Field Study of the Fate of 

Arsenic, Lead, and Zinc at the 

Ground-Water/Surface-Water 

Interface

EPA/600/R-05/161

December 2005

Field Study of the Fate of Arsenic, Lead, 

and Zinc at the Ground-Water/

Surface-Water Interface

Robert G. Ford, Richard T. Wilkin,

Cynthia J. Paul, Frank Beck, Jr., and Tony Lee 

U.S. Environmental Protection Agency 

Office of Research and Development 

National Risk Management Research Laboratory

Ground Water and Ecosystems Restoration Division

Ada, Oklahoma 74820

Kirk G. Scheckel

U. S. Environmental Protection Agency

Office of Research and Development

National Risk Management Research Laboratory

Land Remediation and Pollution Control Division

Cincinnati, Ohio 45268

Patrick Clark

U. S. Environmental Protection Agency

Office of Research and Development

National Risk Management Research Laboratory

Technology Transfer and Support Division

Cincinnati, Ohio 45268

National Risk Management Research Laboratory

Office of Research and Development

U.S. Environmental Protection Agency

Cincinnati, Ohio 45268
ii

Notice

The U. S. Environmental Protection Agency through its Office of Research and 

Development funded the research described herein. This report has been subjected 

to the Agency’s peer and administrative review and has been approved for publication 

as an EPA document. Mention of trade names or commercial products does not 

constitute endorsement or recommendation for use. All research projects making 

conclusions or recommendations based on environmental data and funded by the 

U.S. Environmental Protection Agency are required to participate in the Agency 

Quality Assurance Program. This project was conducted under an approved Quality 

Assurance Project Plan. The procedures specified in this plan were used without 

exception. Information on the plan and documentation of the quality assurance 

activities and results are available from the Principal Investigator.
iii

Foreword

The U.S. Environmental Protection Agency is charged by Congress with protecting the Nation’s land, air, 

and water resources. Under a mandate of national environmental laws, the Agency strives to formulate 

and implement actions leading to a compatible balance between human activities and the ability of natural 

systems to support and nurture life. To meet this mandate, EPA’s research program is providing data 

and technical support for solving environmental problems today and building a science knowledge base 

necessary to manage our ecological resources wisely, understand how pollutants affect our health, and 

prevent or reduce environmental risks in the future.

The National Risk Management Research Laboratory is the Agency’s center for investigation of technological and management approaches for preventing and reducing risks from pollution that threatens human 

health and the environment. The focus of the Laboratory’s research program is on methods and their 

cost-effectiveness for prevention and control of pollution to air, land, water, and subsurface resources; 

protection of water quality in public water systems; remediation of contaminated sites, sediments and 

ground water; prevention and control of indoor air pollution; and restoration of ecosystems. NRMRL 

collaborates with both public and private sector partners to foster technologies that reduce the cost of 

compliance and to anticipate emerging problems. NRMRL’s research provides solutions to environmental 

problems by: developing and promoting technologies that protect and improve the environment; advancing scientific and engineering information to support regulatory and policy decisions; and providing the 

technical support and information transfer to ensure implementation of environmental regulations and 

strategies at the national, state, and community levels.

This publication has been produced as part of the Laboratory’s strategic long-term research plan. It is 

published and made available by EPA’s Office of Research and Development to assist the user community and to link researchers with their clients. Characterization of contaminant transport across the 

ground-water/surface-water transition zone is an important component of risk characterization for sites 

that are a source of contaminants within an urban watershed. Defining the chemical and biological factors that control the fate of ground-water contaminants that are discharged into surface water is critical to 

the design of effective risk management strategies. This report summarizes findings from an extensive 

field investigation conducted to determine the fate of inorganic contaminants within the headwaters of an 

urban watershed and to support the design and evaluation of remedial strategies to mitigate exposure to 

contaminants in surface water and sediments.

Stephen G. Schmelling, Director

 Ground Water and Ecosystems Restoration Division

 National Risk Management Research Laboratory

v

Contents

Foreword .......� iii

Figures .........� vii

Tables ............� ix

Acknowledgments ......................................................................................................................... xi

Executive Summary .....................................................................................................................xiii

Chapter 1

Purpose .....� 1

Study Scope ..............................................................................................................................3

Chapter 2

Site Background ........................................................................................................................5

Study Approach ........................................................................................................................5

Site Characterization Methods ..................................................................................................9

Field Measurements ..........................................................................................................9

Ground Water .............................................................................................................9

Surface Water .............................................................................................................9

Laboratory Measurements ..............................................................................................10

Water Chemistry .......................................................................................................10

Sediments .................................................................................................................10

Sediment Chemistry and Mineralogy ..............................................................................10

Mineralogical Characterization ........................................................................................11

X-ray Absorption Spectroscopy .......................................................................................11

Arsenic XAS ..............................................................................................................11

Pb and Zn XAS .........................................................................................................11

Sediment Extraction Procedures .....................................................................................12

Surficial Sediments ...................................................................................................12

Buried Sediments .....................................................................................................12

Scanning Electron Microscopy-Energy Dispersive Spectroscopy ...................................12

Chapter 3

Site Hydrology .........................................................................................................................15

Media-Specific Contaminant Distributions ..............................................................................24

Ground Water ..................................................................................................................24

Surface Water ..................................................................................................................24

Sediments ........................................................................................................................27

Chemical Speciation of Metals in Sediments .........................................................................32

Sediment Extractions ......................................................................................................32

Surficial Sediments ...................................................................................................32

Buried Sediments .....................................................................................................33

Element Speciation by X-ray Absorption Spectroscopy ........................................................38

Scanning Electron Microscopy ...............................................................................................43

Chapter 4

Stability of Contaminants in HBHA Pond Sediments ..............................................................47

Monitoring Long-Term Behavior (or Performance) of HBHA Pond .........................................49

Relevance to Other Sites .......................................................................................................49

References ....� 51

Appendix A ....� 55

Appendix B ....� 57

Appendix C ...� 63

vii

Figures

Figure 1 Conceptual model of geochemical zones that play a dominant role in governing solid-liquid 

partitioning of arsenic (lead and zinc) across the GW/SW transition zone at the Industri-Plex 

Superfund Site. .........................................................................................................................2

Figure 2 Prominent land features within the Industri-Plex Superfund Site and adjacent to the HBHA 

Pond. ........� 6

Figure 3 (A) Arsenic (As), benzene (Bz), and toluene (Tl) concentrations (ppb) from temporary 

ground-water sampling locations as a function of depth below ground surface (feet) for the 

study site. .� 7

Figure 4 (A) Close-up view of permanent ground-water monitoring network; arrows show approximate 

centerline of arsenic and BTEX plumes discharging into HBHA Pond. ...................................8

Figure 5 (A) Patterns in surface water and ground-water flow budgets within the HBHA Pond as 

documented by Aurilio et al. (1994) and Wick et al. (2000). . ................................................16

Figure 6 Aerial distribution and time-dependent variability for total dissolved arsenic concentrations 

(ppb) detected in tubing wells established adjacent to the HBHA Pond. ...............................17

Figure 7 Aerial distribution and time-dependent variability for total dissolved zinc concentrations (ppb) 

detected in tubing wells established adjacent to the HBHA Pond. ........................................18

Figure 8 Aerial distribution and time-dependent variability for benzene concentrations (ppb) detected 

in tubing wells established adjacent to the HBHA Pond. ......................................................19

Figure 9 Vertical patterns in water quality within the northern part of the HBHA Pond on sampling 

dates in April 2000, August 2000, and April 2001. ................................................................20

Figure 10 Vertical patterns in water quality within the central part of the HBHA Pond on sampling 

dates in April 2000, August 2000, and April 2001. ................................................................21

Figure 11 Vertical patterns in water quality within the southern part of the HBHA Pond on sampling 

dates in April 2000, August 2000, and April 2001. ................................................................22 

Figure 12 Evidence of water column mixing within the HBHA Pond as a result of a large surface 

water flow event. . ....................................................................................................................23

Figure 13 The concentration distribution of (A) alkalinity, (B) sulfate, (C) ammonia-nitrogen, and 

(D) total organic carbon (TOC) and benzene in ground water. ..............................................25

Figure 14 The distribution of dissolved As, Zn, Fe, and total S in shallow ground water adjacent to 

the northern and eastern margins of the HBHA Pond. ..........................................................26

Figure 15 Temporal trends in water chemistry for the NML sampling station and adjacent ground-water 

monitoring locations (TW07 and TW02). ...............................................................................27

Figure 16 The distribution of arsenic (As) in sediments collected from the HBHA Pond. ......................28

Figure 17 The distribution of lead (Pb) and zinc (Zn) in sediments collected from the HBHA Pond. ....29

Figure 18 The distribution of chromium (Cr) and iron (Fe) in sediments collected from the HBHA 

Pond. . .....� 30

Figure 19 The distribution of sulfur (S) and organic carbon (OC) in sediments collected from the 

HBHA Pond. ..........................................................................................................................31
viii

Figure 20 The ratio of As, Fe, Pb, and Zn extracted by HCl for unoxidized and oxidized sediments 

collected from suboxic zones within the HBHA Pond. ............................................................37

Figure 21 (A-D) Comparison of the amount of As and Fe extracted by an ascorbate solution as a 

function of total Fe and S content in sediments 

(oxic = open symbols, suboxic = filled symbols). ...................................................................37

Figure 22 X-ray diffraction data showing the relative proportion of hematite and ferrihydrite in the 

<2 μm size fraction isolated from surficial sediment samples WI01, WI01-NEP, and 

WI02-NEP collected near the north-northwestern margin of the HBHA Pond. ....................38

Figure 23 Locations of a selection of sediments collected from oxic and suboxic zones within the 

HBHA Pond. ..........................................................................................................................40

Figure 24 XANES data for (A) a series of reference compounds and (B) a selection of sediments 

collected within the HBHA Pond. ...........................................................................................41

Figure 25 XANES data for (A) a series of reference compounds and (B) a selection of sediments 

collected within the HBHA Pond. ..........................................................................................41

Figure 26 Aerial distribution of contaminant sediment speciation within the HBHA Pond based on 

LCF-XANES analysis; contour lines delineate depth to sediment (meters). ........................42

Figure 27 Characterization of the mineralogy of the clay-sized sediment fraction and zinc speciation 

in the silt-sized fraction of surficial sediment sample WI02-NEP before and after incubation 

for a period of 2.5 years. .......................................................................................................43

Figure 28 Representative compositional spectra for samples imaged using SEM-EDS; 

UP = unidentified peak. ...........................................................................................................45

Figure 29 Image of a pyrite framboid observed in a surficial (oxic) sediment 

(WI02-NEP, clay-sized fraction). ..............................................................................................45

Figure 30 Images of iron (hydr)oxide precipitates collected near the chemocline within the water 

column of the HBHA Pond (PS-3). ..........................................................................................46

Figure 31 Illustration of the apparent geochemical processes controlling solid-solution partitioning 

of arsenic, lead, and zinc within the HBHA Pond. .................................................................48

Figure C.1 Location of XANES and EXAFS regions of an XAS spectrum. ............................................64

Figure C.2 Aerial photo and architectural diagram of the Advanced Photon Source at Argonne 

National Laboratory, Chicago, IL. ............................................................................................65

Figure C.3 Experimental configuration for transmission data collection. ..................................................65

Figure C.4 Experimental configuration for fluorescence data collection. ..................................................65

Figure C.5 Standard raw XAFS spectra illustrating the three regions: (A) pre-edge, (B) edge step, 

and (C) EXAFS. ....................................................................................................................66

Figure C.6 Solid sample sandwiched between pieces of Kapton tape. ....................................................67

Figure C.7 Sample backfilled into opening of a Teflon block and secured with tape. ...............................67

Figure C.8 Autosampler template for multi-sample analysis. ....................................................................67

Figure C.9 Fluorescence data collection of metal hyperaccumulation in plant leaves. ............................68

Figure C.10 Raw fluorescence data Pb sorption on ferrihydrite. ................................................................68

Figure C.11 Background corrected spectra of Figure C.10. .......................................................................69

Figure C.12 k-space conversion of spectra in Figure C.11. ........................................................................69

Figure C.13 The k3-weighted χ-function of Figure C.12. ...........................................................................70

Figure C.14 Radial distribution (or structure) function of Fourier transformed k-space data for 

Figure C.13. .............................................................................................................................70

Figure C.15 ATOMS input file for magnetoplumbite listing crystallographic information. ...........................71
ix

Figure C.16 FEFF file used to determine fitting paths for EXAFS modeling showing the interaction of 

a central Pb atom (Pb1) with two different oxygen atoms (O5 and O3) and three different 

iron atoms (Fe2, Fe5, and Fe4). .............................................................................................71

Figure C.17 Structural data derived from ab-initio calculated fitting paths for Pb sorption on 

ferrihydrite. .............................................................................................................................72

Figure C.18 XANES spectra (thin line) and derivative of XANES spectra (thick line) for aqueous Zn2+

(blue) and ZnS (red). ...............................................................................................................73

Figure C.19 XANES spectra for arsenite [As(III) - blue] and arsenate [As(V) - red]. .................................73

Figure C.20 Linear combination fitting of X-ray absorption near edge spectroscopy data (LCF-XANES) 

for a sediment (Sediment1) sample with multiple Zn species. .............................................74

Tables

Table 1 Percent of Total Element Released by Selected Wet Chemical Extraction Tests. ................35

Table 2 Percent of Total Element Released by Selected Wet Chemical Extraction Tests. ................36

Table 3 Atomic Percentages of Elements in 5 Samples Determined by SEM-EDS. .........................44

Table A.1 Analytical Methods, Detection Limits, Precision, and Accuracy for Measurement of 

Aqueous Chemistry ................................................................................................................55

Table A.2 Analytical Methods, Detection Limits, Precision, and Accuracy for Measurement of Solid 

Phase Chemistry ....................................................................................................................56

Table B.1 Concentrations of Selected Elements in Halls Brook Holding Area Pond Core NC01

 ................� 57

Table B.2 Concentrations of Selected Elements in Halls Brook Holding Area Pond Core CC02

 ................� 58

Table B.3 Concentrations of Selected Elements in Halls Brook Holding Area Pond Core SC02

 ................� 58

Table B.4 Concentrations of Selected Elements in Halls Brook Holding Area Pond Grab Sediment 

Samples ..� 59

Table B.5 Concentrations of Selected Elements in Halls Brook Holding Area Pond Cores ...................60

Table B.6 Results from LCF-XANES Fits of the Pb XANES Data Collected for Sediments from 

Suboxic and Oxic Zones within the HBHA Pond ....................................................................61

Table B.7 Results from LCF-XANES Fits of the Zn XANES Data Collected for Sediments from 

Suboxic and Oxic Zones within the HBHA Pond ....................................................................62

xi

Acknowledgments

Joseph LeMay (U.S. EPA-Region 1, Boston, MA), Thomas Holdsworth (U.S. EPA/ORD-Cincinnati), and 

Tim Bridges (U.S. EPA-Region 1 Laboratory, Chelmsford, MA) provided valuable assistance and guidance 

during field sampling. Ning Xu, Sandra Saye, and Jihua Hong provided analytical support for determination of metals and arsenic speciation; Lynda Pennington, Kelly Bates, and Brad Scroggins provided 

analytical support for determination of aqueous carbon, nitrogen species and major anions (Contracts 

#68-C-98-138 and #68-C-03-097). Becky Butler (U.S. EPA/ORD-Ada) along with Martha Williams and 

Trina Perry (Contract #68-W-01-032) assisted with editing and formatting for publication. The authors 

greatly appreciate support provided by the staff of DND-CAT and PNC-CAT. DND-CAT is supported by 

the E.I. DuPont de Nemours & Co., The Dow Chemical Company, the U.S. National Science Foundation 

through Grant DMR-9304725, and the State of Illinois through the Department of Commerce and the 

Board of Higher Education Grant IBHE HECA NWU 96. PNC-CAT is supported by the U.S. Department 

of Energy, Basic Energy Sciences, under Contract DE-FG03-97ER45628, the University of Washington, 

and grants from the Natural Sciences and Engineering Research Council of Canada. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Basic Energy Sciences, Office 

of Energy Research under Contract W-31-102-Eng-38. This document benefited significantly from critical and constructive reviews from Christopher Impellitteri (U.S. EPA), Janet Hering (California Institute of 

Technology), Madeline Schreiber (Virginia Polytechnic Institute & State University), and Sonia Nagorski 

(University of Alaska Southeast). 

xiii

Executive Summary

Physical and chemical interactions between adjacent ground-water and surface-water bodies are important factors impacting water budget and contaminant transport within a watershed. These interactions 

are also of importance for hazardous waste site cleanup within the United States, since about 75% of 

sites regulated under the Resource Conservation and Recovery Act (RCRA) and the Comprehensive 

Environmental Response, Compensation, and Liability Act (CERCLA, or Superfund) are located within a 

half mile of a surface water body. The boundary between adjacent ground-water and surface-water bodies is referred to as the ground-water/surface-water (GW/SW) transition zone. The transition zone plays 

a critical role in governing contaminant exchange and transformation during water exchange between the 

two water bodies. The transition zone is host to a wide diversity of aquatic organisms, and it also can 

serve as a sink for contaminants transported in surface water or ground water. Ultimately, the potential 

for human exposure within a watershed and the health of the ecosystem inhabiting the transition zone will 

depend on the bioavailability of accumulated contaminants. The extent of contaminant bioavailability will, 

in part, be dictated by partitioning reactions that control the distribution and speciation of contaminants 

within water and sediments in the GW/SW transition zone.

The purpose of this document is to illustrate some of the chemical processes that govern contaminant 

transport and speciation during water exchange across the GW/SW transition zone. The focus of this 

document is the assessment of metal speciation transformations in contaminated sediments. Results 

from a field investigation of the fate of arsenic, lead, and zinc transported across the GW/SW transition 

zone at a contaminated site are presented in order to illustrate the importance of using a site conceptual 

model and to provide an example of approaches that may be used to characterize the spatial and temporal distributions of inorganic contaminant speciation. The field site described in this report is located 

immediately downgradient from the Industri-Plex Superfund Site in Woburn, MA and is characterized 

by a ground-water contaminant plume that discharges into the Halls Brook Holding Area (HBHA) Pond 

resulting in contamination of surface water and sediments. 

The results from this field investigation provide insight into the source of inorganic contaminants within the 

HBHA Pond and present a conceptual framework relative to the design of strategies to mitigate human 

exposure to site-derived contaminants. Spatial and temporal trends in ground-water data indicate that 

arsenic and zinc observed within surface water and sediments of the HBHA Pond are primarily derived from 

continuing ground-water discharge. In contrast, lead observed in sediments appears either to be derived 

from historical discharges or is currently derived from sediment transport or soil erosion from upgradient 

source areas. In addition, the vertical distribution and temporal patterns in arsenic concentrations within 

the water column of the HBHA Pond indicate that sediment dissolution/desorption processes contribute 

to the overall dissolved concentration of this contaminant. The fate of these inorganic contaminants is 

coupled to the fate of iron and sulfate derived from ground-water discharge. Iron (hydr)oxides are actively 

produced in oxic portions of the HBHA Pond, while iron sulfides are produced in suboxic/anoxic portions 

of the HBHA Pond. The generation of iron (hydr)oxides is a result of oxidation and precipitation of ferrous 

iron upon contact with oxygen within oxic portions of the HBHA Pond. The generation of iron sulfides 

is tied to microbial sulfate reduction coupled with degradation of anthropogenic and naturally occurring 

dissolved organic compounds in discharging ground water within suboxic/anoxic portions of the HBHA 

Pond. These newly formed precipitate phases possess significant sorption capacity for arsenic, lead, 

and zinc. The retention of these solids within deeper portions of the HBHA Pond water column and/or 

sediments helps to mitigate downgradient transport of these freshly deposited contaminants through the 

watershed. Analytical data are presented to define the spatial distribution and chemical speciation of 

arsenic, lead, and zinc within HBHA Pond sediments relative to the spatial distribution of predominant 

redox processes throughout the pond. 
xiv

While the observed distributions of arsenic, lead, and zinc in ground water, sediments, and surface water 

described in this report are ultimately unique to site-specific characteristics, some observed patterns in 

contaminant geochemistry are likely relevant to other sites of contamination. Specifically, the results of 

this study may have application to sites where an anoxic iron-rich ground-water plume encounters an 

oxygenated environment (e.g., anoxic landfill leachates or organic contaminant plumes). The results of 

this intensive field investigation provide useful information on technical approaches to characterize inorganic contaminant transport through subsurface redox gradients that are frequently established at sites 

with co-occurring organic and inorganic contaminant plumes. As demonstrated for this particular site, the 

ultimate fate of inorganic contaminants will depend on both site-specific characteristics (e.g., hydrology 

and soil/sediment mineralogy) and the chemical properties of the contaminant in question.
1

Chapter 1

Purpose

Physical and chemical interactions between adjacent ground-water and surface-water bodies are important factors impacting water budget and nutrient/contaminant transport within a watershed (Winter et al., 1998). These interactions are 

also of importance for hazardous waste site cleanup within the United States, since about 75% of sites regulated under 

the Resource Conservation and Recovery Act (RCRA) and the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA, or Superfund) are located within a half mile of a surface water body (Tomassoni, 2000; 

Biksey and Gross, 2001). The boundary between adjacent ground-water and surface-water bodies is referred to as the 

ground-water/surface-water (GW/SW) transition zone. The transition zone plays a critical role in governing contaminant 

exchange and transformation during water exchange between the two water bodies. The transition zone is host to a 

wide diversity of aquatic organisms, and it also can serve as a sink for contaminants transported in surface water or 

ground water. Ultimately, the health of the ecosystem inhabiting the transition zone will depend on the bioavailability of 

accumulated contaminants. The extent of contaminant bioavailability will, in part, be dictated by partitioning reactions 

that control the distribution and speciation of contaminants within water and sediments in the GW/SW transition zone.

The purpose of this document is to illustrate some of the chemical processes that govern contaminant transport and 

speciation during water exchange across the GW/SW transition zone. The focus of this document is the assessment 

of metal speciation transformations in contaminated sediments. Results from a field investigation of the fate of arsenic, 

lead, and zinc transported across the GW/SW transition zone at a contaminated site are presented in order to illustrate the importance of using a site conceptual model and to provide an example of approaches that may be used to 

characterize the spatial and temporal distributions of inorganic contaminant speciation. The field site described in this 

report is characterized by a ground-water contaminant plume that discharges into a pond resulting in contamination of 

surface water and sediments. While the observed distributions of arsenic, lead, and zinc in ground water, sediments, 

and surface water described in this report are ultimately unique to site-specific characteristics, some observed patterns 

in contaminant geochemistry are likely relevant to other sites of contamination. Specifically, the results of this study 

may have application to sites where an anoxic iron-rich ground-water plume encounters an oxygenated environment 

(e.g., anoxic landfill leachates or contaminant plumes).

Assessment of the factors controlling contaminant transport and distribution between interacting ground-water and 

surface-water bodies will be guided by the conceptual model that delineates the relevant hydrologic and biogeochemical 

processes. Typically, a conceptual model is developed based on site-specific data, which is subsequently revised in an 

iterative fashion with continued accumulation of data that document relevant processes active on site. The complexity 

of the conceptual model and the extent of required refinement or revision will be dictated by site heterogeneity and 

process variability in time and space. 

In a companion publication, a site conceptual model was developed for the GW/SW transition zone at the Industri-Plex 

Superfund Site largely based on temporal measurements of surface water chemistry adjacent to the region of contaminated ground-water discharge within the Halls Brook Holding Area (HBHA) Pond (see Figures 6 and 11 in Ford, 

2005; http://www.epa.gov/ahaazvuc/pubs/rschbrief.html). In general, three geochemical zones that played an important 

role in governing the partitioning of arsenic between liquid and solids across the GW/SW transition zone were identified within the proposed site conceptual model as depicted below in Figure 1. The emphasis of the initial report was 

directed towards illustration of the importance of surface water hydrologic dynamics on observed temporal variations in 

contaminant concentrations in a setting where contaminated ground water was suspected as the primary contaminant 

source. The spatial and temporal dynamics observed for arsenic concentrations within the HBHA Pond water column 

were best explained by considering joint contributions from both contaminated ground-water discharge and sediment 

dissolution/desorption reactions. However, the characteristics of arsenic partitioning to sediments within the HBHA 

Pond were not specifically explored in that document. Additional data are provided in this report to provide greater 

understanding of the characteristics of sediments deposited within the HBHA Pond and the processes that control the 

solid-phase partitioning of arsenic, lead, and zinc.
2

The issue of contaminated sediments within the HBHA Pond is important relative to the desire to reduce contaminant 

exposure and potential health risks. While elimination of the ground-water plume would benefit site restoration, this 

action alone would not eliminate all risks associated with historical contamination due to the recalcitrance of inorganic 

contaminants within the HBHA Pond. Based on the simplified conceptual model of the geochemical processes controlling the fate of arsenic, lead, and zinc shown in Figure 1, three key questions were posed relative to the information 

needed to characterize the site: 

1) What is the chemical composition of the ground water discharging into the HBHA Pond and how does this 

vary spatially and temporally?

2) What are the specific processes that result in the partitioning of arsenic, lead, and zinc to solids that are 

deposited within the HBHA Pond?

3) What is the stability of these solids (and their associated contaminants) and what physicochemical processes 

can disturb the functionality of the HBHA Pond relative to the downgradient migration of contaminants?

Site characterization data on the temporal and spatial distributions of dissolved arsenic in the northern portion of the 

HBHA Pond and the upgradient aquifer were reported by Ford (2005). This information provided partial answers to 

the three questions posed above. Specifically, water chemistry data indicated that: 1) nearly continuous stratification 

(i.e., development of a stable chemocline) within the HBHA Pond caused by discharge of contaminated ground water 

with elevated dissolved solids content lead to the development of an internal redox process that controlled arsenic and 

iron precipitation-dissolution reactions, 2) the concentration of dissolved arsenic within the water column of the HBHA 

Pond was controlled by the balance between ferrous iron oxidation coupled with precipitation of iron (hydr)oxides near 

the chemocline and the reductive dissolution of the newly formed iron (hydr)oxides as they settled through the anoxic 

hypolimnion, and 3) the apparently stable stratification within the HBHA Pond could be disrupted by an exceptionally 

high surface water influx, although the chemocline was re-established through time and the arsenic-iron redox cycle 

restored. However, the results documented by Ford (2005) provided limited detail of the chemical composition of ground 

water, and they provided no specific insight into the extent and characteristics of arsenic (lead and zinc) partitioning 

to sediments for dissolved and particulate contaminants accumulated within the hypolimnion. Accordingly, the objectives of this report are to elaborate on the physicochemical processes controlling arsenic, lead, and zinc partitioning to 

sediments within the HBHA Pond and to evaluate the stability of sediment-associated contaminants along with overall 

performance of the HBHA Pond in mitigating contaminant migration.

Figure 1 Conceptual model of geochemical zones that play a dominant role in governing solid-liquid partitioning of 

arsenic (lead and zinc) across the GW/SW transition zone at the Industri-Plex Superfund Site.
3

Study Scope

As will be discussed later, the fate of arsenic, lead, and zinc is tied to the interaction between ground water and surface 

water within the HBHA Pond. Sediments within the HBHA Pond play a critical role in the biogeochemical linkage of 

these two water sources. These sediments originate from two sources: 1) mineral precipitates derived from discharging 

ground water, e.g., due to ferrous iron oxidation and precipitation of iron (hydr)oxides, and 2) deposition of particulates 

derived from terrestrial sources, e.g., biotic sources within the water column and along the margin of the HBHA Pond 

and eroded soils. Several studies have been conducted to examine the fate of arsenic and other metals across the 

sediment-water interface within the water column of lakes and reservoirs (e.g., Aggett and O’Brien, 1985; Aggett and 

Kriegman, 1988; Seyler and Martin, 1989; Balistrieri et al., 1994; Spliethoff et al., 1995; Sohrin et al., 1997; Kneebone 

et al., 2002; Martin and Pederson, 2002; Senn and Hemond, 2002). In many of these studies, it has been shown that 

the fate of arsenic is tied to the chemical cycling of iron. Iron may be continuously recycled within the water body for 

systems in which anoxia develops within the hypolimnion or underlying sediments. Reduced iron generated in the 

lower regions of a lake can diffuse upward within the water column and re-precipitate upon contact with oxygen or other 

oxidants (e.g., Davison et al., 1982; Sholkovitz and Copland, 1982). Dissolved arsenic within the water column can 

partition to the newly-formed iron (hydr)oxides either through coprecipitation at the time of formation or adsorption to 

settling iron (hydr)oxide particles (Seyler and Martin, 1989; Balistrieri et al., 1994; Spliethoff et al., 1995; Sohrin et al., 

1997; Senn and Hemond, 2002). Arsenic partitioned to the settling iron (hydr)oxides can subsequently be remobilized 

during reductive dissolution within the hypolimnion or following sedimentation depending on the spatial distribution of 

reducing conditions within the lake system. This ‘natural’ biogeochemical cycle may overlap with hydrologically-driven 

contaminant fluxes across the GW/SW transition zone, which can cause misleading interpretations of the environmental 

risk that is directly attributable to discharge of contaminated ground water (See ‘Summary and Conclusions’ in Sholkovitz 

and Copland, 1982.).

The purpose of this study was to evaluate the fate of arsenic, lead, and zinc within the HBHA Pond system immediately 

downgradient from the Industri-Plex Superfund Site, which hosts contaminated soils and ground water resulting from 

industrial activities dating back to the mid-1800s (Davis et al., 1994; Aurilio et al., 1995; Wick and Gschwend, 1998). 

The fate of arsenic and other inorganic contaminants in this system is coupled to the chemical cycling of iron, sulfur, 

and carbon due to the characteristics of the ground-water plume discharging into the surface water system. The results 

of field measurements to ascertain the spatial and temporal variations of the geochemistry within the GW/SW transition 

zone and the chemical speciation of arsenic, lead, and zinc across redox gradients within HBHA Pond sediments are 

examined below to identify the factors controlling the mobility of these contaminants.
4
5

Chapter 2

Site Background

The Aberjona Watershed is an industrialized 65-km2 watershed located in northeastern Massachusetts. The watershed 

boundary lies within the towns of Reading, Woburn, Winchester, and Medford. The Aberjona Watershed extends to the 

Mystic Lakes, which drain into the Boston Harbor via the Mystic River. Historic industrial activities have resulted in 

contamination within the northern extent of the watershed. Specifically, activities at the Industri-Plex Superfund Site, 

a 245-acre industrial park located in Woburn, Massachusetts, resulted in the deposition of inorganic and organic contaminants in soils, sediments, and ground water. In response to public health concerns, a Record of Decision (ROD) 

for the Industri-Plex Superfund Site was signed in 1986, addressing on-site soil, sediment, and hot spot ground water 

contamination.

Contamination at the Industri-Plex Superfund Site was a result of historical industrial activity. From the mid-1800s to 

the 1930s, leather tanning was the dominant industry in Woburn, and the Aberjona River and its tributaries served 

as the main conduits for tannery wastewater (Durant et al., 1990). During the same period, starting in the late 19th 

century, sulfuric acid and arsenical pesticide manufacturing took place near the headwaters of the watershed in an 

area now designated as the Industri-Plex Superfund Site. Estimates suggest that 270 metric tons of arsenic may still 

exist within the site boundaries (Aurilio, 1995). By-products such as unused portions of animal hides from tanning and 

glue-manufacturing operations from 1934-1969 were disposed on site. It is hypothesized that leached degradation 

products from these materials have contributed to the mobilization of arsenic via the ground water from source areas 

to a wetland that serves as a source of surface water to the Aberjona River (Davis et al., 1994). Ground water that, 

in part, originates from within the boundaries of the Industri-Plex Superfund Site discharges into a pond known as the 

Halls Brook Holding Area (HBHA) Pond. This surface water feature was constructed to serve as a hydraulic retention 

basin to mitigate flooding during periods of peak surface water discharge. Sources of surface water to the HBHA Pond 

include Halls Brook, a perennial stream located on the western edge of the pond, and an intermittent stream (Atlantic 

Avenue Drainway) that conveys water from an upgradient wetland and stormwater runoff from Atlantic Avenue and 

nearby parking facilities. The HBHA Pond is located in the northern-most portion of a wetland area referred to as the 

Halls Brook Holding Area. An aerial view of the study location, including portions of the Industri-Plex Superfund Site 

(‘hide piles’) is provided in Figure 2.

The ground water beneath the Industri-Plex Superfund Site and the study area in North Woburn has been designated as 

a non-drinking water source area by the state of Massachusetts. This designation may influence selection of remedial 

alternatives to address the contaminated soils, ground water, surface water, and sediments. It is possible that most, if 

not all, of contaminated ground water discharges to the HBHA Pond down gradient from the Industri-Plex Superfund 

Site. If true, this pond may sequester or retard down gradient transport of arsenic, lead, and zinc. However, there is 

uncertainty as to the speciation of these contaminants in the various environmental media, interactions between the 

surface water and ground water, and stability of contaminated sediments within the HBHA Pond. A critical aspect to this 

site investigation was the resolution of these uncertainties prior to considering remedial action alternatives for contamination at the Industri-Plex Superfund Site. This report expands on the description of the contaminated ground-water 

plume originating from the Industri-Plex Superfund Site and the processes that control the fate of site contaminants as 

they are transported across the GW/SW transition zone (Ford, 2005). 

Study Approach

The field sampling activities and laboratory analyses performed under this study provide a current assessment of arsenic, lead, and zinc distribution in ground water, surface water, and sediments at the Industri-Plex Superfund Site and 

within the HBHA Pond. Site characterization was directed towards identifying the predominant chemical processes 

controlling arsenic, lead, and zinc migration to and sequestration within the HBHA Pond. Field-based sampling was 

carried out over a period of 35 months to assess time-dependent trends in contaminant mobility within the Industri-Plex 

Superfund Site and the HBHA Pond. The data derived from this effort provide a means for 1) assessing the long-term 

assimilative capacity within the HBHA Pond, and 2) the potential for future mobilization of arsenic, lead, and zinc partitioned to sediments.
6

Results from ground water, surface water, and sediment sampling and characterization are summarized and evaluated in the sections and appendices that follow. Collection of ground water, surface water, and sediment samples was 

completed during the following dates: October 13-21, 1999; November 30 - December 3, 1999; March 27 - April 6, 

2000; May 16-18, 2000; August 22-30, 2000; March 26 - April 14, 2001; May 11-17, 2001; September 10-21, 2001; and 

September 20-23, 2004. The study was initiated by defining the extent of the plumes of arsenic and BTEX compounds 

within the aquifer upgradient to the HBHA Pond. This was accomplished by collecting ground-water samples at temporary wells installed at discrete depths using direct-push equipment (Figure 3A). Patterns in water chemistry data 

collected from these sampling locations were then used to guide the installation of permanent wells that were used to 

monitor temporal trends in water chemistry adjacent to and within the HBHA Pond (Figure 3B). An effort was made to 

capture the vertical heterogeneity anticipated within the GW/SW transition zone by the installation of discrete sampling 

points at various locations (illustrated in Figure 4).

Figure 2 Prominent land features within the Industri-Plex Superfund Site and adjacent to the HBHA Pond. Land 

features labeled as ‘hide piles’ are suspected source terms for metal contaminants within the Industri-Plex 

Superfund Site. Image was derived from May 1995 aerial photograph obtained from MassGIS.
7

Figure 3 (A) Arsenic (As), benzene (Bz), and toluene (Tl) concentrations (ppb) from temporary ground-water sampling locations as a function of 

depth below ground surface (feet) for the study site. ND = not detected, BLQ = below limit of quantitation. Image was derived from May 1995 aerial photograph obtained from MassGIS. (B) Permanent ground-water monitoring locations established adjacent to the HBHA Pond. Images were derived from April 2001 aerial photograph obtained from MassGIS.
8

Figure 4 A) Close-up view of permanent ground-water monitoring network; arrows show approximate centerline of 

arsenic and BTEX plumes discharging into HBHA Pond. B) Illustration of the approach used to provide 

depth-resolved spatial coverage of the GW/SW transition zone adjacent to the HBHA Pond. Sampling intervals were completed with 6-inch stainless steel screens connected to 3/8-inch Teflon-lined polyethylene 

tubing, except for nylon membranes used for cells in diffusion sampler.
9

Site Characterization Methods

Water physicochemical parameters were measured in the field along with the collection of water and sediment samples 

during the sampling dates previously listed. The objective of this effort was to define the chemical structure of the 

system with particular attention being given to delineation of the GW/SW transition zone. Historical data had been collected as part of previous site characterization efforts to demonstrate a net decrease in arsenic (and other contaminant) 

concentrations in transitioning from the upgradient aquifer to the outlet of the HBHA Pond. However, limited detailed 

liquid and solid phase characterization data had been collected to aid in establishing the specific process(es) that 

controlled the transport of inorganic contaminants through the system (Aurilio et al., 1994; Davis et al., 1996; Ahmann 

et al., 1997). In particular, insufficient information was available to assess the influence of the seasonal dynamics of 

water flow and microbial processes on the geochemistry of inorganic contaminants within the HBHA Pond. Wick and 

others (Wick and Gschwend, 1998a; Wick and Gschwend, 1998b; Wick et al., 2000) had characterized the HBHA Pond 

system with respect to the fate of synthetic organic compounds that entered the pond via ground-water discharge, and 

this provided a solid basis from which to design a site characterization program. A range of geochemical data was 

collected to refine and support the general site conceptual model that had been developed to describe the function of 

the HBHA Pond relative to arsenic migration, i.e., the sequestration of arsenic, lead, and zinc discharged into the pond 

via partitioning to Fe-bearing minerals that were continuously deposited onto the sediment layer. Details of sample 

collection and characterization protocols are described below.

Field Measurements 

Ground Water 

The major and trace element chemistry and redox characteristics of ground water were assessed through field- and 

laboratory-based measurements on water samples collected from the aquifer immediately upgradient and below the 

HBHA Pond. In addition, the concentration distribution of dissolved contaminants was assessed to determine if there 

were interactions between inorganic and organic contaminants that may influence biogeochemical processes active 

within and adjacent to the GW/SW transition zone. Sufficient data were collected to define the overall redox chemistry 

within ground water and how this influenced the chemical speciation of arsenic prior to its discharge into the HBHA 

Pond. In general, this required measurements to determine the speciation of soluble carbon, iron, and sulfur including 

the spatial and temporal variability in these constituents.

Ground-water samples were extracted from Teflon-lined tubing wells equipped with 6-inch stainless steel screens that 

had been driven to depth using direct-push equipment. Samples of ground water were collected using a peristaltic 

pump through a closed flow cell following stabilization of pH, specific conductance, turbidity, oxidation-reduction potential 

(ORP), and dissolved oxygen (Puls and Paul, 1995). Filtered (0.45-μm, cellulose acetate membrane) ground-water 

samples were collected for laboratory analyses. The concentration of ferrous iron was determined by spectrophotometric detection of its complex with 1,10-phenanthroline at 510 nm (Hach, 1992a). The concentration of ferrous iron plus 

dithionite-reducible ferric iron (total iron) was also assessed by spectrophotometric detection of the ferrous iron complex 

with 1,10-phenanthroline at 510 nm following reduction with dithionite (Hach, 1992b). The alkalinity of water samples 

was determined by titration with standardized sulfuric acid to a colorimetric endpoint near pH 4.5.

Surface Water 

Depth profiling within the northern, central, and southern regions of the HBHA Pond was carried out at various times of 

the year to assess the chemical structure of the water column and to delineate the processes controlling contaminant 

partitioning between solution and solids that subsequently deposit onto the underlying sediments. General water chemistry measurements were conducted as a function of depth either by lowering an instrumented sonde down through the 

water column or by pumping water to the surface through an enclosed and instrumented flow cell (Figure 4B). The sonde 

or flow cell was instrumented to collect readings of temperature, dissolved oxygen, pH, specific conductance, and ORP 

(platinum electrode). Water samples were collected as a function of depth within the water column by pumping water 

to the surface for field determinations (ferrous iron and alkalinity) or laboratory measurements to determine the major 

and trace element chemistry. A permanent multi-screened sampling station (NML) was installed within the northern 

end of the HBHA Pond to facilitate multiple sampling rounds during or following storm events. As shown below and by 

the work of Wick and others (Wick and Gschwend, 1998a; Wick et al., 2000), depth-resolved measurements within the 

HBHA Pond water column were critical towards developing a clear understanding of the physicochemical processes 

that controlled the fate of contaminants discharged from the upgradient aquifer.

Depth-resolved surface water chemistry within the HBHA Pond was assessed using a YSI 6820 Multi-Parameter Water 

Quality Monitor to measure temperature, pH, specific conductance, ORP, and dissolved oxygen. Water samples were 

collected at each depth for determination of turbidity and dissolved/particulate concentrations of arsenic, iron, and 

organic carbon. Suspended solids were collected by in-line pressure filtration using a peristaltic pump from various 

depths within the HBHA Pond water column for mineralogical characterization.
10

Surveys of the distribution of specific conductance throughout the HBHA Pond were conducted prior to and following 

a storm event to understand the dynamics of physicochemical stratification throughout the water column. Continuous 

measurements of specific conductance as a function of depth were collected using a Geoprobe Direct Image® Electrical 

Conductivity System that had been adapted for deployment from a pontoon boat. Transects of specific conductance 

were developed by contouring data from multiple vertical profiles collected within northern and southern portions of the 

HBHA Pond. The locations of vertical profiles were recorded using a Trimble Navigation GeoExplorer GPS Receiver 

with post-processing correction of locational data. Data collection events occurred on April 5, 2001 and May 11, 2001 

following a major surface water flow event at the end of March 2001 (Ford, 2005).

Laboratory Measurements

Water Chemistry 

Dissolved metals in ground water were determined by ICP-OES or ICP-MS for filtered samples acidified to pH<2 with 

concentrated nitric acid (Fisher Optima). The concentration of benzene was determined by purge-and-trap gas chromatography for filtered samples acidified to pH<2 with concentrated sulfuric acid (Fisher Optima). Dissolved sulfate was 

determined by capillary electrophoresis with indirect UV detection (Standard Methods, 1999). Nitrate and ammonium 

were determined by flow injection analysis with a Lachat Quikchem 800 Analyzer (U.S. EPA, 1983; Methods 353.2 

and 350.1, respectively). Dissolved organic carbon was determined by UV catalyzed persulfate digestion with infrared 

detection of evolved CO2 (U.S. EPA, 1983; Method 415.2).

Dissolved (0.2-μm filter, nylon membrane) and total concentrations of metals in surface water samples were determined 

by ICP-OES or ICP-MS for samples acidified to pH<2 with concentrated nitric acid (Fisher Optima). Samples for particulate metals were microwave-digested in 10% v/v nitric acid prior to analysis. Dissolved arsenic speciation (0.2-μm filter, 

nylon membrane) was determined by ion chromatography-hydride generation-atomic fluorescence spectrophotometry 

(IC-HG-AFS) for samples acidified to pH<2 with concentrated hydrochloric acid (Optima) in amber polyethylene containers and stored at 4°C for a period of less than five days (McCleskey et al., 2004). Separate holding time studies using 

a representative range of site-specific water chemistries demonstrated that changes in inorganic arsenic speciation 

were less than the analytical error for a holding time of seven days or less (data not shown). The speciation method 

was optimized for detection of As(III), As(V), MMA(V), and DMA(V) using gradient elution with a phosphate mobile 

phase (Gomez-Ariza et al., 2000). 

Sediments 

The major element and contaminant chemistry was defined in sediments collected from various locations within the 

HBHA Pond during different times of the year. Locations that were deemed important included regions of obvious 

ground-water discharge, i.e., ground-water seeps evidenced by the precipitation of iron (hydr)oxides, as well as regions 

that represented varying stages along the redox gradient from shallow to deeper water levels within the pond. Shallow 

sediment grab samples were collected throughout the HBHA Pond to help define the chemical processes that controlled 

the distribution of contaminants across the sediment-water interface. In addition, sediment cores were collected to 

greater depth in order to define the depositional history and diagenetic processes that exert an influence on inorganic 

contaminants accumulated within the system. The predominant sediment mineralogy at various locations across the 

system redox gradient was assessed along with characterization of the chemical speciation of arsenic, lead, and zinc 

using chemical extraction tests as well as X-ray absorption spectroscopy on preserved sediment samples. Previous 

work had shown that iron monosulfides were a significant component in sediments deposited/formed within suboxic 

regions of the HBHA Pond (Wilkin and Ford, 2002). Results documented below demonstrated the predominance of iron 

(hydr)oxides in the reactive fraction of sediments deposited/formed within oxic regions of the HBHA Pond.

Sediment samples were collected in December 1999, April 2001, and September 2001 from the HBHA Pond. The greatest density of sampling points was along the center axis and along the northeast shore of the pond, where contaminated 

ground water discharges into the HBHA Pond. Sediment samples were retrieved from water depths ranging from 0.5 to 

4.5 m. Cores were collected using a 5-cm diameter piston-coring device. The cores were capped immediately after their 

recovery and kept upright before and during freezing. Surface sediments were collected into N2-purged plastic bags by 

pumping highly fluidized sediments to the surface using a peristaltic pump. Surface sediments and cores were frozen 

within 1 h of collection from the pond bottom. While still frozen, sediment cores were sub-sampled by cutting off 2- to 

5-cm thick sections, which were immediately bagged and kept frozen for subsequent solid-phase analyses. 

Sediment Chemistry and Mineralogy

Surficial (oxic) sediments were dried in air to constant mass prior to chemical measurements or mineralogical determinations. The <2 mm size fraction was isolated from these sediments by sieving prior to further analysis. Buried sediments collected from suboxic zones were thawed and dried at room temperature in an anaerobic glove box (96:4 v/v 

N2-H2 gas mixture). Excess pore water was squeezed out by hand and decanted from the sediments prior to drying. 
11

Dried samples were homogenized with an agate mortar and pestle and retained in the anaerobic glove box. Sample 

splits were removed from the glove box and allowed to oxidize in an oven at 60°C for 72 hours.

Methods used for determination of the elemental composition of sediments are listed in Appendix A along with method 

detection limits, analytical accuracy, and precision. All solid-phase concentrations are reported on a dry weight basis. Total 

carbon and carbonate carbon were determined on oven-dried samples using a UIC (Model CM5014) carbon coulometer 

system (Huffmann, 1977). Organic carbon content was derived from the difference between total carbon and inorganic 

carbon. For total carbon analysis, a 20-300 mg aliquot of powdered sediment was combusted at 950 °C. Inorganic 

carbon was analyzed as carbon dioxide released from a sample after reaction with hot 5% perchloric acid. Total sulfur 

was determined by mixing oven-dried sediment samples with vanadium pentoxide and combusting the mixture in the 

presence of high-purity oxygen-gas at 1050 °C (Atkin and Somerfield, 1994). Sulfur dioxide produced is quantitatively 

titrated using a sulfur coulometer (UIC Model CM5014S). Concentrations of metals and metalloids in the sediments 

were determined by microwave assisted digestion in 10% HNO3 (modified EPA Method 3051), followed by inductively 

coupled plasma optical emission spectrometry (ICP-OES, Perkin Elmer Optima 3300DV). Microwave digestion was 

carried out using a CEM Mars 5 system at a total pressure of 120 psi (8.5 bar) and maximum temperature of 170 °C. 

The reliability of this digestion method was continuously checked using international certified reference materials (NIST 

2710, NIST 2780, and CCRMP LKSD-1). Arsenic recovery from these reference materials, for example, ranged from 

88% to 97% of the certified values.

Mineralogical Characterization

Precipitate mineralogy was determined by powder X-ray diffraction (XRD) using a Rigaku MiniFlex diffractometer. XRD 

data were collected using Fe Kα radiation via continuous scan in 0.02 °2θ steps with a count time of 12 sec/step. The 

<2 μm size fraction of surficial sediment samples was isolated via centrifugation of aqueous suspensions. Air-dried 

samples were prepared as smear mounts with methanol on zero background quartz slides (typically 10-15 mg of sample). 

Mounted samples were allowed to air dry prior to data collection. External calibration using a smear mount of the NIST 

SRM 640b (silicon) was carried out on a regular basis to confirm goniometer angular position accuracy and to estimate 

peak position error due to sample displacement. Peak identification for crystalline phases was achieved by reference 

to the ICDD Powder Diffraction File database (JCPDS, 1993). Identification of 6-line and 2-line ferrihydrite was carried 

out by reference to data reported by Stanjek and Weidler (1992).

X-ray Absorption Spectroscopy

Arsenic XAS 

The oxidation state and bonding environment of arsenic associated with iron oxidation precipitates was examined using X-ray absorption near edge structure (XANES) spectroscopy. Arsenic K-edge spectra were collected at sectors 

20-BM (DuPont-Northwestern-Dow Collaborative Access Team; DND-CAT) and 5-BM (Pacific Northwest Consortium 

Collaborative Access Team; PNC-CAT) at the Advanced Photon Source, Argonne National Laboratory (Argonne, IL). 

Dried sediment samples were loaded into 1-mm-thick plastic sample holders and sealed with strips of Kapton tape. 

Absorption spectra were collected at the As K-edge (11,867 eV) in fluorescence mode using a 13-element solid-state 

Ge-detector. The synchrotron was operated at 7.0 GeV and at a nominal 100 mA fill current. The energy of a Si(220) 

double-crystal monochromator was calibrated using arsenic or gold foil. The monochromator step size was 0.25 eV 

per step in the XANES region (11845-11900 eV). Multiple scans were collected and summed for each sample (3 to 9). 

The XANES fluorescence data were normalized to the edge-jump height, and the K-edge inflection point was determined as the energy at the maximum in the first derivative of the normalized spectra. Mineral reference compounds 

were synthesized in the laboratory, including arsenate adsorbed to hematite (Ford, 2002), arsenite coprecipitated with 

siderite, and arsenite coprecipitated with mackinawite (Wilkin and Ford, 2002). All fluorescence spectra were collected 

at room temperature. For surficial (oxic) sediments and the arsenate-hematite reference sample, no attempt was made 

to exclude sample exposure to air. However, buried (suboxic) sediments and the reference samples prepared using 

arsenite were protected from oxygen exposure in an inert atmosphere prior to and during collection of fluorescence 

data in order to prevent sample oxidation. The relative percentages of As(V)-O, As(III)-O, and As-S species in the study 

samples were assessed by linear combination fitting (LCF) of XANES data employing WinXAS Version 3.1 (Ressler, 

1998) using reference spectra for As(V)-hematite, As(III)-siderite, and As(III) coprecipitated with FeS. The LCF-XANES 

analysis demonstrated that an orpiment-like phase (As2S3) was an insignificant component for all examined samples.

Pb and Zn XAS 

The sediment samples were analyzed for Pb and Zn by XANES spectroscopy to determine metal speciation parameters. 

XANES spectroscopic data were collected at beamline 20-BM (Pacific Northwest Consortium - Collaborative Access 

Team) at the Advanced Photon Source at Argonne National Laboratory, Argonne, IL. The electron storage ring operated 

at 7 GeV with a top-up fill mode. Three to five scans were collected at ambient temperature in fluorescence mode with a 

13-element Ge detector. A 0.5 mm premonochromator slit width and a Si(111) double crystal monochromator detuned 
12

by 20% to reject higher-order harmonics were employed. For XANES analyses, the sediment samples were loaded into 

Teflon sample holders and sealed with Kapton tape within an oxygen-free glovebox. The samples were transferred to 

the experimental hutch in a plastic bag which was purged with Ar during transfer and data collection to avoid oxidation 

from atmospheric oxygen. No beam-induced oxidation was evident. No background Pb or Zn was detected in the purge 

bag material or the Kapton tape. Energy was calibrated to the first inflection of ZnK (9.659 keV) or PbLII (15.200 keV) 

metal foil standards and was collected simultaneously with the spectrum of each sample for the respective metals. Due 

to the relative low concentration of Pb and high concentration of As in the samples, the PbLII edge energy was utilized 

for Pb data collection rather than the usual PbLIII edge energy of 13.055 keV. The PbLIII Lα1 fluorescence energy of X-ray 

emission is 10.552 keV which shouldered on the overwhelming AsK Kα1 emission energy peak at 10.544 keV. The collected scans for a particular sample were merged, the data were then normalized, and the background was removed 

by spline fitting using WinXAS 3.1 (Rehr and Ankudinov, 2001, 2003). The XANES spectra of Pb and Zn in sediment 

samples and references were then assembled in a format suitable for linear combination fitting (Beauchemin et al., 2002; 

Roberts et al., 2002; Scheinost et al., 2002; Scheckel and Ryan, 2004) of the X-ray absorption near edge spectroscopy 

(LCF-XANES) data to identify the major Pb and Zn species present in the sediment samples.

Sediment Extraction Procedures

Surficial Sediments 

A series of single-step chemical extractions were employed to assess the relative stability of metals associated with oxic 

sediments exposed at the surface near visible ground-water seeps along the north-northwestern margin of the HBHA 

Pond. The extraction steps employed in this procedure are documented below. All stock solutions were prepared using 

reagent grade chemicals and deionized water (Millipore® Milli-Q3RO/Milli-Q2 system). All glassware had been previously soaked in dilute HNO3

 and rinsed thoroughly with deionized water.

i) Weakly Adsorbed: A 20 ml aliquot of 1 M MgCl2 (Baker®) at pH 7.0 was added to 45 ml polyethylene centrifuge tubes containing either 0.1 gm solid for WI01-NEP, WI02, WI02-NEP, and WI04 or 1.0 gm for WI01. 

Samples were prepared in duplicate. The tubes were continuously shaken for 1 hr (Tessier et al., 1979) 

and centrifuged for 10 min at 15,000 RPM. The supernatant was carefully pipetted off and filtered through 

0.2 um Nuclepore® filter.

ii) Strongly Adsorbed: The residue from (i) was extracted with 19 ml 0.005 M NaH2PO4 (Sigma®) at both pH 4 

(Bartlett and James, 1988; Welch and Lico, 1998). Samples were shaken for 24 hours, and supernatants 

were collected in the same manner as described in step (i). 

iii) Amorphous Iron Oxides: The ascorbate solution for Step 3 was prepared following the method of Ferdelman (1988). Sodium citrate (0.2 M C6

H5Na3O7·2H2O) and sodium bicarbonate (0.6 M NaHCO3) were added 

to deionized water. The solution was then deaerated with nitrogen and ascorbic acid (0.4 M C6H8O6) was 

added with a resulting pH of 8.0. Sediment samples were extracted for 24 hours with 19 ml of the ascorbate 

solution, and the supernatants were collected in the same manner as described in step (i).

iv) Iron Sulfides: The final step in the sequential extraction scheme involved adding 19 ml 0.5 M HCl (Aldrich®) 

to the sediment samples and shaking for 1 hour (Kostka and Luther, 1994). Supernatants were collected 

in the same manner as described in step (i). 

Buried Sediments 

A series of single-step chemical extractions were performed to better constrain the solid-phase partitioning of arsenic, 

lead, and zinc in buried sediments collected from the bottom of the HBHA Pond. All solutions were deoxygenated prior 

to use, and extractions were carried out using unoxidized sediments in an anaerobic glove box. The following solutions 

were used: 0.5 M MgCl2, 0.1 M Na2CO3, 0.02 M ascorbic acid (buffered to pH 7 in 0.6 M NaHCO3), and 1 M HCl. Dilute 

hydrochloric acid extractions were conducted on both unoxidized and oxidized sediment samples. Supernatant solutions were filtered through 0.22-μm syringe filters and acidified with concentrated nitric acid. The analysis of metals in 

the solutions obtained following chemical extraction was carried out using ICP-OES.

All liquid extractant samples were acidified to pH < 2 using ultra-pure HNO3 and analyzed for the elements of interest 

(arsenic, iron, lead, zinc) by ICP-OES. Samples with low arsenic concentration were also analyzed using GFAAS. Quality assurance measures performed on these analyses included analytical duplicates, known analytical quality controls 

(AQCs), check standards, and blanks. Non-detect results were observed for all blank samples, and the results for all 

other QC samples met the required data quality requirements (Appendix A).

Scanning Electron Microscopy-Energy Dispersive Spectroscopy

Scanning Electron Microscopy was used to evaluate the morphology and spatial relationships among mineral precipitates for two surficial sediments, suspended solids collected adjacent to the chemocline within the HBHA Pond, and 
13

one buried sediment sample. In addition, energy dispersive X-ray spectroscopy (EDS) was conducted on polished and 

unpolished samples to determine the composition of sediments on a semi-quantitative basis. Samples for SEM/EDS 

analysis were stored in an anaerobic glove box and then embedded in an epoxy resin. The sample mounts (1” diameter 

round mounts) were ground and polished using diamond abrasives and coated with a thin layer of gold prior to being 

placed within the SEM sample chamber. The sample of suspended solids was the only unpolished sample analyzed for 

these studies. The grain mount procedure used for this sample is discussed below.

Secondary electron and back-scattered electron images were obtained using a JEOL 6360 SEM. The instrument was 

operated using an 18kV and 20 kV accelerating potential and a beam current of about 8.5 nA. Micrographs were obtained 

at a range of magnifications from 1500X to 10000X. Copper grids obtained from SPI Supplies (West Chester, PA) were 

used to verify quantitative lengths. EDS spectra were acquired using an OXFORD Instruments Model INCA300/GEM 

EDS Unit. Elemental concentrations were calculated using INCA software and cobalt metal as a standard reference 

material to insure semi-quantitative accuracy. 

Polished thin-sections were made from sediments that had previously been imbedded in resin. Thin-section samples 

were sonicated in methanol for 30 minutes prior to mounting, rinsed with RO water, and dried with an air-stream (canned 

air) before proceeding any further. After the drying process, each prepared thin-section was mounted to an aluminum 

stub using double-sided carbon tape, and then a strip of silver paint was applied from the aluminum stub to the top of 

the sample thin-section for conductivity purposes. The silver paint was dried thoroughly with a stream of air before the 

coating procedures were attempted. A 10-second gold sputter coat was applied to these samples in preparation for 

SEM analysis. One of the sediment thin-sections was treated differently in the sample preparation because a pyrite 

framboid was identified under an optical light microscope. The framboid could not be located with the SEM after the first 

sample preparation. The thin-section was stripped of the first 10-second gold coat with a methanol/sonication process 

for 30 minutes, rinsed with RO water, and then followed by a 2% potassium iodide/sonication process for 30 minutes. 

The thin-section sample was then rinsed with RO water, dried with a stream of air (canned air), mounted on an aluminum stub with double-sided carbon tape, two strips of silver paint were applied on opposite sides of the thin-section for 

conductivity reasons, and finally, a fresh 10-second gold coating was applied. By stripping the first gold coat, we were 

able to mark the site of interest so that the framboid could be more easily located in the SEM system.

The suspended solid sample collected from the southern portion of the HBHA Pond was stored in an anaerobic glovebox until ready for SEM/EDS sample preparation. A small sample (<1 mg) was suspended in 1 mL of methanol, sonicated for 10 minutes, agitated, and then sonicated a second time for 10 minutes. A carbon planchet was mounted on 

an aluminum stub with double-sided carbon tape, and then one drop of the suspension was placed on the top surface 

of the planchet. The sample was immediately placed into a dessicator for 2 days of vacuum drying. The sample was 

then removed from the dessicator, two strips of silver paint were applied (opposite sides) and dried immediately with an 

air stream (canned air) before the gold coating could be applied. The sample was then coated with gold for 10 seconds 

to achieve good conductivity for the SEM/EDS analyses.
14
15

Chapter 3

Site Hydrology

A portion of the ground-water plume discharges into the HBHA Pond that also receives surface water inputs from a 

perennial stream (Halls Brook) on its western edge and an intermittent runoff channel at its northwestern end (Figure 2). 

The HBHA Pond was built in the early 1970s for flood control. The HBHA Pond has a maximum depth of approximately 

5 meters (15 feet), and it discharges into a heavily vegetated wetland area. The HBHA Pond discharge reconstitutes 

Halls Brook, which meanders through the wetland until discharge into the Aberjona River. Independent estimates 

of water flux into the HBHA Pond indicate that approximately 7-60% of the water input is derived from ground water 

throughout the year (Aurilio et al., 1994; Wick et al. 2000). Trends in ground water and surface water inputs into the 

HBHA Pond documented by these authors are illustrated in Figure 5. Data shown in Figure 5A and Figure 5B indicate 

that the influx of ground water remains relatively constant, although the fraction of ground-water flow relative to the 

total system flow varies significantly as a function of surface water inputs. For example, during late January 1998, the 

relative fraction of ground-water flow varied from an average of approximately 60% down to 10% following an increase 

in surface-water flow (Figure 5B, see history of water flow in Aberjona River at USGS 01102500). Water level measurements in the HBHA Pond (this study) illustrate rapid fluctuations in water budget in this system in response to changes 

in surface-water influx (Figure 5C).

Based on salt balance calculations, Wick et al. (2000) have estimated that a contaminated ground-water plume (denoted 

‘Qhigh’ by the authors) contributes approximately 1- 9% of the water input into the HBHA Pond throughout the year (3-27% 

of ground-water input). Based on specific conductance measurements throughout the HBHA Pond, these authors 

concluded that the primary location of contaminated ground-water discharge was within the northern end of the lake. 

Independent measurements of ground water and lake water chemistry (including dissolved arsenic) from the current 

study support this finding and show that contaminants are primarily discharged within the north-northeastern portion of 

the pond (Figures 6-8; Pb below detection in ground water). However, arsenic contamination is derived from a larger 

portion of the adjacent ground-water aquifer than the hydrocarbon contaminants studied by Wick et al. (1998, 2000). 

Thus, their estimates of contaminated ground-water flux may be low with respect to the discharging arsenic plume. The 

distribution of arsenic and benzene as a function of depth for three ground-water monitoring locations proximate to the 

north-northeastern shoreline of the HBHA Pond (TW06, TW07, TW08) indicates that the hydrocarbon plume occurs 

over a smaller region than the arsenic plume. While the highest arsenic concentrations were encountered at location 

TW07, there were still substantial concentrations of arsenic in the vicinity of the hydrocarbon plume (TW08). Since 

additional hydrologic data were not collected during this study, there is insufficient information to properly constrain an 

estimate of the flux of arsenic derived from ground water. However, published data of changes in the vertical distribution of dissolved arsenic in the HBHA Pond water column at location NML demonstrate that observed concentrations 

of arsenic within this pond are derived from a combination of ground-water inputs and sediment dissolution processes 

(Ford, 2005).

An important component of the conceptual model developed thus far is the maintenance of a stable chemocline within 

the HBHA Pond that serves to regulate the redox cycling of Fe-bearing solids and associated contaminants. Thus, it 

was important to assess the relative stability of this physical aspect of the HBHA Pond water column during the period 

of study. A series of water quality measurements are shown in Figures 9-11 as a function of depth at north, central, 

and south sampling locations for three sampling dates. In general, the vertical trends in specific conductance, dissolved oxygen, and ORP document relatively uniform stratification (i.e., formation of chemocline) throughout the water 

column. Dissolved oxygen concentrations are low (limit of detection), and measured ORP is negative near the sediment-water interface. In some instances, a turbidity maximum was observed near the chemocline, which was attributed 

to iron (hydr)oxides precipitating as a result of ferrous iron oxidation. The chemocline was disrupted in the central and 

southern portions of the HBHA Pond during the April 2001 sampling date as indicated by a uniform profile for specific 

conductance as a function of depth (Figures 10 and 11). This was due to a significant increase in surface-water influx 
16

(Figure 5C), a fraction of which was contributed by Halls Brook. The chemocline was depressed to greater depth on 

this sampling date in the northern portion of the HBHA Pond (Figure 9). Maintenance of stratification in this portion 

of the pond is likely due to its proximity to the primary point of discharge of contaminated ground water with high dissolved solids. However, as shown in Figure 12 based on contours of specific conductance within the water column in 

the northern and southern portions of the HBHA Pond, the chemocline is re-established throughout the pond to essentially pre-storm conditions within a period of less than a month. Analysis of historical surface water flow records at 

the USGS Winchester Monitoring Station indicates that flow events of sufficient magnitude to disrupt the chemocline 

may occur at a frequency of about every four years (data not shown). Water quality measurements conducted as part 

of this study indicate that any disruption to the chemocline is relatively short-lived, which is consistent with historical 

records for the HBHA Pond (Wick et al., 2000).

Figure 5 (A) Patterns in surface water and ground-water flow budgets within the HBHA Pond as documented by 

Aurilio et al. (1994) and Wick et al. (2000). (B) Concurrent flow variations within the Aberjona River are also 

shown for the USGS 01102500 gaging station located approximately 2.5 miles south of the HBHA Pond 

(Winchester, MA). (C) Water level variations (blue columns) recorded within the HBHA Pond (this study) 

relative to concurrent flow variations observed at the USGS 01102500 gaging station.
17

Figure 6 Aerial distribution and time-dependent variability for total dissolved arsenic concentrations (ppb) detected 

in tubing wells established adjacent to the HBHA Pond. Depths are shown in feet below ground surface 

(ft bgs) and feet below water surface (ft bws) for tubing wells installed on land and within the HBHA Pond, 

respectively. Depths for tubing wells TW01, TW02, TW03, TW04, and TW05 were determined at the time 

of installation. NM = not measured, ND = not detected (<33 ppb). Image derived from May 1995 aerial 

photograph obtained from MassGIS.
18

Figure 7 Aerial distribution and time-dependent variability for total dissolved zinc concentrations (ppb) detected in 

tubing wells established adjacent to the HBHA Pond. Depths are shown in feet below ground surface (ft 

bgs) and feet below water surface (ft bws) for tubing wells installed on land and within the HBHA Pond, 

respectively. Depths for tubing wells TW01, TW02, TW03, TW04, and TW05 were determined at the time 

of installation. NM = not measured, ND = not detected (<14 ppb). Image derived from May 1995 aerial 

photograph obtained from MassGIS.
19

Figure 8 Aerial distribution and time-dependent variability for benzene concentrations (ppb) detected in tubing wells 

established adjacent to the HBHA Pond. Depths are shown in feet below ground surface (ft bgs) and feet 

below water surface (ft bws) for tubing wells installed on land and within the HBHA Pond, respectively. Depths 

for tubing wells TW01, TW02, TW03, TW04, and TW05 were determined at the time of installation. NM = 

not measured, ND = not detected (<1 ppb). Image derived from May 1995 aerial photograph obtained from 

MassGIS.
20

Figure 9 Vertical patterns in water quality within the northern part of the HBHA Pond on sampling dates in April 2000, 

August 2000, and April 2001. Symbols: T = temperature, DO = dissolved oxygen, ORP = oxidation-reduction 

potential, and Sp. Cnd. = specific conductance.
21

Figure 10 Vertical patterns in water quality within the central part of the HBHA Pond on sampling dates in April 2000, 

August 2000, and April 2001. Symbols: T = temperature, DO = dissolved oxygen, ORP = oxidation-reduction 

potential, and Sp. Cnd. = specific conductance.
22

Figure 11 Vertical patterns in water quality within the southern part of the HBHA Pond on sampling dates in April 2000, 

August 2000, and April 2001. Symbols: T = temperature, DO = dissolved oxygen, ORP = oxidation-reduction 

potential, and Sp. Cnd. = specific conductance. 
23

Figure 12 Evidence of water column mixing within the HBHA Pond as a result of a large surface water flow event. (A) Location of conductivity transects 

collected within the north and south portions of the HBHA Pond. Contour lines indicate depth to sediment (meters). (B) Specific conductance (μS cm-1) distribution immediately following (April 2001) and approximately one month (May 2001) after the large surface water flow event at the end of March 2001 (See Winchester flow data in Figure 5C.).
24

Media-Specific Contaminant Distributions

Ground Water

The general distributions of major ion and contaminant metal concentrations in ground water are depicted in Figures 13 

and 14. Patterns in the concentrations of alkalinity, sulfate, and ammonia-nitrogen between well pairs that span the 

GW/SW transition zone point to the stimulation of microbial activity within this region of the aquifer (specifically well 

pairs TW07-TW02 and TW08-TW01). There is a systematic decrease in the concentration of sulfate within ground 

water in moving from a location immediately upgradient to underneath the HBHA Pond (Figure 13B). This decrease 

is accompanied by concurrent increases in alkalinity and ammonia-nitrogen for these well pairs (Figure 13A and 13C). 

These trends appear to be confined to a region along the northeastern portion of the HBHA Pond. This region of the 

pond also corresponds to the location of the BTEX plume (Figure 13D). 

Patterns in inorganic contaminant concentrations within the shallow aquifer appear more complex. The highest concentrations of zinc observed within the upgradient aquifer occur along the northern margin of the HBHA Pond (Figure 14). 

For arsenic, hot spots within ground water occur along the northern margin and also the north-eastern side of the HBHA 

Pond in a similar region as the BTEX plume. The co-occurrence of elevated arsenic, zinc, and sulfate along the northern 

portion of the HBHA Pond points to upgradient oxidative weathering of sulfides as a possible source. This is consistent 

with historical records that document the operation of a sulfuric acid production facility within the Industri-Plex Superfund 

Site boundary (Aurilio et al., 1995). A common process used in the production of sulfuric acid involved burning pyrites 

to generate sulfur dioxide gas. The waste products from this process were disposed on site and pose a long-term 

source of iron, sulfate, and associated inorganic contaminants in shallow ground water (Spanish pyrites with up to 1% 

arsenic by weight were often utilized). The region of elevated arsenic along the north-eastern side of the HBHA Pond 

may be due to an additional buried source or the development of chemical conditions conducive to maintaining arsenic 

in a dissolved state (e.g., iron reduction). There is an apparent source of sulfate at depth at location TW08-3 that may 

contribute to the elevated concentration of this dissolved component at location TW01. However, sulfate concentrations 

are significantly lower at shallow depths within this part of the aquifer (See Site 5, TW08-1, and Site 6.).

In general, the highest concentrations of inorganic contaminants discharging from ground water appear to be confined 

to the north-northeastern shoreline of the HBHA Pond. While inorganic contaminant concentrations appear to be lower 

in the region of the BTEX plume, it is clear that this portion of the aquifer is exerting an influence on the chemistry within 

the northern region of the HBHA Pond. The extent of this influence will be examined in greater detail in the following 

section.

Surface Water

Patterns in surface water chemistry have been reviewed by Ford (2005) for the HBHA Pond. Briefly, the chemistry 

within the pond is controlled by the relative proportions of surface water and ground-water flows. Due to the physical 

configuration of the HBHA Pond and the differences in dissolved solids concentrations between the main surface water 

inflow (Halls Brook) and the ground-water plume, the pond generally remains stratified. This stratification, or chemocline, acts to limit vertical diffusion of dissolved constituents entering the system via ground-water discharge at depth. 

The barrier to vertical transport within the water column plays an important role in maintaining the redox cycling of iron 

and co-precipitated arsenic (and metals). In general, ferrous iron entering the system via ground-water discharge is 

precipitated within the oxic (shallow) portion of the water column, leading to sequestration of dissolved arsenic. The 

iron (hydr)oxides (poorly crystalline ferrihydrite, Ford et al., 2005) settle to the sediment layer and are subsequently dissolved in the reducing conditions that are established by the microbial degradation of natural and contaminant organic 

compounds that accumulate within the HBHA Pond. The mineralogical characteristics of the iron (hydr)oxide precipitates 

and their rate of formation (data not shown) are comparable to those of natural, poorly crystalline ferrihydrite formed 

in terrestrial systems (Schwertmann, and Fischer, 1973; Davison and Seed, 1983; Schwertmann and Murad, 1988; 

Perret et al., 2000)

However, this near-permanent stratification can be interrupted by a sufficient surface water flow event. Such an event 

occurred during the study period (end of March 2001) resulting in complete mixing within the water column at the central 

and southern portions of the HBHA Pond (i.e., below Halls Brook discharge). The northern portion of the pond remained 

partially stratified (chemocline suppressed in depth), since this is the primary location of discharge for the contaminant 

plume. As shown by Ford (2005), the stratification is re-established along with the steady-state redox chemistry that 

controls iron and arsenic cycling. Based on the ground-water characterization program during this study, it became 

apparent that two plumes discharge into the north-northeastern portion of the HBHA Pond. The northern plume is the 

primary source of arsenic and iron into the system. In contrast, the primary input of BTEX compounds is located along 

the northeastern margin of the pond. The input of sulfate is more widely distributed across these two areas of plume 

discharge (Figures 11 and 12). Assessment of the temporal variations of the HBHA Pond water chemistry as a function of depth confirms that two separate plumes intersect to discharge into the north-northeastern portion of the HBHA 
25

Figure 13 The concentration distribution of (A) alkalinity, (B) sulfate, (C) ammonia-nitrogen, and (D) total organic carbon (TOC) and benzene in ground water. The plotted concentrations represent an average of all temporal 

measurements for all depths at each sampling location for the period of study.
26

Pond. Water chemistry data are shown in Figure 15 to illustrate the relationships between surface water chemistry and 

discharging ground water in this portion of the pond. Samples were collected at various times following the large surface 

water flow event (March, 2001) at several vertical locations within the water column and the adjacent shallow aquifer. 

Comparison of the concentrations of dissolved constituents shown in Figure 15 for ground water and surface water 

reveals three general component groupings. The first group includes NH3-N, alkalinity, and sodium (also TOC, data not 

shown). For these components, there does not appear to be a relationship between the concentrations observed within 

the deeper portion of the HBHA Pond water column at the NML sampling station and the adjacent ground-water wells 

(TW07 series and TW02). This disparity can most likely be attributed to a separate ground-water source originating 

from the eastern margin of the HBHA Pond between the TW07 and TW08 well locations. The concentration of these 

components may also be influenced by microbial processes occurring within pond sediments. The second component 

grouping includes sulfate and zinc in that there are general similarities in depth-dependent concentrations within the 

HBHA Pond at location NML and adjacent upgradient ground water. It should also be noted that there is little fluctuation 

in zinc concentrations at this location within the water column immediately above the sediment-water interface. Thus, 

dissolved zinc that is derived from ground-water discharge along the northern margin of the HBHA Pond is efficiently 

sequestered within sediments. The third component grouping includes arsenic and iron, which appear to originate 

primarily from a northern ground-water source (TW07 and further upgradient). However, concentrations of arsenic and 

iron within the deeper portion of the HBHA Pond water column exceed those observed in adjacent upgradient ground 

water (TW07). Since other adjacent ground-water locations (TW08, TW01) showed much lower arsenic concentrations than those observed in the deeper portion of the HBHA Pond water column, this implies that an additional flux of 

arsenic is derived from shallow sediments. This may be due to the reducing conditions established by microbial ironand sulfate-reduction at the bottom of the HBHA Pond. Thus, while the primary source of arsenic discharging into the 

HBHA Pond appears to derive from the north, clearly ground-water inputs from the eastern BTEX plume influences the 

overall chemistry of the surface water within the northern portion of the pond. This is an important observation, since 

the BTEX plume (and associated degradable hydrocarbons) serves to stimulate microbial processes that likely influence 

iron and sulfur redox chemistry within the HBHA Pond.

Figure 14 The distribution of dissolved As, Zn, Fe, and total S in shallow ground water adjacent to the northern and 

eastern margins of the HBHA Pond. Highlighted regions show locations of elevated arsenic (red) and zinc 

(orange). Water samples were collected during the September 2004 field trip. Locations with ‘Site’ designation were temporary sampling points using a push-point sampler. Only data for samples collected near the 

water table are shown for locations TW07.
27

Figure 15 Temporal trends in water chemistry for the NML sampling station and adjacent ground-water monitoring 

locations (TW07 and TW02). NML data are shown as open black symbols connected with lines for multiple 

sampling dates during the period of April 5, 2001, to September 14, 2001; adjacent ground-water data for 

locations TW07-1, TW07-2, TW07-3, and TW02 are shown as open red symbols for multiple sampling dates 

during the period of April 6, 2000, to September 11, 2001. Arsenic data for the NML sampling station are 

from Ford (2005).

Sediments

Spatially-resolved concentration distributions for arsenic, lead, zinc, chromium, iron, sulfur, and organic carbon in the 

sediments of the HBHA Pond are shown in Figures 16-19. Raw concentration data are also tabulated in Appendix B. 

The highest concentrations of arsenic, chromium, and zinc were observed near the location of plume discharge and 

further downgradient within the HBHA Pond. In contrast, the highest lead concentrations are proximate to the location 

of the Atlantic Avenue Drainway discharge (Figure 2), which may be attributed to erosion or runoff from areas within 

the Industri-Plex Superfund Site. Because sediment concentration maxima for arsenic, chromium, and zinc coincide 

with regions where contaminated ground water discharges into the pond, it is reasonable to suspect that ground water 

transport coupled with metal deposition near the ground water-surface water interface is a primary mechanism controlling metal concentrations in the pond sediments. This pattern is particularly evident for sulfur, arsenic, and zinc 

where the highest concentrations of these elements appear to be associated with the area of plume discharge in the 

north-northeastern region of the HBHA Pond. Analysis of sediment cores showed that the highest concentrations of 

arsenic, lead, and zinc were associated with sediments that have been deposited following construction of the HBHA 

Pond (Appendix B). The underlying sandy sediments possess concentrations of carbon, iron, and sulfur that are lower 

by an order-of-magnitude or more.

A comparatively more even concentration distribution is observed for organic carbon (Figure 19). This pattern is likely the 

result of multiple sources of carbon to the pond, including ground water discharge, carbon input from Halls Brook, and 

seasonal deposition of plant materials and woody debris. High concentrations of organic carbon appear to be clustered 

in areas of low relief at the bottom of the pond suggesting that sedimentary deposition processes play an important role 

in regulating concentrations of organic carbon. With the exception of the locations of visible ground-water seeps along 

the north-northeastern margin of the HBHA Pond, iron is also distributed uniformly in shallow sediments throughout 

the pond (Figure 18, typically 8-12 wt%). The lack of iron concentration gradients in the pond sediments may be tied 
28

to bottom-water redox conditions that are characteristically iron-reducing. That is, iron is highly soluble and mobile in 

deep bottom-waters of the HBHA Pond; consequently, there are no focused loci of iron accumulation. The highest sediment concentrations of iron are found in red-colored, shallow sediments (<1 m) over which waters are typically oxidizing 

(>2 mg/L dissolved O2). Under oxidizing conditions, iron is highly insoluble, and iron would be expected to accumulate 

in the surface sediments, especially in areas that receive discharge of reducing, ferrous iron-laden water.

Figure 16 The distribution of arsenic (As) in sediments collected from the HBHA Pond. The top panel shows the 

location of sediment samples for which compositional data are reported. The aerial distribution of arsenic 

is based on grab samples and the top 2-cm interval of core samples; contour lines delineate depth to sediment (meters). The frequency plot includes all grab samples and all sediment core depth intervals; data are 

compiled in Appendix B. 
29

Figure 17 The distribution of lead (Pb) and zinc (Zn) in sediments collected from the HBHA Pond. The aerial distributions of lead and zinc are based on grab samples and the top 2-cm interval of core samples; contour lines 

delineate depth to sediment (meters). The frequency plot includes all grab samples and all sediment core 

depth intervals; data are compiled in Appendix B.
30

Figure 18 The distribution of chromium (Cr) and iron (Fe) in sediments collected from the HBHA Pond. The aerial 

distributions of chromium and iron are based on grab samples and the top 2-cm interval of core samples; 

contour lines delineate depth to sediment (meters). The frequency plot includes all grab samples and all 

sediment core depth intervals; data are compiled in Appendix B.
31

Figure 19 The distribution of sulfur (S) and organic carbon (OC) in sediments collected from the HBHA Pond. The 

aerial distributions of sulfur and organic carbon are based on grab samples and the top 2-cm interval of core 

samples; contour lines delineate depth to sediment (meters). The frequency plot includes all grab samples 

and all sediment core depth intervals; data are compiled in Appendix B.
32

Chemical Speciation of Metals in Sediments

Sediment Extractions

The objective of the chemical extraction tests was to develop a sense of the degree to which arsenic, lead, and zinc 

were partitioned to sediments in a relatively labile form. It was assumed that the fraction of contaminants released 

by mild extractant solutions (i.e., MgCl2, phosphate, or Na2CO3) or extractant solutions designed to solubilized poorly 

crystalline iron oxides or sulfides would represent the most labile pool. From this perspective, MgCl2 was chosen to 

extract lead and zinc present in a readily exchangeable fraction (Tessier et al., 1979); whereas, solutions of phosphate 

or Na2CO3 were chosen to extract arsenic adsorbed on mineral surfaces (Jackson and Miller, 2000). Solutions of 

ascorbate or hydrochloric acid were chosen to dissolve poorly crystalline iron oxides and sulfides (Kostka and Luther, 

1994; Ford et al., 1999). Extraction with HCl was also studied due to its use for the acid volatile sulfide-simultaneously 

extracted metals (AVS-SEM) procedure commonly employed to determine metal speciation and potential bioavailability 

in sediments (Wilkin and Ford, 2002). 

An implicit assumption in the following analysis was the existence of a direct association between extracted iron oxides 

and sulfides and co-extracted arsenic, lead, and zinc. The reliability of this assumption was tested via independent 

assessment of the solid phase speciation of these elements using X-ray absorption spectroscopy (discussed in a later 

section). It should also be noted that the various extraction solutions were applied in parallel. Thus, contaminants 

solubilized in either the ascorbate or HCl extractions could potentially have been derived from a variety of partitioning 

environments such as ion exchange sites and/or more stable environments such as the internal structures of coprecipitates (e.g., zinc incorporated in the structure of an iron oxide) or pure phase precipitates (e.g., zinc sulfide). The slight 

differences in pH or the concentration of the primary component in the extraction solutions employed for surficial (oxic) 

versus buried (suboxic) sediments was due to different personnel handling the two sediment classes. It was assumed 

that these differences were sufficiently insignificant to prevent comparison of extraction trends for the two sediment 

classes. It should also be noted that selectivity and completeness of the HCl and ascorbate extractions were not calibrated against sediments spiked with reference minerals. Rather, reliance was placed on published data where these 

procedures have been employed.

Surficial Sediments 

Sediment samples were collected from the north-northwestern margin of the HBHA Pond in the vicinity of visible groundwater seeps. The sediments collected from this location were reddish-orange and consistent with the precipitation of 

poorly crystalline iron oxides due to the rapid oxidation and precipitation of ferrous iron derived from ground-water discharge. A series of chemical extraction tests were carried out on 5 sediment samples from this location (<2 mm size 

fraction). The solutions used to extract these sediments varied in their aggressiveness for the extraction of contaminants 

and dissolution of iron oxides or iron sulfides. Results of the chemical extraction tests are presented in Table 1 as 

percent recoveries of the total amount of arsenic, lead, zinc, and iron in the sediment samples. Total concentrations of 

these metals in the test sediments are presented in Appendix B. The chemical extraction tests included a spectrum of 

reagents from relatively mild 1 M magnesium chloride to more aggressive 0.5 M hydrochloric acid. 

Test results indicated that arsenic, lead, and zinc were not readily solubilized from the sediments using a 1 M solution of 

MgCl2. Typically, <1% of the total amount of sediment arsenic, lead, or zinc was leached using the magnesium chloride 

solution (Table 1); although, a greater fraction of zinc was extracted by this reagent (mean 15.0 %, max. 25.9%). This 

easily extracted fraction of zinc may be attributed to magnesium competition with zinc for ion exchange sites. Solutions 

containing phosphate were also examined to assess the degree to which arsenic could be desorbed from the sediment 

without significant solid phase dissolution. The fraction of extractable arsenic increased to as much as 15.9% in the 

presence of phosphate at pH 4, but less arsenic was extracted in the presence of phosphate at pH 7. The increase in 

extractable arsenic relative to the MgCl2 solution can be attributed to the strong competition from phosphate for adsorption 

sites on sediment minerals (Jackson and Miller, 2000). A significant fraction of Zn (up to 27.4%) was also extracted in 

the presence of the pH 4 phosphate solution. This result may be attributed to competition with sodium for ion exchange 

sites and/or desorption of zinc from surface complexation sites due to the low pH (McBride, 1994). An insignificant 

fraction of lead was extracted from the oxic sediments in the presence of the MgCl2 or phosphate solutions. In general, 

the highest fractions of arsenic, lead, and zinc were extracted in parallel with significant dissolution of iron-bearing 

minerals in these sediments. Mineralogical characterization of the clay-sized fraction from these sediments indicated 

that ferrihydrite was a dominant iron oxide phase. Ferrihydrite is readily solubilized via reductive- or acid-dissolution 

processes in ascorbate or HCl solutions (Ford et al., 1999; Larsen and Postma, 2001). The lower fraction of extracted 

lead for the ascorbate versus the HCl extraction (and zinc to a lesser extent) may be due, in part, to precipitation of a 

carbonate phase during the procedure. 
33

Buried Sediments 

A series of selected chemical extraction tests were carried out on 14 buried sediment samples collected from the bottom of the HBHA Pond. These tests were conducted to provide insight into contaminant speciation and the conditions 

that could favor metal release from the sediments. Results of the chemical extraction tests are presented in Table 2 as 

percent recoveries of the total amount of arsenic, lead, zinc, and iron in the sediment samples. Total concentrations 

of these metals in the test sediments are presented in Appendix B. The chemical extraction tests included a spectrum 

of reagents from relatively mild 0.5 M magnesium chloride to more aggressive 1 M hydrochloric acid. Test results indicated that arsenic and other selected metals analyzed were not readily solubilized from the sediments using 0.5 M 

magnesium chloride; typically <6% of the total amount of sediment arsenic or metal was leached using the magnesium 

chloride solution (Table 2). These results indicate that arsenic, lead, zinc, and iron are tightly bound to the solid matrix 

of the sediments. As expected, alkaline extraction solutions (0.1 M sodium carbonate) were ineffective in solubilizing 

lead, zinc, and iron; whereas, arsenic was partially recovered using the sodium carbonate solution (Table 2). Similar to 

the surficial sediments, significant fractions of arsenic and iron were extracted by the ascorbate solution, while extracted 

fractions of lead and zinc were below detection in this solution. Significant fractions of arsenic, lead, zinc, and iron were 

extracted in HCl for “unoxidized” and “oxidized” buried sediments. These results suggest the importance of contaminant 

partitioning to poorly crystalline iron (hydr)oxides and/or acid volatile sulfides. Mineralogical characterization of surficial 

sediments (discussed below) and a representative sample of buried sediments from the HBHA Pond (Wilkin and Ford, 

2002) confirm that both mineral types could potentially be present in sediments deposited within the HBHA Pond.

Apparent differences for the extracted fractions of arsenic and lead between “oxidized” and “unoxidized” sediments 

treated with HCl suggest a potential artifact associated with the predominant mineralogy in the treated sediment. As 

shown by Wilkin and Ford (2002), acid volatile sulfides (in particular, iron monosulfides) may be present as a significant 

component of buried sediments. Typically, one would expect formation of poorly crystalline ferrihydrite as the endproduct 

of rapid oxidation of iron monosulfides. The high extractable fractions of arsenic, lead, zinc, and iron in these “oxidized” 

sediments are consistent with an association with either a ‘native’ poorly crystalline iron (hydr)oxide as well as a similar 

phase produced upon oxidation of iron monosulfide. Thus, the observed artifact can be attributed to some chemical 

process that limits the solubility of arsenic and lead in HCl for “unoxidized” sediments. 

The ratios of extracted arsenic, lead, zinc, and iron for “unoxidized” and “oxidized” buried sediments are plotted in Figure 20 

as a function of total sulfur content. In the absence of analytical artifacts, one would anticipate a value close to unity 

for this ratio in sediments dominated by acid volatile sulfides (“unoxidized”) and/or poorly crystalline iron (hydr)oxides 

(“oxidized”), since both mineral phases are extracted in hydrochloric acid. Results plotted for arsenic indicate that this 

metal is less extractable for the “unoxidized” sediment, i.e., extracted ratio << unity. However, as shown by Wilkin and 

Ford (2002) this behavior can be attributed to an analytical artifact of the acid extraction commonly employed in the 

acid-volatile sulfide-simultaneously extracted metals (AVS-SEM) procedure rather than to any inherent difference in 

extractability. Specifically, hydrogen sulfide that is released during acid extraction reacts with co-extracted arsenic to 

form an orpiment-like phase at acidic pH, which is confirmed through calculation of orpiment solubility at the extraction 

pH (Wilkin and Ford, 2002). It appears there may be a related concern for Pb, which again displayed a decreased 

extractability for the “unoxidized” buried sediment with an increase in sulfide content. The extractability ratios clustered 

around unity for Fe and Zn in these sediment samples are consistent with the high solubility of iron and zinc monosulfides at acid pH. Extract ratios that exceed unity value at low sulfur concentrations may result from the conditions 

used to oxidize sediments. Exposure to air in a drying oven at 60°C was employed to oxidize sulfides. The elevated 

drying temperature may have resulted in the production of an iron oxide phase(s) that was resistant to extraction using 

hydrochloric acid (e.g., Stanjek and Weidler, 1992). This would result in extraction ratios greater than unity, since iron 

and zinc associated with an acid-resistant iron oxide phase would be less extractable. This process would be less likely 

for sediments with exceptionally high sulfide concentrations, since the formation of a more extractable ferric sulfate 

phase(s) would be favored. 

Due to the anticipated importance of poorly crystalline iron (hydr)oxides and iron sulfides in HBHA Pond sediments, 

ascorbate and hydrochloric acid extraction solutions were employed to assist in differentiating the relative proportion of 

these minerals. Peltier et al. (2005) have shown that extraction tests designed to target easily reducible iron (hydr)oxides 

via reductive dissolution at acidic pH also extract a significant fraction of acid volatile sulfides. However, this potential 

artifact was avoided by employing reductive dissolution with ascorbate buffered at a near-neutral pH (Kostka and Luther, 

1994). As a point of reference, the results of ascorbate extractions of both surficial and “unoxidized” buried sediments 

for arsenic and iron are shown in Figure 21. Except for sample WI01, arsenic was completely extracted in the presence of ascorbate for all surficial sediments, which corresponded with an extractable iron fraction of more than 80%. 

The lower amount of extractable arsenic and iron in sediment WI01 may be attributed to the crystallinity of the dominant iron oxide in this sample. As shown in Figure 22, hematite is the dominant iron oxide in the <2 μm size fraction 

from this sediment. X-ray diffraction data for the <2 μm size fraction of sediment samples WI01-NEP and WI02-NEP 

show an increased relative proportion of poorly crystalline 6-line ferrihydrite, consistent with the observed change in 
34

the fraction of ascorbate-extractable iron. As shown by Kostka and Luther (1994), hematite is dissolved to a lesser 

extent than ferrihydrite by ascorbate under the conditions employed. The increase in extractable iron for samples WI01, 

WI01-NEP, and WI02-NEP is consistent with the increased relative proportion of a poorly crystalline 6-line ferrihydrite 

in this series of samples. The low extractability of arsenic concurrent with the significant fraction of hematite in sample 

WI01 suggests the incorporation of arsenic in hematite during nucleation and growth of this mineral as suggested by 

experimental studies by Ford (2002). However, this observation remains uncertain without a specific knowledge of the 

diagenetic history leading to hematite formation in this sediment.

The fraction of ascorbate-extractable arsenic varied significantly for the set of buried sediments that was analyzed. 

Based on the results shown in Figure 21A, there were two distinct sediment groups in which arsenic was either >90% 

extractable (6 samples) or <40% extractable (6 samples) with a set of transition sediments between these two limits 

(5 samples). Examination of the fraction of ascorbate-extractable arsenic as a function of the total sulfur content of 

buried sediments indicates a general correlation between these two parameters (Figure 21C). It should be noted that 

a similar correlation (although weaker) apparently exists between ascorbate-extractable iron and total sulfur content for 

buried sediments (Figure 21D). Insignificant extraction of iron sulfides is anticipated for ascorbate extractions (Kostka 

and Luther, 1994), suggesting that arsenic may be partitioned to iron sulfides for buried sediments with low ascorbateextractable arsenic. Sediments in which the iron mineralogy is dominated by iron oxides are anticipated to have similar 

extractable iron fractions in the presence of ascorbate and HCl. This hypothesis is supported by results presented in 

Figure 21E. All surficial sediments have an ascorbate:HCl extractable ratio for iron ≥1 compared to buried sediments 

with ratios of ≤0.5. Ratios greater than unity for surficial sediments can be attributed to extraction, in the presence of 

ascorbate, of iron oxides that are more crystalline than ferrihydrite (Dos Santos Afonso et al., 1990; Larsen and Postma, 

2001). These results point toward the presence of acid volatile iron sulfides in all of the buried sediments. In general, 

it appears that the iron mineralogy for buried sediments is increasingly dominated by acid volatile iron sulfides with 

increasing sulfur content. 

Comparison to observed trends in apparent lead and zinc partitioning in surficial and buried sediments suggests differences in the partitioning environment of these contaminants relative to arsenic (Tables 1 and 2). The ascorbateextractable fractions of lead and zinc were below analytical detection for “unoxidized” buried sediments (Table 2). The 

observation that a majority of lead and zinc (more than 80% and 90% of lead and zinc, respectively) were extracted 

in “oxidized” buried sediments suggests that a significant fraction of these contaminants are partitioned to acid volatile 

sulfides. As for the surficial sediments, the presence of a high concentration of bicarbonate may cause precipitation 

of lead/zinc carbonates during the ascorbate extraction. In addition, re-adsorption of lead/zinc to the surface of unextracted minerals may also have resulted due to the absence of citrate in the procedure applied for buried sediments. 

Citrate would act as a complexant to help maintain extracted metals in solution. Overall, the results from sediment 

extractions employing ascorbate and HCl were less definitive for delineation of lead and zinc speciation. This limitation 

was addressed for a limited set of sediment samples using X-ray absorption spectroscopy to directly determine the 

in-situ speciation of arsenic, lead, and zinc.
35

Table 1 Percent of Total Element Released by Selected Wet Chemical Extraction Tests. Range and Mean Value 

are Reported for Replicate Analyses of 5 Sediment Samples Collected from the North-Northeastern 

Margin of the HBHA Pond

Solution As Pb Zn Fe Association

1 M MgCl2

<1%

mean

0.2%

0.2 to 1.0%

mean

0.5%

7.7 to 

25.9%

mean

15.0%

<1%

mean

0.01%

Water-soluble; labile; easily desorbed (pH 7)

0.005 M NaH2PO4

8.3 to 

15.9%

mean

11.6%

<1%

mean

†

5.8 to 

27.4%

mean

16.2%

<1%

mean

0.3%

Water-soluble; strongly adsorbed

(pH 4)

0.005 M NaH2PO4

2.2 to

5.1%

mean

3.3%

<1%

mean

†

<1%

mean

0.2%

<1%

mean

0.04%

Water-soluble; strongly adsorbed

(pH 7)

Ascorbate

0.2 M Na-citrate

0.6 M NaHCO3

0.4 M Ascorbic acid

39.2 to 

127.0%

mean

101.8%

4.5 to

33.2%

mean

17.6%

20.0 to 

80.3%

mean

57.6%

27.6 to 

115.9%

mean

88.0%

Soluble with a mild reductant

(pH 8)

0.5 M HCl

16.7 to

66.9%

mean

32.0%

17.2 to 

95.7%

mean

61.0%

27.1 to

95.3%

mean

71.4%

16.4 to 

97.0%

mean

70.2%

Acid-soluble

(pH<2)

Notes: Samples used in extraction tests: WI01, WI01-NEP, WI02, WI02-NEP, WI04. † indicates element was undetected or below quantitative limits 

in the extraction solution.
36

Table 2 Percent of Total Element Released by Selected Wet Chemical Extraction Tests. Range and Mean Value 

are Reported for 14 Sediment Samples Collected from the HBHA Pond

Solution As Pb Zn Fe Association

0.5 M MgCl2

1 to 

26.5%

mean

5.8%

<1%

mean

†

0.1 to 

1.9%

mean

0.6%

0.3 to 

11.0%

mean

4.3%

Water-soluble; labile; easily desorbed (pH 6.8)

0.1 M Na2CO3

10.9 to 

73.5%

mean

25.9%

<1%

mean

†

<1%

mean

†

<1%

mean

0.3%

Base-soluble; complexed with 

organic carbon

(pH 10.9)

0.02 M Ascorbic acid 0.6 

M NaHCO3

16.1 to

108%

mean

65.7%

<1%

mean

†

<1%

mean

†

6.1 to 

51.0%

mean

23.2%

Soluble with a mild reductant (pH 

7.0)

1 M HCl

“oxidized” sediment

71 to 

106%

mean

91.7%

80.0 to

113.1%

mean

99.1%

90.3 to 

109.4%

mean

100.8%

56.8 to 

107%

mean

86.7%

Acid-soluble; leaching characteristic of air-exposed sediment at 

pH<2

1 M HCl

“unoxidized” sediment

<1 to

20.9%

mean

13.2%

3.6 to 

105.8%

mean

63.5%

62.2 to

105%

mean

98.2%

83.0 to 

107%

mean

98.5%

Acid-soluble; leaching characteristic of unaltered sediment at pH<2

Notes: Samples used in extraction tests: SC0401-1, SC0401-2, SC0401-3, SC0401-5, SC0401-6, SC0401-7, NC0901-1, NC0901-2, NC0901-3, 

NC0901-4a, NC0901-4b, NC0901-4c, NC0401-1 (0-2cm), NC0401-1 (2-4cm), NC0401-6 (0-2cm), NC0401-6 (2-4cm). “Oxidized” sediments extracted 

in HCl were exposed to air to oxidize sulfide to sulfate prior to extraction, while “unoxidized” sediments were processed in an inert atmosphere to 

prevent oxidation of sulfides. † indicates element was undetected or below quantitative limits in the extraction solution.
37

Figure 20 The ratio of As, Fe, Pb, and Zn extracted by HCl for unoxidized and oxidized sediments collected from suboxic 

zones within the HBHA Pond. 

Figure 21 (A-D) Comparison of the amount of As and Fe extracted by an ascorbate solution as a function of total Fe 

and S content in sediments (oxic = open symbols, suboxic = filled symbols). (E) Changes in the ratio of 

ascorbate-to-HCl extractable Fe as a function of the molar ratio of S and Fe in sediments. X-ray diffraction 

data are shown below for sediment samples WI01, WI01-NEP, and WI02-NEP (Figure 20). X-ray absorption spectroscopic data are shown below for arsenic in sediment samples WI01-NEP, WI04, NC0901-1, and 

NC0901-4c (Figure 23).
38

Figure 22 X-ray diffraction data showing the relative proportion of hematite and ferrihydrite in the <2 μm size fraction 

isolated from surficial sediment samples WI01, WI01-NEP, and WI02-NEP collected near the north-northwestern margin of the HBHA Pond. The starred peaks in the pattern for sample WI01 correspond with the 

dominant peaks in reference hematite (PDF 33-0664). Mean percent iron extracted by ascorbate in duplicate 

samples is shown to the right of the sample labels. The diffraction pattern for sample WI02-NEP is dominated 

by a poorly crystalline 6-line ferrihydrite (see peaks marked with arrows). 

Element Speciation by X-ray Absorption Spectroscopy

To provide a constraint on the interpretation of the extraction results, the in-situ speciation of arsenic, lead, and zinc 

were determined for a representative set of samples using X-ray absorption spectroscopy. The results of contaminant 

speciation analyses employing X-ray absorption spectroscopy are documented in Figures 23-25. Specifically, linear 

combination fitting (LCF) of the X-ray absorption near edge (XANES) region of the spectra was used to delineate element speciation in sediments. Reference spectra collected for model solid phase compounds were used in the fitting 

procedure in order to estimate the relative fraction of representative contaminant-mineral associations in each analyzed 

sediment sample. In aggregate, these results attest to the importance of the redox gradients within the HBHA Pond 

and attest to the apparent degree of disequilibrium that exists within the sediments. 

The results from a selection of samples characterized for arsenic speciation are shown in Figure 23, and the aerial 

distribution of arsenic phase associations within the HBHA Pond are shown in Figure 26. Analysis of these samples 

indicates that the fate of arsenic is directly tied to the iron minerals that dominate within oxic and suboxic portions of 

the HBHA Pond. Arsenic speciation is dominated by adsorption/coprecipitation of As(III) or As(V) with iron (hydr)oxides 

that precipitate in shallow ground-water seeps near the margin of the HBHA Pond (WI01-NEP, WI04). The speciation 

of arsenic transitions to As(III) partitioned to iron monosulfides within the deepest buried sediments (NC0901-4C) with 

a fraction of As(III) remaining bound to iron (hydr)oxides or iron carbonate at shallower depths within the HBHA Pond 

(NC0901-1). It should be noted that arsenic associated with orpiment (As2S3) represented an insignificant fraction 

(<10%) within buried sediments. The position of the arsenic absorption edge in all sediment samples that were evaluated was significantly higher than that observed for mineral specimens and laboratory-prepared As2

S3 (data not shown). 

The arsenic speciation results for these samples are broadly consistent with the extraction results shown in Figure 21A. 

These results are consistent with the extraction data previously discussed where the bulk of the sediment-associated 

arsenic was extracted using reagents that targeted poorly crystalline iron (hydr)oxides and iron sulfides (Tables 1 and 2). 

The relative fraction of arsenic bound to oxygen increases for sediments with higher iron (hydr)oxide content (FeAsc/FeHCl

ratio near unity), while the fraction of arsenic bound to sulfur increases with higher iron sulfide content (low FeAsc/FeHCl

ratio). Therefore, the stability of solid phase arsenic in these sediments will partly be dictated by the stability of these 

metastable minerals.
39

The results from a selection of samples characterized for lead speciation are shown in Figure 24 (all data fits in Appendix B), and the aerial distribution of lead phase associations within the HBHA Pond are shown in Figure 26. Analysis of 

these samples indicates that lead speciation is dominated by partitioning to iron (hydr)oxide minerals. This pattern holds 

even in sediments collected from the bottom of the HBHA Pond where there is evidence for iron- and sulfate-reduction 

(Figures 13 and 15) and formation of iron sulfides (Figure 21E; low FeAsc/FeHCl ratios). A fraction of lead is partitioned 

to a sulfidic phase within buried sediments, but in most cases, this represents a minor fraction of the species of lead 

identified based on the LCF-XANES approach. This observation in combination with FeAsc/FeHCl extraction ratios >25% 

for half of the buried sediment samples shown in Figure 21E clearly indicates that iron (hydr)oxides formed within oxic 

portions of the HBHA Pond survive the reducing conditions established at the bottom of the pond. The persistence of 

iron (hydr)oxides in zones of iron- and sulfate-reduction in sediments has been observed by others (Kostka and Luther, 

1994). Since both poorly crystalline iron (hydr)oxides and iron sulfides are extracted by HCl, this suggests potential 

limitations to the use of tests such as the AVS-SEM procedure to define element speciation.

Finally, the results from a selection of samples characterized for zinc speciation are shown in Figure 25 (all data fits in 

Appendix B) and the aerial distribution of zinc phase associations within the HBHA Pond is shown in Figure 26. The 

results of this analysis contrast with those for lead in that zinc sulfides play a more dominant role in the speciation of 

this metal in buried sediments. Zinc partitioned to an iron (hydr)oxide phase is still observed as a significant component within all sediments indicating that the survival of this mineral fraction still plays a role in zinc speciation within 

the suboxic zone of the HBHA Pond. The decrease in the abundance of the sediment species for zinc relative to lead 

is consistent with the greater stability of the lead sorption complex on iron (hydr)oxide surfaces (Stumm, 1992). In 

general, the importance of sulfide minerals to element speciation within buried sediments appears to follow the order 

As > Zn > Pb. 

A sediment incubation study was conducted to observe the changes in metal speciation that may occur as iron 

(hydr)oxides formed in the oxic zone are deposited in the suboxic zone within the HBHA Pond. In this study, surficial 

sediments from the northern margin of the HBHA Pond were incubated with their native pore water in closed vessels 

within the laboratory at room temperature. No attempt was made to control the biogeochemical conditions within the 

vessel during incubation for a period of 2.5 years. The silt (<53 μm) and clay (<2 μm) fractions of the fresh and incubated sediments were isolated by sieving and centrifugation to facilitate mineralogical characterization of the most 

reactive mineral fraction. Significant alterations of the iron mineralogy were observed as a result of incubation. X-ray 

diffraction data collected on the clay fraction of a representative sediment sample showed that a poorly-crystalline 6-

line ferrihydrite was dominant in the fresh sediment (Figure 27A). However, this mineral was transformed to a mixture 

of goethite and siderite during the incubation period. Zinc speciation data were collected for the silt-sized fraction of 

the fresh and incubated sample to determine the concurrent changes in contaminant speciation that accompanied the 

observed transformations in bulk mineralogy (Figure 27B; arsenic and lead data not collected). Analysis of these data 

using the LCF-XANES approach demonstrated significant changes in zinc speciation during sediment incubation. The 

speciation of zinc was dominated by a combination of a hydroxide-like precipitate and adsorption/coprecitation with 

an iron (hydr)oxide phase in the fresh sample. During incubation, a zinc sulfide phase formed at the expense of the 

zinc-iron (hydr)oxide phase. The results of the XRD and LCF-XANES analyses demonstrate that iron-reducing and 

sulfate-reducing conditions developed during the period of incubation, potentially driven by the growth of native microbial 

communities in the closed environment. Unfortunately, data on the chemical conditions during aging and evidence for 

microbial reactions were not collected during this experiment. However, the results of this experiment are consistent 

with the observed change in zinc speciation in transitioning from surficial (oxic) to buried (suboxic) sediments within the 

HBHA Pond. Specifically, iron- and sulfate-reducing processes play a critical role in determining contaminant speciation 

in sediments deposited within the HBHA Pond. 
40

Figure 23 Locations of a selection of sediments collected from oxic and suboxic zones within the HBHA Pond. XANES 

data are shown for sediments that span the observed redox gradient within the system relative to representative reference compounds.
41

Figure 24 XANES data for (A) a series of reference compounds and (B) a selection of sediments collected within the 

HBHA Pond. The relative percentage of Pb species is shown in Panel B based on the results of LCF-XANES 

data analysis. 

Figure 25 XANES data for (A) a series of reference compounds and (B) a selection of sediments collected within the 

HBHA Pond. The relative percentage of Zn species is shown in Panel B based on the results of LCF-XANES 

data analysis. 
42

Figure 26 Aerial distribution of contaminant sediment speciation within the HBHA Pond based on LCF-XANES analysis; 

contour lines delineate depth to sediment (meters). FeAsc/FeHCl is a measure of the relative fraction of iron 

(hydr)oxide minerals. An ‘x’ denotes a location for which data is not available.
43

Scanning Electron Microscopy

Scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) was employed to derive information 

on mineral textures in HBHA Pond sediments along with elemental associations within sediment particles. The SEMEDS results support the importance of iron-bearing minerals in HBHA Pond sediments (Table 3). The EDS spectra 

for a selection of analyses for these samples are shown in Figure 28. It should be noted that the Lα emission line was 

used for detection and semiquantitation of arsenic. In general, the concentration of arsenic in individual particles was 

below instrumental detection by EDS. This is consistent with the apparent continuous redistribution of arsenic between 

iron (hydr)oxides and iron sulfides. This process would tend to disperse arsenic more uniformly, in contrast to arsenic 

being present in detrital phases eroded from surrounding contaminated soils and deposited into sediments within the 

HBHA Pond. Back-scattered electron images shown in Figures 29 and 30 indicate that the iron minerals present in 

sediments are fine-grained. This is likely a result of the intensive redox cycling of iron that takes place across redox 

gradients established within the HBHA Pond system. Observation of a pyrite framboid in what is presumably an oxic 

(surficial) sediment sample also points to the extent of redox disequilibria within these sediments. The extent and variability of non-equilibrium conditions that exist within the HBHA Pond sediment system are unknown, but it is clear that 

site characterization efforts must be cognizant of this issue in order to circumvent potential analytical artifacts that could 

prevent accurate interpretation of contaminant speciation and potential mobility/bioavailability

Figure 27 Characterization of the mineralogy of the clay-sized sediment fraction and zinc speciation in the silt-sized 

fraction of surficial sediment sample WI02-NEP before and after incubation for a period of 2.5 years. (A) 

XRD data for the <2 μm fraction of the fresh and incubated sediment along with identification of the predominant mineral components. (B) Raw XANES data (lines) and LCF-XANES fit results (data symbols) for 

the 2-53 μm fraction of the same sediment samples
44

Table 3 Atomic Percentages of Elements in 5 Samples Determined by SEM-EDS. Sample PS-3 is Composed 

of 2-line Ferrihydrite; Poorly Crystalline 6-line Ferrihydrite is the Dominant Iron (Hydr)oxide in Samples 

WI02-NEP and WI01-NEP. Atomic Percentages for Fe and O are Shown in Parentheses under the PS-3 

Column for the Ferrihydrite Structure as Proposed by Towe and Bradley (1967)

Element 

(emission 

shell)

WI02-NEPA

<2 μm

“oxic 

sediment”

WI02-NEPA

<2 μm

“pyrite 

framboid”

WI02-NEPB

2 μm < d < 

53 μm

“oxic 

sediment”

PS-3

“water column 

sample”

PS-3

Compositional 

data from acid 

digestion

SC0401-3

“anoxic 

sediment”

S (K) ND 49.63% ± 

1.08% ND ND 2.54 wt% 7.98% ± 0.32%

Fe (K) 11.16% ± 

0.52%

50.37% ± 

1.08%

19.63% ± 

0.80%

61.92% ± 

0.64%

(64.58%)

34.28 wt% 12.08% ± 

0.48%

Si (K) 1.84% ± 

0.20% ND 7.78% ± 0.46% 3.26% ± 0.29% 0.87 wt% 1.48% ± 0.16%

O (K) 37.94% ± 

1.31% ND 25.75% ± 1.54%

30.25% ± 

0.57%

(33.33%)

-- 20.44% ± 

1.13%

Ca (K) ND ND 0.56 ± 0.17% 1.83% ± 0.13% 1.21 wt% *0.44% ± 

0.11%

K (K) *0.36% ± 

0.12% ND ND ND 0.16 wt% ND

Zn (K) ND ND ND ND 0.11 wt% 1.38% ± 

0.36%

Al (K) * 0.49% ± 

0.17% ND 0.92% ± 0.19% 0.89% ± 0.16% 0.21 wt% 1.52% ± 0.17%

As (L) ND ND ND 0.85% ± 0.26% 0.44 wt% ND

Cu (K) ND ND ND ND -- ND

Na (K) ND ND *0.47% ± 

0.21% ND -- ND

magnification 2000x 4300x 3000x 10000x -- 2000x

Notes: ND = Not Detected

 Below Limit of Quantitation (BLQ)
45

Figure 28 Representative compositional spectra for samples imaged using SEM-EDS; UP = unidentified peak.

Figure 29 Image of a pyrite framboid observed in a surficial (oxic) sediment (WI02-NEP, clay-sized fraction).
46

Figure 30 Images of iron (hydr)oxide precipitates collected near the chemocline within the water column of the HBHA 

Pond (PS-3).
47

Chapter 4

Stability of Contaminants in HBHA Pond Sediments

Assessment of ground-water data collected during this study and historic data collected within the study site indicate 

that the primary point of discharge for contaminated ground water is into the HBHA Pond. Long-term monitoring at 

the HBHA Pond surface water discharge indicates that dissolved arsenic is typically below a concentration of 100 ppb. 

This suggests that arsenic and metals discharged via site-derived ground water are partially sequestered within the 

HBHA Pond. The apparent geochemical processes controlling the solid-solution partitioning of arsenic, lead, and zinc 

are illustrated in Figure 31. In general, arsenic solid-solution partitioning is linked to precipitation-dissolution cycles 

that control the distribution of iron between dissolved and oxidic/sulfidic mineral phases. Arsenic remains in a relatively 

labile chemical state, although the physicochemical characteristics of the HBHA Pond result in significant sequestration within the pond boundary. The cycling of lead and zinc within the HBHA Pond water column is less clear due to 

the lack of clear chemical patterns in dissolved concentrations for these contaminants. However, the solid-solution 

partitioning of lead and zinc in sediments also appears to be linked to the production of iron (hydr)oxides and sulfides. 

These contaminants are also sequestered within the boundaries of the HBHA Pond, primarily in association with sediments. As noted previously (Figure 14 and 17), zinc derived from ground-water discharge is efficiently sequestered in 

sediments. The observed release of arsenic from shallow sediments confirms the partial instability of iron (hydr)oxides 

deposited on the bottom of the HBHA Pond (Ford, 2005). Dissolved concentrations of arsenic are a maximum within 

the hypolimion near the sediment-water interface where iron (hydr)oxide dissolution prevails with a limited capacity for 

production of iron sulfides (until sediment burial and more intense sulfate reduction). This observation is consistent with 

projections made by Hounslow (1980), where regions of iron reduction coupled with insufficient sulfate reduction are 

anticipated to result in the most labile/mobile forms of arsenic. The circumneutral pH near the sediment-water interface 

also contributes to selective sequestration of zinc versus arsenic in sulfate-reducing zones due to the higher solubility 

of iron and arsenic sulfides (Wilkin and Ford, 2002). Ultimately, although the HBHA Pond retains a significant fraction 

of arsenic and metals derived from ground-water discharge, it could still supply a continual flux of arsenic and metals to 

downgradient wetlands and the Aberjona River via discharge of dissolved contaminant species or suspended solids.

The critical factors maintaining arsenic, lead, and zinc at depth within the HBHA Pond hypolimnion are 1) the presence of 

a fairly stable chemocline and 2) the high capacity for metal sorption to iron (hydr)oxides and/or iron sulfides formed at 

the oxic-anoxic transition zone (Taillefert et al., 2000; Ford, 2002; Taillefert and Gaillard, 2002). The capacity for arsenic 

and metal sequestration from the HBHA Pond water column is dependent on a continual supply of ferrous iron into the 

system, while the continual production of iron sulfides is dependent on maintenance of sulfate reduction processes 

within the hypolimnion. The arsenic (and potentially lead and zinc) sequestration capacity will decline concurrent with 

a decline in ferrous iron from the upgradient aquifer. Since performance of the HBHA Pond for arsenic removal is also 

dependent on continued stratification of the water column, assessment of the long-term sedimentation capacity is also 

an important factor. Water column monitoring data from this study suggest that the chemocline is typically located 

approximately 100-200 cm above the sediment-water interface depending on the season. While there are no data 

to assess sediment accumulation rates for the HBHA Pond, it is not anticipated that sediment in-filling would cause 

disruption of the chemocline. Regardless, projections of potential sediment erosion or turnover within the HBHA Pond 

will depend on knowledge of modifications to the pond geometry over time. Buried sediments within the HBHA Pond 

have accumulated significant concentrations of metals such as lead and zinc. Mineralogical characterization and 

contaminant speciation indicate that both iron (hydr)oxides and iron sulfides play a significant role in this process within 

the hypolimnion and buried sediments. The relative distribution of these two mineral classes will depend on the extent 

of sulfate-reduction and the rates of dissolution of the iron (hydr)oxides deposited onto buried sediments (Wersin et 

al., 1991; Poulton et al., 2004). The stability of these sediments will depend on the maintenance of iron- and sulfatereducing conditions at depth within the HBHA Pond. Current and historical ground-water data suggest that these 

processes will continue into the future, but consideration should be given to the impact of water column turnover and 

the potential long-term depletion of the reducing capacity within the aquifer as the BTEX plume diminishes over time 

(e.g., Saulnier and Mucci, 2000).
48

Since the location and mass of arsenic (and iron) within contaminated soils upgradient from the HBHA Pond are poorly 

defined, it is difficult to assess the long-term capacity of the HBHA Pond. Given this level of uncertainty, it is recommended that a long-term monitoring strategy be implemented that continuously tracks arsenic and redox chemistry within 

the aquifer and the HBHA Pond. This appears to be less of a concern for lead and zinc, since elevated concentrations 

of these metals were rarely observed within the HBHA Pond water column. A proposed strategy is outlined below. 

Recommendations for site monitoring and potential remediation apply only to discharge of contaminated ground water 

into the HBHA Pond and are not intended to address potential contaminant migration in surface water and ground water 

downgradient from the HBHA Pond.

Figure 31 Illustration of the apparent geochemical processes controlling solid-solution partitioning of arsenic, lead, 

and zinc within the HBHA Pond: 1) Fe(II) oxidation near chemocline upon encountering dissolved oxygen 

and coprecipitation of As with iron (hydr)oxides, 2) settling of precipitated iron (hydr)oxides, 3A) reductive 

dissolution of a fraction of settling iron (hydr)oxides generating dissolved Fe(II) and arsenic, 3B) deposition 

and diagenisis of a fraction of precipitated iron (hydr)oxides – partial conversion to iron sulfides, and 4) 

vertical transport of dissolved Fe(II) and As up to the chemocline. The speciation of arsenic, lead, and zinc 

in sediments located near and distal to the primary zone of contaminated ground-water discharge is shown 

(red = oxide association; black = sulfide association).
49

Monitoring Long-Term Behavior (or Performance) of HBHA Pond

It is evident that the HBHA Pond currently serves to remove a fraction of the metal burden from ground water originating 

from the Industri-Plex Superfund Site. However, the ability of the HBHA Pond to function in this capacity over the long 

term will be controlled by 1) continued maintenance of the chemical stratification within the water column and 2) continued supply of iron and sulfate from site ground water for the production of iron (hydr)oxides and sulfides.

As shown via the HBHA Pond water column sampling during April 2001, high surface water flow events can perturb the 

chemical stratification (Ford, 2005). While the chemocline was re-established within the HBHA Pond, there are insufficient monitoring data to track the impact of these transient flow events on the long-term performance of the HBHA 

Pond for metal sequestration. Forecasting the impact of surface flow events is critical due to site development, which 

can modify patterns in surface runoff into the HBHA Pond. It is recommended that a long-term monitoring program 

incorporate input and output flow measurements for the HBHA Pond in order to establish whether the major flow event 

occurring at the end of March 2001 was anomalous or is likely to occur on a more frequent basis. Note that continued 

discharge from Halls Brook should be maintained, since this continuous source of fresh water helps maintain chemical 

stratification within the HBHA Pond. As demonstrated by depth-resolved measurements, the highest concentration of 

dissolved As occurs at the bottom of the HBHA Pond and is relatively invariant throughout the year. This mass of dissolved As will be distributed throughout the water column during turnover, leading to increased mass flux of As at the 

shallow outlet. This is a critical factor, since the HBHA Pond is a potential long-term source of arsenic transport to the 

Aberjona River. Based on the observed perturbation to the performance of the HBHA Pond during the March 2001 

surface flow event, it is recommended that control structures be installed to better regulate surface water inputs into 

the HBHA Pond. Structures to divert excess flows during precipitation/snow melt events of similar magnitude to that 

observed during March - April 2001 will facilitate maintenance of the chemocline within the HBHA Pond, which appears 

critical to the precipitation and settling characteristics of the iron cycle within the water column.

It is recommended that a more comprehensive site monitoring strategy be implemented in the near-term to better track 

performance of the HBHA Pond system and detect possible failure. The current focus of monitoring efforts outside of 

this study has entailed extensive evaluation of surface water inputs and discharge out of the HBHA Pond. In order to 

properly assess performance of the HBHA Pond to sequester arsenic and metal contaminants, it is recommended that 

a permanent ground-water monitoring network be established within the aquifer upgradient and downgradient from 

the HBHA Pond. The network should be sufficient to document the extent of the arsenic contaminant plume and its 

elimination down gradient to the HBHA Pond. Contaminants of concern and the general redox chemistry of ground 

water should be monitored concurrent with surface water sampling events. In general, quarterly sampling will provide 

sufficient frequency to monitor the dynamics of the ground-water plume and the HBHA system, but it is recommended 

that additional efforts be made to assess system response to storm events. Assessment of seasonal patterns in contaminant mobility and sequestration within the HBHA Pond is required to establish the long-term viability of the metal 

sequestration reactions active within the HBHA Pond to mitigate mobilization of arsenic and metals. 

There is evidence that the chemical stratification within the HBHA Pond can be interrupted during large flow events. 

Ford (2005) has documented the loss of chemical stratification in the central and southern portions of the HBHA Pond 

following a large flow event during March 2001. The flow-induced mixing resulted in distribution of the entire budget of 

hypolimnetic arsenic throughout the water column in these portions of the pond and ultimately resulted in a temporary 

increase in arsenic export from the HBHA Pond. Evaluation of contemporary and historical water flow records at a 

hydrologic monitoring station approximately 2.5 miles south of the HBHA Pond (USGS 01102005) indicates that an 

event of this magnitude could potentially occur on a 3-4 year cycle. These results suggest that efforts to control highflow events into the HBHA Pond will be important to limiting down gradient transport of arsenic and metals derived from 

discharge of contaminated ground water. However, the ultimate performance of the HBHA Pond system will be limited 

by net sedimentation, which will slowly consume the storage capacity for contaminated water and sediments within the 

deeper portions of the pond.

Relevance to Other Sites

The observations of the dynamics of iron mineralogical cycling and the chemical speciation of arsenic, lead, and zinc 

from this study site may be relevant to other sites. The differential influence of iron- and sulfate-reduction processes 

on the solid-solution partitioning of inorganic contaminants observed in this study is important relative to the design of 

the characterization plan for other sites with complex metal and metalloid contaminant mixtures. The overall stability of 

sediment-associated contaminants as well as the technologies that may be employed for their remediation will depend 

on the types of mineral associations controlling contaminant solid-phase speciation. The results of the work described 

in this document indicate that, of the three inorganic contaminants that were studied, arsenic posed the greatest concern 

due to its higher potential mobility. This may apply to other sites where inorganic contaminants are a primary concern. 

The extent to which this could be defined for the HBHA Pond was dependent on development of a comprehensive 

knowledge of the influence of iron- and sulfate-reduction on the solid-phase partitioning of arsenic within the system 
50

boundaries. This study demonstrated a general correspondence between observed patterns in the aqueous distribution 

of arsenic, lead, and zinc relative to the predominant solid-phase speciation patterns for these contaminants based on 

chemical extractions and spectroscopic analyses. In general, this work supports the applicability of using chemical extractions to help define contaminant speciation. However, results for the specific contaminants indicate that care should 

be taken during the design of an extraction protocol relative to observed site conditions. Ultimately, the reliability and 

utility of extraction protocols to define inorganic contaminant speciation will be predicated on concurrently developed 

knowledge of the mineralogical composition of the tested solids.
51

References

Aggett, J., and M.R. Kriegman. The extent of formation of arsenic(III) in sediment interstitial waters and its release to 

hypolimnetic waters in Lake Ohakuri Water Resources 22: 407-411 (1998).

Aggett, J., and G.A. O’Brien. Detailed model for the mobility of arsenic in lacustrine sediments based on measurements 

in Lake Ohakuri. Environmental Science and Technology 19: 321-238 (1985).

Ahmann, D., L.R. Krumholz, H.F. Hemond, D.R. Lovely, and F.M.M. Morel. Microbial mobilization of arsenic from sediments of the Aberjona watershed. Environmental Science and Technology 31: 2923-2930 (1997).

Atkin, B.P. and C. Somerfield. The determination of total sulfur in geological materials by coulometric titration. Chemical Geology 111: 131-134 (1994).

Aurilio, A.C., R.P. Mason, and H.F. Hemond. Speciation and fate of arsenic in three lakes of the Aberjona watershed. 

Environmental Science and Technology 28: 577-585 (1994).

Aurilio, A.C., J.L. Durant, H.F. Hemond, and M.L. Knox. Sources and distribution of arsenic in the Aberjona watershed, 

eastern Massachusetts. Water Air Soil Pollution 78: 1-18 (1995).

Balistrieri, L.S., J.W. Murray, and B. Paul. The geochemical cycling of trace elements in a biogenic meromictic lake. 

Geochimica et Cosmochimica Acta 58: 3993-4008 (1994).

Bartlett, R.J. and R.R. James, Chromium in the Natural and Human Environment, New York: John Wiley & Sons, 

1988.

Beauchemin, S., D. Hesterberg, and M. Beauchemin. Principal component analysis approach for modeling sulfur KXANES spectra of humic acids. American Journal of Soil Science Society 66: 83-91 (2002).

Biksey, T.M., and E.D. Gross. The hyporheic zone: linking groundwater and surface water – understanding the paradigm. 

Remediation 12: 55-62 (2001).

Davis, A., J.H. Kempton, A. Nicholson, and B.Yare. Groundwater transport of arsenic and chromium at a historical tannery, Woburn, Massachusetts, U.S.A. Applied Geochemistry 9: 569-582 (1994).

Davis, A., C. Sellstone, S. Clough, R. Barrick, and B. Yare. Bioaccumulation of arsenic, chromium and lead in fish: constraints imposed by sediment geochemistry. Applied Geochemistry 11: 409-423 (1996).

Davison, W. and G. Seed. The kinetics of the oxidation of ferrous iron in synthetic and natural waters. Geochimica et 

Cosmochimica Acta 47: 67-79 (1983).

Davison, W., C. Woof, and E. Rigg. The dynamics of iron and manganese in a seasonally anoxic lake; direct measurement of fluxes using sediment traps. Limnol. Oceanography 27: 987-1003 (1982).

Dos Santos Afonso, M., P.J. Morando, M.A. Blesa, S. Banwart, and W. Stumm. The reductive dissolution of iron oxides 

by ascorbate: The role of carboxylate anions in accelerating reductive dissolution. Journal of Colloid and Interface 

Science 138: 74-82 (1990).

Durant, J.L., J.J. Zemach, and H.F. Hemond. The history of leather industry waste contamination in the Aberjona watershed: A mass balance approach. Civil Engineering Practice 5: 41-66 (1990).

Ferdelman, T.G. “The distribution of sulfur, iron, manganese, copper, and uranium in a salt marsh sediment core as 

determined by sequential extraction methods.” Masters Thesis, University of Delaware, 1988.

Ford, R.G. Rates of hydrous ferric oxide crystallization and the influence on coprecipitated arsenate. Environmental 

Science and Technology 36: 2459-2463 (2002).

Ford, R.G. The impact of ground water-surface water interactions on contaminant transport with application to an arsenic 

contaminated site, EPA/600/S-05/002. Cincinnati, OH: U.S. Environmental Protection Agency, 2005.

Ford, R.G., K.M. Kemner, and P.M. Bertsch. Influence of sorbate-sorbent interactions on the crystallization kinetics of 

nickel- and lead-ferrihydrite coprecipitates. Geochimica et Cosmochimica Acta 63: 39-48 (1999).
52

Ford, R.G., R.T. Wilkin, and G. Hernandez. Arsenic cycling within the water column of a small lake receiving contaminated ground-water discharge. Chemical Geology (accepted) (2005).

Gomez-Ariza, J.L.,D. Sanchez-Rodas, I. Giraldez, and E. Morales. A comparison between ICP-MS and AFS detection 

for arsenic speciation in environmental samples. Talanta 51: 257-268 (2000).

Hach. “Iron, ferrous.” In Water Analysis Handbook, Loveland, CO: Hach Co., 1992a, 343-346.

Hach. “Iron, total.” In Water Analysis Handbook, Loveland, CO: Hach Co., 1992b, 353-357.

Hounslow, A.W. Ground-water geochemistry: arsenic in landfills. Ground Water 18: 331-333 (1980).

Huffman, E.W.D. Performance of a new carbon dioxide coulometer. Microchemical Journal 22: 567-573 (1977).

Jackson, B.P., and W.P. Miller. Effectiveness of phosphate and hydroxide for desorption of arsenic and selenium species 

from iron oxides. Soil Science Society of America Journal 64: 1616-1622 (2000).

JCPDS. Mineral Powder Diffraction File Databook; Sets 1-42, International Centre for Diffraction Data: Swarthmore, 

PA, 1993.

Kneebone, P.E., P.A. O’Day, N. Jones, and J.G. Hering. Deposition and fate of arsenic in iron- and arsenic-enriched 

reservoir sediments. Environmental Science and Technology 36: 381-386 (2002).

Kostka, J.E., and G.W. Luther III. Partitioning and speciation of solid phase iron in saltmarsh sediments. Geochimica 

et Cosmochima Acta 56: 1701-1710 (1994).

Larsen, O., and D. Postma. Kinetics of reductive bulk dissolution of lepidocrocite, ferrihydrite, and goethite. Geochimica 

et Cosmochimica Acta 65: 1367-1379 (2001).

Martin, A.J., and T.F. Pederson. Seasonal and interannual mobility of arsenic in a lake impacted by metal mining. Environmental Science and Technology 36: 1516-1523 (2002).

McBride, M.B. Environmental Chemistry of Soils, New York: Oxford University Press, 1994.

McCleskey, R.B., D.K. Nordstrom, and A.S. Maest. Preservation of water samples for arsenic(III/V) determinations: an 

evaluation of the literature and new analytical results. Applied Geochemistry 19: 995-1009 (2004).

Peltier, E., A.L. Dahl, and J.-F. Gaillard. Metal speciation in anoxic sediments: When sulfides can be construed as 

oxides. Environmental Science and Technology 39: 311-316 (2005).

Perret, D., J.-F. Gaillard, J. Dominik, and O. Atteia. The diversity of natural hydrous iron oxides. Environmental Science 

and Technology 34: 3540-3546 (2000).

Poulton, S.W., M.D. Krom, and R. Raiswell. A revised scheme for the reactivity of iron (oxyhydr)oxide minerals towards 

dissolved sulfide. Geochimica et Cosmochimica Acta 68: 3703-3715 (2004).

Puls, R.W., and C.J. Paul. Low-flow purging and sampling of ground-water monitoring wells with dedicated systems. 

Ground Water Monitoring and Remediation 15: 116-123 (1995).

Rehr, J.J., and A.L. Ankudinov. Progress and challenges in the theory and interpretation of X-ray spectra. Journal of 

Synchrotron Radiation 8: 61-65 (2001).

Rehr, J.J., and A.L. Ankudinov. New developments in the theory and interpretation of X-ray spectra based on fast parallel calculations. Journal of Synchrotron Radiation 10: 43-45 (2003).

Ressler, T. WinXAS: A program for X-ray absorption spectroscopy data analysis under MS-Windows. Journal of Synchrotron Radiation 5: 118-122 (1998).

Roberts, D.R., A.C. Scheinost, and D.L. Sparks. Zinc speciation in a smelter-contaminated soil profile using bulk and 

microspectroscopic techniques. Environmental Science and Technology 36: 1742-1750 (2002).

Saulnier, I., and A. Mucci. Trace metal remobilization following the resuspension of estuarine sediments: Saguenay 

Fjord, Canada. Applied Geochemistry 15: 191-210 (2000).

Scheckel, K.G., and J.A. Ryan. Spectroscopic speciation and quantification of Pb in phosphate amended soils. Journal 

of Environmental Quality 33: 1288-1295 (2004).

Scheinost, A.C., R. Kretzschmar, and S. Pfister. Combining selective sequential extractions, X-ray absorption spectroscopy, and principal component analysis for quantitative zinc speciation in soil. Environmental Science and 

Technology 36: 5021-5028 (2002).

Schwertmann, U., and W.R. Fischer. Natural “amorphous” ferric hydroxide. Geoderma 10: 237-247 (1973).

Schwertmann, U., and E. Murad. The nature of an iron oxide-organic iron association in a peaty environment. Clay 

Minerals 23: 291-299 (1988).
53

Senn, D.A., and H.F. Hemond. Nitrate controls on iron and arsenic in an urban lake. Science 296: 2373-2376 (2002).

Seyler, P., and J.-M. Martin. Biogeochemical processes affecting arsenic species distribution in a permanently stratified 

lake. Environmental Science and Technology 23: 1258-1263 (1989).

Sholkovitz, E.R., and D. Copland. The chemistry of suspended matter in Esthwaite Water, a biologically productive lake 

with seasonally anoxic hypolimnion. Geochimica et Cosmochimica Acta 46: 393-410 (1982).

Sohrin, Y., M. Matsui, M. Kawashima, M. Hojo, and H. Hasegawa. Arsenic biogeochemistry affected by eutrophication 

in Lake Biwa, Japan. Environmental Science and Technology 31: 2712-2720 (1997).

Spliethoff, H.M., R.P. Mason, and H.F. Hemond. Interannual variability in the speciation and mobility of arsenic in a 

dimictic lake. Environmental Science and Technology 29: 2157-2161 (1995).

Standard Methods for the Examination of Water and Wastewater. Section 4140 B Capillary Ion Electrophoresis with 

Indirect UV Detection, 20th Edition, pp. 1-4 to 1-13 (1999).

Stanjek, H., and P.G. Weidler. The effect of dry heating on the chemistry, surface area, and oxalate solubility of synthetic 

2-line and 6-line ferrihydrites. Clay Miner 27: 397-412 (1992).

Stumm, W. Chemistry of the Solid-Water Interface, New York: John Wiley & Sons, Inc., 1992.

Taillefert, M., C.-P. Lienemann, J.-F. Gaillard, and D. Perret. Speciation, reactivity, and cycling of Fe and Pb in a meromictic 

lake. Geochimica et Cosmochimica Acta 64: 169-183 (2000).

Taillefert, M., and J.-F. Gaillard. Reactive transport modeling of trace elements in the water column of a stratified lake: 

iron cycling and metal scavenging. Journal of Hydrology 256: 16-34 (2002).

Tessier, A., P.G.C. Campbell, and M. Bison. Sequential extraction procedure for the speciation of particulate trace metals. Analytical Chemistry 51: 844-851 (1979).

Tomassoni, G. “A federal statutory/regulatory/policy perspective on remedial decision-making with respect to groundwater/surface-water interaction.” In Proceedings of the Ground-Water/Surface-Water Interactions Workshop, 

Denver, CO, 1999.

Towe, K.M., and W.F. Bradley. Mineralogical constitution of colloidal “hydrous ferric oxides.” Journal of Colloid and Interface Science 24: 384-392 (1967).

U.S. Environmental Protection Agency. Methods for chemical analysis of water and wastes. EPA/600/4-79/020. Washington, DC, 1983.

Welch, A.H., and M.S. Lico. Factors controlling As and U in shallow ground water, southern Carson Desert, Nevada. 

Applied Geochemistry 13: 521-539 (1998).

Wersin, P., P. Hohener, R. Giovanoli, and W. Stumm. Early diagenetic influences on iron transformations in a fresh-water 

lake sediment. Chemical Geology 90: 233-252 (1991).

Wick, L.Y., and P.M. Gschwend. Source and chemodynamic behavior of diphenyl sulfone and ortho- and para-hydroxybiphenyl in a small lake receiving discharges from an adjacent superfund site. Environmental Science and Technology

32: 1319-1328 (1998a).

Wick, L.Y., and P.M. Gschwend. By-products of a former phenol manufacturing site in a small lake adjacent to a Superfund site in the Aberjona watershed. Environmental Health Perspectives 106: 1069-1074 (1998b).

Wick, L.Y., K. McNeill, M. Rojo, E. Medilanski, and P.M. Gschwend. Fate of benzene in a stratified lake receiving contaminated groundwater discharges from a superfund site. Environmental Science and Technology 34: 4354-4362 

(2000).

Wilkin, R.T., and R.G. Ford. Use of hydrochloric acid for determining solid-phase arsenic partitioning in sulfidic sediments. Environmental Science and Technology 36: 4921-4927 (2002).

Winter, T.C., J.W. Harvey, O.L. Franke, and W.M. Alley. Ground water and surface water: a single resource. U.S. Geological Survey Circular 1139: 77 pp (1998).
54
55

Appendix A 

Analytical Performance for Laboratory Methods

Table A.1. Analytical Methods, Detection Limits, Precision, and Accuracy for Measurement of Aqueous Chemistry

Parameter Method Detection Limit Precision Accuracy

Benzene, Toluene Purge-and-Trap Gas 

Chromatography

0.29 μg/L, benzene

0.22 μg/L, toluene

±15% based on 

duplicate analyses 

of unknowns

±20% based on 

spike recovery for unknowns

TOC, DOC

Wet Oxidation/Infrared 

Detection

EPA Method 415.2

0.02 mg C/L

±10% based on 

duplicate analyses 

of unknowns

±20% based on 

spike recovery for unknowns

Fe2+ Colorimetric, 1,10-phenanthroline 0.01 mg/L

±10% based on 

duplicate analyses 

of unknowns

±20% based on 

spike recovery for unknowns

Arsenic, Iron, Sodium, Lead, Sulfur, 

Zinc

ICP-OES

0.033 mg/L, As

0.035 mg/L, Fe

0.479 mg/L, Na

0.015 mg/L, Pb

0.137 mg/L, S

0.014 mg/L, Zn

±10% based on 

duplicate analyses 

of unknowns

±20% based on 

spike recovery for unknowns

NO3-N EPA Method 353.2 0.004 mg N/L

±10% based on 

duplicate analyses 

of unknowns

±20% based on 

spike recovery for unknowns

NH3-N EPA Method 350.1 0.03 mg N/L

±10% based on 

duplicate analyses 

of unknowns

±20% based on 

spike recovery for unknowns

SO4

Capillary electrophoresis 0.1 mg/L

±10% based on 

duplicate analyses 

of unknowns

±20% based on 

spike recovery for unknowns

n.d. not determined
56

Table A.2. Analytical Methods, Detection Limits, Precision, and Accuracy for Measurement of Solid Phase Chemistry

Parameter Method Detection Limit Precision Accuracy

Total Carbon Coulometry/

combustion

0.01 wt% C for a

100-mg sample

±3.2% based on 32 

analyses of CaCO3

±2.8% using CaCO3

(12.0 wt% C)

Total Sulfur Coulometry/

combustion

0.005 wt% S for a 

100-mg sample

±5.1% based on 35 

analyses of NIST 

1646a

±2.5% using

NIST 1646a

(0.35 wt% S)

Arsenic, Chromium, Iron, 

Lead, Zinc

ICP-OES/microwave 

assisted digestion/ 

chemical extraction

1 to 10 ppm for 100-mg 

sample (depending on 

element) and 20 mL 

extraction volume

±5 to 15% based on 

duplicate or triplicate analyses of unknowns 

Variable, depending on element and 

reference material

As (±3 to 12%)

n.d. not determined; standard reference materials used: NIST 1646a Estuary sediment, NIST 2710 Montana soil, NIST 2780 Hard rock mine waste, 

CCRMP Lake sediment-1
57

Appendix B 

Sediment Composition Data

Table B.1. Concentrations of Selected Elements in Halls Brook Holding Area Pond Core NC01

Core Sample Depth cm Fe wt% S

wt%

TOC 

wt%

As 

μg/gm

Pb

μg/gm

Cr

μg/gm

Zn

μg/gm

NC01-1 1.4 9.91 2.20 15.24 836 593 839 5920

NC01-2 2.8 9.65 1.80 15.58 857 610 970 5560

NC01-3 4.2 9.54 2.20 14.27 828 613 1060 5710

NC01-4 5.6 9.38 2.11 14.61 839 628 815 5130

NC01-5 7.0 9.38 2.10 13.32 834 607 856 5290

NC01-6 8.4 10.20 1.77 14.52 952 668 940 6060

NC01-7 10.0 10.10 0.05 5.95 639 606 691 3990

NC01-8 18.3 2.21 0.04 0.27 150 507 501 2850

NC01-9 26.6 1.51 0.04 0.17 101 106 87.4 609

NC01-10 34.9 0.37 0.03 0.03 10 34 13.8 71

Notes: Core collected 4/03/2000. Fe, As, Pb, Cr, and Zn determined by ICP-OES after microwave-assisted extraction 

in 10% nitric acid. TOC=total carbon minus inorganic carbon, determined using an UIC, Inc. carbon coulometer. S 

determined by coulometery (UIC, Inc.).
58

Table B.2. Concentrations of Selected Elements in Halls Brook Holding Area Pond Core CC02

Core Sample Depth cm Fe wt% S

wt% TOC wt% As μg/gm

Pb μg/

gm

Cr μg/gm Zn μg/gm

CC02-1 2.1 9.89 1.36 15.37 1120 429 593 3500

CC02-2 4.1 11.01 1.38 17.06 1101 425 573 3499

CC02-3 6.3 11.40 1.34 14.58 1020 514 830 4170

CC02-4 8.4 11.80 1.43 14.64 1050 528 901 4380

CC02-5 10.5 12.36 1.73 14.26 914 521 826 4420

CC02-6 12.6 14.55 1.31 13.09 945 633 830 4884

CC02-7 14.7 10.10 1.41 11.71 639 606 691 3990

CC02-8 16.8 7.21 1.00 9.81 455 507 501 2850

CC02-9 21.1 1.51 0.20 1.91 101 106 87.4 609

CC02-10 25.4 0.37 0.04 0.33 10.3 34.0 13.8 71

CC02-11 31.9 0.74 0.23 0.33 18.5 49.5 31.5 373

CC02-12 38.4 0.61 0.05 0.29 6.49 9.8 15.5 121

CC02-13 45.9 0.62 0.05 0.12 2.00 4.7 8.9 45

CC02-14 53.4 0.75 0.04 0.05 6.22 3.0 7.4 24

Notes: Core collected 4/03/2000. Fe, As, Pb, Cr, and Zn determined by ICP-OES after microwave-assisted extraction in 10% nitric acid. TOC=total 

carbon minus inorganic carbon, determined using an UIC, Inc. carbon coulometer. S determined by coulometery (UIC, Inc.).

Table B.3. Concentrations of Selected Elements in Halls Brook Holding Area Pond Core SC02

Core 

Sample Depth cm Fe wt% Swt%

TOC 

wt%

As 

μg/gm

Pb 

μg/gm

Cr 

μg/gm

Zn 

μg/gm

SC02-1 3.0 10.20 2.54 10.33 1570 403 529 2780

SC02-2 6.0 11.94 2.25 11.32 1439 455 594 3612

SC02-3 9.0 10.30 2.18 9.51 1510 412 557 3060

SC02-4 12.0 3.40 1.37 5.04 232 315 328 1790

SC02-5 15.0 2.72 0.57 6.46 200 452 462 1860

SC02-6 18.0 1.28 0.17 1.87 66 226 219 357

SC02-7 21.0 0.07 0.03 0.13 10 202 187 255

Notes: Core collected 4/03/2000. Fe, As, Pb, Cr, and Zn determined by ICP-OES after microwave-assisted extraction in 10% nitric acid. TOC=total 

carbon minus inorganic carbon, determined using an UIC, Inc. carbon coulometer. S determined by coulometery (UIC, Inc.).
59

Table B.4. Concentrations of Selected Elements in Halls Brook Holding Area Pond Grab Sediment Samples

Sediment 

Sample

Depth

cm

Fe

wt%

S

Wt%

TOC

wt%

As

μg/gm

Pb

μg/gm

Cr

μg/gm

Zn

μg/gm

WI01 <10 3.80 0.20 3.10 494 794 197 583

WI01-NEP <10 7.70 0.18 3.00 830 572 122 767

WI02 <10 24.60 0.47 4.75 715 203 106 3034

WI02-NEP <10 18.50 0.65 5.24 630 262 162 4589

WI04 <10 26.30 0.35 5.26 840 115 49 1517

SC0401-1 <10 7.46 1.14 5.24 682 274 511 2550

SC0401-2 <10 8.94 2.44 10.26 1150 392 698 3790

SC0401-3 <10 11.90 10.95 6.69 1680 398 250 17500

SC0401-4 <10 5.03 0.37 4.53 539 494 148 3920

SC0401-5 <10 7.59 1.59 14.18 676 526 593 3440

SC0401-6 <10 8.56 2.85 18.61 1070 481 694 3190

SC0401-7 <10 8.65 2.32 17.34 973 360 549 2590

NC0901-1 <10 1.90 0.83 1.50 207 92 65 836

NC0901-2 <10 3.37 2.24 2.29 356 144 107 1580

NC0901-3 <10 1.65 0.51 0.25 117 20 23.6 682

NC0901-4a <20 7.00 6.71 13.32 1430 464 431 4300

NC0901-4b <20 7.04 5.68 12.98 1490 461 437 4780

NC0901-4c <20 8.23 6.89 12.86 1690 649 478 7480

NC0901-4d <20 9.66 9.91 11.09 2050 548 606 9810

Notes: ‘WI’ sediments collected on 8/24/2000; ‘SC0401’ sediments collected on 4/03/2001; and ‘NC0901’ samples collected in September 2001. Fe, 

As, Pb, Cr, and Zn determined by ICP-OES after microwave-assisted extraction in 10% nitric acid. TOC=total carbon minus inorganic carbon, 

determined using an UIC, Inc. carbon coulometer. S determined by coulometery (UIC, Inc.).
60

Table B.5. Concentrations of Selected Elements in Halls Brook Holding Area Pond Cores

Core Sample Depth 

cm

Fe 

wt%

S 

wt% TOC wt% As μg/gm

Pb 

μg/gm

Cr 

μg/gm

Zn 

μg/gm

NC0401-1 1.0 8.87 1.80 11.92 1020 763 549 4270

NC0401-1 3.0 6.47 1.87 12.28 850 755 893 6320

NC0401-1 5.0 1.84 0.31 3.92 183 1550 421 1240

NC0401-1 7.0 0.57 0.10 1.39 68 837 31 77

NC0401-2 1.5 0.35 0.09 1.01 19 1070 11 67

NC0401-2 4.5 0.91 0.41 2.84 154 350 140 1120

NC0401-3 1.0 8.78 1.61 8.82 284 923 640 7170

NC0401-3 3.0 6.61 1.65 7.91 330 957 500 4390

NC0401-3 6.0 2.34 0.18 2.42 126 1430 300 505

NC0401-3 10.0 1.71 0.10 1.56 88 734 221 334

NC0401-3 14.0 0.68 0.05 0.92 6 38 17 46

NC0401-4 1.0 5.94 2.89 13.27 694 369 389 2270

NC0401-4 3.0 7.05 2.17 13.32 1020 506 811 4090

NC0401-4 6.0 0.82 0.16 0.46 20 18 17 104

NC0401-4 9.5 0.44 0.06 0.02 2 2 6 24

NC0401-4 12.5 0.40 0.03 0.03 2 0.5 6 18

NC0401-5 1.0 7.85 5.42 11.62 1060 459 508 3080

NC0401-5 3.0 12.00 4.54 10.21 1960 557 632 10300

NC0401-5 5.0 3.85 0.49 0.69 342 244 120 1900

NC0401-5 7.5 5.70 0.43 2.57 405 681 149 1210

NC0401-5 11.5 1.25 0.04 0.05 21 27 22 185

NC0401-6 1.0 7.87 4.86 12.66 1550 452 617 5520

NC0401-6 3.0 15.60 13.86 7.02 2570 404 359 15500

NC0401-6 5.0 3.85 1.60 0.14 65 27 22 273

NC0401-6 7.0 2.12 0.49 0.26 78 62 37 301

NC0401-6 11.0 1.20 0.12 0.23 32 50 27 162

NC0401-6 13.5 0.32 0.02 0.06 2 6 5 29

NC0401-7 3.0 1.63 0.21 3.18 158 1210 278 2100

NC0401-7 9.0 1.01 0.09 3.08 107 784 368 275

NC0401-8 4.0 0.57 <0.01 0.07 <2 4 8 15

Notes: NC0401 cores collected in April 2001. Fe, As, Pb, Cr, and Zn determined by ICP-OES after microwave-assisted extraction in 10% nitric acid. 

TOC=total carbon minus inorganic carbon, determined using an UIC, Inc. carbon coulometer. S determined by coulometery (UIC, Inc.).
61

Table B.6. Results from LCF-XANES Fits of the Pb XANES Data Collected for Sediments from Suboxic and Oxic 

Zones within the HBHA Pond

Weighted Percent Contribution

Galena s Pb-Sorbed 

Ferrihydrite s F-Test Residual

NC01-1 19.4 1.11 80.6 1.10 1 0.774

NC01-3 12.7 1.09 87.3 1.09 1 0.807

NC01-6 5.0 0.21 95.0 0.21 1 0.875

NC01-7 37.8 0.32 62.3 0.32 1 0.622

NC0401-1 0-2 11.6 1.24 88.4 1.23 1 0.851

NC0401-1 2-4 4.1 0.28 95.9 0.28 1 0.543

NC0401-5 0-2 16.1 0.86 83.9 0.86 1 0.856

NC0401-5 2-4 3.2 0.25 96.8 0.26 1 0.822

NC0401-6 0-2 4.9 0.21 95.2 0.21 1 0.823

NC0901-4B 3.6 0.26 96.4 0.26 1 0.744

NC0901-4C 5.3 0.11 94.7 0.11 1 0.784

NTW4 20.3 1.74 79.7 1.73 1 0.747

SC0401-3 4.2 0.32 95.8 0.32 1 0.872

SC0401-6 4.1 0.15 95.9 0.15 1 0.808

s = standard deviation
62

Table B.7. Results from LCF-XANES Fits of the Zn XANES Data Collected for Sediments from Suboxic and Oxic 

Zones within the HBHA Pond

Weighted Percent Contribution

Sphalerite s Zn-Sorbed 

Ferrihydrite s Smithsonite s Zn(OH)2 s FTest Residual

NC01-1 73.8 0.09 26.2 0.09 -- -- -- -- 1 1.82

NC01-3 70.5 0.09 29.5 0.09 -- -- -- -- 1 1.42

NC01-6 75.7 0.10 24.3 0.09 -- -- -- -- 1 1.65

NC01-7 49.5 0.11 13.6 0.17 -- -- 36.9 1 2.64

NC0401-1 0-2 55.0 0.11 45.0 0.10 -- -- -- -- 1 2.22

NC0401-1 2-4 73.6 0.09 26.4 0.09 -- -- -- -- 1 1.50

NC0401-5 0-2 67.2 0.09 32.8 0.09 -- -- -- -- 1 1.81

NC0401-5 2-4 73.4 0.11 26.6 0.10 -- -- -- -- 1 1.72

NC0401-6 0-2 79.6 0.11 20.4 0.09 -- -- -- -- 1 1.28

NC0401-6 2-4 72.9 0.09 27.2 0.09 -- -- -- -- 1 1.45

NC0901-1 65.3 0.09 34.7 0.09 -- -- -- -- 1 1.51

NC0901-4B 80.6 0.10 19.5 0.03 -- -- -- -- 1 1.41

NC0901-4C 78.3 0.09 21.7 0.09 -- -- -- -- 1 1.58

NTW4 75.1 0.09 24.9 0.09 -- -- -- -- 1 1.71

SC0401-1 68.5 0.09 31.5 0.09 -- -- -- -- 1 1.88

SC0401-3 84.6 0.09 15.4 0.09 -- -- -- -- 1 1.29

SC0401-6 70.7 0.09 29.3 0.09 -- -- -- -- 1 2.26

SC0401-7 69.1 0.09 30.9 0.09 -- -- -- -- 1 1.89

WI02-N1-53D2 -- -- 42.8 0.40 -- -- 57.2 0.40 1 1.59

WI02-N2-53D2 21.7 0.40 30.8 0.60 -- -- 47.4 0.50 1 1.81

WI02-N2B 2um -- -- 24.9 0.39 12.4 0.29 62.8 0.49 1 2.38

WI02-NINE -- -- 75.0 0.50 9.0 0.50 15.9 0.6 1 1.68

s = standard deviation
63

Appendix C

Description of Methods for Data Collection and Analysis to Determine Element Speciation 

Employing X-ray Absorption Spectroscopy

Introduction

X-ray Absorption Spectroscopy (XAS) refers to a technique that employs analysis of X-ray Absorption Fine-Structure 

(XAFS) to determine structural and chemical speciation of elements in various matrices. At characteristic energies 

for a given element, absorption of an x-ray results in ejection of a core level electron. The ejected photo-electron may 

subsequently be scattered from neighboring atoms and interfere with itself resulting in modulation of the energy of the 

photo-electron and, therefore, the absorption event (http://cars9.uchicago.edu/xafs/xas_fun/xas_fundamentals.pdf). In 

practice, XAFS involves probing and analyzing the modulation of the x-ray absorption probability of an atom due to the 

chemical and physical state of the atom. XAFS spectra are especially sensitive to the formal oxidation state, coordination 

chemistry, and the interatomic distances, coordination number, and species of the atoms in the surrounding proximity 

of the selected element of interest. As a result, XAFS provides a practical and straight-forward way to determine the 

chemical state and local atomic structure for a selected atomic species. XAFS can be used in a wide variety of systems 

and bulk physical environments. 

Current practice for collection of XAFS data usually involves the use of a synchrotron as the source of x-rays. The flux 

of x-rays from a synchrotron facility is sufficiently high to prevent the need to conduct experiments in a vacuum or the 

removal of water. Thus, an important aspect from an environmental perspective is that XAFS can be used as an in-situ

spectroscopy allowing for the investigation of samples in their natural state. Another unique aspect of this technique 

is that it provides a means to collect element-specific data. Since each element absorbs x-rays at unique and discrete 

energies, it is possible to selectively collect chemical and structural data for a specific element. In addition, XAFS probes 

the short-range structure of a substance and is, therefore, not limited to analysis of materials possessing long-range 

structural order, e.g., as required for x-ray diffraction analysis. Thus, XAFS measurements can be used to probe the 

chemical and structural speciation of noncrystalline material, disordered compounds, and solutions feasible; matrices 

that are most relevant to environmental systems. 

Though XAFS measurements can be operationally simple, interpretation of XAFS data involves a mixture of modern 

physics and chemistry, and a complete mastery of the data analysis can be somewhat challenging. Though the basic 

phenomena are well understood, an accurate theoretical treatment is fairly involved, and in some respects, still an area 

of active research. 

X-Ray Absorption Fine Structure Spectroscopy

X-ray Absorption Fine-Structure (XAFS) is the modulation of the x-ray absorption coefficient at energies near and above 

an x-ray absorption edge. XAFS is also referred to as X-ray Absorption Spectroscopy (XAS) and is subdivided into 

2 regimes (Figure C.1):

XANES - X-ray Absorption Near-Edge Spectroscopy

EXAFS - Extended X-ray Absorption Fine-Structure

that contain related, but slightly different, information about the local coordination and chemical state of an element.

X-rays (light with wavelength 0.03 ≤ λ ≤ 12 Å or energy 1 ≤ E ≤ 500 keV) are absorbed by all matter through the photoelectric effect: An x-ray is absorbed by an atom, promoting a core-level electron (K, L, or M shell) out of the atom 

and into the continuum. The atom is left in an excited state with an empty electronic level (a core hole). The electron 

ejected from the atom is called the photoelectron. When X-rays are absorbed by the photoelectric effect, the excited 

core-hole will relax back to a “ground state” of the atom. A higher level core electron drops into the core hole, and a 

fluorescent X-ray or Auger electron is emitted.

X-ray Fluorescence: An x-ray with energy equal to the difference of the core levels is emitted.

Auger Effect: An electron is promoted into the continuum from another core level.

X-ray fluorescence occurs at discrete energies that are characteristic of the absorbing atom and can be used to identify 

the absorbing atom.
64

The intensity of an X-ray beam as it passes through a material of thickness, t, is given by the absorption coefficient, 

μ:

I = I0e−μt 

where I0 is the X-ray intensity hitting the material, and I is the intensity transmitted through the material. The absorption 

coefficient depends strongly on X-ray energy, E, and atomic number, Z, and on the density, ρ, and atomic mass, A:

μ ≈ (ρZ4)/(AE3) 

In addition, μ has sharp absorption edges (Figure C.1) corresponding to the characteristic core level energies of the 

atom. The energies of the K-edge absorption edges go roughly as EK ~ Z2. All elements with Z>18 have either a K- or 

L-edge energies between 3 and 35 keV, which can be accessed at many synchrotron sources.

X-ray Absorption Spectroscopy Data Collection

While room-sized accelerators exist for conducting XAS studies, the intensity pales in comparison to accelerators 

found at synchrotron radiation facilities such as Department of Energy National Laboratories (Figure C.2). Figure C.2 

shows the Advanced Photon Source (APS) synchrotron radiation research facility at Argonne National Laboratory in 

the southeastern suburbs of Chicago, IL. Applying components of Figure C.2B to explain structures in Figure C.2A, the 

operation of the APS synchrotron facility entails (A) production of electrons in a linear accelerator which are deposited 

into (B) the booster/injector ring to bring the electron packets near the speed of light. The electron beam is then sent to 

the storage ring (C) from which beamlines (D) as either insertion devices, ID, or bending magnets, BM, are constructed 

for experimental research (E). In-line with the research beamlines are monochromaters that tune the electron beam to 

selected energies via Bragg diffraction and must be capable of energy resolutions of ~ 1 eV at 10 keV. 

XAS data collection can be divided into two realms: 1) configuration of beamline equipment and 2) sample preparation. 

XAS measures the energy dependence of the x-ray absorption coefficient, μ(E), at and above the absorption edge of 

a selected element. μ(E) can be measured in two experimental configuration setups:

Transmission: The absorption is measured directly by measuring what is transmitted through the sample 

(Figure C.3):

I = I0e−μ(E)t

μ(E)t = ln(I/I0) 

Fluorescence: The re-filling of the deep core hole is detected. Typically the fluorescent x-ray is measured 

(Figure C.4).

μ(E) ~ If /I0

Figure C.1 Location of XANES and EXAFS regions of an XAS spectrum. The characteristic energy required to excite 

core level electrons is unique to each element and is known as the absorption edge or edge step.
65

Figure C.2 Aerial photo and architectural diagram of the Advanced Photon Source at Argonne National Laboratory, 

Chicago, IL.

Figure C.3 Experimental configuration for transmission data collection.

Figure C.4 Experimental configuration for fluorescence data collection.
66

Upon commencing an XAS scan to collect data, one has the option to use step-scan, continuous scan, or dispersive 

XAFS modes. Typically, step-scan is used so that regions can be defined, and specific time can be allocated at step 

to ensure adequate counts for statistical purposes.

Typical scan parameters for spectral regions for environmental samples include (Figure C.5):

A. Sample pre-edge to get background trend 

(Range ~ -100 eV to -20 eV, 5 eV sampling)

C. Sample edge region 

(-20 eV to +40 eV, 1 eV)

C. EXAFS region 

Uniform in k-space (to > 12 Å-1, sampling 0.05 Å-1) 

Prefer increased integration time per point at high k

Sample preparation can be as crucial as data collection setup when attempting to collect quality data. Although much 

imagination can be employed for sample preparation, many considerations must be examined to ensure that the integrity 

of the sample is maintained. For example, with redox sensitive materials such as arsenic, one must be mindful of possible oxidation of arsenite either from oxygen in the air, as a result of materials in the sample holder, and over exposure 

of the electron beam. Another important issue to evaluate for sample preparation is the adequacy of the beamline one 

chooses to conduct the experiments. This can be overcome by first working out the absorption lengths of the material at the relevant energies. One should check for beamlines with the needed energy range and focal properties for 

the intended samples. If the X-rays can get through the sample with only a few absorption lengths of attenuation, i.e. 

small, homogeneous particle sizes, one can consider transmission data collection which can be superior to fluorescence 

measurements. However, the edge step must be large enough for a transmission measurement which is hampered by 

concentration of the element of interest. Thus, if the sample is dilute or inhomogeneous, fluorescence data collection 

is better and most often employed.

If the energy is too low, absorption from air and windows can be a problem (a general rule is absorption energy decreases as atomic weight decreases, so lighter elements such as chromium can have artifacts which heavier elements 

such as arsenic can avoid) but can be overcome by enclosing the sample and detection chambers in a non-ionizing gas 

environment. Once an accurate understanding of detector limitations is accomplished for sample preparation, sample 

mounting should be designed with simplicity in mind.

Figure C.5 Standard raw XAFS spectra illustrating the three regions: (A) pre-edge, (B) edge step, and (C) EXAFS. 

The XANES portion of the spectra contains all of edge step and small portions of the pre-edge and EXAFS 

regions (Figure C.1).
67

Placement of the sample within an experimental hutch at a synchrotron facility is outlined in Figures C.3 and C.4, for 

transmission and fluorescence data collection, respectively. In both setups, the sample is placed after the Io detector 

which measures the intensity of the beam before interaction with the sample. The only difference between sample 

mounting for transmission and fluorescence data collection is that the sample must be situated at a 45o angle to the 

incident electron beam for fluorescence mode (Figure C.4) to allow maximum fluorescence detection to occur and minimize elastic scattering. As such, a fluorescence detector for fluorescence data collection must be placed perpendicular 

to the electron beam and in line with the sample (Figure C.4). While transmission data can be collected from samples 

offset of the penetrating beam, data quality is theoretically better by allowing the beam to pass directly through the 

sample placed perpendicular to the electron beam (Figure C.3) with a suitable detector aligned with the electron beam 

(Figure C.3) behind the sample.

Sample holders can vary significantly but follow a basic design. Sample holders can be as simple as smearing a solid 

material on the tacky side of tape and folding the tape back onto itself to secure the sample (Figure C.6). Another common sample holder is a small block of non-reactive material with a depression or hole lathed into the holder to which 

the sample is added with tape securing the opening (Figure C.7). This type of sample holder allows analysis of solids, 

slurries, and solutions. The thickness and size of openings can be customized. An adaptation of this sample holder 

in conjunction with a movable sample stages and automation software yields a basic autosampler (Figure C.8) which 

permits all types of samples for analysis.

Figure C.6 Solid sample sandwiched between pieces 

of Kapton tape. 

Figure C.7 Sample backfilled into opening of a Teflon 

block and secured with tape.

Figure C.8 Autosampler template for multi-sample analysis.
68

Traditional spectroscopic sample holders, such as thin films and thin sections, are often employed as well for specialized projects, but the above mentioned methods are more suitable for typical environmental analyses. However, some 

of the traditional holders are being examined for the purpose of biological specimens and modified approaches can be 

used to deal with samples with unusual physical characteristics (Figure C.9).

Figure C.9 Fluorescence data collection of metal hyperaccumulation in plant leaves.

Extended X-Ray Absorption Fine-Structure Data Analysis

Whether transmission or fluorescence data are collected, the data reduction and analysis are essentially the same. The 

steps to data analysis are 1) reduce the raw spectra (Figure C.10) to k-space (conversion of energy [eV] to wavelength 

[inverse distance]), 2) apply a Fourier transform to convert the k-space data into R-space (conversion of wavelength to 

actual distance), and 3) XAFS data modeling. 

Figure C.10 Raw fluorescence data Pb sorption on ferrihydrite
69

Background correction of a raw spectrum involves assigning a baseline value of zero to the pre-edge region and a 

normalized unity value of one to the EXAFS region (Figure C.11).

Figure C.11 Background corrected spectra of Figure C.10.

The conversion of energy (E) to k-space (Figure C.12) involves first the identification of the threshold energy, Eo, which 

is the energy maximum of the edge step. Thereafter, one isolates the EXAFS region in terms of the wave behavior of 

the photoelectron (k) created in the absorption process by the equation:

( 0 )

2

2m E E

k − = 

where E is energy, Eo is the absorption edge energy, ħ is Planck’s constant, and m is the electron mass.

Figure C.12 k-space conversion of spectra in Figure C.11.
70

By selecting and emphasizing (k-weighting) only positive k-space, one generates oscillations as a function of the photoelectron wave number also known as the (chi) χ(k)-function (Figure C.13). The oscillations in Figure C.13 correspond 

to different near-neighbor coordination shells which, after Fourier transformation (Figure C.14), can be described by 

the EXAFS equation:

( ) ( ) ( )

2 2 2

2 sin 2

k j

k j j

j j

j j

N f k e kR k

kR

σ

χ δ

−

=  +  ∑  

Where f(k) and δ(k) are scattering properties of the atoms neighboring the excited atom, N is the number of neighboring 

atoms, R is the distance to the neighboring atom, and σ2 is the disorder in the neighbor distance. Though complex in 

appearance, the EXAFS equation allows one to determine N, R, and σ2 if the scattering amplitude, f(k), and phase-shift, 

δ(k), are known. Since f(k) and δ(k) depend on the Z of the neighboring atoms, EXAFS is also sensitive to the atomic 

species of the neighboring atom.

Figure C.13 The k3-weighted χ-function of Figure C.12. The oscillations describe the photoelectron wave number as 

the photoelectron constructively and destructively interacts with neighboring atoms around PC.

Now that the energy spectrum in Figure C.11 is converted to k-space (Figure C.13), one can Fourier transform the 

k-space data in R-space (distance). The Fourier transformation is critical to XAFS analysis and is often the area of 

confusion for novice data analyzers. Since the photoelectron effect causes backscattering in XAFS data collection, a 

phase shift causes real values of distance to be offset at least 0.5 Å when determining interatomic bond distances in 

the data modeling techniques. For example, the protruding peak located at approximately 1.8 Å in Figure C.14 for the 

Fourier transformed k-space data is actually determined to be 2.53 Å in the modeling procedure to follow.

Figure C.14 Radial distribution (or structure) function of Fourier transformed k-space data for Figure C.13.
71

The final step in EXAFS data analysis is modeling which is often the most complex and time consuming aspect of the 

procedure. The most common method of modeling is to develop a library of fitting paths generated crystallographic parameters. Figure C.15 shows the crystalline unit cell parameters for magnetoplumbite, an iron oxide with small amounts 

of Pb in the structure. A commercially available software program, ATOMS, is available for this purpose to record what 

and where the information was derived (title and notes sections), space group identity and unit cell angles (abc) of the 

phase, the ab-initio calculation sphere in angstroms (rmax), which element to centralize the calculation around (core), 

and the 3-dimensional positions (xyz) of each element with corresponding tags (Pb1, Fe2, etc).

This ATOMS input file is used to generate possible coordinations of elements within magnetoplumbite as a FEFF 

file (commercially available software) (Figure C.16). (NOTE: The XAS research user community is rather small, and 

many software packages are available free of charge from the international experts.) The FEFF program then uses 

this information to generate, via ab-initio calculations, fitting paths to describe the interatomic bond distances (R) and 

coordination numbers (N) with neighboring elements.

Figure C.15 ATOMS input file for magnetoplumbite 

listing crystallographic information.

Figure C.16 FEFF file used to determine fitting paths for 

EXAFS modeling showing the interaction of 

a central Pb atom (Pb1) with two different 

oxygen atoms (O5 and O3) and three different 

iron atoms (Fe2, Fe5, and Fe4).

Once theoretical fitting paths are generated, one can return to the radial distribution function (Figure C.14) to conduct 

the actual fitting protocol. Again, using one of several software packages for this procedure allows one to evaluate real 

data collected at a synchrotron facility relative to fitting paths from model data. An overlay of the resulting model fit to 

the actual data shows the goodness of the model parameter fitting and determines the relevant information to understand the overall system in terms of the coordination environment (R and N) (Figure C.17). Figure C.17 shows that 

Pb is octahedrally coordinated (N=6) with a bond distance of 2.53 Å to oxygen (Pb-O shell: oxygen is the first nearest 

neighbor) followed by Pb-Fe shell indicating a coordination number of approximately 2 and an interatomic bond distance 

of 3.67 Å. The Pb-Fe data suggest a bidentate (2 bonds) innersphere (oxygen between the Pb and Fe atoms) sorption 

complex for Pb sorption on ferrihydrite.
72

X-Ray Absorption Near Edge Spectroscopy Data Analysis

The XANES portion of the spectrum (Figure C.1) is a much larger signal than EXAFS which allows data collection for 

samples with lower concentrations and less than ideal sample conditions. The XANES portion is typically limited to 

within the range of -100 eV to 300 eV. These values vary depending on the element of interest and shape of the spectrum. The interpretation of XANES is complicated by the fact that there is not a simple chemical or physical description 

of the spectrum since XANES is uniquely different for each element; a fingerprint as an analogy. However, there is 

significant chemical data in the XANES region, notably the formal valence and coordination environment. The valence 

is identifiable by the position of the maximum edge energy by taking the derivative of the XANES region (Figure C.18). 

Typically, the energy difference between oxidation (valence) states of a given element can range significantly. For 

example, identification of arsenite (As3+) and arsenate (As5+) can be easily distinguished (Figure C.19). In the case of 

aqueous Zn and ZnS in Figure 18, Zn is in the divalent (2+) valence; however, there is a slight shift (1 eV) of the ZnS 

spectrum to a lower energy versus the Zn-O coordination of aqueous Zn. One can clearly notice distinctly different 

shapes of the Zn XANES spectra which allow for understanding of the influence of ligand type (first nearest neighbor) 

(Zn-O and Zn-S) on the coordination environment. For the As species in Figure C.19, oxygen is the nearest neighbor 

for both, resulting in similar spectra but offset in E due to different oxidation states. Note that the higher oxidation state 

of arsenate is at a higher E value than arsenite. This is intuitively correct when considering an oxidized atom will exert 

more effort to hang on to remaining electrons.

While analysis of XANES data can provide information on the oxidation state and the first coordination shell, the structural information is much more limited than what is accessible via EXAFS analysis. However, not all cases require one 

to know the level of information provided by EXAFS. XANES data analysis is significantly less labor intensive and in 

some cases can provide the level of information necessary to answer research questions.

Data Analysis of Complex, Heterogeneous Samples

In many instances for environmental samples, the speciation of metals can result in multiple phases. This can make data 

analysis difficult. For example, EXAFS data analysis is providing information on the average coordination numbers and 

bond distances for a given shell. When multiple species are present, these parameters are organized into one value that 

does not represent the complexity of the metal species. The same problem arises when interpreting the coordination 

environment of metals for XANES data analysis. To overcome this issue, one can apply a statistical fitting procedure 

that seeks to strip the multiple components of a sample spectrum into individual parts through the assistance of known 

reference spectra. The two most common methods are linear combination fitting (LCF) and principle component analysis (PCA). LCF analysis of XANES and XAFS spectra (LCF-XANES and LCF-XAFS) is simple to apply to normalized 

XANES spectrum or the k2 or 3-weighted χ-function from EXAFS data reduction. The goal in this procedure is to accuFigure C.17 Structural data derived from ab-initio calculated fitting paths for Pb sorption on ferrihydrite. The red curve is 

the sample data reduced from a raw spectrum, and the blue dots represent the fit from the modeled paths. 

The fitting data can then determine coordination numbers (N) and interatomic bond distances (R).
73

Figure C.18 XANES spectra (thin line) and derivative of XANES spectra (thick line) for aqueous Zn2+ (blue) and ZnS 

(red).

Figure C.19 XANES spectra for arsenite [As(III) - blue] and arsenate [As(V) - red].

mulate enough relevant reference spectra that can explain and represent the unknown environmental sample. Through 

the use of available software, one selects an unknown spectrum to evaluate and multiple known reference spectra to fit 

against the unknown. By repeating the procedure and removing nonessential reference spectra, one can gain a semiquantitative analysis of the major metal species present in the unknown sample (Figure C.20). Detailed information as 

collected in EXAFS analysis is not possible, but identity of multiple species in the sample is accomplished.

For typical environmental samples, the use of finger-printing methods such as LCF-XANES and LCF-XAFS can be a 

very powerful tool to determine metal speciation when multiple phases are present. This approach proves very effective in monitoring contaminated sites to evaluate changes in metal speciation either through in-situ amendments or 

monitored natural attenuation.
74

Figure C.20 Linear combination fitting of X-ray absorption near edge spectroscopy data (LCF-XANES) for a sediment 

(Sediment1) sample with multiple Zn species. The red curve is the sample data, and the blue curve is the 

fitted result. The LCF-XANES results indicate Zn speciation as 87% ZnS, 4% Zn sorbed to ferrihydrite, 

and 9% ZnFe2O4 (franklinite).
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