PREDICTING FUTURE PFAS CONCENTRATIONS IN GROUNDWATER DISCHARGING TO STREAMS NEAR THE FAYETTEVILLE WORKS FACILITY IN NORTH CAROLINA

Widespread PFAS contamination of groundwater in the area surrounding the Fayetteville Works PFAS production facility near Fayetteville, NC has put critical drinking water resources at risk. Analysis of discharging groundwater at the groundwater-surface water interface in October 2020 provides insight into historical PFAS input to the groundwater system and future discharge of PFAS to four tributaries of the Cape Fear River. SF₆, ³H, and ³H/³He age-dating methods were used to estimate the age of groundwater at 20 points below the streambeds of Kirks Mill Creek, Willis Creek, Mines Creek, and Georgia Branch. Groundwater ages show a transit time distribution (TTD) spanning 0 to 39 years with a flow-weighted mean age of 19 years. Groundwater ages and concentrations of 37 different PFAS indicate PFAS input to the groundwater system steadily increased from the documented start of PFAS production at the Fayetteville Works until roughly the present day. The facility has reported deep reductions in PFAS emissions to air since 2019. PFAS concentrations in recharge and two different best-fit regressions of the TTD data (linear and gamma function) were used in convolution modeling to predict future flow-weighted mean concentrations of PFAS in groundwater discharging to the tributaries. The predicted PFAS concentrations represent the spatially-integrated concentration in future output from the contaminated groundwater system to streams. Modeling results under different scenarios of declining PFAS input post-2019 suggest that groundwater will continue to contain total quantified PFAS concentrations above 70 ng/L until at least 2060 under a linear TTD and 2080 under a gamma function TTD.