Formatted Title
In Situ Treatment of the Air-Water Interface within a PFAS Source Zone Using Colloidal Activated Carbon
Background/Objectives
The treatment of PFAS within soil and groundwater is an emerging topic with various technologies being researched and tested. Currently, PFAS-impacted groundwater is typically treated ex situ using sorptive media such as activated carbon and ion exchange resin. Proven in situ remedial approaches for groundwater have been limited to colloidal activated carbon (CAC) with plumes down gradient of the source zones being treated successfully for more than eight years. However, treatment of groundwater within the source zones has not been shown to be feasible. This study looked at using CAC to treat dissolved PFAS at the air-water within the PFAS source zone. Studies have shown that PFAS tends to preferentially accumulate at the air-water interface due to the chemical properties of various PFAS. This accumulation can act as a long-term source for dissolved phase PFAS, thus making down gradient treatment of the groundwater a long-term requirement.
Approach/Activities
A solution of CAC was injected into the source zone using direct push technology delivered on a dense grid that targeted the interface between the air and groundwater. Concentrations of PFAS within the pore and groundwater were collected using a series of nine lysimeters and two monitoring wells installed within the vadoze and saturated zones, respectively. A total of six PFAS were detected in the pore and groundwater including perfluorobutanoic acid (PFBA), perfluoropentanoic acid (PFPeA), perfluorohexanoic acid (PFHxA), perfluoroheptanoic acid (PFHpA), perfluorooctanoic acid (PFOA), and perfluorononanoic acid (PFNA). Detectable concentrations of PFAS within the pore and groundwater prior to treatment ranged from greater than 300 µg/L for PFPeA to less than 3 µg/L for PFNA. Pore and groundwater samples for the PFAS and other organic, inorganic, and biological parameters were collected over a 36-month period (2 pre-injection samples, and 3-,6-, 9-, 12-, 18-, 24- and 36 months post injection). Finally, cores were collected from the vadoze and saturated zone to evaluate the pre- and post-injection distribution of the CAC as well as the horizontal hydraulic conductivity variability of the aquifer.
Results/Lessons Learned
Following the injection of the CAC, monitoring of the pore and groundwater for PFAS was conducted over a 3-year period post injection. The results indicated that the PFAS within the porewater at and near the air-water interface was effectively attenuated over the 3-year monitoring program with PFAS concentrations being below the method detection limits of approximately 10 ng/L with the exception of PFPeA which is a 5-carbon chained PFAS that has been shown to have a lower affinity for sorption onto activated carbon compared to the longer carbon chained PFAS such as PFOA.
Post injection of the CAC, concentrations of the six PFAS within the shallow groundwater underlying the source zone remained relatively constant for the Day 107 sampling event showing little to no changes. The results from the 9-month sampling event showed a decrease in all PFAS concentrations with reductions of approximately 50 percent being measured in the groundwater sampled from both monitoring wells. This decreasing trend continued over the 36-month sampling period with PFAS concentrations continuing to decline suggesting that the injection of CAC at the air-water interface has reduced the mass loading of PFAS to the shallow groundwater.