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

Glossary

Glossary of Metrics

Subsurface Metrics

Geochemical parameters: Measured concentrations of reactants and products of biodegradation reactions in soil gas or groundwater (e.g., O₂, CO₂, CH₄).

Groundwater concentration: Measured concentrations of COCs in wells, which are compared to remedy criteria. As part of a multiple lines of evidence evaluation, time series data of groundwater concentrations of COCs are used to evaluate plume stability.

Groundwater concentration attenuation rate: Analysis of concentration trends through with distance from the source or with time. See description and tools in Table 1 of the Natural Attenuation Factsheet.

Groundwater mass flux, discharge or loading: Collectively referred to as mass-based estimates, the mass flux is the mass of COC flow in groundwater per unit area of interest (perpendicular to the groundwater flow) per unit time. The mass discharge rate is the integrated flux of COC over the cross-sectional area of the source perpendicular to groundwater flow, generally based on measurements near the source zone (e.g. immediately downgradient of the source). Mass loading is similar to mass discharge, except it is estimated closer to the exposure area rather than the source area, for example, at the intersection of a groundwater plume and a surface-water body.

Mass-based estimates are commonly based on COC concentration measurements across a line of wells, from integral pump tests or tracer tests (e.g. finite volume point dilution method).

LNAPL composition: Measured compositional changes in the LNAPL over time by analysis of mole fractions of LNAPL constituents and changes in time as the remediation progresses, and comparison of the COC mole fractions that are actively or naturally depleting with the more recalcitrant chemicals. The approach requires multiple years of measurements for the analysis and estimation of the attenuation rates of COCs in the LNAPL. Reduced mole fraction of volatile or soluble LNAPL constituents can indicate reduced potential for risk drivers that exceed standards.

LNAPL footprint (presence/absence in wells): Time-series evaluation of the LNAPL body footprint (lateral extent) to evaluate whether the footprint is increasing, stable or decreasing. The LNAPL footprint can also be compared before and after remediation.

LNAPL saturation profile: The saturation profile may be obtained from soil sampling and analysis before, during or after system operation. Alternatively, VEQ model can be used to estimate the saturation profile during and after system operation. Comparison of vertical LNAPL saturation profiles before and after treatment can be used to demonstrate reduced saturations.

LNAPL thickness in wells: Apparent LNAPL thickness is often not well correlated to the mobile LNAPL thickness of the formation and LNAPL recovery rate. Apparent LNAPL thickness may not be an accurate indicator of the LNAPL concern. Seasonal data should be obtained to evaluate LNAPL thickness under a range of conditions and should be interpreted based on soil type and whether LNAPL exhibits unconfined, confined and perched behavior. Limited LNAPL thickness or ephemeral LNAPL may indicate conditions close to residual saturation.

LNAPL transmissivity: Hydraulic recovery becomes ineffective for LNAPL bodies exhibiting low LNAPL transmissivity. ITRC (2018) suggests the practical limit of hydraulic recovery corresponds to a transmissivity range of 0.1 to 0.8 ft²/day (3 to 24 cm²/day).

LNAPL velocity: The LNAPL seepage velocity can be estimated from the LNAPL conductivity and LNAPL gradient. The LNAPL conductivity may be estimated from transmissivity measurements or VEQ model. The velocity can be compared to a de-minimis LNAPL velocity of concern. We caution that a theoretical seepage velocity is not typically a robust metric because it does not incorporate NSZD processes that affect the stability of LNAPL body or reduce migration.

Mobile LNAPL: The LNAPL saturation above residual saturation estimated from model (e.g., Vertical Equilibrium (VEQ) model, or measurement data.

NSZD rate (bulk): The NSZD (bulk) rate refers to the total LNAPL depletion through natural processes. NSZD rates can be used in an assessment of LNAPL mobility and for benchmarking to active LNAPL removal rates through an active remediation system.

NSZD rates (COCs): natural attenuation in mass of COC in the source zone that may be assessed through concentration gradients in soil gas or groundwater, or mass-based rate estimates.

Rebound test: The remediation system is turned off temporarily in parts or phases and applicable performance metrics (e.g., LNAPL footprint, LNAPL transmissivity, groundwater concentrations, soil gas concentrations, mass discharge rate of COCs, NSZD rate) are then monitored to determine if there is a rebound in mass or concentration conditions that warrant turning the system back on, or whether the system can remain off because remedial goals are being met. The monitoring duration will depend on the metric considered and time for conditions to come to a quasi-equilibrium. NSZD rates are often measured during this time period for comparison to active recovery rates in the period before the system was turned off.

Transition threshold: Measured data or lines of evidence used to support the transition between one active remediation system and another or from an active system to a passive system. Transition thresholds factor in remedial performance including progress toward, or achievement of, a remediation goal.

Soil vapor concentration: Measured soil gas concentrations of COCs for comparison to remedy criteria. Depending on the site-specific approach to vapor assessment, indoor and outdoor air monitoring may also be required.

Soil vapor discharge or loading rates: Collectively referred to as mass-based estimates, the mass flux is the mass transport of COC in soil vapor per unit area of interest per unit time. Mass discharge is the integrated mass flux of COC in vapor near the source zone. Mass loading is similar to mass discharge in units of mass of COC per unit time crossing an area closer to the exposure point, for example, the intersection between a vapor plume and a building footprint where vapor exposure is of concern.

System Metrics

Concentrations in extracted fluids: Measured concentrations in fluids (gas, water, liquid-phase hydrocarbon) using field detectors such as photoionization detectors, combustible gas detectors, multi-gas detectors (CO₂, O₂, CH₄, N₂), and laboratory analysis. Novel approaches include CO₂ and isotope testing (e.g., ¹⁴C) to assess biodegradation rates, for example in performance assessment of bioventing.

Concentration ratio: Analysis of the measured COC ratios in extracted fluids from the remedial system, where applicable, as an indirect measure of the relatively higher attenuation of the more volatile and/or soluble components and change in the risk profile of the site with respect to the COCs.

Decline curve analysis: consists of 1) the plot of LNAPL recovery rate (LNAPL volume per unit time) versus cumulative LNAPL recovered (LNAPL volume) or the LNAPL transmissivity; and 2) the plot of cumulative LNAPL recovered in log scale, or the LNAPL transmissivity versus time. Decline curve analysis may be used to predict potential additional recoverable LNAPL and timeline for recovery. Such estimates can be compared to NSZD rates.

Extraction flow rates: Monitoring of the fluid flow rates (gas, water, liquid-phase hydrocarbon) as applicable to the selected remedial system (see accompanying factsheets) for the estimation of bulk or COC-specific recovery rates.

Hydrocarbon mass removal: Analysis of the cumulative mass recovery of individual hydrocarbons or range of hydrocarbons over time, or the change in the hydrocarbon recovery rate with time as a multiple lines of evidence approach in assessing system performance and transition (see Figure 3).

LNAPL recovery: Analysis of cumulative LNAPL recovery or recovery rate (LNAPL volume per unit time). A curve reaching an asymptotic limit indicates diminishing effectiveness of recovery.

LNAPL: water ratio: A declining LNAPL to water ratio in extracted fluids is an indicator of diminishing efficiency of a remediation system. The LNAPL : water ratio can be used to estimate the LNAPL transmissivity.

Physical parameters: Monitoring of pressures, temperatures, water levels, and other relevant physical parameters of a specific remediation system (see accompanying factsheets) for system optimization.

System rebound test: Monitoring of system metrics during cyclic operation of a remedial system. For example, comparison of the concentrations in extracted fluid due to rebound after restarting the system following a period during which the system had been turned off. Similar to the monitoring of the subsurface metrics with the exception that measurements are made only during system operation.

Sustainability Metrics

Cost: Tracking the cost of system operation per reduction in risk, which may for example, be volume (gallons) of LNAPL recovered, or reduction in benzene concentration in groundwater. The former is an example for active system targeting saturation concern, and the latter example relates to an active system targeting composition concern. See Figure 3 for data analysis and visualization. As the LNAPL recovery rate approaches an asymptotic level, the cost per gallon of LNAPL recovered increases rapidly (assuming operating costs are relatively fixed).

GHG emissions: Tracking the greenhouse gas (GHG) emissions per unit reduction or change in risk. GHG emissions are typically assessed in mass of CO₂-equivalent (CO₂-e) emitted due to system operation. Mass of CO₂-e can be, for example plotted versus volume (gallons) of LNAPL recovered or reduction in COC concentration in soil gas or groundwater. See Figure 3 for data analysis and example plots. As the LNAPL recovery rate approaches an asymptotic level, the kg-CO₂-e per gallon of LNAPL recovered increases rapidly resulting in a situation where the adverse impact may outweigh benefit of continued LNAPL recovery.

Glossary of Terms

Active remediation: Performed through technologies that rely on engineered solutions that are actively implemented and are typically resource intensive.

Composition concern: Concentrations of individual compounds that comprise contaminant mass within soil, LNAPL, groundwater, and/or soil vapor, mass discharge or mass loading associated with sources that are above generic or risk-based criteria based on protection of human or ecological health.

Mass-based estimates of chemical transport: Mass flux – mass per unit area per time, mass discharge – mass per time crossing plane of interest near the source, and mass loading – mass per time at intersection of plume and receptor.

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.

Multiple Lines of Evidence: US EPA (1999) defines different tiers of site-specific information or “lines of evidence” to evaluate the efficacy of MNA as a remedial alternative: 1) Data analysis that demonstrate a decreasing trend in contaminant mass or COC concentrations over time; 2) hydrogeologic and geochemical data that indirectly demonstrate the occurrence of natural attenuation; and 3) Microbiological data that directly demonstrate natural attenuation through biodegradation.

Natural attenuation: The mass loss of petroleum hydrocarbons 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 petroleum hydrocarbons 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 petroleum hydrocarbons from an LNAPL source present in the subsurface in the area where the release occurred. NSZD rates can also be defined for individual hydrocarbons.

Passive system: Performed through technologies with monitoring-only requirements and little or no energy usage. Examples include monitoring of natural attenuation (including NSZD), phytoremediation, and passive bioventing.

Performance metrics: Are metrics or data used to assess the performance of the remedy and include subsurface metrics and system metrics.

Rebound: Buildup or redistribution of contaminant concentrations (e.g. soil gas or groundwater) or contaminant mass within a treatment area or recovery (extraction) system following temporary, periodic, or permanent system shutdown.

Remedial concern: as per ITRC (2018) definition, it is a LNAPL condition or potential condition that could:
– 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.

Remedial goal: as per ITRC (2018) definition, LNAPL remedial goals are the desired LNAPL condition to be achieved by the remedial strategy or action that constitutes the end of LNAPL management for a specific LNAPL concern.

Remedy criteria: Are criteria that the remedy should achieve, including criteria relating to mobile or migrating LNAPL (e.g., LNAPL presence/absence, transmissivity, recovery) and media concentrations or mass discharge or loading. Remediation criteria may include timelines.

Saturation concern: A migrating or laterally expanding LNAPL body or the presence of mobile LNAPL above a threshold of concern.

Transition threshold: Measured data or lines of evidence used to support the transition between one active remediation system and another or from an active system to a passive system or to no further action. Transition thresholds factor in remedial performance including progress toward, or achievement of, a remediation goal.