By Maggie Radford, P.E., Laura Cook, PG, Michael Zamboni, Jonathan Thorn, and J.D.
As the Department of Defense continues to deal with both the acute and long-term effects of sites contaminated by per- and polyfluoroalkyl substances (PFAS), concerns have been raised regarding potential cross contamination of environmental samples from PFAS-containing field equipment, clothing, and well construction materials. Because some PFAS pose health hazards at very low concentrations, detection, even at low levels, can have important implications and pose liability to the department.
As PFAS liabilities across the entire defense system are both calculated and remediated through the Defense Environmental Restoration Program (DERP), decisions on remedial actions and ways forward will be made based on collected samples. It is important that leadership is able to distinguish between whether PFAS present in a sample is representative of an environmental release or due to field sampling practices or well construction materials.
The implications of false positive results are significant, as anticipated PFAS liability runs into the billions of dollars.
APPROACH & ANALYSIS
A recent study funded by the U.S. Navy sought to determine whether PFAS could leach from coated bentonite pellets that are commonly used for environmental monitoring well seals. The investigation also aimed to identify whether PFAS are present at levels of concern for environmental investigations and whether variability is observed based on the water’s proximity to the coated bentonite pellets. (Previously, a different study evaluated the presence of PFAS in six bentonite products and found that one bentonite formulation contained detectable levels of PFAS. This latest effort corroborated those previous results and further assessed whether the concern is limited to a few products or is more widespread.)
Because coated bentonite can prevent the formation of undesirable void spaces in well seals, it is necessary to know if the presence of PFAS in these products was significant enough to warrant the use of other well sealing materials at the potential expense of increasing the difficulty of being able to install deep wells entirely below the water table.
Evaluating Concentrations. Leachate samples were created in the laboratory to estimate the concentrations of PFAS that could potentially leach from bentonite in the first 24 hours after the installation of a well and evaluate whether the concentrations varied based on where the leachate samples were collected. Bentonite pellet samples from three manufacturers were collected and shipped in PFAS-free high-density polyethylene jars. Samples included 1/4-in single coated pellets; 3/8-in non-coated pellets; two brands of 3/8-in single coated pellets (Brand 1 and Brand 2); and 3/8-in triple coated pellets.
The number of pellets for each test was based on the size and surface area of each pellet. This was applicable because the coating is the only component identified as potentially containing PFAS.
A total of six transport bottles were set up with PFAS-free water and the recommended number of pellets. The bottles were then capped, but not agitated, as the design was to mimic adding bentonite to a well during construction and evaluate aqueous concentrations based on proximity to the bentonite pellets. The bentonite pellets were allowed to expand for 24 hours, which is the minimum time that newly installed monitoring wells with bentonite and grout seals are typically allowed to seal prior to development and sampling activities. A total of 19 samples were collected for extraction and analysis: three samples from each of the five bentonite and water setups; three samples from a control setup of PFAS-free water only; and one source-water blank.
Sampling Consistency. Water samples were extracted following guidance provided in Table B-15 of the DOD/DOE Energy Quality Systems Manual Version 5.3. Water was extracted using a weak anion exchange solid phase extraction cartridge and target analytes extracted from the cartridge with methanol and ammonia. Extracts were further refined using Envi-carb to remove co-extracted interferences. Then, extracts were transferred for liquid chromatography with tandem mass spectrometry analysis. This method is consistent with analysis for a sample collected from a monitoring well during an environmental investigation. The 28 PFAS analyzed and reported were those listed in the Environmental Laboratory Accreditation Program certification at the time of analysis. Samples were analyzed using liquid chromatography with tandem mass spectrometry with multiple reaction monitoring. Target PFAS were quantified using the isotope dilution method where standards exist. Of all the chemicals analyzed, perfluorobutanoic acid (PFBA), perfluoropentanoic acid (PFPeA), and perfluorohexanoic acid (PFHxA) were the only PFAS detected in the bentonite leachate samples. The remaining 25 PFAS were not detected in any leachate or blank sample.
Chemical Frequency. PFBA was detected in all 19 water samples. PFBA is the most prevalent PFAS to be found at sites at trace levels and is a common false positive contaminant at trace levels from sampling equipment or procedures. PFPeA was detected in all three leachate samples for the 3/8-in single coated pellets from Brand 2 and ranged from 0.38J-ng/L to 1.89J-ng/L. PFHxA was found in one of the three leachate samples for the 3/8-in single coated pellets from Brand 2 at 4.42J-ng/L and all three leachate samples for the 3/8-in single coated pellets from Brand 1 at 0.64J-ng/L to 0.87J-ng/L. Values that are J qualified, while confirmed present, should be considered estimates. A mass contribution evaluation was completed to evaluate the potential magnitude of PFAS contribution from the bentonite pellets to the aquifer. This work was based on the maximum detected PFBA concentration level. Calculations indicated that PFAS coated bentonite pellets could result in an aquifer concentration of PFBA up to 0.49-ng/L.
AVOIDING FALSE POSITIVES
This Navy study detected PFBA, PFPeA, and PFHxA in a subset of the coated bentonite pellet leachate samples. PFBA was detected in all samples for the bentonite products tested. Concentrations in the leachate from the non-coated pellets were similar to concentrations in the control. Toxicity values are being developed for PFBA and may be developed in the future for PFPeA and PFHxA. Consequently, utilization of coated bentonite pellets during well installation presents a potential for low-level false positives.
This evaluation does not account for pH or other geochemical factors within the aquifer that may impact the leaching, or for the potential for additional leaching to occur more than 24 hours after installation. Conversely, this evaluation did not account for the possibility that PFAS released from the coating became adsorbed to the clay bentonite particles themselves, as was previously hypothesized, since the bentonite was not included in the analysis, only the aqueous leachate.
While the potential PFBA concentrations within an aquifer based on the assumptions calculated may not be significant, there is the potential for concentrations to be greater than those calculated. Additionally, there is the potential for additional PFAS or precursors to be present that were not included in this analysis that could impact the concentrations of target PFAS present in an aquifer following the use of coated bentonite pellets. Recent Environmental Protection Agency Health Advisories for some PFAS are well below the estimated potential aquifer contributions from coated bentonite pellets. As a result, future toxicological evaluation could trigger findings that low concentrations of PFBA, PFPeA, and PFHxA are of concern. However, there are no health advisories for the detected PFAS at this time.
RECOMMENDED GUIDANCE
The inadvertent introduction of PFAS during the installation of monitoring wells presents a potential concern, particularly during initial site evaluations where a release to the environment has not yet been confirmed and the nature and extent of PFAS impacts have not yet been defined. This could lead to uncertainty regarding the path forward for defense environmental sites. The financial impact may be significant as additional investigation becomes required to reach regulatory concurrence. Remedial action decisions will be based on sampling data. It is critical to have a high degree of confidence that the results are truly representative of environmental conditions.
Based on the findings of this study, it is recommended that coated bentonite pellets be avoided during the installation of monitoring wells at sites identified for PFAS evaluation.