Per- and polyfluoroalkyl substances (PFAS) are a group of manufactured fluorinated organic chemicals that have been used in a wide array of industrial processes and consumer products. Because of their widespread distribution and unique chemical properties, they have been detected throughout urban, farm and even wilderness environments. As a result of their production, use and disposal, PFAS have been released to surface water, groundwater, the atmosphere, soil and sediment. The presence of PFAS in sediment and pore water is significant because sediment is the home for benthic organisms that are the foundation of aquatic and many terrestrial ecosystems. Understanding the behavior and controlling the release of PFAS in aquatic environments are critical for the management and protection of aquatic life.
PFAS can accumulate in sediment through several different pathways, including direct discharge of untreated industrial wastewater, sludges and waste to surface water bodies. In addition, PFAS can reach sediment, then surface water through a groundwater pathway, as the groundwater carries the PFAS from an upland source and discharges to a surface water body. Treatment systems that prevent or disrupt the transport of PFAS from groundwater into sediment and surface water are needed to reduce the discharge of PFAS to aquatic environments.
In order to design an integrated site remedy for PFAS discharging to surface water, TRC performed an evaluation of a capping system using a groundwater model and experimental data for sorption and partitioning of PFAS to a solid sorbent material (i.e., RemBind®). MODFLOW was used to simulate a variety of sediment cap configurations that were specifically designed to passively bind PFAS seepage from a bank or pore water upwelling into a reactive cap, where it could be captured in a removable treatment cartridge. Sorptive properties of RemBind® utilized in the modeling were developed through independent testing performed by BECA in Australia. In addition, published data from numerous trials were utilized to evaluate various cap or passive barrier configurations.
The cap design has also been modified to include air lift technology to the transmission layer of the cap, as a potential pre-treatment for PFAS-impacted sediment and groundwater. Pilot studies have shown that bubbling air or other gases (e.g., ozone) can cause the PFAS to form bubbles and accumulate at the water surface. This effectively concentrates the PFAS at a location where it can be removed and sent for more efficient treatment. Removing PFAS prior to the sorbent material will extend the life of the sorbent, improve water quality and save costs.
Modeling results utilizing lab data and groundwater flow assumptions indicate that a cap can be designed to effectively treat bank seepage and pore water discharge of PFAS to surface water under a variety of conditions. Modeling indicates that under realistic conditions the AquaGate+RemBind® (AquaBlock Ltd.) gabion could last from 2.3 years to several decades. Adding air lift and foam removal could extend the life of the sorbent, especially at sites with higher PFAS concentrations.