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PFAS Sampling Advisory on Aqueous Samples

Elizabeth Denly | March 31, 2021

PFAS Sampling Advisory: Aqueous Samples

If you plan to collect aqueous samples (e.g., groundwater, wastewater, stormwater, etc.) for PFAS analysis, it is important that you read this advisory.

The Problem

Sample preparation inconsistencies could affect reported PFAS results for aqueous samples containing particulates.

PFAS Sampling Advisory on Aqueous Samples
PFAS Sampling Advisory on Aqueous Samples

Current EPA analytical methods for PFAS address solids-free, drinking water only. Sample preparation and extraction procedures of aqueous samples containing suspended solids vary amongst laboratories. Depending on the laboratory and the analyst, solids present in the sample may or may not be included in the final extract for PFAS analysis. Several laboratories centrifuge the samples and decant off the water for subsequent extraction. Some laboratories may also extract the remaining particulates and combine the extract with the aqueous extract. Laboratories that are not including the extraction of the particulates (i.e., water only) are not providing a total PFAS measurement of these aqueous samples; this may be acceptable in certain instances depending on the ultimate project objectives.

The problem with not controlling or not knowing whether solids are included in the analysis is that PFAS may partition to the solids, especially the longer-chain PFAS. If these solids settle out or are removed from the water samples, reported PFAS concentrations in the sample can be lower, possibly by an order of magnitude or more than if the solids were included in the analysis as a “total” sample. Moreover, the reported mixture of PFAS compounds (i.e., fingerprint) may differ. Thus, we need to work with the laboratory in advance to determine the appropriate procedure to be used, depending on the end use of the data (e.g., remedial action, human or ecological risk assessment, compliance, etc.). The project objectives may also vary depending on the water matrix (i.e., groundwater, wastewater, stormwater).

Recommended Solutions

Depending upon project objectives, you may need to work with the laboratory prior to sampling to understand, and possibly request to change, the procedures they will use for handling your samples. Some guidelines to consider, in the absence of regulatory requirements, in the determination of the need for a “total” PFAS measurement are provided below (“total” refers to the extraction of both the aqueous and particulate fractions of the samples).


  • In order to minimize the presence of particulates, use of low-flow groundwater sampling is preferred; no purge samplers (e.g., Hydrasleeve) can be considered with site-specific verification of sample representativeness.
  • If turbidity is >10 NTUs, consider a “total” measurement if sampling groundwater for delineation, remedial design, or risk assessment. Work with or select the laboratory based on how they manage suspended solids expected at your site (e.g., historical information on suspended solids such as turbidity measurements or sampling notes). If sampling for compliance purposes, purge until clear (<10 NTU) or consider alternative quality control techniques (e.g., well re-development, change purge rate, change pump location, etc.).
  • Collect samples for total suspended solids (TSS) analysis or measure turbidity to assist in PFAS data evaluation.

Drinking Water

  • “Total” PFAS measurements will almost always be required; however, drinking water samples rarely have issues with suspended solids.

Surface Water and Wastewater

  • Consider a “total” measurement if surface water sampling or wastewater for remedial/treatment design, compliance, permitting, or risk assessment.
  • A “total” measurement may not be required if sampling for a line of evidence in a source area.
  • Collect samples for TSS or measure turbidity if sampling for compliance and to assist in PFAS data
Elizabeth Denly

Elizabeth Denly serves as TRC’s Vice President, PFAS Initiative Leader. She is also the Quality Assurance & Chemistry Director, responsible for the creation and implementation of the Quality Management Plan and standard operating procedures (SOPs) for field sampling and documentation protocols. Ms. Denly also leads Quality Coordinator networks, which are responsible for the development and communication of quality initiatives within the organization. She is a chemist with 29 years of consulting experience encompassing field and laboratory analyses and audits, QA/QC, data validation, and consulting for regulatory agencies.

Ms. Denly is a leader in ITRC’s PFAS and TPH Risk Work Groups and in TRC’s Center of Research & Expertise (CORE) Emerging Contaminants Team and received the ITRC’s Industry Member of the Year Award in 2017. She is currently focusing on PFAS, specifically the nomenclature, chemistry, sampling procedures, QA/QC, and laboratory analytical methodologies, and has a significant role in educating clients, attorneys, and regulators about PFAS.

As a senior QA specialist at TRC, Ms. Denly is responsible for providing QA/QC oversight in support of a variety of environmental investigations and remediation programs including risk-based soil cleanups, ambient air monitoring, and human health and ecological risk assessments. In this role, she has directed the preparation of QAPPs, coordination with the laboratory, selection of the appropriate analytical methodologies to achieve the desired remedial standards, oversight and performance of the data validation process, and determination of the usability of the data and achievement of data objectives. Ms. Denly has provided this oversight under different regulatory programs.
Read more on Ms. Denly’s bio page. Please contact her at

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