Per- and polyfluoroalkyl substances in groundwater from the Great Miami buried-valley aquifer, southwestern Ohio, 2019–20

Groundwater samples were collected during 2019 and 2020 from 23 wells in the Great Miami buried-valley aquifer (GM-BVA) in southwestern Ohio by the U.S. Geological Survey, in cooperation with the Miami Conservancy District, Dayton, Ohio, to determine concentrations of selected per- and polyfluoroalkyl substances (PFAS). The GM-BVA is a glacial outwash and alluvial fill aquifer that is the sole source of water supply for much of the region. Wells had total depths that ranged from 21 to 101 feet below land surface, and groundwater levels that ranged from 1.39 to 52.15 feet below land surface before sampling in 2019.

Groundwater and related quality-control samples were sequentially collected from 22 of the 23 wells and analyzed for 24 different PFAS by 2 methods that used proprietary isotope-dilution based adaptations of U.S. Environmental Protection Agency (EPA) method 537.1, termed methods 1 and 2. Method 2 had smaller reporting limits (RL) for 22 of 24 PFAS analyzed and smaller detection limits (DLs) for all 24 PFAS analyzed compared with method 1, which made method 2 more sensitive to detect PFAS.

Concentrations of perfluorooctanesulfonate (PFOS) in a groundwater (GW)-method 2 sample from well CL–275 of 1.9 nanograms per liter (ng/L) and perfluorooctanoate (PFOA) in a GW-method 2 sample from well BU–1106 of 2.1 ng/L were greater than their EPA interim health advisory guidances for drinking water (as of June 2022) by about 9,500 and 52,500 percent, respectively. The EPA interim health advisory guidances for PFOS (0.02 ng/L) and PFOA (0.004 ng/L) were also 65 and 215 times less, respectively, than the smallest method 2 DLs for PFOS (1.3 ng/L) and PFOA (0.86 ng/L).

Other PFAS were either not detected in GM-BVA groundwater samples or were detected in concentrations less than Ohio action levels or Federal health-risk-based guidance. The most detected PFAS in groundwater was perfluorobutanesulfonate (PFBS), which had concentrations in samples from eight wells that ranged from 1.0 to 8.0 ng/L or from 0.05 to 0.4 percent of its EPA health advisory of 2,000 ng/L for drinking water.

The similarity of PFBS (7.8 ng/L), perfluoropentanesulfonate (PFPeS; 8.1 ng/L), and perfluorohexanesulfonate (PFHxS; 14 ng/L) concentrations yielded from the GW-method 1 sample from well CL–275 on July 9, 2019, to those of PFBS (8.0 ng/L), PFPeS (7.8 ng/L), and PFHxS (16 ng/L) from the paired GW-method 2 sample demonstrated the capability of both methods to reproduce PFAS concentrations that were greater than their respective DLs. Non-detection of these PFAS in follow-up GW-method 1 and sequential replicate (Rep–GW-method 1) samples from CL–275 on April 21, 2020, indicated that the 2019 results represented a transient detection in groundwater.

Eleven of twenty-three wells sampled in 2019 had from 1 to 4 PFAS detected in one or more groundwater samples or in a paired replicate sample: PFBS in 8 wells and 9 samples; PFHxS in 4 wells and 5 samples; and PFPeS, PFOS, perfluorobutanoate, perfluoropentanoate, PFOA, and perfluorooctanesulfonamide in 1 well and 1 sample each. More PFAS were detected in GW-method 2 samples than GW-method 1 samples because method 2 had smaller RLs and DLs. Results indicate benefits from the analysis of paired samples, sequential replicate samples, and other quality-control samples using analytical methods with sensitive RLs and DLs to verify PFAS concentrations in groundwater.

Groundwater-age estimates indicate that water produced from all sampled wells had infiltrated to the water table within the 1947–present (2022) period of PFAS use or environmental presence. Eight wells with detectable PFBS in groundwater from 2019 samples also had groundwater-recharge dates that ranged from 1991 to 2016. Those ages coincided with the possible environmental presence of PFBS as a PFAS byproduct or use as an alternative to PFOS after about 2002. Two wells that had detections of PFHxS in 2019 groundwater samples also had post-2000 groundwater-recharge dates that coincided with the period of use of PFHxS as an alternative to PFOS. Six of nine wells with more than 66-percent of urban land use that was within 0.3 miles of each well, as of 2012, also had 1 to 4 PFAS detected in one of their groundwater samples. Seven of nine wells that produced groundwater in 2019 with an oxic redox category also had one or more PFAS detected in a sample.