Theme: 07. Per- and Polyfluorinated Alkyl Substances (PFAS)

Topic Area: 7f. In Situ PFAS Treatment Approaches

Formatted Title:

Gas Sparging Directly in Aquifers to Remove or Retain PFAS: Literature, Experiments, and Modeling

Background/Objectives:

Air sparging is a well-known and highly utilized groundwater remediation technology for volatile groundwater contaminants and has an extensive array of design guidance and technology equipment and vendors to implement sparging projects. Up to now, air sparging has not been intentionally applied to remediation of PFAS groundwater plumes because 1) most PFAS have very low volatility and therefore are not transported by the sparged air to the surface; and 2) addition of oxygen does not degrade PFAAs; rather oxygen addition has been known to increase the concentration of regulated PFAAs in groundwater due to precursor transformation. However, many PFAS exhibit strong affinity for air-water interfaces. Leveraging this key characteristic, directly sparging air or other gases directly into aquifers may be able remove PFAS via this process. Rather than volatilizing contaminants like chlorinated solvents or biologically degrading contaminants such as benzene, the sparging process at PFAS-contaminated sites could concentrate PFAS from the aquifer to its upper layer or the peak of a confined aquifer. The concentration of PFAS above and near the water table offers several potential advantages: 1) it diminishes the thickness of the groundwater plume, thereby lessening the volume of groundwater pumping required to capture a plume, and 2) it potentially allows the accumulated PFAS above the water table to be retained for long periods (years or decades), similar in concept to the retention achieved by engineered in situ activated carbon barriers.

Approach/Activities:

Under SERDP Project ER22-3221, a multi-disciplinary team has been evaluating the prospect and efficacy of using gas sparging to help manage PFAS plumes in groundwater, including the potential for air sparging to facilitate precursor conversion to PFAAs. The technical approach is to: A) mine the historical sparging scientific literature from the 1990s onward to define the key processes and key controls on gas migration in natural porous media; B) perform detailed column studies and tank experiments to determine when and how well sparging works to remove PFAS from groundwater; and C) conduct groundwater modeling studies using a variety of models (MODFLOW, UGS-PFAS, and TOUGH) to understand key issues such as how long PFAS are likely to be retained near the water table after sparging is performed.

Results/Lessons Learned:

A) The sparging literature defined two discrete sparging regimes: bubble flow in coarse-grained porous media and channel flow in more fine grained soils, with the transition zone roughly around grain sizes about 0.75-4.0 mm and a media hydraulic conductivity of 0.1 cm/sec. B) Initial column tests performed at Colorado State University indicate strong removal of dyes (used as PFAS surrogates) under the bubble sparging regime but slower removal under a predominantly channel regime. Pulsing to create additional air-water interfaces is likely to be a key step required to remove PFAS from groundwater via sparging channels. Additional testing is on-going, and an extensive array of PFAS results in sparged columns and research tanks is scheduled to be completed by the summer. C) A new groundwater model USG-PFAS that incorporates air-water partitioning effects into the MODFLOW-USG model has been developed by Dr. Sorab Panday of GSI Environmental, and it has been applied to evaluate the nature of PFAS retention around the water table after gas sparging. Modeling results to date indicate that in terms of mechanisms that shortens PFAS retention time after sparging are (in order of less to more impact): 1) water table fluctuation (WTF); 2) low volume episodic recharge events; and 3) high volume episodic recharge events. Concentrating PFAS near the water table in a thin, concentrated PFAS plume may in theory be able to reduce pumping requirements to control expanding PFAS plumes by a factor of 2X to 4X.

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Jordan White