3H/3He Groundwater Ages and Discharge of PFAS from Groundwater to a Coastal Plain Stream in North Carolina

Per- and polyfluoroalkyl substances (PFAS) are manmade chemicals that are persistent, bioaccumulative, and toxic. Since at least 1980, PFAS have been released from a chemical plant in the North Carolina coastal plain through air emissions and process wastewater. Atmospheric deposition of PFAS has caused extensive contamination of surface water and groundwater in the area.

At 24 points in the streambed of Georgia Branch, a tributary of the Cape Fear River, we measured groundwater flux through the streambed, and sampled groundwater for analysis of 27 PFAS and to determine age by tritium-helium (³H/³He) age dating. At each point, water flux through the streambed was upward from groundwater to the stream; the mean specific discharge was 1.53 m d^-1. Mean total PFAS concentration of groundwater at all 24 points was 2039 ng L^-1. Mean total PFAS flux from groundwater to the stream was 2.90 mg m^-2 d^-1, and mean GenX flux was 0.72 mg m^-2 d^-1. Only 4 of the 24 sampled points had GenX concentrations below North Carolina's drinking water health goal for GenX of 140 ng L^-1.

³H/³He groundwater age results were surprising: apparent age was 0-5 yr at 21 of 24 points and 14, 18, and 28 yr at the other three points. The groundwater mean transit time (MTT), calculated as the flow-weighted mean age, was only 7 yr. Ages and MTT near Georgia Branch were much younger than in similar studies in the North Carolina coastal plain and Wisconsin central sand plain. PFAS concentration generally decreased with increasing ³H/³He age, but with a wide range of PFAS concentrations in the 21 samples with very young apparent age. Even the oldest groundwater sample, recharged in 1991, had significant PFAS (summed concentration 313 ng L^-1). The young groundwater ages and absence of PFAS-free groundwater only 39 yr after the acknowledged start of GenX emissions are consistent with rapid movement of PFAS through the groundwater system to the stream. We hypothesize that a shallow clay layer in the surficial aquifer that causes widespread perched saturation is important in routing water and PFAS rapidly to Georgia Branch and the Cape Fear River. Similar examples of a shallow clay horizon potentially leading to very short tritium-helium groundwater transit times are lacking in the literature.