Insights

PFAS on the Move: Fate and Transport in Sediment

Lee Hovey and John Rice | January 3, 2023

Properties of PFAS

Because per- and polyfluoroalkyl substances (PFAS) are complex molecules that can enter surface waters through runoff, direct discharge (e.g., aqueous film forming foam [AFFF]), aerial deposition and groundwater migration, a thorough understanding of both PFAS and surface water hydrology is critical to best assess potential impacts to sediments.

The PFAS Cycle

PFAS tend to sorb to sediment particles and organic matter, which settle at the bottom of a waterbody, creating a reservoir of PFAS that can cycle in the environment for decades.

The Molecular Structure Influences Movement

PFAS are generally defined by a chain of fluorinated carbons, called the “‘tail’ of the molecule, connected to a functional group, like sulfonates or carboxylates, often referred to as the “‘head’” of the molecule. The “‘tail’” of fluorinated carbons is both oleophobic and hydrophobic (repels oil and water) making them good for stain-resistant and water-repellant applications, while the functional “‘head”’ is often hydrophilic (attracted to water). These unique properties influence their fate and transport mechanisms.

PFAS Accumulates on/in Water

PFAS tend to accumulate at the air-water interface and form micelles, which are essentially little bubbles of PFAS, due to both their hydrophobic and hydrophilic properties. These micelles can create a layer of foam on surface water and be transported distances (e.g., miles) downstream.

PFAS Transport

PFAS also have different solubilities and different sorption affinities for soil and organic content. These factors affect the transport of PFAS in surface water and sediments and can play a role in forensic analysis. Typically, longer-chain PFAS or PFAS with a sulfonate functional group are more prone to binding to soil and organic particulates making them more persistent in sediments, while shorter-chain PFAS or PFAS with a carboxylate functional group usually have higher solubilities and are more easily flushed through a surface water system. Understanding these different mechanisms plays an important role in identifying and differentiating potential sources.

Where PFAS Settles

PFAS are often sorbed to the lighter, fine-grained sediment particles composed of silt, clay, and organic material, and can travel farther from the source and settle in low energy environments downstream, like in the deeper water of lakes, in estuarian settings, or even in reservoirs behind dams.

Best Practices for Sampling and Characterization

It is important to understand the potential PFAS sources and adequately characterize the geomorphology of the water body before developing a sampling program for PFAS in sediment. Depositional areas should be identified ahead of time and included in the sampling program to collect representative samples of potential impact. Additionally, potential upstream sources should be identified, and upstream sediment should also be adequately characterized to document any overlapping sources that could affect potential liability or remedial activities.

Is Contaminated Sediment an Issue for Your Facility or Site?

If your facility or site is located near a water body, TRC has the knowledge and expertise in both PFAS chemistry and surface water hydrology to assist with PFAS-related surface water and sediment projects. We can help you design and implement a surface water and sediment sampling program, from sample collection to forensics analysis to differentiation of possible sources and migration pathways. We can also assist with sediment basin cleanouts or dredge material characterization for disposal or beneficial reuse.

Contact Lee Hovey or John Rice, PE, PH. today to discuss your concerns about PFAS in surface water and sediment.

Also keep an eye out for our upcoming Insights on sediment and dredge material characterization for PFAS!

Lee Hovey

Lee Hovey has 6 years of experience and progressive responsibility in environmental and civil engineering consulting. His qualifications include extensive hands-on planning, field investigation and remediation system design, permitting, cost estimating, and project management. Mr. Hovey serves as the deputy co-chair on TRC’s PFAS CORE Team where he provides technical support on PFAS-related projects for various municipal waste, oil and gas, and airport clientele. Please contact Lee at LHovey@TRCcompanies.com

John Rice, PE, PH

John Rice is a consulting engineer and hydrologist working out of TRC’s Madison, Wisconsin office. He has over 26 years of experience in the environment field. John provides technical expertise in surface water and groundwater hy-drology, sediment and groundwater remediation. He has designed and overseen the successful construction of sedi-ment remediation systems, including complex dredging and capping alternatives. He has also designed and installed active groundwater and soil remediation systems that include innovative in-situ remedies. John is a leader in the devel-opment of new conceptual models for migration of liquid coal tar and of recalcitrant organics from sediment. These efforts have yielded new understanding of risks and appropriate remedies. John was awarded a patent for an innova-tive approach to facilitate the in-situ degradation of chlorinated organic compounds and has patent applications awarded and pending for improved sediment cap designs. John is active in the advancement of the profession through the presentations and publication of professional articles. Contact John at JRice@TRCcompanies.com.

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