This former alumina refinery site in St. Croix is impacted by fuel oils from historical power production and machinery fuels, as well as 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 that would cost-effectively support achieving practical and implementable endpoints for the site.
EHS Support was retained to complete a review of all historical geologic gauging, groundwater monitoring, and LNAPL recovery data to develop 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 provided recommendations for 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 conceptual site model 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 U.S. 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 will be developed and a formal decision process for assessment of LNAPL recovery – to the extent practical – will be provided to USEPA. This program of works and a formal decision process for site closure are anticipated to be submitted to USEPA at the end of 2021.