Formatted Title
Ebullition-Facilitated NAPL Transport: Case Studies where Screening Evaluations Changed the Remedial Path
Background/Objectives
Ebullition-facilitated transport (EFT) of non-aqueous phase liquids (NAPL) can be a significant driver of remedial approaches in sediment. The migration of biogenic gases that are produced in sediment and rise to the sediment surface is called gas ebullition. When the gas ebullition comes in contact with NAPL or other contaminant impacts in the sediment, it can bring them up into the surface water column. This migration mechanism is called EFT.
Where sediment remedies that leave the NAPL in place may be considered (e.g., subaqueous cap, monitored natural remediation), evaluating EFT is a critical component of the conceptual site model. As described in ASTM E3300-21, the Standard Guide for NAPL Mobility and Migration in Sediment – Evaluating Ebullition and Associated NAPL/Contaminant Transport, these evaluations typically start as screening level investigations. Depending on the results and site conditions, increasingly complex and comprehensive methods are available to identify and quantify EFT of NAPL.
The presentation will review case studies from two sediment sites impacted with NAPL: wastewater lagoons from a former petroleum site and MGP NAPL in sediment on the bay side of a barrier island.
Approach/Activities
Screening level investigations of EFT of NAPL were performed at each site. The initial process included a data evaluation of historic observations, where EFT may be more/less likely to occur, and the best conditions to observe it (e.g., sediment temperature, tidal elevation). The initial investigation was performed via visual observations from a kayak. Where further investigation was warranted, it was performed via camera monitoring from drones and/or surface mounted cameras.
Results/Lessons Learned
The sites had no record of EFT observations from decades of prior investigation. EFT of NAPL was observed year-round at both locations. Therefore, a purposeful, well-designed screening level EFT investigation is recommended for sediment sites, even where there is a long history of site investigation.
The results of the screening level observations changed the presumptive remedy at each location. Where EFT had not been observed historically, the screening level observations indicated that an amended cap or physical removal might be necessary
Overall, the screening level EFT investigations provided critical data for the conceptual site model and feasibility studies for a nominal cost in comparison to the effect on the overall project plans.