Formatted Title
Colloidal Activated Carbon Barrier to Reduce PFAS Migration
Background/Objectives
Per- and polyfluoroalkyl substances (PFAS) have been detected in groundwater at Alameda Point (Former Naval Air Station Alameda) from the past use of aqueous film forming foam (AFFF) during firefighting drills at Site 14 training area. Due to the close proximity of the training area (Site 14) to Oakland Inner Harbor, a large-scale pilot test is underway to test the effectiveness of an in situ colloidal activated carbon (CAC) barrier for reducing mass flux of PFAS from migrating into the Oakland Harbor.
The concentrations of PFAS in site groundwater range from 7.6 nanograms per liter (ng/L) to over 300,000 ng/L. Perfluorooctanesulfonic acid (PFOS) is generally present at the highest concentration but a range of other typical AFFF PFAS (e.g., perfluorooctanoic acid [PFOA], perfluorobutanesulfonic acid [PFBS]) are present as are perfluoroalkyl acid (PFAA) precursors. Based upon PFAS concentrations, site geochemistry, geology and hydrology, as well as results from other laboratory and field tests, a CAC barrier was considered as a viable option to decrease PFAS concentrations in groundwater and reduce mass flux to the Oakland Inner Harbor.
Approach/Activities
To design and install the CAC barrier, a multi-phased approach was taken. A short-term column experiment was performed to provide data concerning the adsorption capacity of CAC for representative PFAS in Site 14 groundwater using three flow-through columns prepared with homogenized site sediment and amended with 0%, 0.5% or 2% CAC. To delineate the PFAS plume both vertically and horizontally, discrete groundwater samples were collected to determine the ideal placement of the CAC barrier and performance monitoring wells. Passive flux meters (PFMs) were then installed to define the lithologic zones with the highest groundwater velocity and the greatest potential PFAS mass flux. The last step in the design phase of the CAC barrier was to conduct a clear water injection test in the field to confirm and adjust the design volumes, flow rates, and radius of influence. Using the information gained from the design phase, a total of 340,000 pounds of CAC, in the form of PlumeStop, was injected across 280 direct push injection points to create a 720 feet barrier parallel to the shoreline of Oakland Inner Harbor.
Results/Lessons Learned
The short-term column test simulated approximately 20 months of groundwater flow at the site over 3.5 months (by accelerating flow rate in the lab). The data confirmed the effectiveness of CAC under site conditions to appreciably reduce dissolved PFAS concentrations and subsequently limit downgradient mass flux of PFAS to groundwater. The discrete groundwater sampling and PFMs showed that the PFAS mass flux was higher than expected in some areas across the planned barrier location. These data required modification of the barrier design with 2%, 5%, or 7% CAC depending on the estimated mass loading.
Unconsolidated sediments presented some challenges during the injection, including daylighting, which slightly change the shape of the barrier, but these issues were resolved in the field and the barrier was installed within six weeks. Performance monitoring, which includes groundwater sampling, PFMs, and soil core collection is ongoing. The overall performance of the barrier will be discussed, following receipt of the analytical data.