Ground Water Issue 

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 bioavailability. 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 compositions, contaminant-mineral associations, and metal/metalloid 

uptake capacities of subsurface solids or reactive media used for 

in situ treatment 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 environments, such as soils, aquifers, and sediments.

The preservation method evaluated and recommended here for 

solids collected from anoxic environments involves sample freezing (-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 ironand 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. A selection 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 private remediation managers and their technical support personnel 

Mineralogical Preservation of Solid 

Samples Collected from Anoxic Subsurface 

Environments

with information necessary for preparing sampling plans to support 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 Research 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 performance 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 environment. 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 subsequent analyses and interpretations. For samples collected from 

anoxic subsurface environments, oxidation is the primary reaction 

process that leads to 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, uranium, and arsenic (U.S. Environmental Protection Agency, 2002). 

Reduction-oxidation processes involving iron and sulfur compounds, 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, γ-FeOOH; goethite, α-FeOOH). 

Subsequently during batch adsorption tests or sequential extraction tests, ferric-bearing phases should behave differently than the 

original, unoxidized material representative of the natural environment. 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 

Richard T. Wilkin

U.S. Environmental Protection Agency, National Risk Management Research Laboratory, Ground Water and Ecosystems 

Restoration Division, 919 Kerr Research Drive, Ada, OK 74820 

(wilkin.rick@epa.gov)
2

oxidation. Unpreserved samples containing sulfides can undergo 

oxidative transformations, changing sample behavior and outcomes 

of mineralogical, sequential extraction, and batch adsorption tests 

(Bush and Sullivan, 1997; Carbonaro et al., 2005). A good 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 atmosphere. 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 for the collection 

and handling of peat samples for chemical and mineralogical analyses. Their protocol adopts sample freezing at -18 °C to preserve 

samples for subsequent mineralogical analyses. Rapin et al. (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 Phillips 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 materials 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 adsorption 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 oxidation (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 concentration is high (>1 mg/L).

Methods, Results and Discussion of a Preservation 

Study

In order to evaluate the effects of sample preservation on the results of selected solid-phase characterization tests, contaminated 

sediments were collected from a small lake situated adjacent to a 

Superfund 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). Sediments 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 sediments 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 oxidation 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 HCl, total 

sulfur, acid-volatile sulfide, chromium-reducible sulfur, and batch 

adsorption tests with arsenic and zinc. In addition, X-ray absorption 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, 
3

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 HCl and measuring the ferrous iron concentration using 

the 1,10-phenanthroline colorimetric method. In the unpreserved 

samples, the Fe(II)/FeTotal ratio is <0.03 for all determinations. This 

ratio in the unpreserved samples may be overestimated because 

of the possible formation of Fe(III)-phenanthroline complexes 

(Tamura et al., 1974). This interference is significant when the 

concentration of Fe(III) is >10 mg/L, a condition that was avoided 

during the partial extraction tests. In the preserved samples the 

average Fe(II)/FeTotal ratio is 0.83 (Figure 1). In most samples, 

concentrations of total iron and ferrous iron are in close agreement. Several other samples, in particular ED-14 and ED-15, 

were collected from a transitional redox zone so that a mixture of 

Fe(II) and Fe(III) found in these samples is reasonable. These 

data demonstrate that: i ) the freezing procedure for preserving 

sample redox integrity is appropriate for iron-bearing phases; 

and, ii ) samples containing ferrous iron, if left unpreserved, will 

undergo oxidation reactions that result in the conversion of Fe(II) 

to Fe(III) 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 acidvolatile 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 concentrations ranging from 0.01 to 0.18 wt%. 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(II), S(-II) is lost from samples 

that are left unpreserved.

A comparison was made between acid-volatile sulfide concentrations 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 procedures (R = 0.953; n = 8). Freeze-drying may be advantageous for 

sample drying because low temperatures during lyophilization help 

avoid changes in labile components including the loss of volatile 

constituents (e.g., mercury, Muhaya et al., 1998), avoid aggregation 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 concentrations 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 extracFigure 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).
4

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 environment is preferred.

Arsenic Oxidation State

Arsenic may be present in the solid phase in either the As(V) or 

As(III) oxidation states, or as a mixture of As(V) and As(III). In 

general, as predicted from thermodynamic reasoning, As(V) is 

expected to dominate in oxygenated environments and As(III) 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, interactions 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 

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(III) 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 freezing 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 matrices were particularly susceptible to arsenic oxidation. For sanddominated samples, Rowland et al. (2005) recommend analysis 

within two or three 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. 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.

Figure 3. 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; only As(III) is detected in the preserved 

sample. Inset shows the comparison between total 

arsenic concentrations in the preserved and unpreserved samples. 
5

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 preserved 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 concentrations from 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. In these two 

cases the linear Kd

 model would appear to be just as appropriate 

as the 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 provide 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 environmental 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 

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 allowed to oxidize in air. The examples provided in this Issue Paper 

show that 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 preservation 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 

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: Q Q K C

K C

ads

ads

= +









 max  1

, where Q is the 

concentration- d e p e n d e n t 

sorption (mg/g), Qmax is the maximum possible sorption 

by the solid, C is the aqueous concentration of the 

sorbate (mg/L), and Kads is the sorption constant (L/mg). 

Dashed lines show the fit to the linear adsorption 

model: Kd

 = Q/C.
6

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 adsorption 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-sensitive 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 redoxsensitive components.

EPA’s Office of Research and Development is preparing a technical 

resource document for the application of monitored natural attenuation (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 for selected 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 environmental 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 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.

Acknowledgements 

R. Ford and K. Scheckel are thanked for field and laboratory assistance. We also gratefully acknowledge the support provided by 

Shaw Environmental (Contract #68-C-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 suggestions for improvement 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 major facilities access grant from NSERC, Simon 

Fraser University and the Advanced Photon Source.

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EPA/600/R-06/112

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