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

Natural Attenuation Factsheet

This factsheet provides an overview of tools for the quantification of natural attenuation (NA), including methods for estimating NSZD. These tools are essential in forming the baseline metric for assessing remediation performance, the need for and termination of active remediation, and minimum remediation timeframes¹ (e.g., using trend analyses, nomographs, NSZD methods, or more complex models). This factsheet should be read in conjunction with an overarching Compendium that places NA into broader context with the end-to-end process of remedial decision making. The purpose of this factsheet is to highlight the various tools available to help quantify and analyze NA rates. Their use and a multiple-lines-of-evidence (MLE) approach can help to avoid unnecessary active remediation.

Natural attenuation (NA) of petroleum hydrocarbons (PHCs) occurs from the onset of their release as a light non-aqueous phase liquid (LNAPL) into the subsurface. NA occurs in the LNAPL source zone and across the resulting mass of PHCs in the vadose zone (i.e., LNAPL, pore water, vapor plume) and the saturated zone (groundwater plume) via dissolution, volatilization, and biodegradation (see Figure 1).

Natural Attenuation (NA)

The mass loss of PHCs naturally occurring in various phases (LNAPL, vapor, soil, and groundwater) within an area of soil or groundwater contamination. Natural attenuation occurs throughout the subsurface in and outside of the source zone where LNAPL is present.

Natural Source Zone Depletion (NSZD)

The mass loss of PHCs naturally occurring in LNAPL source zones as a result of dissolution, volatilization, and biodegradation. NSZD is a subset of natural attenuation largely focused on the depletion of bulk PHCs from an LNAPL source present in the subsurface in the area where the release occurred. NSZD rates can also be defined for individual hydrocarbons.

Natural Source Zone Depletion (NSZD) is a subset of NA that focuses on NA processes occurring in the source area where the bulk of the PHC mass is present (see Figure 1), as compared to the dissolved and volatilized mass that occurs downgradient from the source zone. Most PHCs and key chemicals of concern (COCs) are readily degradable and can easily be shown to degrade by evaluating the signatures of biodegradation, such as oxygen – O₂, carbon dioxide – CO₂, methane – CH₄; soil temperature; and other geochemical indices (e.g., pH, alkalinity, terminal electron acceptors, etc).

Understanding the rate at which NA occurs is important information to inform and improve remedial decision making (see Compendium), for example:

determining the need for active remediation based on whether rates of NA are sufficient to 1) limit the migration of hydrocarbons to potential receptors (e.g., drinking water wells, current/future buildings overlying the contamination) and/or

2) achieve remedial objectives (saturation- or composition-based) within a reasonable timeline;

inform the optimization or termination of active remediation. The use of NA rates in remedial decision making should be agreed upfront with key stakeholders prior to the onset of active remediation.

NA measurements are commonly used to support the adoption and implementation of monitored natural attenuation (MNA). Such NA rates are typically based on measurements of key COCs, bulk hydrocarbon (TPH), and geochemical indicators of PHC degradation (dissolved oxygen, pH, temperature, alkalinity, electrical conductivity, electron acceptors) in groundwater that are collected over several years from monitoring wells located across a PHC plume.

Monitored Natural Attenuation (MNA)

Passive remediation approach through site characterization and monitoring. Conventionally focused on the assessment of spatiotemporal trends in concentrations of constituents of concern (COCs) in groundwater. Various tools for the estimation of natural attenuation and NSZD rates, specifically, are used to evaluate the effectiveness of MNA and guide the transition from active remediation to less resource intensive measures and/or MNA.

The adoption of NA rates to support the monitoring of NSZD as a remedial option has been more limited, even though numerous tools are available, albeit some have been recently developed. Application of these tools have shown that NSZD rates and the PHC mass loss in the source zone from methanogenesis and PHC volatilization is often significant. The limited adoption of tools to quantify NSZD rates may be partially attributed to the commonly proposed use of NSZD solely as an alternative to active remediation (hydraulic recovery). Advocating for a broader use of NSZD to support remedial decision making prior to the onset of active remediation (e.g., to assess the need for active remediation) or during active remediation (e.g., to optimize and apply active remediation more sustainably), or as the means to ultimately reach remedial targets or endpoints, may help facilitate greater regulatory approval for usage at sites.

This factsheet aims to address some of the commonly cited challenges in estimating the rates, such as:

Unfamiliarity with the methods and lack of consistent standards;
Uncertainty associated with the measurements;
Lack of regulatory guidance on application of the estimated rates;
Current remedies deemed effective; and
Budgetary constraints.

Before making NA rate measurements, it is critical to understand how the information will be used to support remedial decision making. It is also important to align on remedial concerns and goals (saturation-, or composition-based), understanding that there is likely to be a level of uncertainty in NA rate measurements, given they can vary over time and space depending on site complexity (e.g., number of sources, hydrogeology, plume size, etc). Sites with less hydrogeologic complexity, better defined CSMs, and lower risk, will generally require fewer NA rate measurements. PHC source mass estimates, which are needed to predict minimum times to meet a remedial concern (saturation- or composition-based), will also be largely uncertain and only estimated to within an order of magnitude using LNAPL saturation or soil concentration data.

The purpose of this factsheet is to highlight the various tools available to help quantify and analyze NA rates. Their uptake and a multiple-lines-of-evidence (MLE) approach can help to avoid unnecessary, non-sustainable active remediation.

Figure 1 – Conceptual representation of Natural Attenuation (NA) and Natural Source Zone Depletion (NSZD).

Quantification of Natural Attenuation Processes

Quantification of NA involves collecting information on COC or bulk PHC concentrations over time (at specific locations) or space (at specific times). NA rates can be used to document the occurrence of biodegradation and to assess plume stability (stable, increasing, decreasing plume), a potential new release, and overall progress on meeting site-specific remedial objectives. Depending on the remedial objectives and tool used for data collection, the documentation of NA rates can either require long-term monitoring (e.g., multiple years of seasonal data for trend analyses) or be completed over relatively short periods of time (hours to days).

Table 1 provides a summary of widely available tools for the assessment of NA rates. Tools selection will largely depend on:

whether the remedial concerns and goals are saturation- (migration or mobility), and/or composition-based (Table 1 and Figure 1 of Compendium);
the dominant NA process;
source depth and location relative to the water table;
soil stratigraphy; and
ground surface condition (paved, open ground).

It is also important to align with key stakeholders on when to document rates of NA. For example, NA processes can be quantified:

before active remediation is implemented to document baseline conditions and evaluate the need for active remediation;
during active remediation to support performance optimization and assess radius of influence; and,
after active remediation to validate remedial effectiveness (e.g., rebound testing) and potential transitions to MNA, no further action (NFA), or site closure.

Depending on site conditions, multiple measurements and tools (i.e., a MLE approach) may be needed to quantify NA rates from different processes or reduce uncertainty/increase confidence in remedial decision making. It is also important to:

maintain consistency in methods and measurements used for NA assessment throughout the remedial decision-making process, in particular, for the Baseline Assessment (Stage 2) and Transition Assessment and Validation (Stage 4) (see Compendium);
include measurements of NA rates in background locations (i.e., hydraulically up- or side-gradient from the area where PHCs are present in the subsurface), or include radiocarbon (¹⁴C) isotope analyses, depending on the method (see Quantifying NSZD Rates (1) in Table 1);
understand that predicting the minimum time¹ required to meet a remedial goal based on estimates of NA rates also requires estimates of the PHC source mass, where estimation is limited to within an order of magnitude (see Table 1).

The level of data interpretation can also vary depending on whether the objective is to quantify mass flux, mass discharge/removal rates of the remediation system (active or passive), or mass loading rates at the point of exposure (e.g., across a building foundation or at a drinking well or surface water body). Mass-flux estimates (mass per unit area per time) are often made based on data collected at discrete locations within an affected area and then extrapolated site-wide (e.g., across an LNAPL source zone).

Biodegradation plays a significant role in the PHC attenuation that occurs via NA. Biodegradation can be validated and further quantified through an examination of:

changes in redox conditions and concentrations of terminal electron acceptors in groundwater between upgradient and downgradient locations from the LNAPL source;
soil gas concentrations in samples collected from soil gas probes or the headspace of monitoring wells (field or laboratory measurements), in particular:

lower O₂ concentrations relative to background locations;
elevated CO₂ and CH₄ concentrations; and
laboratory measurement of N2 and Ar that are depleted or enriched relative to background locations as a result of CH₄ generation or oxidation, respectively, and pressure-driven flow (ASTM 2016; Amos et al. 2005; and Molins et al. 2010).

changes in the subsurface temperature profile within the LNAPL-impacted areas as an indicator of the zone of aerobic/anaerobic interface (see Appendix X2 of ASTM E3661 2022).

Tool	Data Requirements	Long term monitoring required?	Primary Purpose	Target Media / Zone	Remedial Concern
Quantifying NSZD Rates (1)	CO₂ Efflux	No	Estimating source depletion rates	Vadose zone	Saturation
	Temperature gradient			Vadose zone	Saturation
	Soil gas gradient			Soil gas	Composition/ saturation
	Groundwater gradient			Groundwater	Composition / saturation
	LNAPL Compositional changes	Yes		LNAPL source zone	composition
Regression Analysis (2)	COC concentrations in groundwater	Yes	Trend analysis and time estimate to reach regulatory standard	groundwater	composition
Nomograph for Source Depletion (3)	Average TPH concentration in soil & thickness + biodegradation rate	No	Estimating the minimum source depletion times¹	vadose zone soil	saturation
	LNAPL saturation & thickness + biodegradation rate			vadose zone soil
	Average TPH concentration in soil & thickness + groundwater flow rate			groundwater
	LNAPL saturation and thickness + groundwater flow rate			groundwater
Groundwater Plume / LNAPL Body Stability (4)	COC concentrations in groundwater + water elevation + LNAPL presence/absence	Yes	Assessment of groundwater plume stability	groundwater	composition
Groundwater Geochemical Parameters (5)	groundwater geochemical data collected upgradient, within and downgradient of the source zone	No	Lines of evidence evaluation of biodegradation	groundwater	saturation
Groundwater Attenuation Rates (6)	COC concentrations in groundwater	Yes	Estimates of attenuation rates for plume duration and trend	groundwater	composition
Mass Discharge Estimation (7)	COC concentrations in groundwater + hydrogeological properties	Yes	Mass-based estimate of attenuation rates; trends and timelines	groundwater	composition
Screening Level Source Depletion Rates (8)	groundwater geochemical data collected upgradient, within and downgradient of the source zone; and O₂ concentration profile in soil gas, soil properties and the representative hydrocarbon	No	Estimating source depletion rates	Saturated and unsaturated zones	saturation
Solute Transport Models (1D Analytical) (9)	COC biodegradation rate constants Hydrogeological parameters + LNAPL source properties	No	Groundwater plume prediction and remediation timeline	groundwater	composition
Multi-component / Multi-dimensional Numerical Models (10)	COC physico-chemical properties + hydrogeological properties + boundary & initial conditions	No	Prediction of COC concentrations in time and space	Saturated zone + vadose zone (some models)	composition
Evaluating Remediation Performance (11)	COC concentrations in soil gas or groundwater (application dependent)	Yes	Rebound and respiration testing (SVE); Mass discharge estimates (P&T)	Vadose zone or saturated zone (application dependent)	composition

Table 1 - Summary of tools for the assessment of natural attenuation.

1. Methods for NSZD Rate Estimates

Methods for quantifying NSZD rates commonly involve measurements of (Figure 2, Table 2, and ASTM E3361 2022):

CO₂ released from biodegradation of PHCs in the subsurface and transported through diffusion and advection to the ground surface;
changes in the soil temperature resulting from exothermic biodegradation of PHCs in the subsurface;
changes in soil gas chemistry in the vadose zone resulting from biodegradation and transport of chemical reactants and reaction by-products in the subsurface (mainly O₂, CO₂, PHCs, and CH₄);
changes in groundwater chemistry resulting from dissolution of PHCs, biodegradation, and flow in the saturated zone; and
changes in LNAPL composition.

These measurements are generally made within the source area where LNAPL is present (LNAPL footprint) and in background locations (Figure 2). Measurements of NA rates in background locations outside of area where PHCs are present are needed to differentiate PHC reactants and reaction by-products (O₂, CO₂, CH₄, temperature) that occur within the LNAPL source area from those occur from the decomposition of organic matter that is naturally present in the subsurface. Background measurements are also necessary, if applying the temperature method, to distinguish the heat generated from PHC biodegradation in the subsurface from the heat generated by atmospheric temperatures. It is also important to note that some NA methods target the mass loss of COCs from the LNAPL source, while others (temperature, O₂, CO₂, CH₄) target bulk LNAPL depletion. The term “bulk” does not imply that each COC attenuates at a similar rate; rather, that COC-specific NA rates cannot be obtained from these measurements.

Figure 2 – Overview of natural attenuation and NSZD processes, methods, and measurements.

Method selection depends largely on:

whether the remedial concerns are saturation (migration/mobility) and/or composition² (Table 1 and Figure 1 of Compendium);
the dominant natural attenuation process based on LNAPL distribution above or below the water table;
source depth and location relative to the water table;
soil stratigraphy; and
ground surface condition (paved, open ground).

NSZD has typically been applied to assess bulk hydrocarbon attenuation and saturation (migration/mobility) concerns. However, NSZD measurements can also be made for individual COCs to address composition concerns using the gradient method (see Appendix X3 of ASTM E3361 2022) or measuring changes in LNAPL composition over time (see Appendix X5 of ASTM E3361 2022). For example, NA rates for individual COCs concentrations in soil-gas can be estimated using the gradient method instead of CO₂ or O₂ concentrations without the need for a stoichiometric constant to relate CO₂ production or O₂ utilization to the rate of bulk PHC attenuation. For the LNAPL composition method, mass loss rates of individual COCs are determined by evaluating changes in LNAPL composition for liquid samples collected from monitoring wells at specific times from one or more sampling locations.

More detailed information on NA methods can be found in ASTM E3361 (2022), including:

available tools and methods;
screening or feasibility assessment of the method for the site conditions
data interpretation, key considerations, and challenges, such as:

measurement frequency, locations, and estimating annual average of NA rates (site-wide or the LNAPL source zone) and,
correcting for background sources;

recommendations for QA/QC;
methods for evaluating the performance of enhanced attenuation (or bioremediation) systems; and
use of NA measurements to support source delineation or estimate mass discharge rates in soil gas or groundwater.

Method	Type of Attenuation Measured¹	Location of processes & Pathway	Measurement Location
1. CO₂ Efflux	Bulk LNAPL	Vadose zone²	Ground surface
2. Temperature Gradient	Bulk LNAPL	Vadose zone²	Vertical profile mostly in the vadose zone & straddling the capillary fringe near the source zone
3. Soil Gas Gradient	Bulk LNAPL & COCs	Vadose zone²	Vertical profile in the vadose zone near the source zone
4. Groundwater Monitoring	Bulk LNAPL & COCs	Saturated zone³	Profile along the groundwater flow path up- and down-gradient from the source zone
5. LNAPL Composition	COCs	LNAPL Source zone	Source zone

Table 2 - Summary of general methods and associated measurement locations for the five NSZD methods.

¹ Method selection is directly linked to the identified risks or concerns at the site. The depletion rate of bulk LNAPL is generally associated with a saturation-based concern, while estimates of COC attenuation rates are associated with composition-based concern (Table 1 and Figure 1 of Compendium).

² Includes the transport of CH4 and other hydrocarbons produced from the biodegradation of PHCs in the saturated zone and CH4 oxidation at the aerobic/anaerobic interface.

³ The estimated rates in the saturated zone are typically one to two orders of magnitude lower than rates measured in the vadose zone, except at sites where confined conditions/lower soil gas permeability limit the direct and vertical transport of generated methane from groundwater to the ground surface (Appendix X4 of ASTM E3361 2022).

2. Regression Analysis

Groundwater concentrations of COCs can be plotted over time at a specific location (monitoring well) for trend analysis. The identified trend and associated confidence intervals can be used to track the progress towards reaching the clean-up goals. Examples of tools and plots are provided to illustrate the application of this tool.

Regression Analysis Tool (US EPA 2011)

Figure 3 – Regression MNA tool (Figure A-3 of Golder 2016)

Non-parametric analysis such as Mann-Kendall:

Non-parametric trend estimate (Figure 13 of Jones et al. 2021; GWSDAT Manual version 3.1)

Figure 4 – Non-parametric trend estimate. Figure 13 of Jones et al. 2021; GWSDAT Manual version 3.1.

3. Nomographs for Source Depletion

Nomographs for source depletion can be used to estimate a source depletion time with the caveat that these time estimates provide a minimum timeline assuming a constant depletion rates over time. 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 as with other remedy options. However, the minimum timeline can be estimated for a qualitative assessment of MNA.

Golder (2016 and 2021) Toolkits for Management of Petroleum Hydrocarbon Sites:

Toolkit 1
Summary of Toolkits and Conceptual Site Model and Case Studies (July 8, 2016)
Toolkit 2
Monitoring and Prediction (July 8, 2016)
Toolkit 3
Evaluation of Remediation Technologies for Petroleum Hydrocarbon Sites (April 5, 2021)
Toolkit 4
Methods for Sustainable Remediation (April 6, 2021)
SR Dashboard

Figure 5 – Minimum source depletion time from biodegradation in the vadose zone based on hydrocarbon concentration. Figure 3-3 of Golder (2016) Toolkit 2.

Figure 6 – Minimum source depletion time from biodegradation in the vadose zone based on LNAPL saturation. Figure 3-4 of Golder (2016) Toolkit 2.

For example, if the LNAPL mass loss rate is 1 g-TPH/m²-day, the corresponding LNAPL source depletion time is estimated to be 100 years as shown by the red arrows.

Figure 7 – Source depletion time from dissolution in the saturated zone based on hydrocarbon concentration. Figure 3-5 of Golder (2016) Toolkit 2.

Figure 8 – Source depletion time from dissolution in the saturated zone based on LNAPL saturation. Figure 3-6 of Golder (2016) Toolkit 2.

4. Groundwater Plume Stability Tools

Groundwater concentrations of COCs are integrated in space and evaluated in time for stability assessment of the plume in terms of expanding, stable or shrinking plume. The tools can also be used for interpretation and visualization of the groundwater monitoring data. The Groundwater Spatio-temporal Data Analysis Tool (GWSDAT) (Jones et al., 2021) includes the Ricker (2008) method (total plume mass, total plume area, and average plume concentration versus time) and trends of COC concentrations and LNAPL thicknesses at all monitoring well locations.

Figure 9 – Example plots of groundwater plume mass, area, and average concentration over time (Figure 9 of Jones et al. 2021; GWSDAT Manual version 3.1)

5. Geochemical Parameters to Demonstrate Biodegradation

Field parameters and groundwater sample collection and analysis (Golder 2016 and Christensen et al. 2000)
Qualitative comparison of the reactants and byproducts of biodegradation reactions within and upgradient of the groundwater plume
Variations in redox potential within and upgradient of the groundwater plume

Qualitative assessment of biodegradation across the groundwater plume:

Figure 10 – Figure 4 of Christensen et al. (2000).

Figure 11 – Qualitative assessment of biodegradation across the groundwater plume. Figure 7 of Christensen et al. (2000).

6. Groundwater Attenuation Rate Estimation Tools

Definitions:

Bulk attenuation rate that incorporates biodegradation, dispersion and sorption through analysis of concentration versus distance (k_bulk) or concentration versus time, k_point (Newell et al. 2002; Golder 2016; Enviro Wiki Page on MNA)
Biodegradation or “biodecay” rate constant () estimated through model calibration/fitting and modeling of non-reactive tracer (ASTM 1998; Newell et al. 2002; Golder 2016)

Applications:

K_point can be used to estimate time to reach clean-up goal at single location; if applied to multiple locations over the plume, can be used to assess plume duration and trend
K_bulk can be used to project the plume trend (expanding, stable or shrinking)
can be used to predict the plume trend and for use in solute transport models

Key considerations:
Estimation of rate constants that are statistically significant is difficult with fewer than six sampling dates or monitoring data in less than three years (Newell et al. 2002). It is also important to consider that the rates determined over a few years may change over longer timeframes like decades into the future.

Additional lines of evidence:
Trends and rates of mass-based estimates can also be used to improve decision making such as changes in time of mass discharge rates (Newell et al. 2002). Refer to Mass Discharge Estimation Tools (7) for available resources and guidance for estimating mass discharge rates. Mass-based estimates can be a more direct indicator of risk to receptors such as water supply wells or surface water bodies since they combine hydrogeological and concentration data. They can also be used to better understand the conceptual site model and the groundwater plume behavior.

Figure 12 – Concentration versus time attenuation rate constant. Figure 1 of Newell et al. (2002).

Figure 13 – Concentration versus distance attenuation rate constant. Figure 2 of Newell et al. (2002).

7. Mass Discharge Estimation Tools

Mass discharge estimates are based on COC concentrations and groundwater flow conditions (typically, hydraulic conductivity and gradient). The estimates improve decision-making for remedial design and performance assessment, because mass discharge estimates inform risk assessment in terms of groundwater plume extent and stability relative to receptor location.

These mass-based estimates also complement concentration-based estimates of trends and COC attenuation rates as they take into account the hydrogeological conditions (5). Mass discharge rate is also referred to as contaminant mass discharge (CMD) (Truex et al. 2015). The rate is the sum of mass flux, or the integrated mass flux, across a transect plane perpendicular to the groundwater flow direction (F)³.

In scenarios where active remediation is used to mitigate risks associated with a groundwater plume, the mass-based estimates can be used to evaluate the risk to potential receptors and thereby guide the transition from active remediation to less resource intensive measures and/or MNA. Refer to Table 1-1 in ITRC (2010) for application of mass discharge rates in site remediation and management.

There are various recommended approaches as described in ITRC (2010):

Transect method (most common approach, Farhat et al. (2011) Mass Flux Toolkit)¹
Using existing site data of concentrations and groundwater flow rates (or transects based on isocontours)
Well capture/pump test method or integrated pump test (ITRC 2010 and Truex et al. 2015)
Passive flux meters (ITRC 2010; Truex et al. 2015; Klammler et al. 2012; iFlux)
Solute Transport Models (Table 4-3 of ITRC 2010)

Figure 14 – Conceptual representation of mass flux (J) and mass discharge. Figure 2-2 of ITRC (2010).

Figure 15 – Conceptual representation of uncertainty and cost associated with mass discharge estimates. Figure 2-2 of ITRC (2010).

8. Screening Level Source Depletion Rates

A box model (or control volume) approach can be taken to estimate the source depletion rates, often with available concentration data in groundwater and soil gas. This approach is described as the Control Volume Method in ITRC (2009) (ITRC 2009; Toolkits 2), and the estimates consist of:

Mass depletion rate by dissolution to groundwater
Dissolved source zone mass depletion rate by biodegradation (Table 2-3 of Toolkits 2)
Unsaturated source zone mass depletion rate by volatilization and biodegradation

Figure 16 – Groundwater transport-related NSZD Processes. Figure 2-1 of ITRC (2009).

Figure 17 – Vapor transport-related NSZD Processes. Figure 2-2 of ITRC (2009).

9. Solute Transport Models

There are various solute transport models with varying level of complexity available for the prediction of groundwater plume migration and source depletion. For example, review and comparison of publicly available models BIOSCREEN, REMFuel and LNAST are available in Section 4 of Toolkits 2.

Figure 18 – Baseline and special cases modeling scenarios. Figure 4-1 of Golder (2016) Toolkit 2.

Figure 19 – Predicted benzene plume extent and concentrations for baseline scenarios (MIN3P-Dusty was not run for large source scenario). Figure 4-2 Golder (2016) Toolkit 2.

10. Numerical Models

In general, the use of numerical models in remedial decision making is limited by the lack of available data on key input variables (i.e., requirements for extensive site characterization to support model application) and modelers with sufficient competency to apply the models, lack of sites which would benefit from more sophisticated modeling, and overall challenge in obtaining regulatory “buy-in” because of unfamiliarity or lack of understanding of the model.

A comprehensive review of models relevant to NSZD is provided in Sookhak Lari et al. (2019) and categorized by key geochemical process. Of the 36 models reviewed, only one third simulate volatilization, variably saturated media and NSZD in both the vadose and saturated zones.

Key relevant models are:

PH3D – while it does not include volatilization and vadose zone processes, it is commercially available and notably includes direct outgassing from LNAPL and aqueous phase as demonstrated in the study of Ng et al. (2015)
TMVOC – notably includes multi-phase transport (Sookhak Lari et al. 2018)
MIN3P-Dusty – notably includes variably saturated media and various implementations of gas transport and a version of the code with gas bubble formation and ebullition

Example applications of the MIN3P-Dusty numerical model for petroleum hydrocarbon fate and transport: 1. Molins et al. (2010) Vadose zone attenuation of organic compounds at a crude oil spill site — Interactions between biogeochemical reactions and multicomponent gas transport; 2. Toolkits 2 (Section 4 along with the solute transport models); 3. Jourabchi and Hers (2013) on modeling study of iron and manganese in groundwater from a PHCs (benzene) source.

Figure 20 – Conceptual representation of processes included in the model (Bemidji North pool), particularly those not included in previous studies such as distinct degradation pathways for diverse oil constituents, direct outgassing in addition to outgassing from the aqueous phase, and Fe sorption with H+ exchange. The model domain includes the oil body at and below the water table. Figure 1 of the Ng et al. (2015) study.

Figure 21 – Dissolved aqueous phase concentrations of simulation results at 10 years for baseline scenario (Note distance is shown on different scales). Figure 5 of Jourabchi and Hers (2013).

11. Evaluating Remediation Performance

Guidance on rebound testing for SVE system

US AFCEE (2001) note the importance of collecting VOC concentration data and time period to reach equilibrium concentrations after system shut-off for rebound/equilibrium testing. Respiration tests can also be used to estimate the mass of contaminants that are being biodegraded and removed from the soil during SVE operation.
Truex et al. (2013)
Combination of VOC rebound testing with mass removal rates (Compendium and SVE Factsheet)

Guidance on rebound testing for Pump and Treat system

Assessment of monitored natural attenuation (MNA) as described in Section 6.2.1 of Truex et al. (2015): estimates of attenuation rates to evaluate threshold concentrations or comparison of contaminant mass discharge (CMD) to attenuation capacity.

Figure 22 – Example rebound/equilibrium test of an SVE system. Figures 4.3 and 4.4 of US AFCEE (2001).

Figure 23 – Example respiration measurement of an SVE system. Figure 4.5 of US AFCEE (2001).

Figure 24 – Example respiration measurement of an SVE system. Figure 4.5 of US AFCEE (2001).

Example calculation of concentration-based attenuation rate to compute threshold concentration, C_T (see Section 6.2.1 of Truex et al. (2015) for details).

Step 1. Identify the remedial action objective (RAO) concentration goal (C_RAO) at a selected maximum downgradient plume migration distance, d (i.e., the maximum downgradient plume migration distance acceptable as a zone of MNA with no impact to receptors or other downgradient constraints like property boundaries)

Step 2. Estimate the concentration-based attenuation rate in the downgradient plume (e.g., in units of mg/L/day) (see Groundwater Attenuation Rates (6) for bulk attenuation rate, k_bulk)

Step 3. Estimate the contaminant transport velocity (V_contaminant)

V_contaminant = (q_natural / n) / R_contaminant

q_natural = the Darcy flux (hydraulic conductivity × hydraulic gradient) under natural (non-pumped) conditions
n = effective porosity
R_contaminant = the retardation factor for the contaminant

Step 4. Calculate a threshold concentration, C_T at the “source” zone or location of the P&T system for comparison to concentrations within the upgradient plume or to a concentration measured during a rebound test:

C_T = C_RAO + (k_bulk × d / V_contaminant)

Example calculations for estimating the post-P&T contaminant mass discharge (CMD) (see Section 6.2.1 of Truex et al. (2015) for details).

Depending on the available data at a site, Truex et al. (2015) recommend one of three options for calculation of the post P&T CMD:

i. Post-P&T CMD = C_P&T x q_natural x A_{capture cross section}
ii. Post-P&T CMD = (q_natural x A_{source zone} / Q_P&T) x CMD_P&T
iii. Post-P&T CMD = C_source x q_natural x A_{source zone}

C_P&T = COC concentration from the P&T extraction system for the wells used in the analysis
q_natural = the Darcy flux (hydraulic conductivity × hydraulic gradient) under natural (non-pumped) conditions
A_{capture cross section} = cross sectional area that is captured by the P&T wells used in the analysis
A_{source zone} = cross sectional area of the source zone through which groundwater under a natural gradient would flow
Q_P&T = P&T extraction flow rate for the wells used in the analysis
CM_P&T = C_P&T / Q_P&T
C_source = COC concentration selected to represent the source zone (or plume upgradient of the P&T system, which can be a maximum observed concentration or an average of some designated areas

The post-P&T CMD can be used in an MNA evaluation considering any continuing sources such as from matrix diffusion that persist after termination of P&T.

¹ 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. However, the minimum timeline can be estimated for a qualitative assessment of MNA.

² Bulk LNAPL depletion rates generally address saturation (mobility) concerns; while attenuation of constituents of concern (COCs) addresses composition concerns (see Figure 1 in the Compendium – Stages 1 and 2).

³ It is recommended to use the terminology of “mass flux” and “mass discharge rate” as defined in Section 2 of ITRC 2010. A different definition of “mass flux” is stated in the Introduction of Farhat et al. (2011) Mass Flux Toolkit where it is used interchangeably with “mass discharge rate”.