Collecting Groundwater Samples for PFAS Analysis using Dedicated Teflon® Lined Tubing and Bladders

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Collecting Groundwater Samples for PFAS Analysis using Dedicated Teflon® Lined Tubing and Bladders

By: Doug Hunter

Polytetrafluoroethylene (PTFE) is a strong, tough, waxy synthetic resin, known by such trademarks as Teflon®, Fluon, Hostaflon, and Polyflon. These materials are recognized for their high temperature resistance and their chemical resistance to solvents, acids and bases.  Because PTFE is chemically inert, the US EPA and others have historically preferred the use of PTFE and PTFE lined tubing and pump bladders for the collection of groundwater samples for VOC, SVOC, pesticide, and PCB analysis.  In addition, the corrosion and temperature resistant properties, along with its resistance to UV deteriorization make it the ideal material for long term deployment in dedicated monitoring systems.  As a result, there are a lot of groundwater monitoring networks that employ dedicated sampling/monitoring equipment containing PTFE. 

Unfortunately, because per- and polyfluoralkyl substances (PFAS) compounds are process agents used in making PTFE, there are concerns that it is not an ideal material for collecting PFAS groundwater samples.  In fact, tubing and pump bladders containing PTFE typically top the list of banned sampling materials in most PFAS sampling guidance documents.  The reason for this is that, although the PFAS processing aids are not present in the finished PTFE, trace concentrations may be left behind as a residue.  But what if that residue wore off, over time, with repeated purging/sampling events, would the dedicated PTFE sampling materials still pose a cross-contamination risk?

To attempt to answer this question we conducted a simple study using both used and new PTFE sampling materials.  The used PTFE materials consisted of dedicated sampling equipment obtained from one of our clients.  The dedicated sampling equipment consisted of 46 feet of Teflon lined tubing and a bladder pump.  The bladder pump was fitted with a PTFE check ball and seal and a PTFE lined bladder.  The sampling equipment had been used to collect semiannual groundwater samples from an upgradient monitor well and had been in the well for approximately three years.  The retrieval of the tubing and pump as well as the subsequent sample collection was performed in accordance with our Standard Operating Procedures (SOP) for collecting and handling PFAS groundwater samples and related equipment.

The groundwater source for the study was our onsite monitor well that we use for training purposes.  The onsite monitor well had been used in prior PFAS training and studies.  Based on validated laboratory results, a single PFAS compound (PFBA) had been detected at an estimated concentration of 1.2 J nanograms per liter (ng/L) in one of the prior groundwater samples (which in the PFAS world is considered an uncontaminated well).    

The approach for “sampling” the used and new PTFE materials was simple.  First, a groundwater sample was collected using the dedicated (i,e, used) sampling equipment.  Then the used PTFE materials were replaced with new PTFE materials and a second groundwater sample was collected using the new material.  Prior to collecting the second groundwater sample, an equipment blank was collected from the new PTFE equipment using laboratory-provided PFAS free water.  As a precaution against potentially introducing PFAS into our monitor well from the sampling equipment, a PVC cylinder staged at the wellhead with groundwater from the well was used for the test. During testing, groundwater was continuously pumped from the monitor well into the PVC cylinder, using a peristaltic pump and HDPE tubing. The sampling was performed using low flow procedures and the temperature, pH, specific conductance, and turbidity of the purge water was monitored to determine when groundwater conditions stabilized.  Following stabilization of the field parameters, the groundwater sample was collected and placed into approved containers provided by the laboratory. 

For the used equipment test, the dedicated pump housing was placed in the PVC cylinder and the groundwater from the cylinder was pumped by the dedicated bladder pump through the 46 ft of used tubing into the sample containers. The bladder pump was then fitted with a new Teflon check ball and seal and a new Teflon lined bladder, and the 46 feet of existing tubing was replaced with 46 feet of new Teflon lined tubing.  The PVC cylinder was decontaminated and then filled with laboratory-verified PFAS free water and an “equipment blank” sample was collected by pumping the PFAS free water through the pump and 46 feet of tubing into a sample container.   Following the collection of the equipment blank, a second groundwater sample was collected using the pump configured with the new Teflon lined tubing and bladder.  The sampling procedures, purge rate, volume and duration were identical to those used to collect the initial groundwater sample using the dedicated PTFE sampling materials.    The groundwater and QA/QC samples were submitted to Eurofins TestAmerica for analysis of 24 PFAS compounds using Method 537 (modified).   Following receipt of analytical results, Cox-Colvin reviewed and validated the analytical data.

A summary of the PFAS compounds detected in the samples is provided in the attached Table 1.   As shown in Table 1, PFBA (1.3 J ng/L) and FOSA (0.52 J ng/L) were reported at trace concentrations in the groundwater sample collected with the dedicated/used PTFE equipment (CCAMW-01 Used).   The detection of just two PFAS analytes at trace concentrations indicates that the dedicated PTFE tubing and bladder pump liner were not a significant source of PFAS cross-contamination.  Currently, there are no federal or state standards/action levels for FOSA in groundwater.  With regard to PFBA, the Minnesota Department of Health has developed a guidance value of 7,000 ng/L in drinking water and the Texas Commission of Environmental Quality has established a Groundwater Protection Concentration Level of 71,000 ng/L.   

Four PFAS compounds (PFBA, PFHpA, PFPeA, and PFOA) were detected in the equipment blank sample using the new PTFE tubing and pump bladder (Table 1).  Of these, PFOA was the only constituent with a reported concentration above the laboratory reporting limit.  PFOA is a legacy PFAS compound that for many years was the primary process agent for PTFE.   The reported PFOA concentration of 4.7 ng/L is well below the current US EPA established health advisory level (HALs) of 70 ng/L.    PFHpA and PFPeA were not reported in any of the previous groundwater samples collected from this monitor well and thus are assumed to be related to the new PTFE sampling materials. 

Finally, just two PFAS compounds, PFBA (1.1 J ng/L) and FOSA (0.99 J ng/L), were detected in the groundwater sample collected using the new PTFE sampling materials (CCAMW-01 New; Table 1).  These are the same two compounds that were detected at similar concentrations in the groundwater sample collected with the dedicated/used PTFE sampling materials (CCAMW-01 Used).   It is interesting to note that three of the PFAS compounds detected in the equipment blank (PFHpA, PFPeA, and PFOA) were not detected in the groundwater sample, suggesting that they were rinsed from the new PTFE sampling materials during the purging process. 

Based on the sampling results, it appears that the dedicated PTFE sampling materials used for this study are not a significant source of PTFE cross-contamination. The equipment blank results indicate there may be some residual PFAS compounds associated with the new PTFE sampling materials, it appears that they are rinsed from the sampling materials during the initial purging process.  Considering that this study was limited to a single sampling event, we are careful not to make any sweeping conclusions, however, the results do provide some hope that dedicated PTFE sampling materials may be suitable for collecting groundwater samples for PFAS analysis. 

Doug is a licensed professional geologist in Indiana. As a consulting hydrogeologist, Doug specializes in aquifer characterization and yield determinations, well and wellfield performance evaluations, and the design and testing of both vertical well and horizontal collector well systems. Additional areas of expertise include environmental assessment and remedial system evaluation and design. He has worked throughout the United States on a wide variety of groundwater supply and environmental contamination related projects. Doug’s wide-ranging expertise and extensive experience in the groundwater supply industry add another dimension to Cox-Colvin’s technical staff and provide additional opportunity to support our clients in meeting their needs and reaching their business goals.