The cleanup of petroleum contamination remains a significant challenge at many Environmental Restoration (ER) sites, especially those with the presence of light non-aqueous phase liquid (LNAPL). However, there have been several advancements in the management and remediation of petroleum sites related to new conceptual site models (CSMs), technologies, and tools that have the potential to accelerate cleanup efforts and site closure.
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Petroleum sites include underground storage tanks (USTs), aboveground storage tanks (ASTs), fuel farms, bulk storage, and transportation facilities. Management of Department of the Navy (DON) petroleum sites include both compliance and remediation responsibilities; however, only long-term cleanup of past contamination from petroleum releases is managed under the DON Environmental Restoration Program (ERP).
In accordance with DON policies, immediate or short-term spill response actions caused by current operations must be funded by the operator responsible for the tank maintenance. However, any required long-term remedial actions to address petroleum spill residuals are managed and eligible for funding under the DON ERP. The DON petroleum site cleanup program follows a similar process to other CERCLA sites. State or local regulations may also apply to petroleum site cleanup efforts and provide the most opportunity for Response Complete (RC) acceleration as described below.
LNAPL management remains a persistent and important component to consider in the cleanup of DON petroleum sites. Several opportunities exist to improve LNAPL CSMs used to evaluate risk exposure and remedial options. The LNAPL CSM should help to define the geologic and hydrogeologic features of the site, the LNAPL distribution, if the LNAPL footprint is stable or expanding, chemical composition, and risk to potential receptors. The potential risks at LNAPL sites arise from the toxicity of the chemicals that dissolve from the LNAPL (known as “composition-based” risk) and from the potential for the LNAPL to spread in the subsurface (known as “saturation-based risk”). However, at legacy sites, these risks have often diminished considerably over the years as the LNAPL footprint has stabilized, residual product has weathered, and the more soluble and volatile compounds have degraded. Studies have also shown that the dissolved phase plumes often do not migrate much beyond the LNAPL footprint and are very stable. More information on the key components of an LNAPL CSM can be found in Interstate Technology and Regulatory Council (ITRC) LNAPL Site Management: LNAPL CSM Evolution, Decision Process, and Remedial Technologies (2018).
At LNAPL sites, regulatory drivers typically include removal of LNAPL to the “maximum extent practicable.” LNAPL transmissivity is an important component of the LNAPL CSM and can be used as a metric to determine LNAPL mobility/stability and if LNAPL may continue to be recovered in a cost-effective and efficient manner. Many DON petroleum sites consist of weathered LNAPL from historic operations and have the potential to meet low-threat closure criteria if LNAPL recovery is found to be no longer technically feasible and residual LNAPL does not pose a risk to receptors. Many states allow for LNAPL transmissivity measurements to be used in evaluating if LNAPL recovery has occurred “to the maximum extent practicable” versus a traditional metric such as LNAPL thickness.
A comprehensive LNAPL CSM helps to set remedial goals, informs the selection of appropriate remedial technologies, and defines metrics to support technology transitions. Remedial technologies can often be combined in a “treatment train” approach to accelerate cleanup across a site and/or over time. In addition, a recent advancement in LNAPL remedial technologies includes an improved understanding of the role of natural source zone depletion (NSZD).
NSZD refers to the natural loss of LNAPL due to volatilization, dissolution, and biodegradation. There has been an increased reliance on NSZD to manage LNAPL sites based upon the availability of new tools to measure NSZD rates. These tools include carbon dioxide traps, the dynamic closed chamber method, the gradient method, and temperature-based methods. These new tools all rely on the collection and analysis of gas samples in the unsaturated zone, a previously underappreciated portion of the subsurface responsible for petroleum degradation. The site-specific NSZD rates are then used to determine if LNAPL is degrading at a sufficient rate and/or as a benchmark to determine if active treatment should be discontinued (based on the gallons per acre per year being removed through NSZD).
Ongoing optimization efforts are needed at petroleum sites to accelerate RC through risk-based closure options. Several states have risk management options for managing LNAPL in place at low risk sites. These state UST programs allow for risk-based or low-threat based closure based upon certain conditions being met including: 1) the free product has been recovered to the maximum extent practicable, 2) the LNAPL footprint and associated dissolved plume is stable, 3) there are no uncontrolled vapor intrusion (VI) issues, and 4) appropriate land use controls (LUCs) are in place.
More information on the investigation and remediation of petroleum sites can be found in Chapter 13.2 of the DON Environmental Restoration Program Manual.