ASTM E1943-98(2015)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E1943 − 98 (Reapproved 2015)
Standard Guide for
Remediation of Ground Water by Natural Attenuation at
Petroleum Release Sites
This standard is issued under the fixed designation E1943; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope plume geometry and observed reductions in concentrations of
the constituents of concern at the site.
1.1 This is a guide for determining the appropriateness of
1.3.2 Secondary lines of evidence for remediation by natu-
remediation by natural attenuation and implementing remedia-
ral attenuation are provided by geochemical indicators of
tion by natural attenuation at a given petroleum release site,
naturally occurring degradation and estimates of attenuation
either as a stand alone remedial action or in combination with
rates.
other remedial actions.
1.3.3 Additional optional lines of evidence can be provided
1.2 Naturalattenuationisapotentialremediationalternative
bymicrobiologicalinformationandfurtheranalysisofprimary
forcontainmentandreductionofthemassandconcentrationof
and secondary lines of evidence such as through solute
petroleum hydrocarbons in the environment to protect human
transport modeling or estimates of assimilative capacity.
health and the environment. Remediation by natural attenua-
tion depends upon natural processes such as biodegradation,
1.4 The emphasis in this guide is on the use of remediation
dispersion, dilution, volatilization, hydrolysis, and sorption to
by natural attenuation for petroleum hydrocarbon constituents
attenuate petroleum constituents of concern to achieve reme-
wheregroundwaterisimpacted.Thoughsoilandgroundwater
dial goals.
impacts are often linked, this guide does not address natural
attenuation in soils separate from ground water or in situations
NOTE 1—Remedial goals must be established through another process
wheresoilscontainingconstituentsofconcernexistwithoutan
as determined by the appropriate regulatory agency.
associated ground water impact. Even if natural attenuation is
1.3 In general, remediation by natural attenuation should
selected as the remedial action for ground water, additional
not be considered a presumptive remedy. A determination of
remedial action may be necessary to address other completed
whether remediation by natural attenuation is appropriate for
exposure pathways at the site.
an individual petroleum release site, relative to site-specific
remedial goals, requires site characterization, assessment of 1.5 Thisguidedoesnotaddressenhancedbioremediationor
potential risks, evaluation of the need for source area control, enhanced attenuation.
and evaluation of potential effectiveness similar to other
1.6 Also, while much of what is discussed is relevant to
remedial action technologies.Application and implementation
other organic chemicals or constituents of concern, these
ofremediationbynaturalattenuationrequiresdemonstrationof
situations will involve additional considerations not addressed
remedial progress and attainment of remedial goals by use of
in this guide.
converging lines of evidence obtained through monitoring and
evaluation of resulting data. When properly applied to a site,
1.7 The guide is organized as follows:
remediation by natural attenuation is a process for risk man-
1.7.1 Section 2 lists referenced documents.
agement and achieving remedial goals. Monitoring should be
1.7.2 Section 3 defines terminology used in this guide.
conducted until it has been demonstrated that natural attenua-
1.7.3 Section 4 describes the significance and use of this
tion will continue and eventually meet remedial goals.
guide.
1.3.1 The primary line of evidence for remediation by
1.7.4 Section 5 provides an overview of the use of natural
natural attenuation is provided by observed reductions in
attenuation as a remedial action alternative, including;
1.7.4.1 Advantages of remediation by natural attenuation as
a remedial alternative;
ThisguideisunderthejurisdictionofASTMCommitteeE50onEnvironmental
Assessment, Risk Management and CorrectiveAction and is the direct responsibil- 1.7.4.2 Limitations of remediation by natural attenuation as
ity of Subcommittee E50.04 on Corrective Action.
a remedial alternative; and
Current edition approved April 1, 2015. Published July 2015. Originally
1.7.4.3 Using multiple lines of evidence to demonstrate the
approved in 1998. Last previous edition approved in 2010 as E1943–98 (2010).
DOI: 10.1520/E1943-98R15. appropriateness of remediation by natural remediation.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1943 − 98 (2015)
1.7.5 Section 6 describes the decision process for appropri- Change in Head (Slug) Tests for Determining Hydraulic
ate application and implementation of remediation by natural Properties of Aquifers
attenuation including; D4050Test Method for (Field Procedure) for Withdrawal
1.7.5.1 Initial response, site characterization, selection of and Injection Well Testing for Determining Hydraulic
chemicals of concern, and establishment of remedial goals;
Properties of Aquifer Systems
1.7.5.2 Evaluation of plume status; D4104Test Method (Analytical Procedure) for Determining
1.7.5.3 Collection and evaluation of additional data;
Transmissivity of Nonleaky Confined Aquifers by Over-
1.7.5.4 Comparing remediation by natural attenuation per- damped Well Response to Instantaneous Change in Head
formance to remedial goals;
(Slug Tests)
1.7.5.5 Comparing remediation by natural attenuation to D4105Test Method for (Analytical Procedure) for Deter-
other remedial options;
mining Transmissivity and Storage Coefficient of Non-
1.7.5.6 Implementation of a continued monitoring program;
leaky ConfinedAquifers by the Modified Theis Nonequi-
1.7.5.7 Evaluation of progress of remediation by natural
librium Method
attenuation; and
D4106Test Method for (Analytical Procedure) for Deter-
1.7.5.8 No further action.
mining Transmissivity and Storage Coefficient of Non-
1.7.6 Section 7 lists keywords relevant to this guide.
leaky Confined Aquifers by the Theis Nonequilibrium
1.7.7 Appendix X1 describes natural attenuation processes;
Method
1.7.8 Appendix X2 describes site characterization require-
D4372Specification for Flame-Resistant Materials Used in
ments for evaluating remediation by natural attenuation;
Camping Tentage (Withdrawn 2002)
1.7.9 Appendix X3 describes considerations for designing
D4448GuideforSamplingGround-WaterMonitoringWells
and implementing monitoring for remediation by natural at-
D4658Test Method for Sulfide Ion in Water
tenuation;
D4700Guide for Soil Sampling from the Vadose Zone
1.7.10 Appendix X4 describes sampling considerations and
D4750Test Method for Determining Subsurface Liquid
analytical methods for determining indicator parameters for
Levels in a Borehole or Monitoring Well (Observation
remediation by natural attenuation;
Well) (Withdrawn 2010)
1.7.11 Appendix X5 describes the interpretation of different
D5092Practice for Design and Installation of Groundwater
lines of evidence as indicators of natural attenuation;
Monitoring Wells
1.7.12 Appendix X6 describes methods for evaluation and
D5269TestMethodforDeterminingTransmissivityofNon-
quantification of natural attenuation rates; and
leaky Confined Aquifers by the Theis Recovery Method
1.7.13 Appendix X7 describes example problems illustrat-
D5270Test Method for Determining Transmissivity and
ing the application and implementation of remediation by
Storage Coefficient of Bounded, Nonleaky, Confined
natural attenuation.
Aquifers
D5434Guide for Field Logging of Subsurface Explorations
1.8 This standard does not purport to address all of the
of Soil and Rock
safety concerns, if any, associated with its use. It is the
D5473Test Method for (Analytical Procedure for) Analyz-
responsibility of the user of this standard to establish appro-
ing the Effects of Partial Penetration of Control Well and
priate safety and health practices and determine the applica-
Determining the Horizontal and Vertical Hydraulic Con-
bility of any regulatory limitations prior to use.
ductivity in a Nonleaky Confined Aquifer
2. Referenced Documents E1599Guide for Corrective Action for Petroleum Releases
2 (Withdrawn 2002)
2.1 ASTM Standards:
E1689Guide for Developing Conceptual Site Models for
D888Test Methods for Dissolved Oxygen in Water
Contaminated Sites
D1125Test Methods for Electrical Conductivity and Resis-
E1739Guide for Risk-Based Corrective Action Applied at
tivity of Water
Petroleum Release Sites
D1293Test Methods for pH of Water
E1912Guide forAccelerated Site Characterization for Con-
D1452Practice for Soil Exploration and Sampling byAuger
firmed or Suspected Petroleum Releases (Withdrawn
Borings
2013)
D1498Test Method for Oxidation-Reduction Potential of
Water
3. Terminology
D1586Test Method for Penetration Test (SPT) and Split-
Barrel Sampling of Soils 3.1 Definitions of Terms Specific to This Standard:
D4043Guide for Selection of Aquifer Test Method in
3.1.1 assimilative capacity—a semi-quantitative estimate of
Determining Hydraulic Properties by Well Techniques
the potential mass of hydrocarbons per unit volume of ground
D4044Test Method for (Field Procedure) for Instantaneous
water that can be metabolized by aerobic and anaerobic
biodegradation under existing site conditions.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on The last approved version of this historical standard is referenced on
the ASTM website. www.astm.org.
E1943 − 98 (2015)
3.1.2 attenuation rate—measured reduction in concentra- stable, or expanding similar to the first line of evidence
tionormassofacompoundwithtimeordistanceexpressedas suggested by NRC (1993).
an amount of reduction per unit time or per unit distance.
3.1.15 receptor—persons, structures, utilities, ecological
receptors, and water supply wells that are or may be adversely
3.1.3 conceptual site model—awrittenorpictorialrepresen-
affected by a release.
tation of an environmental system and the biological, physical,
and chemical processes that determine the transport of con-
3.1.16 remedial goals—remediation objectives established
stituentsofconcernfromsourcesthroughenvironmentalmedia to protect human health and the environment. Remedial goals
to environmental receptors within the system.
may be concentration-based target levels applied at specific
points throughout the plume or performance-based criteria,
3.1.4 constituents of concern—specific petroleum constitu-
such as demonstrated containment of the solute plume or
ents that are identified as posing a potential risk to human
demonstrated reduction in concentrations of constituents of
health or the environment.
concern over time within the plume or with distance from the
3.1.5 corrective action—actions taken to identify and clean
source area.
up a release of petroleum. These activities include site
3.1.17 remediation/remedial action—activitiesconductedto
assessment,interimremedialaction,remedialaction,operation
protect human health, safety, and the environment. These
and maintenance of equipment, monitoring of progress, and
activities include evaluating risk, making no further action
termination of the remedial action.
determinations, monitoring, and designing and operating
3.1.6 electron acceptors—elements or compounds that are
cleanup equipment.
reduced by receiving electrons produced by the oxidation of
3.1.18 remediation by natural attenuation—aremedywhere
organic compounds through microbial metabolism or abiotic
naturally occurring physical, chemical, and biological pro-
chemical oxidation processes.
cesses will achieve remedial goals. The use of natural attenu-
ationprocessesasaremedialactionalsohasbeendescribedby
3.1.7 expanding plume—configuration where the solute
a variety of other terms, such as intrinsic remediation, intrinsic
plumemarginiscontinuingtomoveoutwardordowngradient
bioremediation, passive remediation, natural biodegradation,
from the source area.
passive bioremediation, etc. Remediation by natural attenua-
3.1.8 institutional controls—the restriction on use or access
tion does not include remediation methods that require human
(for example, fences, deed restrictions, restrictive zoning) to a
intervention beyond monitoring.
site or facility to eliminate or minimize potential exposure to a
3.1.19 secondary lines of evidence—geochemical indicators
constituent(s) of concern.
of naturally occurring biodegradation and estimates of natural
3.1.9 monitoring points—a monitoring well or other moni-
attenuation rate.
toring device placed in a selected location for observing
3.1.20 sentinel well—monitoring points established at a
parameters such as liquid levels or pressure changes, or for
location(s) between the leading edge of the solute plume and a
collecting liquid samples. The monitoring point may be cased
sensitive receptor (for example, drinking water well) to ensure
or uncased, but if cased the casing should have openings to
thattherewillbetimeforotherremedialactionstobetaken,if
allow flow of borehole liquid into or out of the casing
the plume does migrate beyond predicted boundaries.
(modified from Test Method D4750).
3.1.21 shrinking plume—configuration where the solute
3.1.10 natural attenuation—reductioninmassorconcentra-
plume margin is receding back toward the source area over
tionofacompoundingroundwaterovertimeordistancefrom
time and the concentrations at points within the plume are
thesourceofconstituentsofconcernduetonaturallyoccurring
decreasing over time.
physical, chemical, and biological processes, such as;
3.1.22 source area—thelocationoffreephaseliquidhydro-
biodegradation, dispersion, dilution, sorption, and volatiliza-
carbons or the location of highest soil and ground water
tion.
concentrations of constituents of concern.
3.1.11 optional lines of evidence—solute transport
3.1.23 stable plume—configuration where the solute plume
modeling, estimates of assimilative capacity (to estimate the
margin is stationary over time and concentrations at points
mass of BTEX and other constituents of concern degraded),
within the plume are relatively uniform over time or may
and microbiological studies.
decrease over time.
3.1.12 plume—volume of ground water where constituents
3.1.24 user—an individual or group involved in the correc-
of concern are present.
tive action process at petroleum release sites, which may
include environmental consultants, industry, and state, local,
3.1.13 point of compliance—a location(s) selected between
and federal regulators.
the source area(s) and potential point(s) of exposure where
concentrations of constituents of concern must be at or below
4. Significance and Use
the determined ground water target levels.
4.1 The approach presented in this guide is a practical and
3.1.14 primary lines of evidence—historical concentration
streamlined process for determining the appropriateness of
dataaretheprimarylineofevidencefornaturalattenuationand
are based on measured petroleum hydrocarbon constituent
National Research Council (NRC), 1993, In Situ Bioremediation: When Does
concentrations over time to define the plume as shrinking, It Work? National Academy Press, Washington, DC.
E1943 − 98 (2015)
remediation by natural attenuation and implementing remedia- 4.8 This guide is intended to be consistent with Guide
tion by natural attenuation at a given petroleum release site. E1599 and U.S. EPAguidance for implementation of remedia-
tion by natural attenuation (U.S. EPA, 1995, Chapter 9).
This information can be used to evaluate remediation by
natural attenuation along with other remedial options for each
5. Natural Attenuation as a Remediation Alternative
site.
5.1 At petroleum release sites petroleum migrates outward
4.2 In general, remediation by natural attenuation may be
from a source area through the environment creating a plume
used in the following instances:
of petroleum constituents. The configuration of a solute plume
4.2.1 As the sole remedial action at sites where immediate
is controlled by the source mass-loading rate relative to the
threats to human health, safety and the environment do not
removal rate of natural attenuation processes. Typically, the
exist or have been mitigated, and constituents of concern are
plume will expand until it reaches steady-state where the rate
unlikely to impact a receptor; of petroleum constituents contributed from the source is
balanced with the rate of natural attenuation. At steady-state
4.2.2 As a subsequent phase of remediation after another
the plume stabilizes. The time scale over which this steady-
remedialactionhassufficientlyreducedconcentrations/massin
state condition is reached can vary depending on specific site
thesourceareasothatplumeimpactsonreceptorsareunlikely;
conditions. When the source area is depleted to the point that
or
the rate of natural attenuation exceeds the source input the
4.2.3 As a part of a multi-component remediation plan.
result will be a shrinking plume over time.
4.3 This guide is intended to be used by environmental
5.2 Remediation by natural attenuation relies on natural
consultants, industry, and state and federal regulators involved
attenuation mechanisms to degrade and reduce concentrations
in response actions at petroleum release sites. Activities de- of constituents of concern in ground water. The natural
scribed in this guide should be performed by a person processes involved are physical, chemical, and biological in
naturesuchasdispersion,dilution,volatilization,sorption,and
appropriately trained to conduct the corrective action process.
biodegradation. Biodegradation is the process which accounts
4.4 The implementation of remediation by natural attenua-
for the majority of mass removal and associated concentration
tion requires that the user exercise the same care and profes-
reduction for constituents of concern. Biodegradation actually
sional judgement as with any other remedial alternative by:
reducesthemassofconstituentsthroughmicrobialmetaboliza-
4.4.1 Ensuring that site characterization activities focus on
tion of constituents of concern. The ultimate products of this
collecting information required to evaluate and implement reactionarecarbondioxide,water,andbiomass.Thesemecha-
remediation by natural attenuation; nisms are described in Appendix X1.
4.4.2 Evaluating information to understand natural attenua-
5.3 The processes which contribute to remediation by natu-
tion processes present at the site;
ral attenuation occur to some extent at all sites. remediation by
natural attenuation is effective when these naturally occurring
4.4.3 Determining whether remediation by natural attenua-
attenuationmechanismsachieveremedialgoals.Dependingon
tion is the most appropriate and cost-effective remedial alter-
site conditions, remediation by natural attenuation may be a
native with a reasonable probability of achieving remedial
long-term remedial option. Remediation by natural attenuation
goals; and
is a remedial action approach that is compatible with existing
4.4.4 Monitoring remedial progress.
remedy selection processes. It is not exclusive of other options
and should be evaluated in the same manner as other remedial
4.5 Applicationandimplementationofremediationbynatu-
action options for a site.
ral attenuation is intended to be compatible with Guide E1739
or other risk-based corrective action programs.
5.4 Remediation by natural attenuation should not be con-
sidered to be a presumptive remedy.
4.6 This guide does not address specific technical details of
5.5 Advantages of Remediation by Natural Attenuation as a
remediation by natural attenuation implementation such as site
Remediation Alternative:
characterization (see Guide E1912), sampling, data
5.5.1 Petroleum hydrocarbon constituents of concern which
interpretation, or quantifying rates. For additional discussion
undergo biodegradation can be ultimately transformed to
andguidanceconcerningthesetechnicalissuesforremediation
innocuous products (for example, carbon dioxide and water),
bynaturalattenuationseeAppendixX1throughAppendixX7.
not just transferred to another phase or location within the
4.7 This guide does not specifically address considerations
environment.
and concerns associated with natural attenuation of non-
5.5.2 Remediation by natural attenuation is less intrusive; it
petroleumconstituents,suchaschlorinatedsolvents.Caremust
resultsinminimaldisturbancetothesiteoperationsandallows
be taken to ensure that degradation by-products will not cause continuing use of the site’s infrastructure during remediation.
harm to human health or the environment. In addition, if
constituentsarepresentwhichdonotreadilyattenuate,suchas
U.S.EPA,1995,EvaluatingAlternativeCleanupTechnologiesforUnderground
methyl-t-butyl ether (MTBE), remediation by natural attenua-
Storage Tanks:AGuide for CorrectiveAction Plan Reviewers. U.S. Environmental
tion may not be a suitable remedial alternative or may need to
Protection Agency, Office of Underground Storage Tanks, Washington, DC, EPA
be supplemented with other remedial technologies. 510-B-95-007, May 1995.
E1943 − 98 (2015)
5.5.3 More conventional remedial technologies can pose 5.6.3 In the public perception, remediation by natural at-
greater risk to potential receptors than natural attenuation due tenuation may be viewed as a “do-nothing” remedial alterna-
to site disruption and/or an inability to properly control these tive.
engineered remedial processes (for example, risk to on-site 5.6.4 Long-term monitoring for remediation by natural
workers, releases to atmosphere, fugitive vapors, induced
attenuation can represent significant cost and a continued
migration, etc.). funding commitment.
5.6.5 Applicationofremediationbynaturalattenuationmay
5.5.4 Remediation by natural attenuation can be used in
require supplemental source area removal or more active
conjunction with conventional remedial technologies such as
remediation when exposure pathways are completed or recep-
excavation, pump and treat, soil vapor extraction, bioventing,
and dual-phase extraction. It can also be used at sites where tors are potentially impacted.
other remedial technologies are not technically feasible to use 5.6.6 Technicallimitationsmayobstructtheimplementation
to achieve required cleanup target levels. or progress of remediation by natural attenuation and require
theconsiderationoruseofotherremediationalternatives.Such
5.5.5 Remediation by natural attenuation can be less costly
limitations can include constraints associated with inadequate
than other currently available remedial technologies when
datausedtoconstructthesiteconceptualmodel,theinabilityto
implemented with an appropriate monitoring program.
implement the monitoring program, insufficient data to per-
5.5.6 Remediation by natural attenuation can be evaluated
form predictive solute transport modeling, and changes in site
bycollectingadequateandappropriategeologicandhydrogeo-
conditions.
logic data during the site characterization phase. Data can be
5.6.6.1 The implementation of remediation by natural at-
collected using relatively inexpensive field and laboratory
tenuation fundamentally requires adequate definition of the
analytical methods (see Appendix X2 and Appendix X4). If it
soluteplumeandunderstandingofsitehydrogeology.Thelack
is shown that remediation by natural attenuation is not solely
of necessary site data or inability to obtain representative, or
sufficient to provide adequate protection of potential receptors,
otherwise requisite samples, necessary to construct an accept-
the data collected for the remediation by natural attenuation
able site conceptual model (for example, aquifer parameters,
studycanbeusedtodesignsupplementalremedialalternatives.
ground water and soil chemistry, etc.) and design an adequate
5.5.7 Use of remediation by natural attenuation can help to
long-termmonitoringplancanprecludeappropriateimplemen-
focus funds and efforts on sites which require active remedia-
tation of remediation by natural attenuation.
tion.
5.6.6.2 Remediationbynaturalattenuationreliesonempiri-
5.5.8 Remediation by natural attenuation is not subject to
caldatageneratedbygroundwatermonitoring.Theinabilityto
the limitations imposed by the use of mechanized remediation
place monitoring points and collect ground water samples in
equipment (that is, no equipment down-time) and can be
appropriate locations due to surface obstructions or other
employed for constituents of concern below buildings and
impediments, changes in aquifer water levels rendering moni-
other areas that are not accessible.
toringpointsunusable,andmonitoringwherethesamplingand
5.5.9 Constituents such as benzene, toluene, ethyl benzene,
analytical protocols are not observed can preclude appropriate
and xylenes (BTEX) that typically pose the greatest risk and
implementationofremediationbynaturalattenuation.Also,the
are commonly the major constituents of regulatory concern at
inherent variability of the ground water monitoring data may
petroleum release sites are generally the most susceptible to
preclude effective evaluation of plume behavior.
biodegradation.
5.6.6.3 Remediationbynaturalattenuationrequiresthatsite
conditions persist or do not change adversely. Actual or
5.6 Limitations of Remediation by Natural Attenuation as a
proposed land use changes may result in the site being
Remediation Alternative:
reclassified to a higher risk level.Anew source may introduce
5.6.1 The ability of remediation by natural attenuation to
additional petroleum product to the system at the site or
achieve remedial goals can be sensitive to natural and human-
another up gradient plume may reduce available electron
induced changes in local hydrogeologic conditions and site
acceptors for biodegradation. Changes in aquifer conditions
operations. Potentially important effects include changes in
may alter the long-term ground water transport rates and
ground water gradients/velocity, rainfall, temperature, pH,
direction or produce short-term changes that are unacceptable.
electron acceptor concentrations, exposures not previously
anticipated,orpotentialfuturereleases.Suchchangescouldbe 5.7 Multiple Lines of Evidence to Demonstrate Appropri-
brought about by alterations in land use, changes in the local ateness of Remediation by Natural Attenuation:
pumping regime, removal of an asphalt cap, or third party 5.7.1 The National Research Council (1993) suggests a
impacts, or a change in the location of receptors.
strategy to demonstrate in situ bioremediation which includes
three types of evidence:
5.6.2 Time frames for achieving remedial goals may be
5.7.1.1 Documentedlossofconstituentsofconcernfromthe
relatively long, particularly for heavier petroleum constituents,
site;
compounds which attenuate slowly, and sites with a large
source mass. Remediation by natural attenuation may take 5.7.1.2 Evidence showing bioremediation is actually real-
ized in the field; and
longer to mitigate constituents of concern than for more
aggressive remedial measures. Remediation by natural attenu- 5.7.1.3 Laboratory assays showing that microorganisms in
ationmaynotalwaysachievethedesiredcleanuplevelswithin site samples have the potential to transform constituents of
a manageable time-frame. concern.
E1943 − 98 (2015)
5.7.2 This guide suggests the demonstration of remediation 6.1.5 Development and implementation of an appropriate
by natural attenuation may include primary, secondary, and monitoring program.
optional lines of evidence. At a minimum, primary lines of
6.2 Initial Response, Site Characterization, Determine Con-
evidence are required to demonstrate the effectiveness reme-
stituents of Concern, and Establish Remediation Goals:
diation by natural attenuation. The decision to collect second-
6.2.1 Initial response should be taken in accordance with
ary and optional lines of evidence should be based on the
implementing agency requirements to report any release of
intended use of the data. The cost benefit of obtaining these
petroleumproducts;preventanyfurtherreleaseof,orexposure
lines of evidence should also be considered. The primary lines
to hydrocarbons in vapor, dissolved, or liquid phase; and
of evidence include constituent of concern data, used to define
mitigate fire and safety hazards. Table1 in Guide E1739
theplumeasshrinking,stable,orexpanding,similartothefirst
provides example site classification and initial response ac-
line of evidence suggested by NRC (1993). For sites which
tions.
have sufficient historical monitoring data, the primary lines of
evidence will often be adequate to demonstrate remediation by 6.2.2 The site characterization must provide the user with
adequate information necessary to determine if remediation by
natural attenuation.
natural attenuation is a viable remedial option for the site,
5.7.3 Secondary lines of evidence include geochemical
either used by itself or in conjunction with other technologies.
indicators of naturally occurring biodegradation and estimates
Site characterizations may be conducted in accordance with
of natural attenuation rate. If the primary lines of evidence are
Section 7 of Guide E1599, and Guide E1912 taking into
inconclusive, it may be necessary to obtain secondary lines of
considerationevaluationofsources,pathways,andreceptorsas
evidence. For those sites where assessment efforts have re-
cently been initiated, it may be appropriate to supplement the discussed in 6.2 of Guide E1739. The types of site character-
ization information that may be necessary for remediation by
primary lines of evidence by measuring indicators of naturally
occurring biodegradation, consistent with the second line of natural attenuation are detailed in Appendix X2. Not all the
evidence suggested by NRC (1993). Estimates of attenuation data listed in Appendix X2 may be needed for each site and
ratearebasedontemporaland/orspatialtrendsforconstituents considerations for when and how this data can and should be
of concern. Once this secondary line of evidence has been used is explained in 6.3 and 6.4.
established, the user must continue to monitor and collect data
6.2.2.1 As part of the site characterization process an initial
to substantiate the primary line of evidence.
conceptual model should be developed before beginning any
5.7.4 Optional lines of evidence may be used to more
field work. The conceptual model should focus on specific
rigorously interpret data developed as secondary lines of
features that are relevant to the assessment objectives. For
evidence, particularly if the primary and secondary lines of
example, the features of a conceptual model of a leaking
evidence are inconclusive to demonstrate remediation by
underground storage tank site may include preliminary esti-
natural attenuation. Optional lines of evidence include solute
matesof:(1)sourceareas;(2)threedimensionaldistributionof
transport modeling, estimates of assimilative capacity (to
constituents of concern; (3) distribution of constituents of
estimate the mass of BTEX and other constituents of concern
concern and impacts to ground water; (4) geologic units or
degraded), and microbiological studies. Attenuation rates can
structures that influence migration of constituents of concern;
be used in modeling transport of constituents of concern.
(5) ground water depth, flow direction and velocity; and (6 )
Indicatorsofnaturallyoccurringbiodegradationcanbeusedto
location of potential receptors and migration pathways.
estimate assimilative capacity. Microbiological studies, as
Hydrogeologic, and analytical data collected during the field
suggested in the third line of evidence by NRC (1993),
investigation should be periodically interpreted and used to
confirm the presence of microorganisms in the subsurface.
refine the conceptual model in an iterative process. The
Onceoptionallinesofevidencehavebeenestablished,theuser
components of the conceptual model that are emphasized
must continue to monitor and collect data to substantiate the
depends on the purpose of the assessment (See Guide E1689,
primary line of evidence.
and Guide E1912 5.4 and 5.7).
6.2.3 The determination of constituents of concern is based
6. Decision Process for Appropriate Application and
on the site specific consideration of exposure routes,
Implementation of Remediation by Natural
concentrations, mobilities, toxicological properties, and aes-
Attenuation
thetic characteristics (taste, odor, etc.). In addition, regulatory
6.1 The key components of the remediation by natural
requirements may dictate certain constituents of concern.
attenuationprocessaredescribedinthefollowingsections.The Appendix X1 in Guide E1739 contains additional discussion
major decisions and actions required to determine the appro-
regarding determination of constituents of concern.
priateness of applying and implementing remediation by natu-
6.2.4 Remedial goals for the site should be determined by
ral attenuation at a given site been are summarized in the
applying the risk-based corrective action process in Guide
flowchart shown in Fig. 1.
E1739 or other accepted state-approved method. Remedial
6.1.1 Site characterization and establishment of remedial
goals may take the form of concentration target levels or
goals;
performance criteria, including demonstration of containment
6.1.2 Evaluation of plume status;
ofthepetroleumhydrocarbonplume.Remedialgoalsmayalso
6.1.3 Comparing RNA performance to remedial goals;
have some time frame associated with them.An evaluation of
6.1.4 Comparing RNA to other remedial options; and the need for source area control measures should be integrated
E1943 − 98 (2015)
NOTE 1—Numbers next to boxes in the flowchart refer to sections in the text.
FIG. 1 Remediation by Natural Attenuation Process Flowchart
E1943 − 98 (2015)
intoremedialdecision-makingatallsiteswherenaturalattenu- water table but the natural attenuation rate approximately
ation is under consideration. Source area control measures equals the mass loading rate for constituents of concern to
include physical removal, treatment, and stabilization. ground water.
6.2.4.1 Remedial goals may be concentration-based target 6.3.1.3 In the case of an expanding plume the mass loading
levels applied at specific points throughout the plume or rate of constituents of concern to ground water exceeds the
performance-based criteria, such as demonstrated containment natural attenuation rate. An expanding plume will become
of the solute plume or demonstrated reduction in concentra- stablewhenthemassloadingrateofconstituentsofconcernto
tions of constituents of concern over time within the plume or ground water is balanced by the natural attenuation rate.
with distance from the source area. Both must be protective of 6.3.2 For sites which have sufficient historical monitoring
human health and the environment. In general, remediation by
data, the primary lines of evidence will often be adequate to
natural attenuation is more amenable to achieving demonstrate remediation by natural attenuation. For sites
performance-based remedial goals.Also, remediation by natu-
which have insufficient historical monitoring data, collection
ral attenuation performance can provide verification of natural and evaluation of geochemical data may be appropriate to
attenuation rates used to determine risk-based target cleanup
expedite the demonstration of remediation by natural attenua-
levels developed through predictive solute transport modeling. tion. Paragraph 6.7 and Appendix X3 describe monitoring
When using remediation by natural attenuation as a contain-
considerations.
ment option, institutional controls may be required to manage
6.3.3 The evaluation of plume status can be accomplished
and prevent on- and off-site exposures.
by either of the following methods, which are described in
6.2.5 Once remedial goals have been established, site con-
detail in Appendix X3.2.1 and Appendix X5. The effects of
ditions should be examined to see if these goals have already historical source removal and remediation efforts should be
been met. If remedial goals have already been met at the site,
incorporated into the evaluation of plume status.
the site may be deemed to require no further action. In some
6.3.3.1 Monitoring points or other sampling devices should
cases continued monitoring may be needed to confirm compli-
belocatedtoallowtheconstructionofcontourmapsforBTEX
ance with remedial goals prior to a determination of no further
and other constituents of concern concentrations. Ideally, the
action. If remedial goals have not been met at the site, then
map will include a non-detect or compliance level contour.
additional remedial action will be required.
Based on changes (or lack of changes) over time, the plume
6.2.6 The potential for impacts to human health and the
can be characterized as shrinking, stable, or expanding. The
environment must be determined by conducting surveys of
example problem in X7.1 illustrates this method.
primary and secondary sources, transport mechanisms, viable
6.3.3.2 Concentrations of BTEX and other constituents of
exposure pathways and potential receptors. Guide E1739
concern can be determined over time at appropriately located
provides a standardized approach to this type of analysis.
monitoring points down gradient of the source and oriented
6.2.6.1 If the potential exists for immediate impacts to an
alongthedirectionofgroundwaterflow(see6.7andAppendix
identified receptor (for example, see Guide E1739 Table 1),
X3 for important considerations regarding placement of moni-
thenotherremedialactionsorrisk-managementstrategiesmay
toring points). The trend in BTEX and other constituents of
be required at the site. If risk-management strategies are not concern concentrations at these points will determine whether
sufficient to prevent impacts to an identified receptor, then
theplumeisshrinking,stable,orexpanding(forexample,ifthe
remediation by natural attenuation is inappropriate as a stand- plume is shrinking, concentrations will decrease over time or
alone option.
space; if the plume is stable, concentrations will remain
6.2.6.2 If the potential for a near-term impact to an existing relatively constant over time and space).
receptor is determined to be low, then remediation by natural
6.4 Collect and Evaluate Additional Data:
attenuation may be used as a stand-alone option for meeting
6.4.1 It may be necessary to obtain additional monitoring
remedial goals within the ground water.
databeforeaplumecanbedefinedasstableorshrinking.Inthe
6.3 Evaluate Plume Status (Primary Lines of Evidence):
caseofanewlydiscoveredpetroleumreleasesite,thehistorical
6.3.1 The dissolved petroleum constituent plume is catego- monitoring data necessary to evaluate plume status discussed
rized based on historical constituent of concern concentrations in 6.3 will not be available. Therefore, one of the methods
obtained from monitoring points. These historical data are the describedin6.3.3maybeusedfollowingadditionalmonitoring
primary line of evidence for natural attenuation and are based events.Fornewlydiscoveredsites,collectionandevaluationof
on measured petroleum hydrocarbon concentrations over time geochemical data may be appropriate to expedite the demon-
to define the plume as shrinking, stable, or expanding. Evi- stration of remediation by natural attenuation.
dence of reductions of constituents of concern is also the first
6.4.2 Secondary lines of evidence may be required if the
line of evidence suggested by NRC (1993). The implications
primary line of evidence, the evaluation of plume status, is
of the three plume categories are as follows: inadequate or inconclusive to demonstrate remediation by
6.3.1.1 A shrinking plume is evidence of natural attenua- natural attenuation. This may be the case for sites where only
tion. The natural attenuation rate of a shrinking plume neces- one or two monitoring events have been performed.
sarily exceeds the mass loading rate of constituents of concern
6.4.3 One secondary line of evidence is to estimate the
to ground water.
naturalattenuationrate.Thisestimateisbasedonthesamedata
6.3.1.2 A stable plume is evidence of natural attenuation. used in the evaluation of plume status (see 6.3). Another
Thesourceofconstituentsofconcernmaypersistinsoilsatthe secondary line of evidence includes geochemical data which
E1943 − 98 (2015)
serve as indicators of naturally occurring biodegradation. 6.4.4 Additional optional lines of evidence may be useful
Geochemical parameters are measured in ground water for the a small percentage of sites where the primary and
samples. secondary lines of evidence are inconclusive to demonstrate
6.4.3.1 Theestimateofattenuationratecanbeperformedby remediation by natural attenuation. These optional lines of
several methods. A mass balance approach is described in evidence may include solute transport modeling, estimates of
X6.1. The technique includes a calculation for the constituent assimilative capacity, and microbiological studies.
of concern source rate (mass loading to ground water). This
6.4.4.1 Solute transport models may be used for several
method yields an estimate for attenuation rate depending on purposes. Transient analytical solutions can estimate the time
whether the plume is shrinking, stable, or expanding.
required for a shrinking or expanding plume to reach a
6.4.3.2 Appendix X6.2 presents graphical and regression particular configuration. Steady-state solutions can be used to
techniques to estimate the attenuation rate. These techniques
estimate the extent of a stable plume and aid in selection of
include plots of (1) concentration versus time for individual locations of down gradient monitoring points. Appendix X6.3
monitoring points and (2) concentration versus distance for
describes the use of a steady-state solution which is coupled to
three or more monitoring points approximately oriented with the regression of concentration versus distance (see X6.2.2),
groundwaterflowdirection.Attenuationratescanbeestimated
for a stable plume.
byregressionofconcentrationversustimeordistance,orboth.
6.4.4.2 One,two,andthree-dimensionalanalyticalsolutions
By plotting the log of concentration versus time or distance as
are presented in X6.4. The justification for two or three-
a straight line (semi-log paper), the assumption of first-order
dimensional analytical models should be based on the avail-
decay can be demonstrated. The attenuation rate is graphically
ability of data. Two of the more sensitive input parameters are
determined by the slope of the straight line.These calculations
the decay rate and source term. Site-specific attenuation or
are described in X6.2. An example problem for concentration
decay rates, as determined by one of the Appendix X6
versus distance appears in X7.2.
methods, can be used in the analytical solution. A source of
6.4.3.3 Indicators of naturally occurring biodegradation are
constituents of concern can be defined as a constant or
useful because biological transformation of petroleum hydro-
decaying term.
carbons is the single most important process contributing to
6.4.4.3 Numerical models are appropriate where site char-
naturalattenuationofpetroleumconstituents.Otherattenuation
acterization data are available to describe a complex hydro-
processes (dispersion, sorption, dilution, volatilization) also
geologic system. Numerical models require input parameters
contribute to reductions in concentrations of constituents of
similar to those used for analytical models, but their spatial
concerningroundwatertoalesserextent.Onelineofevidence
distributionsmustbeknowntowarranttheuseofthesemodels
to demonstrate naturally occurring biodegradation, as sug-
(1).
gested by the NRC (1993), includes data which show that
6.4.4.4
...