Formatted Title
Design and Application of a Colloidal Activated Carbon Barrier to Reduce the Migration of Elevated PFAS Concentrations into the Oakland Inner Harbor
Background/Objectives
Background/Objectives. As a result of past firefighting drills performed at Alameda Point (Former Naval Air Station Alameda Site 14) elevated concentrations of per- and polyfluoroalkyl substances (PFAS) ranging up to 300,000 ng/L are present in groundwater.
After an extensive laboratory testing program, colloidal activated carbon (CAC) was selected as the sorption medium for a permeable reactive barrier (PRB).
Approach/Activities
Approach and Activities. Prior to field implementation a series of focused assessment activities were performed to confirm remedial conditions. These activities included detailed soil core logging, injection testing and contaminant mass flux studies. These focused assessments were performed in areas oriented along the proposed 720-foot long PRB. Field soil core logging and injection testing were performed at five locations along the proposed PRB, and flux studies were performed in 10 nearby wells. The results of this assessment program were used to directly inform on application methods as well as flux-based models used to determine CAC quantities necessary to achieve a 15-year PRB service life.
Results/Lessons Learned
Results/Lesson Learned. The preapplication assessment activities significantly reduced the number of design unknowns present within the proposed PRB’s target treatment zone. Specifically, these studies provided tangible information on application-critical factors including feasible application rates, achieved radius of influence, and necessary application volumes. Results of field testing and flux studies led to material changes in the initial design. Design adjustments were significantly informed in the areas of contaminant mass flux, CAC delivery volumes, injection pressure and delivery rates. The results of the flux studies were also helpful in defining the CAC quantities necessary. The contaminant mass flux results were combined with an estimated PFAS mass present on soil to provide a better picture of the CAC mass necessary.
The influence of PFAS mass present in soil on the quantity of CAC necessary was modelled using a proprietary multi-compartmental model. Results of this modelling indicated the contribution of PFAS mass desorbing from soil was significant and necessitated an increase in the CAC quantity required to secure the performance objectives. Depending on the estimated PFAS mass and flux likely to be present the CAC application rates were adjusted to reflect specific demands. The results of the injection studies performed along the proposed PRB brought into focus the volume per vertical foot necessary to achieve the required radius of influence. Field logging combined with mass flux study results helped identify the zones that had significant mass flux as well as were likely able to accommodate the reagent volumes required.
Field application of the PRB was accomplished in 28 days using two field application teams. The site’s shallow groundwater (3 feet below ground surface) required the field application teams to adjust the CAC application by regulating and changing pressures and flow rates as well as to move from those areas where the design volume had been reached or exceeded (surfacing) to less-filled sections of the PRB.