Appendix I – SVE Technology Description

ESTCP (2002) Air Sparging Design Paradigm

ESTCP (2002) based their design paradigm on the conclusion that long-term air sparging performance (cleanup levels and times) cannot be predicted reliably from data collected during short-term pilot tests. As a result, given the importance of air distribution and our inability to predict it, ESTCP proposed an approach where the actual air distribution in the target treatment zone is characterized during the pilot-testing and full-scale implementation phases. Because of their approach, pilot testing and monitoring should be understood in context of the overall design paradigm, which consists of five main steps:

Site characterization and development of site conceptual model
Preliminary feasibility assessment
Pilot testing
Design
Monitoring

The pilot testing is divided into two approaches, 1) standard design approach and 2) site-specific design approach. For each of these two approaches there are default designs and recommended monitoring. The flow chart for air sparging design including pilot testing is shown in Figure B-1.

The standard design approach recommends the following data collection: injection pressure versus flow rate test, transient pressure response test, dissolved oxygen measurements, helium tracer test and soil gas sampling. Based on this data set, injection wells based on 15 ft centers and 20 ft3/min injection rate are recommended. The site-specific design approach recommends the standard methods plus sulphur hexafluoride tracer test to assess air distribution more accurately.

This appendix provides an overview of the technology, site conditions conducive to SVE, generalized site types for applicability of SVE, and SVE design.

Overview of Technology

SVE is the application of a vacuum to unsaturated (vadose zone) soils to induce advective soil vapor flow toward extraction wells and remove volatile and some semi-volatile contaminants from soil. Once removed from the soil, volatile organic compounds (VOCs) are typically removed or destroyed using an aboveground treatment process such as granular activated carbon, or catalytic or thermal oxidation (US EPA 2006). SVE is primarily a composition-based remedy because higher volatility VOCs are removed at greater rates than lower volatility VOCs. Consequently, SVE can be used to target remediation of lighter COCs such as benzene.⁴

The phase change and mass removal typically decrease during the treatment life cycle. During early stages of remediation, the primary mass removal is from air pathways of low resistance (higher permeability soils), where chemicals in adsorbed phase or non-aqueous phase liquids (NAPL) partition into the moving air. Contaminants in lower permeability soils will not be removed by advection because soil vapors will preferentially flow through higher permeability soils. When the mass in higher permeability soils becomes mostly depleted, the rate of mass removal may approach a low value or cumulative mass recovery may approach an asymptotic value. This is because contaminants in lower permeability silts and clays and within the capillary fringe, if present, must desorb and diffuse into an advective flow path before they can be removed by the SVE system. If the rate of diffusion is slow, the time duration for removing VOCs may be significantly extended (US EPA 1991). US EPA (2018) describes a two-compartment model (coarse- and fine-grained) for prediction of concentrations and clean-up times when there are mass transfer limitations resulting from slow diffusive transport in fine-grained soil layers.

The addition of air through SVE may result in enhanced aerobic biodegradation and consequently SVE is closely related to bioventing (refer to Bioventing Factsheet). Because there is often contamination at or below the water table, remediation through air sparging will often be an additional component to the SVE system.

Exit Strategy Toolkit

Compendium

A Roadmap to the Application of the Exit Strategy Toolkit

The Compendium serves as a general framework and contextual document for the Remediation Factsheets, which provide detail on the implementation of specific remedial technologies. The Compendium is presented as a generic four stage process for remedial decision making (Figure 1), as follows:

Stage 1: Risk Evaluation and Identification
Stage 2: Baseline Assessment
Stage 3: Remedy Selection and Implementation
Stage 4: Transition Assessment and Validation.

The development of a conceptual site model (CSM) is an essential and dynamic component of the LNAPL site management process. A CSM sufficient to identify and evaluate risk-drivers must be completed prior to Stage 1. The CSM is updated and refined throughout the site investigation and remediation process, with a goal of identifying and managing current and future risks to human health and the environment. The CSM generally involves characterizing sources, exposure pathways and receptors, and assessing concentration trends for COCs in soil, groundwater, soil gas, and LNAPL. The level of data collection and analysis typically corresponds with the level of site risk and complexity. Guidance on developing good CSMs is provided in ASTM E2531 (2020), CRC CARE (2015), and ITRC (2018).

The overarching objective of the Exit Strategy Toolkit (Compendium + Factsheets) is to provide practical measures for sustainable remediation of sites. It is specifically intended to optimize resource use, while managing risk to human health and the environment. Sustainable remediation includes limiting energy use, cost, and GHG emissions without compromising the risk management of the site by assessing and implementing natural (or enhanced) remedies over active (or engineered) remedies. This includes timely transitions from active to passive remediation.

Stage 1 - Risk Evaluation & Identification

Most regulatory jurisdictions have well-defined and prescriptive processes for evaluating and identifying risk, thus only a brief overview is provided here. Numerous tools have also been published to aid risk evaluation and identification, including NRC (2003), Adamson and Newell (2014), and Golder (2016 and 2021). It is important to note that active remediation may not be required simply because COC concentrations exceed groundwater quality standards or LNAPL is present in a monitoring well. The key for Stage 1 is to ascertain from the CSM whether a) an acute risk is present warranting rapid response, b) the site can be screened out (i.e., meets regulatory criteria), or c) active remediation is necessary (Figure 1).

Establish remedial concerns and goals

Remedial concerns and remedial goals should be agreed upon and aligned upfront with all key stakeholders prior to the selection and implementation of any active remediation (ITRC 2018; CRC CARE 2018, 2020; and Golder 2021). ITRC defines four types of remedial concerns:

pose a risk to health or safety (composition-based),
result in additional LNAPL migration (saturation-based),
address an LNAPL-specific regulatory requirement (regulatory-based), or
create some other physical or aesthetic impact or other specific regulatory or stakeholder requirements.

Mass-based Estimates

Mass-based estimates may represent a more accurate indication of risk than concentration criteria (ITRC 2012). Understanding how they are defined is important, which are:

mass flux – mass per unit area per time
mass discharge – mass per time crossing plane of interest, and
mass loading – mass per time at intersection of plume and receptor.

Mass estimates require characterization of hydrogeological and geologic processes and understanding of advective and diffusive transport processes.

Example tools include GSI Mass Flux Toolkit, which enables estimation of dissolved-phase mass discharge based on transect of groundwater concentration data, and the BioVapor model, which includes estimation of vapor-phase mass loading into a building

For most sites, one must decide whether the remedial concerns are saturation (e.g., LNAPL thickness and total hydrocarbon mass) and/or composition based (e.g., human health exposure to COC above risk-based concentrations). The remedial goal ties directly to the remedial concern, for example to abate LNAPL migration or reduce constituent concentrations below a target level at a point of compliance. The remedial goal also informs a) the choice of remedial technology, b) the criteria (e.g., baseline assessment, performance metrics, transition thresholds), c) associated tools and analyses, and d) data needs. Examples of remedial concerns and goals and their linkage are provided in Table 1. In many cases, the remedial concerns are both saturation- and composition-based. Such concerns are commonly addressed through a treatment-train approach involving the initial application of a saturation-based remedy, such as hydraulic recovery, to remove LNAPL to a practical limit of hydraulic recovery, followed by the application of a composition-based remedy, such as soil-vapor extraction (SVE) or air sparging (AS) designed to reduce remaining hydrocarbon mass for key COCs present in the subsurface below levels of concern at points of compliance. An active system may also be selected with a goal to transition to a passive system (e.g. monitored natural attenuation (MNA)), and ultimately site closure or no further action (NFA).

Figure 1 – Recommended site management process. In Stage 3, an active or passive system is selected based on site assessments in Stages 1 and 2; prior to the implementation of an active remediation system, performance metrics and transition criteria are defined and used to monitor and optimize the system; change to an alternate active system or transition to passive represent the treatment train; In Stage 4, following the system implementation and optimization, the transition criteria are used to assess and validate the transition options or continued operation. No Further Action in Step 4 may constitute site closure, indicating the process is completed with respect to managing site risks and concerns.

Remedial Concerns	Primary Mechanism	Remedial Goal	Typical Remedial Technologies
Saturation: Presence of migrating or mobile LNAPL	Mass removal or recovery (bulk removal/recovery of free-phase mass)	Abate LNAPL migration or reduce LNAPL mobility	Hydraulic Recovery Multi-Phase Extraction
Composition: Concentrations or mass discharge/loading exceeding health-based criteria (human or ecological)	Phase Change and Mass Reduction (removal of constituents based on differing physico-chemical properties and partitioning between source, soil, vapor, and groundwater media)	Reduce concentrations or mass discharge/loading	Soil Vapor Extraction Bioventing Air (and Bio) Sparging Monitored Natural Attenuation (including NSZD) Multi-Phase Extraction

Table 1 - Examples of remedial concerns and goals to address the risks identified in Stage 1 of the site management process.

The remedial concerns and goals should be developed to facilitate stakeholder engagement and input in decision making by factoring the diversity of technical, economic, business, and social drivers. Although certain active remediation systems can address both saturation- and composition-based concerns (e.g., SVE or AS), the goals of active remediation should be defined independently. A distinct set of goals for the active remediation system allows for the system optimization as well as transitioning to passive (Stages 3 and 4 in Figure 1). In addition, saturation or bulk mass removal targets (e.g., x kg/day total recovery) generally affect the minimum timeline to reach clean-up goals rather than directly impacting the concentrations of COCs above levels. It is also important to note that active remediation may not be required simply because COC concentrations exceed groundwater quality standards or LNAPL is present in a monitoring well. For example, certain US states have invoked policies that preclude the need for active remediation at lower risk sites where certain conditions related to groundwater plume stability, COC concentrations, and source-receptor separation distance are met (Florida DEP, 2005; California SWRCB, 2012). Active remediation may also not be required if COC concentrations never exceed drinking water standards in supply wells located downgradient based on mass-flux (or discharge) principles (Farhat et al., 2011).

Stage 2: Baseline Assessment

A baseline assessment of natural attenuation rates is a critical step used to:

benchmark rates of hydrocarbon mass loss (e.g., bulk LNAPL depletion or COC attenuation);
estimate minimum timelines for natural and enhanced depletion;¹
inform remedy selection in Stage 3 (active or passive); and
support termination of active remediation in Stage 4

Baseline Assessment: Natural Attenuation Rate Measurements

The emerging research indicates that natural attenuation including natural source zone depletion (NSZD) is ubiquitous at LNAPL sites and can occur at rates that rival those achieved by some active remediation systems. For example, reported NSZD rates typically range from 700 to 2,800 US gal/acre/yr (Garg et al. 2017; middle 50% of data). In a more recent review of NSZD rates across 40 sites, Kulkarni et al (2022) report that 90% of sites had site-average NSZD rates greater than 170 gal/acre/yr. Consequently, an understanding of natural attenuation (including NSZD) rates is an integral component of a sustainable remedial strategy (CL:AIRE 2019).

The focus and scope of the baseline assessment depends on the remedial concern (Figure 1). For a saturation-based concern, the attenuation rates of “bulk” LNAPL (e.g., NSZD rates) are typically of interest. For a composition-based concern, the attenuation rates of key COCs driving site risk are typically of interest. Natural attenuation rate estimates can be made from existing site data (e.g., evaluation of groundwater monitoring data using tools such as GWSdat) or from data collected as part of an additional site investigation (e.g., surface CO₂ measurements, analysis of data from installed thermistors or vapor probes, etc.). These concepts, methods, tools, analyses, and associated data needs are described in greater detail in a Factsheet on Natural Attenuation. The Factsheet provides resources for estimating natural attenuation rates including the ASTM Standard Guide E3361 (2022).

Stage 3: Remedy Selection & Implementation

Remedy evaluation and selection should link the remedial goals to the technology being considered (Table 1 and Figure 1). Active remedies that address only a saturation-based concern should target bulk hydrocarbon removal. Such remedies include excavation and hydraulic recovery. Active remedies that address both saturation and composition-based concerns should target both bulk hydrocarbon removal and concentrations of COCs in the subsurface (e.g., multi-phase extraction – MPE). Such mass removal remedies generally involve physical phase changes. Active remedies that primarily address composition-based concerns also involve phase changes along with biodegradation, or abiotic reactions (e.g., SVE, air sparging, bioventing, biosparging, and in-situ chemical oxidation). Again, remedial goals may be addressed through a “treatment train” approach where multiple remediation technologies are implemented sequentially.

The selection of an active remediation system should consider amongst other factors:

sustainable practices;
the established remedial timeframe; and
baseline natural attenuation rates determined in Stage 2.

Key guidance and resources for remedy selection include the ITRC LNAPL Guidance (ITRC 2018), the Remediation Technologies Screening Matrix (FRTR, 2020), and the Golder Remediation Toolkits (2016, 2021). The Tookits also include various sustainability indicators and tools for assessing carbon footprints. The EnviroWiki page on Sustainable Remediation provides additional resources (EnviroWiki) specific to site remediation, while Envision® is another resource for sustainable design of infrastructure in general.

Define Performance Metrics & Transition Thresholds

A critical step in Stage 3 following the remedy selection but carried out prior to implementation of an active system is to identify, establish, and align on relevant performance metrics and transition thresholds with all key stakeholders.

Performance Metrics

Performance metrics are applied to optimize performance of active remedies. A list of common performance metrics is provided in the call-out box with details provided in Tables 2 and 3 and the Metrics Glossary. The performance metrics are broadly classified as either subsurface or system related and are specific to the remediation technology and remedial concern (saturation- or composition-based). The performance metrics thus have different data requirements and timelines for data collection and analysis that should be evaluated relative to site-specific factors that affect system performance. System-related performance metrics are generally obtained during the remediation system operation, while subsurface-related performance metrics are obtained before, during and after remediation system operation. In some cases, the data are obtained in between operation cycles of the remediation system, for example, during rebound testing of an SVE system. Please consult the technology specific factsheets for more detail.

Remedy Optimization

The performance evaluation is an important step to identify measures for optimizing the remedial system in order to increase efficiency and improve sustainability (e.g. cost, energy use, and GHG emissions). More information on the optimization of specific remediation technologies is provided in the technology factsheets.

The monitoring requirements should be agreed upfront with all relevant stakeholders. At some sites with large monitoring networks and long-term monitoring, it may be possible to optimize data collection (i.e., location and frequency of sampling) using certain tools and guidance that include statistical analyses, such as, Monitoring and Remediation Optimization System (MAROS), GWSdat, US EPA, FRTR, and ITRC (2014). Remote sensing equipment and telemetry that provide continuous monitoring can also help reduce mobilizations to and from sites, and thereby reduce costs and exposure hours.

Transition Thresholds

Transition thresholds (or decision points) are comprised of remediation goals (established in Stage 1) and defined performance metrics (see Tables 2 and 3 for examples). The transition thresholds are used to support remedial decisions related to transitioning to another remedial technology, that includes natural attenuation (see Stage 4). The transition thresholds are also technology specific, as described in the technology factsheets. Again, it is essential to define and agree on the performance metrics and transition thresholds upfront with relevant stakeholders prior to the implementation of remediation.

Performance Metrics

Performance metrics differ depending on whether the remedial concern is saturation or composition (S = saturation; C = composition) based. There are two types of performance metrics: those related to the subsurface and those related to the remediation system as listed below. Concentrations and mass-based estimates, as well as data analysis to predict plume longevity, may be relevant performance metrics after initial LNAPL migration concerns are addressed.

Example subsurface metrics include:

LNAPL presence/absence in wells (S)
LNAPL transmissivity (S)
LNAPL saturation (mobile fraction remaining) (S)
LNAPL velocity (S)
NSZD (bulk TPH or COC) rate (S & C)
NA rate (C)
Concentration and/or mass discharge (C)
Concentration and/or mass discharge attenuation rate (C)
Geochemical parameters for assessing natural attenuation, electron acceptors (S & C)

Example system metrics include:

LNAPL presence/absence in wells (S)
LNAPL recovery rate vs. time, cost or greenhouse gas (GHG) emissions (S)
LNAPL decline curve analysis
LNAPL/vapor ratio or LNAPL/water ratio (S)
TPH/COC mass recovery vs. time, cost or GHG emissions (C)
TPH/COC concentration attenuation (C)
COC ratios in water or vapor (C)

Transition Thresholds

Example transition thresholds include:

Recovery of 95% of LNAPL based on decline curve analysis (S)
LNAPL transmissivity below ITRC (2018) threshold of 0.1 to 0.8 ft²/day (S)
Concentrations or mass discharge at or approaching criteria within accepted statistical certainty (C)
Active mass recovery rates similar to or less than NSZD (bulk) rates (S)
Active attenuation rates similar to or less than natural attenuation rates (C)
No or limited rebound in concentrations or mass following temporary active system shutdown
Mass removal or concentration attenuation rates by active recovery approaching asymptotic levels while ratio of GHG emissions per unit reduction in mass or concentration is rapidly increasing (S & C)
Mass removal or concentration attenuation rates by active recovery approaching asymptotic levels while ratio of costs per unit reduction in mass or concentration is rapidly increasing (S & C).

Additional information on performance metrics and transition thresholds can be found in the Technology Factsheets.

Metric	Methods	Relative Cost	References/Tools
SUBSURFACE METRICS
LNAPL transmissivity	Bail-down or skimming test LNAPL-water ratio Data from LNAPL recovery system (analysis of LNAPL/water ratio)	Low to Moderate	• ITRC LNAPL Guidance (ITRC, 2018) • Standard Guide - Estimation of LNAPL Transmissivity (ASTM, 2021) • API LNAPL Transmissivity Workbook (API, 2016) and videos • API Baildown Testing Video • LNAPL Toolbox (Strasert et al 2021)
LNAPL footprint (presence/absence in monitoring wells)	Time-series measurements in perimeter monitoring wells	Low	• ITRC LNAPL Guidance (ITRC, 2018)
LNAPL thickness in wells	Time-series measurements in LNAPL body taking into account spatiotemporal variability (e.g. water-table fluctuations)	Low	• ITRC LNAPL Guidance (ITRC, 2018)
Mobile LNAPL	Compare actual to residual LNAPL saturation; estimated from vertical equilibrium (VEQ) model or lab measurements	Moderate to High	• ITRC LNAPL Guidance (ITRC, 2018) • LNAPL Toolbox (Strasert et al 2021) • API Interactive LNAPL Guide (2006) • API LNAPL Distribution and Recovery Model (LDRM) (API 2007)
Migrating LNPAL	Tools used to assess the stability of a mobile LNAPL: LNAPL footprint, thickness, transmissivity and mobility (taking seasonal changes into account)	Low to Moderate	• API Interactive LNAPL Guide (2006) • ASTM Moving Sites to Closure
LNAPL saturation profile	Estimate from saturation in soil samples or estimate from TPH and/or Estimated from vertical equilibrium (VEQ) model during or after system operation	Moderate to High	• ITRC LNAPL Guidance (ITRC, 2018) • LNAPL Toolbox (Strasert et al 2021)
LNAPL velocity	Estimate from transmissivity or vertical equilibrium (VEQ) model	Moderate to High	• API Interactive LNAPL Guide (2006) • API LNAPL Distribution and Recovery Model (LDRM) (API 2007)
NSZD rate (bulk)	Unsaturated zone biodegradation rate (e.g., CO₂ efflux, soil gas gradient, temperature methods)	Low to High	• Natural Attenuation Factsheet (and references therein)
LNAPL movement in sediment (aquatic environment)²	Metrics for advective NAPL movement: measurements to assess pore scale mobility; and/or evaluate migration	Low to High	• ASTM Standard Guide for NAPL Mobility and Migration in Sediment (ASTM, 2021) • Metrics for Evaluating Advective NAPL Movement in Sediments (Reyenga 2021)
Subsurface rebound test	Turn system off temporarily and monitor response (e.g., LNAPL body stability, transmissivity)	Moderate to High	• See technology specific factsheets • ITRC LNAPL Guidance (ITRC, 2018) • A Practitioner’s Guide for LNAPL Management (CRC Care 2015)
Geochemical parameters (e.g., O₂, CH₄) indicative of natural attenuation	Soil gas and/or groundwater sampling and analysis	Low to Moderate	• Remediation Toolkits 1 & 2 (Golder, 2016) • Remediation Toolkits 3 & 4 (Golder, 2021) • ITRC LNAPL Guidance (ITRC, 2018)
SYSTEM METRICS
LNAPL recovery	Cumulative LNAPL recovery LNAPL recovery rate LNAPL recovery analysis (e.g., asymptotic trends, decline curve)	Low	• LNAPL Distribution and Recovery Model (LRDM) (API, 2007) • MADEP LNAPL Guidance (2016) • Methods for Determining Inputs to Environmental Petroleum Hydrocarbon Mobility and Recovery Models (API, 2001) • ITRC LNAPL Guidance (ITRC, 2018) • LNAPL Toolbox (Strasert et al., 2021)
LNAPL: water ratio	Measurement of collected fluid	Low	• ITRC LNAPL Guidance (ITRC, 2018) • Standard Guide - Estimation of LNAPL Transmissivity (ASTM 2021)
System rebound test	Turn system off temporarily and then on and then monitor applicable parameters (e.g., LNAPL recovery)	Low	• See technology specific factsheets • ITRC LNAPL Guidance (ITRC, 2018) • A Practitioner’s Guide for LNAPL Management (CRC Care 2015)
Physical parameters	Pressure, temperature, water levels, etc. to optimize system performance	Low	• See technology specific factsheets
SUSTAINABILITY METRICS³
Cost	Cost per unit reduction or change (e.g., LNAPL recovery volume (gallons) per unit cost)	Low	• See technology specific factsheets • FRTR Remediation Technology Assessment Reports
GHG emissions	Greenhouse Gas (GHG) emissions (or other indicator) per unit reduction or change (e.g., LNAPL recovery volume (gallons) per kg CO₂-e)	Moderate	• Remediation Toolkits 3 & 4 (Golder, 2021) • Sitewise™ • Methodology for Understanding and Reducing a Project’s Environmental Footprint (US EPA, 2012)

Table 2 - Performance Metrics for Saturation-Based Concern

Metric	Methods	Relative Cost	References/Tools
SUBSURFACE METRICS
Groundwater concentration	Monitoring well measurements Multiple Lines of Evidence (MLE) assessment of stability	Low	• Technical Guide for Demonstrating MNA (CRC CARE 2010) • Remediation Toolkits 3 & 4 (Golder 2021) • API GWSDAT (Jones et al 2021) • Ricker Method (Ricker 2008)
Groundwater concentration attenuation rate	Monitoring well measurements Trend analysis Environmental Molecular Diagnostic tools, such as, molecular biological tools (MBT), stable isotope probing (SIP), compound-specific stable isotope analysis (CSIA)	Low to Moderate	• ASTM E3354 (2022) • Remediation Toolkits 1 & 2 (Golder 2016) • Monitoring and Remediation Optimization System (MAROS), US EPA • GSI Mann-Kendall Toolkit • API GWSDAT (Jones et al 2021)
Soil vapor concentration	Monitoring well or soil-gas measurements Rebound tests	Low to Moderate	• Vertical distance screening (Lahvis et al 2013) • BioVapor Model (API, 2012) • ITRC PVI Guidance • US EPA PVI Technical Guide
Groundwater mass flux, discharge or loading rates	Monitoring well measurements along transect Integral pump test Passive flux meter Tracers	Moderate to High	• Use and Measurement of Mass Flux & Mass Discharge (ITRC, 2010) • GSI Mass Flux Toolkit (Farhat et al 2011)
Soil vapor flux from or to groundwater	Monitoring well or soil-gas measurements Sub-slab depressurization exhaust Building measurements	Moderate to High	• BioVapor Model (API, 2012)
NSZD rate (COC)	Gradient method (soil vapor and/or groundwater), compositional change method (LNAPL)	Low to Moderate	• Natural Attenuation Factsheet (and references therein)
LNAPL composition	LNAPL samples and analysis	Moderate to High	• LNAPL Compositional Change Model (DeVaull et al 2020)
Subsurface rebound test	Turn system off temporarily and monitor response (e.g., GW concentration, soil vapor concentration, mass discharge)	Moderate to High	• See technology specific factsheets • Soil Vapor Extraction System Optimization, Transition, and Closure Guidance (Truex et al 2013)
SYSTEM METRICS
Concentrations in extracted or discharged fluids	Measure concentrations in fluids (gas, water, liquid-phase hydrocarbon) by field screening including potential isotopic (e.g., ¹⁴C) for assessing biodegradation	Moderate to High	• See technology specific factsheets
Extraction flow rates	Measure rates of fluids (gas, water, liquid-phase hydrocarbon)	Low	• See technology specific factsheets
Hydrocarbon mass removal	Cumulative mass recovery Hydrocarbon recovery rate	Low to Moderate	• See technology specific factsheets
Concentration ratio	Measured COC ratios in extracted fluids	Low to Moderate	• See technology specific factsheets
System rebound test	Turn system off temporarily and then on and then monitor applicable parameters (e.g., concentrations, mass removal rates)	Moderate	• See technology specific factsheets
Physical parameters	Pressure, temperature, water levels, etc. to optimize system performance	Low to Moderate	• See technology specific factsheets
SUSTAINABILITY METRICS³
Cost	Cost per unit reduction or change (e.g., hydrocarbon mass recovered per unit cost)	Low to Moderate	• See technology specific factsheets • FRTR Remediation Technology Assessment Reports
GHG emissions	Greenhouse Gas (GHG) emissions (or other indicator) per unit reduction or change (e.g., hydrocarbon mass recovered or concentration reduction per kg-CO2-e)	Moderate to High	• Remediation Toolkits 3 & 4 (Golder, 2021) • Sitewise™ • Methodology for Understanding and Reducing a Project's Environmental Footprint (US EPA 2012)

Table 3 - Performance Metrics for Composition-Based Concern

Stage 4: Transition Assessment (Active Systems) & Validation

The final step in the petroleum hydrocarbon site management process (Figure 1) is to compare remedy performance to the transition thresholds and validate the transition for instances where the transition is to MNA, NFA or site closure.

The performance of active remediation should be evaluated relative to remedial goals established in Stage 1 and related performance metrics defined in Stage 3. These goals often shift over the course of remediation. For example, the focus may initially be on LNAPL mobility and recovery, when transition thresholds include LNAPL recovery rate approaching asymptotic levels and/or LNAPL transmissivity reaching a practical limit of hydraulic recovery. During latter stages of active remediation, achieving or approaching compositional thresholds related to concentrations (e.g., MCLs) and/or mass discharge or mass loading may become the focus. Technology limits are reached, for practical purposes, when monitoring data indicate relevant subsurface metrics are asymptotically approaching the defined transition thresholds (Figure 2). Depending on the complexity of site conditions, history and the remedial system, more rigorous statistical analyses may be needed to quantify uncertainties and support decision making.

Transition Assessment

The transition assessment is based on a multiple lines of evidence (MLE) approach depicted in Figure 2 that considers:

technology performance and limits;
a comparison to the baseline assessment; and,
an evaluation of sustainability metrics (cost, GHG emissions, etc.).

As noted in Stage 3, a subset of subsurface and system metrics is selected and agreed on for the transition assessment in Stage 4.

Figure 2 – Linkage between performance evaluation, transition assessment, and validation.

Remedy Validation – is Transition Viable?

A validation assessment is recommended to build confidence in the transition, in particular, when transitioning from active remediation to MNA, NFA, or site closure. Validation testing is generally conducted through rebound testing, to validate that the remedial goals established in Stage 1 can still be met following the termination of active remediation. Rebound tests typically involve turning off the active system for specified time interval and monitoring the response in plume stability (LNAPL or dissolved phase) and/or COC concentrations (or other metrics) over an agreed upon time period within the zone of system operation. The active system can either be terminated in phases (e.g., segments of the system) or in entirety. The process for system shutdown and rebound monitoring, including the duration, location, and criteria used to validate remedial decision, should be agreed upfront with all key stakeholders in Stage 3 before the system is turned off (i.e., as are becoming imminent). Many of the same tools, analysis, and metrics used to gauge remedial performance can be used for this purpose. The validation assessment should also factor in potential vulnerabilities to the changing climate conditions such as drought and flooding.

¹ A range of models are available, from nomographs incorporating measured rates based on simplified source estimates, to analytical or numerical models for source depletion and dissolved- and vapor-phase plume migration (Mayer et al. 2002; Ng et al. 2015; Sookhak Lari et al. 2015; Golder 2016). The LNAPL Toolbox (Strasert et al. 2021) can be used to estimate the LNAPL volume in the subsurface, which together with the attenuation rate estimates can be used to infer the minimum remediation timelines under natural and enhanced attenuation. Given that NA rates can vary and typically decrease over time, there is a significant degree of uncertainty associated with predicting the long-term remediation timeframe under natural attenuation. Minimum timelines can be estimated for qualitative assessments of MNA and to underpin remedial decision making.

² The Compendium primarily addresses LNAPL in the subsurface on land. For addressing LANPL in sediments of aquatic environments, refer to the noted tools and references.

³ Additional sustainability metrics related to resource usage (e.g., water and energy) or waste production (e.g., air pollutants, solid and liquid waste) can be tracked using footprint analysis tools (ASTM E2893 2016).