ASTM E3361-22

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: E3361 − 22
Standard Guide for
Estimating Natural Attenuation Rates for Non-Aqueous
Phase Liquids in the Subsurface
This standard is issued under the fixed designation E3361; 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 1.5 Therateestimatescanbeusedfordevelopingmetricsin
the corrective action decision framework, complementing the
1.1 This is a guide for determining the appropriate method
LNAPL Conceptual Site Model (LCSM) (Guide E2531).
or combination of methods for the estimation of natural
attenuation or depletion rates at sites with non-aqueous phase 1.6 The emphasis in this guide is on the selection and
liquid (NAPL) contamination in the subsurface. This guide application of methods for quantifying rates of NAPL deple-
buildsonanumberofexistingguidancedocumentsworldwide tion or attenuation. It is assumed that the remediation endpoint
and incorporates the advances in methods for estimating the has been defined for the site based on the remedial objectives
natural attenuation rates. to address composition or saturation concerns as defined in
ITRC (2018) (1). While the rates can be used to estimate the
1.2 The guide is focused on hydrocarbon chemicals of
timeframe to reach the remediation endpoint under natural
concern (COCs) that include petroleum hydrocarbons derived
conditions, methods for estimating the total NAPL mass and
from crude oil (for example, motor fuels, jet oils, lubricants,
timeframe are beyond the scope of this standard.
petroleum solvents, and used oils) and other hydrocarbon
NAPLs (for example, creosote and coal tars). While much of 1.7 The users of this guide should be aware of the appro-
what is discussed may be relevant to other organic chemicals, priate regulatory requirements that apply to sites where NAPL
the applicability of the standard to other NAPLs, like chlori- is present or suspected to occur. The user should consult
nated solvents or polychlorinated biphenyls (PCBs), is not applicable regulatory agency requirements to identify appro-
included in this guide. priatetechnicaldecisioncriteriaandseekregulatoryapprovals,
as necessary.
1.3 This guide is intended to evaluate the role of NAPL
natural attenuation towards reaching the remedial objectives 1.8 ASTM standard guides are not regulations; they are
and/orperformancegoalsataspecificsite;andtheselectionof consensus standard guides that may be followed voluntarily to
an appropriate remedy, including remediation through moni- support applicable regulatory requirements.This guide may be
toring of natural or enhanced attenuation, or the remedy used in conjunction with other ASTM guides developed for
transition to natural mechanisms. While the evaluation can sites with NAPL in the subsurface. The guide supplements
support some aspects of site characterization, the development characterization and remedial efforts performed under
of the conceptual site model and risk assessment, it is not international, federal, state, and local environmental programs,
intended to replace risk assessment and mitigation, such as but it does not replace regulatory agency requirements.
addressingpotentialimpacttohumanhealthorenvironment,or
1.9 SI units are primarily used in the standard, however,
need for source control.
unitsmorecommonlyusedintheindustryarealsorepresented.
1.4 Estimation of NAPL natural attenuation rates in the
1.10 The guide is organized as follows:
subsurface relies on indirect measurements of environmental
1.10.1 Section 2 lists referenced documents.
indicators and their variation in time and space. Available
1.10.2 Section 3 defines terminology used in this guide.
methods described in this standard are based on evaluation of
1.10.3 Section 4 describes the significance and use of this
biogeochemical reactions and physical transport processes
guide.
combined with data analysis to infer and quantify the natural
1.10.4 Section 5 provides the conceptual model of natural
attenuation rates for NAPL present in the vadose and/or
attenuation processes and pathways.
saturated zones.
1.10.5 Section 6 provides an overview and description of
methods for the estimation of natural attenuation rates, includ-
ThisguideisunderthejurisdictionofASTMCommitteeE50onEnvironmental ing:
Assessment, Risk Management and CorrectiveAction and is the direct responsibil-
ity of Subcommittee E50.04 on Corrective Action.
Current edition approved Oct. 1, 2022. Published December 2022. DOI: Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
10.1520/E3361–22 this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E3361 − 22
1.10.5.1 Descriptionofmethodsandavailabletechnologies: D4448GuideforSamplingGround-WaterMonitoringWells
(1)CO efflux method D4700Guide for Soil Sampling from the Vadose Zone
(2)Temperature gradient method D6866Test Methods for Determining the Biobased Content
(3)Soil gas gradient method of Solid, Liquid, and Gaseous Samples Using Radiocar-
(4)Groundwater monitoring method bon Analysis
(5)NAPL composition method D7648/D7648MPractice for Active Soil Gas Sampling for
1.10.5.2 Screening or feasibility assessment of the method DirectPushorManual-DrivenHand-SamplingEquipment
for the site conditions; D7663/D7663MPractice for Active Soil Gas Sampling in
1.10.5.3 Background sources and correction methods; the Vadose Zone for Vapor Intrusion Evaluations
1.10.5.4 Data interpretation, key considerations and chal- E1943Guide for Remediation of Ground Water by Natural
lenges(forexample,measurementfrequencyandlocationsand Attenuation at Petroleum Release Sites
spatial/temporal averaging); E2531Guide for Development of Conceptual Site Models
1.10.5.5 Applicability of methods for evaluating the perfor- and Remediation Strategies for Light Nonaqueous-Phase
mance of enhanced natural attenuation (bioremediation) sys- Liquids Released to the Subsurface
tems; E2856Guide for Estimation of LNAPL Transmissivity
1.10.5.6 Other method applications (for example, source E2876Guide for Integrating Sustainable Objectives into
delineation or estimating mass discharge rates). Cleanup
1.10.6 Section7providesguidanceonselectionofamethod E2993Guide for Evaluating Potential Hazard as a Result of
or combination of methods applicable to site-specific condi- Methane in the Vadose Zone
tions. 2.2 API Documents:
1.10.7 Section 8 provides example applications through API, 2010BioVapor, A 1-D Vapor Intrusion Model with
case studies. Oxygen-limited Aerobic Biodegradation
1.10.8 Section 9 lists keywords relevant to this guide. API, 2017Quantification of Vapor Phase-related Natural
1.10.9 Appendix X1 describes details of the CO Efflux Source Zone Depletion Processes. American Petroleum
Method. Institute. Publication No. 4784
1.10.10 Appendix X2 describes details of the Temperature API, 2018Managing Risk at LNAPL Sites. Frequently
Gradient Method. Asked Questions, Second Edition, Soil and Groundwater
1.10.11 Appendix X3 describes details of the Soil Gas Research Bulletin No. 18, May 2018, updated May 8,
Gradient Method. 2019
1.10.12 Appendix X4 describes details of the Groundwater 2.3 US EPA Standards:
Monitoring Method. EPA Method 8015 (SW-846), 2003Nonhalogenated Organ-
1.10.13 Appendix X5 describes details of the NAPL Com- ics Using GC/FID, Washington, DC
position Method. EPA Method 8260 (SW-846), 2018Volatile Organic Com-
1.10.14 Appendix X6 provides details of case studies dis- pounds byGas Chromatography/Mass Spectrometry (GC/
cussed in Section 8. MS), Washington, DC
1.10.15 Appendix X7 provides example estimates of NAPL US EPA, 2015Technical Guide for Addressing Petroleum
quantity. Vapor Intrusion at Leaking Underground Storage Tank
Sites.
1.11 This standard does not purport to address all of the
US EPA, 2016Petroleum vapor intrusion modeling assess-
safety concerns, if any, associated with its use. It is the
ment with PVIScreen. U.S. Environmental Protection
responsibility of the user of this standard to establish appro-
Agency Office of Research and Development. US EPA
priate safety, health, and environmental practices and deter-
Report# EPA/600/R-16/175, p. 34
mine the applicability of regulatory limitations prior to use.
US EPA, 2017Documentation for EPA’s Implementation of
1.12 This international standard was developed in accor-
the Johnson and Ettinger Model to Evaluate Site Specific
dance with internationally recognized principles on standard-
Vapor Intrusion into Buildings. Version 6.0, Revised
ization established in the Decision on Principles for the
September 2017
Development of International Standards, Guides and Recom-
2.4 USGS Document:
mendations issued by the World Trade Organization Technical
USGS, 2021Dissolved Gas N /Ar Sample Collection Pro-
Barriers to Trade (TBT) Committee.
cedure
2. Referenced Documents
3. Terminology
2.1 ASTM Standards:
3.1 Definitions:
D3328Test Methods for Comparison of Waterborne Petro-
leum Oils by Gas Chromatography
Available from American Petroleum Institute (API), 200 Massachusetts Ave.
NW, Suite 1100, Washington, DC 20001, http://www.api.org.
AvailablefromUnitedStatesEnvironmentalProtectionAgency(EPA),William
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Jefferson Clinton Bldg., 1200 Pennsylvania Ave., NW, Washington, DC 20460,
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM http://www.epa.gov.
Standards volume information, refer to the standard’s Document Summary page on Available from U.S. Geological Survey (USGS), 12201 Sunrise Valley Drive
the ASTM website. Reston, VA 20192, https://www.usgs.gov.
E3361 − 22
3.1.1 CO efflux method, n—a method for quantifying the biodegradationinboththevadosezoneandthesaturatedzone.
natural source zone depletion rate that relies on measurements Therefore, methods for natural source zone depletion (NSZD)
of CO released from NAPL biodegradation in the subsurface rateestimatesfortheNAPLarebecomingmorewidelyusedto
and transported through diffusion and advection to the ground evaluate the effectiveness of MNA, in addition to those that
surface. historically addressed the monitoring of groundwater only.
3.1.1.1 Discussion—The upward flux or efflux of CO Estimating the rate of natural attenuation processes in the
measuredatthegroundsurfaceabovetheNAPLfootprintisan vadose zone and the saturated zone can also be used to guide
indicator of the NAPL source depletion and can be used to the transition from an engineered remedy to a natural remedy
estimatethenaturalsourcezonedepletionratewithappropriate such as MNA.
correction for background sources of CO . The method is
3.1.5 NAPL composition method, n—amethodforassessing
described in Section 6 and Appendix X1.
natural source zone depletion based on monitoring and data
3.1.2 engineered remedy, n—also referred to in other guid- analysis of changes in NAPL composition over time.
ance documents as active remediation, is generally considered 3.1.5.1 Discussion—The natural attenuation processes such
to be more resource intensive in terms of cost, energy use and as dissolution, chemical and biological degradation, and vola-
greenhouse gas (GHG) emissions (Guide E2876). tilization all contribute to the NAPL weathering and hence a
3.1.2.1 Discussion—There can be a transition to natural quantifiable change in the mass fractions of NAPL constitu-
remedy following the operation of an engineered remedy. ents.The method relies on assessment of the relative depletion
Natural Remedy is commonly contrasted with Engineered of COCs or bulk NAPL as compared to marker constituents
Remedy as the two end members in the spectrum of remedia- thatarelesssusceptibletoweathering.Themethodisdescribed
tion systems as shown in Fig. 1. in Section 6 and Appendix X5.
3.1.3 groundwater monitoring method, n—a method for 3.1.6 natural attenuation, n—the naturally occurring mass
quantifyingnaturalattenuationratesthatreliesongroundwater loss of hydrocarbons in various phases and media (NAPL,
sampling and analyses. vapor, soil, and groundwater) within a volume of soil or
3.1.3.1 Discussion—The method can be used to obtain groundwater contamination.
estimates of the attenuation rates of either bulk NAPL or 3.1.6.1 Discussion—Natural attenuation occurs in and out-
groundwater COCs depending on the data collection and side of the source zone where NAPL is present.
interpretation. Groundwater concentrations of hydrocarbons
3.1.7 natural remedy, n—also referred to in other guidance
and geochemical indicators of redox reactions and dissolved
documents as passive or knowledge-driven remediation, is
gases, as well as hydrogeological parameters are used in the
generally a less resource intensive remediation system mainly
estimation of mass loss rates. The method is based on mecha-
relying on natural or in-situ and enhanced bioremediation
nisms contributing to mass loss in the saturated zone such as
measures.
hydrocarbon dissolution and flow; biodegradation; degassing,
3.1.7.1 Discussion—They are generally considered to have
bubble formation and ebullition. The method is described in
a lower cost, and environmental footprint in terms of energy
Section 6 and Appendix X4.
consumption and GHG emissions (Guide E2876). Natural
3.1.4 monitored natural attenuation (MNA), n—a natural remediesmaybeselectedastheonlyremediationmeasureata
remedydocumentedthroughsitecharacterizationandmonitor- site with minimal engineered intervention such as institutional
ing. controls. More typically, however, there is a transition to
3.1.4.1 Discussion—MNA has historically been focused on natural remedy after an engineered remedy has been in
the assessment of spatiotemporal trends in concentrations of operation. MNA is an example of a natural remedy. Natural
COCs in groundwater. Various tools for the estimation of Remedy is commonly contrasted with Engineered Remedy as
natural attenuation have been advanced that consider all the two end members in the spectrum of remediation systems
relevant processes including dissolution, volatilization, and as shown in Fig. 1.
FIG. 1 Natural and Engineered Remedies Defined in Relative Terms as End-Members Across a Spectrum of Attributes for Remediation
Systems
E3361 − 22
3.1.8 natural source zone depletion (NSZD), n—the natu- degraded, and results in thermal conduction and increase in
rally occurring mass loss of hydrocarbons in NAPL source in-situsoiltemperature.Verticalprofilesofsoiltemperatureare
zones as a result of dissolution, volatilization, and biodegrada- used to infer the NAPL attenuation through biodegradation.
tion. Themethodrequirescorrectionforbackgroundsourcesofheat
3.1.8.1 Discussion—NSZDisasubsetofnaturalattenuation including heat exchange with the atmosphere at the ground
largely focused on the depletion of bulk hydrocarbons from a surface. The method is described in Section 6 and Appendix
NAPL source present in the subsurface. NSZD rates can also X2.
be estimated for individual hydrocarbons depending on the
selected method. The biogeochemical reactions and transport
4. Significance and Use
processesinthevadosezoneandthesaturationzoneareshown
4.1 Guidance on management of NAPL sites and a large
in Fig. 2. These include dissolution and flow in the groundwa-
body of research effort contributing to their development (for
ter and biodegradation; dissolution into the porewater, volatil-
example, ITRC 2018 (1); CRC CARE 2018 (2); CL:AIRE
izationandbiodegradationinthevadosezone;andtransportof
2019 (3)andCRCCARE2020 (4))pointtothesignificanceof
gases across the capillary fringe. NSZD results in changes in
naturalattenuationandNSZDintheevolutionofNAPLsource
the composition of the NAPL over time, which can impact
and the resulting distributions of COCs in soil, groundwater
NAPL forensics, and the risks associated with the NAPL such
and vapor.
asinvaporintrusion,NAPLmigration,andgroundwaterplume
extent and stability.
4.2 Examplesofreportedrangesinestimatednaturalattenu-
ation rates are 300 – 7700 gallons of NAPL/acre/year (Garg et
3.1.9 soil gas gradient method, n—amethodforquantifying
al. 2017 (5)); and 0.4 – 280 metric tons of NAPL/year (CRC
the NSZD rate based on measurements of changes in soil gas
CARE 2020 (4)).
composition with depth (vertical gradient) in the vadose zone
resulting from biodegradation and transport of terminal elec-
4.3 The intent of this guide is to provide a standardized
tron acceptors (TEAs) and reaction byproducts (mainly O ,
approach for the estimation of natural attenuation rates for
CO , hydrocarbons, and CH ).
2 4
NAPLinthesubsurface.Theratescanbeusedforestablishing
3.1.9.1 Discussion—The most common application of this
a baseline metric for those involved in the remedial decision-
method is based on aerobic biodegradation of hydrocarbons in
making process.There is a need for a systematic approach and
the vadose zone with diffusive oxygen transport from the
refinement in data collection and interpretation for quantifying
ground surface. There are also variations on this approach for
the spatially and temporally variable rates. Providing quality
calculating COC-specific attenuation rate. The method is
assurance in estimation of this metric will enable the assess-
described in Section 6 and Appendix X3.
ment of relatively more engineered remedies as compared to
3.1.10 temperature gradient method, n—amethodforquan- natural remedies or MNA(Fig. 1), as well as estimation of the
tifying the NSZD rate based on measurements of temperature remediation timeframe. This comparison, when performed
and estimates of heat flux resulting from aerobic biodegrada- throughastandardizedapproach,canleadtoactionablemetrics
tion of the NAPLand byproducts (methane) in the subsurface. for transition to sustainable remedies through well-defined and
3.1.10.1 Discussion—The heat released is proportional to
transparentcriteria.Inthecontextofaspectrumofremediation
the rate of biodegradation and amount of hydrocarbon options in terms of engineered and natural remedies (Fig. 1),
FIG. 2 Overview of Natural Attenuation and NSZD Processes, Methods, and Measurements
E3361 − 22
the transition is from a relatively more engineered (or active rates in both the saturated zone and the vadose zone and
remediation) to a relatively more nature-based remedy. When complements previous standards (Guide E1943) focused on
considered in the remedial decision-making process, estimates MNAin the saturated zone by inclusion of methods related to
of natural attenuation rates can be used: the vadose zone (Section 6).
4.3.1 Before active remediation (as baseline to assess
4.9 Thenaturalattenuationprocesses(Section5)canimpact
whether active remediation is needed);
remedial objectives in terms of addressing NAPL saturation
4.3.2 During active remediation (as performance/
(mobilityormigration)orcomposition(COCconcentrationsin
optimization metric); and
soil, groundwater or vapor), and therefore need to be included
4.3.3 At the end of active remediation (support transition to
in the CSM. Natural attenuation, including NSZD, can reduce
MNA or site closure).
both NAPL saturation and constituent-specific mass.
4.4 SincenaturalattenuationresultsinchangestotheNAPL
4.10 Integration of natural attenuation rate estimate at the
composition over time, methods to estimate the natural attenu-
earlystagesofsitemanagement(thatis,intheCSM)canresult
ationratealsoinformNAPLforensics,andtherisksassociated
in its proper application to the remedial decision-making
with the NAPL such as in vapor intrusion, NAPL migration,
process, since natural attenuation can result in exposure risk
and groundwater plume extent and stability.
reduction, as well as overall source mass reduction.
4.5 In addition, understanding of the magnitude of natural
4.10.1 In most cases, identifying the occurrence of natural
attenuation rates can contribute to addressing overarching
attenuation at a site or measuring the rate at a site is not
questions in NAPL sites management, following initial char-
sufficient in itself to accomplish remedial goals and regulatory
acterization and risk assessment, such as:
requirements.
4.5.1 What is the remediation timeframe under natural
4.10.2 This guide provides methods for identifying the
attenuation and how does it compare with the remedial
occurrenceofnaturalattenuation,measuringtherateofnatural
timeframe of engineered remedies?
attenuation and demonstrating how this data can be used for
4.5.2 What are the current and future estimates of NAPL
achieving remedial goals and regulatory requirements.
mass (or volume) remaining on site? The remaining mass can
impact compositional concerns.
4.11 The advantages of estimating natural attenuation rates
4.5.3 Under what scenarios (for example, size of release
at sites impacted by hydrocarbon-based NAPL including
and/or presence of NAPL); and site conditions are the rates of
petroleum, coal tars, or creosote is evidenced by examples
NAPL natural attenuation significant in terms of reaching
whereoneormultiplemethodsfortherateestimateshavebeen
remedial objectives in accordance with regulatory criteria and
applied.
remedial timeframe?
4.12 US EPA and State regulations or guidance that high-
4.5.4 How do the rate estimates of natural attenuation
light the significance of natural attenuation at NAPL sites
change over time?
include:
4.6 Common challenges encountered in the management of
4.12.1 Role of natural attenuation and specifically biodeg-
NAPL sites are:
radationinthevadosezoneisdemonstratedthroughanalysisof
4.6.1 Sites that remain under engineered (active) remedia-
datasetstosubstantiatetheapplicabilityofscreeningdistances
tion over extended periods of time without reaching an
for petroleum vapor intrusion (US EPA, 2015, ITRC, 2014
acceptable endpoint.
(6)).
4.6.2 Understanding what the long-term fate of NAPL
4.12.2 Adoption of MNA as a means to ensure long-term
bodies would be with and without engineered remedies.
containment and reduction of dissolved phase plumes (Guide
4.6.3 Understanding the long-term fate of NAPL-related
E1943, WI-DNR 2014 (7), ITRC 2018 (1)).
dissolved organic carbon (DOC) plumes.
4.12.3 Additional technical aspects of NSZD pertaining to
4.6.4 Understanding NAPL movement and demonstrating
forensic evidence and weathering patterns have previously
stability.
been employed by environmental professionals, regulatory
4.7 A major obstacle in answering the questions in 4.5 and
agencies and legal courts on site specific projects.
addressing the challenges in 4.6 is the availability of methods
4.13 Comparison of the natural attenuation rates to the
for estimation of reliable and quantifiable NAPL attenuation
removalratesachievedthroughengineeredremediesovertime,
rates that can be implemented and reviewed by site managers,
if applicable, and defining a threshold for transition from more
siteownersandregulators.Toaddressthischallenge,theintent
engineered to more natural remedies has the potential to
of this standard is to describe the available methods and their
improve remedial decisions as demonstrated through case
selection and application based on site conditions.
studies presented in this standard guide. This includes termi-
4.8 Itisimportanttounderstandtheapplicabilityanduseof
nationofarelativelyengineeredremedyandrelianceonMNA.
the NAPL natural attenuation rates in decision making with
regards to the requirement for an endpoint of an engineered
5. Conceptual Model of Natural Attenuation Processes
remediation system. A merited transition from engineered to
and Pathways
natural remedy, including MNA would result in a more
sustainable approach to site management. MNAin the context 5.1 The natural attenuation processes that affect the distri-
of this standard includes the monitoring of natural attenuation bution and evolution of a NAPL source zone are largely
E3361 − 22
defined by the LCSM (Guide E2531; Guide E1943; Garg et al. particularly the significance of volatilization and aerobic
2017 (5); ITRC 2018 (1); CL:AIRE 2019 (3); CRC CARE biodegradation, driven in large part by the potential impact on
2020 (4)). vapor intrusion, have led to a greater emphasis on vadose zone
processes.
5.2 An overview of natural attenuation processes including
NSZD is provided in Fig. 3.
5.4 Methods used to assess natural attenuation rates using
5.2.1 Natural attenuation processes begin at the onset of a soil gas profiles (for example, Lahvis and Baehr 1996 (8);
NAPL release to the subsurface and involve both physical
Lahvis et al. 1999 (9); and Johnson et al. 2006 (10)), CO flux
transport processes, as well as biological and chemical reac- at ground surface (for example, Sihota et al. 2011 (11); Sihota
tions.
and Mayer 2012 (12); and McCoy et al. 2014 (13))or
5.2.2 ReductionofNAPLmassinthesourcezonecanoccur temperature profiles (for example, Sweeney and Ririe 2014
in the vadose zone as well as the saturated zone depending on
(14))abovetheNAPLsourcehaveindicatedagreaterrangeof
the location of the source zone with respect to the water table. natural attenuation rates than those estimated in the saturated
5.2.3 Natural attenuation is defined here as a broader
zone (detailed review provided in Garg et al. 2017 (5)).
concept that includes processes occurring within and away
5.5 A number of guidance documents (for example, API
from the NAPL source zone in the vapor and/or groundwater
2017; ITRC 2018 (1); and CRC CARE 2018 (2) and 2020 (4))
plume (Fig. 3). These processes include:
have emerged in a relatively short span with research and
5.2.3.1 Biodegradation;
publications improving the state of practice, particularly with
5.2.3.2 Degassing, bubble formation and ebullition in
respect to improved accuracy provided by new approaches to
groundwater;
data analysis.
5.2.3.3 Volatilization and transport in soil gas in the vadose
5.6 To the extent that risk drivers are the more volatile,
zone;
soluble,andchemicallyorbiologicallydegradablecomponents
5.2.3.4 Dissolution and flow in groundwater;
of the NAPL source, estimates of bulk NAPL natural attenua-
5.2.3.5 Sorption;
tion can be useful in remedial decision making and managing
5.2.3.6 Back diffusion; and
risk at sites (see Sections 4 and 6.4).
5.2.3.7 Outgassing from direct biodegradation of NAPL
source without dissolution into the aqueous phase.
5.7 While most methods address the depletion of bulk
5.3 Historicalrepresentationsofprocessesandpathwaysfor NAPL(see 6.2 and 6.3), which ultimately reduce risks associ-
natural attenuation of NAPL have focused on the saturated ated with both saturation and composition-based concerns,
zone due to its impact on the extent and stability of ground- some methods can more directly address composition-based
water plume of COCs (Guide E1943). Research studies on concerns by estimating the natural attenuation rate of COCs
behavior of petroleum hydrocarbons in the vadose zone, (notably one of the approaches presented for the soil gas
NOTE 1—The mass loss of petroleum hydrocarbons naturally occurring in any of the phases (NAPL, vapor, soil, and groundwater) within an area of
soil or groundwater contamination. Natural attenuation occurs in and outside of the source zone where NAPL is present.
NOTE 2—The mass loss of petroleum hydrocarbons naturally occurring in NAPL source zones as a result of dissolution, volatilization, and
biodegradation. NSZD is a subset of natural attenuation largely focused on the depletion of bulk petroleum hydrocarbons from a NAPL source present
near the water table. NSZD rates can also be defined for individual hydrocarbons.
FIG. 3 Conceptual Representation of Natural Attenuation (Note 1) and Natural Source Zone Depletion (NSZD) (Note 2)
E3361 − 22
TABLE 1 Summary of the Five Methods for Estimating Natural
gradient method in Appendix X3; and the compositional
Attenuation Rates
change method in Appendix X5).
Type of Location of
5.8 Detailed descriptions of available methods for estimat-
Method Attenuation Processes & Measurement Location
A
Measured Pathway
ing natural attenuation rates and key processes considered for
B
each method are provided in Section 6. 1. CO Efflux Bulk NAPL Vadose zone Ground surface
B
2. Temperature Bulk NAPL Vadose zone Vertical profile mostly
6. Natural Attenuation Estimation Methods
Gradient in the vadose zone &
straddling the capillary
6.1 As described in the conceptual model of natural attenu-
fringe above the
ationofNAPL(Section5),thereareacomplexsetofphysical, source zone
chemical, and biological processes that shape the distribution
B
3. Soil Gas Bulk NAPL & Vadose zone Vertical profile in the
andlongevityoftheNAPLbodyanditschemicalconstituents,
Gradient COCs vadose zone above
some of which may be identified as COCs at a given site. the source zone
6.2 In order to untangle the interplay of these processes and
4. Groundwater Bulk NAPL & Saturated zone Profile along the
Monitoring COCs groundwater flow path
enable the practical estimation of natural attenuation rates
up- and down-gradient
using readily available, or easy to obtain environmental data,
from the source zone;
five generalized methods based on key processes, target media
includes monitoring of
dissolved gases
andtransportpathwaysarepresentedandareillustratedinFig.
2.
5. NAPL COCs NAPL Source Source zone
Composition zone
6.3 For the various methods, it is important to note that
A
The depletion rate of bulk NAPL directly addresses saturation-based concern.
some approaches are directly related to mass loss of specific
While estimates of COC attenuation rates have a more direct impact on
COCs from the NAPL body, while others (temperature, O ,
composition-based concern, both bulk depletion of NAPL and COC attenuation
CO,CH ) are used to estimate “bulk” NAPL depletion. impact the extent and longevity of the COCs in soil vapor and groundwater.
2 4
B
Includes the transport of methane and other hydrocarbons produced from the
6.4 Bulk NAPL depletion refers to the depletion of an
biodegradation of NAPL in the saturated zone; and methane oxidation at the
aerobic/anaerobic interface.
unknown set of NAPL constituents. The term “bulk” does not
imply uniform rates of depletion of the NAPL constituents.
These methods do not provide information on hydrocarbon-
specific rate of attenuation.
6.5 The five methods are defined as: (1) CO flux at ground
trationscanbeusedtoderiveCOCspecificattenuationratesas
surface; (2) Vertical temperature profiles; (3) Gradients in soil
described in Appendix X3 and Appendix X4, respectively.
gas concentrations; (4) Gradients in dissolved phase concen-
6.7.3 All methods use data collected over a short-term
trations (including degassing); and (5) Changes in NAPL
(minutes to several days) to estimate the natural attenuation
composition. Fig. 2 and Table 1 provide a summary of the
rates, except for Method 5 using NAPL composition. This
methods with detailed descriptions in Appendix X1 to Appen-
Method uses long-term monitoring data (that is, years) to infer
dix X5.
rates and can be used to estimate the fraction of NAPL
6.6 An overview of methods is provided in this section remainingatanypointintimeduringwhichmonitoringdatais
while detailed descriptions for each of the five methods can be
available.All other methods may require repeat measurements
found in Appendix X1 to Appendix X5. and data analysis to account for seasonal variations, while
some can also be configured for continuous monitoring (for
6.7 Description of method and available technologies: A
example, CO flux and temperature).
wide range of technologies and data analysis approaches are
6.7.4 Methods 1 to 4 utilize a change in a proxy measure-
availabledependingonthespecificmethod.Fordetails,referto
ment over space, for example a concentration gradient deter-
Appendix X1 to Appendix X5. In general terms, the methods
minedduringasinglemonitoringevent.Method5usingNAPL
are compared as follows:
composition utilizes long-term monitoring at a single location.
6.7.1 The methods can be divided based on measurement
Spatial coverage of this Method for site-wide estimation
location (that is, ground surface or below ground surface) of
requires evaluation at multiple locations. This also applies to
data used to infer the rates. Method 1 using CO flux requires
variations of Method 4 based on long-term monitoring and
measurements at the ground surface and is the least invasive
trend analysis of concentrations or mass discharge.
method to employ at a site. All other methods require subsur-
face installations for sampling and analysis of relevant param- 6.8 Screening or feasibility assessment of the method based
eters. It is noted that some (and in some cases all) required on site conditions:
subsurface installations may already be in place as part of site 6.8.1 All methods except for Method 5 on NAPL composi-
characterization, for example, existing monitoring wells and tion are primarily applicable to site conditions where biodeg-
soil gas probes. radation plays a significant role in the natural attenuation
6.7.2 The estimated natural attenuation rates represent estimates.Therefore,itisimportanttoconductanevaluationof
depletion of bulk NAPL-only for all methods except Method 5 NAPLbiodegradation.Thesearegenerallybasedonchangesin
using NAPL composition. Variations on Method 3 using soil thedistributionofthereactantsandproductsofbiodegradation
gas concentrations and Method 4 using groundwater concen- reactions in groundwater and soil vapor such as:
E3361 − 22
6.8.1.1 Change in redox conditions and concentrations of standing of the LCSM and the conceptual model of natural
terminalelectronacceptorsingroundwaterbetweenupgradient attenuation processes and pathways (Section 5) is essential for
and downgradient locations from the NAPL source; all methods.
6.8.1.2 Soil gas concentrations in samples collected from 6.10.2 ThefactorsdetailedforeachmethodinAppendixX1
soil gas probes or headspace of monitoring wells (field or
to Appendix X5 generally relate to subsurface and ground
laboratory measurements): surface conditions. Method 1 using CO flux is particularly
(1)Lower oxygen concentrations relative to atmospheric;
sensitive to ground surface conditions; whereas Method 2
(2)Elevated carbon dioxide and methane concentrations; usingtemperatureprofilesandMethod3usingsoilgasgradient
and
are indirectly affected by these conditions in terms of O
(3)LaboratorymeasurementofN andArthataredepleted availability and its effect on aerobic biodegradation; Method 4
or enriched relative to atmospheric (related to methane gen-
using groundwater gradient and Method 5 using NAPL com-
eration and pressure-driven flow) (Guide E2993; Amos et al. position are not affected.
2005 (15); and Molins et al. 2010 (16)).
6.10.3 Spatial variability in subsurface and ground surface
6.8.1.3 Screening of the subsurface temperature profile
conditions is expected to result in variability in the estimated
within the NAPL-impacted areas for relative increase in the
rates across the NAPL footprint. For this reason, all methods
zone of aerobic/anaerobic interface (see Appendix X2).
generally require measurements at multiple locations and a
6.8.2 Methods based on measurements in the vadose zone
form of weighted spatial averaging or integration.
and dependent on downward flux of O can be affected by
6.10.4 Considerationsaffectingmeasurementfrequencyand
ground surface covers or soil layers with low gas permeability
temporal variability are changes in groundwater elevation and
such as a high clay or high moisture content. Low gas
seasonalvariationsintemperatureandprecipitationalongwith
permeability also impacts the upward flux of CO or hydro-
2 resulting changes in soil moisture content in the vadose zone.
carbons in soil gas. These factors specifically impact the
Temporalvariabilityisexpectedtohavethegreatestimpacton
applicability of the CO Efflux, the Temperature Gradient and
2 vadose zone Methods 1 to 3, and least impact on methods
the Soil Gas Gradient Methods.
basedonlong-termmonitoringdatasuchasMethod5basedon
6.8.3 Method specific factors are discussed in Appendix X1
NAPL composition.
to Appendix X5.
6.10.5 Method specific recommendations for addressing
spatialandtemporalvariabilityareprovidedinAppendixX1to
6.9 Background sources and correction methods:
Appendix X5.
6.9.1 Aerobicbiodegradationofmethaneorotherhydrocar-
bons originating from the NAPL source zone can have a
6.11 Applicability of the method for evaluating the perfor-
significant contribution to the rates estimated using Methods 1
mance of enhanced attenuation (bioremediation) systems.
to 3. Soil respiration, or aerobic biodegradation of natural soil
6.11.1 Biodegradation forms the basis or the major compo-
organics, can confound the signature of measurement proxies
nent of Methods 1 to 4. The specific technology used in each
for estimating the NAPL attenuation rate. There are generally
method may be affected by the operation of a bioremediation
two approaches for distinguishing between natural soil respi-
system.Forexample,changesintemperature,soilgasandCO
ration (NSR) and respiration attributable to hydrocarbon or
flux data can be used for rebound testing of a soil vapor
contaminant soil respiration (CSR).
extraction (SVE) system.
6.9.1.1 One approach for determining the CSR portion of
6.11.2 Further details are provided in Appendix X1 to
the attenuation rate is based on subtracting the contribution of
Appendix X5.
NSR based on data obtained from an unimpacted area of the
6.12 Other method applications:
site (background location) from the data obtained from the
6.12.1 Data obtained for site investigation and LCSM de-
NAPL footprint. Key considerations are the similarities be-
velopment can be used to assess the applicability of methods
tween the subsurface soil and ground surface conditions
for estimating natural attenuation rates. Likewise, data ob-
between the NAPL area and the background location.
tainedforMethods1to5canbeusedtoinformtheLCSMand
6.9.1.2 The alternative approach is the use of radiocarbon
remedial decision making. Examples include:
( C) analysis to correct for the portion of a rate attributable to
6.12.1.1 Some specific technologies for CO flux measure-
CSR. This approach has been well demonstrated for Method 1 2
ments can be used for NAPL source delineation.
based on the assumption that CO derived from fossil fuel is
14 14
6.12.1.2 Data obtained for Method 4 on groundwater moni-
C depleted, while CO derived from modern organics is C
toring can be used to estimate dissolved mass discharge rates.
rich.
6.12.2 Method specific suggestions are provided in Appen-
6.9.1.3 Key factors are the presence of soil layers rich in
organics such as peat, vegetation and seasonal variability. dix X1 to Appendix X5.
6.9.2 Further details on the application of the two ap-
proaches are discussed in Appendix X1 to Appendix X3. 7. Decision Process for Appropriate Selection of Method
and Application
6.10 Data interpretation and key considerations and chal-
lenges:
7.1 Thissectionisintendedtoprovideguidanceonselection
6.10.1 Considerationsfordataanalysisandinterpretationof of a method or combination of methods for estimating natural
estimatedattenuationratesarehighlydependentonthespecific attenuation rates at sites with NAPLin the subsurface with the
method, in addition to site-specific conditions. A solid under- following considerations:
E3361 − 22
TABLE 2 Summary of the Method Assumptions And Site-Specific
7.1.1 Site conditions are unique and in addition to technical
A
Considerations
challenges, the logistics of implementation and availability of
Method Underlying Assumptions Site Conditions
resources factor into the method selection, for example, access
B
CO Efflux • Attenuation of NAPL • Ground surface cover
to available technology, instruments, or laboratory services;
• Vegetation
constituents through
• High natural organics (for
knowledge and expertise for data interpretation; and computa-
biodegradation
example, peat)
• Complete mineralization of
tional resources.
• High permeability soils and
NAPL constituents to CO
barometric pumping
7.1.2 Ultimately, the application of any of the five methods
• Low gas permeability soils
•CO transport in soil gas
• Preferential pathways (for
presentedcanbeusedfordecision-makingtowardssustainable
from the source to the ground
example, utilities)
surface (point of
remediation and risk reduction.
measurement)
7.2 Applicabilityofeachmethoddependsontheunderlying • Background source: CO
produced from natural soil
assumptions in the context of site conditions (Table 2).
respiration
• Estimate the portion of CO
7.3 Multiplelinesofevidenceapproachthroughcomparison 2
efflux attributable to
of rates derived from different methods may increase level of
contaminant biodegradation
confidence in estimates.
Temperature • Attenuation of NAPL • Low gas permeability
Gradient constituents through aerobic surface cover that could limit
B
biodegradation and oxygen soil gas transport
8. Example Problems
• High natural organics (for
availability
example, peat)
• Production of biogenic heat
• Confined NAPL conditions
8.1 Thereareseveraltechniquesavailableforestimatingthe
from aerobic oxidation of
(Guide E2856)
natural attenuation rates depending on site conditions, avail-
hydrocarbons (notably
• Geologic or anthropogenic
methane) sources of heat not related to
able resources and remedial concerns. These techniques are
the NAPL
• Background correction for
categorizedintofivegeneralmethodsasdescribedinTable1of
heat exchange with the
this standard guide and described in detail in Appendix X1 to atmosphere and other
sources of heat in the
Appendix X5. This section provides practical, step-wise rec-
subsurface
ommendations for example implementation of these methods,
Soil Gas Gradi- • Spatial changes in soil gas • Low gas permeability sur-
ent composition – vertical profile face cover that could limit O
in addition to seven case studies of site-specific applications.
B
in the vadose zone resulting ingress
8.1.1 Exampleimplementationsforeachofthefivemethods
• Low gas permeability soils
from biodegradation of NAPL
• Soil gas advection from
are provided in Figs. 4-8, noting that there are other variations
constituents
barometric pumping effects or
• Vertical gradients in O ,
in technologies and approaches than these examples. These 2
high methane concentrations
CO , or hydrocarbon concen-
examplesandalternativeapproachesarecoveredintherespec-
trations in soil gas
tive Appendix X1 to Appendix X5.
• Diffusive gas transport in
the vadose zone
8.1.2 Fig. 4 shows an example of the CO Efflux Method
Groundwater • Spatial (up-and down- • Availability of groundwater
using the dynamic closed chamber (DCC) for CO efflux
Monitoring gradient of the source) monitoring data and hydro-
measurements. This is one of the various technologies de-
changes in the groundwater geologic parameters
chemistry including dissolved • Assessment of confined
scribed in Appendix X1.
gas concentrations resulting NAPL conditions (Guide
8.1.3 Fig. 5 shows an example approach for estimating the
from biodegradation of NAPL E2856) for data interpreta-
C
naturalattenuationratefromtemperaturemonitoringdata.This constituents in the saturated tion
zone
is one of the various approaches described in Appendix X2.
• Dissolution and flow of
8.1.4 Fig. 6 shows an example approach for estimating the
NAPL constituents in ground-
water
natural attenuation rate of bulk NAPL using soil gas monitor-
NAPL • Changes in the composition • Finite NAPL mass with no
ing data. This is one of the various approaches described in
Composition of NAPL constituents over additional releases during the
Appendix X3, which also includes approaches for estimating
time assessment period
• NAPL sampled consecu- • Availability of NAPL compo-
natural attenuation rates of specific COCs.
tively from a single location is sitional data over time (mini-
8.1.5 Fig. 7 shows an example approach for estimating the
representative of the same mum of
...