Regulatory Update: EPA’s New Clarification on the Federal Underground Storage Tank Regulation on Free Product Removal
Volume 11, Issue 4 | November 2023

Lisa Reyenga, P.E., GEI Consultants, Inc.

The United States Environmental Protection Agency issued a clarification to the Federal Underground Storage Tank (UST) Regulation on Free Product Removal regarding removing free product to the maximum extent practicable. The clarification indicates that only migrating LNAPL must be removed to the maximum extent practicable, not mobile or residual LNAPL.

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Use of Dissolved-Phase Plume Stability to Evaluate LNAPL Body Stability with GWSDAT
Volume 11, Issue 3 | August 2023

Lisa Reyenga, P.E., GEI Consultants, Inc.

LNAPL body stability is a prerequisite to a risk-based approach to LNAPL management. Dissolved-phase stability is a key line of evidence for LNAPL body stability, and it can be efficiently evaluated using the free Groundwater Spatiotemporal Data Analysis Tool (GWSDAT).

Evaluating LNAPL body stability is critical to understand potential risks and the potential need for engineered LNAPL remediation. A stable LNAPL body does not have the potential to create new risks as the result of LNAPL migration into new areas. Stable LNAPL bodies are potential candidates for a risk-based and/or natural remediation approach to LNAPL management.

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Simulating Dissolution of the Most Soluble Compounds from Complex NAPLs Using Equilibrium Partitioning
Volume 11, Issue 2 | April 2023

Michael J. Gefell and Deviyani Gurung
Anchor QEA, LLC

Equilibrium partitioning simulations can be used to estimate the time required to deplete the most soluble components from complex NAPLs that contain a significant insoluble fraction.

Non-aqueous phase liquid (NAPL) dissolution can create persistent plumes of dissolved NAPL components in groundwater. Dissolution of multicomponent NAPLs is complex, and numerical models that explicitly simulate it at a site scale are not widely available. This study introduces an equilibrium partitioning method to simulate, as a first approximation, the time required to dissolve the most soluble chemical components from a multicomponent NAPL that contains a significant fraction of relatively insoluble mass. The effective distribution coefficient that describes depletion of a specific soluble compound from NAPL is calculated based on the NAPL and soil properties.

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Part 2 of Managing NAPL Heterogeneity’s Hijinks
Volume 11, Issue 1 | March 2023

Randy St. Germain, Dakota Technologies, Inc.

In Part 1 of this series, we introduced heterogeneity and how it makes nonaqueous phase liquid (NAPL) characterization inherently difficult. Part 2 examines two relatively different scales of heterogeneity; larger scale heterogeneity (10s to 100s of feet) and smaller scale (inches to feet) because these scales are commonly encountered.

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Managing NAPL Heterogeneity’s Hijinks
Volume 10, Issue 8 | December 2022

Randy St. Germain, Dakota Technologies, Inc.

Heterogeneity, we’re all at least loosely familiar with the term. Most of us don’t bat an eye when we hear it, even though it should get our blood pressure up a bit when we do. In this three part series we’ll explore NAPL heterogeneity, have an honest look at the surprising degree to which it affects our work, and some of the things we can do to manage its inescapable influence.

Anyone involved in non-aqueous phase liquid (NAPL) characterization eventually runs headlong into bewilderment caused by spatial heterogeneity of NAPL in soil. NAPL’s spatial and chemical complexity is particularly troublesome when we’re attempting to characterize NAPL and just the NAPL alone, i.e. the source term.

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Addressing PFAS Contamination in Aquatic Sediments
Volume 10, Issue 7 | November 2022

Kendrick Jaglal, P.E.

When PFAS emerged as a contaminant of concern, the focus was initially on drinking water, soil and then groundwater. With sediment being the eventual sink for most anthropogenic chemicals, sediment began to receive increasing attention and many related questions started to arise. One key observation was the affinity of PFAS to adsorb to NAPL in sediment which means that NAPL remediation may also involve PFAS. This article discusses current considerations that would be associated with assessing and remediating PFAS contamination in sediment and sediment-containing NAPL.

Sediment has historically been a sink for legacy pollutants contained in industrial discharges to surface water. These pollutants have primarily been polychlorinated biphenyls (PCBs), polynuclear aromatic hydrocarbons (PAHs), metals, organic solvents and non-aqueous phase liquids (NAPL). However, over the past several years perfluoroalkyl and polyfluoroalkyl substances (PFAS) began receiving attention for their presence in soil, groundwater and air and the focus later expanded to sediment.

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PFAS and LNAPL/PFAS Microemulsions at NAPL Remediation Sites
Volume 10, Issue 5 | August 2022

Sid Park, Jacobs

Where historical NAPL fires were extinguished using AFFF, PFAS may be retained in the subsurface in residual NAPL mass, at the air- or NAPL-water interface, or as a viscous LNAPL/PFAS microemulsion (LPME). PFAS retention in-situ and potential presence of LPME with phase behavior that is very different from both the LNAPL and PFAS require special consideration for management or treatment. This article summarizes the current understanding PFAS partitioning to NAPL and potential LPME formation for consideration in-situ characterization and remedial design.

Aqueous film-forming foam (AFFF) contains high concentrations (grams per liter) of per- and polyfluoroalkyl substances (PFAS) and was historically used to combat fires where the fuel source was non-aqueous phase liquids (NAPLs) including light non-aqueous phase liquids (LNAPLs) such as diesel, gasoline, or waste oil and/or dense non-aqueous phase liquids (DNAPLs) such as trichloroethene (TCE) (Figure 1). These events occurred at fire-training areas, fuel spill locations, in hangars or on runways, and at bulk fuel storage areas. Conceptually, after being sprayed on the fire, AFFF foam eventually collapsed and AFFF liquid infiltrated the subsurface. PFAS dissolved in water also infiltrated the subsurface. Although several studies showed preferential association of PFAS to NAPL and at the air-water interface (AWI), evidence from one study indicated PFAS can form an immobile LNAPL/PFAS microemulsion (LPME) with milligrams per liter (mg/L) concentrations of PFAS (Figure 2) at the NAPL-water interface (NWI). PFAS retained in NAPL, at the AWI, or as LPME may represent an ongoing source of PFAS to groundwater and limit disposal options for recovered LNAPL, which was previously treated through traditional means that are likely ineffective for PFAS treatment. PFAS persistence in the subsurface and potential treatment difficulty can increase remedial durations and associated costs. Therefore, understanding the presence and behavior of PFAS at sites with NAPL or where LPME may have formed is necessary for site characterization, remediation, and revitalization.

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Using Flux Chambers to Quantify Ebullition Facilitated NAPL and Contaminant Transport
Volume 10, Issue 4 | June 2022

Amy L. Corp, P. Chem, EP, Anchor QEA, LLC

Accurately quantifying gas ebullition and related mass transport of NAPL and other contaminants from sediment to the overlying water column is an important issue at some contaminated sediment sites. This article summarizes procedures for building and using flux chambers to collect technically defensible data.

Gas ebullition is gas bubble formation and growth in sediment, followed by sediment fracture and the subsequent upward migration of gas bubbles through sediment to the surface water column. Migration of gas bubbles through nonaqueous phase liquid (NAPL) or other organic contaminants in sediments may result in the transport of NAPL/contaminants from sediments to surface water (i.e., ebullition-facilitated transport [EFT]). This can be an important transport mechanism for contaminated sediment sites, moving isolated contaminants into the biologically active zone (Viana and Rockne 2021, 2022 ; Fendinger et al. 1992; Viana et al. 2012, 2018; Yuan et al. 2009). Also, at sites where sediment capping is planned as part of the remedy, ebullition should be evaluated to understand potential impacts to cap design such as contaminant transport and potential gas buildup (which could potentially destabilize a remedial cap).

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Assessment and Control of Ebullition Facilitated NAPL and Contaminant Transport in Sediment
Volume 10, Issue 3 | May 2022

Priscilla Z. Viana, Ph.D., Arcadis U.S., Inc.
Karl J. Rockne, Ph.D., PE, BCEE, University of Illinois at Chicago

Gas ebullition can be a significant mechanism for mass transport of NAPL and other contaminants from sediments to the surface water column. Accurate evaluation of the presence and potential of gas ebullition to facilitate NAPL and contaminant migration from sediment to the water column is a critical step in contaminated sediment site characterization, conceptual site model development, assessment of remedial alternatives, and remediation design.

The migration of gas bubbles through sediment and surface water is called gas ebullition. In many organic-rich sediment environments, microorganisms can produce gas comprised of methane with a typical range from 40 to 90%, and lesser amounts of carbon dioxide and other gaseous end products (Casper 2000, Huttunen et al. 2001, Rockne et al. 2011, and Viana et al. 2012). Gas ebullition requires that the gas production rate must be sufficiently rapid to cause gas bubbles to grow, fracture the sediment, and subsequently migrate upward (Boudreau 2012, Zamanpour et al. 2020). Gas ebullition can represent an important mass transport mechanism of NAPL and/or other contaminants to the water column (Fendinger et al. 1992, Viana et al. 2012, Viana et al. 2018, Yuan et al. 2009, Zamanpour et al. 2020). Viana and Rockne (2021) summarize the mechanisms of biogenic gas production in sediment, ebullition processes, and NAPL transport via ebullition. They also provide a summary of site-specific conditions and characteristics that affect gas production, gas ebullition, and associated NAPL and contaminant transport.

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Benefits of Including Climate Change Considerations in a Sediment NAPL Remedy
Volume 10, Issue 2 | March 2022

Breanna Moak, Melissa Harclerode, Sean Sheldrake, Cannon Silver

CDM Smith

For sediment NAPL corrective action to be both effective and adaptive to climate change impacts, resiliency plans that address vulnerabilities for both the site and the remedy are essential. Many state and federal regulatory programs either require or recommend an evaluation of resiliency as an important component of evaluating long-term effectiveness and remedy permanence to protect human health and the environment.

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