Stream restoration has become a widely accepted nature-based approach to addressing nitrogen and phosphorus contamination in surface water. While these contaminants have been the primary focus of regulatory efforts in several US states, per- and polyfluoroalkyl substances (PFAS), are rapidly moving to the forefront of regulatory and mitigation efforts due to their prevalence, persistent nature and associated negative health consequences.

Used in a variety of consumer and industrial products, PFAS dissolve in water and move easily through aquatic environments. Occurring widely in surface water and soils near rivers and streams, PFAS are showing up in small aquatic organisms and fish tissues, due to their ability to bioaccumulate.

How can stream restoration address PFAS?

Stream restoration projects are usually undertaken to improve the health and function of streams and their associated ecosystems. For example, they may be employed to reduce the risk of flooding, provide recreational activities or restore diverse habitats. An additional role being explored is a project’s ability to reduce PFAS in contaminated water bodies. Here are some ways in which stream restoration can address PFAS contamination:

1. Absorption: While research in this area is ongoing, there is evidence that some riverbank and aquatic plants, such as cattails, water hyacinth and duckweed, can absorb certain PFAS chemicals, reducing their concentrations in the water and protecting aquatic fauna.
2. Bank stabilization: Unstable riverbanks are prone to erosion, which can release sediment into the water column. If these sediments contain PFAS, they can contribute to the contamination of downstream water bodies. Riverbank stabilization techniques, such as vegetative stabilization, bioengineering, or engineered structures, can prevent erosion and reduce the release of PFAS-laden sediment.
3. Sediment management: Sediments can contain PFAS compounds. Restoration efforts that address sediment management can help prevent the release of PFAS-laden sediment into the water column.
4. Biodegradation: Some microorganisms can break down certain types of PFAS compounds through natural processes collectively known as biodegradation. Stream restoration projects can promote the growth of microbial communities that have the potential to break down PFAS compounds.
5. Education and outreach: Stream restoration projects provide opportunities for education and outreach to communities about the risks of PFAS contamination, how it spreads, and what actions individuals and industries can take to reduce PFAS releases.

Careful stream restoration planning and monitoring is essential to prevent the spread of PFAS contaminated sediment downstream or to adjacent areas. Without full knowledge of the potential impacts, stream restoration projects could do more harm than good. This is where modelling can prove invaluable.

What role can hydrological modelling play in removing PFAS from rivers and streams?

Stormwater and hydrological modelling programs can help us understand the impacts of stream restoration on PFAS contamination in streams and surrounding environments. Such software enables us to:

• Simulate the movement of PFAS in streams.
• Identify potential sources of PFAS contamination by tracing back the transport pathway of contaminants.
• Model how changes in stream flow, erosion and restoration techniques can influence PFAS distribution and concentration.
• Predict how PFAS concentrations might change over time.

Providing the input data is accurate, incorporating modelling software into PFAS stream restoration efforts can enhance decision-making, improve the efficiency of restoration projects, and contribute to a more comprehensive understanding of PFAS contamination dynamics.

Water and sediment quality monitoring, and aquatic life tissue sampling can be used to assess the effectiveness of restoration efforts and track changes in PFAS levels over time.

What are the main treatment methods for removing PFAS from water?

Ideally, PFAS would never enter a stream and eliminating transport of contaminated groundwater and surface water near the source is a good way to lessen stream impacts. The choice of treatment method depends on flow rates, the specific PFAS compounds present, water quality parameters and treatment goals. The main treatment methods for water matrices include activated carbon adsorption, ion exchange, membrane filtration such as reverse osmosis and nanofiltration, and advanced oxidation processes that use chemical reactions to break down PFAS compounds. Although these methods are effective in removing PFAS accumulation, nature-based approaches, such as stream restoration, can also play a role for surface water.

Final thoughts

PFAS contamination is one of the greatest environmental challenges of our time. While stream restoration can contribute to limiting PFAS from entering ecological systems by utilizing natural processes to reduce PFAS concentrations, a comprehensive approach that combines restoration with source control, regulation, and other remediation strategies is necessary for effectively managing and mitigating PFAS contamination.