

Cohas 

Brook

CB-1

CB-1

Bedrock

Outwash and(or) till

EXPLANATION

CB-1

Iron and(or) sulfate-

 reducing zone

Manganese- 

 reducing zone

90

MW-102B

0

0

25

50

0

0

0

99

0

0

mixing with oxygenated 

groundwater

400 FEET

0

0 200

40 80 120 METERS

Well and identifier

Arsenic concentration, in micrograms per liter

Land surface

Potentiometric surface of overburden

Bedrock surface

Groundwater-flow direction

B

A

Line of section

300

250

200

Altitude, in feet above National Geodetic Vertical Datum of 1929 

Figure 10. A, Aerial view of the location of the cross section and wells that are projected on the cross section and B, cross section 

showing redox conditions and concentrations of arsenic in groundwater along a flow path from the town-dump landfill to Cohas 

Brook, spring 2008, Auburn Road Landfill Superfund Site, Londonderry, New Hampshire.
Hydrogeologic Framework and Groundwater Geochemistry 21

0

500400300200100

C-1

1994

1996

2002

2006

2010

Year

0

500400300200100

GZ-6-3R

0

500400300200100

GZ-6-2R

0

500400300200100

MW-302BR

1994

1996

2002

2006

2010

Year

0

500400300200100

NUS-1-2

0

500400300200100

NUS-2-2

1994

1996

2002

2006

2010

Year

0

900800700600500400300200100

1,000

Concentration of arsenic, in micrograms per liter 

PZ-218

A

B

C

MW-302BR

EXPLANATION

Well numberConcentration of arsenic,  in micrograms per liter 

Figure 11. Concentrations of arsenic in groundwater by year and well A, between landfills, B, in the gravel pit, and C, beneath Whispering Pines Pond, Auburn Road Landfill Superfund Site, Londonderry, New Hampshire.
22 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

Back-diffusion of contaminants at waste sites is a 

well-recognized process (Parker and others, 2008). Fine 

sand and silt zones that have a low permeability can become 

loaded with contaminants through diffusion as a plume 

passes by or through the low-permeability zone. (appendix 1, 

table 2; MW-104B). Differential rates of transport will 

allow high-permeability zones to flush contaminants more 

rapidly than low-permeability zones, promoting backdiffusion of contaminants, such as leachate and TOC, from 

low- to high-permeability zones. Back-diffusion may be 

affecting concentrations in well MW-104B, where arsenic 

concentrations are often above 100 μg/L. Thus, the contrast in 

flow between low- and high-transmissivity zones can promote 

back-diffusion from low- to high-transmissivity zones and 

provide a long-term continuous source of leachate and arsenic.

The relative importance of several processes and factors 

affecting arsenic concentrations at the site are important to 

the decisions that will be made regarding (1) what additional 

data need to be collected, (2) what additional studies might be 

warranted, and (3) ultimately, what remedial strategies should 

be considered. In order to start the process of understanding 

the relative importance of various factors, a simple onedimensional, uncalibrated, reactive-transport model was 

developed, and various factors were assessed. 

The main objective of the model was to assess the sensitivity of input parameters (representing components of physical and chemical processes) in affecting arsenic attenuation. 

Such a sensitivity analysis can lead to a clearer understanding 

of the factors that are most important in governing arsenic 

transport. The model was constructed to investigate arsenic 

mobility under a hypothetical scenario where clean water is 

flushed through the glacial-sediment aquifer after a hypothetical instantaneous removal of the leachate source. The model 

primarily simulated redox-driven reactions between the solutes, including ferric oxide, carbon, and arsenic, and mineral 

surfaces (in the form of surface complexation). The effects of 

pH and ion activity on arsenic mobility were accounted for 

in the surface-complexation reactions; however, these factors 

had a small effect on dissolved-arsenic concentrations in these 

reactions. The importance of mineral-exchange or gas-phase 

reactions, where pH and ion-activity are known to be important factors, could be assessed with additional geochemicaldata collection and further modeling efforts.

Currently (2012), parts of the solid-waste, tire-pile, and 

town-dump landfills are under water. The simulations showed 

that if the source of leachate is removed, the time required 

for the arsenic concentration to reach the standard is in part 

dependent on the position along the transport path in relation 

to the position of the leachate source. Concentrations in the 

area by Cohas Brook may take the longest to decline because 

this area is the farthest downgradient. Arsenic concentrations 

in areas upgradient of Cohas Brook may decrease earlier 

because the clean water flushes through these areas first.

The number of oxyhydroxide surface-complexation 

sites on minerals and the velocity of groundwater are key 

parameters in controlling the time for arsenic concentrations 

in groundwater to drop below the MCL. A bar graph showing 

model-parameter sensitivities illustrates the effect of adjusting 

these parameter values (relative to a base simulation) on the 

final simulated arsenic (III) concentration (fig. 12A). Arsenic (III) is presented here because it is the primary dissolved 

arsenic species in the model (appendix 6). The simulated 

arsenic (III) concentrations in the graphs are from the area by 

Whispering Pond (fig. 12). Parameters were adjusted typically 

by less than one order of magnitude from the base simulation 

except for groundwater velocity, which was adjusted by only 

30 percent, to coincide with estimated adjective-transport 

rates (Sevee and Maher Engineering, 1993a, b). The amount 

of parameter adjustment was based on either reported literature values or estimated variations. The simulations are 

grouped according to the parameter adjusted (called coupled 

parameter simulations).

The sensitivity of a given parameter is partly a function 

of the parameter adjustment (difference) from the value used 

in the base simulation. The relation between the time difference and parameter sensitivity for coupled parameter simulations is nonuniform because parameters were not equally 

adjusted. The model was most sensitive to the potential range 

in groundwater velocities (fig. 12A), but the largest time difference between coupled simulations occurred when the number of weakly bonded surface sites was increased (fig. 12B). 

Two types of surface complexation sites—weakly bonded 

and strongly bonded—were modeled. PHREEQC allows for 

the simulation of different surface affinities to adsorption 

and desorption (Parkhurst and Appelo, 1999). Variations in 

the number of weakly bonded surface sites affected the time 

to reach the MCL more than the variations in the number of 

strongly bonded surface sites.

The relative percent time difference to achieve the MCL 

for arsenic concentrations between the coupled parameter 

simulations ranged from -33 to 90 percent from the base 

simulation (fig. 12B). This wide range highlights the need 

to obtain information on the unconsolidated sediment at the 

site, particularly the physical and chemical composition. Soil 

coring in the middle of the plume would allow more accurate 

determination of the number of binding surfaces available 

for surface complexation. Higher rates of transport, a lower 

number of binding surfaces available for surface-complexation 

reactions, or an increase in the volumes and (or) frequencies of 

influxes of oxygenated waters would reduce the time to reach 

the arsenic standard. 

Back-diffusion processes—whereby leachate can diffuse 

out of fine-grained sediments after the leachate plume has 

been flushed through the bulk of the system—and mixing 

of both oxic and anoxic groundwater were simulated in the 

model by use of an additional model layer (parameter mixing, fig. 12B). The mixing of groundwater with high arsenic concentrations diffusing from a modeled till layer (also 

containing fine-grained sediments) increased cleanup times 

by 40 percent (parameter mixing; fig. 12B) near Whispering 

Pines Pond, suggesting that diffusion could significantly prolong the cleanup times at this site. In contrast, the mixing of 
Hydrogeologic Framework and Groundwater Geochemistry 23

EXPLANATION

Abbreviation for parameter Description

As3_initial_I Increase in arsenic concentration of initial groundwater

toc_flush_D Decreased TOC concentration of flushed groundwater

toc_initial_D Decreased TOC concentration of initial groundwater

toc_initial_I Increased TOC concentration of initial groundwater

surfboh_I Increase in the number of sites with strongly bonded surface complexation, with arsenic

surboh_D Decrease in the number of sites with strongly bonded surface complexation, with arsenic

surfwoh_I Increase in the number of sites with weakly bonded surface complexation, with arsenic

velocity_D Decrease in groundwater velocity

velocity_I Increase in groundwater velocity

mixing Simulation of mixing of oxic or anoxic groundwater, location dependent

-40

-30

-20

-10

0

10

20

30

40

50

60

1

As3_initial_I

toc_flush_D

toc_initial_D

toc_initial_I

surfboh_I

surboh_D (indicates no effect)

surfwoh_I

velocity_D

velocity_I

mixing

A

Parameter effect on dissolved arsenic (III) concentration at Whispering Pond.

Equation = 100*(difference in years from base)/(difference in par values/base par value)

Sensitivity of parameter from base, in percent difference ‐40

‐20

0

20

40

60

80

100

1

As3_initial_I

toc_flush_D

toc_initial_D

toc_initial_I

surfboh_I

surboh_D (indicates no effect) 

surfwoh_I

velocity_D

velocity_I

mixing

B Parameter effect on dissolved arsenic (III) concentration at Whispering Pond

Equation = 100*(difference in years from base)/(mean years of coupled simulations)

EXPLANATION

EXPLANATION

Simulation difference from base, in relative percent difference

Figure 12. Results of sensitivity analyses of the A, relative effect of parameter adjustment on the dimensionless arsenic 

concentration, and B, the percent difference between the simulated times to achieve the standard for dissolved arsenic at 

Whispering Pines Pond, Auburn Road Landfill Superfund Site, Londonderry, New Hampshire. TOC, total organic carbon.
24 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

oxygenated groundwater from precipitation recharge with low 

arsenic concentrations decreased cleanup times by 10 percent 

in the middle of the transport path by the excavated sand area 

(fig. 12B; former gravel mine in fig. 1). Simulated recharge of 

oxygenated water occurs near the excavated sand area and is 

consistent with an increase in observed DO in this area (fig. 9).

Simulation results show that increasing groundwater 

velocities by 30 percent reduced the time to achieve the arsenic standard after removal of the leachate source or leachate 

by -33 percent (parameter velocity 1; fig. 12B). This result 

indicates that increasing hydraulic gradients and velocities 

could be an effective part of the remedial strategy for the site.

The simulations point to a lack of suitable information 

to constrain model parameters. For example, equilibrium constants, which control the exchange rates between the surface 

and aqueous arsenic concentrations, could be as important 

to determine as the number of surface-complexation sites. 

Equilibrium constants in the simulations were kept at rates 

specified for the Saco landfill study in Maine (Stollenwerk 

and Colman, 2004). Batch column experiments from soil 

cores at the Auburn landfill would allow estimation of specific 

equilibrium constants for this site and thus reduce uncertainty 

in the model. Mineral and gas-exchange reactions were not 

simulated but could be important factors in dissolved arsenic 

concentrations. Additionally, the model indicated strong sensitivity to groundwater- flow velocities. To accurately define 

velocities, a three-dimensional model could be developed. 

Furthermore, processes such as back-diffusion could be better 

simulated with a fully three-dimensional model because the 

total arsenic mass from the till or other low-permeability zones 

could be better replicated. The development of the model and 

testing of model parameters highlighted the need for further 

model development, calibration, and field-data collection to 

better replicate field conditions.

Data Gaps

Despite the amount of data that has been collected, gaps 

remain in the temporal and spatial hydrogeologic and geochemical data for the Auburn Road Landfill Superfund Site. 

More frequent sampling or sampling driven by hydrologic 

events, such as changes in water levels, seasons, and precipitation, may explain some of the variations in concentrations in 

the wells. In addition, the current monitoring network may be 

inadequate to fully monitor the arsenic and leachate plume. 

These gaps in data were identified on the basis of results of 

the new surveys and a reexamination of existing information. 

Several locations in the deepest part of the glacial-sediment 

aquifer, which fills bedrock troughs downgradient of the 

landfills, are not being monitored with existing wells—yet 

surface resistivity surveys indicate possible contamination 

with leachate. 

Additional data collection and interpretation could be 

used in the development of a three-dimensional geochemical 

model of the glacial-sediment and bedrock aquifers to predict 

the persistence and location of the arsenic plume. Water-level 

monitoring from surface-water bodies and wells in the existing network could be used to further refine flow directions 

and gradients. Although past studies have indicated that the 

groundwater-flow directions are generally to the north and 

northwest, a variety of localized flow directions and velocities are possible. Continuous water-level-monitoring and 

horizontal flowmeter measurements will provide flow velocity 

and directions that can be used to estimate transport rates and 

the extent of leachate and arsenic contamination. Geophysical data collected for this study indicate the occurrence of 

leachate-affected groundwater as low-resistivity zones in the 

glacial-sediment aquifer. Spatial information on the chemistry 

of the leachate plume mapped with geophysical methods could 

be determined by calculating the electrical conductivity of the 

groundwater on the basis of water-quality data by using the 

methods of McCleskey and others (2012).

Aquifer and Landfill-Matrix Solids

Little information on the geochemistry of aquifer- and 

landfill-matrix solids, which is important for assessing the 

rates of arsenic desorption at the Auburn Road Superfund 

Site, had been collected at the time this report was completed 

(2013). Soil cores extracted from the middle of the plume, as 

well as along boundaries between fine- and coarse-grained 

sediments, could provide data with which to more accurately 

determine the number of binding surfaces available for surface 

complexation. Also, determining the distribution of TOC in 

each of the hydrogeologic units identified in the conceptual 

model (fig. 2) would enable a more thorough geochemical 

evaluation of the arsenic plume. For example, aquifer material 

consisting of fine sand and silt of low hydraulic conductivity 

may be a source of leachate and arsenic through backdiffusion from the fine-grained into the coarse-grained parts 

of the aquifer.

Indicators of Leachate and Groundwater Redox

Data indicating groundwater redox status and leachate 

concentrations are not available for many existing and new 

monitoring wells. These data may be useful in fingerprinting 

groundwater from individual landfills or portions of landfills. In 1994, prior to the installation of the landfill caps, a 

detailed monitoring survey of groundwater geochemistry was 

completed in the glacial-sediment aquifer by using microwells (Sevee and Maher Engineering,1993b). Such a survey, if 

repeated, would help to determine the effects of capping and 

two decades of natural attenuation. In addition to the previously measured constituents, analysis of parameters related 

to the redox state of the groundwater, general groundwater 

geochemistry, and stable isotopes would facilitate source 

characterization. This would include, but not be limited to, 

measurements of groundwater samples for field parameters, 

isotopes of dissolved organic and inorganic carbon, arsenic, 
Hydrogeologic Framework and Groundwater Geochemistry 25

VOCs, dissolved gases, and major ions in the glacial-sediment 

aquifer and the bedrock aquifer.

Surface-Water/Groundwater Interactions

Groundwater discharge (iron-stained seeps) has been 

observed, and groundwater/surface-water interactions may 

control arsenic transport. An understanding of the timing and 

interaction of groundwater flow with the wetlands, Whispering 

Pines Pond, and Cohas Brook would be necessary to understand the effects of surface water on groundwater arsenic concentrations at the site. The fate of precipitated compounds that 

contain arsenic in surface-water sediments and their potential 

for remobilization are not well understood. Hydrologic variations may affect contaminant transport at several wetland 

groundwater-discharge locations at the Auburn Road Landfill 

Superfund Site. The needed surface-water stage, flow, and 

geochemical data are not available to evaluate these interactions. For example, water-level measurements are needed for 

Cohas Brook, Whispering Pines Pond, and nearby wetlands. 

Beaver dams in the pond just north of the town-dump landfill, 

in the middle of Whispering Pines Pond, and the stream that 

runs between the tire-pile and solid-waste landfills appear to 

hold the water at a higher elevation than that of the pond near 

the dam and outlet to Cohas Brook. Hydrologic and waterquality information collected for specific areas of sediment 

beneath the pond in the location of geophysical anomalies 

identified in this study would provide a better determination 

of groundwater-flow paths. Specifically, drive-point wells 

from which measurements of water levels and water chemistry 

could be obtained over a range of depths would provide the 

type of information needed.

Hydrogeologic Framework of the Bedrock 

Aquifer

Bedrock formations, lithology, and fracture patterns, if 

evaluated, would provide needed information relating to flow 

into and out of the bedrock aquifer. The bedrock aquifer was 

not specifically addressed in this study, but some information 

was acquired during geophysical surveys. Preliminary results 

from DC resistivity surveys indicate that the underlying 

fractured bedrock is in hydraulic connection with the glacialsediment aquifer above the bedrock aquifer. Additional 

geophysical surveys could be designed to determine the 

orientation and extent of the bedrock fractures. 

A detailed assessment of the fractured bedrock aquifer 

could answer many questions about flow and arsenic 

contamination at this site. For example, bedrock-borehole 

and outcrop-fracture characterization along with surface 

geophysical surveys designed to determine the locations 

and orientations of bedrock fractures could identify possible 

contaminant pathways and be used to design a more effective 

monitoring-well network for the fractured bedrock. Wells 

installed in these areas would allow for more precise definition 

of the hydrogeology at specific locations in the bedrock. 

Wells already in place (currently not monitored) may 

be available for inclusion in the enhanced monitoring and 

bedrock-characterization effort. For example, an open bedrock 

borehole (NUS-3) on the north side of Whispering Pines Pond 

is installed deep into the bedrock and could be used to assess 

arsenic contamination. The results of sampling groundwater 

at multiple depths in this well could be used to assess whether 

contaminants are present and what fractures are transporting them. Borehole-flowmeter characterization of the open 

bedrock could indicate whether cross contamination between 

the fractures is currently occurring and determine what, if any, 

preventative actions are needed. 

Mapping ductile and brittle features and precisely locating bedrock outcrops at the site would provide information 

that could be used in the determination of structural patterns of 

the rock—this information would be used to further map the 

bedrock surface and to develop model input representing the 

hydraulic properties of the bedrock. By combining geophysical results with information about the locations of remotely 

sensed lineaments and fractures, the orientations of larger 

fracture zones could be determined. Detailed information 

about locations of outcrops could be used to map the bedrock 

surface between wells more precisely.

Hydrogeologic Framework of the GlacialSediment Aquifer

The determination of hydrogeologic layers representing hydraulic features in the glacial-sediment aquifer could 

be remapped with greater resolution. Detailed and current 

site topography with 2-ft contour intervals could be used to 

represent the land-surface layer. Prominent topographic (U.S. 

Geological Survey, 1950) glacial features such as large kame 

deltas interpreted from quadrangle-scale maps no longer exist. 

Although their topographic form is no longer identifiable on 

the land surface, and their influence on precipitation infiltration has been altered, the subsurface remnants of the internal 

bedding structures of these features have a strong influence 

on groundwater flow. Conversely, other features that were not 

represented on older maps are now visible at the land surface, 

such as bedrock outcrops. The contour of the bedrock surface (figs. 3–5, 9, 10) could be further refined on the basis of 

seismic-refraction data collected as part of site investigations 

in 1985 and 1992; data from microwells, borings installed 

during precapping investigations, fracture patterns, and ductile 

structures; and GPS locations of bedrock outcrops around 

the site. Additionally, data collected from monitoring well 

MW-213 (drilled in 2009) and seismic data (Sevee and Maher 

Engineering, 1993) indicate high points in the bedrock surface 

that may represent a buried bedrock ridge or ridges between 

Whispering Pines Pond and Cohas Brook. This feature is near 

the trace of the Flint Hill Fault, which is marked by silicified zones that would represent a significant geologic control 
26 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

affecting groundwater flow at the site. Additional geophysical 

surveys could be collected in areas for which well data are 

not available to complete the map of the bedrock surface. The 

organic muck and peat at the bottom of the pond may affect 

surface-water/groundwater interactions and can be modeled 

by using available GPR data collected by this study. Estimates 

of hydraulic conductivity, porosity, and permeability of the 

aquifer would be useful in the development of a more detailed 

groundwater-flow model to support a three-dimensional 

geochemical model. This information could be obtained from 

available borehole geophysical data (appendix 1) and from 

additional hydraulic data, including slug-test and nuclear magnetic resonance data, to assign aquifer properties to the entire 

borehole. An aquifer test would be useful in determining the 

connectivity between the glacial-sediment aquifer and surface 

water. Determining the location and extent of high hydraulic 

conductivity features, such as eskers and other ice-contact 

glaciofluvial coarse-grained sediments, would be important for 

modeling preferential pathways for groundwater flow. Additional synthesis of currently available data could be used to 

create surfaces of stratigraphic units that could inform future 

modeling efforts.

Groundwater Flow and Three-Dimensional 

Geochemical Model

A detailed groundwater-flow model with site-specific data 

and three-dimensional information about aquifer geochemistry 

could be used to accurately assess the persistence of arsenic and to test potential remediation schemes. Incorporating 

reactive-transport into the flow model (coupled flow and transport model) would result in more accurate and representative 

simulations of conditions across the site. The simulation of 

processes such as the recharge of groundwater by oxygenated 

water in the gravel pit would also be possible with the development of a three-dimensional model. The fractured-bedrock 

aquifer should be included in the model because it is likely an 

important source of arsenic, according to regional studies of 

arsenic in New Hampshire (Montgomery and others, 2003).

Summary and Conclusions

Based on previous investigations and on characterization 

done for this study, arsenic mobility at the Auburn Road 

Landfill Superfund Site is attributed largely to reductivedissolution processes involving manganese and iron oxides 

and (or) reductive desorption of arsenic from those oxides. 

Also, relatively high dissolved arsenic (V) concentrations 

at wells (Weston Solutions, 2008) suggest a complex 

mobilization process. Persistent high arsenic concentrations 

at the Auburn landfill are produced, in part, from continuous 

leachate generation from partially submerged waste under 

the landfills. 

Mapping of the arsenic plume is complicated by the 

complex aquifer heterogeneity at the site; the hydrogeologic 

framework plays a large role in the spatiotemporal variation 

of arsenic concentrations. Data from previous studies and 

new surface and borehole geophysical surveys were used 

to characterize the hydrogeologic framework of the glacialsediment aquifer between the three capped landfills and 

downgradient areas at the site. Changes in water levels are 

affected by recharge (precipitation infiltration) and by varying 

impoundment of surface water as a result of beaver dams. 

Leachate-affected groundwater, which contained arsenic, 

had an electrically conductive signature at this site. This 

signature allowed for the delineation of leachate by means 

of geophysical techniques that are sensitive to electrical 

properties of the groundwater. Sorption and desorption 

reactions that control arsenic concentrations were studied 

through the development of a simple uncalibrated, onedimensional, reactive-transport model.

The hydrogeologic framework of the glacial-sediment 

aquifer is the result of a complex system of till, glaciofluvial, and glaciolacustrine sediments from three overlapping 

glacial lakes and is further complicated by the topography 

of the underlying bedrock surface. The remnants of kame 

deltas (east of the town-dump and solid-waste landfills) form 

high-permeability zones that lack vegetative cover and provide rapid infiltration of precipitation in the aquifer. Layered 

low-hydraulic conductivity glaciolacustrine sediment at the 

site limits infiltration and slows groundwater flow in areas surrounding the kame deltas. The site is underlain by a bedrock 

trough south of the tire-pile landfill that trends and deepens to 

the north-northeast towards Whispering Pines Pond. The deep 

part of the bedrock trough underlies coarse-grained sediments 

that contain leachate-affected groundwater. A plume of red-tobrown iron oxide in the pond water and iron oxides precipitated on the pond bottom indicate that iron-laden groundwater 

is discharging to the pond, where the iron is oxidized and 

precipitated among the pond sediments. It is assumed that 

groundwater normally discharges to the pond, but it is unclear 

whether the pond level is sometimes higher than the groundwater level, potentially inducing recharge to the aquifer. 

Groundwater recharged by precipitation and unaffected by 

leachate was observed in shallow parts of the aquifer beneath 

muck on the pond bottom and upgradient (south) of the pond, 

suggesting that the muck is controlling groundwater discharge 

to the pond. A northeast-trending, steep, buried bedrock ridge 

includes erosion-resistant low-permeability silicified zones 

associated with the Flint Hill Fault. This ridge may be continuous, resulting in a limited saturated thickness that inhibits 

groundwater flow in the glacial-sediment aquifer. 

Groundwater-level data and interpretations from the 

current monitoring network indicate that groundwater was 

in contact with wastes at all three landfills, likely generating 

leachate. Arsenic values were highest at wells adjacent to the 

solid-waste and the tire-pile landfills and were correlated with 

iron-reducing and methanogenic redox conditions. A plume of 

methanogenic groundwater was mapped (by using data from 
References Cited 27

2008) from the tire-pile and solid-waste landfills along the 

bedrock surface to Whispering Pines Pond. Iron- and sulfatereducing conditions were inferred in the outer edges of this 

plume. A plume of iron- and sulfate-reducing groundwater was 

flowing from the town-dump landfill under Whispering Pines 

Pond towards Cohas Brook; the plumes may mix before flowing through fractured bedrock and discharging at the edge of 

Cohas Brook. The arsenic concentrations were highest in the 

inferred methane-producing zone.

As part of the assessment, a one-dimensional uncalibrated 

reactive-transport model was constructed to measure the 

relative influence of several parameters on dissolved arsenic 

concentrations. The model simulated hypothetical scenarios 

that included the removal of the leachate source. Surface 

complexation (adsorption-desorption of arsenic from 

oxyhydroxides) was simulated, and the sensitivity of arsenic 

attenuation to changes in model parameters was evaluated. 

The results of simulations indicated a high sensitivity to the 

number of surface sites available for adsorption-desorption. 

Coring and analysis of aquifer sediments within the arsenic 

plume could allow investigators to quantify available carbon 

and the area available on surface sites for adsorptiondesorption to constrain model-parameter values. Groundwater 

velocity was identified as an important parameter; increasing 

hydraulic gradients at the site could reduce the time for 

groundwater to reach arsenic standards if waste generation 

were terminated. The model does not incorporate aquifer 

heterogeneity, but results of simulations provide some insight 

into the potential effect of heterogeneity on remediation 

and the need to map the subsurface to accurately assess 

the time required for complete remediation. Enhancements 

to the monitoring-well network to capture the center and 

edges (which move according to the varying hydraulics 

and geochemistry) of the plume as well as to assess the 

potential contamination of the underlying bedrock would 

greatly improve the characterization of the arsenic plume. 

Samples from wells currently (2013) used in the monitoring 

network that are screened in the top or edge of the conductive 

leachate plume may detect greater variability in arsenic 

concentrations as a result of the fluctuating water table or the 

mixing of geochemically different recharge waters. Without 

an improved understanding of the hydrogeologic framework 

and geochemical reactions at this site, projections of long-term 

trends in arsenic concentrations will have a large uncertainty.

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127 p.

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6941.2009.00797.x.

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Appendixes 1–7
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Appendix 1 35

Appendix 1. Summary of Results of Geophysical-Data Processing and

Borehole Geophysical Log Analysis

A total of about 9 miles of electromagnetic (EM) surveys, 

16 line segments of ground-penetrating radar (GPR), 14 direct 

current (DC)-resistivity lines, and 23 borehole logs were completed at the site as part of this study. The Geophex GEM–2 

Plus, a portable multifrequency electromagnetic sensor, was 

used for frequency-domain electromagnetic (FDEM) surveys 

in this study (Geophex, Ltd., 2007). Fifteen frequencies ranging from 2010 to 40,050 Hertz were chosen; lower frequencies 

were used to increase the depth of the subsurface materials 

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

Won and others, 1996). Before and after each FDEM survey, 

one of three common base stations was measured to provide 

data for the linear correction of drift (shift in instrument 

response with time). Data collection and processing methods 

used in this study were similar to those used in Abraham and 

others (2006). The shifted data were calibrated to the same 

scale with the results of inverted DC-resistivity data. Total 

conductivity and inverted data (to provide interpretations with 

depth) were produced from the processed EM data by using 

WinGEM version 3, 0, 0, 14 (Geophex, Ltd., 2007).

Table 1–1. Root mean square error of direct-current resistivity 

inversion.

[Lines are shown on figure 1–1]

Line identifier Date Root mean square error

1 1/31/2011 5.6

2 2/1/2011 5

3 2/3/2011 10.3

4.1 2/3/2011 12.9

4.2 2/3/2011 5

5.1 2/3/2011 6.6

7.1 2/9/2011 4.1

7.2 2/9/2011 3.2

8 7/25/2011 23.8

9 7/26/2011 18.5

10 7/27/2011 22.8

11 7/27/2011 4.2

12 7/28/2011 31

13 7/29/2011 15.2

DC-resistivity-array types included dipole-dipole and 

Schlumberger arrays (Zohdy and others, 1974). Reverse 

dipole-dipole and reciprocal Schlumberger surveys served as 

quality-check surveys for this study. Results provided by the 

dipole-dipole array have the greatest lateral resolution but the 

lowest signal-to-noise ratio. Some data points were removed 

because of large errors in the data generated in some of the 

driest parts of the survey lines, where current application was 

difficult. Reverse and forward dipole-dipole survey data that 

were combined and inverted together five times are interpreted 

and presented in this report. Root mean square error from the 

inversions is a measure of quality and provided a means to 

assess the model match to the field data (apparent resistivity) (table 1–1). DC-resistivity data were processed (inverted) 

by using RES2DINV version 3.55 (Loke, 1999) to provide 

estimates of resistivity at specific depths. The smaller number 

of data points at the ends of the survey lines made results for 

the line ends slightly less reliable. The quality-check (reverse) 

survey results were compared to results from the other surveys 

to ensure that the same general patterns and interpretations 

would be made.

As part of this study, new borehole geophysical logs were 

collected in August 2011 in 22 polyvinyl-chloride-cased wells 

(already installed in up to 60 feet (ft) of unconsolidated sand 

and gravel). One deep (210-ft) borehole in bedrock with a steel 

casing also was logged (NUS-3). All results are given in feet 

below top of casing. Logs collected include natural gamma 

radiation, EM induction, and neutron porosity. Hydraulic 

conductivities measured by slug tests in groundwater were 

available from 17 wells (Sevee and Maher Engineering, 1992); 

8 of these wells were logged, or an adjacent well was logged, 

as part of this study. A total of 33 wells were logged, or an 

adjacent well was logged, in 13 locations, 2 of which had 

3 adjacent wells (table 1–2). Hydraulic conductivity values 

within the glacial-sediment aquifer ranged from 0.074 to 

140 ft per day. Geophysical-logging results were compared to 

geologic logs and water-quality results for analysis. Specific 

conductance data obtained from groundwater within specific 

screened intervals of monitoring wells were summarized to 

assess EM logs.

Acoustic televiewer (ATV) logs, which provide images 

of the borehole wall, were collected in a bedrock well at this 

site in 1985 as a part of a previous investigation of superfund 

sites (Bruce Hanson, formerly of the U.S. Geological Survey, 

written commun., 1985). The ATV log was interpreted in 

concert with EM and gamma logs. The methods of collection 

and analysis of ATV logs are described by Johnson and 

others (2005).
36 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

Bedrock outcrops

Direct current resistivity transect

 and identifier (DC_Lines_NHSPM)

Well and identifier

Seismic refraction line transect

 and identifier (RefractionLine),

 (Sevee and Maher, 1992) 

Ground penetrating radar transect 

 and identifier (GPR_Lines_NHSPM)

Electromagnetic survey points,

 Geophex, Ltd. model 2 (GEM-2)

EXPLANATION

GZ-6-3R

Whispering 

Pines Pond

Cohas Brook

Auburn Road

9

2

8

1

3

12

10

13

11

7.2

7.1

C-1

C-3

C-2

CB-1

MW-B

WPP-1

WPP-3

WPP-2

NUS-3

PZ-218

MW-202

MW-201

MW-204

NUS-12

NUS-2-2

MW-304B

MW-303B

MW-212B

MW-212A

MW-108B

MW-106B

MW-104B

MW-102B

GZ-1-3R

MW-303A

MW-304A

MW-102A

MW-104A

MW-106A

NUS-1-2

NUS-9-1

MW-302A

GZ-6-2R

GZ-6-3R

GZ-10-3R

GZ-10-3R

MW-302BR

71°21'15" 71°21'

42°56'15"

42°56'

Solid-waste landfill

Tire-pile 

landfill

Town-dymp

landfill

3

8

9 7

5

6

4

1

2

21

16

11

19

12

1

9

2

3

8

7

5

10

4

4.1

4.2

5.1

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

0 500 1,000 FEET

0 100 200 300 METERS

Figure 1–1. The locations of additional wells, outcrops, and geophysical surveys not specifically named elsewhere in this report, 

Auburn Road Landfill Superfund Site, Londonderry, New Hampshire.
Appendix 1 37

Table 1–2. Summary of borehole geophysical measurements at screened intervals, August 2011, Auburn Road Landfill Site, 

Londonderry, New Hampshire.

[Data in red are estimated screen intervals. EM, electromagnetic; K, hydraulic conductivity in feet per day (Sevee and Maher, 1992, 1993b); SC, specific conductance in microsiemens per square centimeter per day at 25 degrees Celsius]

Well identifier Geologic log 

available1

Well logged1 Adjacent well identifier 

or log data source

Screen, 

in feet

EM anomaly, 

in feet Contrast

EM Gamma Neutron Top Bottom Top Bottom

C-1 1 1 1 MW-212A 29.5 34.5 Metal?

C-2 1 1 1 46.3 56.3 22 36 High

C-3 1 18 28

GZ-9-4R 1 MW-202 40 48 38 48 High

GZ-10-3R MW-201 24.6 29.6 34 36 Low

GZ-1-2R GZ-1-3R 7.5 17.5

GZ-1-3R 1 1 1 1 GZ-1-2R 25 30

GZ-6-2R 1 1 1 1 GZ-6-3R 30.5 35.5 24 35 Medium

GZ-6-3R 1 1 1 1 GZ-6-2R 45 50 30 47 High

MW-102A 1 1 1 1 MW-102B 51.5 61.5 23 32 Medium

MW-102B 1 MW-102A 24 34 23 32 Medium

MW-104A 1 1 1 1 MW-104B 69.8 79.8 17 37 High

MW-104B 1 MW-104A 29 40 17 37 High

MW-106A 1 1 1 1 MW-106B 48.7 58.7 22 23 Low

MW-106B 1 MW-106A 17 27 22 23

MW-108A 1 1 1 1 MW-108B 62.9 72.9 27 33 Medium

MW-108B 1 MW-108A 48 51 27 33

MW-201 1 1 1 GZ-10-3R 65 75 34 36 Low

MW-202 1 1 1 GZ-9-4R 76 86 38 48 High

MW-202 53 59 High

MW-204 1 1 1 MW-B, NUS-9-1 88.9 98.9 17 25 Low

MW-212A 1 1 1 1 MW-212B, C-1 40 60 12 18 Low

MW-212B 1 C-1, MW-212A 12.2 22.2 12 18

MW-302A 1 1 1 MW-302BR 54.5 59.3 34 42 High

MW-302BR 1 1 1 MW-302A 36.5 41.5 32 42 High

MW-303A 1 1 1 MW-303B 32 37

MW-303B MW-303A 8.5 13.8

MW-304A 1 1 1 MW-304B 32.4 37.4

MW-304B MW-304A 10.6 14.6

MW-B 1 1 1 MW-204, NUS-9-1 55 65 56 59 High

NUS-12 1 1 1 1 23.3 33.3

NUS-1-2 1 1 1 1 37.5 47.5 38 40 Low

NUS-9-1 1 1 1 1 MW-B, MW-204 13 38

NUS-3 1 1 1 50 210
38 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

Table 1–2. Summary of borehole geophysical measurements at screened intervals, August 2011, Auburn Road Landfill Site, 

Londonderry, New Hampshire.—Continued

[Data in red are estimated screen intervals. EM, electromagnetic; K, hydraulic conductivity in feet per day (Sevee and Maher, 1992, 1993b); SC, specific conductance in microsiemens per square centimeter per day at 25 degrees Celsius]

Well 

identifier

Well log 

inferred2

Overburden 

K, 

in feet 

per day

Bedrock 

K, 

in feet 

per day

Type of 

geologic 

material

Average over screen interval SC 

spring 

2011

SC 

fall 

2011

SC 

fall 

2010 EM Gamma Neutron

C-1 1 Bedrock 7.2 83.4 4,657 406 323 419

C-2 Bedrock 10.1 108.1 2,605

C-3 Overburden 15.2 Blocked Blocked

GZ-9-4R 62 Overburden

GZ-10-3R 1 45 Overburden 14.7 111.8 991

GZ-1-2R 1 Overburden 13.7 108.7 851 343 286 392

GZ-1-3R 0.43 Bedrock 9.1 213.7 4,761 372 322 363

GZ-6-2R 1 27 Overburden 20.2 75.8 517 207 298 205

GZ-6-3R 21 Overburden 17.6 122.7 1,331 394 500 476

MW-102A 0.13 Bedrock Obstructed Obstructed Obstructed

MW-102B 1 140 Overburden 15.5 81.7 751

MW-104A 0.028 Bedrock 9.9 91.2 3,187

MW-104B 1 0.074 Overburden 18.6 95.5 1,010 316 409 391

MW-106A 0.1 Bedrock 11.4 357.3 4,111 353

MW-106B 1 1.1 Overburden 15.8 143.5 907 255

MW-108A 0.034 Bedrock 12.9 193.2 3,441 414

MW-108B 1 140 Overburden 14 201.5 1,278 258

MW-201 0.012 Bedrock 16.4 245.2 3,363

MW-202 0.034 Bedrock 11.3 101 3,014

MW-202

MW-204 0.026 Bedrock 10.8 102.7 2,837 367

MW-212A Bedrock 10.2 124 Not logged 334 412 312

MW-212B 1 Overburden Metal? 89.1 1,099 470 481 445

MW-302A Bedrock 8.7 Blocked Screened interval not logged468 622 618

MW-302BR Overburden 15.1 72.1 1,017 224 362 407

MW-303A Bedrock 13.3 79.6 3,485 225 271 218

MW-303B 1 Overburden 15.6 72.9 1,004 322 423 401

MW-304A Bedrock 9.6 87.1 5,390

MW-304B 1 Overburden Metal casing 146 869.6

MW-B Bedrock 16 113 1,430

NUS-12 Bedrock 7.9 Blocked Blocked

NUS-1-2 Overburden 15.5 92.9 1,114

NUS-9-1 1 Overburden 12.5 74.6 837 226

NUS-3 Bedrock 10.8 249 3,404

1

Where “1” is shown, logs are available. Blank cells indicate no logs are available.

2

For wells where no logs are available, logs are inferred from adjacent well (indicated with “1” in column). In addition, for certain wells with logs, logs are 

also inferred from adjacent wells (also indicated with “1”).
Appendix 1 39

References Cited

Abraham, Jared, Deszcz-Pan, Maria, Fitterman, David, and 

Burton, Bethany, 2006, Use of a handheld broadband 

FDEM induction system for deriving resistivity depth 

images, in Symposium on the Application of Geophysics to 

Engineering and Environmental Problems, 19, Las Vegas, 

Nevada, February 10–14, 2002, Proceedings: Denver, Colo., 

Environmental and Engineering Geophysical Society 

CD–ROM, p. 1782–1799.

Geophex, Ltd., 2007, DEM–2 broadband EMI sensor: 

Geophex, Ltd., accessed June 14, 2011, at http://www.

geophex.com/GEM_2%20Handheld.html.

Huang, Haoping, and Won, I.J., 2000, Conductivity and 

susceptibility mapping using broadband electromagnetic 

sensors: Journal of Environmental and Engineering Geo-

physics, v. 5, no. 4, p. 31–41.

Johnson, C.D., Joesten, P.K., and Mondazzi, R.A., 2005, 

Borehole-geophysical and hydraulic investigation of the 

fractured-rock aquifer near the University of Connecticut 

landfill, Storrs, Connecticut, 2000 to 2001: U.S. Geological 

Survey Water-Resources Investigations Report 03–4125, 

133 p.

Loke, M.H., 1999, Electrical imaging surveys for environmen-

tal and engineering studies—A practical guide to 2–D and 

3–D surveys: Stanford University, School of Earth Sciences, 

57 p., accessed October 2, 2009, at http://www.agiusa.

com/literature.shtml. (Also accessed February 15, 2013, 

at https://pangea.stanford.edu/research/groups/sfmf/docs/

DCResistivity_Notes2.pdf.)

Sevee and Maher Engineering, 1992, Pre-design investigation 

report for remediation of groundwater, Auburn road landfill 

site: Londonderry, N.H., Sevee and Maher Engineering, 

2 vols.

U.S. Geological Survey, variously dated, National field man-

ual for the collection of water-quality data: U.S. Geologi-

cal Survey Techniques of Water-Resources Investigations, 

book 9, chaps. A1–A9, accessed May 30, 2013, at 

http://pubs.water.usgs.gov/twri9A.

Won, I.J., Keiswetter, D.A., Fields, G.R.A., and Sutton, L.C., 

1996, GEM–2—A new multifrequency electromagnetic sen-

sor: Journal of Environmental and Engineering Geophysics, 

v. 1, no. 2, p. 129–137.

Zohdy, A.A.R., Eaton, G.P., and Mabey, D.R., 1974, Applica-

tion of surface geophysics to ground-water investigations: 

U.S. Geological Survey Techniques of Water-Resources 

Investigations, book 2, chap. D1, 86 p., accessed Febru-

ary 15, 2013, at http://pubs.usgs.gov/twri/twri2-d1/.
THIS PAGE INTENTIONALLY LEFT BLANK
Appendix 2 41

Appendix 2. Three-Dimensional Diagram Showing Locations of Wells, 

Bedrock-Surface Altitudes from Seismic Refraction Surveys, and Results from 

Electromagnetic and Direct-Current Resistivity Geophysical Surveys

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You must download and then open the file with Acrobat Pro or Acrobat Reader. 

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440

720

880

1,060

1,360

1,790

2,450

3,630

5,740

9,400

17,500

30,200

Resistivity, in ohm meters

160

EXPLANATION 
THIS PAGE INTENTIONALLY LEFT BLANK
Appendix 3 43

Appendix 3. Preliminary Bedrock-Surface Map
44 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

78

80

82

84

86

88

84

82

80

78

76

74

86

76

90

90

74

92

94

96

72

74

98

96

70

68

66

64

100

102

104

102

62

104

108

106

110

76

68

72

72

72

74

72

70

84

72

78

82

82

68

72

70

80

78

96

82

74

94

70

78

82

74

70

74

78

72 76

76

92

82

80

74

80

70

82

78

76

78

80

74

72

82

84 94

92

86

76

72

80

72

70

90

88

Base modified from United States Department of Agriculture

National Agriculture Imagery Program, 2008

Digital orthophoto quarter-quadrangle

Universal Transverse Mercator projection, zone 19

National Geodetic Vertical Datum of 1929

EXPLANATION

Wells

1991 seismic refraction

Direct current resistivity

Outcrops

Bedrock-surface altitude, 

in meters. Contour interval 

is 2 meters (6.6 feet) 

Flint Hill Fault

82

71°21'15" 71°21' 71°20'45"

42°56'15"

42°56'

42°55'45"

Solid-waste landfill

Tire-pile

landfill

Town-dump landfill

Former septage 

lagoon

Former gravel 

pit

Whispering

Pines Pond

Cohas Brook

Auburn Road

0 500 1,000 FEET

0 100 200 300 METERS

Figure 3–1. Bedrock-surface altitudes, outcrops, and seismic refraction and direct-current resistivity survey lines, Auburn Road 

Landfill Superfund Site, Londonderry, New Hampshire.
Appendix 4 45

Appendix 4. Geochemical Data Used for Redox-Zone Classification, Spring 2008

Table 4–1. Geochemical data used for redox-zone classification, spring 2008.

[Numbers in boldface in the first line of the table are threshold values. Wells shown on figure 1–1. Concentrations given as 0 were below the detection limit. 

mg/L, milligrams per liter; μg/L, micrograms per liter; O2, oxygen reduction; Fe(III), iron reduction; SO4, sulfate reduction; CH4, methanogenesis; Mn, manganese reduction; NO3

, nitrate reduction]

Well identifier Redox 

variables

Dissolved 

oxygen, 

in mg/L

Nitrate 

(as nitrogen), 

in mg/L

Manganese, 

in μg/L

Iron, 

in μg/L

Sulfate, 

in mg/L

General redox 

category Redox process Arsenic, in μg/L

1 0.5 50 100 0.5

A-31 6.54 0.26 94.6 8,810 7.6 Mixed O2-Fe(III)/SO4 0

A-33 0.23 0.22 1,610 25,300 6.5 Anoxic Fe(III)/SO4 68

A-46 9.65 0 0 0 0 Oxic O2 0

C-1 0.28 0 3,560 48,900 0 Anoxic CH4 236

GZ-10-3R 0.31 0.19 3,550 4,050 10.6 Anoxic Fe(III)/SO4 16.8

GZ-1-2 0.63 0 3,990 34,600 0 Anoxic CH4 33.9

GZ-1-3R 0.38 0.09 481 4,930 0 Anoxic CH4 28.7

GZ-6-2R 0.25 0.16 3,440 26,600 0 Anoxic CH4 205

GZ-6-3R 0.58 0.16 5,590 37,600 5.5 Anoxic Fe(III)/SO4 169

GZ-9-4R 2.04 0.28 4,300 29,500 6.6 Mixed O2-Fe(III)/SO4 54.6

MW-102A 3.33 0.12 0 0 14.7 Oxic O2 0

MW-102B 0.38 0 4,820 15,700 11.6 Anoxic Fe(III)/SO4 49.8

MW-104B 1.48 0.18 2,960 51,800 0 Mixed O2-CH4 115

MW-106A 7.03 0 93.8 0 12.9 Mixed O2-Mn(IV) 0

MW-106B 1.42 0.46 2,560 5,850 23.1 Mixed O2-Fe(III)/SO4 0

MW-108A 2.44 0 0 0 17.4 Oxic O2 0

MW-108B 0.4 0 3,660 1,890 15.7 Anoxic Fe(III)/SO4 0

MW-109A 0.7 0 0 151 13.2 Anoxic Fe(III)/SO4 0

MW-109B 7.14 6.3 15.8 0 6.9 Oxic O2 0

MW-201 2.23 0 174 101 16.3 Mixed O2-Fe(III)/SO4 0

MW-202 7.73 0.36 933 5,660 0 Mixed O2-CH4 0

MW-204 9.67 1.7 0 0 11.3 Oxic O2 0

MW-301A 4.29 0 0 0 14.8 Oxic O2 14.7

MW-302A 0.52 0.23 4,920 4,550 0 Anoxic CH4 20

MW-302B 0.59 1.8 669 361 11 Mixed (anoxic) NO3-Fe(III)/SO4 0

MW-303A 0.8 0 33.5 0 9.4 Suboxic Suboxic 0

MW-303B 0.5 0.2 3,450 23,100 10.8 Anoxic Fe(III)/SO4 24.1

MW-304A 5.91 0 0 0 16.2 Oxic O2 0

MW-304B 5.58 2.9 0 0 15.7 Oxic O2 0

NUS-1-2 1.31 0.22 4,040 23,800 10.6 Mixed O2-Fe(III)/SO4 206

NUS-2-2 0.6 0.18 1,870 11,400 9 Anoxic Fe(III)/SO4 99.2

NUS-9-1 9.4 4.2 0 0 8.3 Oxic O2 0

PZ-218 0.24 0.22 4,810 36,800 6 Anoxic Fe(III)/SO4 363
THIS PAGE INTENTIONALLY LEFT BLANK
Appendix 5 47

Appendix 5. Preliminary Spatial Distribution of Redox Zones, Water Table, and 

Flow Directions in the Glacial-Sediment Aquifer, Spring 2008
48 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

86

88

84

82

82

82

90

80

90

Towndump 

landfill

Tire-pile 

landfill

Solid-waste 

landfill

Former 

septage 

lagoon

Former 

gravel

pit

Auburn Road

Whispering

Pines Pond

Cohas Brook

Former 

gravel

pit

71°21'15" 71°21'

42°56'15"

42°56'

EXPLANATION

CH4gen

O2-CH4gen

Fe(III)/SO4

O2-Fe(III)/SO4

Mn(IV)

O2-Mn(IV)

O2

Suboxic 

Interpreted glacial aquifer 

water table, spring 2008. 

Two meter (6.6 feet) contour 

interval

82

Transmissivity greater than 

1,000 feet per day

Redox

Iron and(or) sulfate-reducing zone

Methanogenesis zone

See appendix figure 1-1 for well locations and identifiers.

Interpreted flow direction

0 500 1,000 FEET

0 100 200 300 METERS

Base modified from United States Department of Agriculture

National Agriculture Imagery Program, 2008

Digital orthophoto quarter-quadrangle

Universal Transverse Mercator projection, zone 19

National Geodetic Vertical Datum of 1929

Figure 5–1. Preliminary spatial distribution of redox zones, water table, and flow directions in the glacial-sediment aquifer, 

spring 2008.
Appendix 5 49

Table 5–1. Estimated saturated thickness of wastes and altitudes of the waste 

and water table, spring 2008.

[Altitude of bottom of waste from Sevee and Maher, 1993, figure 4–1. Estimate of altitude 

of spring 2008 preliminary potentiometric surface shown on the map in appendix 3. Vertical 

coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 

of 1929). Altitude, as used in this report, refers to distance above the vertical datum]

Piezometer 

identifier

Altitude, 

in feet Thickness of 

saturated waste, 

Bottom of waste in feet Estimate of spring 2008 

potentiometric surface

Landfill

Town dump

PZ-206 1274.0 270.0

A-26 276.0 277.2 1.2

PZ-212 1280.0 280.2 0.2

A-29 274.5 272.0

PZ-210 1269.0 269.0

PZ-209 277.0 274.0

PZ-208 276.0 275.6

2

270.0 2275.0 5.0

Tire pile

PZ-205 285.0 277.2

PZ-213 287.0 277.2

PZ-204 286.0 279.5

PZ-203 279.5 284.1 4.6

A-15 282.0 279.5

Solid waste landfill

PZ-214 303.0 300.9

PZ-202 307.0 301.2

PZ-207 307.0 298.9

A-12 305.0 308.4 3.4

PZ-201 NA 302.8

1

Altitude of bottom of waste from Weston, 2007.

2

Altitude of bottom of waste and water table from Weston, 2009a.
THIS PAGE INTENTIONALLY LEFT BLANK
Appendix 6 51

Appendix 6. Reactive-Transport Model

Concentrations of arsenic in groundwater at the Auburn 

Road Landfill Superfund Site were simulated by using the 

PHREEQC computer code (Parkhurst and Appelo, 1999). The 

general conditions tested and methods employed during the 

simulations are discussed in this appendix. Parameter sensitivity analysis is discussed in the main part of the report. 

Simulation results show that most of the dissolved 

arsenic in groundwater at the site is in the arsenic (III) form. 

Dissolved arsenic (V) concentrations were typically much 

lower (three orders of magnitude or less) than concentrations 

of arsenic (III). Recent (2008) data collected at the site show 

appreciable (20 percent of total) concentrations of arsenic (V) 

in the groundwater in certain wells (Weston Solutions, 2008 

and 2009), indicating the presence of a more complex dissolution process. 

Simulated results are presented for dissolved arsenic 

(III) concentrations (table 6–1). Graphs of dissolved arsenic 

(III) concentrations show similar patterns along the onedimensional transport path regardless of the simulation, 

but the time required to flush the arsenic varies. Several 

simulations tested the effects of various initial conditions and 

parameter values on model results.

All simulations started with an initial increase in arsenic concentration caused by a disequilibrium state between 

the specified aqueous and surface concentrations (this effect 

could be reduced by use of a more complete geochemical 

dataset, including information on the amount of sorbed arsenic 

on aquifer sediments, from the site). Reducing the period of 

disequilibrium is desirable because initial conditions can affect 

the final results. An example showing concentration increases 

(reported as dimensionless concentrations) at the beginning 

of the simulation is shown in figure 6–1. The initial starting 

arsenic (III) concentration increases by almost a factor of five 

before it declines to the initial concentration. This disequilibrium condition is partly caused by the uncertainty regarding 

the sorbed and aqueous concentrations, reaction rates, and 

mass of sorbed arsenic.

An initial reactive-transport model (base) was based on 

the assignment of likely parameter values and the objective 

of minimizing the early disequilibrium period. Groundwater 

transport was simulated through an iterative trial-and-error 

process. The initial static geochemistry representing leachateaffected groundwater was iteratively simulated as flowing 

through the aquifer for 10 to 20 years (called static geochemistry) prior to being flushed by clean water. The static geochemical conditions were then used as input to the model as 

the initial conditions for subsequent flushing simulations. The 

largest change from initial geochemical conditions was the 

decrease in pE (table 6–1). Redox potential expressed as pE 

is used widely in geochemical literature and is expressed as 

pE=Eh/0.0592 when Eh is in volts (Hem, 1985). The disequilibrium period was also reduced by decreasing the number of 

surface-complexation sites by one-half of an order of magnitude overall and by one-quarter of the value from the simulation shown in figure 6–1. The initial estimate of the number 

of surface-complexation sites was based on data from the 

Saco landfill study in Maine (Stollenwerk and Colman, 2004). 

Concentrations during the initial simulations returned to their 

initial values after 25 percent of the simulation period had 

passed (fig. 6–1). In the base model, this duration was reduced 

to less than 10 percent.

Equations 6–1 through 6–5 describe the modeled surfacecomplexation reactions. Equilibrium constants (log k) are 

higher for arsenate (AsV) in equation (6–2) than arsenite 

(AsIII) in equation (6–1) because arsenite is more readily dissolved from surfaces.

Surf_wOH + H3AsO3 = Surf_wH2AsO3 + H2O (6–1)

log_k 5.41

Surf_wOH + AsO4

-3 + 3H+ = Surf_wH2

AsO4 + H2O (6–2)

log_k 29.31

Surf_wOH + AsO4

-3 + 2H+ = Surf_wHAsO4-

 + H2O (6–3)

log_k 23.51

Surf_wOH + AsO4

-3 = Surf_wOHAsO4-3 (6–4)

log_k 10.58

SurfbOH + Toc = SurfbOHToc (6–5)

log_k1.90
52 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire Table 6–1. Aqueous geochemistry of uncontaminated groundwater, contaminated groundwater specified in the initial simulation shown in figure 6–1, and contaminated groundwater calculated by the model from simulations of static contamination. [Units for chemical entries are in micromoles per liter. temp-C, temperature in degrees Celsius; USGS, U.S. Geological Survey; OFR, Open-File Report; TOC, total organic carbon] Concentrations in uncontaminated groundwater Comments Concentrations in contaminated groundwater Comments Concentrations in contaminated groundwater Comments Physical properties pH 6.5 MW101b (fall 2007) 6.7 GZ-1-2 (fall 2007) 6.7 From model output of equilibrium conditions pE 0.3 MW101b (fall 2007) 0 GZ-1-2 (fall 2007) -2.9 From model output temp-C 15.1 MW101b (fall 2007) 11 Estimated 11 Estimated Chemical constituents Calcium 299 5USGS OFR 89–390, well LRW68 875 8Saco Landfill data 875 8Saco Landfill data Magnesium 58 6Fall 1995 (allmereged.xls data) 325 8Saco Landfill data 325 8Saco Landfill data Sodium 309 5USGS OFR 89–390, well LRW68 1,260 8Saco Landfill data 1,260 8Saco Landfill data Iron -- 68 2GZ6-2R (fall 2006) 68 2GZ6-2R (fall 2006) Manganese -- 2.9 Estimated mean 2.9 Estimated mean Silicon 356 5USGS OFR 89–390, well LRW68 356 5USGS OFR 89–390, well LRW68 356 5USGS OFR 89–390, well LRW68 Chloride 153 7Spring 2008, table 21, 2008 163 7Spring 2008, table 21, 2008 2,892 2From Weston 2006 Sulfur 78 2Fall 2006, table 4–1 78 No change from uncontaminated 78 No change from uncontaminated Carbon 459 5USGS OFR 89–390, well LRW68 3,015 2Fall 2006, table 4–1 3,015 2Fall 2006, table 4–1 Ammonium 29 4Sevee and Maher Engineering, 1993 1,238 4SME (1993) 1,238 4SME (1993) Nitrogen 5 2Fall 2006, table 4–1 0.9 2Fall 2006, table 4–1 0 From model output Oxygen 294 3MW101b (fall 2007) 19 2Fall 2006, table 4–1 0 From model output TOC 16 3MW101b (fall 2007) 108 3GZ6-2R (fall 2007) 32 From model output Arsenic (III) 0 2.5 3GZ6-2R, table 3–7, 2007 1.5 From model output Well data from:

1Weston Solutions, Inc., 2006, 2005 annual report, Long Term Environmental Monitoring Program at the Auburn Road Landfill Site, Londonderry, New Hampshire, February 17.2Weston Solutions, Inc., 2007, 2006 annual report, Long Term Environmental Monitoring Program at the Auburn Road Landfill Site, Londonderry, New Hampshire, March 2.3Weston Solutions, Inc., 2008, 2007 revised final annual report, Long Term Environmental Monitoring Program at the Auburn Road Landfill Site, Londonderry, New Hampshire, May 30.4Sevee and Maher Engineering, 1993, Supplement II investigation report, supplemental pre-design investigation for remediation of groundwater, volumes I and II, Auburn Road landfill site, Londonderry, New 

Hampshire, August 1993. Includes March 4, 1994 updated solute transport model simulations.

5Flanagan, S.M., and Stekl, P.J., 1990, Geohydrologic, ground-water-quality, and streamflow data for the stratified-drift aquifers in the lower Merrimack and coastal river basins, southeastern New Hampshire: 

U.S. Geological Survey Open-File Report 89–390, 130 p., 3 pls. (Also available at http://pubs.water.usgs.gov/ofr89-390.)

6Sevee and Maher Engineering, 2000, 1999 annual report, environmental monitoring program, Auburn Road superfund site: Londonderry, Sevee and Maher Engineering, variously paginated.7Weston Solutions, Inc., 2009a, 2008 annual report, long-term environmental monitoring program Auburn Road Landfill site Londonderry, New Hampshire: Weston Solutions, Inc., variously paginated.8Ken Stollenwerk, U.S. Geological Survey, written commun., 2012.
Appendix 6 53

0.0

0.0 0.2 0.4

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Ratio of computed arsenic(III) concentration to 

 initial assigned for arsenic(III)

0.6 0.8 1.0 1.2

Ratio of total time

Figure 6–1. Decrease of the dimensionless arsenic concentration with time in a simulation by the reactive-transport 

model of the Auburn Landfill Superfund site, Londonderry, New Hampshire.

References Cited

Hem, J.D., 1985, Study and interpretation of the chemical 

characteristics of natural water (3d ed.): U.S. Geological 

Survey Water-Supply Paper 2254, 263 p., 3 pls. (Reprinted 

in 1986 and 1989.) (Also available online at http://pubs.

usgs.gov/wsp/wsp2254/.)

HydroQual, Inc., 2009, Results of model simulations predicted 

arsenic cleanup time at the Auburn Road landfill, 

Londonderry, New Hampshire: Mahwah, N.J., HydroQual, 

Inc., 69 p.

Parkhurst, D.L., and Appelo, C.A.J., 1999, User’s guide to 

PHREEQC (version 2)—A computer program for speciation, batch-reaction, one-dimensional transport, and inverse 

geochemical calculations: U.S. Geological Survey WaterResources Investigations Report 99–4259, 312 p., accessed 

February 15, 2013, at http://pubs.usgs.gov/wri/1999/4259/

report.pdf.

Stollenwerk, K.G., and Colman, J.A., 2004, Natural remediation of arsenic contaminated ground water associated 

with landfill leachate: U.S. Geological Survey Fact Sheet 

2004–6057, 4 p., accessed February 15, 2013, at http://pubs.

usgs.gov/fs/2004/3057/.

Weston Solutions, Inc., 2009, 2008 annual report, long-term 

environmental monitoring program Auburn Road Landfill 

site Londonderry, New Hampshire: Weston Solutions, Inc., 

variously paginated.
THIS PAGE INTENTIONALLY LEFT BLANK
Appendix 7 55

Appendix 7. Piezometer Installation and Results of Analysis of Groundwater 

Samples, 2012

Three minipiezometers were installed to determine 

groundwater heads, vertical gradients between the groundwater and surface water, and the quality of groundwater 

beneath discharge points to surface water. All piezometers 

show upward gradients, which are indicative of groundwater 

discharging to surface water. Wells WPP-1, WPP-2, and 

CB-1 were installed where iron oxide was observed on the 

bottom sediment. A fourth piezometer (WPP-3) targeted 

a conductive anomaly (possible leachate) as identified in 

surface geophysical surveys. The final depth of WPP-3 fell 

short of the anomaly because of a subsurface feature that 

obstructed penetration.

Results

Data from Table 7–1 indicate that

• Deuterium-oxygen-18 data indicate that sampled water 

represents primarily groundwater with little surfacewater recharge.

• Concentrations of methane correlated with those of 

arsenic. This correlation is a good indicator of reductive dissolution (Harte and others, 2012).

• Higher CO2 concentrations at WPP-2 with lower 

calculated recharge temperatures indicate that the 

sampled water was mixed from shallower and deeper 

flow paths.

• Isotopically heavier inorganic carbon-13 corresponds 

to arsenic detections, indicating groundwater that is 

derived from deeper flow paths.

Reference Cited

Harte, P.T., Ayotte, J.D., Hoffman, Andrew, Révész, K.M., 

Belaval, Marcel, Lamb, Steven, and Böhlke, J.K., 2012, 

Heterogeneous redox conditions, arsenic mobility, and 

groundwater flow in a fractured-rock aquifer near a waste 

repository site in New Hampshire, USA: Hydrogeology 

Journal, v. 20, no. 6, p. 1189–1201, accessed February 

15, 2013, at http://link.springer.com/article/10.1007%

2Fs10040-012-0844-4.
56 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

Table 7–1. Laboratory analysis of groundwater samples from piezometers installed in 2012.

[All concentrations are given in milligrams per liter (mg/L) except for recharge temperature and delta notation. Recharge temperature is given in degrees 

Celsius (°C); shaded recharge temperatures indicate estimates; repeated rows are duplicate samples; WPP-1DUP is a duplicate quality control sample taken 

from piezometer WPP-1; ND, not detected; CH4, methane dissolved; CO2, carbon dioxide dissolved; N2, nitrogen dissolved; O2, oxygen dissolved; Ar, argon dissolved; δ, ratio of the isotope in the sample to a known standard; 15N, nitrogen isotope; 18O, oxygen isotope; 13C, carbon isotope; ‰, Per mil (parts per thousand)]

Chemical species or property CB-1 WPP-1 WPP-2 WPP-3 WPP-1DUP Detection limit

Dissolved arsenic 0.0786 0.0059 0.0012 ND 0.0059 0.001

Total organic carbon 1.8 1.4 1.3 0.67 1.3 0.5

Alkalinity 84.8 68 119 62.8 69.2 1

Chloride 33 42 42 47 43 3

Bromide ND ND ND ND ND 0.1

Sulfate (as SO4) 7.4 5.5 11 8.9 5.5 1

Fluoride ND ND ND ND ND 0.2

Nitrite-nitrogen ND ND -- ND ND 0.05

Nitrate-nitrogen ND ND -- 0.34 ND 0.05

Boron 0.114 0.164 0.062 ND 0.159 0.05

Calcium 23.6 18.1 33.9 20.3 18.2 1

Iron 25.5 34.8 0.497 0.289 35.5 0.25

Magnesium 5.33 2.66 5.92 3.46 2.67 0.1

Manganese 6.42 0.682 3.49 0.228 0.683 0.01

Sodium 25.3 28.8 30 31.1 28.8 1

Arsenic 0.0705 0.006 0.0015 ND 0.0061 0.001

Arsenic III 0.0755 0.0058 0.0014 ND 0.0058 0.001

Arsenic V 0 0.0002 0.0001 ND 0.0003 0.001

CH4 1.93 2.53 0.0935 0.0200 2.50

CO2 42.4 58.5 104 53.6 58.5

N2 19.8 20.6 23.4 19.3467 20.8

O2 0.242 0.2606 0.2839 0.2456 0.271

Ar 0.692 0.6930 0.7706 0.6797 0.696

Recharge temperature, in °C 12.3 11.9 8.4 12.9 11.8

1

δ15N ‰ 8.92

1

δ18O ‰ 3.2

2

δ13C ‰ -10 -11.0 -19.7 -17.5 -10.5

1

From nitrate.

2

From dissolved inorganic carbon.

3

From dissolved organic carbon.
Appendix 7 57

Table 7–2. Construction data for drive-point piezometers.[The length of the drive point is 16 inches, the screened length is 9 inches, the center of the screen is 8 inches below the top of the drive point or bottom of steel pipe. Well OTID, Well other identifier; GWSI ID, Groundwater Site Inventory identifier; Local ID, local identifier; TIC, top of inner casing; TOC, top of outer casing; bls, below land surface; amsl, above mean sea level. Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD of 1929). Altitude, as used in this report, refers to distance above the vertical datum]

Well 

OTID GWSI ID Local ID

Well 

diameter, 

in inches

Inner 

casing, 

in inches

Length of 

1-inch steel 

outer casing, 

in feet

Stick up 

TIC to land 

surface, 

in inches

Screen 

height from 

bottom, 

in inches

Distance between TOC 

and TIC, 

in inches

Well 

depth, 

in feet

Depth to 

screen 

top, 

in feet

Depth to 

screen 

bottom, 

in feet

Measuring 

point

Measuring-point 

altitude, 

in feet

Landsurface 

altitude, 

in feet

Targeted 

well 

depth, 

in feet bls

WPP1 425607071211401 NH-LRW 941 1 5/8 10 35 4 3.375 8.6 7.5 8.3 TIC 275.37 272.45 8WPP2 425608071211701 NH-LRW 939 1 5/8 10 32.75 4 3.625 8.8 7.7 8.5 TIC 272.94 270.21 8WPP3 425609071211701 NH-LRW 940 1 5/8 22.9 63.5 4 3.5 19.2 18.2 19.0 TIC 273.18 267.89 28CB1 425618071211501 NH-LRW 942 1 5/8 10 36 4 3.5 8.5 7.5 8.2 TIC 262.03 259.03 8
58 Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire

Table 7–3. Water-quality parameters for groundwater samples collected from piezometers installed in 2012.

[NTU, nephelometric turbidity units; DTW, depth to water, in feet; MP, measuring point; LS, land surface]

Well identifier CB-1 WPP-1 WPP-2 WPP-3 Units

Specific conductance 311 389 372 309 Microsiemens per centimeter

Temperature 14.7 12.1 12.2 13.7 Degrees Celsius

Dissolved oxygen1 1.2 0.4 0.8 0.8 Milligrams per liter

Dissolved oxygen2 0.1 0.1 0.1 0.2 Milligrams per liter

pH 6.2 6.3 6.1 6.3

Turbidity 298 22 0.43 4 NTU

Ferrous iron3 3.2 4.2 0.9 0.5 Micrograms per liter

Ferrous iron2 0.7 Micrograms per liter

DTW in well 3.63 2.95 2.92 3.51 Feet

DTW outside well 3.98 Feet

MP stick up from LS 3.89 3 3.08 Feet

1

Measured in flow through cell.

2

Measured by colormetric analysis.

3

Measured by photometric analysis.
Prepared by the Pembroke Publishing Service Center.

For more information concerning this report, contact:

Office Chief

U.S. Geological Survey

New England Water Science Center 

New Hampshire-Vermont Office

331 Commerce Way, Suite 2

Pembroke, NH 03275

dc_nh@usgs.gov

or visit our Web site at:

http://nh.water.usgs.gov
Degnan and Harte—Hydrogeologic Framework and Arsenic Distribution at the Auburn Road Landfill Superfund Site, New Hampshire—SIR 2013–5123