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.