The hypothesis for this project is that sparging gas directly into an impacted aquifer can reduce the mass flux of per- and polyfluoroalkyl substances (PFAS) in groundwater plumes below regulatory levels. Due to buoyancy effects, sparging will concentrate PFAS near the top of a water-bearing unit or capillary fringe where it can then be removed by conventional technologies such as pump-and-treat, vacuum extraction, adsorption, or phytoremediation. Therefore, gas sparging can be an effective and efficient first stage of a treatment train solution where PFAS are first concentrated, extracted, and destroyed. Alternatively, because of the extremely high partitioning of PFAS to air-water interfaces, PFAS could be left in place as part of an enhanced monitored natural attenuation (MNA) strategy where the PFAS are not biodegraded, but retained in the capillary fringe and thereby isolated from moving groundwater. PFAS retention is already an accepted practice as shown by a few projects where particulate sorbents have been injected into PFAS plumes for the purpose of retention. Gas sparging could be a less expensive and more versatile method to sequester PFAS for the purpose of enhancing PFAS MNA, given mobilization by precursor transformation is avoided.
The technical approach is based on the following themes and elements:
- Air sparging is one of the mostly commonly used technologies for remediating groundwater and has been used successfully since 1985.
- Because PFAS are surfactants, they partition strongly to air-water interfaces.
- A proof-of-concept column experiment showed sparging mobilizes dissolved surfactants in a saturated porous media and transports them directly to the top of the water-bearing media.
The project team plans to combine the existing knowledge about sparging as a remediation technology, the well understood chemistry of PFAS partitioning to gas/water interfaces, and the results of the preliminary column study to perform an integrated multi-faceted research study on gas sparging directly in aquifers. Nine key sub-hypotheses will be tested, such as “Can gas sparging remove PFAS from groundwater to an environmentally significant degree?” and “Is pulsed sparging more effective than continuous sparging?”
Three different research arms will be designed, performed, and then synthesized to greatly expand the scientific community’s understanding of sparging for the purpose of managing PFAS sites:
- A detailed laboratory study using batch, column, and tank experiments;
- Development and application of several different mathematical models constructed in Excel, Python and/or R that will help illuminate key unknowns about the PFAS sparging process; and
- If the results from the laboratory and modeling studies are positive and lead to a “go” decision, then a small-scale field trial will be performed at an actual PFAS-impacted groundwater site where PFAS will first be concentrated by sparging, extracted, and destroyed via sonolysis.
If successful, site managers would secure a well-known, relatively inexpensive in situ remediation technology for managing their PFAS groundwater plumes that could greatly reduce the cost or even preclude the need for expensive groundwater pump-and-treat systems at some or many PFAS sites. This research could provide a novel way to avoid expensive treatment of PFAS-impacted subsurface media by allowing for long-term retention of PFAS in the subsurface under an enhanced-MNA type framework.