Tracking the Transport and Transformation of an Emergency Use, Single Event, AFFF Release in a Sandy Aquifer

ABSTRACT: Per- and polyfluoroalkyl substances (PFAS) are being scrutinized due to their persistence in the environment and adverse health effects. Groundwater is of particular concern, because of the use of aqueous film forming foam (AFFF), which allows PFAS constituents to quickly infiltrate into the subsurface. Speculation on fate and transport has been largely derived from assessments at historical and ongoing release sites, such as firefighting training areas (FFTAs). Data from these sites suggest that many PFAS can sequester in the vadose zone at depths approaching 3 to 6 m. This is supported by modelling of the distribution at soil/water/air interfaces. Laboratory studies are starting to populate these models with the necessary parameters for transport predictions. A challenge for transport modelling in field settings is the lack of knowledge regarding the numerous AFFF formulations and volumes used over time that have left significant concentrations of both quantifiable and unquantifiable PFAS. This study examines PFAS from a single, emergency use of AFFF, to characterize the distribution and transport. The site is located overtop a shallow, unconfined sandy aquifer with underlying clay lens. A suite of 26 PFAS compounds were monitored in the vertical profile soon after release and a year later. Elevated concentrations were initially observed in the vadose zone to a depth of 30 cm, followed by transformation to more mobile species. PFAS constituents were found to be travelling at least 0.00063 cm/s horizontally in the fine-to-medium sand aquifer one year following release while concentrations in the vadose zone were below guideline concentrations. It is believed that vertical fractures in the clay as well as seasonal changes to hydraulic gradients and resulting changes to water saturation were the driving factors in PFAS transport.