Standard Guide for Moving Sites to Closure (MStC) for Petroleum Underground Storage Tank (UST) Releases

Importancia y uso:

4.1 This guide is intended for use primarily by responsible parties, corrective action professionals, regulators, and others who desire to secure regulatory closure or an NFA certification from the applicable regulatory oversight agency at petroleum UST sites requiring corrective actions. While the focus is on sites located in the United States (US), this guide may also be applicable in other jurisdictions.

4.2 The EPA’s Office of Underground Storage Tanks reports that as of March 31, 2024, there were 56 432 leaking UST releases in the US remaining to be cleaned up. These include sites undergoing active remediation or sites in long-term monitored natural attenuation. The goal of this guide is to provide a framework (and tools) for assisting and addressing many of the common barriers that hinder these sites from getting to closure. These barriers are often rooted in a lack of awareness of applicable, technically defensible approaches for achieving regulatory closure or NFA or ways to improve a corrective action that is not making progress toward closure. The barriers may be technical (such as, limited site assessment or ineffective remediation) or non-technical (such as the site being too low on a state priority list or risk ranking to receive funding, unknown or unwilling owners, or site access issues).

4.3 In addition, decarbonizing the US economy is expected to lead to an increase in the number of petroleum UST cleanups over the 56 432 open petroleum UST corrective action sites identified as of March 31, 2024. It is expected that eventually most of the over 500 000 active retail motor fuel UST systems will eventually close as electric vehicles replace petroleum-powered vehicles. It is expected that older, less profitable service stations will close earlier as investors, wary of eventual reductions in gasoline use, will be reluctant to invest in upgrading and replacing the tank systems at these facilities. Releases discovered at these facilities will add to the 56 432 petroleum UST remediation sites that remain to be remediated and obtain closure in the US.

4.4 Guidance is needed to help raise awareness within the regulatory and practitioner communities of best practices based on the latest scientific understanding for the systematic review of remedial progress and facilitation of UST site closure.

4.5 It is important that users make appropriate decisions on these remediation projects to ensure that human and ecological receptors are continuing to be protected and that these decisions should be consistent with the best practices and the latest scientific understanding, including sustainability considerations. Over the last 30 or more years, scientific studies and professional experience have documented site conditions that pose low threat to human and ecological receptors. There have also been numerous scientific tools and methods developed over this time that can be used to demonstrate when site closure is appropriate. Experience has shown that it is technically not feasible, prudent, or necessary to remove all the petroleum at many release sites to protect human and ecological receptors.

4.5.1 The basis for improved LNAPL body stability criteria continue to leverage historic literature and accounts for recent publications. Natural source zone depletion has been demonstrated in literature to exhibit sufficient magnitude to balance out the migration flux at the leading edge of LNAPL bodies resulting in stability (1 and 2)5 of older LNAPL bodies. Assimilative capacity (3) includes biodegradation mechanisms included in natural source zone depletion but also includes losses of mobile LNAPL to a residual or immobile state. The losses to residual have been shown to be a dominant loss mechanism within the initial days of a release where the majority of LNAPL can transition from mobile to residual within a few months (4).

4.5.2 It is important for communities that petroleum release sites are returned to productive use as soon as possible; this is easier if the uncertainties associated with open remediation projects do not impede lending or redevelopment, allowing restoration of the site to productive use.

4.5.3 Regulatory agency and owner staff time and money should not be spent on continued remediation or monitoring of releases that are no longer a threat to human and ecological receptors. The owners and the public deserve to know that the corrective action projects for sites that pose significant risks to human and ecological receptors will be managed as priorities, while sites that pose a low threat obtain closure as soon as possible. Closing corrective action for releases that no longer pose a threat to human and ecological receptors frees up resources for other cleanup projects.

4.5.4 For many petroleum UST releases, allowing petroleum that poses a low threat to human and ecological receptors to degrade in place is the most sustainable corrective action. This approach minimizes negative secondary effects, such as carbon dioxide emissions and waste generation, that can result from active remedies.

4.5.5 In this guide, both active and passive remedies are addressed. Both types of remedies can be appropriate for reducing the threats from petroleum releases. Active remedies, which are also often referred to as engineered remedies, generally operate on a shorter timeframe than passive remedies but require more resources to implement and often do not reach desired concentrations. Passive remedies, which are often, but not always, natural or not engineered, generally operate on longer timeframes, have the advantage of requiring less resources, and can achieve more complete degradation of the petroleum, especially when they are distributed throughout the contaminated media and do not rely on the support of mechanical systems. For example, microbes in the subsurface soils can over time reach and degrade petroleum that is trapped in very small pore spaces that cannot be reached by mechanical means.

4.6 Experience has also shown that there is no single criterion that can determine whether a site no longer poses a threat to human and ecological receptors. In most cases, a multiple lines of evidence approach will compensate for uncertainties in individual criterion, increasing confidence in decisions.

4.7 Experience has demonstrated that it is often better to reevaluate and make changes to a corrective action that is not making progress toward closure rather than continuing to stay the course in an unsustainable manner. While modifying the corrective action takes time and effort, in the end, the increased understanding of the site and improved remedy proficiency will usually save time and money in achieving closure. High-resolution site characterization (HRSC) has been shown to aid in many reevaluations. These methods, while the upfront cost may be greater than traditional boring and monitoring well programs, result in reduced uncertainty and an improved CSM, more targeted remedy selection, faster decision-making, and more efficient site cleanup.

4.8 The petroleum UST cleanup programs across the US are diverse and have developed a variety of approaches to overcoming non-technical barriers such as limited funding, access issues, and recalcitrant or missing responsible parties. While every jurisdiction has unique laws and regulations, the user can apply some of these best practices to keep the corrective action moving toward closure.

4.9 The user may need to adapt the approaches and criteria described in this guide to the regulatory requirements of the regulatory program that governs the corrective action and closure decisions at each petroleum release site.

4.10 The user should check with their regulatory jurisdiction to determine which elements of this framework are allowed for bringing petroleum UST release corrective actions to closure.

4.10.1 Some regulatory programs have the flexibility to adopt or adapt the closure approaches described in this guide. This guide can be used to add options to those regulatory structures. If the jurisdiction allows individual regulators to approve alternative closure criteria at their discretion on a site-by-site basis, this guide can bring certainty to the process of making alternative closure decisions.

4.10.2 Other regulatory programs may need to seek revisions to their guidance, regulations, or legal authorities to implement some or all the approaches described in this guide. In these jurisdictions, this guide can provide a framework for such revisions.

4.11 Activities described in this guide should be conducted by a person familiar with current risk and exposure assessment methodologies.

4.12 This guide is intended to compliment and give context to other available guidance and practices. Where possible, this guide references other guidance and practices and the user is encouraged to seek additional detail in these documents.

Subcomité:

E50.04

Volúmen:

11.05

Palabras clave:

petroleum; RBCA; risk-based corrective action;