Formatted Title
In Situ Treatment of PFAS-Impacted Groundwater: Do We See Desorption or Competitive Sorption Occurring in the Field
Background/Objectives
The in situ treatment of PFAS within groundwater is an emerging topic with various technologies being researched and tested. To date a number of PFAS-impacted aquifers have been treated in situ using colloidal activated carbon (CAC) with monitoring results showing effective treatment over short and intermediate time frames. At one site, monitoring post injection has shown that the PFAS has remained below the method detection limits for over eight years. Numerical modelling and laboratory studies suggest that CAC and other sorbants can effectively attenuate long-chained PFAS effectively over long time frames depending on the mass flux, however, attenuation effectiveness decreases with carbon chain length. These laboratory studies have shown that short-chained PFAS have a lower affinity for sorption onto activated carbon and other absorbents compared to the longer carbon chained PFAS such as PFOA and therefore can be desorbed or affected by competitive sorption reactions with other longer carbon chained PFAS and organic compounds like petroleum hydrocarbons and chlorinated organic compounds that may be commingled with the PFAS. This study was completed to evaluate over a short and moderate monitoring period if desorption reactions were occurring at four sites where CAC had been injected to treat PFAS and other commingled organic compounds within groundwater.
Approach/Activities
Pilot-scale studies at four sites where CAC was injected were monitored for up to eight years looking at possible desorption and competitive sorption reactions associated with the CAC. The sites included commingled plumes with PFAS mixed with petroleum hydrocarbons including benzene, toluene, ethylbenzene, and xylene (BTEX) present, plumes with PFAS and chlorinated ethenes including tetrachloroethene (PCE), trichloroethene (TCE), cis- and trans-1,2-dichloroethene (cis- and trans-1,2-DCE) and vinyl chloride present as well as plumes in groundwater having various ionic strengths ranging from fresh to saline groundwater. The plumes contained a wide variety of PFAS including precursors, sulfonic acids, and carboxylic acids with carbon chains ranging from C4 to C9 in length with total PFAS measurable concentrations of up to 59,070 ng/L. Groundwater flow velocities within the four aquifers ranged up to 300 m/year with the geology of the aquifers ranging from silty sand to medium grained sand. All four aquifers were heterogenetic with horizontal hydraulic conductivity varying by at least two orders of magnitude within the impacted aquifer zone. One area of interest for using CAC and other adsorbents for the attenuation of PFAS in situ is the life span of the adsorbents and the potential for desorption to occur over the long term as well as the effects of competitive desorption. Both processes have the potential to release PFAS and other organic compounds of concern over the long term at concentrations that may be of concern. Numerical modeling and laboratory studies have shown that these processes may occur, however, only limited field studies have been implemented with results only available for short monitoring timeframes in a limited number of geological, hydrogeological, and geochemical settings.
Results/Lessons Learned
Following the injection of the CAC at four sites, monitoring of the groundwater within and down gradient of the CAC-injected zones was conducted for PFAS for up to eight years post injection. The results to date indicated that the PFAS within the groundwater at the four sites has been effectively attenuated for up to eight years with no indication of desorption of the PFAS including the short chained PFAS such as PFPeA, PFHxA, and PFBA during the monitoring periods. The one exception was PFPeA which is a 5-carbon chained PFAS that was detected at low concentrations (i.e., 55 ng/L) after 18 months at the site where CAC was injected at the air-water interface. Based on the observations made to date at the four study sites desorption reactions including competitive desorption does not appear to be a significant concern over the short to medium term in aquifers that have a low to moderate mass flux of PFAS commingled with other organic compounds such as petroleum hydrocarbons and chlorinated ethenes. Monitoring of other short-chained organic molecules which have been shown to have a lower affinity to activated carbon such as benzene and vinyl chloride have also not been shown in these field studies to be desorbed over the monitoring periods. However, this may be influenced by biological and chemical degradation reactions associated with other remedial reagents injected at the same time as the CAC.