Formatted Title

Background/Objectives

Light non-aqueous phase liquid (LNAPL) seeping beneath a bulkhead (seawall) at low tides was causing petroleum sheening on a tidally-influenced river adjacent to an active petroleum product storage and distribution facility in Pennsylvania.  Initial mechanical recovery efforts removed a substantial volume of LNAPL, however, sheening continued on occasion, typically at low tides.

To achieve the overall goal of project closure under the Pennsylvania Corrective Action Process, the first remedial objective was to prevent the seepage of LNAPL at two areas along the seawall.  The second objective involved removing LNAPL and reducing dissolved-phase concentrations to acceptable levels to facilitate project closure.  An extensive evaluation of potential remedial technologies was completed, taking into account the conceptual site model, the structural stability of the seawall, responsiveness, and cost effectiveness.  A permeable reactive barrier (PRB) was selected as the most appropriate remedy for the two seep areas considering the various limitations, as the PRB would minimize hydraulic pressure on the seawall. A high-resolution pre-design investigation (HR-PDI) was performed to optimize the design of the two PRBs.  An adaptive management approach has been implemented for decision making regarding potential future expansion of the PRBs to continue remediation of dissolved-phase hydrocarbons and residual LNAPL.

Approach/Activities

Two PRBs (North and South) were installed via direct push technology injection of a slurry composed of BOS200+™ activated carbon, electron acceptors (calcium sulfate, magnesium sulfate), nutrients (corn starch and yeast extract), and microbes. The flow-through barriers are designed to stop LNAPL seepage into the river via a trap-and-treat approach, in which the material adsorbs dissolved phase hydrocarbons (DPH) which are degraded via microbiological processes within the activated carbon pore space. To maintain responsiveness, the North and South PRBs were installed in a phased approach in 2021, with the first event serving as a constructability evaluation to assess the slurry distribution and optimize future PRB injections. Based on the successful installation and immediate and sustained results of the first-round injection, the second-round injections were completed in April 2023 for both PRBs. Performance monitoring data included volatile organics, anions, and dissolved gasses, and post-installation monitoring was conducted one day, one week, one month, and quarterly following each round of injections, and is on-going as of 4^th Quarter 2023.

Results/Lessons Learned

The activated carbon slurry PRBs were effective in elimination of the LNAPL seeps present along the seawall.  LNAPL presence within the PRBs has also been eliminated and performance groundwater monitoring indicates a greater than 90% reduction in volatile organics.  Elevated carbon dioxide and methane concentrations demonstrate increased microbial activity after injection. Sulfate concentrations observed in the post-injection samples support continued effectiveness, and will be monitored to determine the necessity for supplemental injections of electron acceptors, nutrients, and microbes. The adaptive management of the selected remedial measure allowed for multiple site closure goals to be pursued via one remedial technology. Overall, the primary remedial goal has been achieved by the elimination of the LNAPL seeps along the seawall, and secondarily the reduction in dissolved-phase concentrations will allow pursuit of project closure.