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

Tools and Methods to Support the Optimization and Termination of Active Remediation Systems

Authors
Parisa Jourabchi, ARIS Environmental Ltd.
Ian Hers, Hers Environmental Consulting, Inc.
Matthew Lahvis, Shell Oil Products (US)

Active remediation systems are often operated at petroleum underground storage tank (UST) sites impacted by light non-aqueous phase liquids (LNAPLs) beyond where they effectively reduce risk or provide net environmental benefit. Such instances arise for a variety of reasons, including:

the failure to set and agree on remedial concerns and performance criteria;
lack of guidance and framework in clear and concise language for the practical application of available methods to inform remedial decision making; and
uncertainty in the natural assimilative capacity of the aquifer system to attenuate key constituents of concern (COCs).

This Exit Strategy Toolkit was developed to support remedial decision making at sites with active remediation by:

a) improving the understanding of remedial concerns, tools, methods, and data needs;
b) establishing and implementing remediation metrics that can be applied throughout the lifecycle of active remediation; and
c) incorporating natural attenuation, including natural source-zone depletion (NSZD) into the remedial paradigm.

The Toolkit provides a systematic, non-prescriptive approach to improve sustainable, risk-based decision making by recommending four essential elements of a remedial framework:

baseline assessments to quantify existing rates of hydrocarbon attenuation (i.e., natural attenuation rates) prior to active remediation;
performance metrics to assess whether active remediation is performing as intended (e.g., reducing the overall time to achieve remediation goals relative to those achieved through natural attenuation) and providing a net environmental benefit;
transition thresholds to inform the transitions between active remediation systems or to monitored natural attenuation (MNA) or no-further action (NFA); and
validation testing to confirm transitions to MNA or NFA.

The Exit Strategy Toolkit consists of a Compendium and series of Remediation Factsheets:

Soil Vapor Extraction (SVE)
LNAPL Hydraulic Recovery
Bioventing
Air Sparging
Natural attenuation, including NSZD methods

a) CO₂ Efflux
b) Temperature Gradient
c) Soil Gas Gradient
d) Groundwater Monitoring
e) LNAPL Composition

The Compendium serves as a general framework and contextual document for the Remediation Factsheets, which provide detail on the implementation of specific remedial technologies. The Compendium and Remediation Factsheets are motivated by numerous instances of active remediation being operated beyond a period of net environmental benefit, which is largely attributed to the lack of establishment, alignment, and implementation on various metrics, tools, methods, and data needs with key stakeholders prior to implementation. The Toolkit is intended for sites, where the conceptual site model (CSM) is well developed and refined. The Compendium builds off the concepts and terminology presented in the ITRC LNAPL guidance (ITRC, 2018) and reinforces the importance of having well-defined remedial goals and endpoints.

The Toolkit is intended for key stakeholders involved in remedial decision making, including industry representatives, consultants and environmental regulators overseeing the management of sites impacted by LNAPL. Although intended for application prior to the onset of active remediation, the Exit Strategy Toolkit can also benefit sites where active remediation has already been implemented. The Toolkit is intended to be illustrative (i.e., provide links to additional information rather than a checklist) and leverages the latest science, especially on NSZD. The CSM is assumed to be sufficient to identify all exposure pathways and support remedial decision making. Ultimately, application of the Exit Strategy Toolkit is expected to:

provide a more systematic approach to initiating, evaluating, and terminating active remediation;
minimize unnecessary active remediation that provides no net environmental benefit (e.g., lower carbon dioxide (CO₂) emissions, energy use, and cost);
facilitate widespread use of available tools and existing science;
provide more confident remedial decision making;
improve stakeholder communication; and
focus limited resources on sites posing the greatest risk.