&EPA
Directive 9283.1-36
_ _ August 2015
United States Office of Solid Waste and Emergency Response
Environmental Protection
Agency
USE OF MONITORED NATURAL ATTENUATION FOR INORGANIC
CONTAMINANTS IN GROUNDWATER AT SUPERFUND SITES
U.S. Environmental Protection Agency
Office of Solid Waste and Emergency Response
Directive 9283.1-36
August 2015
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
TABLE OF CONTENTS
Section Page
ACRONYMS AND ABBREVIATIONS iv
NOTICE/DISCLAIMER vii
EXECUTIVE SUMMARY ES-1
1.0 INTRODUCTION 1
1.1 Intended Use of Document 4
1.2 Tiered Analysis Approach for Developing Multiple Lines of Evidence 4
1.3 Conceptual Site Model 5
1.4 Definition of MNAin Groundwater 7
1.5 Overview of the 1999 OSWER Directive 7
1.6 Relationship of MNAto Remedial Action Objectives 10
1.7 MNA vs. Treatment as a Response Action for Inorganic Contaminants 10
1.8 Primary Differences between Organic and Inorganic MNA 11
2.0 IMPLEMENTATION 13
2.1 Plume Management 13
2.2 Dispersion and Dilution 14
2.3 Site Monitoring 15
2.4 Plume Stability 17
2.5 Complex Geologic Regimes 19
2.6 Reasonable Timeframe 19
2.7 Cleanup Levels for MNA 22
2.8 Relationship to Technical Impracticability (TI) Waiver 23
2.9 Documentation 24
3.0 RECOMMENDED TIERED ANALYSIS APPROACH TO DEVELOP MULTIPLE
LINES OF EVIDENCE 26
3.1 Phase I: Demonstration that the groundwater plume is not expanding 29
3.2 Phase II: Determination that the mechanism and rate of the attenuation process
are sufficient 30
3.3 Phase III: Determination that the capacity of the aquifer is sufficient to attenuate
the mass of contaminant within the plume and the stability of the immobilized
contaminant is sufficient to resist re-mobilization 31
3.4 Phase IV: Design of a performance monitoring program based on an
understanding of the mechanism of the attenuation process, and establishment of
contingency remedies ailored to site-specific characteristics 32
4.0 SITE CHARACTERIZATION 34
4.1 Hydrogeologic and Contaminant Distribution Characterization 34
4.2 Determination of Attenuation Rates 35
4.3 Geochemical Considerations 35
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
TABLE OF CONTENTS (Continued)
Section Page
4.4 Groundwater Geochemistry Characterization 36
4.5 Solid Phase Characterization 36
4.6 Special Considerations for Radionuclides 37
5.0 ATTENUATION PROCESSES FOR INORGANIC CONTAMINANTS 39
5.1 Microbial Degradation 39
5.2 Chemical Transformation/Redox 40
5.3 Sorption and Precipitation 41
5.4 Radioactive Decay 43
6.0 USE OF MODELS 46
6.1 Types of Models 47
6.2 Model Calibration 48
6.3 Interpreting Model Results 49
6.4 Site-Specific Data 50
7.0 SUMMARY 51
8.0 REFERENCES 52
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
TABLE OF CONTENTS (Continued)
FIGURES
Figure Page
1.1 Conceptual distinction between organic and inorganic plume behavior 12
5.1 Illustration of four decay ingrowth scenarios in groundwater plumes contaminated with
radionuclides 44
TABLES
Table
3.1
Page
Recommended site characterization objectives, potential supporting data types and
analysis approaches associated with each phase 28
5.1 Radiochemical information for select radionuclide isotopes 44
APPENDICES
Appendix A Recommended Groundwater Analyses
Appendix B Recommended Soil Analyses
Appendix C Recommended Analytical Methods and Data Quality Requirements for
Groundwater Analyses
Appendix D Recommended Analytical Methods and Data Quality Requirements for Soil
Analyses
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
ACRONYMS AND ABBREVIATIONS
Acronym Definition
a Alpha
Am Americium
ARAR Applicable or relevant and appropriate requirement
As Arsenic
AVS Acid volatile sulfide
Bq Becquerel
Br Bromine
P Beta
C Celsius
Ca Calcium
Cd Cadmium
CEC Cation exchange capacity
CERCLA Comprehensive Environmental Response, Compensation, and Liability Act
CFR Code of Federal Regulations
CH4 Methane
Cl Chlorine
C1O4 Perchlorate
Co Initial groundwater concentration
CO2 Carbon dioxide
COC Contaminant of concern
Cr Chromium
Cs Cesium
CSM Conceptual site model
Cu Copper
DO Dissolved oxygen
DOC Dissolved organic carbon
DOE U.S. Department of Energy
DQO Data quality objective
EDTA Ethylenediamine tetraacetic acid
Eh Redox potential
EPA U.S. Environmental Protection Agency
ESD Explanation of significant differences
F Flourine
Fe Iron
FR Federal Register
FS Feasibility study
y Gamma
Fb Hydrogen
FbS Hydrogen sulfide
HS- Bisulfide
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
HTRW
I
1C
ITRC
K
Kd
kg
K.QC
MCL
MCLG
MeV
Mg
mg
\iS
|iS/cm2
MNA
ms
mV
Na
NCP
Ni
NO3
Np
NPL
NRC
OE
ORP
OSWER
Pa
Pb
pCi
pCi/L
PRO
Pu
QA
Ra
RAO
RCRA
RfD
RI
Rn
ROD
Hazardous, Toxic, and Radioactive Waste
Iodine
Institutional control
Interstate Technical Regulatory Council
Potassium
Distribution/partition coefficient
Kilogram
Organic carbon soil-water partition coefficient
Maximum contaminant level
Maximum contaminant level goal
Megaelectronvolt
Magnesium
Milligram
Micrograms per liter
Microsecond
Microsecond per square centimeter
Monitored natural attenuation
Millisecond
Megavolt
Sodium
National Oil and Hazardous Substances Pollution Contingency Plan
Nickel
Nitrite
Nitrate
Neptunium
National Priorities List
Nuclear Regulatory Commission
Ordnance and explosives
Oxidation-reduction potential
Office of Solid Waste and Emergency Response
Protactinium
Lead
Picocurie
Picocurie per liter
Preliminary remediation goal
Plutonium
Quality assurance
Radium
Remedial action objective
Resource Conservation and Recovery Act
Reference dose
Remedial investigation
Radon
Record of decision
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
SCM Surface complexation model
SDWA Safe Drinking Water Act
Se Selenium
SEM Simultaneously extracted metals
SEP Sequential extraction procedure
Sr Strontium
Tc Technetium
Th Thorium
TI Technical impracticability
TIC Total inorganic carbon
TOC Total organic carbon
TPP Technical project planning
U Uranium
UMTRCA Uranium Mill Tailings Radiation Control Act of 1978
USAGE U.S. Army Corps of Engineers
USGS U.S. Geological Survey
VOC Volatile organic compounds
Xe Xenon
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
NOTICE/DISCLAIMER
This document was developed through the cooperative efforts of a team of Headquarters and
regional staff inside the U.S. Environmental Protection Agency (EPA) and relies on peer-
reviewed literature, EPA reports, Web sources, current research, and other pertinent information.
This document has been through a thorough internal EPA peer-review process, which included
comments from the Office of Solid Waste and Emergency Response (OSWER) and the Office of
General Counsel. References and Web links are provided for readers interested in additional
information; these Web links, verified as accurate at the time of publication, are subject to
change by Web sponsors. Note that the mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
This guidance is designed to help promote consistent national approach for implementation of
Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) response
actions at private party and federal facility sites. It does not, however, substitute for CERCLA or
EPA's regulations, nor is it a regulation itself. Thus, it does not impose legally binding
requirements on EPA, states, tribes or the regulated community, and may not apply to a
particular situation based on the circumstances. EPA, state, tribal and local decision-makers
retain the discretion to adopt approaches on a case-by-case basis that differ from this guidance
where appropriate. Any decisions regarding a particular facility will be made based on the
applicable statutes and regulations.
In working with other federal agencies to make cleanup decisions for groundwater at sites where
the other federal agency is lead for cleanup, EPA Regions should use the recommendations in
this document to the same extent as at non-federal facility sites. Section 120(a)(2) of CERCLA
provides that all guidelines, rules, regulations, and criteria for preliminary assessments, site
investigations, National Priorities List (NPL) listing, and remedial actions are applicable to
federal facilities to the same extent as they are applicable to other facilities. It states the
following: "No department, agency or instrumentality of the United States may adopt or utilize
any such guidelines, rules, regulations, or criteria which are inconsistent with the guidelines,
rules, regulations, and criteria established by the Administrator under this Act."
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
EXECUTIVE SUMMARY
This new monitored natural attenuation (MNA) policy document for inorganic contaminants
("2015 MNA guidance") expands on and is designed to be a companion to the 1999 MNA
guidance.1 The 1999 MNA guidance, which clarified "EPA's policy regarding the use of
monitored natural attenuation (MNA) for the cleanup of contaminated soil and groundwaten in
the Superfund, RCRA Corrective Action, and Underground Storage Tank programs,2" focused
primarily on organic contaminants; however, the 1999 MNA guidance does address inorganic
contaminants to some extent (see for example, pp. 8-9). Together, these two policy documents
provide guidance on the consideration of MNA for a broad range of contaminants at Superfund
sites. The two MNA policy documents are supported by a three-volume set of technical reports
issued by the U.S. Environmental Protection Agency's (EPA) Office of Research and
Development (2007-2010).3
Regions should continue to consider the overall recommendations in the 1999 MNA guidance
when evaluating all sites (those with organic and inorganic contaminants). Consistent with the
1999 MNA guidance, the 2015 MNA guidance document discusses in more detail below that
MNA for inorganic contaminants: (1) is not intended to constitute a treatment process for
inorganic contaminants; (2) when appropriately implemented, can help to restore an aquifer to
beneficial uses by immobilizing contaminants onto aquifer solids and providing the primary
means for attenuation of contaminants in groundwater; and (3) is not intended to be a "do
nothing" response.
Furthermore, as discussed in the 1999 MNA guidance and in more detail below, the Agency's
longstanding policy is that MNA is generally not an appropriate response action if a receptor is
currently being exposed to a contaminant or the contaminant plume is expanding. In addition,
MNA, whether selected as the sole remedial action or as a finishing step, may be appropriate
when it can achieve a site's remedial action objectives in a reasonable timeframe; thus, MNA
remedies should not extend over very long timeframes, and the anticipated timeframes should be
reasonable compared with other potential alternatives being considered. However, the document
acknowledges that longer timeframes may be needed for some contaminants that degrade or
decay over a long time period.
As also discussed in the 1999 MNA guidance and in more detail below, an MNA approach for
groundwater may not be appropriate for ensuring protectiveness of human health and the
environment at Superfund sites. Regions should evaluate specific site conditions in determining
1 Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank
Sites (OSWER Directive 9200.4-17P, April 21, 1999) (EPA 1999c).
2 1999 MNA guidance (page 1).
3 To the extent it is consistent with CERCLA, the NCP and this and other EPA CERCLA guidance documents,
Regions also may find useful information in the Interstate Technical Regulatory Council (ITRC) Guidance on
MNA for metals and radionuclides (2010).
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
whether MNA can be a viable cleanup approach (for example, the groundwater plume should be
stable or shrinking, geochemical evidence of attenuation should be documented in the
administrative record, there should be no exposure to the contaminated groundwater, and the
source of contaminants should be identified and addressed).
This 2015 MNA guidance, consistent with the 1999 MNA guidance, indicates that multiple
"lines of evidence" should be obtained to evaluate whether MNA should be considered as part of
the site's selected response action. As a related matter, the 1999 MNA guidance also
recommends use of a tiered analysis approach for considering MNA, which typically involves a
detailed analysis of site characteristics that control and sustain attenuation. The 2015 MNA
guidance builds on this tiered approach and recommends a phased analytical approach tailored
specifically for inorganic contaminants. Where natural attenuation leads to daughter products
that are more toxic than the parent compounds, Regions should ensure protectiveness of human
health and the environment by taking steps to make sure that the more toxic compounds do not
increase over time or are addressed by changes to the existing remedy.
In conclusion, while the 1999 MNA guidance continues to provide overall recommendations on
evaluating MNA, the 2015 MNA guidance (generally) offers more specific recommendations
intended to assist the Regions in evaluating whether MNA for inorganic contaminants is
appropriate. If MNA is considered as an appropriate cleanup approach at Superfund sites, the
guidance can assist in identifying steps that can be taken to ensure that the risk to human health
and the environment is adequately reduced and managed in a timely manner.
OSWER Directive 9283.1-36 ES-2
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
1.0 INTRODUCTION
This guidance document provides recommendations for evaluating monitored natural attenuation
(MNA) as a potential component of a remedial action approach for cleaning up inorganic
contaminants (including radionuclides) in groundwater at Comprehensive Environmental
Response, Compensation, and Liability Act (CERCLA) sites. This document uses "inorganic
contaminants" as a generic term for metals and metalloids (such as arsenic); the phrase also
refers to radioactive as well as non-radioactive isotopes. The purpose of this document is to
provide additional guidance, generally consistent with the 1999 MNA guidance, on considering
the use of MNA for inorganic contaminants (as well as nitrate and perchlorate) in groundwater as
a way to ensure protectiveness of human health and the environment. With regard to inorganic
contaminant plumes in groundwater, it describes the primary processes that typically govern
MNA and offers a recommended framework for assessing the potential effectiveness of MNA as
a cleanup approach.
More detailed discussion of the scientific principles and processes described in this policy may
be found in the following three documents, which are referenced frequently in this guidance:
• Monitored Natural Attenuation of Inorganic Contaminants in Ground Water, Volume I
Technical Basis for Assessment, EPA 600-R-07-139 (EPA 2007a).
• Monitored Natural Attenuation of Inorganic Contaminants in Ground Water, Volume II
Assessment for Non-Radionuclides Including Arsenic, Cadmium, Chromium, Copper,
Lead, Nickel, Nitrate, Perchlorate, and Selenium, EPA 600-R-07-140 (EPA 2007b).
• Monitored Natural Attenuation of Inorganic Contaminants in Ground Water, Volume III
- Assessment for Radionuclides Including Americium, Cesium, Iodine, Plutonium,
Radium, Radon, Strontium, Technetium, Thorium, Tritium, and Uranium, EPA600-R-10-
093 (EPA2010a).
As discussed in the National Oil and Hazardous Substances Pollution Contingency Plan (NCP)
and in various associated EPA CERCLA guidance documents, "[t]he EPA expects to return
usable groundwaters to their beneficial uses whenever practicable, within a timeframe that is
reasonable given the particular circumstances of the site" (see 40 Code of Federal Regulations
[CFR] §300.430(a)(l)(iii)(F)).
In general, five key principles stem from the overarching expectations for groundwater
restoration.4 As discussed in "Summary of Key Existing EPA CERCLA Policies for
Groundwater Restoration" (OSWER Directive Number 9283.1-33, June 26, 2009), these
expectations are as follows:
4 See "Summary of Key Existing EPA CERCLA Policies for Groundwater Restoration" (OSWER Directive
Number 9283.1-33, June 26, 2009) (See pages 3-4.) at
www.epa.gov/superfund/health/conmedia/gwdocs/pdfs/9283 l-33.pdf.
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
(1) "If groundwater that is a current or potential source of drinking water5 is
contaminated above protective levels (that is, for drinking water aquifers,
contamination exceeds federal or state maximum contaminant levels [MCLs] or
non-zero maximum contaminant level goals [MCLGs]), a remedial action under
CERCLA should seek to restore the aquifer to beneficial use (that is, drinking
water standards) wherever practicable.
(2) "Groundwater contamination should not be allowed to migrate and further
contaminate the aquifer or other media (for example, indoor air via vapor
intrusion into buildings; sediment; surface water; or wetland).
(3) "Technical impracticability waivers and other waivers may be considered and,
under appropriate circumstances, granted if the statutory criteria are met, when
groundwater cleanup is impracticable. The waiver decision should be
scientifically supported and clearly documented.
(4) "Early actions6 (such as source removal, plume containment or provision of an
alternative water supply7) should be considered as soon as possible. Institutional
controls (ICs) related to groundwater use or even surface water use may be useful
to protect the public in the short term, as well as in the long term.
(5) "ICs should not be relied on as the only response to contaminated groundwater or
as a justification for not taking action under CERCLA.8 To ensure protective
remedies, CERCLA response action cleanup levels for contaminated groundwater
should generally address all pathways of exposure that pose an actual or potential
risk to human health and the environment."
5 The EPA generally considers potential source for drinking water as Class II under EPA's Groundwater
Classification System in "Guidelines for Groundwater Classification Under the EPA Groundwater Protection
Strategy" (Final Draft, December 1986) and the National Contingency Plan (NCP) where Class I and II generally
are considered to be current and potential drinking water aquifers (See 55 FR [Federal Register] 8732 (March 8,
1990).
6 See "Considerations in Groundwater Remediation at Superfund Sites and RCRA Facilities - Update" (Directive
Number 9283.1-06, May 27, 1992) for a more complete discussion of early actions. (See pages 6-8.) at
www.epa. gov/superfund/policv/remedv/pdfs/92-83106-s.pdf.
7 See 55 FR 8865 (March 8, 1990) for a list of potential ways of providing an alternative water supply
(Appendix D).
8 See 40 CFR § 300.430(a)(iii)(D) ("The use of institutional controls shall not substitute for active response
measures (for example, treatment and/or containment of source material, restoration of groundwaters to their
beneficial uses) as the sole remedy unless such active measures are determined not to be practicable, based on
the balancing of trade-offs among alternatives that is conducted during the selection of remedy"). Also see 40
CFR § 300.430(a)(iii)(A) related to the expectation for treatment.
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Cleanup levels for response actions under CERCLA generally are developed based on applicable
or relevant and appropriate requirements (ARARs) where they are available and sufficiently
protective of human health,9 and on site-specific risk assessments where ARARs do not exist.
The determination of whether a requirement is an ARAR, as stated in the NCP, is made on a site-
specific basis (see 40 CFR§300.400(g)). In general, drinking water standards provide relevant
and appropriate cleanup levels for groundwater that is a current or potential source of drinking
water; drinking water standards include federal or state MCLs or non-zero MCLGs established
under the Safe Drinking Water Act (SDWA) or more stringent state drinking water standards.10
Depending on site-specific circumstances, however, drinking water standards may not be
relevant and appropriate for groundwater that is not a current or potential source of drinking
water (see 55 Federal Register [FR] 8732, March 8, 1990).
Selection of Contaminants
The series of technical resource documents addressing MNA for inorganic contaminants
referenced in this policy includes a discussion of a specific list of contaminants. The
contaminants addressed in the technical resource documents were selected based on the
frequency of occurrence at contaminated sites and to represent the range of contaminant
properties that can influence the efficiency of natural attenuation processes to achieve site
cleanup goals. The recommendations in this guidance should be considered for all non-
radiological or radiological inorganic contaminants in groundwater, regardless of their inclusion
in the technical resource documents.
The non-radionuclide contaminants addressed in the technical documents include the following:
arsenic, cadmium, chromium, copper, lead, nickel, nitrate, perchlorate and selenium. These
contaminants are commonly found at Superfund sites throughout the nation and reflect toxicity,
industrial use, and frequency of occurrence at Superfund sites. They represent a broad range of
geochemical traits such as the following: ion charge (cation vs. anion), transport behavior
(conservative vs. non-conservative) and oxidation-reduction (redox) chemistry (EPA 1999a,
1999b and 2004c). Conservative behavior typically is exhibited by non-reactive contaminants
that tend to move readily with groundwater flow, while non-conservative behavior typically is
exhibited by contaminants whose transport is retarded by any number of different mechanisms.
Finally, the EPA regional staff members were asked to nominate inorganic contaminants that
occurred frequently or that were problematic in their Regions. The above list of nine inorganic
contaminants reflects this process.
9 See e.g., "Clarification of the Role of Applicable, or Relevant and Appropriate Requirements in Establishing
Preliminary Remediation Goals under CERCLA," OSWER Directive No. 9200.4-23 (August 22, 1997) ("It
remains EPA's policy that ARARs will generally be considered protective absent multiple contaminants or
pathways of exposure. However, this Directive clarifies that, in rare situations, EPA regional offices should
establish PRGs at levels more protective than required by a given ARAR, even absent multiple pathways or
contaminants, where application of the ARAR would not be protective of human health or the environment.").
10 Other regulations may also be ARARs for purposes of CERCLA §121(d)(2)(B).
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
A similar process was used to identify the radionuclide contaminants in the technical documents,
including the following: americium, cesium, iodine, neptunium, plutonium, radium, radon,
technetium, thorium, tritium, strontium and uranium. These radionuclides are daughter and
fission products that result from radioactive decay and are commonly found at Superfund sites.
The decay of radioisotopes can produce daughter products that may differ both physically and
chemically from parent isotopes. The radionuclide contaminants addressed in the technical
document also represent a broad range of geochemical traits and environmental characteristics.
1.1 Intended Use of Document
Users of this document may include the EPA and state cleanup program personnel and their
contractors, especially those individuals responsible for evaluating alternative cleanup methods
for a given site or facility. Depending on site-specific circumstances (for example, which
hazardous substances are being addressed), the recommendations in both this 2015 MNA
guidance and the 1999 MNA guidance may be useful. For more information on MNA for
groundwater cleanups, see www.epa.gov/superfund/health/conmedia/gwdocs/monit.htm.
Additional guidance may also be found at
www.cluin.org/techfocus/default.focus/sec/Natural_Attenuation/cat/Guidance.
The potential attenuation processes affecting inorganic contaminants generally should be the
same for both radioactive and non-radioactive inorganic contaminants contaminant types, except
for radioactive decay. As a result, the decision-making approach and process for establishing
cleanup levels at CERCLA sites normally should be the same for sites with radioactive and non-
radioactive inorganic contaminants, except where there are technical differences between the two
types of contaminants (such as external exposure from gamma radiation vs. dermal exposure).
1.2 Tiered Analysis Approach for Developing Multiple Lines of Evidence
As discussed in the 1999 MNA guidance (pp. 15-16):
Once site characterization data have been collected and a conceptual model
developed, the next step is to evaluate the potential efficacy of MNA as a
remedial alternative. This involves collection of site-specific data sufficient to
estimate with an acceptable level of confidence both the rate of attenuation
processes and the anticipated time required to achieve remediation objectives. A
three-tiered approach to such an evaluation is becoming more widely practiced
and accepted. In this approach, successively more detailed information is
collected as necessary to provide a specified level of confidence on the estimates
of attenuation rates and remediation timeframe. These three tiers of site-specific
information, or "lines of evidence", are:
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(1) Historical groundwater and/or soil chemistry data that demonstrate a clear
and meaningful trend [footnote in original deleted] of decreasing
contaminant mass and/or concentration over time at appropriate
monitoring or sampling points. (In the case of a groundwater plume,
decreasing concentrations should not be solely the result of plume
migration. In the case of inorganic contaminants, the primary attenuating
mechanism should also be understood.)
(2) Hydrogeologic and geochemical data that can be used to demonstrate
indirectly the type(s) of natural attenuation processes active at the site,
and the rate at which such processes will reduce contaminant
concentrations to required levels. For example, characterization data may
be used to quantify the rates of contaminant sorption, dilution, or
volatilization, or to demonstrate and quantify the rates of biological
degradation processes occurring at the site.
(3) Data from field or microcosm studies (conducted in or with actual
contaminated site media) which directly demonstrate the occurrence of a
particular natural attenuation process at the site and its ability to degrade
the contaminants of concern (typically used to demonstrate biological
degradation processes only).
For inorganic contaminant plumes, the evaluation and selection of MNA as part of a cleanup
action in groundwater typically involves a detailed analysis of site-specific data and
characteristics that control and sustain attenuation. Developing multiple lines of evidence (as
discussed in the 1999 MNA guidance) to support this analysis can require significant resource
outlays. Thus, site characterization should be approached in a step-wise manner to collect data
for inclusion in the administrative record that support the evaluation of existing natural
attenuation processes within the aquifer and the analysis of potential long-term effectiveness.
The 2015 MNA guidance builds on the tiered analysis approach discussed in the 1999 MNA
guidance for inorganic contaminants as a way to provide a cost-effective way to screen sites for
MNA because it is designed to prioritize and focus the characterization needs for decision
making at each screening step. Conceptually, a tiered analysis approach is designed to
progressively reduce uncertainty as more and more site-specific data are collected. The
recommended tiered analysis approach is discussed in more detail in Section 3 of this document
involves obtaining progressively more information. The recommended approach is designed to
acquire lines of evidence that can be used to assess the likely effectiveness of MNA as a
remedial action alternative for inorganic contaminants in groundwater. The EPA generally
recommends following the tiered approach outlined in this document for inorganic contaminants.
1.3 Conceptual Site Model
As stated in the 1999 MNA guidance (p. 14), "EPA recommends the use of conceptual site
models to integrate data and guide both investigative and remedial actions."
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Assessing the suitability of MNA as a component of a groundwater response action for sites with
inorganic contaminants is helped by development of a conceptual site model11 (CSM). Regions
should refer to existing EPA guidance on CSMs (see, for example, Environmental Cleanup Best
Management Practices: Effective Use of the Project Life Cycle Conceptual Site Model., EPA
542-F-l 1-011, OSWER, July 2011; Performance Monitoring of MNA Remedies for VOCs in
Ground Water, EPA/600/R-04/027 April 2004; A Guide To Preparing Superfund Proposed
Plans, Records Of Decision, and other Remedy Selection Decision Documents, O SWER 9200.1 -
23P, July 1999; Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents
in Ground Water. EPA 600-R-98-128, Office of Research and Development, 1998).
Generally, the CSM is a representation (written, graphical or pictorial) of the environmental
system at a site and the biological, physical, and chemical processes (and relationships between
them) that affect contaminant transport. The CSM is designed to identify potential pathways that
may expose receptors to site contaminants. The CSM should also quantify fluxes of
contaminants and describe the conditions that may affect or alter the MNA processes. The CSM
should include an understanding of the attenuation mechanisms, the geochemical conditions
governing these mechanisms, the capacity of the aquifer to sustain attenuation of the contaminant
mass and prevent future contaminant migration, and indicators that can be used to monitor MNA
performance. Uncertainties and assumptions should be listed with specific strategies to describe
and minimize their impact on qualitative and quantitative models. Data collection should be
focused on complete or potentially complete exposure pathways, based on both current and
reasonably anticipated future land use, to avoid collecting unnecessary data that do not contribute
to site closeout. A well-formed CSM can be important in the development of sound data quality
objectives (DQOs). DQOs should be developed to ensure that all appropriate data are collected
with sufficient quantity, sensitivity, and precision to meet the needs of the project (EPA 2002b
and 2006a). Finally, the CSM serves as a planning instrument and data interpretation aid as well
as a communication device between and among project staff and the public.12
11 As stated in the 1999 MNA guidance, A conceptual site model (CSM) is a three-dimensional representation that
conveys what is known or suspected about contamination sources, release mechanisms, and the transport and fate
of those contaminants. The conceptual model provides the basis for assessing potential remedial technologies at
the site. "Conceptual site model" is not synonymous with "computer model"; however, a computer model may
be helpful for understanding and visualizing current site conditions or for predictive simulations of potential
future conditions. Computer models, which simulate site processes mathematically, should in turn be based on
sound conceptual site models to provide meaningful information. Computer models typically require a lot of
data, and the quality of the output from computer models is directly related to the quality of the input data.
Because of the complexity of natural systems, models necessarily rely on simplifying assumptions that may or
may not accurately represent the dynamics of the natural system. Calibration and sensitivity analyses are
important steps in appropriate use of models. Even so, the results of computer models should be carefully
interpreted and continuously verified with adequate field data.
12 To the extent it is consistent with CERCLA, the NCP and existing EPA CERCLA guidance documents, Regions
may find useful information in documents prepared by the U.S. Army Corps of Engineers (USAGE 1998 and
2003) and the Interstate Technical Regulatory Council (ITRC) (ITRC 2003).
OSWER Directive 9283.1-36 6
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Although the focus of this document is on groundwater, the vadose zone often is another source
of contaminants to groundwater at CERCLA sites. Thus, both the vadose and saturated zones
normally should both be carefully characterized. Regions should consider developing a CSM that
adequately characterizes both the saturated and vadose zone.
Initially, the CSM is developed based on existing knowledge of groundwater and vadose zone
fate and transport characteristics, as well as known properties of the specific contaminants
potentially present at the site. The CSM should be updated in an iterative fashion as
progressively more is learned about the site.
1.4 Definition of MNA in Groundwater
The term "monitored natural attenuation," as used in the 1999 MNA guidance and this
document, refers to
" ...[t]he reliance on natural attenuation 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. The 'natural attenuation processes' that are
at work in such a remediation approach include a variety of physical, chemical, or
biological processes that, under favorable conditions, act without human
intervention to reduce the mass, toxicity, mobility, volume, or concentration of
contaminants in soil or groundwater. These in-situ processes include
biodegradation; dispersion; dilution; sorption; volatilization; radioactive decay;
and chemical or biological stabilization, transformation, or destruction of
contaminants." (EPA 1999c, page 3)
1.5 Overview of the 1999 OSWER Directive
The 1999 MNA guidance provides recommendations related to the consideration of MNA
generally (for example, for both organic and inorganic contaminants). This 2015 MNA guidance
provides additional information and recommendations regarding site characterization, data
quality and attenuation processes related specifically to inorganic contaminants.
Although several physical, chemical and biological processes are included in the definition of
MNA mentioned above, the 1999 MNA guidance recommends using processes that permanently
degrade or destroy contaminants and using MNA only for stable or shrinking plumes, as noted
below:
When relying on natural attenuation processes for site remediation, the EPA
prefers those processes that degrade or destroy contaminants. Also, the EPA
generally expects that MNA will only be appropriate for sites that have a low
potential for contaminant migration. (EPA 1999c, page 3)
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MNA should not be used where such an approach would result in either plume
migration13 or impacts to environmental resources that would be unacceptable to
the overseeing regulatory authority. Therefore, sites where the contaminant
plumes are no longer increasing in extent, or are shrinking, would be the most
appropriate candidates for MNA remedies. (EPA 1999c, page 18)
Control of contaminant sources also is an important aspect of the 1999 MNA guidance:
Control of source materials is the most effective means of ensuring the timely
attainment of remediation objectives. EPA, therefore, expects that source control
measures will be evaluated for all contaminated sites and that source control
measures will be taken at most sites where practicable. At many sites it will be
appropriate to implement source control measures during the initial stages of site
remediation ('phased remedial approach'), while collecting additional data to
determine the most appropriate groundwater remedy. (EPA 1999c, page 22)
The 1999 MNA guidance (see for example, pp. 8-9) provides a few general recommendations
for use of MNA as a remedial approach for inorganic contaminants. For example, these general
recommendations include (1) the specific mechanisms responsible for attenuation of inorganic
contaminants should be known at a particular site; (2) the stability of the process should be
evaluated and shown to be protective under anticipated changes in site conditions; and (3) fate
and transport characteristics of any daughter products should be understood. Thus:
MNA may, under certain conditions (e.g., through sorption or oxidation-reduction
reactions), effectively reduce the dissolved concentrations and/or toxic forms of
inorganic contaminants in groundwater and soil. Both metals and non-metals
(including radionuclides) may be attenuated by sorption14 reactions such as
precipitation, adsorption on the surfaces of soil minerals, absorption into the
matrix of soil minerals, or partitioning into organic matter. Oxidation-reduction
13 As stated on p. 18 of the 1999 MNA guidance: "In determining whether a plume is stable or migrating, users of
this Directive should consider the uncertainty associated with defining the limits of contaminant plumes. For
example, a plume is typically delineated for each contaminant of concern as a 2- or 3-dimensional feature.
Plumes are commonly drawn by computer contouring programs which estimate concentrations between actual
data points. The EPA recognizes that a plume boundary is more realistically defined by a zone rather than a line.
Fluctuations within this zone are likely to occur as a result of a number of factors (such as analytical, seasonal, or
spatial), which may or may not be indicative of a trend in plume migration. Therefore, site characterization
activities and performance monitoring should focus on collection of data of sufficient quality to enable decisions
to be made with a high level of confidence."
14 As stated on p. 8 of the 1999 MNA guidance: "When a contaminant is associated with a solid phase, it is usually
not known if the contaminant is precipitated as a three-dimensional molecular coating on the surface of the solid,
adsorbed onto the surface of the solid, absorbed into the structure of the solid, or partitioned into organic matter.
"Sorption" will be used in this Directive to describe, in a generic sense (i.e., without regard to the precise
mechanism) the partitioning of aqueous phase constituents to a solid phase."
OSWER Directive 9283.1-36 8
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(redox) reactions can transform the valence states of some inorganic contaminants
to less soluble and thus less mobile forms (e.g., hexavalent uranium to tetravalent
uranium) and/or to less toxic forms (e.g., hexavalent chromium to trivalent
chromium). Sorption and redox reactions are the dominant mechanisms
responsible for the reduction of mobility, toxicity, or bioavailability of inorganic
contaminants. It is necessary to know what specific mechanism (type of sorption
or redox reaction) is responsible for the attenuation of inorganics so that the
stability of the mechanism can be evaluated. For example, precipitation reactions
and absorption into a soil's solid structure (e.g., cesium into specific clay
minerals) are generally stable, whereas surface adsorption (e.g., uranium on iron-
oxide minerals) and organic partitioning (complexation reactions) are more
reversible. Complexation of metals or radionuclides with carrier (chelating)
agents (e.g., trivalent chromium with EDTA) may increase their concentrations in
water and thus enhance their mobility. Changes in a contaminant's concentration,
pH, redox potential, and chemical speciation may reduce a contaminant's stability
at a site and release it into the environment. Determining the existence and
demonstrating the irreversibility, of these mechanisms is important to show that a
MNA remedy is sufficiently protective.
In addition to sorption and redox reactions, radionuclides exhibit radioactive
decay and, for some, a parent-daughter radioactive decay series. For example, the
dominant attenuating mechanism of tritium (a radioactive isotopic form of
hydrogen with a short half-life) is radioactive decay rather than sorption.
Although tritium does not generate radioactive daughter products, those generated
by some radionuclides (e.g., Am-241 and Np-237 from Pu-241) may be more
toxic, have longer half-lives, and/or be more mobile than the parent in the decay
series. Also, it is important that the near surface or surface soil pathways be
carefully evaluated and eliminated as potential sources of external direct radiation
exposure.15 (EPA 1999c, pages 8-9)
The 1999 MNA guidance provides context for the Agency's recommendations regarding the
feasibility of employing MNA as part of a cleanup for contaminated groundwater. As indicated
by the sections transcribed above, the 1999 MNA guidance also points out some key specific
issues associated with what constitutes natural attenuation for inorganic contaminants:
15 As stated on p. 9 of the 1999 MNA guidance: "External direct radiation exposure refers to the penetrating
radiation (i.e., primarily gamma radiation and x-rays) that may be an important exposure pathway for certain
radionuclides in near surface soils. Unlike chemicals, radionuclides can have deleterious effects on humans
without being taken into or brought in contact with the body due to high energy particles emitted from near
surface soils. Even though the radionuclides that emit penetrating radiation may be immobilized due to sorption
or redox reactions, the resulting contaminated near surface soil may not be a candidate for a MNA remedy as a
result of this exposure risk."
OSWER Directive 9283.1-36 9
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Inorganic contaminants persist in the subsurface because, except for radioactive
decay, they are not degraded by the other natural attenuation processes. Often,
however, they may exist in forms that have low mobility, toxicity or
bioavailability such that they pose a relatively low level of risk. Therefore, natural
attenuation of inorganic contaminants is most applicable to sites where
immobilization or radioactive decay is demonstrated to be in effect and the
process/mechanism is irreversible. (EPA 1999c, page 9)
1.6 Relationship of MNA to Remedial Action Objectives
Existing guidance on the development of remedial action objectives (RAOs) and the relationship
of MNA to RAOs may be found in the EPA's 1999 record of decision (ROD) guidance titled, A
Guide to Preparing Superfund Proposed Plans, Records of Decision, and Other Remedy
Selection Decisions Documents, OSWER Directive 9200.1-23P, page 6-26 (EPA 1999d).
If the ROD includes an RAO that addresses restoration of groundwater for sites with inorganic
contaminants in groundwater it may be appropriate to include MNA as a component of a general
remedial approach. However, MNA may not be an appropriate response action to ensure
protectiveness at the site if the ROD does not include an RAO addressing restoration of
groundwater but rather includes RAOs addressing exposure control and prevention of migration.
Where the RAOs include restoring groundwater to beneficial use by meeting ARARs or MCLs
and the lines of evidence supporting MNA are documented sufficiently in the administrative
record, then MNA may be a viable option used in conjunction with other remedial actions or
independently to meet the restoration RAO.
1.7 MNA vs. Treatment as a Response Action for Inorganic Contaminants
As stated in the 1999 MNA guidance on p. 3: "The 'natural attenuation processes' that are at
work in such a remediation approach include a variety of physical, chemical, or biological
processes that, under favorable conditions, act without human intervention to reduce the mass,
toxicity, mobility, volume, or concentration of contaminants in soil or groundwater." Inorganic
contaminants can be transferred between solid, liquid or gaseous phases and these phase transfers
may reduce the aqueous concentration and mobility of inorganic contaminants in groundwater.
Mass reduction through degradation generally is not a viable process for most of the inorganic
contaminants discussed in this document. The exception is radioactive decay, which is a well-
understood attenuation process that may result in decreased contaminant mass, as described in
Section 5.4. There are also limited examples where degradation of nonradiological inorganic
contaminants may reduce contaminant mass (for example, biological degradation of nitrate or
perchlorate). Thus, while attenuation can reduce the aqueous concentration and mobility of
inorganic contaminants in groundwater, it should not be considered a treatment process for most
inorganic contaminants, such as zinc and cadmium.
OSWER Directive 9283.1-36 10
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1.8 Primary Differences between Organic and Inorganic MNA
As discussed in the 1999 MNA guidance (p. 13): "Decisions to employ MNA as a remedy or
remedy component should be thoroughly and adequately supported with site-specific
characterization data and analysis" (emphasis in original).
When the potential use of an MNA approach is evaluated, site characterization for organic
contaminants typically is focused on evaluating the mechanism of contaminant degradation,
quantifying the risks associated with transformation products16, and calculating the capacity of
site conditions to sustain degradation of contaminant mass to achieve cleanup levels throughout
the plume. Much of the emphasis on site characterization for MNA of organic contaminants has
been directed toward collection and analysis of groundwater samples.
Characterization of the solid substrate within the aquifer normally plays a more significant role
during site assessment for inorganic contaminants (other than nitrate and tritium), where
immobilization onto aquifer solids provides the primary means for attenuation of the
groundwater plume. In this case, concentrations in groundwater typically are reduced through
sorption of the inorganic contaminant onto aquifer solids in combination with the long-term
stability of the immobilized contaminant to resist remobilization because of changes in
groundwater chemistry. Precipitation also can be a primary attenuation mechanism for inorganic
contaminants, whereas it generally is an insignificant mechanism for organic contaminants. The
approach and data and information supporting site characterization for nonradiological inorganic
contaminants subject to degradation or reductive transformation processes (for example, nitrate)
will likely be consistent with the approach employed to assess MNA for organic contaminant
plumes (EPA 1998 and 2001). Figure 1.1 illustrates the conceptual distinction between organic
and inorganic plume behavior and the degradation of organic contaminants versus
immobilization of inorganic contaminants on aquifer solids. When contaminants of concern
(COCs) include radionuclides, it generally is important to identify specific isotopes and
associated daughter products present in site groundwater and to include both in the assessment of
plume stability.
16 As discussed on p. 6 of the 1999 MNA guidance: "The term "transformation products" in the Directive includes
intermediate products resulting frombiotic or abiotic processes (e.g., TCE, DCE, vinyl chloride), decay chain
daughter products from radioactive decay, and inorganic elements that become methylated compounds (e.g.,
methyl mercury) in soil or sediment. Some transformation products are quickly transformed to other products
while others are longer lived."
OSWER Directive 9283.1-36 11
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Organic Inorganic
Time
Mobile plume
shrinkage due to
degradation
Original Plume
Boundary
EH
Mobile Contaminant
—i
'_ _ j
Immobile Contaminant
Mobile plume
shrinkage due to
immobilization onto
aquifer sediments
Immobilized inorganic
contaminant still
present on aquifer
solids
Figure 1.1. Conceptual distinction between organic and inorganic plume behavior illustrating
degradation of organic contaminants and immobilization of inorganic contaminants.
Immobilization of inorganic contaminants generally may be a viable component of an MNA
where the immobilized contaminant remains stable and resistant to remobilization if there are
any changes in groundwater chemistry.
OSWER Directive 9283.1-36
12
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
2.0 IMPLEMENTATION
Implementation of MNA typically incorporates and balances several factors. It generally is
critical to understand the subsurface geologic system and avoid conditions where MNA is not
suitable. The Region should obtain data and information to adequately support multiples lines of
evidence and a determination of plume stability, which indicate an MNA approach will ensure
protectiveness of human health and the environment within a reasonable timeframe.
2.1 Plume Management
As discussed on p. 5 of the 1999 MNA guidance:
It is common practice in conducting remedial actions to focus on the most
obvious contaminants of concern, but other contaminants may also be of
significant concern in the context of MNA remedies. In general, since engineering
controls are not used to control plume migration in an MNA remedy, decision
makers need to ensure that MNA is appropriate to address all contaminants that
represent an actual or potential threat to human health or the environment
(emphasis in original).
Furthermore, as discussed on p. 18 of the 1999 MNA guidance:
MNA should not be used where such an approach would result in either plume
migration17 or impacts to environmental resources that would be unacceptable to
the overseeing regulatory authority. Therefore, sites where the contaminant
plumes are no longer increasing in extent, or are shrinking, would be the
most appropriate candidates for MNA remedies (emphasis in original)
MNA may be implemented in a variety of ways for inorganic contaminants, depending on the
nature of the chemical composition of the contaminant plumes, subsurface geology and potential
exposures that are addressed. EPA policy allows MNA to be selected as a response action for
one plume or comingled plumes; addressing commingled plumes may be significantly more
17 "In determining whether a plume is stable or migrating, users of this [1999 MNA guidance] Directive should
consider the uncertainty associated with defining the limits of contaminant plumes. For example, a plume is
typically delineated for each contaminant of concern as a 2- or 3-dimensional feature. Plumes are commonly
drawn by computer contouring programs which estimate concentrations between actual data points. The EPA
recognizes that a plume boundary is more realistically defined by a zone rather than a line. Fluctuations within
this zone are likely to occur due to a number of factors (e.g., analytical, seasonal, spatial, etc.) which may or may
not be indicative of a trend in plume migration. Therefore, site characterization activities and performance
monitoring should focus on collection of data of sufficient quality to enable decisions to be made with a high
level of confidence." See USEPA, 1993b, USEPA, 1993c, USEPA, 1994b andUSEPA, 1998b, for additional
guidance (citations in original).
OSWER Directive 9283.1-36 13
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complex than addressing similar separate plumes and may involve additional risk management
considerations and expertise.
It may also be appropriate to select MNA for a particular contaminant while another response
action is selected for other contaminants within the same plume. Likewise, it may be appropriate
to select MNA for a particular contaminant in a portion of the plume and another remedy for the
same contaminant in another portion of the plume. For example, enhanced bioremediation of a
plume containing petroleum hydrocarbons can produce reducing conditions if bioremediation
results in consumption of dissolved oxygen and other electron acceptors in the aquifer. These
reducing conditions may result in the increased mobilization or solubility of redox-sensitive
inorganic contaminants such as iron or manganese, which may also be associated with arsenic.
MNA may be an appropriate response action for the inorganic plume where it can be shown that
the geochemistry downgradient of the hydrocarbon plume reverts to oxidizing conditions that
would immobilize the inorganic contaminants. Similarly, an active remedy may be selected for
one portion of a plume (for example, near a source area) while MNA may be selected for the
same contaminant at the lower-concentration portion of the same plume farther downgradient.
2.2 Dispersion and Dilution
As discussed on p. 18 of the 1999 MNA guidance:
An example of a situation where MNA may be appropriate is a remedy that
includes source control, a pump-and-treat system to mitigate the highly-
contaminated plume areas, and MNA in the lower concentration portions of the
plume. In combination, these methods would maximize groundwater restored to
beneficial use in a timeframe consistent with future demand on the aquifer, while
utilizing natural attenuation processes to reduce the reliance on active remediation
methods and reduce remedy cost. If, at such a site, the plume was either
expanding or threatening downgradient wells or other environmental resources,
then MNA would not be an appropriate remedy (emphasis in original).
Dispersion and dilution resulting from mixing with influent precipitation, up- or cross-gradient
groundwater or leakage from overlying surface water bodies may be elements of an MNA
response action for inorganic contaminants. However, dilution and dispersion generally are
not appropriate as primary MNA mechanisms because they reduce concentrations through
dispersal of contaminant mass rather than destruction or immobilization of contaminant
mass. Dilution and dispersion may be appropriate as a "polishing step" for distal portions of a
plume when an active remedy is being used at a site, source control is complete and appropriate
land use and ground water use controls are in place. Results of conservative tracer studies can be
used to estimate the contribution of dilution and dispersion to contaminant attenuation rates.
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2.3 Site Monitoring
As discussed on p. 20 of the 1999 MNA guidance:
It should be noted that the timeframe required for MNA remedies is often longer
than that required for more active remedies. As a consequence, the uncertainty
associated with the above factors increases dramatically. Adequate
performance monitoring and contingency remedies (both discussed in later
sections of this Directive) should be utilized because of this higher level of
uncertainty (emphasis in original).
Furthermore, as discussed in the 1999 MNA guidance (pp. 22-23):
Performance monitoring to evaluate remedy effectiveness and to ensure protection of
human health and the environment is a critical element of all response actions.
Performance monitoring is of even greater importance for MNA than for other types of
remedies due to the potentially longer remediation timeframes, potential for ongoing
contaminant migration, and other uncertainties associated with using MNA. This
emphasis is underscored by EPA's reference to "monitored natural attenuation."
The monitoring program developed for each site should specify the location, frequency,
and type of samples and measurements necessary to evaluate whether the remedy is
performing as expected and is capable of attaining remediation objectives. In addition, all
monitoring programs should be designed to accomplish the following:
• Demonstrate that natural attenuation is occurring according to expectations;
• Detect changes in environmental conditions (e.g., hydrogeologic, geochemical,
microbiological or other changes) that may reduce the efficacy of any of the
natural attenuation processes [footnote in original deleted];
• Identify any potentially toxic and/or mobile transformation products;
• Verify that the plume(s) is not expanding (either downgradient, laterally or
vertically);
• Verify no unacceptable impact to downgradient receptors;
• Detect new releases of contaminants to the environment that could impact the
effectiveness of the natural attenuation remedy;
• Demonstrate the efficacy of institutional controls that were put in place to protect
potential receptors; and
• Verify attainment of remediation objectives.
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In addition to the 1999 MNA guidance, other existing EPA CERCLA guidance discusses
development of a performance monitoring framework and monitoring plan (see Performance
Monitoring of MNA Remedies for VOCs [volatile organic compounds] in Ground Water [EPA
2004b]). Although that guidance focuses on attenuation of common organic contaminants, the
recommended framework and many of the recommendations regarding plan development also
may be useful at sites with inorganic constituents.
The performance of an MNA response action should be monitored to determine whether site-
specific RAOs identified in remedy decision documents are achieved.18 Where the time horizons
for successful implementation of an MNA response action are expected to be long, Regions
should pay particular attention to long-term monitoring plans. Monitoring trends in groundwater
COCs through time and space in a carefully designed monitoring network typically is a key part
of informed decision making for both (1) selecting MNA as an appropriate response action for a
site, and (2) assessing the effectiveness of MNA over time.
Initial assessments of whether the aquifer is generally oxidizing or reducing, shallow or deep,
and whether it is influenced by external hydrologic forces (for example, interactions between
groundwater and surface water, recharge from meteoric precipitation or episodic regional
withdrawals from the aquifer) should be considered in designing the dimensions of the
monitoring network and the frequency of data collection to characterize site chemistry and
hydrology (EPA 2008).
With the exception of nitrate, perchlorate and radioactive decay, inorganic contaminant mass
generally is not reduced with most attenuation mechanisms. Therefore, performance monitoring
for these chemicals typically is designed to demonstrate geochemical alteration of COCs to
lower-risk or lower-mobility compounds or species (for example, Fe2+ to Fe3+). A determination
that cleanup levels have been achieved should be based on data and information contained in the
administrative record that demonstrate degradation and immobilization, in addition to showing
that decreasing concentrations are within the risk level or in compliance with ARARs specified
in the remedy decision (for example, MCLs attained throughout the plume). The data and
information collected by the Region also should demonstrate that site conditions and
contaminant concentrations have long-term stability (so that there will be no remobilization of
contamination in the future).
Much of the monitoring to evaluate performance of MNA usually falls into three basic
categories: (1) ambient monitoring to assess background contaminant levels and the status of
relevant ambient geochemical indicators (for example, redox potential [Eh] and pH);
(2) monitoring to assure the viability and efficacy of attenuation processes; and (3) monitoring to
18 As stated on p. 23 of the 1999 MNA guidance: "Performance monitoring should continue until remediation
objectives have been achieved, and longer if necessary to verify that the site no longer poses a threat to
human health or the environment" (emphasis in original).
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detect any plume expansion (EPA 2007a). Identifying the solid phase components' contribution
to attenuation of the contaminant plume often can be critical to reducing the level of uncertainty
in selecting MNA for sites where immobilization is the dominant attenuation process. These
solid-phase components can be grouped into the following three categories: (1) components that
serve as a source of contaminants within the plume; (2) components (biotic and abiotic) that
participate directly or indirectly during the attenuation process; and (3) the chemical form of the
immobilized contaminant and its long-term stability considering future changes in groundwater
chemistry.
The specific recommended objectives for an MNA performance monitoring program discussed
in the 1999 MNA guidance usually can be met by implementing a performance monitoring
program that measures contaminant concentrations, geochemical parameters and hydrologic
parameters (for example, hydraulic gradients). Much of the monitoring typically focuses on
groundwater and should be used to evaluate changes in plume distribution in three dimensions as
well as changes in redox state that may affect the rate and extent of natural attenuation. Data on
groundwater can often be used to evaluate mobile contaminant mass and concentration
reductions that would indicate progress toward RAOs (EPA 2007a). However, periodic sampling
of aquifer solids, through soil coring, generally will be warranted in most situations to evaluate
potential reduction in the capacity of aquifer materials to immobilize contaminants.
Ultimately, monitoring programs should be designed to demonstrate continued stability of the
plume over time and to identify changes in groundwater chemistry that may lead to decreases in
rates or capacity of the aquifer to attenuate the contaminant of concern or changes that may lead
to re-mobilization of attenuated compounds. Changes in indicator parameters or compounds such
as pH, dissolved iron, or sulfate may indicate dissolution of important sorptive phases within the
aquifer. These changes may be detected before observed changes in concentrations of COCs and
thus often serve as indicators of potential MNA failure.
Demonstrating that the inorganic contaminant immobilized onto aquifer solids will not
remobilize typically depends on identifying the chemical speciation of the inorganic contaminant
partitioned to the solid phase. This information often is critical for identifying the mechanism
controlling attenuation and evaluating the long-term stability of the immobilized contaminant in
light of observed or anticipated changes in groundwater chemistry.
2.4 Plume Stability
As stated on p. 18 of the 1999 MNA guidance:
Of the above factors, the most important considerations regarding the suitability
of MNA as a remedy include: whether the contaminants are likely to be
effectively addressed by natural attenuation processes, the stability of the
groundwater contaminant plume and its potential for migration, and the potential
for unacceptable risks to human health or environmental resources by the
contamination. MNA should not be used where such an approach would result in
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either plume migration^ or impacts to environmental resources that would be
unacceptable to the overseeing regulatory authority. Therefore, sites where the
contaminant plumes are no longer increasing in extent, or are shrinking,
would be the most appropriate candidates for MNA remedies (emphasis in
original).
Demonstration of plume stability generally is a critical factor for selecting MNA and typically
involves delineating a plume in all three dimensions and designing a monitoring network to
assess the plume over time. In general, a plume may be considered stable if the monitoring
network shows that groundwater contaminant concentrations (in unfiltered samples) do not
increase in space or time. The demonstration of plume stability normally should consider both
the aqueous mobile and the enhanced colloid transport phases, if present, throughout the plume.
If the attenuation rate is less than the rate of concentration increase within the plume, then the
plume could expand. MNA normally would not be considered suitable for an expanding plume.
It is possible that expanding plume conditions could develop over time because of formation of
daughter products or unforeseen geochemical or other site changes. (See Section 5 for additional
information on conditions that affect plume stability.) Such a situation would warrant further or
additional sampling and analysis to determine if MNA is still a suitable action. Therefore, MNA
remedies for stable plumes should be evaluated systematically (that is, quarterly to yearly), and
an appropriate contingency remedy should be identified if conditions no longer conform to those
defined as necessary for MNA.
As discussed on p. 22 of the 1999 MNA guidance:
Control of source materials is the most effective means of ensuring the timely
attainment of remediation objectives. EPA, therefore, expects that source
control measures will be evaluated for all contaminated sites and that source
control measures will be taken at most sites where practicable. At many sites
it will be appropriate to implement source control measures during the initial
stages of site remediation ("phased remedial approach"), while collecting
additional data to determine the most appropriate groundwater remedy (emphasis
in original).
Although source control will likely reduce contaminant mass flux, the plume may still continue
to expand or migrate. It is therefore generally not appropriate to demonstrate plume stability after
source control has been accomplished only by showing a decrease in contaminant mass flux.
Instead, plume stability generally should be demonstrated by showing decreasing concentration
trends at all wells and static or contracting plume boundaries. See Sections 3 and 4 of this
document for further discussion of mass flux.
MNA is generally not appropriate for plumes that are considered stable, yet there is
confirmed discharge to surface water bodies or potential human or ecological receptor
exposure.
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2.5 Complex Geologic Regimes
As discussed on p. 15 of the 1999 MNA guidance:
MNA may not be appropriate as a remedial option at many sites for technological
or economic reasons. For example, in some complex geologic systems,
technological limitations may preclude adequate monitoring of a natural
attenuation remedy to ensure with a high degree of confidence that potential
receptors will not be impacted. This situation typically occurs in many karstic,
structured, and/or fractured rock aquifers where groundwater moves preferentially
through discrete pathways (e.g., solution channels, fractures, joints, foliations).
The direction of groundwater flow through such heterogeneous (and often
anisotropic) materials cannot be predicted directly from the hydraulic gradient,
and existing techniques may not be capable of identifying the pathway along
which contaminated groundwater moves through the subsurface. MNA will not
generally be appropriate where site complexities preclude adequate monitoring. In
some other situations where it may be technically feasible to monitor the progress
of natural attenuation, the cost of site characterization and long-term monitoring
required for the implementation of MNA may be higher than the cost of other
remedial alternatives. Under such circumstances, MNA may not be less costly
than other alternatives
MNA generally should not be considered at sites with zones where groundwater flow is rapid or
overwhelms biotic and abiotic attenuation mechanisms. The particular situation may be
problematic in specific fractured rock and karst environments because of high flow regimes and
inadequate reaction times. Sites with these conditions generally are characterized by very rapid
groundwater transport and, thus, attenuation mechanisms are unlikely to occur at a rate
commensurate with or exceeding the rate of contaminant transport. MNA generally will not be
effective or protective under these conditions. In addition, technological limitations in such
complex geologic systems may preclude adequate monitoring of MNA to ensure with a high
degree of confidence that potential receptors will not be affected.
2.6 Reasonable Timeframe
The 1999 MNA guidance (p. 2) states that natural attenuation should "achieve site-specific
remediation objectives within a time frame that is reasonable compared to that offered by other
more active methods" (EPA 1999c). In the "Reasonable Timeframe for Remediation" section,
the 1999 MNA guidance (p. 19) goes on to state that "determination of the most appropriate
timeframe is achieved through a comparison of estimates of remediation timeframe for all
appropriate remedy alternatives."
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Furthermore, the 1999 MNA guidance states (on pp. 19 - 20) states:
Whether a particular remediation timeframe is appropriate and reasonable for a given site
is determined by balancing tradeoffs among many factors which include:
• Classification of the affected resource (e.g., drinking water source, agricultural
water source) and value of the resource19;
• Relative timeframe in which the affected portions of the aquifer might be needed
for future water supply (including the availability of alternate supplies);
• Subsurface conditions and plume stability which can change over an extended
timeframe;
• Whether the contamination, either by itself or as an accumulation with other
nearby sources (on-site or off-site), will exert a long-term detrimental impact on
available water supplies or other environmental resources;
• Uncertainties regarding the mass of contaminants in the subsurface and
predictive analyses (e.g., remediation timeframe, timing of future demand, and
travel time for contaminants to reach points of exposure appropriate for the site);
• Reliability of monitoring and of institutional controls over long time periods;
• Public acceptance of the timeframe required to reach remediation objectives; and
• Provisions by the responsible party for adequate funding of monitoring and
performance evaluation over the time period required for remediation (emphasis
in original).
In evaluating what is a "reasonable timeframe" for achieving RAOs at a site with inorganic
contaminants in groundwater, Regions should consider a number of factors that may affect the
timeframe. The EPA recommends that Regions also consider additional factors, including
contaminant properties, exposure risk, classification of the protected resource (for example, a
source of drinking water), the potential for plume stability and the relative timeframe for active
remediation methods to achieve RAOs.
Some radionuclides have long decay half-lives, and substantially longer timeframes generally
will be required that may exceed both the remediation timeframe and active treatment if
radioactive decay is used as the primary natural attenuation mechanism. In these situations,
19 "In determining whether an extended remediation timeframe may be appropriate for the site, the EPA and other
regulatory authorities should consider state groundwater resource classifications, priorities and/or valuations
where available, in addition to relevant federal guidelines. Individual states may provide information and
guidance relevant to groundwater classifications or use designations as part of a Comprehensive State
Groundwater Protection Program (CSGWPP)." (SeeUSEPA, 1992aandUSEPA, 1997b) (citations in original).
OSWER Directive 9283.1-36 20
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
MNA may not be reasonable for radionuclides that have a long half-life and decay daughters that
are long lived, have other properties affecting mobility or that emit other particles that increase
risk.
While remediation timeframes for organic plumes may be on the order of a few tens of years to
more than a hundred years, remediation timeframes for inorganic plumes may be substantially
longer. Ultimately, the timeframe for remediation will be based on site-specific conditions and
chemical characteristics. The longer timeframes for inorganic plumes may be reasonable if the
source term has already been addressed, the plume is stable or shrinking, the exposure risks for
the source term and daughter products are acceptable, and when active measures have similar
timeframes. Multiple lines of evidence are recommended for demonstrating "reasonable
timeframe" considering the above factors in conjunction with the following: source control or
removal is complete; there is high confidence in the attenuation mechanisms, rates and capacity
identified; and contingency plans are included for both the monitoring program and containment
or treatment approaches. Ultimately, consistent with CERCLA and the NCP, an MNA remedial
action must be protective of human health and the environment over the selected timeframe of
the site cleanup (until RAOs are met).
MNA may be particularly useful for radionuclides that have a short half-life (that is, less than 50
years), depending on the total timeframe required for MNA to achieve RAOs and reach cleanup
levels. The initial concentrations of the radionuclides and daughter products should be
considered and reflected in the Region's evaluation of MNA as a potential response action.
Using the equation below normally should be useful in evaluating the reasonableness of the time
required to reach the MCL (or, in the absence of an MCL, the risk-based number) using the total
radionuclide contaminant concentration from groundwater (or selected media). Depending on the
initial groundwater concentration (C0), many half-lives may be required to reach the MCL (or
risk-based cleanup level). Failure to account for this potential lag may lead to inappropriate
consideration of MNA as a potential response action (that is, a longer response action timeframe
than is reasonable or the groundwater plume migrates and contaminates a larger area).
The time required to reach the cleanup concentration for radionuclides in groundwater generally
may be calculated as follows:
C
t = 3323*tl/2*\og— Eq. 1
where ti/2 is the half-life of the radionuclide, C represents the target cleanup level, and C0
represents the initial chemical concentration in groundwater (Smith and Smith 1971). For
example, if the initial concentration of uranium-234 (ti/2=2.4x!05 yrs) in groundwater was 700
micrograms per liter (|ig/L), the time required to reach the 30 |ig/L MCL for uranium-234 would
be more than 1 million years, clearly not generally considered a reasonable timeframe. This
example was calculated using mass concentration units but may be calculated using activity units
(picocuries per liter, for example). This recommended simple equation can allow time required
OSWER Directive 9283.1-36 21
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
to reach a desired concentration C to be estimated; it also may be rearranged to determine the
final concentration at a specified time t. Note that additional calculations would be needed to
quantify concentrations of daughter products generated and their associated decay timeframes.
In the event of long-duration MNA remediation timeframes, ICs may be needed to help ensure
protectiveness of human health as a short-term tool to supplement MNA, consistent with
CERCLA and the NCP (40 CFR 300.430(a)(l)(iii)). ICs generally should remain in place and be
maintained and enforced effectively until the groundwater concentrations allow for an acceptable
level of risk for all resources uses (EPA 2012).
2.7 Cleanup Levels for MNA
As discussed in the 1999 MNA guidance (p. 12):
The NCP Preamble also specifies that cleanup levels appropriate for the expected
beneficial use (e.g., MCLs for drinking water) "should generally be attained
throughout the contaminated plume, or at and beyond the edge of the waste
management area when waste is left in place" (USEPA, 1990a, p.8713) (citation
in original).
An MNA remedial action should attain the same cleanup levels that would be defined for active
remedies and be consistent with the NCP's expectation for achieving restoration of groundwater
to beneficial use;20 site-specific decision documents typically include RAOs, preliminary
remediation goals (PRGs), and cleanup levels that reflect groundwater restoration when that is
the selected remedy.21 MCLs defined in the Safe Drinking Water Act are typically used as
ARARs and cleanup levels for groundwater. The groundwater standards for uranium-234 and
238 under the Uranium Mill Tailings Radiation Control Act of 1978 (UMTRCA) generally are
potential ARARs (see Table 1 of Subpart A, 40 CFR 192). If promulgated ARARs do not exist
for the inorganic compound being remediated, risk-based cleanup levels should be established
such that exposure to the contaminant will not result in unacceptable risk to human health or the
environment at the calculated cleanup level.22
To help evaluate the performance of an MNA remedial action, a site-specific groundwater exit
strategy should be developed early in the cleanup process.23
20 See "Summary of Key Existing EPA CERCLA Policies for Groundwater Restoration" (OSWER Directive
Number 9283.1-33, June 26, 2009).
21 See "A Guide to Preparing Superfund Proposed Plans, Records of Decision, and Other Remedy Selection
Decision Documents," OSWER Directive 9200.1-23P, 1999).
22 See "Clarification of the Role of Applicable, or Relevant and Appropriate Requirements in Establishing
Preliminary Remediation Goals under CERCLA," OSWER Directive No. 9200.4-23 (August 22, 1997).
23 More detailed OSWER guidance regarding development of an exit strategy at CERCLA sites is currently under
development.
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
2.8 Relationship to Technical Impracticability (TI) Waiver
The 1999 MNA guidance discusses TI waivers on p. 13 as follows:
It also should be emphasized that the selection of MNA as a remedy does not
imply that active remediation measures are infeasible, or are "technically
impracticable" from an engineering perspective. Technical impracticability (TI)
determinations are used to justify a departure from cleanup levels that would
otherwise be required at a Superfund site or RCRA facility based on the inability
to achieve such cleanup levels using available remedial technologies (USEPA,
1993a) (citation in original). Such a TI determination does not imply that there
will be no active remediation at the site, nor that MNA will be used at the site.
Rather, such a TI determination simply indicates that the cleanup levels and
objectives which would otherwise be required cannot practicably be attained
using available remediation technologies. In such cases, an alternative cleanup
strategy that is fully protective of human health and the environment must be
identified. Such an alternative strategy may still include engineered remediation
components, such as recovery of free phase NAPLs and containment of residual
contaminants, in addition to approaches intended to restore some portion of the
contaminated groundwater to beneficial uses. Several remedial approaches could
be appropriate to address the dissolved plume, one of which could be MNA under
suitable conditions. However, the evaluation of natural attenuation processes and
the decision to rely upon MNA for the dissolved plume should be distinct from
the recognition that restoration of a portion of the plume is technically
impracticable (i.e., MNA should not be viewed as a direct or presumptive
outcome of a technical impracticability determination.) (emphasis in original).
The EPA's response actions may be designed to achieve several objectives, including to remove
or treat source materials, contain non-restorable source areas, and restore contaminated
groundwater to beneficial uses at CERCLA sites with inorganic contaminants in the
groundwater. Complete restoration of the contaminated groundwater (for example, achieving
MCLs throughout the plume) may not be technically practicable at some sites, however.
Depending on site conditions, groundwater restoration may be impractical because of a
combination of hydrogeologic factors (such as fractured rock or karst conditions, or matrix
diffusion) and contaminant-related factors (such as low solubility).
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Under CERCLA and the NCP, one of the potentially available ARAR waivers in such
circumstances is a technical impracticability (TI) waiver. EPA's TI guidance24 discusses the
circumstances where it may be appropriate for Regions to consider a TI waiver, as well as the
data and information that should be collected to ensure the administrative record contains
sufficient information and data to support the Agency's determination.
The utilization of a TI waiver for a portion of the site does not preclude the use of MNA at the
site where MNA would potentially being appropriate outside of the TI zone. Data collected as
part of the overall site characterization for contaminant and hydrogeologic factors may be helpful
in evaluating whether MNA may be an appropriate approach at another area of the site. Both TI
waiver and MNA decisions should be supported by sufficient data and information in the
administrative record (multiple lines of evidence).
2.9 Documentation
As discussed in the 1999 MNA guidance (pp. 13-14):
Decisions to employ MNA as a remedy or remedy component should be
thoroughly and adequately supported with site-specific characterization data
and analysis. In general, the level of site characterization necessary to support a
comprehensive evaluation of MNA is more detailed than that needed to support
active remediation. Site characterizations for natural attenuation generally warrant
a quantitative understanding of source mass; groundwater flow (including
preferential pathways); contaminant phase distribution and partitioning between
soil, groundwater, and soil gas; rates of biological and non-biological
transformation; and an understanding of how all of these factors are likely to vary
with time. This information is generally necessary since contaminant behavior is
governed by dynamic processes which must be well understood before MNA can
be appropriately applied at a site. Demonstrating the efficacy of MNA may
require analytical or numerical simulation of complex attenuation processes. Such
analyses, which are critical to demonstrate natural attenuation's ability to meet
remediation objectives, generally require a detailed conceptual site model as a
foundation (emphasis in original).
Consideration of MNA in the remedy selection process at a site where inorganic contaminants
are present in the groundwater should be documented and supported like any other CERCLA
response action, consistent with the statute, NCP and existing guidance (such as the 1999 ROD
guidance). Thus, for example, data and information to support evaluation and selection of MNA
24 Guidance for evaluating the technical impracticability of ground-water restoration, OSWER Directive 9234.2-
25, EPA/540-R-93-080 (1993). See also A Guide To Preparing Superfund Proposed Planned, Records Of
Decision, and other Remedy Selection Decision Documents, OSWER 9200.1-23P July 1999 (section 9.5).
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
should be collected and documented in the administrative record, starting with the remedial
investigation (RI) phase of a project and continuing throughout the remedy selection and remedy
implementation phases of the cleanup.25 MNA typically should be identified as a potential
response action in the feasibility study (FS) and included as a free-standing alternative or as a
component of an alternative that involves other technologies (for example, source removal via
excavation, in situ chemical oxidation in high concentration areas, or ICs). Supporting rationale
for selecting MNA, if it is part of the preferred alternative, should be included in the proposed
plan, and final selection should be documented in the ROD for a site. In general, when MNA is
selected, contingency ROD language may be appropriate (see 1999 MNA guidance, p. 24).
2.10 Five Year Reviews
Consistent with CERCLA § 121(c), the NCP at 40 CFR § 300.430(f)(4)(ii), states:
If a remedial action is selected that results in hazardous substances, pollutants, or
contaminants remaining at the site above levels that allow for unlimited use and
unrestricted exposure, the lead agency shall review such action no less often than
every five years after the initiation of the selected remedial action.
At sites where MNA is selected in the ROD, five-year reviews evaluate the continued
protectiveness of the remedy until cleanup levels are met because MNA does not immediately
allow for unlimited use and unrestricted exposure of groundwater. In general, it is important to
understand the attenuation mechanisms so that the risk for contaminant mobilization or
remobilization can be anticipated, incorporated into the long-term monitoring plan, and
addressed in a manner that ensures protectiveness of human health and the environment.
25 Refer to Subpart I of the NCP and the EPA guidance (EPA 20 lOb) regarding preparation of the administrative
record.
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
3.0 RECOMMENDED TIERED ANALYSIS APPROACH TO DEVELOP MULTIPLE
LINES OF EVIDENCE
As discussed earlier, the 1999 MNA guidance recommends a three- tiered evaluation approach.
A tiered analysis approach to site characterization to develop multiple lines of evidence for
evaluation of MNA may have the advantage of potentially saving significant resources because it
is designed to prioritize and focus the data used for decision making at each screening step.
Uncertainty typically also is reduced as site-specific data are collected. Information and data
collection and evaluation within the tiered analysis approach typically should be developed in the
following four phases:
Phase I: Demonstration that the groundwater plume is not expanding.
26
• Phase II: Determination that the mechanism and rate of the attenuation process are
sufficient.27
• Phase III: Determination that the capacity of the aquifer is sufficient to attenuate the
mass of contaminant within the plume and the stability of the immobilized
contaminant is sufficient to resist re-mobilization.28
• Phase IV: Design of a performance monitoring program based on an understanding
of the mechanism of the attenuation process, and establishment of contingency
remedies tailored to site-specific characteristics. This phase in effect reflects
recommendations in the 1999 MNA guidance, but consolidated into a single,
additional phase.29
Obtaining data and information for inclusion in the administrative record to support a
demonstration that a groundwater plume is not expanding (Phase I) and determination that the
mechanism and rate of attenuation are sufficient (Phase II) are the recommended initial steps in
evaluating MNA. Successful demonstration of Phase III generally involves predicting future
MNA performance, which may be difficult to accomplish with confidence at sites with complex
hydrogeology and contaminant geochemistry. Developing multiple lines of evidence reflecting
26 In the 1999 MNA guidance, this tier is described as: "Historical groundwater and/or soil chemistry data that
demonstrate a clear and meaningful trend of decreasing contaminant mass and/or concentration over time at
appropriate monitoring or sampling points."
27 In the 1999 MNA guidance, this tier is described as: "Hydrogeologic and geochemical data that can be used to
demonstrate indirectly the type(s) of natural attenuation processes active at the site, and the rate at which such
processes will reduce contaminant concentrations to required levels." (emphasis in original).
28 In the 1999 MNA guidance, this tier is described as: "Data from field or microcosm studies (conducted in or with
actual contaminated site media) which directly demonstrate the occurrence of a particular natural attenuation
process at the site and its ability to degrade the contaminants of concern (typically used to demonstrate biological
degradation processes only)" (emphasis in original).
29 Refer to Table 1.1 in Monitor e d Natural A ttenuation of Inorganic Contaminants in Ground Water, Volume I-
Technical Basis for Assessment, EPA600-R-07-139 (EPA2007a).
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
these three phases should be considered at any site with inorganic contaminants in the
groundwater where MNA is evaluated as a component of the groundwater remedy.
The technical knowledge obtained through the first three phases generally may be useful in
designing a monitoring program (Phase IV) that tracks MNA performance.
In discussing the three tiers, the 1999 MNA guidance (p. 16) states:
Unless the EPA or the overseeing regulatory authority determines that
historical data (Number 1 above) are of sufficient quality and duration to
support a decision to use MNA, data characterizing the nature and rates of
natural attenuation processes at the site (Number 2 above) should be
provided. Where the latter are also inadequate or inconclusive, data from
microcosm studies (Number 3 above) may also be necessary.30 In general,
more supporting information may be required to demonstrate the efficacy of
MNA at those sites with contaminants which do not readily degrade through
biological processes (e.g., most non-petroleum compounds, inorganics), or that
transform into more toxic and/or mobile forms than the parent contaminant, or
where monitoring has been performed for a relatively short period of time. The
amount and type of information needed for such a demonstration will depend
upon a number of site-specific factors, such as the size and nature of the
contamination problem, the proximity of receptors and the potential risk to those
receptors, and other characteristics of the environmental setting (e.g.,
hydrogeology, ground cover, climatic conditions) (emphasis in original).
30 As stated on p. 16 of the 1999 MNA guidance:
(1) Historical groundwater and/or soil chemistry data that demonstrate a clear and meaningful trendlS of
decreasing contaminant mass and/or concentration over time at appropriate monitoring or sampling
points. (In the case of a groundwater plume, decreasing concentrations should not be solely the result of
plume migration. In the case of inorganic contaminants, the primary attenuating mechanism should also
be understood.)
(2) Hydrogeologic and geochemical data that can be used to demonstrate indirectly the type(s) of natural
attenuation processes active at the site, and the rate at which such processes will reduce contaminant
concentrations to required levels. For example, characterization data may be used to quantify the rates of
contaminant sorption, dilution, or volatilization, or to demonstrate and quantify the rates of biological
degradation processes occurring at the site.
(3) Data from field or microcosm studies (conducted in or with actual contaminated site media) which
directly demonstrate the occurrence of a particular natural attenuation process at the site and its ability to
degrade the contaminants of concern (typically used to demonstrate biological degradation processes
only).
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Furthermore, as discussed in the "Reasonable Timeframe for Remediation" section of the 1999
MNA guidance (p. 21):
Thus, the EPA or other regulatory authorities should consider a number of factors
when evaluating reasonable timeframes for MNA at a given site. These factors, on
the whole, should allow the overseeing regulatory authority to determine whether
a natural attenuation remedy (including institutional controls where applicable)
will fully protect potential human and environmental receptors, and whether the
site remediation objectives and the time needed to meet them are consistent with
the regulatory expectation that contaminated groundwaters will be restored to
beneficial uses within a reasonable timeframe. When these conditions cannot be
met using MNA, a remedial alternative that more likely would meet these
expectations should be selected (emphasis in original).
Consistent with the 1999 MNA guidance, MNA may normally be considered a feasible
groundwater alternative if data and information obtained pursuant to Phases I through III suggest
cleanup goals can be achieved within a reasonable time frame. Recommended objectives that
generally should be addressed and the types of site-specific data that generally should be
collected under each successive phase are described below (EPA 2007a).
The primary objectives of progressing through the tiered site analysis are to reduce uncertainty in
the MNA remedy selection process and to compile data and information in the administrative
record supporting the Agency's remedy selection decision. The recommended tiered analysis
process can provide a means to organize the data collection effort in a cost-effective manner that
allows sites to be evaluated at intermediate stages of the site characterization effort. A general
synopsis of the recommended objectives along with possible analysis approaches and data types
to collect under each phase is provided in Table 3.1. Data collected for assessment of MNA are
often similar to data collected to evaluate engineered remedies such as pump and treat or in situ
treatment methods. This recommended approach is designed to optimize site characterization and
data collection, facilitate development of multiple lines of evidence, and ensure adequate
administrative record support for remedy selection decisions.
Table 3.1. Synopsis of the recommended site characterization objectives to address throughout
the tiered analysis process and potential supporting data types and analysis approaches associated
with each phase.
PHASE
I
RECOMMENDED
OBJECTIVE
Demonstrate plume stability
POTENTIAL DATA TYPES AND ANALYSIS
• Groundwater flow direction (calculation of hydraulic
gradients); aquifer hydrostratigraphy
• Contaminant concentrations in groundwater
• General groundwater chemistry data for preliminary
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PHASE
RECOMMENDED
OBJECTIVE
POTENTIAL DATA TYPES AND ANALYSIS
evaluation of contaminant degradation
II
Determine mechanism and
rate of attenuation
Detailed characterization of system hydrology (spatial
and temporal heterogeneity; flow model development)
Detailed characterization of groundwater chemistry
Subsurface mineralogy and microbiology
Contaminant speciation (groundwater and aquifer solids)
Evaluate reaction mechanism (site data, laboratory
testing, develop chemical reaction model)
III
Determine system capacity
and stability
Determine contaminant and dissolved reactant fluxes
(concentration data and water flux determinations)
Determine mass of available solid phase reactants
Laboratory testing of immobilized contaminant stability
(ambient groundwater; sequential extraction solutions)
Perform model analyses to characterize aquifer capacity
and to test immobilized contaminant stability (hand
calculations, chemical reaction models, reaction-
transport models)
IV
Design performance
monitoring program and
identify alternative remedy
Select monitoring locations and frequency consistent
with site heterogeneity
Select monitoring parameters to assess consistency in
hydrology, attenuation efficiency, and attenuation
mechanism
Select monitored conditions that "trigger" re-evaluation
of adequacy of monitoring program (frequency,
locations, data types)
Select alternative remedy best suited for site-specific
conditions
.1
Phase I: Demonstration that the groundwater plume is not expanding
As stated in the 1999 MNA guidance (p. 18): "Therefore, sites where the contaminant plumes are
no longer increasing in extent, or are shrinking, would be the most appropriate candidates for
MNA remedies."
The recommended objective under Phase I analysis is to obtain data and information that can be
used to evaluate whether MNA should be eliminated from further consideration for sites where
the groundwater plume is not stable or continuing to expand. Efforts generally should focus on
delineating the areal and vertical extent of plume boundaries. Time-series data collected from
monitoring wells normally can be used to evaluate whether concentrations are increasing or
decreasing at monitoring locations downgradient from a source area. An increasing concentration
trend generally indicates that sufficient attenuation is not occurring and the groundwater plume is
expanding and, as a result, MNA is generally not appropriate. However, short-term increases in
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
contaminant concentration may not automatically indicate an expanding groundwater plume and
should be evaluated in the context of a longer-term monitoring trend analysis. This approach is
recommended to account for variations in groundwater contaminant concentration because of
natural subsurface variability and seasonal fluctuations.
3.2 Phase II: Determination that the mechanism and rate of the attenuation process are
sufficient
As discussed in the 1999 MNA guidance (p. 20): "A decision on whether or not MNA is an
appropriate remedy for a given site is usually based on estimates of the rates of natural
attenuation processes. Site characterization (and monitoring) data are typically used for
estimating attenuation rates."
Furthermore, the 1999 MNA guidance states (p. 21):
As an example, analysis of natural attenuation rates from many sites indicates that a
measured decrease in contaminant concentrations of at least one order of magnitude is
necessary to determine the appropriate rate law to describe the rate of attenuation, and to
demonstrate that the estimated rate is statistically different from zero at a 95% level of
confidence (Wilson, 1998) (citation in original). Due to variability resulting from
sampling and analysis, as well as plume variability over time, smaller apparent reductions
are often insufficient to demonstrate (with 95% level of confidence) that attenuation has
in fact occurred at all.
The recommended objectives under Phase II analysis are to obtain data and information that can
be used to accomplish the following: (1) evaluate the mechanism and rate of the attenuation
process or processes, and (2) evaluate whether MNA should be eliminated from further
consideration. This second consideration normally is appropriate for sites where further analysis
shows that attenuation rates are insufficient for attaining site cleanup objectives within a
timeframe that is reasonable compared with other remedial alternatives (EPA 1999c). Data
should be collected to define groundwater chemistry, aquifer solids composition and mineralogy,
and the chemical speciation of the contaminant in groundwater and associated aquifer solids to
evaluate the attenuation mechanism. Radioisotopes and associated daughter products should be
identified for radionuclide-contaminated sites, as these may have different fate and transport
properties. This site-specific data collection effort may be significant, but it is intended
ultimately to provide the underpinning for further evaluation of MNA performance to be
addressed in subsequent stages of site characterization. Data collection efforts may include water
quality data collected in the field (for example, pH, dissolved oxygen, alkalinity, ferrous iron and
dissolved sulfide); laboratory measurements of groundwater and aquifer solids chemical
composition; microbial characteristics and mineralogy of the aquifer solids (as relevant to
degradation or immobilization); and determination of contaminant speciation in groundwater and
the aquifer solids (EPA 2007a). Contaminant speciation for this recommended analysis refers to
both oxidation state characterizations (for example, As[III] vs. As[V]; U[IV] vs. U[VI]) as well
as specific associations with chemical constituents in aquifer solids (for example, precipitation of
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
lead carbonate vs. adsorption of lead to iron oxides). Evaluation of subsurface microbiology may
be necessary in situations where biotic processes play a direct or indirect role in governing
contaminant attenuation. Microbial influences may be predominant in plumes where readily
degradable organic contaminants, such as hydrocarbons, also are present. Ultimately, knowledge
of the attenuation process along with a detailed knowledge of the groundwater flow field can
provide the basis for subsequent evaluations to assess the long-term capacity of the aquifer to
sustain contaminant attenuation.
An estimate of attenuation rates for inorganic contaminants typically will involve calculations of
apparent mass transfer from the aqueous to the solid phase, based on sampling of groundwater or
aquifer solids.31 These estimates should be based as much as possible on field measurements
rather than on modeling predictions (EPA 2007a). Where radioactive decay is a primary
attenuation process, both intermediate and terminal decay products should be identified, and the
time to reach cleanup levels should be estimated as described in Section 2.1 of this document.
3.3 Phase III: Determination that the capacity of the aquifer is sufficient to attenuate the mass
of contaminant within the plume and the stability of the immobilized contaminant is
sufficient to resist re-mobilization
Sites that possess insufficient capacity to adequately attenuate the groundwater plume generally
are not suitable candidates for MNA.
The recommended objective under Phase III is to obtain data and information that can be used to
evaluate whether MNA should be eliminated from further consideration for sites where there is
insufficient capacity in the aquifer to attenuate contaminant mass to groundwater cleanup levels.
Likewise, the data may show that the stability of the immobilized contaminant is insufficient to
prevent re-mobilization caused by future changes in groundwater chemistry (EPA 2007a).
Possible factors that could result in an insufficient capacity for attenuation include the following:
(1) changes in groundwater chemistry that result in slower rates of attenuation or re-mobilization
of contaminants, and (2) insufficient mass of solid constituents in the aquifer solids that
participate in the attenuation reaction. These factors may apply to situations where either
degradation or immobilization is the primary attenuation process. For example, contaminant
desorption could be caused by changes in groundwater pH, because the degree of adsorption
typically is sensitive to this parameter.
31 With regard to consideration of modeling as a general matter, the following preamble language may be useful in
the context of MNA specifically: "However, limited fate and transport modeling and site information may be used to
establish cleanup levels for contaminated soils and waste materials remaining at the site. For example, the ground-
water route of exposure would be protected by determining a level in the soils that would be consistent with the
levels established for ground water. Typically, monitoring will be necessary after the completion of the remedial
measure to verify that the levels established at the site are protective of ground water and other routes of exposure"
(53 Fed. Reg. at p. 51446, December 21, 1988).
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Assessment of attenuation capacity usually depends on knowledge of the mass flux of
contaminants and associated reactants in groundwater, as well as the mass distribution of reactive
aquifer solids along groundwater flow paths. Mass flux for this recommended analysis is defined
as the contaminant mass per time passing by a plume transect perpendicular to groundwater flow
(Farhat and others 2006). The general approach that can be taken is to estimate the attenuation
capacity within the plume boundaries and compare this capacity with the estimated mass flux of
aqueous-phase contaminants emanating from source areas, assuming source zone removal or
containment has been completed to the extent practicable (EPA 2007a). It is recommended that a
detailed characterization of the site's hydrology be performed to ensure that sufficient data are
available to determine system capacity in the subsurface environment.
The stability of an immobilized contaminant can be evaluated through a combination of
laboratory testing and chemical reaction modeling considering existing and anticipated site
conditions. Both analysis approaches normally can be developed based on the information
gathered during recommended Phase II efforts to characterize the specific attenuation process
active within the groundwater plume (EPA 2007a).
The sensitivity to contaminant re-mobilization typically can be assessed with laboratory tests
employing aquifer solids collected from within the plume boundaries. These solids can be
exposed to solutions that mimic anticipated groundwater chemistries (for example, ambient
groundwater samples or laboratory-created solutions in which the concentrations of specific
dissolved constituents can be varied). A supplementary approach to test contaminant stability
could include use of chemical reaction models to efficiently explore contaminant solubility under
a range of hypothetical groundwater conditions to identify the groundwater parameters to which
the attenuation reaction may be most sensitive.
3.4 Phase IV: Design of a performance monitoring program based on an understanding of the
mechanism of the attenuation process, and establishment of contingency remedies
tailored to site-specific characteristics
As discussed in the 1999 MNA guidance (p. 20):
It should be noted that the timeframe required for MNA remedies is often longer
than that required for more active remedies. As a consequence, the uncertainty
associated with the above factors increases dramatically. Adequate
performance monitoring and contingency remedies (both discussed in later
sections of this Directive) should be utilized because of this higher level of
uncertainty. When determining reasonable timeframes, the uncertainty in
estimated timeframes should be considered, as well as the ability to establish
performance monitoring programs capable of verifying the performance expected
from natural attenuation in a timely manner (e.g., as would be required in a
Superfund five-year remedy review) (emphasis in original).
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The recommended objective under Phase IV analysis is to develop a monitoring program to
assess long-term performance of MNA and to identify alternative remedies that could be
implemented in case MNA fails. Site data collected during the previous phases should focus on
identification of alternative remedies that best match site-specific conditions.
The 1999 MNA guidance includes recommendations in the section on Performance Monitoring
and Evaluation (pp. 22-23). The monitoring program for inorganic contaminants should consist
of establishing a network of wells that meet the following criteria: (1) can provide adequate areal
and vertical coverage to verify that the groundwater plume remains static or shrinks, and (2) can
provide the ability to monitor groundwater chemistry throughout the zones where contaminant
attenuation is occurring. The monitoring program generally should include an assessment of
groundwater flow patterns so the monitoring network can be adjusted to evaluate the influence of
potential flow changes within the plume. Monitoring should include continued verification of
contaminant removal from groundwater, but also should include tracking trends in other
reactants that participate in the attenuation reaction (for example, pH, alkalinity, ferrous iron,
oxidation-reduction potential and sulfate). Where radioactive decay is a contributing attenuation
process, the monitoring program should also track concentrations of daughter products in
groundwater. Periodic collection of aquifer solids may be warranted to verify consistency in
reaction mechanisms for sites where contaminant immobilization is the primary attenuation
process. Groundwater parameters should be selected to monitor constituents that provide
information on continued stability of the solid phase that is associated with an immobilized
contaminant. Examples include ferrous iron or sulfate to track dissolution of iron oxides or
sulfide precipitates. Non-contaminant performance parameters such as these are likely to serve as
"triggers" to alert site managers to potential remedy failure or performance losses, since the
attenuation reaction should respond to these changed conditions. Monitoring these indicator
parameters may improve the ability of site managers to evaluate and address the potential for
groundwater plume expansion because increases in mobile contaminant concentrations may be
delayed relative to changes in site conditions.
With regard to developing contingency remedies as part of the Phase IV analysis, please refer to
the Contingency Remedies section of the 1999 MNA guidance (pp. 24-25).
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4.0 SITE CHARACTERIZATION
As discussed in the 1999 MNA guidance (pp. 13-14):
Site characterization should include collecting data to define (in three spatial
dimensions over time) the nature and distribution of contaminants of concern and
contaminant sources as well as potential impacts on receptors (see "Background"
section for further discussion pertaining to "Contaminants of Concern").
However, where MNA will be considered as a remedial approach, certain aspects
of site characterization may require more detail or additional elements. For
example, to assess the contributions of sorption, dilution, and dispersion to natural
attenuation of contaminated groundwater, a very detailed understanding of aquifer
hydraulics, recharge and discharge areas and volumes, and chemical properties is
necessary. Where biodegradation will be assessed, characterization also should
include evaluation of the nutrients and electron donors and acceptors present in
the groundwater, the concentrations of co-metabolites and metabolic byproducts,
and perhaps specific analyses to identify the microbial populations present. The
findings of these, and any other analyses pertinent to characterizing natural
attenuation processes, should be incorporated into the conceptual model of
contaminant fate and transport developed for the site.
The primary objective of site characterization at sites with inorganic contaminants in the
groundwater generally is to obtain data and information that can be used to identify attenuation
mechanisms at a given site. This characterization effort should emphasize direct measurements
of groundwater conditions and the associated solid phase characteristics of the aquifer.
Measurements or tests conducted with subsurface samples retrieved within the zones where
attenuation occurs should provide the most direct means to evaluate ongoing reaction processes.
This knowledge may guide approaches to assess the capacity of the aquifer to sustain
contaminant attenuation within the plume and to evaluate the long-term stability of immobilized
contaminants. Evaluations conducted on subsurface samples also have the potential advantage of
incorporating actual characteristics and factors of groundwater and aquifer solids that may be
difficult to adequately parameterize within geochemical models.
Delineation of the inorganic plume in three dimensions and subsequent monitoring of the
groundwater plume with time generally should be a central component of the recommended
tiered analysis. The following sections describe some of the key site characterization objectives
relevant to evaluating the potential use of MNA for inorganic contaminants.
4.1 Hydrogeologic and Contaminant Distribution Characterization
The recommended first step (Phase I) in any natural attenuation evaluation is obtaining a
thorough working knowledge of site hydrogeology, including direction and rate of groundwater
flow, potential impact of interactions between groundwater and surface water or sediment, and
potential impact of active pumping, if applicable (EPA 2007a). Information on the nature and
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extent of contamination and potential contaminant loading to groundwater also is needed,
including the existence and distribution of both organic and inorganic plumes in soil and
groundwater. These data then may be used to create or update a three-dimensional CSM
describing site conditions.
4.2 Determination of Attenuation Rates
The plume should be demonstrated to be stable or shrinking for MNA to be viable for inorganic
contaminants in groundwater. The hydrogeologic and contaminant distribution data
recommended above (collected during the recommended Phase I evaluations) normally can be
used to estimate attenuation rates during Phase II evaluations. Calculation and Use of First-
Order Rate Constants for Monitored Natural Attenuation Studies discusses the following two
methods to determine rate estimates: (1) plume concentration vs. distance from a source, and
(2) plume concentration measured over time at a point (EPA 2002c). Estimates of mass flux may
be used as additional supporting information in determining the rate of contaminant attenuation
because these estimates may be affected by changing directions and rates of groundwater flow. It
is important to note that determination of mass flux normally is constrained by the same
limitations that exist for determining attenuation rate estimates, namely that groundwater flow
rates can change in both space and time. An additional factor is that mass flux generally is
estimated only for horizontal flow. If vertical gradients are present, the typical estimates of mass
flux will likely be incorrect. Multiple lines of evidence should be used to identify whether
attenuation is occurring. Therefore, neither the attenuation rate nor the mass flux estimates
should be used as the primary supporting evidence that attenuation is occurring.
Demonstration of decreasing concentrations is the primary supporting evidence that
attenuation is occurring.
Both direct measurements and indirect evidence may be used to identify the mechanism and rate
of attenuation in groundwater. For example, decreased nickel concentrations collocated with
decreased ferrous iron or sulfide concentrations in groundwater would suggest potential co-
precipitation of nickel with iron sulfide. Various types of data can provide multiple lines of
evidence to assess the likelihood of inorganic attenuation and the viability of MNA. An analysis
of groundwater concentration data alone generally will not be adequate to confirm any
precipitation or co-precipitation mechanism of attenuation (EPA 2007a).
4.3 Geochemical Considerations
Generally, hydrogeology and groundwater and aquifer geochemistry together form the
framework for understanding contaminant fate and transport at a site. Evaluation of aquifer
mineralogy and solid-phase contaminant speciation is typically an important part of
identification of the contaminant immobilization process (EPA 2007a) for inorganic
contaminants. Both groundwater and aquifer solids samples collected using methods that
preserve the in situ integrity of the samples help to support this evaluation. Appendices 1 and 2
summarize the recommended physical and chemical analyses and data uses for development of a
CSM to support an MNA evaluation for inorganic contaminants. Appendices 3 and 4 provide
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recommended analytical methods and data quality objectives and goals for the analyses
suggested in Appendices 1 and 2. Determining aquifer capacity and the stability of reactions
likely will include use of laboratory-based tests using site groundwater and aquifer solids.
4.4 Groundwater Geochemistry Characterization
Data collected during the recommended Phase II evaluation for geochemical characterization of
groundwater generally should include pH, oxidation-reduction potential and dissolved oxygen,
dissolved organic and inorganic carbon, major cations and anions, and chemical speciation of the
contaminants and key reactants in groundwater. Sufficient data should be collected to understand
both the temporal and spatial variability of these parameters (EPA 2007a).
4.5 Solid Phase Characterization
Solid phase characterization often is an important aspect of evaluating natural attenuation of
inorganic contaminants during the recommended Phase II and Phase III analysis. Procedures for
characterizing aquifer materials include the following: X-ray diffraction or X-ray fluorescence
for characterizing mineralogy; sequential extraction procedures (SEP) for characterizing the
solid-phase components the contaminants are associated with; geochemical speciation analysis
for determining the redox conditions of the aquifer; and laboratory batch and flow-through
column tests for determining the sorptive capacity of the aquifer materials.
In SEP, contaminated soils are subjected to successively harsher solutions in an attempt to
sequentially leach soil contaminants. While environmental risk may be assessed using the results
of the water soluble or exchangeable soil fractions (step 1 of the multiple-step SEP process),
such an approach will generally be inadequate for developing the necessary understanding of
attenuation mechanisms and long-term contaminant behavior to support selection of MNA (EPA
2007a). As a result, partial SEP analyses are not the sole line of evidence that can be used to
demonstrate inorganic attenuation. Therefore, the results should not be accepted without question
because of the wide variety of methods used to implement the SEP.32
If redox processes are believed to be an important component of attenuation mechanisms, special
attention should be given to preserving the redox status of materials after they are retrieved from
the subsurface. For example, if anoxic materials are collected, they should be frozen after
collection or stored in evacuated containers that have been purged with inert gas to preserve
primary mineralogy (EPA 2002a and 2006b). Methods for characterizing the oxidation capacity
and reducing capacity of aquifer solids are summarized in Workshop on Monitoring Oxidation-
Reduction Processes for Ground-water Restoration (EPA 2002a).
32 Refer to Section IIIB.2.4.2 m Monitored Natural Attenuation of Inorganic Contaminants in Ground Water,
Volume I (EPA 2007a) for a discussion of sequential extraction considerations.
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Aquifer capacity for contaminant attenuation is often estimated during the Phase III evaluation.
As thoroughly described in Understanding Variation in Partitioning Coefficient, Kd Values,
Volumes I-III (EPA 1999a, 1999b and 2004c), there are multiple approaches to measure or
estimate distribution/partition coefficient (Kd) values, such as laboratory batch tests, in situ batch
tests or flow-through column tests. Each has its inherent advantages and limitations, and each
involves a unique set of assumptions. The Kd, for this guidance, is considered the ratio of
contaminant mass per unit mass of solid to the mass of contaminant remaining in solution at
equilibrium (EPA 1999b).
Contaminant sorption behavior can be simulated by geochemical models. The use of the
Langmuir and Freundlich isotherms (also known as empirical models) and surface complexation
models (SCM, known as mechanistic models) can be used to develop the Kd value (EPA 1999a
and 2007a). The predictive capability of empirical models (using empirically derived values of
Kd) are limited to the range of experimental data when Kd is determined. However, mechanistic
models like SCMs have the advantage of being able to modify input parameters and account for
changes in groundwater chemistry, such as solution pH and the impact of major ions in solution
on available sorption sites. Thus, SCMs are potentially more robust in their predictive
capabilities to evaluate the impact of changing chemical conditions in the system. Further
discussion of the use of models in the assessment of natural attenuation of inorganic
contaminants may be found in Section 6 of this document.
It is normally important that solid samples be representative of aquifer materials and contaminant
concentrations. Contaminant attenuation is not linear in that the attenuation rate does not increase
in direct proportion to the concentration. Therefore, solid samples used to define solid-phase
attenuation should contain a range of contaminant concentrations. In addition, they should
represent a range of soil textures and other factors that affect attenuation. Measured contaminant
concentrations represent a mean concentration of the soil volume sampled and can be determined
for each of the contaminants of interest. SEP may be used to differentiate contaminant
concentrations associated with different phases of soil (for example labile vs. sorbed vs.
structural) in the designated representative sampling area based on the CSM. This same
designated representative sampling area can be re-sampled during subsequent sampling events—
again at random locations and depths below the water table and within the same lithology—and
SEP results compared to determine if a significant change in concentrations has occurred over
time. Ideally, this approach provides a representative and repeatable simulation of aquifer
materials and minimizes the negative impact of soil heterogeneity on the evaluation of
attenuation processes.
4.6 Special Considerations for Radionuclides
The amount of radioactive material in soil or water is typically measured in units of decay rate or
activity and reported as picocuries (pCi) per mass of soil or volume of water — for example,
picocuries per liter (pCi/L) (1 picocurie equals 0.037 becquerels [Bq]). Activity units are used to
evaluate exposure risk that forms the basis for remediation, whereas mass-based concentration
levels (for example, |ig/L) are used in selecting and designing a remediation technology. The
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activity corresponding to a given mass of radioactive material varies depending on the
radionuclide. For example, 1 gram of uranium-238 has an activity of 0.33 pCi, whereas 1 gram
of uranium-234 has an activity of 6,200 pCi.
Decay rate-based activities are different than mass-based concentrations, and neither should be
used alone to identify potential remedial components For example, transport models that are
employed to understand the solid-liquid partitioning and fate and transport of a radionuclide are
developed using mass concentration units and mass-action reaction expressions. Isotopic
composition and activity of radioactive material generally are important for risk assessment and
plume decay predictions (EPA 2010a).
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5.0 ATTENUATION PROCESSES FOR INORGANIC CONTAMINANTS
The following sections briefly describe microbial, chemical and physical attenuation processes
for metals and other inorganic contaminants, as well as radioactive decay as an attenuation
process for radionuclides. These attenuation processes may act in isolation or together to retard
or arrest migration of inorganic contaminants in an aquifer. Factors that can help evaluate which
process is likely to dominate contaminant attenuation include chemical properties of the
contaminant, chemical characteristics of the groundwater, and properties of the aquifer solids.
Microbial activity may exert a significant but primarily indirect influence on contaminant
attenuation for many of the inorganic contaminants discussed in this document; however, in the
case of both nitrate and perchlorate, direct microbial degradation should be the controlling
attenuation process (EPA 2007b). Redox conditions in an aquifer normally are a key controlling
factor of contaminant fate and transport of inorganic contaminants in groundwater and will be
mentioned repeatedly in the discussions that follow.
5.1 Microbial Degradation
Subsurface microbes typically play an important and dynamic role in controlling aquifer
geochemistry and fate of inorganic contaminants in situ; they tend to alter most attenuation
processes in groundwater. Subsurface microorganisms exhibit a remarkable array of metabolic
capabilities. For example, microbes derive energy through oxidation of organic or inorganic
compounds as electron donors. The electrons are transferred to an electron acceptor which, in the
case of aerobic respiration, is oxygen. Anaerobic respiration is also possible, whereby chemically
reducible inorganic compounds (such as nitrate, sulfate, ferric iron or iron/manganese
oxyhydroxides) are used as electron acceptors (EPA 2007a). The coupled reactions of electron
acceptors and donors are termed oxidation-reduction or "redox" reactions. These redox reactions
are often mediated by microbes in situ. In general, microbes preferentially consume oxygen and
nitrate as the most favorable electron acceptors, followed by manganese and iron oxyhydroxides,
sulfate, and finally, carbon dioxide. An aquifer progresses from oxidizing (aerobic) to reducing
(anaerobic) redox conditions as microbes consume this series of electron acceptors.
In some situations, intense local microbial activity may be entirely responsible for the redox
status of the aquifer. The nature of the active microbial population (for example, iron-reducing,
sulfate-reducing, or sulfur-oxidizing bacteria) can often be inferred from geochemical data. Thus,
trends in the concentration of organic substrates (dissolved organic carbon) and their metabolites
(for example, Fb, FbS, CFU, CO2, NC>2~, HS" or Fe2+) can indicate whether and which
microorganisms are active in a particular subsurface region.
Conversion of dissolved organic carbon by microbial activity can create and replenish the
reductive capacity of a site. In some instances, direct and specific determination of microbial
population by culturing or genetic analysis (for example, messenger ribonucleic acid profiles) of
aquifer solids extracts may be warranted.
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Although metals and radionuclides may change valence state or form different anion complexes
in response to microbial activity or redox conditions, they generally are not degraded. Microbial
degradation involves breaking chemical bonds in a contaminant compound, the subsequent
formation of new bonds and, ultimately, creation of another compound that may be more or less
toxic.
Both nitrate and perchlorate are highly soluble and thus mobile contaminants in groundwater that
may directly serve as electron acceptors for subsurface microorganisms in situ. However,
subsurface microbes may be limited in situ by organic carbon, energy substrate, or trace
nutrients. Given the high solubility and mobility of nitrate and perchlorate in groundwater under
organic carbon-limited conditions, MNA by itself may not be appropriate for these contaminants.
It may be appropriate to consider MNA in combination with active remedies such as in situ
bioremediation through organic carbon substrate injections or other treatments, however.
Soil microbes may be responsible for the methylation of iodine-129 to form methyl-iodide in
wetland environments under low redox conditions and in the presence of high concentrations of
organic matter (EPA 2010a). Methyl-iodide is subject to volatilization. While the microbial
activity is not directly responsible for destruction of the iodine, it is an example of the indirect
impact of microbial activity on chemical fate. Failure to account for microbially induced
methylation can result in misinterpretation of the volatilized iodine-129 as sorbed iodine-129,
which in turn can result in an overestimation of contaminant mass sorbed to aquifer solids.
5.2 Chemical Transformation/Redox
An understanding of redox conditions in the aquifer often is important, as redox processes have a
significant impact on the aqueous and solid phase speciation of inorganic contaminants.
Although most metals generally are not degraded through microbial action, some can change
oxidation state, which in turn significantly influences their solubility and transport in
groundwater. Changes in oxidation state of a metal occur through abiotic or microbially
mediated redox reactions where the metal serves as an electron acceptor or donor. This section is
focused on redox transformations of inorganic contaminants (metals, metalloids and
radionuclides).
Ferrous sulfide rich formations may promote abiotic reduction of soluble metal species to their
less mobile lower oxidation states. Alternatively, microbes may deplete oxygen and other highly
energetic electron acceptors from groundwater under organic carbon rich conditions. In this way,
they promote anaerobic or reducing conditions that favor reduction and immobilization of some
metals. Under iron- and sulfate-reducing conditions, metals such as chromium(VI), selenium(VI
or IV), and copper(II) may be reduced to lower valence states, which may form sparingly soluble
metal-oxide minerals or may co-precipitate with sulfides. Likewise, radionuclides such as
uranium(VI) and technetium(VII) become favorable electron acceptors under iron- or sulfate-
reducing conditions and may precipitate as radionuclide-oxide minerals or may co-precipitate
with sulfides. These redox sensitive metals and radionuclides are generally less soluble and less
mobile in their reduced oxidation states; however, there are exceptions to this generalization.
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Studies have shown that addition of organic carbon to stimulate iron reduction results in
transformation of some contaminants, such as U(VI), to less mobile forms and as a consequence
result in decreased groundwater concentrations in metal or radionuclide contaminated plumes
where organic carbon is limited (Anderson and others 2003; Istok and others 2004; Michalsen
and others 2006). However, these same reduced metal- or radionuclide-oxide precipitates may be
vulnerable to oxidation by nitrate and other oxidants, which could reverse the process, causing a
related increase in contaminant concentrations in groundwater (Anderson and others 2003;
Senko and others 2002). Shifts in groundwater pH or bulk geochemistry may affect the metal
solubility and could reverse the attenuation process. Thus, the stability of the attenuated
contaminant will ultimately be governed by the type of contaminant-solid phase association and
by the stability of groundwater geochemistry.
It is important to recognize that reversals in oxidation state of inorganic contaminants may result
from attempts to remediate other contaminants present at a site. For example, the use of in situ
chemical oxidation methods to remediate high concentrations of hydrocarbon-based compounds
may result in a related increase in soluble U(VI). Similarly, the injection of organic electron
donors intended to create reducing conditions in an aquifer may result in the reductive
dissolution of arsenic species, with a corresponding increase in groundwater concentrations. SEP
analysis may also be used to assess changes in concentrations associated with different soil
phases (for example, sorbed vs. precipitated).
5.3 Sorption and Precipitation
Physical partitioning of a contaminant from a soluble and mobile form in groundwater to a less
mobile form on aquifer solids is a primary natural attenuation process for many metals and
radionuclides. This partitioning process generally involves the following three primary
mechanisms: (1) adsorption, which is the accumulation of a contaminant ion at the aqueous and
solid phase adsorbent interface; (2) precipitation, which is the growth of solid phase containing
repeated molecular units in three dimensions; and (3) absorption, which is diffusion of the
aqueous or adsorbed contaminant ion into the solid phase (Sposito 1986). "Sorption" will be
used in this guidance to describe, in a generic sense (that is, adsorption and absorption
mechanisms), the partitioning of aqueous phase constituents to a solid phase. One or more
sorption mechanisms are likely important if the inorganic contaminant of concern at a site is a
metal or a radionuclide.
In general, adsorption or desorption of metal or radionuclide cations onto and off of aquifer
materials is pH dependent and increases with increasing pH, typically reaching a maximum
under circumneutral pH conditions, depending on groundwater chemistry and properties of the
adsorbent surface (Sparks 2003). Important adsorbent phases commonly found in the
environment include phyllosilicate minerals ("clays"), metal oxyhydroxide phases, and natural
organic matter (Dixon and Schulze 2002; EPA 2007a, Section HE. 1.1). However, most clay
minerals possess a permanent negative charge as a result of the substitution of lower valence
cations within mineral layers of the clay. This permanent negative charge is unaffected by
changes in groundwater pH and is typically balanced through ion exchange reactions involving
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major cations in groundwater (for example, Na+, K+, Ca2+ or Mg2+) but potentially also
contaminant metal or radionuclide ions.
Precipitation is another important attenuation mechanism for removal of metals and radionuclide
contaminants from groundwater. Contaminant ions may precipitate as a pure phase (for example,
CdCOsfs]) or may co-precipitate by incorporation of the contaminant ion within the structure of
another mineral phase. Examples of co-precipitation include Cr(III) in hydrous ferric oxide and
Cd(II) in calcium carbonate. Strontium may co-precipitate during formation of calcium or
ferrous iron carbonates (for example, in the presence of elevated alkalinity or ferrous iron formed
during microbial degradation of organic compounds) (Fujita and others 2004; Roden and others
2002; EPA 2010a, Strontium Chapter). Precipitation is also an important attenuation pathway for
radium, which may also co-precipitate as a sulfate mineral (for example, as RaSO4 or
BaRa[SO4]2, in the presence of moderate sulfate concentrations) (Langmuir and Reise 1985);
however, under sulfate reducing conditions, these minerals may dissolve and result in radium
release to groundwater (Huck and Anderson 1990; Pardue and Guo 1998; EPA 2010a, Radium
Chapter).
Most precipitation reactions have a strong dependence on solution chemistry and pH. The
tendency for a system to support a specific precipitation or dissolution reaction can be evaluated
through comparison of the equilibrium solubility constant for a given solid phase mineral to the
ion activity product calculated using the site groundwater geochemical data. The ion activity
product is useful for evaluating the potential for contaminant precipitation; however, it is not
unequivocal evidence that a given phase is at equilibrium or even present in the system (Sposito
1984; EPA 2007a, Section IIB.2.1).
Physical partitioning is a particularly important attenuation process for cadmium, lead, nickel,
and copper because these metals are stable in their +2 valence state and are not subject to direct
chemical transformation or changes in valence state, which can significantly alter the solubility
of metals. However, these metals may form stable precipitates with redox-sensitive elements
such as sulfur and iron; thus, the solubility and mobility of these metals are indirectly tied to
redox conditions. For example, if sorption to iron oxides is a primary attenuation pathway and
the redox conditions change such that reductive iron dissolution occurs, this process could
mobilize or remobilize the metal of concern in groundwater. Likewise, if the primary attenuating
phase of the metal of concern is a metal-sulfide precipitate and the groundwater redox conditions
shift such that oxidative dissolution of sulfides occurs, this shift could also mobilize or
remobilize the metal of concern in groundwater. Furthermore, shifts in groundwater pH or bulk
geochemistry may alter the metal partitioning and could reverse the attenuation process. For
example, sorption to carbonate minerals may be an important attenuation process for thorium
(EPA 1999a) and americium (Shanbhag and Morse 1982; EPA 2010a, Americium-Strontium
Chapter). However, decreases in pH can destabilize carbonate minerals and result in increased
radionuclide concentrations in groundwater. Thus, the stability of the immobilized contaminant
(precipitated or sorbed) will ultimately be governed by the type of contaminant-solid phase
association and by groundwater geochemistry.
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In general, the absorption of metal or radionuclide cations is limited to Group 1 (Alkali metals),
mainly potassium, cesium and rubidium. These elements exhibit low hydration energy and
unique hydrated radii that allow them to diffuse into the structure of vermiculite minerals. This
process promotes the "fixation" of the cations by the subsequent collapse of the vermiculite into
a mica-type structure.
5.4 Radioactive Decay
Radioactive contaminants share many fate and transport properties in common with metals, as
illustrated in previous sections. However, radioactive decay is a unique attenuation process
specific to this group of contaminants that warrants special discussion. Radioactive decay
typically functions in conjunction with other attenuating processes as part of MNA, but it is the
primary attenuating process for radon and tritium, as they are generally considered unreactive in
groundwater and have relatively short half-lives (EPA 2010a).
Radioactive decay ultimately decreases the concentration of parent atoms or compounds in
groundwater but can result in increased concentrations of daughter products, which are the
products of parent isotope decay. Eventually, a stable daughter product is created and no further
radioactive decay follows.
If the decay rate of the daughter product is less than the decay rate for the parent isotope or is
infinite because the daughter is stable, then the daughter product may accumulate and affect the
activity of the plume in a process called ingrowth. Ingrowth is a particularly important concept
when use of MNA is evaluated for radionuclides because daughter products may exhibit
increased toxicity and solubility, which may affect plume fate and transport (EPA 1999c).
Radioactive decay can be simple (for example, decay of 1-129 to stable Xe-129); however,
radionuclides with complex, multi-step decay series (for example, decay of Ra-226 to Pb-210)
are most commonly encountered at National Priorities List (NPL) sites (EPA 1993). Table 5.1
below provides a summary of radiochemical information for select radionuclides, including half-
lives and energy levels of emitted radiation, as well as associated decay chains and terminal
products. It is important to identify specific isotopes present in groundwater so that associated
decay chains, intermediate daughter, and terminal daughter products can be identified and
accounted for during remedy selection and monitoring program development.
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Table 5.1. Radiochemical information for select radionuclide isotopes (EPA 1993 and 2000).
Select
Radionuclides
Nuclide35
Am-241
Am-243+D
Cs-134
Cs-135
Cs-137+D
H-3
1-129
Pu-238
Pu-239
Pu-240
Pu-244+D
Ra-226+D
Ra-228+D
Rn-222
Sr-90+D
Tc-99
Th-228+D
Th-229+D
Half-
life (yr)
432
7,400
2
3E+6
30
12
1.6E+7
88
2.4E+4
6,500
9.3E+7
1,600
8
l.OE-2
29
2.1E+5
2
7,300
Radiation Type (MeV)
a
5.486
5.3
5.499
5.156
5.168
4.6
4.784
5.490
5.423
4.9
P(avg)
0.022
0.1520
0.0570
0.1950
0.0050
0.0400
0.0071
0.0140
0.2000
0.0850
0.12
Y
0.0595
0.055
0.605
0.787
0.662
0.0516
0.0012
0.1861
0.5100
0.096
Associated Decay
Chain33
-
Np-239 (2d)
-
-
Ba-137m (95%, 3min)
-
-
-
-
-
U-240 -100%
Np-240
Rn-222 (4 d)
Po-218(3min)
Pb-214(~100%, 27min)
Bi-214(20min)
Po-214(~100%, Imin)
Ac-228 (6 h)
Po-218(3min)
Po-2 14 (0.2ms)
Po-210(138d)
Y-90 (64 h)
-
Ra-224 (4 d)
Rn-220 (56 s)
Po-216(0.2s)
Pb-212(llh)
Bi-212(61min)
[Po-2 12 (64%, 0.3 (is)
Tl-208 (36%, 3 min)]
Ra-225 (15d)
Ac-225(10d)
Terminal Nuclide
or Radionuclide34
Nuclide
Np-237
Pu-239
Ba-134
(-100%)
Ba-135
Ba-137
He-3
Xe-129
U-234
U-235
U-236
Pu-240
Pb-210
Th-228
Pb-206
Zr-90
Ru-99
Pb-208
Bi-209
Half-
life (yr)
2.1E+6
2.4E+4
stable
stable
stable
stable
stable
2.4E+5
7E+8
2.3E+6
6,500
22
2
stable
stable
stable
stable
stable
34
35
The chain of decay products of a principal radionuclide extending to (but not including) the next principal
radionuclide or a stable nuclide. Half-lives are given in parentheses. Radioactive ingrowth branches are indicated
by square brackets with branching ratios in parentheses.
The principal radionuclide or stable nuclide that terminates an associated decay chain.
"+D" designates radionuclides with associated decay chain.
OSWER Directive 9283.1-36
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Select
Radionuclides
Nuclide35
Th-230
Th-232
U-234
U-235+D
U-238+D
Half-
life (yr)
7.7E+4
1.4E+10
2.4E+5
7.0E+8
4.5E+9
Radiation Type (MeV)
a
4.688
4.013
4.776
4.400
4.197
P(avg)
Y
0.0677
0.0532
0.1857
Associated Decay
Chain33
Fr-22 (5 min)
At-217(32ms)
Bi-213(46min)
[Po-213(98%, 4 (is)
Tl-209 (2%, 2 min)]
Pd-209 (3 h)
-
-
-
Th-231(26h)
Th-234 (24 d)
Pa-234m (99.8%, 1 min)
Pa-234 (0.2%, 7 h)
Terminal Nuclide
or Radionuclide34
Nuclide
Ra-226
Ra-228
Th-230
Pa-231
U-234
Half-
life (yr)
1,600
6
8E+4
3.4E+4
2.4E+5
a
P
Y
d
Alpha
Beta
Gamma
day
h hour
s second
MeV Megaelectronvolt
us
ms
microsecond
millisecond
mm
minute
Unstable parent radionuclides decay to form either unstable or stable radionuclide daughter
products. The decay of an unstable radionuclide parent to a stable radionuclide daughter results
in ingrowth. An unstable radionuclide daughter results in one of three equilibrium conditions, all
exhibiting a period of ingrowth. The three parent/daughter equilibrium relationships are
identified as "secular," "transient," and "no equilibrium." The "ingrowth only" example in
Figure 5.1 A demonstrates the decay of the unstable parent leading to the ingrowth in the stable
daughter (for example, Cs-137 decay to stable Ba-137). The first of the equilibrium cases is the
limiting "secular equilibrium," where the half-life of the parent is much larger (approximately
10~4) than the daughter (for example, Ra-226 decay to Rn-222). The period of the daughter
product's ingrowth occurs until its activity reaches that of the initial parent activity
(Figure 5.IB).
Thereafter, the daughter decays at the same rate that it is formed. The second equilibrium case is
"transient equilibrium," where the half-life of the parent is somewhat larger (approximately 10
times) than the daughter (for example, Th-227 decay to Ra-223). The period of the daughter
product ingrowth initially reaches a maximum followed by a decrease until both parent and
daughter decay become constant (Figure 5.1C). The last equilibrium case is "no equilibrium,"
where the half-life of the parent is smaller than the daughter (example, Am-241 decay to Np-
.237). The period of daughter ingrowth peaks later than for "transient equilibrium" and
eventually decays according to the daughter half-life characteristics (Figure 5.ID).
OSWER Directive 9283.1-36
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
A.
Parent (unstable) - Daughter (stable)
original radionuclide
time
Parent (unstable) - Daughter (unstable)
k combined activity
^original radionuclide
kdecay product
period of transient equilibrium
ingrowth
B.
Parent (unstable) - Daughter (unstable)
Ti/2,P»T1/2,d
combined activity
. — • —
original radionuclide
•- >
secular
equilibrium
rouc
i in
time |
period of ingrowth
Parent (unstable) - Daughter (unstable)
decay product
i i i in
11/2,p
original radionuclide
, decay product
combined activity
period of
ingrowth
no equilibrium
Figure 5.1. Illustration of four decay ingrowth scenarios in groundwater plumes contaminated
with radionuclides. Ti/2,P=decay half-life of parent radionuclide, Ti/2,d=decay half-life of daughter
radionuclide. Illustrations derived from the EPA website
www.epa.gov/radiation/understand/equilibrium.html
It can be seen that the production of daughter products can influence plume composition,
potential radiological risks, and the dimensions of the plume if, as is typical, the daughter
product displays radiological or chemical risk and transport characteristics different from that of
the parent radionuclide. It is important to note that, as with organics, daughter products of
radionuclides may pose greater risk, be more mobile, and have longer half-lives than the parent
in the decay series. Radionuclide ingrowth corrections may be important for accurate
descriptions of plume characteristics over time because of the relatively low regulatory
benchmarks for activity- (picocuries per liter, pCi/L) and mass-based concentrations
(micrograms per liter, |ig/L) in groundwater. Such radioactive decay relationships may be used
to predict increases in decay products and activity or ingrowth, with or without consideration of
groundwater transport.
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Alpha recoil is a decay-related physical fractionation process that may impede achievement of
groundwater cleanup levels for some radionuclides. Briefly, alpha recoil occurs when ejection of
an alpha particle propels the daughter product away from the decay site (Kigoshi 1971; EPA
2010a, Front Matter, ID. 1.3) — for example, from a mineral surface into groundwater. Ejection
of an alpha particle can destabilize the host solid (Fleischer 1980; EPA 2010a, Front Matter,
ID. 1.3) and increase its susceptibility to dissolution (Eyal and Fleischer 1985). Potential
influence of alpha recoil on contaminant fate is exemplified by the behavior of U-238 and U-234
solid-solution partitioning in groundwater systems, which results in U-234 enriched groundwater
(Ivanovich 1994; EPA 2010a, Front Matter, ID. 1.3). U-238 decay produces Th-234 plus an alpha
particle of sufficient energy to cause ejection of Th-234 into groundwater. Subsequent serial
decay of Th-234 (24.1 day half-life) to Pa-234 (6.7 hour half-life) and ultimately U-234 results in
an elevated activity/concentration of U-234 relative to what would be anticipated based strictly
on the solid-liquid partitioning for uranium or thorium. Additional examples of decay chains that
may produce recoil effects include Th-228, Th-229, and Ra-226 (Sun and Semkow 1998; EPA
2010a, Front Matter, ID. 1.3). In general, the impact of this process is difficult to predict in an
aquifer and may play a minor role in contaminant plumes with concentrations that greatly exceed
natural levels; however, alpha recoil may impart a large contribution to radioactive-enriched
groundwater when concentrations of alpha emitters are large.
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
6.0 USE OF MODELS
Models that rely solely on estimated or computer-derived parameters, rather than on site-
specific measured parameters, are generally inappropriate as the dominant justification for
MNA. Modeling can be used to support or corroborate observed field conditions or can be used
as another line of evidence in support of MNA. However, modeling simulations generally should
not be the sole line of evidence used to support MNA. There should be site-specific monitoring
data that provide evidence of contaminant attenuation, such as stable or declining groundwater
concentrations or evidence of increasing contaminant concentrations in the solid phase (either via
precipitation or sorption to aquifer solids). Site-specific information supporting continued
attenuation of site contaminants should be obtained. Modeling should be supported with actual
site data, and multiple lines of evidence should be used to indicate MNA is appropriate.
As noted previously, a CSM is not the same as a computer model, but a valid and sound CSM
generally should be used to obtain meaningful computer modeling results. In the discussion that
follows, the term "model" or "modeling" refers to a computer model or other mathematical
representation of reality, whereas references to the CSM will be explicit. If a correct and robust
CSM is not derived, any computer modeling results, no matter how detailed or extensive, may
contribute little to understanding the site. EPA guidance generally recommends not relying on
modeling as the sole criterion for determining whether MNA is an appropriate response action.
Any model predictions should be substantiated by performance monitoring.
The modeling effort should begin with the careful identification of processes that can play
significant roles in contaminant migration and attenuation at the site. Fundamental data regarding
the rate and direction of groundwater flow, degree of aquifer heterogeneity, and current
distribution of contamination typically should be included in the CSM. Identification of specific
reaction mechanisms that may be active in the plume (for example, precipitation or sorption to
solid surfaces, complexation with other chemical constituents, or microbially induced changes in
groundwater chemistry) can provide the basis for developing models that allow projection of
contaminant transport into the future.
Planning for computer modeling should occur early in the site assessment process so that the
modeling can be integrated with the evaluation of the site and the appropriate data can be
collected. In all the models, it is always important to characterize the assumptions, boundary
conditions, and uncertainty. The EPA often uses a tiered modeling approach. Generally, more
complex models require more site-specific data. When radionuclides are modeled, a model that
can account for parent-daughter decay chains and the accompanying change in fate and transport
parameters as the radionuclides change needs to be selected.
For further information on modeling at radioactively contaminated sites, see Documenting
Ground Water Modeling at Sites Contaminated with Radioactive Substances [EPA 540-R-96-
003] January, 1996. (www.epa.gov/rpdwebOO/docs/cleanup/540-r-96-003.pdf) and the associated
fact sheet (www.epa.gov/rpdwebOO/docs/cleanup/540-f-96-002.pdf).
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
6.1 Types of Models
Several types of models may prove useful for characterizing attenuation processes at a site.
Initiating the modeling effort with the simplest possible models is recommended. Highly
complex models usually are difficult to work with, expensive to produce and difficult to
interpret. A more efficient strategy normally is to begin with simple models of various aspects of
the system, combining these as necessary into progressively more complex models, until a
satisfactory final result has been reached, one that reproduces the salient aspects of the system's
behavior without introducing unnecessary complexity.
Simple Calculations. Simple calculations performed by hand or via computer applications may
serve as an important component of the overall modeling strategy. These calculations may be
useful in any of the four phases in the tiered analysis approach. An example of a simplified
approach would be calculation of the mass of contaminant and the mass of reactant within a
predefined volume of the aquifer to assess whether sufficient reactant mass is available for an
identified attenuation process. This calculation provides a general sense of the relative degree to
which the aquifer could support attenuation and may provide some perspective as to the relative
importance of investing resources to fully characterize reactant mass or flux. This calculation
does not, however, likely provide any insight into the efficiency of the attenuation process.
Another example of simplified calculations that may be used is input parameters for more
complex transport or reaction models using specific mathematical formulas. Several examples of
these types of calculations are provided at the following the EPA website:
www.epa.gov/athens/learn2model/part-two/onsite/index.html. These calculations may support
analysis of the adequacy of monitoring network design in addition to estimating model input
parameters and hence play an important role in the site characterization effort.
Mass Transport Models. Mass transport models seek to describe the flow of groundwater at a
site and the transport of chemical species by the groundwater. Because mass transport models
typically consider migration of non-reacting species, they seldom can be relied on to accurately
describe natural attenuation. However, they can still be useful for estimating the transit time of
contaminants within the site, absent attenuating processes. This "worst case" transport scenario
has value in evaluating a site's potential for MNA. Mass transport models are best suited for
application in Phase I or Phase II of the tiered analysis approach.
Speciation and Reaction Models. Speciation models seek to describe the distribution of
chemical mass between solution, minerals, mineral surfaces, gases, and biomass. Models of this
class are useful because they can predict the conditions that might attenuate contaminants by
sequestration, and those in which they are likely to be mobile in the groundwater flow. For
example, a speciation model might demonstrate that a contaminant is likely to adsorb to the
surface of a component of the aquifer solids over the pH range of interest. Alternatively, the
model might show that the contaminant will tend to complex strongly with dissolved chemical
species, leaving it mobile and resistant to attenuation. Speciation models assume the modeled
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
system is in partial or complete chemical equilibrium. One example of a speciation model is
MINTEQA2, which can be found at www2.epa.gov/exposure-assessment-models/minteqa2.
Reaction models are similar to speciation models in that they consider the distribution of
chemical mass, but have the additional ability of modeling the chemical evolution of the system
with changing site conditions. Examples of uses for reaction models include estimating
sequestration of contaminants onto a mineral surface as the mineral forms, or estimating
precipitation of contaminant-bearing minerals as water chemistry changes.
Speciation models and reaction models would typically be used in Phase II or Phase III of the
tiered analysis approach. However, they can also be helpful in Phase IV (monitoring) to identify
critical chemical parameters to which the attenuation process is sensitive.
Reactive Transport Models. Reactive transport models, as the name suggests, couple reaction
models to transport models. Unlike a reaction model, a reactive transport model predicts not only
the reactions that occur as the groundwater flows, but how those reactions vary in space and
change through time. A reactive transport model may have several advantages over a simple
reaction model, including the ability to account for heterogeneity at the site, such as an uneven
distribution of a sorbing mineral or variation in pH conditions.
Reactive transport modeling is a relatively complex and time-consuming undertaking, since it
combines the data needs and uncertainties inherent in modeling reaction as well as transport of
contaminants. As such, reactive transport models are typically reserved for use in Phase III
analysis. It may be the capstone of the modeling effort but is seldom the best tool for initial
scoping of the attenuation capabilities at a site. This modeling, on the other hand, may play an
important role in the site characterization effort because it represents the integration of all of the
components of the conceptual site model.
6.2 Model Calibration
Because of the uncertainties discussed above, it generally is important to calibrate a model to
observations and to verify that the model behaves in a manner that adequately describes the
natural system. Calibration is typically designed to bring the model into alignment with observed
data. To have optimal confidence in results, models are recommended that (1) utilize to the
greatest extent possible parameter values specific to the site, and (2) are calibrated to the
observed evolution and distribution of the contaminant plume. It is further recommended that
steps taken to calibrate the model application be documented and provided for review to build
confidence in the use of the model as an assessment tool.
More direct lines of evidence should be included in the recommended tiered analysis process
because of the complexity of modeling efforts and the potential level of uncertainty associated
with model predictions. The acquisition of these data often depends on establishing a network of
monitoring locations throughout the aquifer. The site-specific data collected from these
monitoring locations should provide a reliable way to identify the attenuation process and assess
OSWER Directive 9283.1-36 48
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
the performance characteristics of MNA. As with any technology used as part of a cleanup,
continued assessment of performance is normally important for ensuring cleanup goals will be
attained.
6.3 Interpreting Model Results
It is generally not possible to account for all variability in a modeling study because of the
heterogeneity of most geologic systems. Modeling results should therefore be interpreted in
realistic rather than absolute terms. Modeling is often most helpful for identifying relative
changes in contaminant speciation and distribution in response to geochemical changes in the
system. Models can provide an indication of whether a particular reaction or system response is
expected under specific conditions. When the potential for MNA of inorganic contaminants is
evaluated, modeling should be validated with observational study to confirm whether the
expected reactions occur. Discrepancies between modeled and actual conditions can lead to new
insights into the geochemical system and may result in changes to the CSM.
Discrepancies between modeled and actual conditions can result from uncertainty introduced at
several points in the modeling process. Geochemical modeling applications generally require
complete chemical analyses, including not only the contaminants of interest, but the major ion
chemistry, pH, and distribution of metals among their mobile redox states. Errors in chemical
analysis may therefore alter model results. Similarly, errors in measuring hydrologic parameters
may result in differences between measured and modeled distributions of chemical species.
Errors or omissions in sampling also affect model results. Sample choice and dataset size can
introduce error through sampling bias. Fluid samples may be collected from monitoring wells
completed in highly conductive layers, where they can be extracted rapidly, leaving unaccounted
significant quantities of residual contamination in slightly less conductive layers. In addition,
samples may not be collected from upgradient or downgradient stations located outside the
immediate plume, which precludes an accurate evaluation of the groundwater chemistry of
unaffected portions of the aquifer. Uncontaminated groundwater migrating onto a site can induce
changes in groundwater chemistry that may affect the stability of attenuated compounds.
Similarly, as contaminated groundwater mixes with uncontaminated groun dwater
downgradient of a site, changes in groundwater chemistry may occur, with impacts to the
stability of the attenuated compound. Collecting samples upgradient and downgradient of the site
is recommended to accurately evaluate site-induced chemistry changes.
Geochemical models rely on thermodynamic databases that contain data on aqueous species and
potential reactions between them. These databases, and the thermodynamic data contained within
them, vary widely in breadth and accuracy. Modeling results will vary depending on the
thermodynamic database used in the model.
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
For best results, data used as input in a model must be accurate and representative of site
conditions. Inaccurate information will lead to skewed results. Where differences in modeled and
measured site conditions are observed, changes in the CSM for a site may be needed or new
information obtained.
6.4 Site-Specific Data
Site-specific data should be collected to define the physical, chemical, and biological
characteristics of the aquifer to derive meaningful modeling results and test the validity of model
predictions. It is important to calibrate models to observations and to verify that the model
adequately describes the natural system. Steps taken to calibrate any models used to support
selection of MNA should be documented and available for review to increase confidence in the
use of the model.
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
7.0 SUMMARY
The EPA remains fully committed to its goals of protecting human health and the
environment by remediating contaminated soils, restoring contaminated groundwaters to
their beneficial uses, preventing migration of contaminant plumes, and protecting
groundwater and other environmental resources. The EPA does not view MNA as a "no
action" remedy, but rather considers it a potential means of addressing contamination under a
limited set of site circumstances where its use is consistent with CERCLA and the NCP. In
general, MNA should not be considered as a "presumptive" or "default" remediation alternative,
but rather should be evaluated and compared with other viable remediation methods (including
innovative technologies) during the assessment phases leading to the selection of a remedy. The
evaluation of MNA should include a comprehensive site characterization, risk assessment where
appropriate and measures to control sources. In addition, the progress of MNA toward a site's
cleanup levels should be carefully monitored and compared with expectations to ensure that it
will meet RAOs within a timeframe that is reasonable compared with timeframes associated with
other methods. Where MNA's ability to meet these expectations is uncertain and based primarily
on lines of evidence other than documented trends of decreasing contaminant concentrations,
decision-makers should incorporate contingency measures into the remedy.
In summary, there are several key issues and ideas to note regarding MNA for inorganic
contaminants:
• Because MNA does not use any active remedial measures, MNA does not constitute a
treatment process for inorganic contaminants.
• Dilution and dispersion generally are not appropriate as primary MNA mechanisms
because they accomplish concentration reduction through dispersal of contaminant mass
rather than mass destruction or immobilization.
• MNA is generally not appropriate for plumes that are considered stable, yet there is
confirmed discharge to surface water bodies or potential human or ecological receptor
exposure.
• MNA should be supported by actual site data and information in the administrative
record demonstrating a decreasing trend of the contaminant concentration.
• Attenuation rates and mass flux estimates can be used as supporting lines of evidence but
should not be used as the primary supporting evidence that attenuation is occurring.
• Reliance on models without monitoring data to demonstrate continued attenuation would
generally be inconsistent with this guidance.
The EPA is confident that MNA can be, at many sites, a reasonable and protective component of
a broader remediation strategy. However, the EPA also believes that there may be many other
sites where either the uncertainties are too great or there is a need for a more rapid remediation
that precludes the use of MNA as a stand-alone remedy because it would not ensure
protectiveness of human health and the environment. This guidance is intended to help promote
consistency in how MNA remedies for inorganic contaminants are, evaluated, and if appropriate,
proposed and selected as remedial actions.
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8.0 REFERENCES
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S. Marutsky, D. R. Metzler, A. Peacock, D. C. White, M. Low, and D. R. Lovely. 2003.
Stimulating the in situ activity of geobacter species to remove uranium from the
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Dixon, J.B. andD.G. Schulze (Eds.). 2002. Soil Mineralogy with Environmental Applications.
Soil Science Society of America, Madison, Wisconsin.
Eyal, Y. and R.L. Fleischer. 1985. Preferential leaching and the age of radiation damage from
alpha decay in minerals. Geochimica et Cosmochimica Acta 49:1155-1164.
Farhat, S.K., C.J. Newell, and E.M. Nichols. 2006. User's Guide: Mass Flux Tool Kit.
Environmental Security Technology Certification Program, Washington, DC.
Fleischer, R.L. 1980. Isotopic disequilibrium of uranium: alpha-recoil damage and preferential
solution effects. Science 207:979-981.
Fujita, Y., G. D. Redden, J. C. Ingram, M. M. Cortez, F. G. Ferris, and R. W. Smith. 2004.
Strontium incorporation into calcite generated by bacterial ureolysis. Geochimica et
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Huck, P. M., and W. B. Anderson. 1990. "Removal of 226Ra from Uranium Mining Effluents and
Leaching from Sludges." In The Environmental Behavior of Radium, Volume 2, pp. 135-
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Radionuclides. (www.itrcweb.org/Guidance/GetDocument?documentID=5)
Istok, J. D., J. M. Senko, L. R. Krumholz, D. Watson, M. A. Bogle, A. Peacock, Y. J. Chang, and
D. C. White. 2004. In situ bio-reduction of technetium and uranium in a nitrate-
contaminated aquifer. Environmental Science and Technology 38:468-475.
Ivanovich, M. 1994. Uranium series disequilibrium - concepts and applications. Radiochimica
Acta 64:81-94.
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Langmuir, D. and A.C. Riese. 1985. The thermodynamic properties of radium. Geochimica et
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Michalsen, M. M., B. A. Goodman, S. D. Kelly, K. M. Kemner, J. P. McKinley, J. W. Stucki,
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physical models of an in situ bio-barrier. Environmental Science and Technology 40:7048-
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Pardue, J. H., and T. Z. Guo. 1998. Biochemistry of 226Ra in contaminated bottom sediments and
oilfield waste pits. Journal of Environmental Radioactivity 39:239-253.
Roden, E. E., M. R. Leonardo, and F. G. Ferris. 2002. Immobilization of strontium during iron
biomineralization coupled to dissimilatory hydrous ferrous oxide reduction. Geochimica
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Shanbhag P. M. and J. W. Morse. 1982. Americium interaction with calcite and aragonite
surfaces in seawater. Geochimica et Cosmochimica Acta 46:241-246.
Senko, J. M., J. D. Istok, J. M. Suflita, and L. R. Krumholz. 2002. In-situ evidence for uranium
immobilization and remobilization. Environmental Science and Technology 36:1491-
1496.
Smith, C. and N. Smith. 1971. General Chemistry Workbook, Nuclear Chemistry, Chapter 24 pp
308-309. W.H. Freeman and Co.
Sparks, D. L. 2003. Environmental Soil Chemistry. Academic Press, San Diego, California.
Sposito, G. 1984. The Surface Chemistry of Soils. Oxford University Press, New York, New
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Sposito, G. 1986. Distinguishing adsorption from surface precipitation. In Geochemical
Processes at Mineral Surf aces. J.A. Davis and K.F. Hayes (Eds.), American Chemical
Society Symposium Series 323:217-228.
Sun, H. and T.M. Semkow. 1998. Mobilization of thorium, radium and radon radionuclides in
groundwater by successive alpha-recoils. Journal of Hydrology 205:126-136.
Tessier, A., P. G. C. Campbell, and M. Bisson. 1979. Sequential Extraction Procedure for the
Speciation of Particulate Trace Metals. Analytical Chemistry 51:844-851.
U. S. Army Corps of Engineers (USAGE). 1998. Technical Project Planning (TPP) Process.
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
USAGE. 2003. Conceptual Site Models for Ordnance and Explosives (OE) and Hazardous,
Toxic, and Radioactive Waste (HTRW) Projects. Engineer Manual 1110-1-1200, U.S.
Army Corps of Engineers, Washington, DC. (www.epa.gov/tio/download/char/eml 110-1-
1200.pdf)
U.S. Environmental Protection Agency (EPA). 1993. Environmental Characteristics of EPA
NRC [Nuclear Regulatory Commission], and DOE [U.S. Department of Energy] Sites
Contaminated with Radioactive Substances. EPA 402-R-93-011, Office of Radiation and
Indoor Air, Washington, DC. (www.epa.gov/radiation/docs/cleanup/402-r-93-011 .pdf)
EPA. 1998. Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in
Ground Water. EPA 600-R-98-128, Office of Research and Development, Washington,
DC. (www.epa.gov/tio/download/remed/protocol.pdf)
EPA. 1999a. Understanding Variation in Partition Coefficient, Kd, Values: Volume I—Kd Model,
Measurement Methods, and Application of Chemical Reaction Codes. EPA 402-R-99-
004A, Office of Radiation and Indoor Air, Washington, DC.
(www.epa.gov/radiation/docs/kdreport/voll/402-r-99-004a.pdf)
EPA. 1999b. Understanding Variation in Partition Coefficient, Kd, Values: Volume II—
Geochemistry and Available Kd Values for Selected Inorganic Contaminants. EPA 402-R-
99-004B, Office of Radiation and Indoor Air, Washington, DC.
(www.epa.gov/radiation/docs/kdreport/vol2/402-r-99-004b.pdf)
EPA. 1999c. Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and
Underground Storage Tank Sites. OSWER Directive 9200.4-17P, Office of Solid Waste
and Emergency Response, Washington DC.
(www.epa.gov/swerustl/directiv/d9200417.pdf)
EPA. 1999d. A Guide to Preparing Superfund Proposed Plans, Records of Decision, and Other
Remedy Selection Decision Documents. OSWER Directive 9200.1-23P, Office of Solid
Waste and Emergency Response, Washington, DC.
(www.epa.gov/superfund/policv/remedy/rods/pdfs/guide decision documents 071999.pdf)
EPA. 2000. Soil Screening Guidance for Radionuclides: Technical Background Document.
OSWER Directive 9355.4-16, Office of Radiation and Indoor Air, Office of Solid Waste
and Emergency Response, Washington, DC.
(www.epa.gov/superfund/health/contaminants/radiation/pdfs/sstbd.pdf)
EPA. 2001. Evaluation of the Protocol for Natural Attenuation of Chlorinated Solvents: Case
Study at the Twin Cities Army Ammunition Plant. EPA 600-R-01-025, Office of Research
and Development, Washington, DC. (nepis.epa.gov/Adobe/PDF/P1000F25.pdf)
OSWER Directive 9283.1-36 54
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
EPA. 2002a. Workshop on Monitoring Oxidation-Reduction Processes for Ground-water
Restoration. EPA 600-R-02-002, Office of Research and Development, Cincinnati, Ohio.
(nepis.epa.gov/Adobe/PDF/10003Z26.pdf)
EPA. 2002b. Guidance for Quality Assurance Project Plans. EPA QA/G-5, EPA 240-R-02-009,
Office of Environmental Information, Washington, DC. (www.epa.gov/OUALITY/qs-
docs/g5-fmal.pdf)
EPA. 2002c. Calculation and Use of First-Order Rate Constants for Monitored Natural
Attenuation Studies. EPA 540-S-02-500, National Risk Management Research
Laboratory, Cincinnati, Ohio).
EPA. 2004a. Multi-Agency Radiological Laboratory Analytical Protocols Volume II.
EPA. 2004b. Performance Monitoring ofMNA Remedies for VOCs in Ground Water. EPA 600-
R-04-027, Office of Research and Development, Washington, DC. (clu-in.org/download/
contaminantfocus/dnapl/Treatment Technologies/performance monitoring mns600R040
27.pdf)
EPA. 2004c. Understanding Variation in Partition Coefficient, Kd, Values: Volume III—Review
oj"Geochemistry andAvailable Kd Values for Americium, Arsenic, Curium, Iodine,
Neptunium, Radium, and Technetium. EPA 402-R-04-002C, Office of Radiation and
Indoor Air, Washington, DC. (www.epa.gov/radiation/docs/kdreport/vol3/402-r-04-
002c.pdf)
EPA. 2006a. Guidance on Systematic Planning Using the Data Quality Objectives Process. EPA
QA/G-4, EPA 240-B-06-001, Office of Environmental Information, Washington, DC.
(www.epa.gov/quality/qs-docs/g4-fmal.pdf)
EPA. 2006b. Miner alogical Preservation of Solid Samples Collected from Anoxic Subsurface
Environments. EPA 600-R-06-112, Office of Research and Development, Cincinnati,
Ohio. (nepis.epa.gov/Adobe/PDF/600003Il.pdf)
EPA. 2006c. Inventory of Radiological Methodologies. EPA 402-R-06-007, Office of Radiation
and Indoor Air, Montgomery, Alabama. (www.epa.gov/narel/IRM Final.pdf)
EPA. 2007a. Monitored Natural Attenuation of Inorganic Contaminants in Ground Water,
Volume I—Technical Basis for Assessment. EPA 600-R-07-139, Office of Research and
Development, Washington, DC. (nepis.epa.gov/Adobe/PDF/60000N4K.pdf)
EPA. 2007b. Monitored Natural Attenuation of Inorganic Contaminants in Ground Water,
Volume II Assessment for Non-Radionuclides Including Arsenic, Cadmium, Chromium,
Copper, Lead, Nickel, Nitrate, Perchlorate, and Selenium. EPA 600-R-07-140, Office of
Research and Development, Washington, DC. (nepis.epa.gov/Adobe/PDF/60000N76.pdf)
OSWER Directive 9283.1-36 55
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
EPA. 2008. Ground Water Issue: Site Characterization to Support Use of Monitored Natural
Attenuation for Remediation of Inorganic Contaminants in Ground Water. EPA 600-R-
08-114, National Risk Management Research Laboratory, Cincinnati, Ohio.
(nepis.epa.gov/Adobe/PDF/P1002X3C.pdf)
EPA. 2009. Standard Analytical Methods for Environmental Restoration following Homeland
Security Events Volume 5.0.
(nepis.epa.gov/EPA/html/DLwait.htm?url=/Exe/ZyPDF.cgi?Dockev=P1005B4P.PDF)
EPA. 2010a. Monitored Natural Attenuation of Inorganic Contaminants in Ground Water,
Volume III—Assessment for Radionuclides Including Americium, Cesium, Iodine,
Plutonium, Radium, Radon, Strontium, Technetium, Thorium, Tritium, and Uranium. EPA
600-R-10-093, Office of Research and Development, Washington, DC.
EPA. 201 Ob. Revised Guidance on Compiling Administrative Records for CERCLA Response
Actions. Office of Solid Waste and Emergency Response, Washington, DC. (www.clu-
in.org/conf/tio/CECOSC4 121813/AR-Guidance-dated-9.20.2010.pdf)
EPA. 2012. Institutional Controls: A Guide to Planning, Implementing, Maintaining, and
Enforcing Institutional Controls at Contaminated Sites. OSWER 9355.0-89, EPA 540-R-
09-001. Office of Solid Waste and Emergency Response, Washington, DC. December
2012.
(www.epa.gov/superfund/policy/ic/guide/Final%20PIME%20Guidance%20December%2
02012.pdf)
OSWER Directive 9283.1-36 56
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APPENDIX A
RECOMMENDED GROUNDWATER ANALYSES
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Appendix A. Recommended Groundwater Analyses (EPA 2007a and 2007b)
Analysis
Associated
Phase
Data Use
Recommended
Frequency of
Analysis1
Comments
Water level
data
I, II, III, IV
Can be used to determine
groundwater flow directions and
rate of flow.
Each sampling
round
Water levels from a sufficient number of wells on a
project should be collected to determine groundwater
flow direction. Measurements should be taken over a
time period to minimize impacts from changing
hydrogeologic conditions (for example, tidal
influences, pumping drawdown).
Total Metals
I, II, III, IV
Can be used in defining plume
boundary, confirming static or
shrinking plume conditions,
modeling geochemical speciation/
environmental fate and transport,
and determining overall
appropriateness of MNA for
meeting remediation objectives
within the specified regulatory
time frame. Data is ultimately
useful to MNA performance/
compliance monitoring program.
Each sampling
round
Exposure of samples to air should be prevented and
samples should be preserved to prevent sample
oxidation and potential metal co-precipitation with
iron oxides. For further discussion of total and
dissolved metal sampling, refer to Monitored
Natural Attenuation of Inorganic Contaminants
in Ground Water, Volume I (EPA 2007a).
Notes:
1. Recommended frequency of analysis may need to be adjusted to meet site-specific data needs.
OSWER Directive 9283.1-36
A-l
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Analysis
Associated
Phase
Data Use
Recommended
Frequency of
Analysis1
Comments
Radionuclides
I, II, III, IV
Same as total metals in water. In
addition, some analytical methods
for radionuclides measure total
mass while others measure
specific isotopes or activity,
which may be important for
evaluating ingrowth contributions
to plume activity and mobility.
Each sampling
round
Same as total metals in water. Keep anaerobic
samples anaerobic to prevent potential co-
precipitation of target radionuclides with iron or
other oxides. Measurement of activity is more
appropriate for short half-life radionuclides where
decay processes dominate, while measurement of
total mass is more appropriate for long half-life
radionuclides where attenuation is dominated by
physicochemical processes such as sorption or
precipitation.
pH
I, II, III, IV
Can be used in modeling
geochemical speciation/
environmental fate and transport.
Important in evaluating sorption
capacity of soil.
Each sampling
round
Improperly calibrated electrodes could impair results;
should measure immediately or in a flow-through
cell to prevent equilibration of groundwater with
atmosphere.
Major Anions
II, III, IV
Includes Br, Cl, F, NO3 / NO2, o-
PC>4 and SC>4. Used in modeling
geochemical speciation and
environmental fate and transport.
Trends in nitrate/nitrite and
sulfate/ sulfide concentrations
also indicate redox conditions.
Each sampling
round
Relative amounts of NO3- / NO2- and SO42- / S2'
provide additional lines of evidence of oxidizing/
reducing conditions in the aquifer and provide data
on changing aquifer conditions. Note: Br, F and o-
PC>4 are typically present at trace concentrations and
are not considered "major anions."
Notes:
1. Recommended frequency of analysis may need to be adjusted to meet site-specific data needs.
OSWER Directive 9283.1-36
A-2
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Analysis
Major Cations
Ferrous iron
and Sulfide
Temperature
Oxidation-
Reduction
Potential
(ORP)
Dissolved
Oxygen (DO)
Associated
Phase
II, III, IV
II, III, IV
I, II, III, IV
I, II, III, IV
I, II, III, IV
Data Use
Includes Na, K, Ca, Mg and Fe.
Used in modeling geochemical
speciation/ environmental fate
and transport. Ca and Mg in
solution may compete with other
metals for sorption sites and
thereby reduce sorptive capacity
of soils.
Can be used to determine whether
a system is reducing. Also used to
determine the potential for
contaminant precipitation or co-
precipitation and for geochemical
modeling.
Standard parameter; may be used
to evaluate reaction kinetics.
Indicates whether oxidizing or
reducing conditions prevail in site
groundwater. Some redox-
sensitive metals and radionuclides
may form sparingly soluble
complexes or precipitates under
reducing conditions
Use with ORP to determine if
system is oxic or anoxic.
Recommended
Frequency of
Analysis1
Each sampling
round
Initial sampling
round, then semi-
annually
Each sampling
round
Each sampling
round
Each sampling
round
Comments
Relative amounts of Fe(II)/Fe(III) provide additional
lines of evidence of oxidizing/reducing conditions in
the aquifer and provide data on changing aquifer
conditions.
Field measurement. Can determine if ferric iron is
present by subtracting ferrous iron concentration
from total iron concentration.
Groundwater system redox affects mineral solubility
and relative concentrations of redox-sensitive
elements in groundwater.
Measure in a flow-through cell to prevent
equilibration of groundwater with atmosphere.
Notes:
1. Recommended frequency of analysis may need to be adjusted to meet site-specific data needs.
OSWER Directive 9283.1-36
A-3
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Analysis
Associated
Phase
Data Use
Recommended
Frequency of
Analysis1
Comments
Dissolved
Organic
Carbon (DOC)
II, III, IV
Elevated DOC may promote
metal complexation with humic
acids and affect sorption to soil;
DOC may also promote reducing
aquifer conditions that could
impact speciation of redox-
sensitive metals.
Initial sampling
round, then
annually
Can be used initially to help understand the
attenuation process at work and aquifer capacity for
contaminant reduction. Used during long-term
monitoring to signify changes in water chemistry that
may impact attenuation sustainability.
Total Inorganic
Carbon (TIC)
II, IV
Can be used with pH data to
determine carbonate speciation
and buffering capacity of
groundwater. Some metals may
form sparingly soluble complexes
with carbonates (for example,
lead) while others become more
mobile when complexed with
carbonate (for example, uranium).
Initial sampling
round, then semi-
annually
Total inorganic carbon is similar to alkalinity, but
includes additional species (for example, CO2 or
IrbCOs) not typically accounted for in alkalinity
measurement. Alkalinity may be an acceptable
substitute for TIC. Used during long-term monitoring
to signify changes in water chemistry that may
impact attenuation.
Conductivity
I, II, III, IV
Conductivity can provide
indications of ionic strength and
total dissolved solids.
Each sampling
round
Notes:
1. Recommended frequency of analysis may need to be adjusted to meet site-specific data needs.
OSWER Directive 9283.1-36
A-4
-------
APPENDIX B
RECOMMENDED SOIL ANALYSES
-------
Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Appendix B. Recommended Soil Analyses (EPA [2007a, 2007b])
Analysis
Associated
Phase
Data Use
Recommended
Frequency of
Analysis1
Comments
Hydraulic
conductivity
I
Can be used with groundwater
level data to determine
groundwater flow velocity.
Initial sampling
round
Component in the development of the conceptual
site model and sampling frequency.
Total Metals
II, III, IV
Can be used in determining the
extent of soil contamination,
contaminant mass present, and
potential for source
attenuation.
Initial site
characterization
and MNA
evaluation
Frequency of soil sampling and analysis during
MNA program should be established as part of the
performance and compliance monitoring program.
Radionuclides
II, III, IV
Same as total metals in soils.
In addition, some analytical
methods for radionuclides
measure total mass while
others measure specific
isotopes and/or activity. Mass
measurements are collected for
determining soil uptake
capacity.
Initial site
characterization
and MNA
evaluation
Analytical data should be adjusted to account for
decay losses during the interval between sample
collection and analysis for radionuclides with short
half-lives. Measurement of activity for short half-
life radionuclides or total mass for long half-life
radionuclides is same as for groundwater.
Mineralogy
II, III
Can be used to evaluate
attenuation capacity of aquifer;
identification of site-specific
metal and mineral associations
may be used to evaluate long-
term metal retention capacity
of soil.
Initial site
characterization
and MNA
evaluation
Both mineralogical composition and contaminant
distribution in soil may be highly heterogeneous at
the field scale. Data can also be used to support
geochemical modeling of the site.
Notes:
1. Recommended frequency of analysis may need to be adjusted to meet site-specific data needs.
OSWER Directive 9283.1-36
B-l
-------
Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Analysis
Associated
Phase
Data Use
Recommended
Frequency of
Analysis1
Comments
Cation Exchange
Capacity (CEC)
II, III, IV
Provides relative indication of
sorptive capacity of soil under
specific solution conditions.
Initial site
characterization
and MNA
evaluation
Dependent on pH and soil organic matter.
Total Organic
Carbon (TOC)
II
Can be used along with cation
exchange capacity to obtain
relative indication of soil
sorptive capacity.
Initial site
characterization
and MNA
evaluation
Can be used with the organic carbon soil-water
partition coefficient, Koc, to determine contaminant
partitioning coefficient.
Acid Volatile
Sulfides (AVS)/
Simultaneously
Extracted Metals
(SEM)
II, III, IV
Can be used to assess metal
and metalloid partitioning to
the sulfide fraction of soils and
sediments, and to estimate
potential metal toxicity in
aquatic sediments.
Initial site
characterization
and MNA
evaluation
Outside the source area samples are generally taken
from the saturated zone to determine potential
reaction between dissolved contaminants and
mineral solids.
Sequential
Extraction
Procedures (SEP)
II, III, IV
Can be used to obtain a
relative assessment of
contaminant environmental
availability. Site samples are
subjected to successively
harsher extracting solutions;
contaminants released earlier
are presumed to be relatively
more available.
Initial MNA
evaluation;
thereafter at
intervals based on
groundwater flow
velocity
Can provide quantitative information on the
capacity of a material to attenuate inorganic
contaminants. Measurements done overtime can
provide data on changes in available contaminant
mass sorbed to or precipitated in the soil matrix.
Assumes components released in earlier extractions
are more environmentally mobile.
Notes:
1. Recommended frequency of analysis may need to be adjusted to meet site-specific data needs.
OSWER Directive 9283.1-36
B-2
-------
APPENDIX C
RECOMMENDED ANALYTICAL METHODS AND DATA QUALITY
REQUIREMENTS FOR GROUNDWATER ANALYSES
-------
Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Appendix C. Recommended Analytical Methods and Data Quality Requirements for Groundwater Analyses (EPA [2007a, 2007b])
Analysis
Method Reference and/or
Description (most recent
revision or date)
Sample Volume,
Sample Container,
Sample Preservation
Minimum
Required Limit
of Quantitation
or Resolution
Comments
Total Metals/
Inorganic
SW 60IOC (Revision 3, February
2007)/6020A (Revision 1,
February 2007): As, Cr, Cu, Pb,
Ni, Se
EPA 200.8 (Revision 5.4)/SW
6020A (Revision 1, February
2007): Cd
SW 6850 (Revision 0, January
2007/6860 (Revision 0, January
2007): CIO4
EPA 2007a
250 mL, polycarbonate
bottle, nitric acid (pH<2),
cool to 4° C
If aquifer is anaerobic, exposure of
samples to air should be prevented or
samples should be preserved to prevent
sample oxidation and potential co-
precipitation with iron oxides.
Radionuclides
Standard Analytical Methods for
Environmental Restoration
Following Homeland Security
Events v. 5.0 (EPA 2009);
Inventory of Radiological
Methodologies (EPA 2006c);
Multi-Agency Radiological
Laboratory Analytical Protocols
Vol. II (EPA 2004a)
250 mL, polycarbonate
bottle, cool to 4°C; also
see method-specific
recommendations
Analytical data should be adjusted to
account for decay losses during the
interval between sample collection and
analysis for radionuclides with short
half-lives. Use of either activity- or
mass-based analytical methods may be
appropriate depending on data needs.
Notes:
1.
2.
3.
Most recent version of listed method is assumed.
"SW" refers to the Test Methods for Evaluating Solid Waste, Physical, and Chemical Methods, SW-846, EPA, most recent edition.
"SM" refers to Standard Methods for the Examination of Water and Wastewater, most recent edition.
4. "EPA" refers to Methods for Chemical Analysis of Water and Wastes, EPA, most recent edition.
OSWER Directive 9283.1-36
C-l
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Analysis
Method Reference and/or
Description (most recent
revision or date)
Sample Volume,
Sample Container,
Sample Preservation
Minimum
Required Limit
of Quantitation
or Resolution
Comments
EPA 150.2 (Revision 0,
December 1982) or field probe
with direct reading meter
Measure in field
following procedures in
the EPA 150.2 or per
manufacturer instructions
0.2 units
Improperly calibrated electrodes could
impair results; should measure
immediately to prevent equilibration of
groundwater with atmosphere.
Major Anions
EPA300.1/SW9056A
(Revision 1, February 2007)
250 mL, Polycarbonate
bottle, cool to 4°C
100
Major Cations
SW 6010CC (Revision 3,
February 2007)/7000B (Revision
2, February 2007)
250 mL, polycarbonate
bottle, Nitric acid
(pH<2), cool to 4°C
100
Ferrous Iron,
Sulfide
EPA 2002a
25 mL, glass bottle,
unpreserved
Img/L
Measured in field using colorimetric
method. Analysis should be done
quickly as the analytes are sensitive to
exposure to atmospheric oxygen.
Temperature
Field temperature probe with
direct reading meter
Measure in the field
using a flow-through cell
or an overflow cell filled
from the bottom
0.5°C
Oxidation-
Reduction
Potential
(ORP)
SM2580B(1997)
EPA 2002a
Measure in the field
using a flow-through cell
or an overflow cell filled
from the bottom
+/-100mV
Field measurement should be in situ or
in flow-through cell to prevent
introduction of atmospheric oxygen.
Improperly calibrated electrodes or
introduction of atmospheric oxygen
during sampling could impair results.
Notes:
1.
2.
3.
Most recent version of listed method is assumed.
"SW" refers to the Test Methods for Evaluating Solid Waste, Physical, and Chemical Methods, SW-846, EPA, most recent edition.
"SM" refers to Standard Methods for the Examination of Water and Wastewater, most recent edition.
4. "EPA" refers to Methods for Chemical Analysis of Water and Wastes, EPA, most recent edition.
OSWER Directive 9283.1-36
C-2
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Analysis
Method Reference and/or
Description (most recent
revision or date)
Sample Volume,
Sample Container,
Sample Preservation
Minimum
Required Limit
of Quantitation
or Resolution
Comments
Dissolved
Oxygen (DO)
Dissolved oxygen meter
calibrated between each well
according to the supplier's
specifications
Measure in the field
using a flow-through cell
or an overflow cell filled
from the bottom
0.2 mg/L
Field measurement should be in situ or
in flow-through cell to prevent
introduction of atmospheric oxygen.
Dissolved
Organic
Carbon (DOC)
EPA 415.3 (Revision 1.0, June
2003)
250 mL, polycarbonate
bottle, 0.45 jam field
filtered and H2SO4
(pH<2), cool to 4°C
Total Inorganic
Carbon (TIC)
SM 2320 (1997)
250 mL, zero headspace
glass bottle, cool to 4° C
Quickly collect sample to prevent
equilibration of groundwater with
atmosphere; Hach alkalinity test kit
model AL AP MG-L could be
considered as an alternative method.
Conductivity
EPA 120.1 (1982), SW9050A
(Revision 1, December 1996), or
direct reading meter
250 mL plastic or glass
bottle, cool to 4°C; or
measure in the field
50 (iS/cm2
Notes:
1.
2.
3.
4.
Most recent version of listed method is assumed.
"SW" refers to the Test Methods for Evaluating Solid Waste, Physical, and Chemical Methods, SW-846, EPA, most recent edition.
"SM" refers to Standard Methods for the Examination of Water and Wastewater, most recent edition.
"EPA" refers to Methods for Chemical Analysis of Water and Wastes, EPA, most recent edition.
OSWER Directive 9283.1-36
C-3
-------
APPENDIX D
RECOMMENDED ANALYTICAL METHODS AND DATA QUALITY
REQUIREMENTS FOR SOIL ANALYSES
-------
Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Appendix D. Recommended Analytical Methods and Data Quality Requirements for Soil Analyses (EPA [2007a, 2007b])
Analysis
Method Reference and/or
Description (most recent
revision or date)
Sample Volume,
Sample Container,
Sample
Preservation
Minimum
Required Limit
of Quantitation
or Resolution
Comments
Total Metals
SW 60IOC (Revision 3,
February 2007)/7000B
(Revision 2, February 2007)
To be updated as necessary
8 oz. wide-mouth
glass jar, cool to 4 C
0.01 mg/kg
Sample prep methods to be chosen from
SW 3050, 3051, or 3052; or from U.S.
Geological Survey (USGS) Open File
Report 02-223-1. Heterogeneity of metals
concentrations in soils can be significant
at the field scale. Should include reporting
of Fe, Mn, Na, K, Ca, Mg and Al at a
minimum.
Radionuclides
Standard Analytical Methods
for Environmental Restoration
Following Homeland Security
Events v. 5.0 (EPA 2009);
Inventory of Radiological
Methodologies (EPA 2006c);
Multi-Agency Radiological
Laboratory Analytical
Protocols Vol. II (EPA 2004a)
8 oz. wide-mouth
glass jar, cool to 4°C;
also see method-
specific
recommendations
It is critical that analytical data be
adjusted to account for decay losses
during the interval between sample
collection and analysis for radionuclides
with short half-lives. Use of either
activity- or mass-based analytical
methods may be appropriate depending
on data needs.
Mineralogy
X-ray fluorescence and X-ray
diffraction
8 oz. wide-mouth
glass jar, cool to 4 C
N/A
Petrographic analysis may also be
appropriate.
Cation Exchange
Capacity (CEC)
SW 9081 (Revision 0,
September 1986)
8 oz. wide-mouth
glass jar, cool to 4 C
Perform under various pH conditions to
determine pH dependence of exchange
capacity.
Notes:
1. Most recent version of listed method is assumed.
2. "SW" refers to the Test Methods for Evaluating Solid Waste, Physical, and Chemical Methods, SW-846, EPA, most recent edition.
OSWER Directive 9283.1-36
D-l
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Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites
Analysis
Total Organic
Carbon (TOC)
Acid Volatile
Sulfide (AVS)/
Simultaneously
Extracted Metals
(SEM)
Sequential
Extraction
Procedures (SEP)
Method Reference and/or
Description (most recent
revision or date)
Modified SW 9060A
(Revision 1, November 2004)
EPA 82 l-R-9 1-100
Tessier and others. (1979)
Sample Volume,
Sample Container,
Sample
Preservation
8 oz. wide -mouth
glass jar, cool to 4 C
8 oz. wide -mouth
glass jar, cool to 4 C
8 oz. wide -mouth
glass jar, cool to 4 C
Minimum
Required Limit
of Quantitation
or Resolution
Comments
Method not standard between labs. Can
also consider organic matter loss on
ignition with subsequent calculation of
TOC.
Notes:
1. Most recent version of listed method is assumed.
2. "SW" refers to the Test Methods for Evaluating Solid Waste, Physical, and Chemical Methods, SW-846, EPA, most recent edition.
OSWER Directive 9283.1-36
D-2
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