Editor: Lisa Reyenga, GEI Consultants, Inc.
ANSR Scientific Advisory Board
Lisa Reyenga, PE, Board Chair, GEI Consultants, Inc.
J. Michael Hawthorne, PG, GEI Consultants, Inc.
Andrew J. Kirkman, PE, BP Corporation North America
Robert Frank, RG, Jacobs
Paul Cho, PG, CA Regional Water Quality Control Board-LA
Randy St. Germain, Dakota Technologies, Inc.
Dr. Terrence Johnson, USEPA
Brent Stafford, Shell Oil Co.
Douglas Blue, Ph.D., Imperial Oil Environmental & Property Solutions (Retired)
Natasha Sihota, Ph.D., Chevron
Kyle Waldron, Marathon Petroleum
Danny D. Reible, Professor at Texas Tech University
Reeti Doshi, National Grid
Mahsa, Shayan, Ph.D., PE, AECOM Technical Services
Chis Marks, Ph.D., Arizona Department of Environmental Quality
Kammy Sra, Ph.D., Chevron
David Edgerton, USEPA
Applied NAPL Science Review (ANSR) is a scientific ejournal that provides insight into the science behind the characterization and remediation of Non-Aqueous Phase Liquids (NAPLs) using plain English. We welcome feedback, suggestions for future topics, questions, and recommended links to NAPL resources. All submittals should be sent to the editor.

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DISCLAIMER: This article was prepared by the author(s) in their personal capacity. The opinions expressed in this article are the author’s own and do not necessarily reflect the views of Applied NAPL Science Review (ANSR) or of the ANSR Review Board members.

Regulatory Update: EPA’s New Clarification on the Federal Underground Storage Tank Regulation on Free Product Removal
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.
The goal of any environmental remediation project is to do the most good (reduce the potential harmful effects of the impacts and minimize risk) while causing the least harm (the negative consequences of the remedial activities themselves). A recent regulation clarification from the United States Environmental Protection Agency (EPA) has the potential to improve environmental practitioners’ ability to weigh these factors and make the most sustainable decisions possible at light non-aqueous phase liquid (LNAPL) remediation sites.

On January 23, 2023, the EPA released a clarification on the Federal Underground Storage Tank (UST) Regulation on Free Product Removal, 40 CFR 280.64. The clarification provides information on the EPA’s intent for liability owners to remove “free product” to the “maximum extent practicable”. While the clarification directly applies to Federal UST remediation sites, it has implications across the industry. The specific criteria for what free product removal to the maximum extent practicable means in practice was entrusted to the implementing agencies. The term maximum extent practicable has also subsequently been adopted in many state and federal regulations applying to a wide range of hydrocarbon release sites. Therefore, this clarification has the potential to change how this requirement is interpreted across the industry.

When there is a release of petroleum hydrocarbons (e.g., gasoline, diesel), it may impact the soil, soil gas, and/or groundwater underlying the release location. The overall Federal UST regulation details all the requirements to address the release. One aspect of the clarification is to identify that the industry understanding of free product has incorporated new science and become more nuanced. Free product is now characterized as residual, mobile, or migrating LNAPL, consistent with industry best practice documents from the Interstate Technology Regulatory Council, the Cooperative Research Centre for Contamination Assessment and Remediation of the Environment, Contaminated Land: Applications in Real Environments, and others. Each category is further defined in the clarification as:

  • Residual – LNAPL that is bound in the soil and will not move into monitoring wells or smear with a rising or falling water table.
  • Mobile – LNAPL that exists above residual saturation levels such that it can accumulate in monitoring wells constructed within its footprint or smear vertically with a rising or falling water table but will not migrate or spread from its current footprint (i.e., move into monitoring wells beyond its current footprint).
  • Migrating – A LNAPL body that is expanding laterally into areas previously not impacted by LNAPL.
Figure 1 – Residual, mobile, and migrating LNAPL (ITRC 2018)
An additional element of the clarification was to define the EPA’s intent behind the requirement for removal to the maximum extent practicable. The clarification states:

  • The federal regulation was written to require owners to remove free product during the early phases of a response to a confirmed release. EPA’s intention was to mitigate the risk of free product spreading to uncontaminated areas of a site.
  • UST Sites may have “migrating LNAPL” as well as “mobile LNAPL” and “residual LNAPL.” Mobile and residual LNAPL are both distinct from migrating LNAPL in that they do not spread laterally and do not increase the LNAPL footprint. Abatement of mobile and residual LNAPL is not necessary to meet the requirement of 40 CFR 280.64 to prevent the migration of “free product” into previously uncontaminated soil or groundwater.

Many implementing agencies currently interpret removal to the extent practicable to mean criteria such as removal of any measurable LNAPL or removal to a maximum thickness such as 1/8”, while others may recognize criteria such as diminishing returns (asymptotic recovery) or an LNAPL transmissivity threshold. The intent of these requirements is assumed to be to ensure that the liability owner for the release fulfills their duty to address the release and remediate the impacts. However, in practice, achieving these criteria can have unintended consequences.
A common scenario at hydrocarbon release sites is:

  • an older release where the source is no longer active;
  • the LNAPL is not migrating and has been stable for many years;
  • the LNAPL is weathered and is not contributing to dissolved or vapor phase contaminants that cause a risk to human health or ecological receptors; and
  • despite remediation, LNAPL is still present at the site and does not meet the implementing agency’s definition of removal to the maximum extent practicable.

In this scenario, the liability owner may still be required to implement engineered remedial activities. These activities consume resources (e.g., electricity, fuel, or water), produce wastes (e.g., contaminated material for disposal, air emissions, greenhouse gases), yet frequently do not meaningfully improve the environment nor reduce any actual risk. Sites can be stuck in this loop for decades where site closure is not possible under the regulations, but no practical options are available to achieve removal to the maximum extent practicable.

This clarification from the EPA has the potential to change the industry and allow for a holistic, sustainable perspective to be applied more broadly to LNAPL remediation sites. The base requirement of remediation is to abate migration and prevent further spread of contamination. After that, requirements for further remediation can be defined based on mitigating risk to human health and the environment and identifying a sustainable remedial approach. Ultimately, this strategy can guide practitioners towards the least environmental harm resulting from LNAPL releases.

A Word of Caution

The EPA Office of Underground Storage Tanks (OUST) directly regulates USTs in Indian country. Each state has its own UST program with its own regulations. The EPA OUST’s position on interpretations of the Federal rules are not binding to the states. Likewise, USTs are only one type of LNAPL remediation site. Other types of sites will be regulated through different programs. The definition for maximum extent practicable for any individual site is decided by the applicable regulatory organization.


UST Technical Compendium: Release Investigation, Confirmation, and Corrective Action, Updated February 23, 2023. https://www.epa.gov/ust/ust-technical-compendium-release-investigation-confirmation-and-corrective-action

Interstate Technology and Regulatory Council (2018). Light Non-Aqueous Phase Liquid Site Management: LCSM Evolution, Decision Process, and Remedial technologies. March 2018.

Research Corner

Jane Ye
Masters Thesis
University of Waterloo


Petroleum hydrocarbons (PHCs) are essential to the functioning of the industrialized world yet serve a potential threat to human and ecosystem health when they inadvertently enter the environment. In recent decades, recognition of natural attenuation as a viable approach to PHC remediation is increasing. Natural attenuation includes the biodegradation of PHCs through respiration, fermentation, and methanogenesis, processes which are also central to the biodegradation of natural background soil organic matter. Biodegradation of both PHCs and natural soil organic matter are a major component of the global carbon cycle and an important source of atmospheric greenhouse gases (GHGs). As a biologically mediated set of reactions, environmental factors like temperature and moisture are important controls on the rates and pathways of biodegradation. It is therefore important to understand the influence of these environmental factors on PHC biodegradation and associated carbon dioxide (CO2) and methane (CH4) effluxes to improve predictions of PHC remediation efficiency and soil GHG emissions under ongoing and future climate change. In Chapter 2, I investigated the effect of soil moisture on PHC biodegradation kinetics, using naphthalene as a representative PHC compound. I performed microcosm incubations with naphthalene-spiked soil at 60, 80, and 100% water-filled pore space (WFPS) under oxic headspace, and at 100% WFPS under anoxic headspace. Incubations lasted 44 days. The results showed that the total naphthalene in soil decreased to below detection after Day 9, 17, and 44 in incubations at 60, 80, and 100% WFPS under oxic headspace, respectively. At 100% WFPS under anoxic headspace, total soil naphthalene concentrations decreased over time but were still detectable past Day 44. Fitting of the naphthalene data to first order decay equations revealed two distinct kinetic regimes of degradation in the oxic incubations: an initial fast regime characterized by an apparent first order rate constant on the order of 10-1 day-1 followed by dominance of a slower degradation regime. In the anoxic incubations, only the slow end-member regime was observed with a corresponding rate constant of 10-2 day-1. Porewater electron acceptor and organic acid data indicated that in the fast regime, naphthalene degradation was dominated by microbial respiration pathways, while in the slow regime fermentative pathways dominated. Results from Chapter 2 imply that spatial and temporal fluctuations in soil moisture – and the associated oxygen (O2) availability – can cause order-of-magnitude variability in the degradation kinetics of PHCs in the vadose zone. In Chapter 3, I investigated the effect of temperature and O2 availability on CO2 and CH4 accumulations in the presence of naphthalene. I performed naphthalene-spiked microcosm incubations under oxic or anoxic headspace at temperatures of 10, 20, and 30°C. Time series data of net accumulated CH4, accumulated CO2, consumed O2, accumulated dissolved inorganic carbon (DIC), and consumed organic acids (OAs) were analyzed using Arrhenius temperature sensitivity curve-fitting. Q10 temperature sensitivity quotients were estimated from this analysis, indicating a greater temperature sensitivity of anaerobic CO2 and CH4 production processes than their aerobic equivalents. I observed that methanogenesis under anoxic conditions had a particularly high Q10 of 9. Overall, findings from this research confirm our understanding of field biodegradation rates. PHC biodegradation in oxic, drier zones is expected to be 10 times faster than in anoxic, saturated zones. The two distinct regimes of biodegradation activity identified in Chapter 2 could also be used as simplified representations of PHC biodegradation when modelling variable moisture and oxygen conditions. Chapter 3 additionally suggests that the CH4:CO2 ratio of soil carbon emissions from anoxic soils may potentially increase with warming temperature. Thus, PHC contaminated sites may see increasing GHG emissions potential, but also increasing contaminant biodegradation rates, in a warming climate, especially those located in saturated soils and cold regions. These expected alterations in soil carbon fluxes are important for the consideration of site managers concerned with site-scale carbon cycling and GHG emissions.

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