Formatted Title
PFAS in Groundwater and Porewater in San Francisco Bay: Estimating Attenuation and Industrial Background
Background/Objectives
Per- and polyfluoroalkyl substances (PFAS) are present in groundwater at a facility located along San Franciso Bay (Bay), next to a harbor with a century-long history of industrial activity. We performed a hydrogeologic investigation to estimate PFAS fluxes, and the groundwater discharge zone in the harbor. Subsequently, we designed and implemented the first-ever investigation of PFAS in porewater in the Bay to (a) evaluate whether PFAS are migrating with groundwater to the discharge zone; (b) determine whether PFAS concentrations in porewater exceed screening levels; and (c) calculate dilution-attenuation factors (DAFs) for PFAS between shoreline wells and sediment porewater.
Approach/Activities
We conducted hydrogeologic investigations, including a tidal study and slug tests, to develop a conceptual site model and to estimate the location of the groundwater discharge zone. We collected porewater samples from locations along transects perpendicular to shore, downgradient of shoreline groundwater monitoring wells, and within the estimated discharge zone. “Near-shore” and “step-out” porewater sampling locations along each transect were 5 to 15 feet and 30 to 50 feet from shore, respectively. At each location, we collected two porewater samples (0.5–1 and 2.5–3 feet below sediment surface) using a modified, PFAS-free, Trident probe. Shallow porewater samples represent transition-zone conditions; deeper porewater samples represent groundwater conditions below the transition zone. We also collected surface water samples to evaluate potential introduction of surface water during porewater sampling. Samples were analyzed for PFAS using EPA Method 537M, and for geochemical parameters. We screened PFAS data against San Francisco Bay Regional Water Quality Control Board Interim Aquatic Environmental Screening Levels (ESLs).
Results/Lessons Learned
Perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) concentrations in groundwater ranged from 17 to 850 nanograms per liter (ng/L) and 11 to 2,400 ng/L, respectively. PFOS concentrations in several shoreline wells exceeded the ESL (75 ng/L). PFOS and PFOA concentrations in porewater ranged from <0.51 to 16 ng/L and 1.4 to 97 ng/L, respectively. None of the PFAS concentrations in porewater exceeded screening levels. PFAS concentrations in porewater were generally slightly higher in the near-shore locations than in the step-out locations. Geochemical data confirmed that porewater samples were representative of porewater and not surface water conditions. There were no consistent PFAS concentration trends with depth. Data indicate substantial attenuation of PFAS between groundwater and porewater, with DAFs ranging from 10 to >1,000. Tidal pumping and salting out may be responsible for the observed attenuation.
Most PFOS and PFOA concentrations in porewater fell within the range of Bay-wide PFOS and PFOA concentrations in surface water (2 to 9 ng/L; San Francisco Estuary Institute, 2018). These findings suggest that most of the PFAS in porewater downgradient may be industrial background. We performed statistical analysis of porewater data to develop site-specific industrial background threshold values (BTVs). After outlier removal, the data for PFOS and PFOA were normally distributed and yielded BTVs of 4.5 and 2.9 ng/L, respectively. These BTVs are within the range of PFOS and PFOA concentrations in Bay-wide surface water.