The goal of this project is to demonstrate and validate the Osorb® passive sampler to determine concentrations of per- and polyfluoroalkyl substances (PFAS) in porewater at impacted sediment sites. Measuring PFAS in sediment porewater is important because freely dissolved concentrations in porewater represent the potentially bioavailable fraction. While a variety of demonstrated porewater sampling methods exist, there are important limitations with existing methods. With some methods, there is potential for overestimating dissolved constituents in porewater while other methods are limited by commercial availability, ease of deployment, and implementation costs. Porewater also tends to be highly heterogeneous, increasing the cost and time required for collecting representative samples.

Passive sampling commonly offers cost and time-saving advantages over grab sampling. Recently, a new simple passive sampler for PFAS in aqueous systems using the Osorb® media was developed (SERDP project ER20-1127) and tested in groundwater and surface water. This project showed the sampler reliably measured PFAS concentrations in a variety of water types, behaved predictably as an integrative-type sampler, and yielded field results highly comparable to conventional surface water and groundwater sampling methods. This demonstration activity will extend the application of the Osorb® passive sampler to the sediment porewater environment. Additional work is needed to (1) verify the calibration of samplers for porewater sites where flow rates through sediment may be low, (2) evaluate deployment duration in sediment, and (3) provide estimates of precision and accuracy for sediment systems. Laboratory and field deployments will address these issues.

The Osorb® passive sampler design is simple and robust, using an organosilica resin modified with crosslinked amine polymer in a high-density polyethylene housing with polypropylene mesh. The design is currently being modified for sediment applications incorporating a metal housing for strength. The addition of amine groups as a weak ion-exchange resin in combination with Cu²⁺ was designed to promote binding of short-chain PFAS. Passive sampler uptake rates (i.e., sampling rate [Rs]) were robust in laboratory tests even under conditions of extreme ionic strength and natural organic matter concentrations, indicating potential applicability to a wide range of environmental water types. Integrative performance for most analytes showed a linear response to concentration with time, which simplifies calculation of aqueous concentrations. Sampling times of one to two weeks achieved detection limits of approximately two nanograms per liter (ng/L). Flow rates previously evaluated in the laboratory ranged from approximately 0.01 cm/min (simulated aquifer) to 1.9 cm/day (laboratory column), while flow rates in sediment environments may be lower, potentially resulting in lower passive sampler Rs values. The scope therefore includes evaluation of Rs values and deployment times applicable for sediment environments, and will consider design modifications, if needed. The scope also explores the use of the passive sampler in vertical profiling of PFAS concentrations in sediment porewater, as well as in PFAS flux calculations.

The development and demonstration of standardized procedures for field sampling of PFAS in environmental waters has been identified as a critical research priority. Releases of PFAS are known or suspected at hundreds of sites and in the coming years, hundreds of millions of dollars will be spent in sampling, analysis, and remediation. Passive sampling tends to reduce time spent in the field and reduces the volume of investigation derived waste, providing health and safety and sustainability benefits, and cost savings. The small size of the Osorb® passive sampler further reduces sample shipping weight, which saves cost and reduces the carbon footprint. The Osorb® passive sampler is designed to be flexible for use in a variety of water types and range of PFAS concentrations, from low ng/L to µg/L levels. A successful porewater passive sampler for PFAS furthermore will allow measurement of the freely dissolved PFAS concentration in sediment. Measurement of this fraction will improve decision-making for site investigation, risk assessment, and remediation to focus on the bioavailable fraction of PFAS in sediment. (Anticipated Project Completion 2025)