Abstract
Per-and polyfluoroalkyl substances (PFAS) have been identified as emerging contaminants of concern in the environment in a wide variety of media including groundwater. Typically, PFAS-impacted groundwater is extracted by pump and treat systems and treated using sorptive media such as activated carbon and ion exchange resin. Pump and treat systems are generally considered ineffective for the remediation of dissolved phase contaminants including PFAS but instead are considered applicable for plume containment. An alternative to pump and treat is in-situ treatment. The demonstrated use of in-situ treatment for PFAS-impacted groundwater is limited with only colloidal activated carbon (CAC) being shown to effectively attenuate PFAS over short and moderate time periods. Active research topics for the in-situ treatment of PFAS include the effect of heterogeneity on the distribution of the CAC, the lifespan of the CAC itself, the effect of competitive adsorption/absorption, and the effect of other geochemical conditions on the removal process. This study looked at the effect of heterogeneity on the distribution of CAC and subsequent treatment of PFAS at a site with a multiple aquifer system. The site’s geology varied from a silty sand to sand to fractured bedrock with all three units being impacted by PFAS and benzene (B), toluene (T), ethylbenzene (E), and xylene (X). Parameters evaluated included the distribution of the CAC as well as the subsequent treatment of the PFAS and BTEX. Results of groundwater sampling indicated that the PFAS detected within the groundwater were treated effectively to below their respective reporting limits for the duration of the 1-year test in both the silty sand and sand aquifers. The PFAS in the fractured rock aquifer showed a different treatment profile with longer carbon chained PFAS being attenuated preferentially compared to the shorter carbon chained PFAS. These results suggest that competitive sorptive reactions were occurring on the CAC within the fractured rock. Analysis of the unconsolidated aquifer materials determined that direct push injection of the CAC was effective at delivering the CAC to the target injection zones with post-injection total organic carbon (TOC) concentrations increasing by up to three orders of magnitude compared to pre-injection TOC concentrations. Heterogeneity did have an impact on the CAC distribution with higher hydraulic conductivity zones receiving more CAC mass than lower hydraulic conductivity zones.
Highlights
The in-situ treatment of groundwater impacted by petroleum hydrocarbons and chlorinated solvents is well established (Interstate Technology and Regulatory Council ITRC, 2005; Huling and Pivetz, 2006; Petri et al, 2011; Usman et al, 2012; Sra et al, 2013; Kashir and McGregor, 2015; Saeed et al, 2021) with in-situ chemical oxidation (ISCO), in-situ chemical reduction (ISCR), and enhanced bioremediation (EBR) being the most applied
The in-situ treatment of emerging compounds of concern such as per and polyfluoroalkyl substances (PFAS), synthetic dyes and synthetic musks have become a topic of interest over the past few years as stakeholders look for more economical and technically feasible options for treating groundwater impacted by these compounds (McGregor, 2018; McGregor and Maziarz, 2021; McGregor and Zhao, 2021; McGregor and Carey, 2019)
The presence of PFAS within groundwater has been identified by many regulatory agencies as being a threat to human health and the ecology as some of PFAS have been confirmed or suspected of being cariogenic
Summary
The in-situ treatment of groundwater impacted by petroleum hydrocarbons and chlorinated solvents is well established (Interstate Technology and Regulatory Council ITRC, 2005; Huling and Pivetz, 2006; Petri et al, 2011; Usman et al, 2012; Sra et al, 2013; Kashir and McGregor, 2015; Saeed et al, 2021) with in-situ chemical oxidation (ISCO), in-situ chemical reduction (ISCR), and enhanced bioremediation (EBR) being the most applied.The in-situ treatment of emerging compounds of concern such as per and polyfluoroalkyl substances (PFAS), synthetic dyes and synthetic musks have become a topic of interest over the past few years as stakeholders look for more economical and technically feasible options for treating groundwater impacted by these compounds (McGregor, 2018; McGregor and Maziarz, 2021; McGregor and Zhao, 2021; McGregor and Carey, 2019). The treatment of PFAS-impacted groundwater is complex due to numerous physical and chemical factors These include PFAS’ resistance to biological and chemical degradation due to the strong carbon-fluoride bonding (National Ground Water Association National Ground Water Association Press, 2017); the low regulatory guidelines for treatment; poorly understood environmental fate and transport processes; and limited proven remedial approaches (Simon et al, 2019). These factors combined with other technical challenges including distribution, required contact between the PFAS and the remedial reagent, and back/matrix diffusion can all influence the outcome of in-situ treatment
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