A charcoal-like material made from leaves and branches that collect on forest floors could be a cheap, sustainable way to keep pollution from washing off roadways and into Georgia’s lakes and rivers.

Engineers at Georgia Tech and Georgia Southern University have found that this biological charcoal, or biochar, can be mixed with soil and used along roadways to catch grimy rainwater and filter it naturally before it pollutes surface water.

Their tests found the biochar effectively cleans contaminants from the rainwater and works just as well in the sandy soils of the coastal plain as in the clays of north Georgia. Their biochar-soil mixture can be easily substituted for expensive material mined from the earth that’s typically used on roads. 

Though they focused on Georgia, the researchers said the findings could easily apply across the U.S., providing a simple, natural way to keep road pollutants out of water sources. They published their approach in the Journal of Environmental Management.

“We have a lot of forest residues, meaning that sourcing the raw material is not a problem,” Yunus said. “So we can use an abundant natural resource to create a process that actually protects another resource, our surface water.”

Biochar is often used to improve soil for agricultural uses, and it’s available at lawn and garden stores. It’s also becoming a popular way to sequester carbon: Microsoft recently purchased more than a million tons of carbon credits from a company producing biochar in Bolivia.

Yunus has been exploring the idea of using the material mixed with soil to clean roadway runoff since his environmental engineering master’s studies with Georgia Southern Professor George Yuzhu Fu. At Georgia Tech, he’s continued the work with his Ph.D. advisors, Yongsheng Chen and Joe Bozeman. Meanwhile, Fu’s team is complementing Yunus’ lab studies with a full-scale trial of the material along roads on the Georgia Southern campus.

Though this study focused on soil and road design standards in Georgia, Yunus said translating their approach to neighboring states or across the country is a simple matter of applying their data to the characteristics of local soil.

“The insight here for engineers is that if you are building a road and you want to apply this design, you have to understand the soils around you. Some geotechnical engineers already do that,” Yunus said. “Our study presents design data about how to effectively amend the soil by adding the right amount of biochar — for example, 10% or 5% or 2%. You can make that decision based on your site-specific information.”