SIMULATION OF THE PFAS AIR DEPOSITION PATHWAY TO GROUNDWATER CONTAMINATION

Background/Objectives. Groundwater contamination is often the result of concentrated releases of hazardous substances to the land surface or the shallow subsurface. However, even highly dispersed releases of PFAS at the land surface, such as deposition of air emissions from industrial facilities, can result in groundwater contamination above the current screening values and health advisory limits. Trace quantities of PFAS deposited over large areas may produce groundwater concentrations of low part-per-trillion levels.

Approach/Activities. Barr has modeled transport of PFOA through air, unsaturated soil, and water in settings in which the primary sources of PFOA were determined to be air emissions of PFOA from industrial facilities. PFOA was deposited on the surrounding land surface over hundreds of square kilometers and then leached through the unsaturated zone to the water table. A complex modeling sequence linking AERMOD (air deposition), SWB (infiltration rate), MODFLOW-UZF (unsaturated zone flow), MODFLOW-NWT (saturated zone flow), and MT3D-USGS (unsaturated and saturated transport) was used to simulate the complete pathway from the facility stacks to groundwater receptors. The models were calibrated to available water level, water quality, and soil concentration data. A key process captured in the modeling is the affinity of PFOA molecules for organic carbon, which results in lagged transport times through the unsaturated zone.

Results/Lessons Learned. The model results were used to inform subsequent investigations and remediation efforts. These same soil and groundwater modeling techniques are also applicable to sites contaminated by other types of PFAS releases to the land surface, such as the use of firefighting foams at airports and fire training facilities. The results of this study confirm that an integrated, multi-media modeling approach for PFAS fate and transport provides significant value in assessing the potential for contamination, predicting future concentration trends, designing remedies, and estimating cleanup tim