This former alumina refinery site in St. Croix is impacted by fuel oils from historical power production and machinery fuels, as well as from the abutting refinery site. Light non-aqueous phase liquid (LNAPL) remediation has been occurring at the site for more than 35 years, and a strategic review was requested to develop a final LNAPL remediation and closure strategy for the site. The focus of the participating group was to define clear endpoints for the site and develop a strategy to achieve those endpoints in a practical, implementable, and cost-effective manner.
EHS Support was retained to review all historical geologic gauging, groundwater monitoring, and LNAPL recovery data to develop an LNAPL Conceptual Site Model (CSM) and LNAPL Management Plan for the site. The review of data indicated a more geologically complex setting than previous assessments, with lagoonal deposits, marl, and limestone with various degrees of cementing and widely varying hydraulic conductivities. Our experts recommended modifications of the existing LNAPL recovery approach, and supplemental investigation/assessment works for the site focused on obtaining additional LNAPL transmissivity/recoverability data and completing a natural source zone depletion (NSZD) assessment. The focus of this data collection was to validate the hydrogeologic CSM and support the hypothesis that natural mass losses in large areas of the site were just as, if not more, effective than ongoing LNAPL recovery.
While the approach proposed had been soundly supported by science, inherent stakeholder concerns in the US Virgin Islands and from the United States Environmental Protection Agency (USEPA) resulted in the need to develop an iterative assessment process, where education and advocacy could be part of a transition to a new remedial approach.
The LNAPL CSM has been developed and the work plan for NSZD assessments completed leveraging multiple lines of evidence, including:
- Assessment of geochemical properties of groundwater in contact with LNAPL (to define the predominant mass loss mechanisms for these low-solubility oils)
- Temperature profiling in unsaturated and saturated zones to provide preliminary quantification of mass loss mechanisms
- Vertical profile soil-gas surveys to define the key mechanisms of mass loss, demonstrate that methane is being degraded to carbon dioxide (to aid with the selection of Phase 2 NSZD assessment methods), and provide quantification of mass losses
- Use of E-Flux canisters and Licor Flux chamber methods to quantify mass losses across the plume area
Based on this program, an updated LNAPL CSM was developed, and a formal decision process for assessment of LNAPL recovery—to the extent practical—was provided to USEPA. This program of works and a formal decision process for site closure were submitted to USEPA at the end of 2021, and the approach was approved in January 2022. The program of works was implemented and reported to USEPA later in 2022, along with a subsequent verification study completed in 2023 and reported in February 2024. Since 2024, recovery wells that reach the agreed-upon shutdown metrics are iteratively being shut down. The site is transitioning into a NSZD monitoring program that documents the magnitude of natural mass losses and confirms the decline in active recovery efficiency through assessment of recoverability data.