&EPA Ground Water Issue
United States
Environmental Protection
Agency
Site Characterization to Support Use of
Monitored Natural Attenuation for
Remediation of Inorganic Contaminants in
Ground Water
Robert G. Ford, Richard T. Wilkin, and Steven Acree
Background
The term "monitored natural attenuation," as used in the
following discussion and in the Office of Solid Waste and
Emergency Response (OSWER) Directive 9200.4-17P
(hereafter referred to as the 1999 OSWER Directive;
USEPA, 1999), refers to "the reliance on natural attenua-
tion processes (within the context of a carefully controlled
and monitored site cleanup approach) to achieve site-
specific remediation objectives within a time frame that
is reasonable compared to that offered by other more
active methods." When properly employed, monitored
natural attenuation (M N A) may provide an effective remedy
for ground water where a thorough engineering analysis
informs the understanding, monitoring, predicting, and
documenting of the natural processes. In principle,
MNA provides a reasonable remedy for attaining ground-
water cleanup objectives for some sites with inorganic
contaminants (typically metals and radionuclides). Due
to potential limitations in attenuation capacity within an
aquifer, MNA is likely to be more applicable as a polishing
step and/or under more dilute plume concentrations as
compared to situations encountered in source zones or
in more concentrated regions of a ground-water plume.
The objective of site characterization for assessing the
viability of MNA as a component of ground-water cleanup
is determination of the performance characteristics of
the subsurface system with respect to achieving cleanup
goals. As stated within the 1999 OSWER Directive,
one of the primary processes that may result in natural
attenuation of an inorganic contaminant in ground water
is the transfer of the mobile contaminant into an immobile
form within the aquifer solids; this process is generally
referred to as "sorption", inclusive of adsorption, co-
precipitation, and precipitation reactions (See page 8
of USEPA, 1999; illustrative reactions shown in Table 1).
The presumption for sites where "sorption" (hereafter
referred to as immobilization) appears to result in
Forfurther information contact Robert G. Ford (513) 569-7501 [ford.
robert@epa.gov] at the Land Remediation and Pollution Control
Division of the National Risk Management Research Labora-
tory, Office of Research and Development, U. S. Environmental
Protection Agency, 26 W Martin Luther King Dr, Cincinnati, Ohio
45268 or Richard T. Wilkin (580) 436-8872 [wilkin.rick@epa.gov]
and Steven Acree (580) 436-8609 [acree.steven@epa.gov] at
the Ground Water and Ecosystems Restoration Division, Robert
S. Kerr Environmental Research Center, 919 Kerr Research Dr,
Ada, Oklahoma 74820.
contaminant attenuation is that a specific mechanism
(or mechanisms) controls contaminant partitioning to
aquifer solids. Thus, in order to reliably evaluate the
capacity for and stability of contaminant immobilization
within the aquifer, the mechanistic characteristics of
the partitioning process and the identification of the
subsurface components that influence the extent of the
immobilization reaction need to be understood. This
requires information on the abundance and chemical
speciation of solid phase reactants and products that
participate in the immobilization reaction. The purpose
of this Issue Paper is to highlight at what stage of the
process solid phase characterization techniques need
to be implemented in the site characterization process
and to describe two case studies where the results of
such techniques were critical to evaluation of MNA as a
potential component of ground-water cleanup.
Introduction
The technical framework for evaluating the potential vi-
ability of MNA as a component of a ground-water remedy
is presented in the document entitled "Monitored Natural
Attenuation of Inorganic Contaminants in Ground Water:
Volume 1 - Technical Basis for Assessment" (USEPA,
2007a). This document provides a detailed description
of the objectives of the site characterization effort relative
to the tiered analysis approach recommended in USEPA
(2007a; Section 1C). Specific data requirements and
monitoring approaches to establish the existenceof natural
attenuation mechanism(s) for a range of non-radioactive,
inorganic contaminants is provided in USEPA (2007b).
As described in these documents, site characterization
is conducted to develop and validate the conceptual site
model and to evaluate performance characteristics of the
natural attenuation process(es) that may be active within
the aquifer. The data collection and analysis process is
intended to support:
• development of a detailed knowledge of the system
hydrogeology to establish transport pathway(s),
• determination of the mechanism(s) and rate(s) of
contaminant attenuation,
• determination of the capacity of the aquifer to sus-
tain attenuation of the mass of contaminant within
the ground-water plume needed to achieve cleanup
goals, and
• evaluation of the long-term stability of immobilized
contaminants.
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Hydrogeologic characterization is used to determine the
spatial and temporal variabilities in ground-water transport
that ultimately dictate the types and amounts of aque-
ous- and solid-phase reactants which the contaminant
will contact. Determination of attenuation mechanism(s)
can be realized through evaluation of the ground-water
chemical setting (e.g., pH, oxidation-reduction potential,
and alkalinity), assessment of the chemical speciation of
the contaminant and key reactants in ground water, and
identification of the solid components within the aquifer
with which the contaminant reacts (e.g., iron oxyhydroxides,
represented by [=FeOH]) or are products of the overall reac-
tion (e.g., Pb carbonate, Ni coprecipitated with FeS, or Cu
adsorbed onto iron oxyhydroxide). In general, this aspect of
the characterization effort is directed toward elucidation of
thegeochemicalreaction(s)controlling contaminant removal
from ground water, where the reaction products include the
specific form of the attenuated contaminant (Table 1).
To support this evaluation effort, samples of both ground
water and aquifer solids will need to be collected employing
methods that maintain the in situ integrity of the samples.
Field and laboratorymeasurementsmustthen be employed
to determine concentrations of reactants, the chemical
speciation of the contaminant and/or reactant(s) in solution
that participate in the reaction, and the chemical speciation
of solid phase components that participate in and are pro-
duced from the reaction. This determination of the critical
reaction parameters underpins the subsequent design of
approaches to assess the capacity of the aquifer to sustain
the attenuation reaction, as well as approaches to assess
the stability of an immobilized contaminant relative to antici-
pated changes in ground-water chemistry. Determinations
of capacity and stability will likely include the application
of laboratory-based tests employing site ground water and
aquifer solids, as well as the application of models that
adequately capture the details of ground-water transport
and governing biogeochemical reactions. Ultimately,
defining site-specific aspects of reaction mechanism(s),
rates, capacity, and stability will rely on the development
of multiple lines of evidence based on direct and indirect
observations of the ground-water system.
Mechanisms for immobilization of inorganic contaminants
can be grouped into three general categories: precipitation,
coprecipitation, and adsorption (including ion exchange
under limited circumstances). The tendency for precipitation
of a contam inant to occur will depend on the concentrations
of dissolved reactants in ground water relative to the solu-
bility of potential precipitation products. For example, the
precipitation of lead carbonate (cerussite or hydrocerussite)
from ground water will depend on the concentrations of
dissolved lead and inorganic carbon as well as the pH. Pre-
cipitation may occur under conditions in which the ground
water is chemically oversaturated relative to the solubility
of this solid phase. However, reliance on attenuation of
lead via precipitation of a carbonate mineral will only be
viable for conditions where this solid phase can maintain
ground-water concentration below actionable levels and
can be reasonably expected to resist dissolution that may
be induced by future changes in ground-water pH and/or
alkalinity. Coprecipitation reactions are distinguished from
precipitation reactions, in that the inorganic contaminant is
removed from ground water as a m inor com ponent of some
other precipitating (or host) solid phase. For example, the
host solid phase may be an iron oxyhydroxide that precipi-
Table 1. Examples of immobilization reactions that may be active within a contaminant plume for non-radioactive
inorganic contaminants or radionuclides with long radioactive decay half-life (See also USEPA, 2007b
and Rittmann et al., 2007.). Each attenuation process is organized according to the convention for
depicting chemical reactions, i.e., Contaminant + Reactant(s) = Product(s), with notations to specify
the sampled medium for each [(GW) = ground-water component, (S) = component in aquifer solids;
[=FeOH](S) represents site of adsorption onto iron oxyhydroxide mineral]. The species containing
the immobilized contaminant are highlighted with blue text.
Process
Precipitation
Precipitation (redox)
Coprecipitation
Adsorption
Contaminant
Pb2+(GW) +
Cr(V"042-(GW +
xN'2Vj +
Cu2+(GW) +
Reactant(s)
HCO3"(GMO =
3Fe^(GW) + 40H-(GW) + 4H20 =
d-x)Fe2+(GW) + HS-(GW) =
2[=FeOH](S) =
Product(s)
PbC03(S) + H+(GW)
Cr(|")(OH)3(S) + 3Fe(OH)3(S)
Fe,1-x)N'xS(S) + H+,GW
2[=FeO]Cu(S) + 2H+(GW)
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tates when ferrous iron, transported from an upgradient,
reduced portion of a plume, encounters dissolved oxygen
in more oxidizing, downgradient portions of the plume. An
example of this type of attenuation reaction would be the
coprecipitaton of nickel during oxidation-precipitation of
ferrous iron to produce the mineral, ferrihydrite. Likewise,
one could envision an alternative scenario where nickel
is coprecipitated with a ferrous sulfide mineral, such as
mackinawite, in a zone where sulfate-reducing conditions
have developed. Finally, adsorption reactions are defined
as those where the inorganic contaminant partitions to the
surface of a solid phase that is an existing component of
the aquifer solids (i.e., the sorbent). Adsorption reactions
will be governed by specific properties of the contaminant
(i.e., the adsorbate), the abundance and properties of the
sorbent(s) within the aquifer, and by the ground-water
chemistry (e.g., pH, competing adsorbates, or dissolved
complexing agents). Elucidation of thespecific immobiliza-
tion processes that are active within the aquifer provides
the basis for evaluating the site-specific performance
characteristics of the MNA remedy.
Site Characterization to Define Performance
Characteristics
Developing a detailed site characterization strategy is
facilitated by first considering general site characteristics
that can influence the type of equipment and methods
needed for sample collection, as well as the locations and
frequency of sampling for the monitoring network. This
type of information can often be derived from knowledge
of the regional hydrogeologic setting in which the con-
taminant plume is located as well as historical records
of the types and quantity of contaminant release(s). For
example, knowledge of whether the plume is comprised
of one or more inorganic contaminants or also contains
organic contaminants needs to be factored into the selec-
tion of sampling and analysis protocols. This situation is
illustrated in Figure 1, which shows how biodegradation of
an organic co-contaminant within the plume may influence
the type of immobilization reactions that might occur. The
degradation of the organic can result in localized changes
in redox conditions within the aquifer. It could also result
in the production of ligands that affect speciation of the
Methanogenesis
CH3COOH + H2O = CH4 + HCO3- + H+
Sulfate Reducing
SO/- + 9H* + Be- = HS- + 4H2O
Cu2+ + 2[=FeOH](s) = 2[=FeO]Cu(s) + 2H+
CrO42- + SFe2* + 4OH- + 4H2O = 3Fe(OH)3(s) + Cr(OH)3(s)
Figure 1. Illustration of ground-water geochemical conditions that may develop due to the presence of a biodegrad-
able organic co-contaminant within the plume and potential immobilization reactions for a range of inorganic
contaminants. Pertinent microbially-mediated reactions occurring under methanogenic, sulfate reducing, and
iron reducing reactions are shown. Products of these reactions serve as reactants for a range of potential
immobilization reactions listed at the bottom of the figure; immobilized contaminants are highlighted with
blue text (=FeOH and =FeO represent sorption sites on iron oxyhydroxide mineral surfaces; "e~" refers to an
electron transfer process involving other aquifer components;. More detailed discussion of the impact
of microbial processes on ground-water chemistry is provided in USEPA (1998, 1999b, 2002, and 2007'a).
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inorganic contaminant. The presence of multiple inorganic
contaminants may necessitate use of analytical methodolo-
gies with different sensitivity and/or different approaches
to sample collection and preservation. Relative to the sub-
surface setting, the spatial and temporal characteristics of
ground-water flow and chemistry may also factor into the
characterization strategy. Initial assessments of whether
the aquifer is generally oxidizing or reducing, a shallow or
deep system, and is or is not influenced by external hydro-
logic forces (e.g., ground water/surface water interactions,
recharge from meteoric precipitation, or episodic regional
withdrawals from the aquifer) need to be considered in de-
signing the dimensions of the monitoring network and the
frequency of data collection to characterize site chemistry
and hydrology.
For sites where immobilization is the dominant attenuation
process, understanding the types of solid phase compo-
nents that participate in the evolution of the contaminant
plume is critical to reducing the level of uncertainty in the
selection of MNA as a component of the ground-water
remedy. These solid phase components can be grouped
into three different categories: 1) components that contrib-
ute to the source of contaminant mass within the plume, 2)
components (biotic and abiotic) that participate directly or
indirectly during the attenuation process, and 3) the solid
form of the immobilized contaminant. Relative to the first
category, the solid phase speciation of the contaminant
in source areas does not necessarily affect the specific
mechanism(s) of attenuation in the downgradient plume.
However, the total mass and rate of contaminant release
from concentrated source areas can exert direct impact
on the adequacy of the inherent capacity for the aquifer
to attenuate contaminant transport. As illustrated in one
of the case studies presented later, consideration of the
residual mass of contaminant within source areas following
removal efforts is important to evaluation of attenuation
capacity within the aquifer. The latter two solid phase
component categories bear directly on evaluation of the
mechanism, capacity, and stability of the immobilization
process. Solid components that participate indirectly
include aquifer minerals that buffer ground-water geo-
chemistry (e.g., alkalinity buffering by carbonate minerals
or ion exchange with clay minerals) and/or subsurface
microbial communities whose activity can directly affect
ground-water chemistry (e.g., iron- or sulfate-reduction)
and precipitation/dissolution of new reactive minerals (e.g.,
iron sulfides). Finally, evaluation of the chemical form of
the attenuated contaminant provides confirmation of the
active mechanism and provides the basis for assessing the
long-term stability of the immobilized contaminant relative
to future changes in ground-water chemistry that might be
reasonably anticipated. For convenience, a summary of
several important data objectives to consider relative to
the three identified solid phase component categories is
provided in Table 2.
Table 2. Data objectives to consider relative to solid phase characterization activities supporting evaluation
of the mechanism, capacity, and stability of contaminant immobilization. More detailed discussions
of data requirements and methodologies for data acquisition are provided in USEPA (2007a).
Solid Phase Category
Contaminant Source
Reactants
Abiotic
Biotic
Immobilized Contaminant
Data Objectives Relevant to Site Characterization
• Mass of contaminant and rate of release
• Interaction dynamics of contaminant source with system hydrology and chemistry
• Chemical form of contaminant transported from source area
• Identity and accessible/available mass of aquifer solids that control concentrations of
soluble reactants
• Identity and accessible/available mass of aquifer solids that react directly with contami-
nant during immobilization
• Spatial distribution of solid components that participate directly or indirectly in contami-
nant immobilization along critical ground-water flow paths
• Microbial community activities that affect ground-water chemistry and indirectly impact
contaminant immobilization
• Direct interactions of microbial community with contaminant during immobilization
• Type and mass of substrates and bioavailable terminal electron acceptors used by micro-
bial community
• Chemical speciation of immobilized contaminant
• Stability of immobilized contaminant relative to current and potential future ground-water
chemical conditions
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In order to illustrate the issues one may need to consider
during development of the site characterization strategy, a
hypothetical situation will be considered in which nickel in
ground water is attenuated as a result of iron suIfide mineral
precipitation within a plume. The process is presented in
Table 1 as an example of a coprecipitation reaction. In the
following analysis, questions will be posed relative to threeof
the general information needs identified in the Background
section (i.e., "mechanism and rate", "capacity", and "stabil-
ity"). Potential measurements that one may undertake to
address these questions will then be identified. Measure-
ments that require the collection and characterization of
solid materials from the aquifer are highlighted to illustrate
the importance of solid phase characterization.
As previously stated, identification of the mechanism con-
trolling contaminant attenuation depends on determ ination
of the key components that participate in the reaction.
As indicated by the entries in Table 1 for this hypotheti-
cal scenario, iron, sulfur, and ground-water pH may exert
direct control on nickel attenuation. This leads to some
specific questions that need to be addressed in order to
confirm that the proposed process is actually occurring
within the aquifer:
Question
Is dissolved sulfide in
ground water? If not, is
sulfate in ground water?
Is ferrous iron in ground
water?
Does decrease in
ground-water nickel
concentration coincide
with decreases in dis-
solved sulfide/sulfate
and/or ferrous iron?
Is iron sulfide present in
aquifer solids?
Is nickel associated
with iron sulfides in
aquifer sediments?
What is the rate of
nickel attenuation in
locations where iron
sulfide precipitation is
occurring?
Potential Measurements
1 Ground-water chemistry (field
colorimetry for sulfide, field or
laboratory measurement for
sulfate)
1 Ground-water chemistry (field
colorimetry)
1 Ground-water chemistry
(laboratory measurement for
nickel; see above for sulfide,
sulfate, and ferrous iron)
1 Laboratory measurement
for iron sulfide content
(acid volatile sulfide extrac-
tion of preserved solids)
Laboratory measurement
(acid volatile sulfide extrac-
tion with measurement of
co-extracted nickel)
1 Electron microscopy with
element mapping to dem-
onstrate Fe-Ni-S associa-
tion
1 Spectroscopic methods to
directly identify nickel solid
phase speciation
1 Examination of spatial or time
trends in nickel and iron/sul-
fide loss from ground water
1 Controlled laboratory tests
using site-derived ground
water or synthetic water solu-
tions that simulate ground-
water chemistry
The results from these measurements will provide both
direct and indirect lines of evidence to determine whether
the hypothesized attenuation mechanism is active within
the plume. Since the geochemical setting in this situation
is likely reducing with little or no oxygen, using proper pro-
cedures to acquire and preserve samples of ground water
and aquifer solids is critical to insuring data accuracy (e.g.,
USEPA, 2006a). For ground-water samples, one approach
to minimize potential alterations to the chemical specia-
tion of ferrous iron and dissolved sulfide is to conduct the
measurement in the field with a sampling procedure that
minimizes air contact (USEPA, 2002). Available methods
and equipment forthese field measurements are sufficiently
sensitive to evaluate concentration trends of relevance to
a contaminant plume.
Subsequent to identifying the mechanism of contaminant
attenuation, it will then be necessary to determine if suf-
ficient capacity exists within the aquifer to attenuate the
mass of nickel that will be transported through the aquifer.
In simple terms, the goal of this effort is to account for the
mass of aquifer components available to react with the
contaminant to result in a sufficient level of immobilization
to achieve cleanup goals (e.g., drinking water standards,
ambient water quality criteria, or risk-based criteria). In
addition, thespatial distribution of the relevant aquifer com-
ponents relative to the spatial distribution of contaminant
flux needs to be considered during this accounting exer-
cise. Thus, in addition to the use of appropriate sampling,
preservation, and analytical methodologies, the monitoring
network from which environmental samples are retrieved
needs to adequately capture the dimensions and physical
heterogeneity of the plume. Relative to the hypothetical
immobilization mechanism currently being considered,
the following questions need to be addressed in order to
assess capacity:
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Question
• What is the source of
ferrous iron and dis-
solved sulfide?
If source is from
microbial reduction,
is there sufficient
electron donor and
acceptor supply?
• Is the mass flux of
ferrous iron and
dissolved sulfide
sufficient to capture
nickel given knowl-
edge of process
efficiency?
Potential Measurements
1 Site information on source
area waste constituents (liq-
uid and/or solid) as potential
source
• Ground-water chemistry data
to map out spatial distribution
of ferrous iron and/or dis-
solved sulfide
• Ground-water chemistry data
to infer microbial processes
(USEPA, 2007a; Section NIC)
1 Direct identification of
microbial community char-
acteristics (USEPA, 2007a;
Section IIIC.4)
• Ground-water or aquifer
solids chemistry data to
estimate available mass of
electron donor (e.g., dis-
solved and/or solid organic
carbon)
• Ground-water or aquifer
solids chemistry data to
estimate available mass
of electron acceptor (e.g.,
dissolved sulfate and/or iron
oxyhydroxide minerals)
• Ground-water concentration
and flow data for reactants
• Laboratory tests to assess
efficiency of nickel coprecipi-
tation with iron sulfides under
chemical conditions expected
to be encountered along
transport pathway(s)
Finally, the stability of the immobilized contaminant needsto
be assessed relative to potential fluctuations or anticipated
long-term changes in ground-water chemistry. Potential
fluctuations in ground-water chemistry may result from
natural hydrologic events such as recharge from surface
infiltration or water level fluctuations in surface water bod ies
that are hydraulically connected to the aquifer. Alterna-
tively, fluctuations in ground-water chemistry may result
from changes induced by active remediation (Sutherson
and Horst, 2008), which may precede implementation of
MNAas a follow-on component of theoverall ground-water
remedy. Long-term changes may result from dissipation
of the contaminant plume accompanied by the influx of
upgradient ground water into the portion of the aquifer
previously occupied by the plume. Assessing the long-
term stability is critical because contaminant immobilization
may occur under ground-water chemical conditions that
are far removed from future conditions following plume
diminution. For example, precipitation or co-precipitation
reactions often occur where plume characteristics induce
dramatic changes in ground-water chemistry parameters
such as oxidation-reduction potential, pH, or alkalinity. In
these situations, the immobilization process may result in
accumulation of a new solid component that, while stable
under existing plume conditions, is unstable with respect
to natural conditions in the aquifer outside of the plume.
Contaminant immobilization may also be temporary in situ-
ations where adsorption to existing aquifer solids controls
contaminant retardation, since these reactions are often
sensitive to changes in ground-water pH or major ion
chemistry. Ultimately, analysis of contaminant stability
must be guided by knowledge of the chemical conditions
driving immobilization relative to natural or anticipated fu-
ture ground-water conditions. Relative to the hypothetical
immobilization mechanism currently being considered, the
following questions need to be addressed:
Question
1 Are natural ground-
water pH and/or
major ion chemistry
significantly different
as compared to exist-
ing plume conditions?
1 Is natural ground
water oxidizing?
1 Is nickel released
from iron sulfide upon
exposure to upgradi-
ent ground-water
chemistry?
1 Does chemical form
of immobilized nickel
change upon expo-
sure to upgradient
ground-water chem-
istry?
Potential Measurements
1 pH and/or major ion chemistry
of upgradient ground water
or other sources of recharge
along relevant transport path-
ways
1 Dissolved oxygen concentra-
tion and/or oxidation-reduction
potential of upgradient ground
water or other sources of re-
charge along relevant transport
pathways
Laboratory tests using
contaminated aquifer solids
with samples of upgradi-
ent ground water or other
sources of recharge
> Chemical extraction meth-
odologies to examine nickel
solid phase association and/
or speciation in laboratory
test samples and/or sampled
aquifer solids
1 Spectroscopic and/or elec-
tron microscopy to examine
nickel solid phase asso-
ciation and/or speciation in
laboratory test samples and/
or sampled aquifer solids
It should be noted that the last recommended measure-
ments imply additional collection and analysis of the solid
phase speciation of the immobilized contaminant during
performance monitoring of the MNA remedy. A change in
contaminant solid phase speciation indicates a change in
immobilization mechanism. Knowledge of this change is
important both to the conceptual understanding of factors
controlling the long-term stability of the immobilized con-
taminant and to the development of chemical speciation
and/or reactive transport models that may be employed
to estimate attenuation capacity and project permanence
of the MNA remedy.
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Case Studies Illustrating Importance of Solid
Phase Characterization
The preceding descriptions of solid phase characterization
measurements and their application as part of the tiered
analysis approach for site characterization illustrate the
rigorous effort that is needed to reduce the uncertainty in
selecting MN A as a component of the ground-water remedy
(USEPA, 2007a; Section 1C). This level of effort is justified
sincetheperformancecharacteristicsof the natural attenu-
ation process are not known in the absence of site-specific
data. It should also be noted that many of the measurements
undertaken to characterize conditions within the plume are
similarto those that would be implemented during selection,
optimization, and performance monitoring of an engineered
remedy that is designed to manipulatesubsurface chemistry.
Thus, the high level of effort required for conducting these
characterization tests will likely prove beneficial regardless
of the final remedy selected. To illustrate the importance
of solid phase characterization measurements, summary
descriptions of two case studies are provided for sites
where MNA was under consideration as a component of
a remedy to address inorganic contaminants in ground
water. These case studies provide clear examples of the
importance of the characterization of solids from the sub-
surface and the potential impacts of hydrologic dynamics
on the effectiveness of natural attenuation reactions for
controlling contaminant mobility.
Industri-Plex Site Arsenic Plume
The Industri-Plex Superfund Site is a former chemical and
glue manufacturing facility in Massachusetts. Chemicals
such as lead-arsenic insecticides, acetic acid, and sulfuric
acid were produced for local textile, leather, and paper
manufacturing industries from 1853 to 1931 (Durant et al.,
1990). Chemicals manufactured by other facilities at the
site include phenol, benzene, and toluene. Industri-Plex
was also used to manufacture glue from raw animal hide
and chrome-tanned hide wastes from 1934 to 1969. The
by-products and residues from these industries contami-
nated the soils at the site with elevated levels of metals,
such as arsenic, lead and chromium. During the 1970s,
the site was redeveloped for industrial use. Excavations
uncovered and mixed industrial by-products and wastes
accumulated over 130 years. During this period, residues
from animal hide wastes used in the manufacture of glue
were relocated on-site from buried pits topiles near swampy
areas on the property. Many of the hide piles and lagoons
were leaching toxic metals into the underlying shallow aqui-
fer. Estimates suggest that 270 metric tons of arsenic may
still exist within the site boundaries (Aurilio et al, 1995). In
response to public health concerns, a Record of Decision
(ROD) for the Industri-Plex Superfund Site was signed in
1986, addressing the stabilization of on-site soil, sediment,
and hot spot ground-water contamination under Operable
Unit 01 (USEPA, 1986). It is hypothesized that leached or-
ganic degradation products from site disposal areas have
contributed to the transport of arsenic via ground water
to a wetland that flows to the Aberjona River (Davis et al.,
1994). TheaquiferunderlyingNorthWoburn, MA, including
the Industri-Plex Superfund Site, has been designated as
a non-drinking water source area by the state of Massa-
chusetts. Under the 1986 ROD, the USEPA was required
to conduct a Multiple Source Groundwater Response Plan
(MSGRP) which served as a second operable unit (OU-2)
for the Industri-Plex site. The MSGRP was required to
investigate other potential contamination impacts on the
regional aquifer, and determine if additional remedies were
necessary to clean up the aquifer within the Industri-Plex
Study Area.
Characterization of the ground-water plume that migrates
beyond the southern boundary of the Industri-Plex site was
documented as part of the MSGRP Remedial Investigation
(USEPA, 2005a). Information from site monitoring revealed
that the plume primarily discharges into a pond known as
the Halls Brook Holding Area (HBHA) Pond. This surface
water feature was constructed during the early 1970s 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.
Historical surface water data from the outlet of the HBHA
Pond indicated significant reduction in arsenic concentra-
tion from that expected for the discharging ground-water
plume, presumably dueto oxidation-precipitation of ferrous
iron and sequestration of arsenic by iron oxyhydroxides that
are subsequently deposited onto sediments in the HBHA
Pond (e.g., Aurilio et al., 1994; Davis et al., 1996). These
observations suggested that natural attenuation processes
occurring within the HBHA Pond could contribute to the
overall ground-water remedy. During the period 1999-
2001, a field investigation was conducted to characterize
the chemical and hydrologic processes impacting the fate
and mobility of arsenic entering the HBHA Pond following
plume discharge. The results of this site characterization
effort are documented in a series of publications (Wilkin
and Ford, 2002; USEPA, 2005b; USEPA, 2005c; Ford et
al., 2006; USEPA, 2006a; Wilkin et al., 2006).
In summary, the results of this field investigation revealed
that natural attenuation of arsenic was not occurring within
the aquifer and that the HBHA Pond attenuated only a
fraction of the arsenic derived from plume discharge.
Consistent with historical observations, ground-water geo-
chemical conditionsalong flow paths from the Industri-Plex
source areas to the HBHA Pond were sufficiently reduc-
ing to maintain arsenic (and iron) in a mobile form. Upon
discharge to the HBHA Pond, ferrous iron did undergo
oxidation-precipitation in the presence of dissolved oxygen,
efficiently sequestering arsenic from the plume (Figure 2A).
However, upon settling to the sediment layer in anoxic
zones within the bottom of the HBHA Pond, much of the
arsenic-laden iron oxyhydroxides re-dissolved to release
arsenic and ferrous iron to the water column. These dis-
solved constituents could diffuse backuptowardsthe water
surface, whereoxidation-precipitation of ferrous iron would
again sequester dissolved arsenic via coprecipitation with
-------
newly formed iron oxyhydroxides. The internal recycling
of iron and arsenic within the HBHA Pond following plume
discharge is illustrated in Figure 2B. Characterization of
arsenic speciation in sediments indicated that a fraction
of the arsenic load to the sediment layer was transferred
to iron sulfides due to sulfate reducing conditions. The
generally low stability of the co-precipitated arsenic within
theHBHAPond resulted in high concentrations of dissolved
arsenic within the deep surface water (Figure 2A). During
base flow conditions, arsenic was primarily retained within
the pond due to stratification that limited vertical transport
of arsenic to shallow surface water. However, during large
surface water flows, portions of the pond were mixed lead-
ing to increased flux of arsenic at the pond outlet (USEPA,
2005a; USEPA, 2005b). These observations supported
the conclusion that MNA was not a viable option for treat-
ment of the arsenic plume. However, characterization of
the physicochemical controls on arsenic fate within the
HBHA Pond enabled use of the surface water body as a
component of the selected remedy, in which engineered
enhancements to control system hydraulics and oxidation
processes will be incorporated (USEPA, 2006b).
Differences in supporting information available before
("Original") and after the 1999-2001 investigation ("Revised")
are listed in Table 2. Significant limitations of the initial
site characterization effort included the lack of information
supporting analysis of system capacity for attenuation and
analysis of the stability of arsenic partitioned to solids across
the transition zone from ground water to surface water, as
well as improper collection/preservation methods for solid
samples that contained oxygen-sensitive components.
Improper collection/preservation techniques led to an
incomplete analysis of arsenic attenuation processes and
overestimation of attenuation capacity. While there were
active processes sequestering arsenic derived from plume
discharge, the instability of arsenic solid phase associations
in concert with the periodic occurrence of hydrologic events
that disturbed system functionality precluded use of MNA
as a remedy at this site. This example illustrates how cost
savings may be realized during site characterization, since
reiteration of the characterization effort to address lim itations
in the original conceptual site model (CSM) entailed signifi-
cant added cost. Specifically, many of the shortcomings
of the original site data compilation could be attributed to
improper collection, preservation, and processing methods
for site-derived samples. Knowledge of general site char-
acteristics, in this case a biologically productive wetland
setting, should alert those designing the characterization
effort to consider sources of potential analytical bias from
use of inappropriate sampling or analysis protocols.
Upgradient -
anaerobic
hide piles &
wetland soils
Flow path for
geochemical
measurements
S°4(aq)
Fe(M)aq
H3As03(aq)
Solid
Phase
FeS
(s)
Fe(OH)3(8)
Internal Recycling Process
Surface Water
As-HFO,.
©t
O'
As-H FO,S, + H2S(aq) (or other reductants)
Figure 2. Conceptual depiction of (A) the spatial distribution of aqueous and solid components along a flow path (solid
blue arrow) across the ground water/surface water (GW/SW) transition zone at the Industri-Plex Superfund Site
(blue dotted rectangle highlights area of detail shown in Panel B) and (B) solid-solution partitioning dynamics
of arsenic within the Halls Brook Holding Area (HBHA) Pond.
-------
Table 2. Comparison of site characterization efforts during original and revised consideration of MNA as a
component of the ground-water remedy for the Industri-Plex Superfund Site (OU-2). Solid phase
characterization measurements are highlighted in bold.
Data Category
Site Characterization to Support MNA Evaluation
Original
Revised
Hydrology
1 Particle track modeling using measured water
levels to characterize plume discharge and cap-
ture within HBHA Pond
1 Direct measurement of ground-water flux into
HBHA Pond
1 Measurement of surface water flow inputs and
outputs to calculate ground-water flow contribu-
tion
1 Measurement of ground-water chemistry in tem-
porary and permanent nested well clusters to map
out plume dimensions adjacent to and underneath
HBHA Pond
Reaction
Process
1 Characterization of elemental arsenic and iron
association in pond sediments using chemical
digestion
1 Scanning electron microscopy with energy dis-
persive spectroscopy for a subset of sediment
samples to evaluate microscale As-Fe associa-
tion and infer controlling mineralogy
1 Determination of the presence of sulfide miner-
als using acid volatile sulfide (AVS) extraction on
unpreserved sediment samples (USEPA, 2006a)
• Measurement of redox chemistry in water within
the plume, sediment pore space, and HBHA Pond
water column using in-situ techniques or sampling
procedures that avoided air-exposure
• Direct measurement of arsenic speciation in
water by HPLC-ICP-MS and in solids samples
using X-ray absorption spectroscopy in combi-
nation with chemical extraction methodologies
• Field measurement of ferrous iron using colorimet-
ric techniques in combination with total iron by
atomic emission spectroscopy
• Identification of iron-bearing minerals in sus-
pended solids and sediments, along with ele-
ment correlations in solids via bulk extraction
and SEM-EDS measurements
Attenuation
Capacity
1 Laboratory measurements of arsenic seques-
tration capacity using unpreserved sediment
samples
1 Depth-discrete measurements of dissolved and
particulate arsenic and iron throughout entire
water column and sediment layer within the
HBHA Pond at locations within and outside of
the influence of plume discharge
Stability of
Attenuation
1 Historical measurements of arsenic concentra-
tions in plume discharge and depth-integrated
pond water samples to infer negligible release of
sorbed arsenic
1 Field measurement of arsenic release during
deposition of arsenic-iron oxyhydroxide coprecipi-
tates on the pond bottom via time-series sampling
across the ground water-surface water transition
zone following a large surface water flow event
Mineralogical identification of components
controlling arsenic attenuation in suspended
solids and shallow sediments in combination
with geochemical models to assess theoreti-
cal stability under measured field geochemical
conditions
-------
Hanford 300 Area Uranium Plume
The U.S. Department of Energy's Hanford Site is a
586-square-mile federal facility located in southeastern
Washington along the Columbia River (Duncan, 2007).
The Hanford Site was established during World War II, as
part of the Manhattan Project, to produce plutonium for
nuclear weapons. One of the primary waste components
from this production effort was uranium. The 300 Area,
which encompasses approximately 1.35 square kilometer
(0.52 square mile), is adjacent to the Columbia River and
approximately 1.6 kilometer (1 mile) north of the city limits
of Richland, WA. The 300 Area terrain is generally level, with
a steep embankment dropping to the river. The 300 Area
was established as a fuels fabrication complex in 1943.
Liquid process wastes generated during these operations
were disposed in unlined surface ponds and trenches that
are included in the 300-FF-1 Operable Unit under CERCLA.
A portion of these liquid wastes containing elevated levels
of uranium infiltrated into the vadose zone and underlying
unconfined aquifer adjacent to the Columbia River. Subse-
quently, a plume of uranium has developed in the unconfined
aquifer, which migrates towards and discharges into the
river (Figure 3A). Selection of the remedy for cleanup of
surface and subsurface contamination within the 300 Area
was documented in the 1996 Record of Decision (ROD;
USEPA, 1996). Removal of contaminated soil and debris
was selected as a component of the remedy for Operable
Unit 300-FF-1. Ground-water contamination (Operable
Unit 300-FF-5) was addressed through selection of an
interim remedial action that included institutional controls
to restrict use of ground water and continued monitoring
of ground water to verify decreasing trends in contaminant
concentrations (uranium, trichloroethene, and 1,2-Dichlo-
roethene) with future review to determine the need for ac-
tive remedial measures.1 Selection of the interim remedy
for ground water was supported by contaminant transport
model projections that indicated uranium concentrations
would decrease to <20 |ag/L due to natural processes over
a period of 3-10 years following removal of contaminated
surface soils (DOE, 1994; DOE, 1995).
The development and implementation of a contaminant
transport model for uranium in the unconfined aquifer played
a key role in the selection of the interim remedial action
for Operable Unit 300-FF-5. It is instructive to evaluate
what assumptions were made in constructing the model
descriptions of water transport and uranium partitioning
to aquifer solids. Waichler and Yabusaki (2005) provide
a useful description of the assumptions inherent to the
original transport model and the associated limitations for
describing uranium transport in the unconfined aquifer. In
general, the following assumptions appear to have been
critical relative to disparities between model projections and
the subsequent observed behavior of the plume: 1) removal
1 The 1996 ROD was prepared prior to publication of the 1999
OSWER Directive in which expectations for assessment and
use of MNA as a remedial action alternative were documented.
Documentation of more recent site investigations is available at
http://www.hanford.gov/cp/gpp/library/programdocs-300.cfm or
http://ifchanford.pnl.gov/publications/.
of contaminated surface soils eliminated the source of
uranium from which the plume developed; 2) ground-water
flow was unidirectional towards the Columbia River, which
was treated as a constant head boundary; and 3) partition-
ing of uranium to aquifer solids could be described using
a constant equilibrium sorption isotherm (Kd) independent
of water chemistry and aquifer mineralogy. Subsequent
investigationsto determine the causeof sustained elevated
uranium concentrations in the plume have demonstrated that
a lack of information on the spatial distribution of uranium
within subsurface solids and the chemical speciation of
solid phase uranium were important limitations to the reli-
ability of the contaminant transport model. First, failure to
consider that a fraction of the uranium transported through
the vadose zone was retained in the subsurface in a range
of solid forms (Wang et al., 2005; Catalano et al., 2006; Aral
etal., 2007; McKinleyetal., 2007) led to overly conservative
projections of the long-term flux of uranium that could be
transported through the saturated aquifer. Characterization
of subsurface solids collected from under former waste
process areas indicated elevated uranium concentrations
bound to the solids in both precipitated (co-precipitated
in CaC03, uranium-phosphate precipitates) and adsorbed
(e.g., muscovite in aquifer solids) forms. As demonstrated
by subsequent hydrologic investigations, water level fluc-
tuations induced by periods of recharge from the Columbia
River, causes a portion of residual uranium solids within the
lower vadose zone (i.e., the smear zone in Figure 3A) to be
cyclically leached into the underlying plume (Qafoku et al.,
2005; Zachara et al., 2005; Bond et al., 2007; Peterson et
al., 2008; Yabusaki et al., 2008). Finally, aquifer recharge
by river water with low dissolved carbonate concentrations
results in cyclical increases in uranium adsorption to aqui-
fer solids within the plume, which has resulted in slower
dissipation of the plume due to flushing from the aquifer.
Tests with contaminated subsurface solids have demon-
strated that dissolved carbonate is the most important
parameter in site ground-water chemistry with respect to
impacting uranium mobility; uranium adsorption to aquifer
solids decreases with increasing alkalinity (e.g., Serne et
al., 2002; Bond et al., 2007). The revised conceptual site
model including identification of the important subsurface
zones is provided in Figure 3A (Peterson et al., 2005; Nim-
mons, 2007), and the solid phase distribution and chemical
speciation of uranium as a function of depth below source
areas are shown in Figure 3B.
Asummary of site characterization efforts supporting interim
remedy selection under the 1996 ROD and subsequent
solid phase characterization studies for the ground-
water system is provided in Table 3. Comparison of the
information collected under the "Original" and "Revised"
column listings illustrates the disparity between what
was known at the time of the 1996 ROD and the present.
The site-specific knowledge of hydrologic dynamics
and uranium geochemistry within the vadose zone and
ground-water plume gained from the various tests and
characterization methods employed using field samples
has significantly reduced the level of uncertainty relative
to the processes controlling plume dynamics. Based on
10
-------
Han ford 300 Area
Historical Disposal Unit M
Contaminated
Vadose
Zone
Transient
Flow
Reversal
daily fluctuation
B
c
0
W
J» NP4-1
f^ NP. -2
:a& NFi-45
I I O NPP2-0.5
O
I hOI NPP2-4
] NP1-6
*—NP1-NP4-NPP1
O NPP2
U(VI)-CaCO3
Metatorbernite
U(VI)-Muscovite
NPP2-Fines
Water
U Species Cone, (mg/kg)
Figure 3. Subsurface uranium contamination within the Hanford 300 Process Waste Sites area. A) Primary hydrogeologic features impacting plume
migration to the Columbia Region (vertical dimension is exaggerated; CSM zones identified with circled numbers after Nimmons, 2007). B)
Concentrations and speciation of solid phase uranium in vadose zone above (North Process Ponds solids and contaminated aquifer solids)
and within the influence of ground-water level fluctuations (smear zone). Note break in x-axis scale; "Water Table" is approximate location of
ground-water surface at the time of subsurface excavations; closed and open symbols represent two sampling locations; graph generated
using information from Wang et al. (2005), Catalano et al. (2006), Aral et al. (2007), McKinley et al. (2007).
-------
the current site knowledge, a decision has been made
to use active remedial technologies to control the flux of
uranium entering into ground water from the vadose zone
and the zone of fluctuating water table (i.e., "smear zone"
in Figure 3A). It is anticipated that by minimizing the flux of
uranium contributing to concentrations in the ground-water
plume, natural attenuation processes may be successful
in achieving cleanup objectives in more dilute portions
of the plume. In general, the revised information on the
mass and speciation of uranium in aquifer solids relative
to relevant transport pathways has contributed to more
realistic expectations of the role that MNA may play as a
component of the ground-water remedy.
Tables. Comparison of site characterization efforts during original and revised consideration of MNA as
a component of the ground-water remedy for the Hanford 300 Area (OU 300-FF-5). Solid phase
characterization measurements highlighted in bold.
Data
Category
Site Characterization to Support MNA Evaluation
Original
Revised
Hydrology
• Water level measurements in
aquifer and stage measurements in
Columbia River
• Modeled ground-water transport
assuming average river stage as
downgradient boundary condition
Higher time frequency measurements of river stage and ground-
water level
Modeled ground-water transport to account for time-variant
changes in water flux and direction
Reaction
Process
Modeled sorption of uranium to
aquifer solids assuming fixed Kd in-
dependent of ground-water chem-
istry and aquifer solids mineralogy
1 Laboratory tests with aquifer solids from various depths to
assess influence of ground-water chemistry on the extent and
rate of uranium sorption-desorption
• Direct measurement of uranium speciation in vadose zone pore
water using fluorescence spectroscopy to identify mobile aqueous
species
1 Direct measurement of uranium speciation in aquifer solids as
a function of depth using bulk and microfocused X-ray spec-
troscopy/diffraction and electron microscopy in combination
with chemical extraction methodologies
1 Determination of uranium distribution as a function of particle
size in aquifer solids
1 Identification of reactive clay minerals in aquifer solids
• Modeled contaminant transport to account for rate-limited sorption-
desorption processes and influence of ground-water chemistry on
uranium sorption to aquifer solids
Attenuation
Capacity
Estimated sorption contribution
based on Kd and mass of aquifer
solids along ground-water flow
path
Assumed no additional inputs of
uranium from overlying vadose
zone into plume
1 Laboratory measurements with aquifer solids to estimate mass
flux of uranium derived from smear zone
Stability of
Attenuation
No measurement of stability of ura-
nium partitioned to aquifer solids
1 Laboratory tests to evaluate reversibility of uranium sorption as
a function of contact time and/or uranium solid phase specia-
tion
• Geochemical modeling to assess theoretical stability of identi-
fied solid phase uranium species under relevant field geochemical
conditions
12
-------
Future Technical Development Issues
The case studies reviewed above illustrate several types
of measurements that provide information on the charac-
teristics and behavior of solid phase components involved
in contaminant transport and attenuation. For both stud-
ies, identification of the solid components participating in
contaminant attenuation wasclearly important. In addition,
knowledge of the chemical speciation of the contaminant
(beyond its mineral association) was important for identify-
ing the specific reaction mechanism(s) controlling attenu-
ation, as well as understanding factors that may influence
the efficiency and stability of the immobilization process.
It should also be evident that selection of 1) sampling
locations and frequencies, 2) sampling procedures and
3) the specific measurement techniques needs to be con-
ducted with knowledge of the ground-water hydrology
and biogeochemical characteristics of the plume. While
not discussed in detail within this document, information
derived from characterization of the flow system (pathways
and dynamics in velocity and/or direction), as well as the
general chemical conditions within the plume bear directly
on design of the sampling and analysis program applied
to solid phase characterization for a site (USEPA, 2007a;
Sections IMA and NIB, respectively). As an example, sub-
surface redox conditions will dictate to a large extent what
sampling, preservation, and processing procedures are
most suitable to ensure that the in situ characteristics of
the solid samples are preserved prior to analysis (USEPA,
2002; USEPA, 2006a).
The analytical techniques employed for characterization of
solid samples at the two sites varied in their commercial
availability and technical complexity. Procedures used to
determ ine element com positions in solid sam pies (e.g., total
digestion or acid-extractable with element-specific detec-
tion, X-ray fluorescence spectroscopy; Amonette, 2002) are
generally routine and readily available through commercial
laboratories. There are a range of laboratory-based instru-
ments that can be used to measure bulk m ineralogical char-
acteristics of solid samples (Hawthorne, 1988; Amonette,
2002; Ulery and Drees, 2008). Frequently, the detection
capability of these instruments can be improved through
physical manipulation of the solid sample to concentrate
the targeted component of interest. While the general
availability of these types of instruments varies across the
commercial laboratory network, the accessibility to these
methods and theavai lability of technical personnel capable
of collecting and interpreting the resultant solid phase
characterization data are not anticipated to constrain more
routine application for site characterization. In contrast, the
use of X-ray absorption spectroscopy or related advanced
spectroscopic techniques is generally not routine and re-
quires a high level of technical knowledge and experience
in data interpretation. However, these limitations should
not eliminate these techniques from consideration. Rather,
it may be necessary to consider use of these techniques
for a more limited number of high-value samples with cor-
relation of information to more conventional techniques
that can be applied to a greater number of samples to
fully characterize site conditions. The value of advanced
spectroscopic techniques is that they provide improved
capability for directly assessing chemical characteristics
of the contaminant even at very low concentrations; this is
critical because a contaminant is often present only as a
minor component for even highly contaminated samples.
The scarcity of studies evaluating correlation of data from
a range of direct and indirect measurements for the pur-
pose of defining contaminant speciation in solid samples
represents a current limitation for more routine application
of solid phase characterization to support selection of MNA
as a component of ground-water remedies. A specific
example is the complementary use of chemical extraction
methodologies and spectroscopic techniques tocharacter-
ize solid phase components within an aquifer. Chemical
extraction methodologies are generally sensitive and can
be applied in a cost-effective manner to a large number
of samples. However, there are analytical limitations to
the accuracy of the information derived from these tests
(USEPA, 2007a, Section IIIB.2.4; USEPA, 2007b, individual
contaminant chapters). While specific chemical extraction
methodologies that might be used to characterize con-
taminant speciation in solid samples have been proposed,
there has been limited effort to confirm the applicability of
these tests with direct evaluationsof in situ speciation using
spectroscopic techniques. Additional work is needed to
validate the appropriate use of chemical extraction meth-
odologies in order to support the more routine application
of solid phase characterization analysis in the evaluation of
MNA as a potential remedy. As reflected in the two case
studies reviewed above, research organizations across the
federal sector are actively pursuing work to address this
limitation. The goal of these efforts is to provide regula-
tory agencies and site remediation managers with the
necessary tools for conducting rigorous assessment of
MNA as a component for ground-water restoration. The
underlying basis of this assessment is a good conceptual
site model derived from a site characterization plan that
considers potential reactions of the contaminant with the
solid phase, as well as changes to the solid phase that
may impact contaminant mobility. As described earlier in
this report, acquisition of this site-specific information is
critical for developing technical and public acceptance of
MNA as a viable remedy.
Notice
The U.S. Environmental Protection Agency through its Office of
Research and Development funded and managed the research
described for the Industri-Plex Superfund Site. This report and
referenced USEPA publications were subjected to the Agency's
peer and administrative review and approved for publication as
an EPA document.
Quality Assurance Statement
All research projects making conclusions or recommendations
based on environmentally-related measurements and funded by
the Environmental Protection Agency are required to participate in
the Agency Quality Assurance (QA) program. Preparation of this
report did not involve the collection or use of environmental data
and, as such, did not require a Quality Assurance Project Plan.
13
-------
Acknowledgement
This document benefited from critical and constructive reviews
from Miles Denham (Savannah River National Laboratory), Yoshiko
Fujita (Idaho National Laboratory), and Steve Yabusaki (Pacific
Northwest National Laboratory).
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