Leaching and transport of PFAS from aqueous film-forming foam (AFFF) in the unsaturated soil at a firefighting training facility under cold climatic conditions

The contaminant situation at a Norwegian firefighting training facility (FTF) was investigated 15 years after the use of perfluorooctanesulfonic acid (PFOS) based aqueous film forming foams (AFFF) products had ceased. Detailed mapping of the soil and groundwater at the FTF field site in 2016, revealed high concentrations of per and polyfluoroalkyl substances (PFAS). PFOS accounted for 96% of the total PFAS concentration in the soil with concentrations ranging from < 0.3 mu g/kg to 6500 mu g/kg. The average concentration of PFOS in the groundwater down-gradient of the site was 22 mu g/l (6.5-44.4 mu g/l), accounting for 71% of the total PFAS concentration. To get a better understanding of the historic fate of AFFF used at the site, unsaturated column studies were performed with pristine soil with a similar texture and mineralogy as found at the FTF and the same PFOS containing AFFF used at the site. Transport and attenuation processes governing PFAS behavior were studied with focus on cold climate conditions and infiltration during snow melting, the main groundwater recharge process at the FTF. Low and high water infiltration rates of respectively 4.9 and 9.7 mm/day were applied for 14 and 7 weeks, thereby applying the same amount of water, but changing the aqueous saturation of the soil columns. The low infiltration rate represented 2 years of snow melting, while the high infiltration rate can be considered to mimic the extra water added in the areas with intensive firefighting training. In the low infiltration experiment PFOS was not detected in the column leachate over the complete 14 weeks. With high infiltration PFOS was detected after 14 days and concentrations increased from 20 ng/l to 2200 ng/l at the end of the experiment (49 days). Soil was extracted from the columns in 5 cm layers and showed PFOS concentrations in the range < 0.21-1700 mu g/kg in the low infiltration column. A clear maximum was observed at a soil depth of 30 cm. No PFOS was detected below 60 cm depth. In the high infiltration column PFOS concentration ranged from 7.4 to 1000 mu g/kg, with highest concentrations found at 22-32 cm depth. In this case PFOS was detected down to the deepest sample (similar to 90 cm). Based on the field study, retardation factors for the average vertical transport of PFOS in the unsaturated zone were estimated to be 33-42 and 16-21 for the areas with a low and high AFFF impact, respectively. The estimated retardation factors for the column experiments were much lower at 6.5 and 5.8 for low and high infiltration, respectively. This study showed that PFOS is strongly attenuated in the unsaturated zone and mobility is dependent on infiltration rate. The results also suggest that the attenuation rate increases with time.