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

Air Sparging Factsheet

This factsheet provides information to support remedial decision-making on air / bio sparging (AS). The information is intended to help:

a) optimize AS remediation performance, and
b) transition from AS to natural attenuation, passive remediation, or “no further action”.

This factsheet addresses both air sparging and bio sparging (enhanced bioremediation). For simplicity, these technologies are collectively referred to as AS in this factsheet. This factsheet should be read in conjunction with the other air-based remediation factsheets on soil vapor extraction (SVE) (see Soil Vapor Extraction Factsheet) and bioventing (see Bioventing Factsheet), because of the common use of AS with SVE and similarities in application. In addition, this factsheet should be read with the overarching Compendium Factsheet, which provides a broader context for the application of AS within a framework for remedial decision making. This factsheet is not intended to provide detailed guidance on AS and assumes that a sufficiently detailed conceptual site model (CSM) has been developed and AS has been selected as an appropriate technology to meet agreed remedial concerns and objectives. The scope of this factsheet is primarily injection of air to treat petroleum hydrocarbon (PHC) contamination.

Details of the AS technology and its application are contained in the appendices. Appendix I provides a summary of the AS technology. Appendix II provides guidance on pilot testing. Appendix III provides methods to optimize AS implementation. Additional details on AS design and implementation are also found in Johnson et al. (1993), Hinchee (1994), NAVFAC (2001), ESTCP (2002), US ACE (2013) and CRC Care (2018b). The information found in the appendices and reference materials can be extremely helpful in supporting remedy implementation and facilitating the uptake of risk-based approaches. Note that there are analogous technologies to AS that supply amendments to the subsurface to enhance in-situ remediation such as oxygen-release compounds or chemical oxidants. These technologies are beyond the scope of this factsheet but may warrant consideration.

AS - Technology Summary

AS is a remedial technology primarily intended to address composition-based concerns for petroleum hydrocarbons (PHCs) distributed below the water table. AS is implemented by injecting air or oxygen (O₂) (as pure O₂, ozone, or via other injectants) below the water table, which enhances volatilization and removal of the more volatile constituents of concern (COCs) contained in light non-aqueous phase liquid (LNAPL) from the saturated zone. These COCs are generally lighter in molecular weight relative to other LNAPL COCs and tend to be risk drivers (e.g., benzene, toluene, ethylbenzene, and xylenes) for key exposure pathways. The volatilized COCs move upward because of pressurized air flow in channels and the buoyancy of air (as bubbles) to the vadose zone, where either a SVE system is used to capture the COCs before they potentially migrate to land surface or buildings (see SVE Factsheet) or biodegraded, if safe. The O₂ in the injected air also enhances aerobic biodegradation.

AS may be configured to treat PHC sources and/or associated groundwater plumes. AS design should be based on remedial goals and site-specific conditions which affect the PHC source distribution in the subsurface. The AS design should be based on maximizing the mass transfer of O₂ to the subsurface, which depends largely on injection flow rate, system operation (e.g., pulsed versus continuous), and the hydrogeology (heterogeneity) of the formation. Additional information on AS technology is provided in Appendix I.

Remedial Concerns and Objectives

Remedial concerns and goals or objectives associated with AS are either to reduce COC concentrations below a risk-based threshold at a point of compliance (e.g., groundwater monitoring well), or to achieve criteria for risk-based contaminant mass discharge or mass loading. Although AS is primarily used to address composition-based concerns, AS can also be effective in the treatment of bulk (residual) LNAPL to varying degrees, although some early research and guidance on use of AS for bulk remediation is considered overly optimistic. US ACE (2013) provides a balanced assessment of the practicality of AS for remediating bulk LNAPL recognizing success is highly site-specific. At sites with mobile LNAPL, the potential for AS to cause redistribution of LNAPL should also be considered because AS-induced pressures or gradients may move mobile LNAPL laterally. The remedial objectives may also include target timelines to achieve the risk-based threshold. Broader remedial decision making should also factor in potential liabilities associated with site risk, regulatory requirements, business plans and timelines, future redevelopment potential, sustainability, and economic factors. The remedial objectives should also incorporate the notion of technical practicability.

Generally, volatilization is the primary PHC attenuation mechanism at early stages of AS implementation when volatile COCs present in LNAPL, aqueous, or sorbed phases are in direct contact with air channels. As AS continues, volatilization expressed by the mass partitioning of COCs to the gaseous phase becomes more rate limited and concentrations of the more volatile COCs decrease. Meantime, the microbial community within the area of AS influence becomes more established because of the added presence of an electron acceptor (O₂) and biodegradation becomes the primary PHC attenuation mechanism. Biodegradation may also be the more dominant mass removal process in aquifers at lower dissolved PHC concentrations less than about 1 mg/L (Johnson 1998). Diagnostic tools (see call-out box) are an additional line of evidence that can help distinguish the dominant PHC attenuation mechanism and inform the selection of performance metrics and transition thresholds.

Performance Metrics

Performance metrics are used to optimize AS performance and gauge progress toward meeting a transition threshold (discussed later) or remedial objective. These metrics should be established and agreed upfront by all key stakeholders before AS is implemented. Performance metrics are broadly classified as system or subsurface related (see Table 1 and Figure 1). Data to assess performance can be collected before, during, and after system operation.

Figure 1 – General subsurface and system-related performance metrics associated with AS remediation.

Performance Metrics

Subsurface:

C_s = Soil concentration
C_g = Groundwater concentration
C_v = Soil vapor concentration
CR_s CR_v CR_g = Concentration ratios
M_g = Groundwater mass flux or discharge
M_v = Soil vapor mass flux or discharge

System:

Q = Injection flow rate
Pi = Injection pressure
Ps = Pressure in soil
DO = Dissolved oxygen
H = Water level mounding and bubbles
GCHEM = Geochemistry data
He / SF₆ = tracers (optional)
ZOI = Zone of influence

System Metrics

Recommended performance metrics obtained during system implementation include:

Injection air pressures at individual injection wells and the system header
Injection air flows at individual injection wells and the system header
Pressures in unsaturated and saturated soil zone at monitoring points
Dissolved oxygen concentrations at monitoring wells
Water level mounding at injection wells and monitoring wells
Geochemistry data at injection wells and monitoring wells
Monitoring of air bubbles in wells and at surface
Helium (He) tracer test (optional, may be conducted during pilot testing)¹
Sulphur hexafluoride (SF₆) air distribution test (optional, may be conducted during pilot testing)

Diagnostic Tools

Diagnostic tools can be used with AS to demonstrate remediation effectiveness, differentiate contaminant removal processes (volatilization versus biodegradation), assist in system optimization, and to support transition to another active remediation strategy, monitored natural attenuation (MNA), no further action (NFA), or site closure. Diagnostic tools to evaluate AS performance include compound-specific isotope analysis (CSIA), functional gene (mRNA) expression, and presence of metabolites that are characteristic of aerobic and anaerobic biodegradation. In an AS field study, the above tools were used to demonstrate the occurrence and relative importance of biodegradation and physical (volatilization) removal of PHCs (Bouchard et al. 2018). Isotopic shifts were used to identify shifts in predominant in-situ PHC attenuation processes. Differentiation of PHC mass removal via volatilization and biodegradation can also be achieved using multi-tracer push-pull tests (ESTCP 2002).

Note that AS implementation is defined to include pre-operation baseline testing of the above metrics (when applicable), including geochemistry data, pressures and water levels. The geochemistry data should also be reviewed to assess the potential for reduced zone (or radius) of influence (ZOI) associated with the clogging of air channels due to iron precipitation and/or biofouling. Sites with groundwater under reducing conditions with high iron content are more prone to iron precipitation.

Pilot testing is a valuable step to identify whether AS is potentially infeasible (e.g., because of confining layers that prevent air flow) and to provide data for design and optimization (Appendices II and III). Testing often includes the measurement of injection pressures and injection volumes versus flow rates at injection wells and pressures exhibited by water-table mounding and dissipation (if injection is pulsed) within the aquifer using pressure transducers installed in monitoring wells. Pulsed AS operation has significant potential to increase contaminant mass removal rates through improved air distribution as new air channels are formed during each pulse and possible increased AS zone of influence (Appendix III). Helium tracer testing (see call-out box) and sulphur hexafluoride (SF6) tracer testing of air distribution may also be conducted during pilot testing to assess ROI.

Subsurface Metrics

Recommended subsurface performance metrics include:

Concentrations in soil, groundwater, and soil vapor (Table 1)
Mass flux or discharge in groundwater and/or soil vapor

The primary subsurface metrics used to assess AS performance are typically those related to groundwater, either concentrations or mass flux/discharge of COCs or general water-quality parameters (e.g., dissolved O₂ and other geochemical parameters). If SVE is implemented together with AS, then performance will also be gauged based on soil-gas pressures, concentrations, and mass flux/discharge data and measurements of fixed gas concentrations that are evidence of biodegradation (e.g., O₂, carbon dioxide – CO₂, and methane – CH₄). CSIA and other diagnostic tools (see call-out box) can also be used to document biodegradation and the primary process for PHC attenuation (e.g., volatilization or biodegradation), which as noted previously, can shift over time following AS implementation. The monitoring of soil gas concentrations and pressures should be optimized so that needed data on both SVE and AS are obtained (e.g., radius of influence, migration pathways, sparging air capture).

Evaluation of Air Sparging Performance

In general, AS performance should be evaluated relative to the following key metrics:

time to achieve objectives
cost (e.g., $/kg HC removed or treated)
greenhouse gas emissions (GHG) or other quantifiable sustainability metrics (e.g., kg CO₂ emissions/kg HC removed or treated)

All performance metrics should be interpreted relative to site specific factors, such as, water table fluctuations, groundwater flow, and seasonal climatic conditions. Monitoring programs should take into consideration spatial and temporal variability in performance metrics data caused by the above factors. Additional information on AS system pilot testing, monitoring, and optimization is provided in Appendix II and Appendix III, respectively.

Transition Thresholds

Transition thresholds are comprised of remedial objectives and relevant performance metrics. The transition thresholds are used to support remedial decisions related to transitioning to another remedial technology or terminating operation of active remediation altogether. Like performance metrics, transition thresholds should be established and agreed with all relevant stakeholders prior to the onset of AS.

The decision to terminate AS requires confidence (multiple lines of evidence – MLE) that rates of PHC attenuation are sufficient to meet remedial goals after the system has been turned off. Eight transition thresholds (T1 to T8) are provided below and in Table 1 and Figure 2 which are similar to those in the SVE Factsheet, given that the remedies are air-based technologies and often implemented together:

T1. Stable or shrinking dissolved groundwater and soil vapor plumes.

T2. Subsurface concentrations and/or mass flux, mass discharge or mass loading less than or approaching defined risk-based or regulatory criteria (i.e., where MNA is expected to “finish the job”)².

T3. System total mass removal and/or mass removal rates approaching asymptotic levels or risk-based thresholds indicating reducing efficacy of remediation.

T4. Minimal or acceptable rebound (i.e., stability) in groundwater or vapor concentrations based on quantitative analysis of plume concentrations, mass discharge or mass loading.

T5. Mass removal rates similar to natural attenuation or NSZD rates obtained from the baseline assessment³ for either total hydrocarbon or individual COCs. The mass removal rates can be compared based on an area normalized rate in units of mass per time-area or for a representative LNAPL-impacted area or “site” in units of mass per time.

T6. Decreasing mass removal rates and corresponding GHG emissions (or other quantifiable sustainability indicators) per mass of contaminant removed that are increasing exponentially.

T7. Decreasing mass removal rates and corresponding costs per mass of contaminant removed that are increasing exponentially.

T8. Preferential loss of lighter molecular weight hydrocarbons (e.g., risk-drivers such as benzene) from groundwater or LNAPL and/or ratios of light to heavy molecular weight hydrocarbons that have reached asymptotic levels and/or risk-based thresholds.⁴

The transition thresholds should be selected depending on site conditions and project requirements. While a comprehensive list of thresholds is provided to provide flexibility in options, typically only a smaller subset of thresholds or in some cases a single primary threshold may be warranted. Depending on the site, a MLE approach can improve confidence and acceptance of the transition, and often can be achieved through analysis of readily available data collected during the remediation life cycle.

Transition Threshold T3 can be measured from SVE monitoring data, assuming SVE is also applied. The contribution of mass recovery from AS compared to SVE can also be quantified by using helium as a tracer. A given mass of helium is placed into the injected air and then measured in the SVE exhaust with and without the AS system on (Johnston et al. 2002).

Tracer Tests

Tracer tests have historically been an important element of AS pilot testing and design. For example, ESTCP (2002) recommends the use of helium (He) as a tracer to 1) assess the effectiveness of the SVE system for capturing vapors caused by AS and 2) identify locations where sparge gas moves from the saturated zone to the unsaturated zone. The % Recovery of the sparge gas can be quantified as follows:

where Q_SVE is the SVE flow rate; C_off-gas is the He concentration in the SVE off-gas; Q_tracer is the flow rate of the injected He; and C_{gas tank} is the initial concentration of the He injected. ESTCP (2002) also provides a simplified recovery calculation based on concentrations where any % Recovery > 80% is considered good recovery and any % Recovery < 30% indicates the likely presence of a hydrostratigraphic barrier preventing upward migration of air. % Recovery values in-between 30 and 80% indicate fair performance. The use of tracers in AS and SVE applications may become more limited with increasing scarcity of tracers such as He and concerns with GHG emissions.

Metric	Applicable (Y/N)?	Locations/Frequency
Performance Metrics
System
P1. Injection air pressures at injection wells and system header
P2. Injection air flows at injection wells and system header
P3. Pressures in unsaturated and saturated zones at monitoring points
P4. Dissolved oxygen concentrations at monitoring wells
P5. Water level mounding
P6. Geochemistry data
P7. Monitoring of air bubbles
P7. Helium tracer test (optional)
P9. Sulphur hexafluoride air distribution test (optional)
Subsurface
P10. Soil, groundwater, and soil gas concentrations* at monitoring locations
P11. Groundwater and soil vapor mass flux and mass discharge
Transition Thresholds
T1. Stable or shrinking groundwater and soil vapor plumes (consider statistical tools such as Mann-Kendall, regression and spaciotemporal plume or mass analytics)
T2. Subsurface concentrations and mass flux, discharge or loading are at or approaching criteria
T3. System mass removal rates approaching asymptotic level
T4. Minimal or acceptable rebound
T5. Mass removal rates during AS operation approaching natural attenuation (i.e., NSZD) rates
T6. Decreasing mass removal rates while normalized GHG emissions are increasing
T7. Decreasing mass removal rates while normalized costs are increasing
T8. Compositional change indicating decreasing light fraction as indicated by concentration ratios

* VOCs and TPH in soil, groundwater, and soil vapor; natural attenuation parameters (e.g., O₂, CO₂ and CH₄) in soil gas and/or dissolved phase.

Table 1 - Summary of Example Performance Metrics and Transition Thresholds Checklist (select applicable metrics and thresholds as needed)

Figure 2 – Conceptual examples of transition thresholds (Toolkits refer to Golder 2016, 2021).

Air sparging may, in some cases, be conducted in absence of SVE, for example, when the water table is relatively shallow (because to the vadose zone is not sufficiently thick to capture vapors with SVE) and if there are no health and safety concerns related to vapor migration (e.g., no buildings or enclosed spaces present). Without the SVE system monitoring data, AS mass removal (Threshold T3) is challenging to measure, which consequently has implications for Thresholds T5 to T7. The use of surface efflux measurements of PHCs and CO₂ may be used to characterize hydrocarbon depletion.

A full accounting of the biodegradation mass loss associated with AS requires consideration of how the technology affects processes in both the unsaturated and saturated zones. Historical research on AS (Appendices I and II) has focused on estimation of biodegradation mass loss in the saturated zone. Mass transfer of VOCs and unconsumed oxygen to the unsaturated zone and subsequent biodegradation is potentially unaccounted for unless respiration type tests are performed (see Bioventing Factsheet). Depending on whether AS is paired with SVE, there are options for characterizing biodegradation rates. If there is no SVE system, efflux can be monitored at ground surface. While no literature could be found on specific application of efflux for AS monitoring, there are extensive literature and guidance available on NSZD (e.g., Garg et al. 2017; API 2017, see the Natural Attenuation Factsheet). If SVE is occurring, the CO₂ flux from aerobic biodegradation can be quantified through radiocarbon monitoring of SVE exhaust (see the Bioventing Factsheet).

Validation

Validation testing for AS is generally conducted by turning off the active system, either in phases or in entirety, and monitoring the potential response in plume concentrations or mass discharge over a specified period within the zone of system operation. Many of the same tools, analysis, and metrics used to gauge remedial performance can be used for this purpose. Constituents to be monitored can include individual hydrocarbons or signatures of hydrocarbon biodegradation (e.g., dissolved O₂, dissolved CH₄).

Groundwater quality monitoring data obtained from conventional monitoring wells may be compromised by AS operation (due to localized aeration) and a slow rise in hydrocarbon constituent concentrations may occur over periods of 1 to 12 months after cessation of the AS system (Johnson 1998). The criteria for rebound in Bass et al. (2000) is recommended to guide interpretation of data:

where Cr is the dissolved concentration during post AS monitoring (rebound phase), Co is the dissolved concentration at remediation initiation, and Cf is the dissolved concentration at remediation completion. A rebound value of less than 0.2 has been determined to be representative of permanent reduction, while a rebound value of greater than 0.5 has been determined to indicate that there is significant rebound based on a database review (Bass and Brown 1996).

Bass et al. (2000) found that rebound was particularly prevalent at PHC sites where there was a LNAPL smear zone and seasonal water table fluctuations that exposed the groundwater to new sources of contamination. NAVFAC (2001) recommend turning off the AS system for 12 to 18 months while monitoring for rebound. While the timeframe for rebound assessment is site-specific, these guidance and factors should be considered in the monitoring design. Again, the process for system shutdown and rebound monitoring should be agreed upfront with all key stakeholders before the AS system is first turned on and at a minimum prior to turning the system off (i.e., as transition thresholds are becoming imminent).

Appendix I – AS Technology Description

This appendix provides an overview of AS technology, discussion on AS air distribution, AS implementation and biosparging considerations.

Overview of Technology

AS is a remedial technology primarily intended to address composition-based concerns for groundwater plumes. Some contaminant mass removal is also achieved. AS is implemented by injecting air below the water table, which enhances volatilization and removal of volatile constituents in light non-aqueous phase liquid (LNAPL) and petroleum hydrocarbon contamination in the saturated zone. Volatilized VOCs move upward because of pressurized air flow in channels or by buoyancy in air bubbles to the vadose zone, where typically a SVE system is used to capture the VOCs. AS targets remediation of lighter molecular weight contaminants of concern (COCs), including risk-drivers such as benzene. Additionally, oxygen in injected air will stimulate contaminant mass destruction through aerobic biodegradation. AS, when specifically targeting biodegradation is referred to as biosparging, and may involve use of air, pure oxygen, ozone or other injectants.

Three main factors affecting air sparging performance are (ESTCP 2002):

The air distribution in the target treatment zone.
The distribution (location and concentration) of contaminants relative to the air distribution.
The contaminant characteristics (composition and chemical properties).

All other factors being equal, AS remediation is more effective in settings having a higher density of air channels in the treatment zone. The conclusion of ESTCP (2002) was that it is not possible to predict the air distribution, except in a gross or approximate sense for simple geology. Consequently, an approach based on pilot testing and empirical knowledge is emphasized for air sparging design (Appendix II). Select research on air distribution is summarized below given the importance of this factor.

AS Air Distribution

Multi-phase fluid flow in the saturated soil zone is highly complex. Understanding fluid flow is important because of its effect on the efficiency and extent of contaminant mass transfer.

Overview of Historical AS Research

Historical AS research has demonstrated that air flow in saturated soil from AS in typical depth wells primarily occurs in the form of pressurized air flow in channels where air continuously displaces water in channels (Johnson et. al. 1993; Ahlfeld et. al. 1994; Hinchee 1994). Only a small percentage of sites have an average grain size of 2.0 mm or larger, which is necessary for bubble transport in soil through buoyancy effects (Ahlfeld et. al. 1994).

The physical processes of groundwater upwelling are also important to understand. When air is injected in a well, air displaces the water within the aquifer near the well screen as air channels are formed. The water is then displaced upwards and laterally away from the zone surrounding the well screen. Once the air channels are formed and stable, the water table then returns to near static levels. After air injection ceases, the water flows back into the voids with higher air content as the remaining air rises to the water table (NAVFAC 2001; US ACE 2013).

The physical configuration of air channels has a significant effect on contaminant mass transfer. The reason for this is because for volatilization to occur, VOCs must migrate primarily by molecular diffusion in water to the nearest air channel (NAVFAC 2001; ESTCP 2002). As the air channels are small, and the distance between air channels can vary from centimeters to meters, the mass transfer rates will also consequently vary.

Research on AS Air-Channelling

Since the rejection of the bubble flow conceptual model for in situ air sparging, most practitioners have adopted the conceptual model of air channeling, which generally implies the development of widely spaced, discreet air channels that bypass large regions of the subsurface. Clayton (1998) presented field and laboratory data that supported a site-specific conceptual model where channelized air flow occurred in coarse sand deposits with lower bulk air saturation but where more uniform air flow occurred in fine sand deposits with higher bulk air saturation that contrasted with the widely spaced air channel model. Breakthrough air saturations, which represent the minimum air saturations that will conduct air flow, of approximately 0.02 to 0.04 were observed in coarse sands. In contrast, breakthrough air saturations of 0.10 to 0.13 were observed in fine and medium sands. The transition between these behaviors falls at about 15 to 20 cm water air entry pressure.

The research by Clayton (1998) indicates that, at both the field and laboratory scale, coarse sands are more prone to air channeling and bypassing than fine sands. Additionally, the larger air gradients and capillary pressures in fine sands result in a less buoyancy-dominated flow pattern, with a larger lateral extent of air flow. Mortensen et al. (2000) in a study of sparging at a MTBE site reported similar results to Clayton (1998) and additionally determined from comparison of modeling to laboratory studies that kinetic mass transfer limitations may control the volatilization and treatment rate. This research indicates the universality of widely spaced air channels in homogeneous media is not supported by available evidence and that the air distribution is highly dependent on the site geology.

Effects of Macro-scale Geology

Macro-scale geology and soil layering can also affect gas migration pathways as demonstrated by the research on fugitive gas migration from integrity compromised energy wells (Cahill et al. 2017). Vertical migration of free phase gas migration by buoyancy tends to occur in the saturated zone, but gas migration is highly influenced by layering of soil and permeability contrasts. Mostly vertical upward migration of free phase gas occurs in homogeneous soil deposits, but significant lateral migration of free-phase gas occurs when there is soil layering and permeability contrasts (Forde et al. 2018).

Gas Ebullition in Sediments

The outgassing or degassing of methane and carbon dioxide and ebullition (upward gas transport) of these gases in aquifer porous media to the water table is described by Amos and Mayer (2006) and Garg et al. (2017). There is also an extensive body of literature on ebullition of gases in often fine-grained sediments present near the sediment/surface water interface (Zamanpour et al. 2020; Viana and Rockne 2021). The controlling processes and factors for gas migration in sediments may be different than in aquifer materials. When sparging in sediments, this literature should be consulted.

AS Implementation

Air injection can be performed using vertical or horizontal drilled wells, and from injection points in trenches or reactive barriers. AS may be configured to treat PHC sources and/or associated groundwater plumes. Air compressors are used to convey air under pressure. Pure oxygen injection while less frequently used than air may be considered when the primary objective is biosparging (see Appendix II). When SVE is used simultaneously, a network of vapour extraction wells is also provided in the vadose zone and vacuum pumps are used to create a negative pressure to extract the sparged air and volatilized vadose zone constituents.

The zone of influence of air sparging and well spacing is an important consideration in AS design. US ACE (2013) recommends that injection well spacing be designed to meet a minimum bulk air content of 3% in the target saturated zone. While geophysical methods and tracers can be used to measure air content, these methods may not be practical at some sites. Monitoring of water levels, dissolved oxygen and air pressures can also be used to provide insight on the approximate zone of influence but limitations in these data and potential to overestimate the zone of influence should be understood (ESTCP 2002; US ACE 2013). The effect of soil heterogeneity on air distribution should also be understood.

Biosparging Considerations

Commonly, at the beginning of air sparging, a higher initial mass removal rate is achieved under near-equilibrium conditions where volatilization of constituents that are in direct contact with air channels occurs. During the later stages of air injection, the mass partitioning from NAPL, sorbed or liquid phase to gaseous phase can become rate limited by diffusion in water-filled pores and mass recovery may approach an asymptotic limit. Under this condition, biodegradation rather than volatilization can become the dominant mass depletion process. Pulsed operation of AS can increase mass recovery rates (Appendix III). Under a biosparging approach, injectant rates should be optimized to increase efficiency. ESTCP (2002) present design calculations for biosparging based on key factors including the estimated rate of oxygen mass transfer from air to water, hydrocarbon mass flux and stoichiometric requirements for oxygen in aerobic biodegradation reactions. Oxygen mass transfer rates can be conceptualized as the oxygen delivery rate to groundwater outside of air channels. It is a difficult to estimate this parameter although it can be measured using a SF6-based diagnostic tool for assessing air distributions and oxygen transfer rates (Bruce et al. 2001).

Appendix II – Pilot Testing and Monitoring

Guidance on pilot testing and monitoring of AS is provided in NAVFAC (2001), ESTCP (2002), USACE (2013), Shell (2007) and CRC Care (2018b). The summaries of these guidance documents are intended to provide practitioners with an overview of decision frameworks and data collection activities for both air sparging and bio sparging. Because of the technical nature of air sparging design, it is recommended that the practices in these documents be well understood. The differing remedial goals and objectives that may be achieved between air sparging and bio sparging systems should be recognized, and the practically of AS remediation depending on the goals / objectives and site size should be recognized. Depending on the site, the remediation may focus on air sparging or bio sparging, or a combination or treatment train approach involving both technologies.

NAVFAC (2001) Air Sparging Guidance Document

NAVFAC (2001) provides a basic description of a single-well AS pilot test. A useful summary of pilot test activities is provided together with questions to be answered, as follows: 1) baseline sampling, 2) injection pressure/flow rate test, 3) groundwater pressure response test, 4) helium tracer test of air extent, 5) soil vapor/SVE off-gas sampling, 6) dissolved oxygen (DO) measurements, 7) direct observations (e.g., odours, bubbles, etc.) that could indicate unintended conditions, 8) sulphur hexafluoride (SF6) test of air distribution and 9) other geophysical tests of air distribution. This guidance is a good starting point for gaining an understanding of pilot testing prior to immersion in details provided in ESTCP (2002) and US ACE (2013).

ESTCP (2002) Air Sparging Design Paradigm

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

Figure II-1 – Sequence of Activities During Implementation of the Air Sparging Design Paradigm in ESTCP (2002)

US ACE (2013) In-situ Air Sparging

US ACE (2013) presents detailed information that adds to the knowledge base in ESTCP (2002). A relatively simple framework for pilot testing parameters is presented (Figure B-2). Useful technical information is provided on conduct of pilot tests and interpretation of monitoring data, including:

Interpretation of water table mounding and pressure data including avoiding overestimation of zone of influence from these data
Pressure versus injection response including air entry pressure to evaluate aquifer characteristics for air sparging
Interpretation of dissolved oxygen data including issues with in-well aeration
Conduct of tracer tests including use of helium and sulphur hexafluoride
Short term versus long-term pilot testing

Figure II-2 – Process for Implementation of In-situ Air Sparging in US ACE (2013) (IAS = in-situ air sparging; TDR = time-domain reflectometry; ERT = electrical resistance tomography).

Shell (2007) Protocol for Oxygen Pulse Injection System (OPIS)

Shell has developed a protocol for biosparging using an Oxygen Pulse Injection System (OPIS) (Shell 2007). Dissolved oxygen concentrations in the range of 10-40 mg/L are possible since nearly pure oxygen is being injected. OPIS can address dissolved contamination or residually contaminated areas. While it can be applied as a barrier to downgradient contamination or as a source treatment technology, the two basic implementation pathways have different well designs. Barrier systems are installed as a fence line of wells extending the length of the plume’s width. Residually contaminated areas and entire dissolved plumes can be treated by wells covering the entire areal extent of contamination. While focused on biosparging and optimization using oxygen, many concepts and aspects of the protocol are generally applicable to AS.

Before applying OPIS, the following site characterization data are necessary:

Closure goals
Depth to water (ft bgs)
Historic water table fluctuation (seasonal, long-term)
Groundwater flow direction
Vadose geology (lithology, permeability, type of cover, layering, etc.)
Saturated geology (lithology, permeability, layering, etc.)
Aquifer characteristics (saturated hydraulic conductivity, anisotropy, gradient/direction, number of zones, etc.)
Vadose zone impacts (PHC type, current maximum concentrations, LNAPL)
Groundwater (GW) impacts (PHC type, is there LNAPL, number of zones impacted, current maximum concentrations)
Location of PHC sources
Date/type of known release(s)
Delineated? (if yes, describe magnitude/size, on-site versus off-site)

The remediation implementation process involves 1) determining design objective (barrier or areal treatment), 2) conducting pilot test, 3) designing system; and 4) conducting monitoring.

Two types of pilot tests are recommended: 1) Default Design (wells on 5-10 ft centers depending on remediation objective) where monitoring consists of injection pressure vs. flow rate test, injection volume vs. flow rate test and transient pressure transducer response or 2) Site Specific Design (other well spacing) where monitoring consists of default parameters plus dissolved oxygen response (monthly for at least 3 months). OPIS equipment consists of oxygen generators, controllers, Program Logic Controller (PLC), piping, solenoid valves and injection points.

Monitoring includes process control variables (operating pressures, injection duration) and components (e.g., dissolved oxygen, oxidation-reduction potential (ORP) and contaminants of concern).

Figure III-3 – Oxygen Pulsed Injection System Design Process in Shell (2007)

CRC Care (2018) Technology guide: In-situ air sparging

CRC Care (2018) identify the objectives of treatability or pilot testing as:

Identify indicators of infeasibility (showstoppers);
Characterise the distribution of air that is likely to occur (i.e., the effectiveness of AS); and
Identify safety hazards to be addressed in the full-scale design.

The process for AS design and pilot testing process in CRC Care (2018) is almost the same as the process described in ESTCP (2002).

Summary

The primary pilot test objectives are typically associated with the injection of air and air distribution, as follows:

Evaluate ability to introduce air into the zone of contamination (i.e., is AS feasible?)
Estimate the air distribution in the target treatment zone (extent and spatial orientation)
Estimate the distribution (location and concentration) of the contaminants relative to the air distribution

Pilot testing can provide data on potential red flags, for example, where water table mounding is not observed or is highly transient, or where pressure data is non-uniform, potentially indicating short-circuiting. When detailed pilot testing involving tracer testing is conducted, the data can provide more nuanced evaluation of potential variability in air distribution due to geologic variability or other factors.

Pilot test objectives typically are not focussed on long-term contaminant reduction trends (because for practical purposes pilot testing is typically conducted over a few days) but can provide useful data on potential maximal increases in initial mass recovery.

Pilot testing in historical guidance has been integrated with empirical approaches for air sparging well spacing and flow rate design. For example, NAVFAC (2001) recommend a well spacing of 30 feet apart (and smaller spacing for shallow sparge wells) with flow rates of 5 to 20 cfm, while ESTCP (2002) recommend a relatively dense default well spacing of 15 ft and flow rate of 20 cfm.

Appendix III - Optimization of Air Sparging Systems

Optimization activities to improve performance and sustainability of AS systems can include:

Pulsed operation of the AS system operation to increase mass recovery through improved air distribution thereby reducing remediation timelines. Additionally, pulsed operation of multiple banks of wells can reduce equipment and operation costs. When AS is intended as a treatment barrier, pulsed operation can reduce the potential for groundwater diversion and flow pass.
Rebalancing or addition of air flow injection wells to increase flow rates in zones with highest mass removal rates as measured by SVE system or surface efflux monitoring if there is no SVE system present.
Limiting or reducing the number of air injection wells that are connected to a single flow controller to increase the ability to target select zones and increase uniformity of treatment within the target zone.
Hydraulic or pneumatic fracturing to enhance remediation by increasing the effective (interconnected) porosity of subsurface materials, which may be particularly effective at sites with low-permeability soil and geologic media.
Conversion of an air sparging system to a lower energy biosparging system with lower air flow rates, which could reduce operational costs.
Sparging of pure oxygen to increase dissolved oxygen (DO) concentrations and thus increase biodegradation rates (Shell 2007). Use of pure oxygen can potentially increase DO concentrations up to 40 mg/L in groundwater. Pure oxygen requires specialized equipment, requires that appropriate health and safety measures be followed, and may increase potential for clogging (e.g., when groundwater contains high iron content).
Use of packers to isolate zones of the air sparging well screen to increase removal efficiency (Drucker 2007).
Porous sintered polyethylene tips are considered by some practitioners to increase mass transfer efficiency of air to water surrounding the well (US ACE 2013) but there is limited research on their effectiveness. Most applications of air sparging use conventional wells with slotted screens.

Review of Pulsed AS Operation

Pulsed AS operation has significant potential to increase contaminant mass removal rates through improved air distribution (i.e., new air channels are formed during each pulse) and possible increased AS zone of influence (NAVFAC 2001). Several examples of improved performance described in the literature are:

Bruce et al. (2001) reported results of field studies where there was about a 30% increase in cumulative removal resulting from pulsed operation versus steady injection conducted at a single well where operation was varied. The AS system at the site was operated in a pulsed mode with a cycle of 3 hr on and 3 hr off, which allowed for pressure to return to near hydrostatic conditions before initiation of the next pulse cycle.
Johnson et al. (1999) compared mass removal rates of hexane and octane for continuous and pulsed air sparging in laboratory sand tank experiments. The mass recovery under pulsed air flow was up to about 2X the rate observed under continuous air flow. Higher recovery was obtained for octane compared to hexane, which was attributed to the higher solubility of octane.
Yang et al. (2005) report on a field study at a gasoline contaminated site where an AS system operated in continuous mode for 3 years was converted to pulsed operation. Monitoring of AS parameters in fine sand aquifer indicated optimal pulsing frequency of 4 hrs on and 4 hrs off based on maximizing the DO and hydrocarbon concentrations when the system was on and allowing the groundwater table to recover while the system was off. Pulsed operation increased the average hydrocarbon removal rate (kg/day) by a factor of up to 3X as compared to the previous continuous operation.
Neriah and Pastor (2017) report results of sand tank experiments where air injection pressures were increased in short bursts (15 sec) over a 10-min cycle (this optimization was referred to as boxcar). Comparing the boxcar to conventional continuous air-injection shows up to a three-fold increase in the single well radius of influence, dependent on the intensity of the short-duration pressure-pulses. The cleanup efficiency of toluene from the water was 95% higher than that achieved under continuous injection with the same average conditions.

Guidance on pulsed AS operation is provided in NAVFAC (2001) and ESTCP (2002). NAVFAC (2001), citing a database by Bass et al. (2000), indicated the projects that implemented pulsing had pulse times ranging from 0.5 to 24 hours, with an average time of 10 hours. NAVFAC (2001) citing Gordon (1998) indicate hydraulic mounding can be mitigated by setting up a network of sparge wells, and sparging the downgradient wells first.

Recommended criteria for pulsing frequency is based on visual observation of aquifer relaxation following the start and end of air injection, where pulse times can be determined based on time required for hydraulic gradients (local groundwater mounding) to dissipate after the onset or termination of an interval of air sparging. Evaluation of pulsing frequency can be assessed as part of pilot testing. As new air channels are formed during each pulse, a system that is initially continually operated, will still see benefit when switched to pulsed operation.

Since the seminal work on AS in late 1990’s and early 2000’s, there has been limited study of optimization of AS through pulsed operation. US ACE (2013) provides important information on technology factors for pulsing including design features to optimize operation of equipment and avoiding well siltation through use of check valves.

Neriah and Paster (2018) evaluated how air sparging could be optimized through pulsation frequency by conducting laboratory studies. Small differences in pulse duration (5 minutes) had a modest effect on mass removal rate (12%). Neriah and Paster (2018) also showed how numerical modeling could potentially be used to optimize pulsing duration and that modeling resulted in different criteria compared to the pulse duration based on measurements of groundwater pressure response (GPR) to air injection.

Other Optimizations

Surfactant enhanced air sparging has been researched for close to two decades (Qin et al. 2013; Xu et al. 2022) and has promise for increasing air saturation but there appears to be little field experience with this technology. Hot-air injection has been identified as possible technique to increase contaminant volatilization⁵, which may be beneficial at sites with semi-volatile contaminants. There are few studies in literature on hot-air injection and while there could be some benefit in increasing temperature, the effectiveness and application of this technology is not considered proven. Injection of nutrients such as ammonia in the air stream has also been proposed but most studies of intrinsic and enhanced bioremediation suggest nutrients are generally not rate limiting at sites.

Complementary Technologies to Air Sparging

There are complementary technologies that share similarities to air sparging that are briefly summarized below.

Ozone gas sparging may be considered as an in situ chemical oxidation (ISCO) technology (ITRC 2005), although it also promotes physical removal through stripping near the injection point and biodegradation through conversion to oxygen. A detailed review of use of ozone is provided in Clayton (2011). A briefer summary of advantages and disadvantages is provided in US ACE (2013). Ozone is a strong oxidant but is highly unstable (and consequently must be generated on-site) and has a short half-life. It requires special precautions for use. While there are potential advantages with use of ozone, it’s use for treatment of petroleum hydrocarbon is less common, possibly because of disadvantages identified here.

Injection of water that is super-saturated with carbon dioxide is a technology with potential novel features that could increase efficiency of treatment. When injected, carbon dioxide will come out of solution and result in a high density of bubbles or a multi-phase water and air mixture that potentially could increase the zone of influence relative to conventional air sparging and increase efficiency of gas stripping of contaminants (Nelson et al. 2009; Zhao and Ioannidis 2011; Enouy et al. 2011).

¹ There are potential disadvantages with both He and SF6 as tracers. There are reports that He is becoming scarce. SF6 is a known greenhouse gas. Unfortunately, there do not appear to be alternative gases that have been researched in context of AS testing. The current recommendation is to use as small amount of these gases in tracer tests as able.

² Remedial clean-up standards may be less stringent at sites where PHC plumes are well delineated, concentrations are decreasing, and certain natural attenuation conditions are met. For example, Florida has established Natural Attenuation Default Concentrations (NADCs) that are 10 to 100 times the groundwater clean-up criteria for select substances (Florida DoS, 2010).

³ A baseline assessment can consist of testing of natural attenuation (or NSZD) rates for total (“bulk”) hydrocarbons or individual COCs either prior to remedy implementation or during rebound testing (see Natural Attenuation Factsheet for additional details).

⁴ Concentration ratios can be readily calculated from typically available data. Threshold concentration ratios, for example, can be estimated for ratios of benzene to TPH vapor concentrations to identify risk drivers and thresholds associated with vapor intrusion (Brewer et al. 2013). A concentration ratio approach could also be used as a supporting line of evidence interpreted in conjunction with other transition thresholds.