Quality Assurance in PFAS Site Investigations – Part 3: Field Blanks

Field blanks: what are they, why do we collect them, and what does our data from dozens of site investigations show?

By Scott Bell, Senior Environmental Engineer & Vice President, Christopher Behnke, Field Services Manager & Environmental Geologist, and Jessica Bleha, Hydrogeologist, PG (Ann Arbor, MI)

August 28, 2024

PFAS Site Investigations and Lessons Learned

This article is the third in our series on the value of quality assurance samples during poly- and perfluoroalkylated substances (PFAS) investigations and lessons learned (see Part 1: Source Water Sampling and Part 2: Equipment Blanks). Here, we share some lessons learned from our field blank data. We’ll discuss what field blanks are, why we collect them, and what our data from dozens of site investigations show.

What are Field Blanks, and Why Do We Collect Them?

Field blanks are samples of analyte-free water (i.e., the chemical compound(s) being measured is not present in the sample, in this case, PFAS) exposed to the environment during sample collection to determine the potential for sample contamination from ambient (or surrounding) sources. There are two main categories of ambient sample contamination. First, it is possible that the attire worn or personal care products used by field personnel (items such as soaps, cosmetics, sunscreen, and insect repellant) may contain leachable PFAS. The concern is that precipitation or perspiration might leach PFAS from these materials and make its way into a sample container during sampling, thereby contaminating the sample. This concern continues to be a significant feature in many PFAS sampling guidance documents (1).

The second category of ambient sample contamination is environmental sources, primarily precipitation in various forms and windblown dust. Field blanks are the primary quality assurance method of identifying possible sample contamination from these ambient sources because they are known to be PFAS-free before exposure to the sampling location and environment, and they are not exposed to the media (e.g., groundwater, surface water, soil, etc.) being sampled.

Field Blank Collection Procedures

Our method for field blank collection follows the standard approach described in most published PFAS sampling guidance. As stated in the Michigan EGLE guidance, the field blank is prepared “by placing an aliquot [a portion] of PFAS-free reagent water in a sample container and treating it as a sample in all respects, including shipment to the sampling site, exposure to sampling site conditions, transfer to a clean sample container in the field, storage, preservation, and all analytical procedures.”

PFAS-free water is purchased from the labs we use for PFAS analysis, and also used in equipment blank collection, as discussed in previous articles in this series (Part 2: Equipment Blanks). The PFAS-free water is typically supplied in sealed one-gallon containers, which we transport to the investigation site unopened. When the time comes to collect the first field blank of the event, field personnel open the container and use it to fill the sample container for the field blank. That sample container is then closed, stored, and transported in the same container as other samples for that event.

In preparing this article, we compiled and reviewed data from 196 field blanks collected from 18 PFAS investigation sites between 2018 and 2023.

In our PFAS investigation experience, we’ve deviated from this procedure on only one site. In that case, our client required us to use a laboratory already under contract with them and whom we had not worked with previously. For field blanks, that laboratory provided sample containers pre-filled with PFAS-free water, and those containers were transported to the site to be used during sampling. During sampling, the field personnel would open the lab-provided field blanks at a selected sample location to expose them to ambient conditions during the sample collection process. Once sampling was complete, the field blank was re-capped and placed in the sample cooler for transport back to the lab. Although this is not our standard procedure, this alternate approach is valid and not uncommon.

Observations on Our Field Blank Data

In preparing this article, we compiled and reviewed data from 196 field blanks collected from 18 PFAS investigation sites between 2018 and 2023. General observations on the data are presented below. Of the 196 field blank samples we reviewed, 177 (~90%) were collected using our standard approach described in the previous section, and 19 (~10%) were collected using the alternate approach described above.

The first question we wanted to answer was whether we had any positive PFAS detections in our field blank data, and if so, did they occur more commonly in our standard approach or the alternate approach? Of the 196 field blanks reviewed, no PFAS were detected in 165 (~84%) field blank samples. At least one PFAS was detected in 31 (~16%) field blank samples.

An additional observation is that PFAS were detected in only 14% (or 25 of 177 samples) of the field blanks collected using our standard method, compared to PFAS detection in 32% (or six of 19 samples) of the field blanks collected using the alternate approach. However, this observation should not be construed as a concrete finding due to the relatively small sample size using the alternate approach and the fact that all those samples were collected from one site.

A review of the data from the 31 field blanks with positive PFAS detections yields some interesting observations, including the following:

• As shown in Figure 1, the most commonly detected PFAS in field blanks was perfluorohexanoic acid (PFHxA), measured in 48% of the field blanks with detections. This observation is interesting because PFHxA was also the most commonly detected PFAS in our review of equipment blank data, as discussed in the Part 2: Equipment Blanks article.
• PFHxA was followed by perfluorobutanoic acid (PFBA) and perfluorooctane sulfonic acid (PFOS), which were detected in 26% and 19% of the field blanks with detected PFAS, respectively.

Figure 1. The frequency of individual PFAS detection in field blanks with positive detections.

• Most detected PFAS were measured in single-digit parts per trillion (ppt) concentrations, but six different PFAS were reported in the double-digit ppt range at least once. The highest PFAS concentration measured in a field blank was 44.9 ppt for 8:2 fluorotelemer sulfonic acid (8:2 FTS). The concentration distributions of all PFAS detected in field blanks are shown in Figure 2.
• Four field blanks were found to contain PFOS and/or perfluorooctanoic acid (PFOA) at concentrations greater than 4 ppt, the recently promulgated federal safe drinking water limits.

Figure 2. The concentration distribution of PFAS detected in field blanks.

The frequent detection of PFHxA in these blanks prompted a discussion with one of the analytical labs we routinely work with.

The lab shared that based on their experience, PFPeA and PFHxA are probably the two most common contaminants not related to high-level samples. They noted that they see these compounds periodically in “Field” and “Equipment” blanks and occasionally in their laboratory method blanks; however, with the method blanks, they generally see one or the other, and not both compounds.

One theory the lab shared is that the compounds leach out of some of the sampling supplies or lab containers, but they noted that they have not been able to duplicate it. The lab stated it is possible that the compounds could have been picked up from something the field personnel was wearing or had sitting nearby, but that it is extremely difficult to pinpoint this level of contamination.

Although this does not provide conclusive evidence of the source of the detected PFHxA, it is relevant information and worth noting here.

Recommendations for Use of Field Blanks in PFAS Site Investigations

Field blanks are essential for data quality assurance in PFAS investigations, as they are the best way to isolate and identify possible sample contamination from ambient sources. Here are a few recommendations for collecting field blanks based on our experience:

• At a minimum, one field blank should be collected for every 20 samples collected during a PFAS investigation. In addition, we typically recommend collecting one field blank for each day of an investigation to account for variations in weather conditions.
• Field blanks should be collected at the same time and location as matrix samples (i.e., groundwater, surface water, soil, etc.) are collected. The general guideline is that the field blanks should be exposed to the same sampling environment as the matrix samples.
• Without exception, PFAS-free water from a certified lab should be used when collecting field blanks.

We hope you found this article informative and helpful in considering and building quality assurance protocols for collecting field blanks during PFAS site investigations.

If you have any questions about PFAS analysis methods, regulations, or site investigations or would like to discuss your PFAS-related needs, contact Scott Bell at sbell@limno.com.

Citations:
(1) Michigan Department of Environment, Great Lakes, and Energy (EGLE). “General PFAS Sampling Guidance.” January 2024.

This article is the fourteenth in a series of articles authored by LimnoTech staff on PFAS-related issues. Follow us on LinkedIn and check the Insights & Perspectives page on our website for more information and updates. Links to the other PFAS articles in this series are provided below:

PFAS – Emerging, But Not New
Sampling for PFAS Requires Caution
PFAS Analysis – The New Wild West
Aviation and PFAS – What’s the Connection?
PFAS – The Next Wastewater Utility Challenge?
Should Municipalities Worry About PFAS?
PFAS – How Low Can You Go?
Key Considerations for PFAS Field Investigations
What Would A PFOS And PFOA Hazardous Substance Designation Under CERCLA Mean For The Business Sector?
PFAS – How Low Can You Go? Now We Know!
Method 1633 For PFAS In Aqueous Samples
Quality Assurance in PFAS Site Investigations – Part 1: Source Water Sampling
Quality Assurance in PFAS Site Investigations – Part 2: Equipment Blanks

Scott Bell, PE, is the Vice President of Business Operations and a Senior Environmental Engineer at LimnoTech. Scott has been with LimnoTech since 1992. He manages projects for various industrial, municipal, and federal clients. Scott has technical expertise in environmental remediation and restoration and is LimnoTech’s practice leader for PFAS and emerging contaminants. In that role, he has overseen and directed PFAS-related investigations at two dozen airports and industrial sites. Throughout his career at LimnoTech, Scott has planned and executed scores of hydrologic and hydraulic engineering, water quality management, wastewater discharge impact analysis, stormwater management, and engineering feasibility evaluation projects across North America. He has taught professional development courses for the Engineering Society of Detroit, delivered numerous technical papers and presentations, and served on several professional boards and committees.

Christopher Behnke serves as the Field Services Manager at LimnoTech. Chris has 23 years of experience as a Geologist conducting environmental sampling of various types, including surface water, groundwater, soil, sediment, and air. Each collection method requires consideration for a distinct, site-specific approach to collect the most representative sample possible based on existing conditions. Chris has experience with GIS, Excel, and various instrument-related programs used for field data collection. These skills are utilized in the planning, implementing, and reporting numerous data collection efforts. Chris’ experience included the collection of soil, groundwater, and stormwater runoff samples around airfields to characterize PFAS impacts from past use of AFFF and provide oversight for field investigations of PFAS. He has conducted fieldwork at several airports to monitor the environmental impacts of aircraft deicing and has also conducted investigations of PFAS and other contaminants at numerous industrial sites.

Jessica Bleha, PG, is a Hydrogeologist with more than 15 years of environmental consulting experience and over a decade of work in project management, planning, and coordination. Jessica’s expertise includes hydrogeological assessments of surface and groundwater resources, environmental site assessments, conceptual modeling, data management systems, mine remediation and permitting, monitoring wells, soil boring, and remediation system installations. She has supported investigations of PFAS impacts in groundwater, soil, and stormwater from past use of aqueous film-forming foam (AFFF) at 11 airports in Michigan. Her work includes oversight of drilling subcontractors, installation of groundwater wells, and sample collection. Jessica has performed and overseen soil, groundwater, and surface water investigations for PFAS and other contaminants, evaluated municipal well-pumping test data, and provided remedial design and support for a Superfund site. She is proficient in various environmental and engineering software (ArcGIS, AQTESOLV, LogPlot, RockWorks20).