Background/Objectives. Globally, there are hundreds of sites that have significant risks posed by PFAS to human health and the environment. The major challenges associated with PFAS is their relatively high mobility, extreme persistence in the subsurface, their potential for bioaccumulation, and their significant toxicological impacts to human health and the environment at extremely low concentrations (i.e., parts per trillion (ppt)). To date, there are limited in-situ remediation options for PFAS‐impacted sites. With the in-situ methods of absorption onto GAC by various permanent reactive barriers for groundwater plume control, these technologies do not represent cost-effective or long-term remedial solutions due to the low absorption of PFAS to these materials and the high volumes of impacted groundwater. A more effective solution of gas injection into the lower portion of a well enables the extraction of PFAS compounds from groundwater by foaming them to the phreatic surface and then vacuum extracting them. The method is further enhanced by the well construction where multi-azimuth highly permeable vertical planes are propagated, creating homogeneous planes for movement of the PFAS-impacted groundwater to extraction as well as initiating distinct higher permeability zones within lower permeability formations.
Approach/Activities. A field trial of this single well where six (6) propped planes of sand on differing azimuths were created and then coalesced with six (6) propped planes at another depth was conducted. The coalesced vertical planes were constructed from two (2) expanded split casing sections cemented at differing depths in the well. The field demonstration was conducted in a sandy silt uncontaminated formation to demonstrate the functionality of the casing construction and methodology and then quantify the attributes of the constructed system.
Column treatability studies have been performed to quantify the removal rates of PFAS compounds from groundwater by foam fractionation, both at source and plume level concentrations and at differing gas bubble sizes. Combining these results with the field trial demonstration, multi-phase models have been developed to determine the system’s in-situ radius of influence, flow phenomena within the propped planes, and transport behavior of the foam bubbles from the diffuser to the collector, resulting in a PFAS hyper-concentrate removed from the well for destruction.
Results/Lessons Learned. The field trials showed conclusively that multiple vertical propped planes on various azimuths can be constructed and coalesced from differing depths in a single wellbore. The geometry and hydraulic properties of the planes were quantified by real time active resistivity monitoring, hydraulic pulse interference testing and excavation down to the groundwater table. The in-placed permeability of the 12/20 sand proppant was determined to be in excess of 2,500 Darcy. The column treatability studies quantified the removal rates of PFAS compounds (i.e., carbon #4 to #14) by foam fractionation as a function of gas type, gas bubble size and contaminant concentration. The multi-phase models estimated that in-situ removal of most of the surface active, priority PFAS contaminants to low (ppt) concentrations is possible during field operation of the system. Following these laboratory and field trial activities, the design, construction and operation of the system at various PFAS contaminated sites are planned.