146-3 Heterogeneous Distribution of PFAS in Groundwater and Sediments Adjacent to a Groundwater-Surface Water Interface

Session: Contaminants Near Groundwater-Surface Water Interfaces (Posters)

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Abstract:

Per-and polyfluoroalkyl substances (PFAS) are recalcitrant in groundwater and threaten aquatic life and ecosystem health at low concentrations (ppt). As contaminated groundwater crosses the groundwater-surface water interface (GSI), PFAS can sorb to sediments creating a long-term source of contamination to the overlying freshwater system.  In this study, we examined the distribution of PFAS in surface water, sediments, and shallow groundwater in and adjacent to two fish rearing ponds operated by the Michigan Department of Natural Resources (MDNR). Up until 2024, the MDNR filled the man-made ponds each spring to rear walleye and fathead minnows during the summer and then drained the ponds each fall after transferring the fish to lakes.  Water in the ponds was sourced with a combination of surface water diverted from a nearby spring and groundwater springs beneath the ponds.

In May 2023, total PFAS concentrations observed in five shallow groundwater monitoring wells ranged from 40 to 1270 ng/L, averaging 504 ng/L. Total PFAS concentrations in the fish rearing ponds measured at the same time ranged from 58 to 66 ng/L while surface water concentrations in the inlet stream totaled 10 ng/L PFAS. Differences in the observed PFAS compounds and their concentrations generated estimates that 10 to 30% of the pond was sourced from groundwater.

In May 2025, we measured PFAS concentrations in two groundwater seeps at the bottom of the largest pond and took a 36 cm sediment core within 10 cm of one of the seeps.  Groundwater emanating from the seeps contained 11 PFAS compounds at low concentrations (total PFAS 240 to 430 ng/L, consistent with average monitoring well concentrations). PFAS in the sediment core, analyzed in 6 cm increments, varied vertically from 370 to 4,820 ng/kg. Nine different PFAS compounds were detected, but detection limits were two orders of magnitude greater than those for the aqueous samples.

Observations from this site, which is located approximately 3 km (2 mi) downgradient from the PFAS source zone, indicate that PFAS distributions can be highly heterogeneous in both groundwater and sediment distributions. This conclusion has important implications for sampling strategies when characterizing contaminant distributions at a GSI.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8878

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