Nature-based solutions are succeeding against PFAS contamination at the town of Alpena in the US state of Michigan – where site-specific constraints meant that traditional methods such as pump-and-treat of contaminated water were impractical.

In solving these issues, WSP had three objectives: to capture and contain the PFAS on site, reduce PFAS migration to the storm sewers, and enhance the site by dealing with both the flooding and the PFAS contamination. 

In many such situations, extraction wells are drilled on the site, and pumps then pull the contaminated groundwater to surface for treatment by carbon or specialty resins.  Handling and disposing of this waste is becoming increasingly difficult and expensive.  

But in this case, pump-and-treat was unlikely to work. PFAS is widely distributed on the site and must be brought to low concentration levels to meet the State’s requirements, so it would be a lengthy process. Site conditions meant that pumps and wellheads would need to be repaired or replaced frequently. And, the State regulator was looking for a way to avoid pump-related emissions and expenses over the decades needed to bring the site to regulatory standard. 

Finding a solution in pavement, biochar and trees 

WSP developed a solution that involved three components. 

Pavement to reduce the flooding problem 
One aspect involved paving part of the property, with three main purposes in mind: 

• Prevent aquifer water from surging to surface, so as to manage the flooding problem, and direct the groundwater to a place where it could be dealt with 
• Help reduce the flow of rainwater that would recharge the aquifer, to reduce the amount of groundwater needing to be treated and to reduce the flushing of residual PFAS from the soil into the groundwater
• Put that paving in places that would meet the business needs of the property owners 

Binding PFAS with biochar 

The second aspect involved engineered biochar, a by-product of pyrolysis, which is the burning of biomass such as wood scraps or animal manure at high temperature and without oxygen. This black powder, a type of charcoal, is particularly rich in carbon. In this instance, WSP worked with a vendor that had identified “waste” woody material that they were able to recycle and use in reuse in this application. 

Biochar works in PFAS remediation because one end of the PFAS molecule is hydrophilic – attracted to water – and the other hydrophobic, repelling water. But the end of the molecule that doesn’t like water, happens to like carbon. This means that carbon-rich biochar, placed in the soil, can be used to bind the PFAS molecules in place, so they don’t migrate. 

WSP worked with a biochar manufacturer to identify, test and engineer biochar blends that would optimize PFAS uptake.  WSP then worked with general contractors and used ordinary construction equipment to work the biochar into the soil on some parts of the property, which helped reduce the tendency of the PFAS to migrate in groundwater towards the river and lake. The engineered biochar was also pumped into the ground using low-pressure equipment – between 60 and 80 psi. 

But there was still the question of the PFAS in the groundwater that was flowing into the storm sewer system. The team needed a way to bring the PFAS in the groundwater to a location where it could be trapped in place using biochar and to further reduce the potential for PFAS-enriched groundwater to flood to the groundwater surface. 

For each tree a borehole 3 feet across and up to 8-feet deep (1 by 2.6 meters) deep was dug, lined with a sleeve of impervious plastic, and a tree planted in the hole. The sleeve comes all the way to the surface and seals around the base of the tree. This keeps precipitation away from the roots, forcing the trees to send their roots deep into the soil to find the water they need to grow. These deep roots pull groundwater to the trees – water that contains PFAS. That PFAS is then bound in place by the biochar in the soil. 

This project is still in pilot mode, so there are still questions to be answered. For example, it’s not yet known how much PFAS eventually reaches the trees’ leaves and needles, with potential to be dispersed when the growing season ends. Also yet to be determined is how much PFAS-laden sap is pulled back into the roots when the trees go dormant in the fall. By pulling PFAS back into the roots with the sap, the trees themselves may be reducing the potential for exposure to PFAS. 

In effect, the biochar and trees are working in synergy to deal with the PFAS impacts to the groundwater. Some types of PFAS have longer molecules, and these varieties – PFOS and PFOA – are those for which biochar fixing is most effective. This characteristic of biochar remediation is most helpful, because it’s those longer-chain PFAS varieties that have the lowest concentration thresholds for effects on human health and the environment. Biochar tends to be less effective in hanging onto shorter-chain PFAS varieties, and this is the type of PFAS for which phytoremediation, using the trees, is most effective.  

WSP expects that on maturity, the miniature forest will pump some 30 to 50 gallons of water per day per tree. Currently, the trees are still small but will increase their capacity each year. Results so far are promising – in the first year of operation, percent reductions in total PFAS in groundwater range from 81% to 98%.  And even in their young states, the tree’s water use is producing measurable drops in the water table that indicate they are effectively “pumping” at a rate that is drawing nearby groundwater toward them and the biochar treatment.  

Lessons learned include: 

• Biochar and phytoremediation can work in tandem to remove a wide range of PFAS – both long and short-chain molecules alike
• Nature-based solutions are particularly effective at PFAS remediation, where the regulations require low levels in the parts-per-trillion category
• It’s important to work with other considerations for the property – such as making sure that the paving that capped the aquifer is also effective from a property-use perspective
• Where applied, this nature-based approach has transformed the aesthetic of the property, greening it up, while also helping to clean it up.