v°/EPA Ground Water Issue
United States
Environmental Protection
Agency
Mineralogical Preservation of Solid
Samples Collected from Anoxic Subsurface
Environments
Richard T.Wilkin
Background
Remedial technologies utilized at hazardous waste sites for the
treatment of metal and metalloid contaminants often take advantage
of reduction-oxidation (redox) processes to reach ground water
clean up goals (Barcelona and Holm, 1991; U.S. Environmental
Protection Agency, 2002). This is because redox reactions, in
many cases, govern the biogeochemical behavior of inorganic
contaminants by affecting their solubility, reactivity, and bioavail-
ability. Site characterization efforts, remedial investigations, and
long-term post-remedial monitoring often involve sampling and
analysis of solids. Solid-phase studies are needed to evaluate
contaminant partitioning to various mineral fractions, to develop
site conceptual models of contaminant transport and fate, and
to assess whether or not remedial mechanisms are occurring as
expected. Measurements to determine mineralogical composi-
tions, contaminant-mineral associations, and metal/metalloid
uptake capacities of subsurface solids or reactive media used for
in s/futreatment of the subsurface all depend upon proper sample
collection and preservation practices. This Issue Paper discusses
mineralogical preservation methods for solid samples that can be
applied during site characterization studies and assessments of
remedial performance. A preservation protocol is presented that
is applicable to solids collected from anoxic subsurface environ-
ments, such as soils, aquifers, and sediments.
The preservation method evaluated and recommended here for
solids collected from anoxic environments involves sample freez-
ing (-18 °C), transportation of frozen samples on dry ice, and
laboratory processing of solids in an anaerobic glove box. This
method was found to preserve the redox integrity of reduced iron-
and sulfur-bearing compounds, which are typically predominant
redox-sensitive inorganic constituents in environmental materials
and are important in controlling contaminant behavior at hazardous
waste sites. Aselection of solid-phase measurements was carried
out on preserved anoxic sediments collected from a contaminated
lake and compared to identical measurements on sample splits in
which no preservation protocol was adopted, i.e., the unpreserved
samples were allowed to oxidize in ambient air. An analysis of
results illustrates the importance of proper sample preservation
for obtaining meaningful solid-phase characterization. This Issue
Paper provides remedial project managers and other state or pri-
vate remediation managers and their technical support personnel
U.S. Environmental Protection Agency, National Risk Manage-
ment Research Laboratory, Ground Water and Ecosystems
Restoration Division, 919 Kerr Research Drive, Ada, OK 74820
(wilkin.rick@epa.gov)
with information necessary for preparing sampling plans to sup-
port site characterization, remedy selection, and post-remedial
monitoring efforts.
For further information contact Richard T. Wilkin (580) 436-8874
at the Ground Water and Ecosystems Restoration Division of the
National Risk Management Research Laboratory, Office of Re-
search and Development, U.S. Environmental Protection Agency,
Ada, Oklahoma.
Introduction
Solid phase samples may be collected for physical, chemical, or
biological tests during site characterization and remedial perfor-
mance monitoring studies. The principal objective of any sampling
program is to collect and deliver materials to the laboratory that
are representative of the original material present in the environ-
ment. If samples are collected for the purpose of determining total
element concentrations, then the mode of preservation may not
be important unless the contaminant is a volatile or semi-volatile
component. However, when solid samples are collected for more
sensitive or detailed analyses, such as sequential extraction tests,
solid-phase speciation tests, or batch adsorption tests, preservation
methods become critical and may direct the outcome of all subse-
quent analyses and interpretations. For samples collected from
anoxic subsurface environments, oxidation is the primary reaction
process that leadsto unrepresentative samples. Therefore, proper
sample preservation will ideally minimize the undesirable effects
of oxidation. Unfortunately, the literature is not extensive on the
assessment of procedures for handling anoxic materials. Lacking
general guidance, sampling and preservation protocols are usually
developed to best suit needs on a project-by-project basis.
Redox-sensitive elements commonly important in environmental
studies include iron, manganese, sulfur, chromium, copper, ura-
nium, and arsenic (U.S. Environmental Protection Agency, 2002).
Reduction-oxidation processes involving iron and sulfur com-
pounds, in particular, have significant impacts on the partitioning
of metals to solids and these impacts must be considered when
collecting and preserving field samples. For example, minerals
containing ferrous iron (e.g., siderite, FeCO3; mackinawite, FeS;
pyrite, FeS2) may undergo rapid oxidation reactions during air
exposure and transform to ferric-iron phases (e.g., ferrihydrite,
Fe(OH)3-nH2O; lepidocrocite, y-FeOOH; goethite, a-FeOOH).
Subsequently during batch adsorption tests or sequential extrac-
tion tests, ferric-bearing phases should behave differently than the
original, unoxidized material representative of the natural environ-
ment. Oxidative mineral transformations may result in changes in
reactive surface area, influence precipitation and co-precipitation
reactions, and/or trigger different surface adsorption reactions.
Similarly, sulfide minerals are in general highly susceptible to
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oxidation. Unpreserved samples containing sulfides can undergo
oxidativetransformations, changing sample behavior and outcomes
of mineralogical, sequential extraction, and batch adsorption tests
(Bush and Sullivan, 1997; Carbonaro et al., 2005). Agood example
of how sample preservation practices can affect the outcome of
sediment analyses is discussed in Harrington et al. (1999).
Adequate preservation of samples must encompass the chain of
events from sample collection, to sample transport back to the
laboratory, to sample storage, and finally to sample preparation
and analysis in the laboratory. The strategies most often adopted
for preserving the redox status of freshly collected solid materials
include sample freezing and/or sample storage in an inert atmo-
sphere. Sample freezing typically involves collection of core or
grab samples, placement of samples in containers, followed by
freezing in a freezer or flash-freezing using liquid nitrogen. Freezing
preserves the redox integrity of samples by decreasing the rate
of reaction between reduced solids and atmospheric oxygen or
other oxidants. The other principal approach for preserving redox
status is to eliminate or minimize sample interactions with oxygen
by transferring samples after their collection into an evacuated
container or a container purged with an inert gas, such as nitrogen,
helium, or argon. Transportation of samples back to the laboratory
is an especially vulnerable process for maintaining sample integrity.
Once samples are frozen, they must be kept in a frozen state, or
samples stored in gas-purged containers must be transported in
secondary air-tight containers. After samples have arrived at the
laboratory, they may be again transferred to a laboratory freezer
or to an anaerobic chamber for drying and homogenization.
Givelet et al. (2004) recently developed a protocol forthe collection
and handling of peat samplesfor chemical and mineralogical analy-
ses. Their protocol adopts sample freezing at -18 °C to preserve
samplesforsubsequent mineralogical analyses. Rapinetal.(1986)
examined the impact of freezing and other preparation methods
on the results of sequential extraction analyses for determining
solid-phase partitioning of metals in sediment. Their conclusions
were that freeze-drying and oven-drying should be avoided, but
that freezing was acceptable for sequential extraction tests. They
also noted that partial extraction tests for copper, iron, and zinc
were especially sensitive to sample handling protocols. Mudroch
and Bourbonniere (1994) proposed that when applying sequential
extraction procedures to anoxic sediments all manipulations and
extraction steps should be carried out in an anaerobic glove box.
Mineralogical studies on the corrosion products in zero-valent
iron permeable reactive barriers (PRBs) were conducted by Phil-
lips et al. (2003). After collecting cores from subsurface PRBs,
these investigators placed the core materials in PVC chambers
purged with argon gas. The cores were stored for up to 2 weeks
and argon was recharged into the PVC tubes every 2 to 3 days.
Other long-term performance studies of zero-valent iron PRBs
have successfully utilized sample freezing to preserve core materi-
als for mineralogical and chemical analyses (U.S. Environmental
Protection Agency, 2003).
Several studies have examined the effects of air-drying versus
oven-drying on the behavior of soils and sediments in batch ad-
sorption tests. Physical and chemical properties of materials are
altered depending on the mode of drying. For example, differences
have been observed in sample pH, partition coefficients, and the
exchangeable metals fraction depending on whether samples are
field-moist, frozen, air-dried, freeze-dried, or oven-dried. Based
upon a review of previous work, U.S. Environmental Protection
Agency (1992) recommends air-drying of samples over oven-drying
in order to minimize changes to the physico-chemical properties
of solids used in batch tests for estimating adsorption parameters.
The endpoint of air-drying is achieved when the sample moisture
content reaches equilibrium with room atmosphere conditions and
in practice can be assessed by tracking sample mass to a steady
state. Reduced solids collected from anoxic environments should
be dried in an anaerobic glove box or glove bag to prevent oxida-
tion (U.S. Environmental Protection Agency, 1992). The analysis
of solid materials for inorganic species can be performed on wet,
freeze-dried, or air-dried samples. In general, however, sample
drying is preferred to eliminate sample homogeneity issues in
relating element concentrations from a wet-weight to a dry-weight
basis (Muhaya et al., 1998). Water removal may be achieved
through various means, including decanting, gravity filtration,
vacuum filtration, pressure filtration (e.g., Bottcher et al., 1997),
and centrifugation. It is important to note that pore-water solutes
can significantly contribute to total element concentrations in dried
solids, especially in situations where the solid-phase concentration
of the element is low (<10 mg/kg) and the pore-water concentra-
tion is high (>1 mg/L).
Methods, Results and Discussion of a Preservation
Study
In order to evaluate the effects of sample preservation on the re-
sults of selected solid-phase characterization tests, contaminated
sediments were collected from a small lake situated adjacent to a
Superf und Site located approximately 16 km northwest of downtown
Boston, Massachusetts (Industri-Plex Superfund Site). The lake
receives discharge of ground water with elevated concentrations
of arsenic, ferrous iron, sulfate, and petroleum hydrocarbons.
The site has been used to develop an improved understanding of
arsenic geochemical cycling at the ground water-surface water
interface (U.S. Environmental Protection Agency, 2005). Sedi-
ments were retrieved from depths ranging from 0.5 to 4.5 meters
using an Eckman dredge. One half of each sample retrieved from
the lake bottom was immediately bagged and frozen; the other
half was bagged and left unfrozen. During each sampling event
approximately 1 L of sediment plus water was collected. The
mixture was transferred from the dredge to polyethylene bags
and excess air was displaced. Frozen sediment samples were
transported back to the laboratory on dry ice. Frozen samples
were subsequently thawed and dried at room temperature in an
anaerobic glove box (96:4 v/v N2-H2 gas mixture). The dried sedi-
ments were homogenized with an agate mortar and pestle and
kept in the glove box. Unpreserved samples were dried in air
and homogenized using an agate mortar and pestle. The color of
the Unpreserved samples was red, presumably due to the oxida-
tion of ferrous iron and production of ferric oxyhydroxides. Color
changes in the Unpreserved samples were noted within the first
several hours after sample collection. The preserved samples
kept in the glove box remained black in color. Solid-phase tests
carried out on the preserved and Unpreserved sediments included
total metals concentrations, metal extractability with 1 M HCI, total
sulfur, acid-volatile sulfide, chromium-reducible sulfur, and batch
adsorption tests with arsenic and zinc. In addition, X-ray absorp-
tion near-edge structure (XANES) spectroscopy was carried out
to determine the oxidation state of arsenic in the preserved and
unpreserved samples.
Iron and Sulfur Partitioning
Total element concentrations were determined by microwave
assisted digestion in nitric acid followed by inductively coupled
plasma-optical emission spectroscopy (ICP-OES; modified EPA
Method 3051). Figure 1 is a bar graph that shows a comparison of
total iron concentrations in the preserved and unpreserved sediment
samples. Concentrations of total iron in the sediments range from
1.0 to 11.5 wt%. Total iron concentrations are independent of the
mode of preservation; values in the preserved and unpreserved
samples deviate within ±10%. Similar correlations are observed
for other major and trace elements. As a general rule, therefore,
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^m Total Fe, preserved
I ~1 Total Fe, unpreserved
I l Fe(ll), preserved
SZH Fe(ll), unpreserved
Figure 1. Comparison of solid-phase concentrations of total iron and ferrous iron (wt%) in a series of sediments with and without
preservation. Map showing the distribution of sampling points within the Hall's Brook Holding Area pond, located adjacent
to the Industri-Plex Superfund Site (for site background see U.S. Environmental Protection Agency, 2005 and references
therein).
the total concentration of inorganic components is conservative
and independent of the mode of sample preservation. If total
concentrations in solid samples are the data objective of a specific
site investigation, then it may not be necessary to expend the extra
effort and cost to ensure preservation of the sample redox state.
Sample preservation may be necessary, however, to maintain
solid-phase concentrations of volatile or semi-volatile inorganic
components, such as mercury (Muhaya et al., 1998).
Although total concentrations of iron are independent of the mode
of sample preservation, the oxidation state of iron in the preserved
and unpreserved samples is completely different. Figure 1 shows
the amount of ferrous iron in the solid phase compared to total iron
concentrations in the preserved versus unpreserved sediments.
Ferrous iron content was determined by extracting the sediments
in 1 M HCI and measuring the ferrous iron concentration using
the 1,10-phenanthroline colorimetric method. In the unpreserved
samples, the Fe(ll)/Fe ratio is <0.03 for all determinations. This
ratio in the unpreserved samples may be overestimated because
of the possible formation of Fe(lll)-phenanthroline complexes
(Tamura et al., 1974). This interference is significant when the
concentration of Fe(lll) is>10 mg/L, a condition that was avoided
during the partial extraction tests, in the preserved samples the
average Fe(ll)/FeTotal ratio is 0.83 (Figure 1). In most samples,
concentrations of total iron and ferrous iron are in close agree-
ment. Several other samples, in particular ED-14 and ED-15,
were collected from a transitional redox zone so that a mixture of
Fe(ll) and Fe(lll) found in these samples is reasonable. These
data demonstrate that: /) the freezing procedure for preserving
sample redox integrity is appropriate for iron-bearing phases;
and, //') samples containing ferrous iron, if left unpreserved, will
undergo oxidation reactions that result in the conversion of Fe(ll)
to Fe(lll) in the solid phase.
Similar results are observed for sulfur. In Figure 2 data are
presented that show the concentration of acid-volatile sulfide in
preserved and unpreserved sediment samples compared to total
sulfur concentrations. Methods used for determining total sulfur
and reduced sulfur partitioning are reported in Wilkin and Bischoff
(2006). In the preserved set of samples, concentrations of acid-
volatile sulfide range from 0.05 to 5.1 wt% or from about 10 to
79% of the total amount of sulfur contained in the samples. In
contrast, the unpreserved samples have acid-volatile sulfide con-
centrations ranging from 0.01 to0.18wt%. Losses of acid-volatile
sulfide concentrations range from 95 to 100% in the unpreserved
samples. More detailed sulfur partitioning studies indicate that
the balance of sulfur in the preserved samples is composed of
mixed reduced and oxidized species including chromium-reducible
sulfur, sulfate-sulfur, and minor quantities of organic-sulfur (Wilkin
and Bischoff, 2006). Similarly to Fe(ll), S(-ll) is lost from samples
that are left unpreserved.
A comparison was made between acid-volatile sulfide concentra-
tions obtained in sediment samples that were thawed and dried in
an anaerobic chamber and concentrations in freeze dried samples.
Very good agreement was found between the two drying proce-
dures (R= 0.953;n=8). Freeze-drying maybe advantageous for
sample drying because lowtemperatures during lyophilization help
avoid changes in labile components including the loss of volatile
constituents (e.g., mercury, Muhaya et al., 1998), avoid aggrega-
tion of particles, and minimize oxidation reactions. A previous
study showed, however, that freeze-drying was not effective for
samples with low acid-volatile sulfide concentrations (Brumbaugh
and Arms, 1996). At acid-volatile sulfide concentrations below
0.2 wt%, Brumbaugh and Arms (1996) noted reductions in con-
centrations following freeze-drying of up to 95%. They proposed
that increases in sample surface area of freeze dried materials
render such materials highly susceptible to air-oxidation. Hjorth
(2004) also suggests that freeze-drying does not preserve the
speciation pattern of major elements, trace metals, and sulfur in
anoxic sediments as determined by a 3-step sequential extrac-
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^m Total S
I I Acid-volatile sulfide, preserved
l__ | Acid-volatile sulfide, unpreserved
Figure 2.
Comparison of solid-phase concentrations of total
sulfur (wt%) and acid-volatile sulfide (wt%) in a series
of sediments with and without preservation.
preserved sample. The unpreserved sample shows two features,
a shoulder at 11871 eV and an absorption maximum at about
11874 eV. The second energy feature is characteristic of arsenic
in the pentavalent state. Linear combination fitting of the measured
spectra indicate that the unpreserved sample contains a mixture
consisting of about 54% As(lll) and 46% As(V).
Similar to iron and sulfur, the oxidation state of arsenic in the solid
phase is highly dependent on the mode of sample preservation.
Unless preserved, solid matrices containing arsenic in the trivalent
state will likely oxidize to form arsenate. As an example, Bostick
et al. (2004) documented arsenic oxidation artifacts encountered
during spectroscopic measurements. In this study, sample freez-
ing followed by sample preparation and analysis under an anoxic
atmosphere was found to preserve the reduced arsenic oxidation
state in solid samples. These findings are consistent with a recent
study by Rowland et al. (2005). They noted substantial oxidation of
solid-phase arsenic in unpreserved samples and that sandy matri-
ces were particularly susceptible to arsenic oxidation. For sand-
dominated samples, Rowland et al. (2005) recommend analysis
within two orthree weeks of sample collection to minimize oxidation
artifacts. The issue of holding time was not specifically examined
as an experimental variable in this study. Arsenic XANES data
reported here were collected 5 months after sample collection, so
over a 5-month period the arsenic oxidation state was maintained
in the redox-preserved samples by freezing.
tion procedure. Although more data are needed, results available
in the literature suggest that freeze-drying may not be an ideal
approach for samples to be used in redox-sensitive solid-phase
measurements; room-temperature drying in an anaerobic environ-
ment is preferred.
Arsenic Oxidation State
Arsenic may be present in the solid phase in either the As(V) or
As(lll) oxidation states, or as a mixture of As(V) and As(lll). In
general, as predicted from thermodynamic reasoning, As(V) is
expected to dominate in oxygenated environments and As(lll) is
expected to dominate in suboxic to anoxic environments. Various
mechanisms of arsenic mobilization and immobilization in the
environment have been proposed including abiotic and microbially
mediated redox processes. Determination of the oxidation state
of arsenic in the solid phase is an important component of risk
assessments and remediation strategies because both the toxicity
and the geochemical mobility of arsenic are strongly dependent on
its solid and aqueous phase speciation (e.g., Cullen and Reimer,
1989; Smedley and Kinniburgh, 2002).
Changes in the natural distribution of arsenic species in a sample
collected from the field can come about due to several factors
including chemical reactions with sample components, interac-
tions with the container material, and microbial activity. All of
these factors may in turn be affected by parameters such as
temperature, light levels, and pH (Rowland et al., 2005). In this
study, XANES spectra were collected to evaluate the oxidation
state of arsenic in the preserved and unpreserved sediments. A
discussion of data collection and data analysis methods relating
to X-ray absorption spectroscopy is presented in a separate report
(U.S. Environmental Protection Agency, 2005). As an example,
X-ray absorption spectra for preserved and unpreserved samples
of ED-03 are shown in Figure 3. The preserved sample shows
a single absorption maximum at about 11871 eV. This energy
is characteristic of arsenic in the trivalent state dominant in the
11860
11870 11880
Energy, eV
11890
Figures.
Arsenic K-edge XANES spectra for sample ED-03.
The bold blue line is the spectrum collected from
the preserved sample and the open circles show the
spectrum collected from the unpreserved sample.
Sample ED-03 contains a total arsenic concentration
of 490 mg/kg; onlyAs(lll) is detected in the preserved
sample. Inset shows the comparison between total
arsenic concentrations in the preserved and unpre-
served samples.
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Batch Adsorption Tests
The capacity of soils, sediments, or aquifer solids to attenuate
pollutants is often assessed by using batch-adsorption or static
equilibrium tests (U.S. Environmental Protection Agency, 1992). It
is reasonable to suspect that the results of such tests will depend
on the ways in which sample matrices are preserved and handled
after their collection. To examine the effects of preservation on
batch-adsorption experiments, tests with zinc and arsenic were
carried out using sub-samples of the preserved and unpreserved
sediments. Zinc sorption onto the preserved samples was about
5 to 30 times greater than zinc sorption onto the unpreserved
samples (Figure 4A). Interestingly, the reverse trend is evident
for arsenic, i.e., the unpreserved samples are about 4 times more
efficient in removing arsenic from solution compared to the pre-
served samples (Figure 4B). Figure 4 shows batch sorption data
plotted in terms of the aqueous concentrations of zinc or arsenic
in mg/L versus the solid-phase concentration of zinc or arsenic in
mg/g, respectively. The solid-phase concentration of the inorganic
contaminant is calculated based upon the dry sample mass used
in the batch adsorption test and the time-dependent loss of zinc or
arsenic concentrationsfrom solution. The solid lines are the data fit
to the Langmuir isotherm equation; the fitting equation is provided
in the caption for Figure 4. The dashed lines in Figure 4 represent
the linear distribution coefficient (Kd) for zinc and arsenic uptake by
the unpreserved and preserved samples, respectively. Inthesetwo
cases the linear Kd model would appear to be just as appropriate
asthe non-linear Langmuir model. Results of these tests, however,
demonstrate that data collected in batch adsorption experiments
are entirely dependent on how samples are preserved. Sample
preservation would be just as important for column experiments. If
solid-phase testing is used in the context of developing predictive
models of contaminant transport and fate or for developing site
remediation strategies, it is imperative that the solid-phase tests
be carried out only on redox-preserved materials.
Arsenic is preferentially retained on the unpreserved sediment
matrix. This behavior is likely due to the fact that both arsenite
and arsenate are more favorably adsorbed by ferric oxyhydroxides
or hydroxides present in the unpreserved samples as compared
to ferrous sulfides that are present in the preserved samples. On
the other hand, zinc is preferentially retained on the preserved
sediment sample relative to the unpreserved sample. The high
acid-volatile sulfide concentrations in the preserved samples pro-
vide reactive sulfide for precipitation of insoluble zinc sulfide(ZnS),
which is a more effective process for removing zinc from solution
than adsorption by ferric oxyhydroxides or hydroxides.
Summary and Conclusions
Unless preserved, samples collected from suboxic to anoxic
environments should not be submitted for solid-phase tests to
assess contaminant partitioning or for determining contaminant
uptake capacity. Results of such tests on improperly preserved
samples will be unrepresentative at best and misleading in the
worst case.
The preservation method tested and recommended here for
samples collected from suboxic to anoxic environments involves
collection of samples followed by freezing (-18 °C), transporting
frozen samples on dry ice, and laboratory processing of solids in
an anaerobic glove box. This method was found to preserve the
redox integrity of reduced iron- and sulfur- bearing compounds
which are typically abundant redox-sensitive constituents in en-
vironmental samples.
The method is relatively simple and inexpensive to apply in the
field compared to other possible methods of preservation that
require liquid nitrogen or compressed gas cylinders containing
100
200 300 400 500
Zn, mg L"1
b)
50 100 150 200 250 300 350
As, mg L
Figure 4. Adsorption isotherms of a) zinc and b) arsenic for
preserved (blue) and unpreserved (black) sediment.
Batch adsorption experiments were carried out using
sample ED-10; pH of adsorption varied between 5.7
and 7.1. Solid lines show the fit to the Langmuir
isotherm equation: O = Q ( Kaitc \ ; where Q is the
concentration- ~ m(i+K^c) dependent
sorption (mg/g), Qmax is the maximum possible sorption
by the solid, C is the aqueous concentration of the
sorbate (mg/L), andKads is thesorption constant (L/mg).
Dashed lines show the fit to the linear adsorption
model: Kd = Q/C.
nitrogen or argon. A selection of solid-phase measurements
was carried out on preserved anoxic sediments collected from a
contaminated wetland and compared to sample splits in which no
preservation was adopted, i.e., the unpreserved samples were al-
lowed to oxidize in air. The examples provided in this Issue Paper
showthat attention must be paid to sample preservation protocols,
especially in site assessments that focus on the details of metal
or metalloid partitioning to the solid matrix. Improper preserva-
tion practices prior to metal partitioning or batch adsorption tests
may result in misleading data that are unrepresentative of site
conditions. Changes in the oxidation state of iron and sulfur result
in mineralogical changes that significantly impact contaminant
behavior during characterization tests. Freezing was found to be
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an adequate method for preserving samples containing reduced
iron, sulfur, and arsenic. When solid-phase tests such as metal
speciation analyses, sequential extraction tests, or batch adsorp-
tion experiments are carried out on samples collected from anoxic
environments, sample preparation and testing must be conducted
in an oxygen-free atmosphere.
Although this study focused on a limited set of redox-sensitive
elements from only one environment (i.e., freshwater sediment),
it is reasonable to expect that the methods employed would be
appropriate in other environmental media and for other redox-sen-
sitive elements of interest (e.g., Mn, Se, U, V). Additional studies
are needed to address redox preservation over a more complete
range of contaminant types and environmental conditions. Other
specific issues that require more study include an analysis of
methods for preserving organic carbon fractions such as humic
substances and an evaluation of storage times for specific redox-
sensitive components.
EPA's Office of Research and Development is preparing a technical
resource documentforthe application of monitored natural attenu-
ation (MNA) to inorganic contaminants in ground water (see, e.g.,
Reisinger et al., 2005). The technical resource document presents
a four-tiered analysis for assessing MNA as a viable remediation
option forselected metal, metalloid, and radionuclide contaminants
encountered in ground water. Components of the tiered approach
include demonstrating contaminant sequestration mechanisms,
estimating attenuation rates and the attenuation capacity of aquifer
solids, and determining potential reversibility issues. All of these
issues require samples that are representative of actual environ-
mental conditions in order to evaluate MNA as a possible remedy
for restoring ground water resources. Redox preservation of solids
collected from the field will necessarily be a key component of
MNA assessments for inorganic contaminants.
Notice
The U.S. Environmental Protection Agency through its Office of
Research and Development funded and managed the research
described here. It 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.
Quality Assurance Statement
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 Quality Assur-
ance 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.
Acknowledgements
R. Ford and K. Scheckel are thanked for field and laboratory as-
sistance. We also gratefully acknowledge the support provided by
Shaw Environmental (Contract #68-0-03-097). This Issue Paper
was reviewed by C. Stein (Gannett Fleming, Inc.), C. Cooper
(Idaho National Laboratory), R. Ford (USEPA/ORD), and D. Frank
(USEPA/Region 10); their comments and suggestionsfor improve-
ment of the manuscript are greatly appreciated. Arsenic K-edge
spectra were collected at beamline 20-BM at the Advanced Photon
Source, Argonne National Laboratory (Argonne, IL). Use of the
Advanced Photon Source is supported by the U. S. Department of
Energy, Office of Science, Office of Basic Energy Sciences, under
Contract #W-31-109-Eng-38. Advanced Photon Source research
facilities at beamline 20-BM are also supported by the US DOE
Office of Science Grant No. DEFG03-97ER45628, the University
of Washington, a majorfacilities access grantfrom NSERC, Simon
Fraser University and the Advanced Photon Source.
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