Per-and Polyfluoroalkyl Substances in Groundwater from the Great Miami Buried-Valley Aquifer, Southwestern Ohio, 2019–20

Abstract

Groundwater samples collected during summer and autumn of 2019 and spring of 2020 from 23 previously sampled 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, were evaluated to determine concentrations of selected per-and polyfluoroalkyl substances (PFAS) in groundwater. The GM-BVA is a glacial outwash and alluvial fill aquifer that is the sole source of water supply for much of the region. Sampled wells had total depths that ranged from 21 to 101 feet (ft) below land surface. Wells were completed with well screen in lengths that ranged from 2 to 11 ft (18 wells) or open hole in which the base of casing was left open against aquifer material (5 wells). Groundwater levels in the wells before sampling in 2019 ranged from 1.39 to 52.15 ft below land surface. Groundwater and related quality-control samples were sequentially collected from 22 of the 23 wells and analyzed for 24 different PFAS by two 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 in groundwater and quality-control samples as compared with method 1, which made method 2 the more sensitive method. Quality-control sample results indicated that protocols and reagents for equipment cleaning and rinsing did not contribute to PFAS results in GM-BVA groundwater samples. Concentrations of perfluorooctanesulfonate (PFOS) in a groundwater (GW)-method 2 sample from well CL–275 of 1.9 nanograms per liter (ng/L) and of perfluorooctanoate (PFOA) in a GW-method 2 sample from well BU–1106 of 2.1 ng/L were considerably greater than their EPA interim health advisory guidance 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) as of June 2022 were also 65 and 215 times less, respectively, than the smallest DLs for PFOS (1.3 ng/L) and PFOA (0.86 ng/L) reported for method 2, the more sensitive of the two methods used in this study. 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. A 16 ng/L concentration of perfluorohexanesulfonate (PFHxS) in the GW-method 2 sample from well CL–275 was the largesconcentration of any PFAS in GM-BVA groundwater samples from this study and was about 11.4 percent of the Ohio action level of 140 ng/L for PFHxS in drinking water. 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 0.40 percent of its EPA health advisory of 2,000 ng/L for PFBS in drinking water. The PFOS concentration of 1.9 ng/L in a GW-method 2 sample from well CL–275 and a PFOA concentration of 2.1 ng/L in a GW-method 2 sample from well BU–1106 were about 2.7 and 3.0 percent, respectively, of their Ohio action levels in drinking water. Most PFAS targeted for analysis were not detected in groundwater or their paired samples. The GW-method 2 sample from well CL–275 on July 9, 2019, had the largest number of different PFAS detected in groundwater, including PFBS, perfluoropentanesulfonate (PFPeS), PFHxS, and PFOS. The similarity of PFBS (7.8 ng/L), PFPeS (8.1 ng/L), and PFHxS (14 ng/L) concentrations yielded from the GW-method 1 sample from that well, 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 demon-strated 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. Results indicated that repeated sampling of a well on multiple dates and analysis of those samples using an analytical method with sensitive RLs and DLs are needed to assess persistence and fluctuations of PFAS concentrations. Eleven of the 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. The PFAS detected in groundwater samples included PFBS in 8 wells and 9 samples, PFHxS in 4 wells and 5 samples, and PFPeS, PFOS, perfluorobutanoate, perfluoropentanoate, PFOA and perfluo-rooctanesulfonamide 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 for those compounds. Several PFAS compounds that were detected in GW-method 2 samples and not in paired GW-method 1 samples had concentrations that were less than their corresponding DLs in method 1, including PFBS at 7 wells; PFHxS at 3 wells; and PFOS, perfluorobutanoate, and PFOA at 1 well each. 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 concentrations of 1 to 4 PFAS detected in one of their groundwater samples. The same 6 wells also had from 4 to 10 facility or industry points of interest that may have used PFASas of 2012, that were within 2 miles or less of those wells. Groundwater-age estimates indicate that water produced from all sampled wells had infiltrated and recharged the water table within the 1947–present (2022) period of PFAS use or environmental presence. Eight wells with detectable PFBS concentrations in groundwater samples from 2019 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 as an alternative to PFOS after 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. Results from wells with modern groundwater-recharge dates within the post-1947 period of common use or presence of many PFAS and that had no detections of those PFAS in groundwater samples indicate that those samples were unlikely to have been affected by a PFAS source. Seven of nine wells that produced groundwater in 2019 with an oxic redox category also had detections of one or more PFAS in a sample. No apparent association between redox category and detections of PFBS and PFHxS in groundwater samples from 2019 was discernable. Groundwater samples with specific conductance values greater than or equal to the median of samples collected in 2019 (779 microsiemens per centimeter) were more likely to have detectable concentrations of PFAS (9 of 12 wells) than groundwater from wells with specific conductance values less than that median amount (2 of 11 wells). Groundwater levels and depths to the top of the well screen had no apparent relation to PFAS concentrations in groundwater. Results from this study indicate the benefits of analyzing paired and sequential replicate samples and other qualitycontrol samples using a method with sensitive RLs and DLs to verify PFAS concentrations in groundwater. Groundwaterage estimates, predominant urban land use proximate to the well, and larger specific conductance values were identified as factors to consider when selecting wells to sample to evaluate PFAS concentrations in the groundwater of the GM-BVA.