ASTM E2531-06(2020)

Standard Guide for Development of Conceptual Site Models and Remediation Strategies for Light Nonaqueous-Phase Liquids Released to the Subsurface

SIGNIFICANCE AND USE
5.1 This guide will help users answer simple and fundamental questions about the LNAPL occurrence and behavior in the subsurface. It will help users to identify specific risk-based drivers and non-risk factors for action at a site and prioritize resources consistent with these drivers and factors.  
5.2 The site management decision process described in this guide includes several features that are only examples of standardized approaches to addressing the objectives of the particular activity. For example, Table 1 provides example indicators of the presence of LNAPL. Table 1 should be customized by the user with a modified list of LNAPL indicators as technically appropriate for the site or group of sites being addressed.  
5.3 This guide advocates use of simple analyses and available data for the LCSM in Tier 1 to make use of existing data and to interpret existing data potentially in new ways. The Tier 1 LCSM is designed to identify where additional data may be needed and where decisions can be made using existing data and bounding estimates.  
5.4 This guide expands the LCSM in Tier 2 and Tier 3 to a detailed, dynamic description that considers three-dimensional plume geometry, chemistry, and fluxes associated with the LNAPL that are both chemical- and location-specific.  
5.5 This guide fosters effective use of existing site data, while recognizing that information may be only indirectly related to the LNAPL body conditions. This guide also provides a framework for collecting additional data and defining the value of improving the LCSM for remedial decisions.  
5.6 By defining the key components of the LCSM, this guide helps identify the framework for understanding LNAPL occurrence and behavior at a site. This guide recommends that specific LNAPL site objectives be identified by the user and stakeholders and remediation metrics be based on the LNAPL site objectives. The LNAPL site objectives should be based on a variety of issues, including:  
5.6.1 Potenti...
SCOPE
1.1 This guide applies to sites with LNAPL present as residual, free, or mobile phases, and anywhere that LNAPL is a source for impacts in soil, ground water, and soil vapor. Use of this guide may show LNAPL to be present where it was previously unrecognized. Information about LNAPL phases and methods for evaluating its potential presence are included in 4.3, guide terminology is in Section 3, and technical glossaries are in Appendix X7 and Appendix X8. Fig. 1 is a flowchart that summarizes the procedures of this guide.  
1.2 This guide is intended to supplement the conceptual site model developed in the RBCA process (Guides E1739 and E2081) and in the conceptual site model standard (Guide E1689) by considering LNAPL conditions in sufficient detail to evaluate risks and remedial action options.  
1.3 Federal, state, and local regulatory policies and statutes should be followed and form the basis of determining the remedial objectives, whether risk-based or otherwise. Fig. 1 illustrates the interaction between this guide and other related guidance and references.  
1.4 Petroleum and other chemical LNAPLs are the primary focus of this guide. Certain technical aspects apply to dense NAPL (DNAPL), but this guide does not address the additional complexities of DNAPLs.  
1.5 The composite chemical and physical properties of an LNAPL are a function of the individual chemicals that make-up an LNAPL. The properties of the LNAPL and the subsurface conditions in which it may be present vary widely from site to site. The complexity and level of detail needed in the LCSM varies depending on the exposure pathways and risks and the scope and extent of the remedial actions that are needed. The LCSM follows a tiered development of sufficient detail for risk assessment and remedial action decisions to be made. Additional data collection or technical analysis is typically needed when fundamental questions about the LNAPL cannot be answ...

General Information

• Status: Published
• Publication Date: 31-Oct-2020
• ICS: 13.080.05 - Examination of soils in general
• Technical Committee: E50 - Environmental Assessment, Risk Management and Corrective Action
• Drafting Committee: E50.04 - Corrective Action

Relations

• Refers: ASTM E2348-24 - Standard Guide for Framework for a Consensus-based Environmental Decision-making Process
• Effective Date: 01-Feb-2024
• Refers: ASTM E1903-19 - Standard Practice for Environmental Site Assessments: Phase II Environmental Site Assessment Process
• Effective Date: 15-Nov-2019
• Refers: ASTM D6235-18 - Standard Practice for Expedited Site Characterization of Vadose Zone and Groundwater Contamination at Hazardous Waste Contaminated Sites
• Effective Date: 15-Dec-2018
• Refers: ASTM E2091-17 - Standard Guide for Use of Activity and Use Limitations, Including Institutional and Engineering Controls
• Effective Date: 01-Nov-2017
• Refers: ASTM E2348-17 - Standard Guide for Framework for a Consensus-based Environmental Decision-making Process
• Effective Date: 01-Jan-2017
• Refers: ASTM E1739-95(2015) - Standard Guide for Risk-Based Corrective Action Applied at Petroleum Release Sites (Withdrawn 2024)
• Effective Date: 01-Apr-2015
• Refers: ASTM E1943-98(2015) - Standard Guide for Remediation of Ground Water by Natural Attenuation at Petroleum Release Sites
• Effective Date: 01-Apr-2015
• Refers: ASTM D653-14 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Aug-2014
• Refers: ASTM D653-11 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Sep-2011
• Refers: ASTM E2091-11 - Standard Guide for Use of Activity and Use Limitations, Including Institutional and Engineering Controls
• Effective Date: 01-May-2011
• Refers: ASTM E1739-95(2010)e1 - Standard Guide for Risk-Based Corrective Action Applied at Petroleum Release Sites
• Effective Date: 01-Sep-2010
• Refers: ASTM E1943-98(2010) - Standard Guide for Remediation of Ground Water by Natural Attenuation at Petroleum Release Sites
• Effective Date: 01-Sep-2010
• Refers: ASTM E2081-00(2010)e1 - Standard Guide for Risk-Based Corrective Action
• Effective Date: 01-Sep-2010
• Refers: ASTM E2348-06(2010) - Standard Guide for Framework for a Consensus-based Environmental Decision-making Process
• Effective Date: 01-Sep-2010
• Refers: ASTM D6235-04(2010) - Standard Practice for Expedited Site Characterization of Vadose Zone and Groundwater Contamination at Hazardous Waste Contaminated Sites
• Effective Date: 01-May-2010

Overview

ASTM E2531-06(2020) - Standard Guide for Development of Conceptual Site Models and Remediation Strategies for Light Nonaqueous-Phase Liquids (LNAPL) Released to the Subsurface provides a systematic approach to assess, model, and strategize remediation for sites impacted by light nonaqueous-phase liquids. LNAPLs, such as petroleum products, pose risks to soil, groundwater, and soil vapor, and may go unrecognized without proper evaluation.

This guide supports users in understanding the occurrence and behavior of LNAPLs beneath the surface and in developing effective remediation strategies. The standard emphasizes a tiered, risk-based approach to site characterization and decision-making, encouraging the use of existing data and identifying when further data collection is needed for informed remedial action.

Key Topics

• Conceptual Site Model (LCSM):

  • Provides a dynamic, three-dimensional understanding of the distribution, behavior, and evolution of LNAPLs at contaminated sites.
  • Supports identification and evaluation of exposure pathways and potential site risks.

• Tiered Approach to Site Assessment:

  • Tier 1: Utilizes simple analysis and available data to develop an initial LCSM and identify needs for further data.
  • Tier 2 & 3: Expands to detailed, location-specific analysis considering plume geometry, chemistry, and fluxes.

• Remediation Strategies:

  • Focuses on site-specific objectives, risk-based and non-risk drivers, and establishing appropriate remediation metrics.
  • Supports evaluation and prioritization of remedial options based on technical feasibility and regulatory requirements.

• Stakeholder Engagement:

  • Emphasizes the importance of involving stakeholders-owners, regulators, community, and professionals-in defining site objectives and decision-making.

• Framework for Data Collection and Evaluation:

  • Highlights methods to identify LNAPL presence, including direct and indirect site indicators.
  • Recommends updating the LCSM as new information is obtained during site management.

Applications

ASTM E2531-06(2020) is designed for use on sites where LNAPL is present in any phase-residual, free, or mobile-and may impact soil, groundwater, or vapor. The standard is particularly valuable for:

• Environmental Consultants & Site Managers:

  • Developing and refining LNAPL conceptual site models for impacted properties.
  • Designing effective, risk-oriented remediation strategies for petroleum release sites.

• Regulatory Compliance:

  • Supplementing risk-based corrective action (RBCA) approaches and meeting federal, state, or local environmental regulations.

• Risk Assessment and Decision Making:

  • Identifying exposure pathways, assessing risks to human health and ecological receptors, and establishing measurable site remediation goals.
  • Selecting appropriate remedial actions based on site-specific data and risk evaluations.

• Stakeholder Communication:

  • Facilitating transparent site assessments and remediation plans for regulators, property owners, and community stakeholders.

Related Standards

ASTM E2531-06(2020) is intended to be used alongside other key ASTM and regulatory standards to ensure thorough and compliant site assessment and remediation, including:

• ASTM E1689: Developing Conceptual Site Models for Contaminated Sites.
• ASTM E1739 & E2081: Risk-Based Corrective Action (RBCA) processes applied at petroleum release sites.
• ASTM E1903: Phase II Environmental Site Assessment Process.
• ASTM E1943: Remediation of Ground Water by Natural Attenuation at Petroleum Release Sites.
• ASTM D653, D6235: Terminology and site characterization practices for soil, rock, and groundwater contamination.
• EPA Method 8021B: Analysis of aromatic and halogenated volatiles.

By following the guidance in ASTM E2531, environmental professionals can ensure their approach to LNAPL assessment and remediation is consistent, efficient, and aligned with industry best practices for environmental protection and regulatory compliance.

Keywords: ASTM E2531, LNAPL, conceptual site model, remediation strategies, petroleum contamination, risk-based corrective action, subsurface contamination, environmental site assessment, regulatory compliance, risk assessment.

Standards Content (Sample)

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: E2531 − 06 (Reapproved 2020)
Standard Guide for
Development of Conceptual Site Models and Remediation
Strategies for Light Nonaqueous-Phase Liquids Released to
the Subsurface
This standard is issued under the fixed designation E2531; 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.
INTRODUCTION
This guide provides a framework for developing a light nonaqueous phase liquid (LNAPL)
conceptual site model (LCSM) and for using that LCSM in a corrective action decision framework.
LNAPLs are most commonly petroleum or petroleum products liquids. Historically, subsurface
LNAPL distribution has been conceptualized based on the thickness observed in monitoring wells.
However, these conceptualizations often result in an insufficient risk analysis and frequently lead to
poor remedial strategies. By using this guide, the user will be able to perform a more appropriate
assessment and develop an LCSM from which better remedial decisions can be made.
The design of this guide is a “tiered” approach, similar to the risk-based corrective action (RBCA)
process (Guides E1739 and E2081), where an increase in tiers results from an increase in the site
complexity and site-specific information required for the decision-making process.The RBCAguides
apply to LNAPL and to dissolved and vapor phases. This guide supplements the RBCA guides by
providing more information about identifying LNAPL, linking the LCSM to the RBCAprocess, and
describing how the presence of LNAPL impacts corrective action at sites.
In addition to developing the LCSM, the components of this guide will support the user in
identifying site objectives, determining risk-based drivers and non-risk factors, defining remediation
metrics, evaluating remedial strategies, and preparing a site for closure. If the processes in this guide
are adequately followed for sites with LNAPL, it is expected that more efficient, consistent,
economical, and environmentally protective decisions will be made.
1. Scope E1689)byconsideringLNAPLconditionsinsufficientdetailto
evaluate risks and remedial action options.
1.1 This guide applies to sites with LNAPL present as
residual, free, or mobile phases, and anywhere that LNAPL is
1.3 Federal, state, and local regulatory policies and statutes
a source for impacts in soil, ground water, and soil vapor. Use
should be followed and form the basis of determining the
of this guide may show LNAPL to be present where it was
remedial objectives, whether risk-based or otherwise. Fig.1
previously unrecognized. Information about LNAPL phases
illustrates the interaction between this guide and other related
and methods for evaluating its potential presence are included
guidance and references.
in 4.3, guide terminology is in Section 3, and technical
1.4 Petroleum and other chemical LNAPLs are the primary
glossaries are in Appendix X7 and Appendix X8. Fig.1 is a
focus of this guide. Certain technical aspects apply to dense
flowchart that summarizes the procedures of this guide.
NAPL(DNAPL),butthisguidedoesnotaddresstheadditional
1.2 Thisguideisintendedtosupplementtheconceptualsite
complexities of DNAPLs.
model developed in the RBCA process (Guides E1739 and
1.5 The composite chemical and physical properties of an
E2081) and in the conceptual site model standard (Guide
LNAPL are a function of the individual chemicals that
make-up an LNAPL. The properties of the LNAPL and the
subsurface conditions in which it may be present vary widely
ThisguideisunderthejurisdictionofASTMCommitteeE50onEnvironmental
Assessment, Risk Management and CorrectiveAction and is the direct responsibil-
from site to site. The complexity and level of detail needed in
ity of Subcommittee E50.04 on Corrective Action.
the LCSM varies depending on the exposure pathways and
Current edition approved Nov. 1, 2020. Published November 2020. Originally
risks and the scope and extent of the remedial actions that are
approvedin2006.Lastpreviouseditionapprovedin2014asE2531–06(2014).DOI:
10.1520/E2531-06R20. needed. The LCSM follows a tiered development of sufficient
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2531 − 06 (2020)
detail for risk assessment and remedial action decisions to be 1.14.2 Remedial actions taken should be protective of
made. Additional data collection or technical analysis is human health and the environment now and in the future.
typically needed when fundamental questions about the
1.14.3 Remedial actions should have a reasonable probabil-
LNAPL cannot be answered with existing information. ity of meeting the LNAPL site objectives.
1.14.4 Remedial actions implemented should not result in
1.6 This guide does not develop new risk assessment
greater site risk than existed before taking actions.
protocols.Itisintendedtobeusedinconjunctionwithexisting
1.14.5 Applicable federal, state, and local regulations
risk-based corrective action guidance (for example, Guides
should be followed (for example, waste management
E1739 and E2081) and regulatory agency requirements (for
requirements, ground water designations, worker protection).
example, USEPA 1989, 1991, 1992, 1996, 1997).
1.15 This guide is organized as follows:
1.7 ThisguideassiststheuserindevelopinganLCSMupon
1.15.1 Section 2 lists associated and pertinentASTM docu-
which a decision framework is applied to assist the user in
ments.
selecting remedial action options.
1.15.2 Section 3 defines terminology used in this guide.
1.8 The goal of this guide is to provide sound technical
1.15.3 Section 4 includes a summary of this guide.
underpinning to LNAPL corrective action using appropriately
1.15.4 Section 5 provides the significance and use of this
scaled, site-specific knowledge of the physical and chemical
guide.
processes controlling LNAPL and the associated plumes in
1.15.5 Section 6 presents the components of the LCSM.
ground water and soil vapor.
1.15.6 Section 7 offers step-by-step procedures.
1.9 This guide provides flexibility and assists the user in 1.15.7 Nonmandatory appendices are supplied for the fol-
developing general LNAPL site objectives based on the
lowing additional information:
LCSM. This guide recognizes LNAPL site objectives are
1.15.7.1 Appendix X1 provides additional LNAPLreading.
determined by regulatory, business, regional, social, and other
1.15.7.2 Appendix X2 provides an overview of multiphase
site-specific factors. Within the context of the Guide E2081
modeling.
RBCA process, these factors are called the technical policy
1.15.7.3 Appendix X3 provides example screening level
decisions.
calculations pertaining to the LCSM.
1.15.7.4 Appendix X4 provides information about data
1.10 Remediation metrics are defined based on the site
collection techniques.
objectives and are measurable attributes of a remedial action.
1.15.7.5 Appendix X5 provides example remediation met-
Remediationmetricsmayincludeenvironmentalbenefits,such
rics.
asfluxcontrol,riskreduction,orchemicallongevityreduction.
1.15.7.6 Appendix X6 provides two simplified examples of
Remediation metrics may also include costs, such as installa-
the use of the LNAPL guide.
tioncosts,energyuse,businessimpairments,wastegeneration,
1.15.7.7 Appendix X7 and Appendix X8 are glossaries of
waterdisposal,andothers.Remediationmetricsareusedinthe
technical terminology relevant for LNAPL decision-making.
decision analysis for remedial options and in tracking the
performance of implemented remedial action alternatives. 1.15.8 A reference list is included at the end of the docu-
ment.
1.11 This guide does not provide procedures for selecting
one type of remedial technology over another. Rather, it 1.16 Theappendicesareprovidedforadditionalinformation
and are not included as mandatory sections of this guide.
recommends that technology selection decisions be based on
the LCSM, sound professional judgment, and the LNAPL site
1.17 This standard does not purport to address all of the
objectives. These facets are complex and interdisciplinary.
safety concerns, if any, associated with its use. It is the
Appropriateuserknowledge,skills,andjudgmentarerequired.
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
1.12 This guide is not a detailed procedure for engineering
mine the applicability of regulatory limitations prior to use.
analysisanddesignofremedialactionsystems.Itisintendedto
1.18 This guide offers an organized collection of informa-
be used by qualified professionals to develop a remediation
tion or a series of options and does not recommend a specific
strategythatisbasedonthescientificandtechnicalinformation
course of action. This document cannot replace education or
contained in the LCSM. The remediation strategy should be
experienceandshouldbeusedinconjunctionwithprofessional
consistent with the site objectives. Supporting engineering
judgment. Not all aspects of this guide may be applicable in all
analysis and design should be conducted in accordance with
circumstances. This ASTM standard is not intended to repre-
relevant professional engineering standards, codes, and re-
sent or replace the standard of care by which the adequacy of
quirements.
a given professional service must be judged, nor should this
1.13 ASTM standards are not federal or state regulations;
document be applied without consideration of a project’s many
they are voluntary consensus standards.
unique aspects. The word “Standard” in the title of this
1.14 The following principles should be followed when
document means only that the document has been approved
using this guide: through the ASTM consensus process.
1.14.1 Data and information collected should be relevant to 1.19 This international standard was developed in accor-
and of sufficient quantity and quality to develop a technically- dance with internationally recognized principles on standard-
sound LCSM. ization established in the Decision on Principles for the
E2531 − 06 (2020)
Development of International Standards, Guides and Recom- time due to processes such as diffusion, dispersion, sorption,
mendations issued by the World Trade Organization Technical chemical degradation, and biodegradation.
Barriers to Trade (TBT) Committee.
3.1.3 chemicals of concern, n—specific chemicals that are
identified for evaluation in the corrective action process that
2. Referenced Documents
may be associated with a given LNAPL release and are a
2.1 ASTM Standards:
concern because of potential risk or aesthetic issues.
D653Terminology Relating to Soil, Rock, and Contained
3.1.3.1 Discussion—Identification can be based on their
Fluids
historical and current use at a site, detected concentrations in
D6235Practice for Expedited Site Characterization of Va-
environmental media and their mobility, toxicity, and persis-
dose Zone and Groundwater Contamination at Hazardous
tence in the environment. Because chemicals of concern may
Waste Contaminated Sites
be identified at many points in the corrective action process,
D5717Guide for Design of Ground-Water Monitoring Sys-
includingbeforeanydeterminationthattheyposeanunaccept-
tems in Karst and Fractured-Rock Aquifers (Withdrawn
able risk to human health or the environment, the term should
2005)
not automatically be construed to be associated with increased
E1689Guide for Developing Conceptual Site Models for
or unacceptable risk.
Contaminated Sites
3.1.4 conceptual model, n—integration of site information
E1739Guide for Risk-Based Corrective Action Applied at
and interpretations generally including facets pertaining to the
Petroleum Release Sites
physical,chemical,transport,andreceptorcharacteristicspres-
E1903Practice for Environmental Site Assessments: Phase
ent at a specific site.
II Environmental Site Assessment Process
3.1.4.1 Discussion—Aconceptual model is used to describe
E1912Guide forAccelerated Site Characterization for Con-
comprehensively the sources and chemicals of concern in
firmed or Suspected Petroleum Releases (Withdrawn
environmental media and the associated risks for particular
2013)
locations, both now and in the future, as appropriate, at a site.
E1943Guide for Remediation of Ground Water by Natural
3.1.5 corrective action, n—sequence of actions taken to
Attenuation at Petroleum Release Sites
address LNAPL releases, protect receptors, and meet other
E2081Guide for Risk-Based Corrective Action
environmental goals.
E2091Guide for Use of Activity and Use Limitations,
3.1.5.1 Discussion—Corrective actions may include site
Including Institutional and Engineering Controls
assessment and investigation, risk assessment, response
E2205Guide for Risk-Based Corrective Action for Protec-
actions,interimremedialaction,remedialaction,operationand
tion of Ecological Resources
maintenance of equipment, monitoring of progress, making
E2348Guide for Framework for a Consensus-based Envi-
no-further-action determinations, and termination of the reme-
ronmental Decision-making Process
dial action.
2.2 EPA Standard:
3.1.6 dense nonaqueous phase liquids (DNAPL),
EPAMethod 8021BAromatic and Halogenated Volatiles by
n—nonaqueousphaseliquidwithaspecificgravitygreaterthan
Gas Chromatography Using Photoionization and/or Elec-
one (for example, a chlorinated solvent, creosote, polychlori-
trolytic Conductivity Detectors
nated biphenyls).
3. Terminology
3.1.7 engineering controls, n—physical modifications to a
site or facility (for example, slurry walls, capping, and point-
3.1 Definitions—Definitions of terms specific to this stan-
of-use water treatment) to reduce or eliminate the potential for
dard are included in this section, with additional technical
exposure to LNAPLor chemicals of concern in environmental
terminology provided for reference in Appendix X7 and
media.
Appendix X8.
3.1.1 active remediation, n—actions taken to reduce or
3.1.8 entrapped LNAPL, n—residual LNAPLin the form of
control LNAPLsource flux or the concentrations of chemicals discontinuous blobs in the void space of a porous medium in a
of concern in dissolved- or vapor-phase plumes. Active reme-
submerged portion of a smear zone resulting from the upward
diation could be implemented when the no-further-action and movement of the water table into an LNAPL body.
passive remediation courses of action are not appropriate.
3.1.8.1 Discussion—At a residual condition, however, a
transient fall of the water table can result in local area
3.1.2 attenuation, n—the reduction in concentrations of
redistribution of LNAPL that is no longer in a residual
chemicals of concern in the environment with distance and
condition.
3.1.9 exposure pathway, n—course a chemical of concern
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
takes from the source area to a receptor or relevant ecological
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
receptor and habitat.
Standardsvolume information, refer to the standard’s Document Summary page on
3.1.9.1 Discussion—An exposure pathway describes the
the ASTM website.
The last approved version of this historical standard is referenced on
mechanism by which an individual or population is exposed to
www.astm.org.
a chemical of concern originating from a site. Each exposure
Available from United States Environmental Protection Association (EPA),
pathway includes a source or release from a source (for
Ariel Rios Bldg., 1200 Pennsylvania Ave., NW, Washington, DC 20460, http://
www.epa.gov. example, LNAPL released from a tank or pipeline), a point of
E2531 − 06 (2020)
exposure, an exposure route, and the potential receptors or engineered processes altering LNAPL body and ground water
relevantecologicalreceptorsandhabitats.Iftheexposurepoint and vapor plume conditions. The LCSM can be presented as
is not at the source, a transport or exposure medium (for text or figures, or both.
example, air), or both, are also included.
3.1.20 LNAPL properties, n—physical and chemical prop-
3.1.10 facility, n—property containing the source of the erties of a specific LNAPL.
LNAPL or chemical of concern where a release has occurred. 3.1.20.1 Discussion—Since many petroleum products are
3.1.10.1 Discussion—A facility may include multiple composed of multiple chemicals, and because of environmen-
sources and, therefore, multiple sites. tal interactions, both physical and chemical properties can be
quite variable between LNAPLs and over time for an LNAPL
3.1.11 flux, n—masscrossingaunitareaperunittimeinany
body at a site, as are the associated potential environmental
phase (for example, LNAPL, dissolved-phase, vapor-phase).
risks and amenability to different remedial actions.
3.1.11.1 Discussion—Mass flux controls the concentrations
potentiallyreachingreceptorsandaccountsforthedepletionof 3.1.21 LNAPL site objectives, n—specific set of well-
defined, desired outcomes that serve as a basis for remedial
LNAPLbodies through time. See Fig. 5 and Appendix X2 for
action.
more information.
3.1.21.1 Discussion—For instance, performing an appropri-
3.1.12 free LNAPL, n—LNAPL that is hydraulically con-
ate remedial action should protect human health and relevant
nected in the pore space and has the potential to be mobile in
ecological receptors and habitats. The corrective action goals
the environment.
defined under a RBCAprocess are a subset of the LNAPLsite
3.1.12.1 Discussion—Often exhibited by LNAPL accumu-
objectives. Remediation metrics (specific measurements of the
lations in wells. Free LNAPL exceeds the residual saturation.
results of the remedial action) are developed to be consistent
Not all free LNAPL is mobile LNAPL.
with the site objectives. Section 7.5 discusses the LNAPL site
3.1.13 institutional controls, n—legal or administrative re-
objectives in more detail.
striction on the use of, or access to, a property so as to
3.1.22 LNAPL type-area, n—type-area is a description,
eliminate or minimize potential exposure to a chemical of
which may include text, or figures or both, of the geologic,
concern(forexample,restrictivecovenants,restrictivezoning).
chemical, and LNAPL conditions for a sub-area of a site that
3.1.14 interim remedial action, n—remedial action taken in
represents, or may conservatively represent, the remainder of
the near-term before designing a final remedy to reduce
the site.
migration of chemicals of concern in the vapor phase, dis-
3.1.22.1 Discussion—Multiple type-areas may be defined
solved phase, or LNAPL, or to reduce the concentrations of
forlargesitesorsiteswithmultiplesources.Theintentofusing
chemicals of concern or the mass of LNAPL at a source area.
a type-area is to constrain key questions in adequate detail for
3.1.15 LNAPL, n—alightnonaqueousphaseliquidhavinga
the type-area, and then apply those findings elsewhere at the
specific gravity less than one and composed of one or more
site, as appropriate.
organic compounds that are immiscible or sparingly soluble in
3.1.23 mobile LNAPL, n—free LNAPL that is moving
water and the term encompasses all potential occurrences of
laterally or vertically in the environment under prevailing
LNAPL (for example, free, residual, mobile, entrapped). (See
hydraulic conditions.
Fig. 2.)
3.1.23.1 Discussion—TheresultoftheLNAPLmovementis
3.1.16 LNAPL body, n—three-dimensional form and distri- a net mass flux from one point to another. Not all free LNAPL
bution of LNAPL in the subsurface existing in all phases (for
is mobile, but all mobile LNAPL is free LNAPL.
example, free, residual, mobile, entrapped).
3.1.24 multi-component, n—refers to petroleum products or
3.1.17 LNAPL body footprint, n—two-dimensional form
other mixtures composed of many different individual chemi-
and distribution of LNAPL in the subsurface existing in all cals at varying molar fractions, such as in most petroleum-
phases (for example, free, residual, mobile, entrapped).
based fuels, solvents, petrochemicals, and other products.
3.1.18 LNAPL body state, n—status and conditions of the 3.1.25 natural attenuation, n—reduction in the mass or
LNAPL body now and in the future, including whether it is
concentration of chemicals of concern in environmental media
geographically stable, mobile, or recoverable. as a result of naturally occurring physical, chemical, and
3.1.18.1 Discussion—The estimates of vapor phase and
biological processes (for example, diffusion, dispersion,
dissolved phase flux from the LNAPL body are also included adsorption, chemical degradation, and biodegradation).
in the description of the LNAPL body state. It is a dynamic
3.1.26 non-risk factors, n—these are a subset of the desired
description of the LNAPL body used in risk assessment and
outcomes that determine the site objectives and they are not
remedial action evaluations.
strictly based on risks to human health or the environment,
3.1.19 LNAPLconceptual site model (LCSM), n—describes although they may have an impact on the risk at a site.
thephysicalproperties,chemicalcomposition,occurrence,and
3.1.26.1 Discussion—They are often determined by regula-
geologic setting of the LNAPL body from which estimates of tions or statutes that are applicable to a site. Examples of
flux, risk, and potential remedial action can be generated.
non-riskfactorsincludeeliminationofnuisanceconditionsand
3.1.19.1 Discussion—The LCSM should be a dynamic, reduction of LNAPL in wells. The non-risk factors should be
living conceptual model (see 3.1.4) that changes through time secondary to risk-based drivers at a site. Section 7.7 provides
as new knowledge is gained or as a result of natural or additional discussion of the non-risk factors.
E2531 − 06 (2020)
(After Guide E1739 and USEPA 2005 (Ref 1))
NOTE 1—The user is directed to Fig. 6 for details of the decision process beginning with identifying LNAPL site objectives.
FIG. 1 Summary of the LCSM Guide
3.1.27 petroleum, n—including crude oil or any fraction 3.1.27.1 Discussion—The term includes petroleum-based
thereof that is liquid at standard conditions of temperature and substances comprised of a complex blend of hydrocarbons
pressure. derived from crude oil through processes of separation,
E2531 − 06 (2020)
LNAPL = light nonaqueous phase liquid
COC = chemicals of concern
(From Huntley and Beckett 2002 (Ref 2))
NOTE 1—During the early stages of an LNAPL release, LNAPL can be mobile (free) in all zones.
NOTE 2—The schematic is intended to convey generalized zones, not the dynamics of an active LNAPL release.
FIG. 2 Illustration of LNAPL Zones

E2531 − 06 (2020)
LNAPL = light nonaqueous-phase liquid
(credit: John L. Wilson, 1990)
NOTE 1—Wettability aspects are discussed in Appendix X2.
FIG. 3 Illustration of Residual LNAPL (Immobile) as Identified in a Photomicrograph
conversion,upgrading,andfinishing(forexample,motorfuels, 3.1.30 point of exposure, n—point at which an individual or
jet oils, lubricants, petroleum solvents, and used oils). population may come in contact with a chemical of concern
originating from a site.
3.1.28 plume stability, n—lack of significant geographic
movement in the dissolved phase or vapor phase.
3.1.31 reasonably anticipated future use, n—futureuseofa
3.1.28.1 Discussion—The significance of the movement
site or facility that can be predicted with a high degree of
wouldtypicallybemeasuredatascalepertinenttoLNAPLsite
certainty given current use, local government planning, and
objectives. For example, if a receptor is nearby, then stability
zoning.
would be demonstrated at a finer-scale than if a receptor is at
3.1.32 receptors, n—persons that are or may be affected by
a more distant location in order to meet the LNAPL site
a release (see relevant ecological receptors and habitats for
objectives. Different phases can have different stability condi-
non-human receptor definition).
tions. For example, the LNAPL body may be geographically
3.1.33 recover ability, n—general term for the degree to
stable,butdissolved-phasefluxemanatingfromthatbodymay
which LNAPL can be removed from the subsurface, often
not be stable.
definedasthefractionofthetotalinsituLNAPLmassorofthe
3.1.29 point of compliance, n—location selected between
free or residual volumes.
the source area and the potential point of exposure, or other
3.1.33.1 Discussion—The recoverability is a function of the
relevant location, where remediation metrics are demonstrated
in situ LNAPL conditions, the hydrogeologic setting, the type
to be met (for example, concentrations of chemical of concern
oftechnologytobeused,andthemannerinwhichitisapplied.
at or below the determined site-specific target levels).
3.1.34 release area, n—area in and around the location
3.1.29.1 Discussion—Depending on site conditions, mul-
where LNAPL was first released to the subsurface.
tiple points of compliance may be selected for one source area
and point of exposure. 3.1.34.1 Discussion—The source zone is the subsequent
E2531 − 06 (2020)
NOTE 1—This is an example list that is not exhaustive, the boundary between tiers is subjective and based on user judgment.
NOTE 2—(Concept after Sale 2002. (Ref 3))
FIG. 4 Example Factors Affecting LCSM Complexity (see also Table 2)

E2531 − 06 (2020)
(After Huntley and Beckett 2002 (Ref 2))
FIG. 5 Fluxes in Groundwater and Vapor Emanate from the LNAPL

E2531 − 06 (2020)
subsurface distribution of LNAPL that forms the source term people in indoor environments and controlling ground water
for dissolved- and vapor-phase plumes, as applicable. migration to protect drinking water wells. The risk-based
drivers should generally be the priority, while recognizing
3.1.35 relevant ecological receptors and habitats,
other factors exist as well.
n—ecological resources that are valued at the site.
3.1.41 risk reduction, n—lowering or elimination of the
3.1.35.1 Discussion—Identification of relevant ecological
receptors and habitats is dependent on site-specific factors and level of risk posed to human health or relevant ecological
receptors and habitats through interim remedial action, reme-
technical policy decisions. Examples may include species or
communities afforded special protection by law or regulation; dial action, or institutional or engineering controls.
recreationally,commercially,orculturallyimportantresources;
3.1.42 site, n—area defined by the likely physical distribu-
regionally or nationally rare communities; communities with
tion of LNAPL and chemicals of concern from a source.
high aesthetic quality; and habitats, species, or communities
3.1.42.1 Discussion—A site could be an entire property or
thatareimportantinmaintainingtheintegrityandbio-diversity
facility, a defined area or portion of a facility or property, or
oftheenvironment.SeeGuideE2205foradditionaldiscussion.
multiple facilities or properties. One facility may contain
multiple sites. Multiple sites at one facility may be addressed
3.1.36 remedial action/remediation, n—activities conducted
individually or as a group.
to protect human health, safety, and the environment.
3.1.36.1 Discussion—Included in remedial actions are
3.1.43 site assessment, n—characterization of a site through
monitoring programs, activity and use limitations, engineering
an evaluation of its physical and environmental context (for
controls and active clean up systems.Associated with each of
example, subsurface geology, soil properties and structures,
the remedial actions are the applicable implementing, operat-
hydrology,andsurfacecharacteristics)todetermineifarelease
ing and monitoring tasks. Remedial actions include activities
has occurred, including the levels of the chemicals of concern
that are conducted to recover LNAPL, reduce fluxes of
in environmental media, the likely physical distribution of
chemicals of concern from the LNAPL, reduce sources of
LNAPLandchemicalsofconcern,andLNAPLcharacteristics.
exposure, sever exposure pathways, or make other changes to
3.1.43.1 Discussion—As an example, the site assessment
meet LNAPL site objectives.
collects data on soil, ground water and surface water quality,
land and resource use, potential receptors, and potential rel-
3.1.37 remediation metric, n—specific measurement associ-
evant ecological receptors and habitats. It also generates
ated with progress or performance of a remedial action.
information to develop the LCSM and to support corrective
3.1.37.1 Discussion—Remediation metrics can be cost met-
action decision-making. The user is referred to Guide E1912
rics or benefit metrics. For example, if chemical flux reduction
and Practice D6235, and other references in Appendix X1 for
to a receptor were an LNAPL site objective, measurements of
more information.
flux before, during, and after remediation would be a metric of
that remedial action. Other remediation metrics might be a
3.1.44 site-specific, adj—activities, information, and data
measurement to determine the minimum mobility potential for
unique to a particular site.
observableLNAPL,amaximumallowableconcentrationofan
3.1.45 smear zone, n—zoneinandaroundthehistoricwater
LNAPL chemical of concern at a point of compliance, or a
table where there is residual and potentially free LNAPL that
percentile of the potentially recoverable LNAPL.
may be above or below the current water table.
3.1.38 residual LNAPL, n—LNAPL that is hydraulically 3.1.45.1 Discussion—The smear zone results from fluctua-
discontinuous and immobile under prevailing conditions.
tions of the water table and redistribution of free LNAPL in
3.1.38.1 Discussion—Residual LNAPL that cannot move that zone at sometime in the past or present.
through hydraulic mechanisms (unless prevailing conditions
3.1.46 source zone, n—three-dimensional zone in the sub-
change), but is a source for chemicals of concern dissolved in
surface associated with the release area where LNAPL acts as
ground water or in the vapor-phase in soil gas. The residual
source for dissolved-phase and vapor-phase plumes of chemi-
LNAPL saturation is a function of the initial (or maximum)
cals of concern.
LNAPL saturation and the porous medium. (See Fig. 3.)
3.1.47 stakeholders, n—individuals, organizations, or other
3.1.39 risk assessment, n—analysis of the potential for
entities that directly affect or are directly affected by a
adverse human health effects or adverse effects to ecological
corrective action.
receptors and habitats caused by the LNAPL or chemicals of
3.1.47.1 Discussion—Stakeholders include, but are not lim-
concern from a site to determine the need for remedial action
ited to, owners, buyers, developers, lenders, insurers, govern-
or the development of LNAPL site objectives (for example,
ment agencies, and community members and groups.
corrective action goals under a RBCA process) in which
3.1.48 user, n—individualorgroupusingthisLNAPLguide
remedial action is required.
including owners, operators, regulators, underground storage
3.1.40 risk-based drivers, n—these are remedial require-
tank (UST) fund managers, federal or state government case
ments that are based solely on the potential risk to human
managers, attorneys, consultants, legislators, and other stake-
health or ecological receptors and habitats, as compared to
holders.
remedial requirements based on other factors (for instance,
4. Summary of Guide
nondegradation of ground water).
3.1.40.1 Discussion—Examples of risk-based drivers in- 4.1 This LNAPL guide assists in developing an LCSM for
clude reduction of vapor-phase concentrations to protect making site management decisions. Fig. 1 and the following
E2531 − 06 (2020)
TABLE 1 Example LNAPL Indicators
NOTE 1—Items 1 through 3 are direct indicators of LNAPL presence.
NOTE 2—Items 4 through 9 are indirect indicators of potential LNAPL presence.
NOTE 3—The user is encouraged to include additional indicators, as needed.
NOTE4—PositiveresponsesonindirectindicatorsincreasethelikelihoodofthepresenceofLNAPL;additionaltestingshouldbeconductedtoconfirm
LNAPL presence.
NOTE5—Foranymeasurementdevicethereliabilityoftheequipmentshouldbeunderstood(forexample,rateoffalsenegatives,rateoffalsepositives)
in order to interpret the results.
Measures Yes/No Site Information
1. Known LNAPL release
2. Observed LNAPL (for example, in wells or other discharges)
3. Visible LNAPL or other direct indicator in samples
4. Fluorescence response in LNAPL range
5. Near effective solubility or volatility limits in dissolved or vapor phases.
6. Dissolved plume persistence and center-of mass stability
7. TPH concentrations in soil or groundwater indicative of LNAPL presence
8. Organic vapor analyzer (OVA) and other field observations
9. Field screening tests positive (for example, paint filter test, dye test, shake test)
sections summarize the procedure. The figure and text may 4.3.3 A schematic of different LNAPL occurrences consid-
indicate a linear process; however, as additional data are ered by this guide is shown in Fig. 2. A photomicrograph
collected, remedial action is conducted, and knowledge is showing observed residual, immobile LNAPLin soil is shown
gained about the LNAPL and the site, the LCSM should be in Fig. 3.
updated and the evaluation processes revisited to incorporate
4.4 Develop aTier 1 LCSM based on available information
this new information.
and procedures outlined in this guide. Table 2 is an example
4.2 Ensure that immediate or eminent threats and hazards
evaluation that provides information to identify the potential
are mitigated. These are conditions such as explosive vapors,
level of complexity that may be needed for the LCSM. If key
flammablematerials,orotherthreateningconditions.Stateand
elements of the LCSM cannot be developed because of an
local regulations and other guidance materials address these
absence of information, and those elements are necessary to
facets, as warranted.
estimate risks to human health or ecological receptors and
habitats, then either additional data collection or a remedial
4.3 Define the presence or absence of LNAPL based on
action is warranted.
existing data, if applicable. Table 1 presents some example
indicatorsthatindividually,orincombination,maysuggestthe
4.5 Determinewhetherimmediateresponseactionsorinitial
presence of LNAPL at a given site. These are examples only;
remedial actions are needed based on Guides E1739 and
the list is not comprehensive.The user may develop additional
E2081, and federal, state, and local regulations and policies.
LNAPL screening indicators as technically appropriate. This
4.6 Determine the appropriate activities for stakeholder
guide is pertinent to all occurrences of LNAPL, including
involvement and public participation for the site, see Guide
conditions where it is observable in monitoring wells and
E2348 and USEPA 2005 (1) for additional information.
whereitisnotvisible,butratherheldbycapillaryforcesinthe
pore space.
4.7 DetermineiftheTier1LCSMisadequatetoanswerrisk
4.3.1 LNAPL, where present, is typically the source zone
questions and remedial action questions. Collect additional
for dissolved- and vapor-phase plumes (that is, assuming that
information and upgrade to a Tier 2 LCSM, if appropriate, or
the chemicals of concern that are dissolved in ground water or
alternatively, elect to perform a remedial action. For theTier 2
are volatilized to soil vapor are components of the LNAPL).
LCSM, define the LNAPL type-area based on LNAPL
The LNAPL is often conceptualized as an infinite mass with
occurrence, characteristics of the chemicals of concern, and
respect to the dissolved and vapor phases; additional back-
physical properties of the soil and rock. Guide E1903 contains
ground is included in Appendix X2 and Appendix X4. While
additional information about environmental site assessments.
the infinite mass concept is useful, it is clear that the LNAPL
4.8 Determine whether risks to human health or ecological
is in fact a finite mass that will change in character through
receptors or habitats are present using the site-specific LCSM
time as a result of natural processes and remedial actions.
and the RBCA process detailed in Guides E1739 and E2081.
4.3.2 Dissolved- and vapor-phase concentrations of chemi-
Identify the risk-based drivers for the LNAPL site objectives
cals of concern, which are components of the LNAPL, will
(for example, risk-based screening levels (RBSL), site-specific
remain elevated and be complexly and non-linearly related to
the concentration or saturation of LNAPL until the amount of
LNAPL remaining is less than the mass capacity in other
phases (for example, sorbed, dissolved, vapor). When LNAPL
The boldface numbers in parentheses refer to a list of references at the end of
ceases to be present, this guide no longer applies. this standard.
E2531 − 06 (2020)
TABLE 2 Example LNAPL Conceptual Site Model Adequacy Checklist
NOTE 1—The use of the scoring is site- and regulation-specific.As the complexity of the site increases, the benefit of a detailed LSCM increases.This
table is designed to help the user identify what level of complexity, or what tier, for the LSCM is likely to be beneficial to the site. See also Fig. 4.
NOTE 2—The factors should be used to develop a weight-of-evidence to suggest the level of complexity for the LSCM. Sites that have a majority of
low scores on the factors would likely fall into a Tier 1 LSCM; sites with mostly low and medium scores on the factors would fall into a Tier 2 LSCM;
sites with mostly medium and high scores would fall into a Tier 3 LSCM.
NOTE 3—The user is encouraged to include additional, site-specifc factors as needed.
Data
Factors Score Site Information
Available
Potential Risk Factors
1. Exposure pathways complete H/M/L Y/N
1a. Risk magnitudes H/M/L Y/N
1b. Toxicity H/M/L Y/N
1c. Sensitive receptors H/M/L Y/N
2. Business issues H/M/L Y/N
3. Community issues H/M/L Y/N
Hydrogeologic and Plume Factors
4. Chemicals of concern H/M/L Y/N
4a. Degradation H/M/L Y/N
4b. Persistence H/M/L Y/N
5. Plume characteristics H/M/L Y/N
5a. Plume COC/mass distribution H/M/L Y/N
5b. Offsite plume H/M/L Y/N
5c. Uncertainty in LNAPL body H/M/L Y/N
6. Geologic complexity H/M/L Y/N
6a. Conductivity/ grain-size H/M/L Y/N
6b. Degree of heterogeneity H/M/L Y/N
6c. Uncertainty in hydrogeologic condi- H/M/L Y/N
tions
Remediation Factors
9. Groundwater classification H/M/L Y/N
11. Land use H/M/L Y/N
12. Challenges of remediation H/M/L Y/N
13. Cost of remediation H/M/L Y/N
14. Uncertainty in remediation H/M/L Y/N
Applicable factors
Total score
target levels (SSTL), other relevant measurable criteria benefits and costs of the considered LNAPL remedial action
(ORMC)). See Guide E2081 for further information about options and the overall site context of site objectives and
risk-based drivers. remediation metrics.
4.9 Determineiftherearenon-riskfactors,inadditiontothe 4.13 Use appropriate technical resources to properly design
risk-based drivers, for the LNAPLsite objectives and remedial andinstalltheremedialactionelementswithintheremediation
action. strategy.Theseremedialengineeringaspectsarenotcoveredin
this guide.
4.10 Enumerate the LNAPL site objectives for the risk-
based drivers and non-risk factors in adequate detail such that 4.14 Monitor the remedial action systems; verify the reme-
a remediation strategy may be developed based on the LCSM. diation metrics are met.
Define the remediation metrics and determine which remedial
4.15 Complete the remedial action or implement long-term
action alternatives may be suitable to achieve the LNAPL site
monitoring and site management, depending on site context
objectives.TheLNAPLsiteobjectivesandremediationmetrics
and the remedial action evaluations described in this guide.
should be consistent with the overall site context and other
management or remedial goals that may exist for conditions 5. Significance and Use
other than the LNAPL and associated plumes.
5.1 Thisguidewillhelpusersanswersimpleandfundamen-
4.11 Develop a higher tier LCSM or revise LNAPL site tal questions about the LNAPLoccurrence and behavior in the
objectives if none of the remedial action options appears to subsurface. It will help users to identify specific risk-based
address the LNAPL site objectives, or if there is unacceptable drivers and non-risk factors for action at a site and prioritize
uncertainty in the LNAPL remedial action evaluation. resources consistent with these drivers and factors.
4.12 Developaremediationstrategyusingaremedialaction 5.2 The site management decision process described in this
option, or set of options. The remediation strategy should be guide includes several features that are only examples of
holisticinthatitaddressestherisksandconsiderschemicalsof standardized approaches to addressing the objectives of the
concerninthesolublephase,thevaporphase,andtheLNAPL. particular activity. For example, Table 1 provides example
The remediation strategy is based on the evaluation of the indicators of the presence of LNAPL. Table 1 should be
E2531 − 06 (2020)
customized by the user with a modified list of LNAPL 5.12 Current risk assessment methods often assume the
indicators as technically appropriate for the site or group of LNAPL is an infinite source of chemicals of concern. The
sites being addressed. remediation decision-making may be better defined by consid-
ering the LNAPL as the source material for chemicals of
5.3 This guide advocates use of simple analyses and avail-
concern by explicitly characterizing the chemical composition
able data for the LCSM in Tier 1 to make use of existing data
and physical characteristics of the LNAPL body.
andtointerpretexistingdatapotentiallyinnewways.TheTier
1 LCSM is designed to identify where additional data may be 5.13 When LNAPL presents the main source of risk, the
needed and where decisions can be made using existing data
LNAPL should be the primary target of remedial actions and
and bounding estimates. those remedial actions should be determined by following the
decision evaluations described in this guide.
5.4 This guide expands the LCSM in Tier 2 and Tier 3 to a
detailed, dynamic description that considers three-dimensional
5.14 LNAPL regulatory policies that define remediation
plume geometry, chemistry, and fluxes associated with the metrics by small LNAPL thicknesses in wells are, on a
LNAPL that are both chemical- and location-specific.
site-specific basis, often inconsistent with risk-based screening
levels (RBSLs) and with current technical knowledge regard-
5.5 This guide fosters effective use of existing site data,
ing LNAPL mobility and recoverability. LNAPL remediation
while recognizing that information may be only indirectly
metrics should be connected to the current or potential future
related to the LNAPL body conditions. This guide also
exposuresandrisks,aswellastoothernon-riskdriverspresent
provides a framework for collecting additional data and defin-
for a particular site.
ing the value of improving the LCSM for remedial decisions.
5.15 The user of this guide is encouraged to identify the
5.6 By defining the key components of the LCSM, this
appropriate process for public involvement and stakeholder
guide helps identify the framework for understanding LNAPL
participation in the development of the LCSM and the site
occurrence and behavior at a site.This guide recommends that
management decision process.
specific LNAPL site objectives be identified by the user and
stakeholders and remediation metrics be based on the LNAPL
5.16 By providing a flexible framework, this guidance will
site objectives.The LNAPLsite objectives should be based on
continue to be applicable in principle while the many un-
a variety of issues, including:
knowns and uncertainties in LNAPL movement and the asso-
5.6.1 Potential human health risks and risks to relevant ciated risks in all plume phases (for example, sorbed,
dissolved, vapor) are studied through future resear
...