An entirely new “green” technology is proposed that has the potential to dramatically reduce roadways’ stormwater runoff volume. The stormwater volume reduction is achieved by enhancing a roadway soil’s hydrologic performance (i.e., water retention and infiltration) by amending it with biochar. Hence, the existing highway greenways can provide stormwater treatment without the requirement for adding a new infrastructure. Biochar’s impact on soil hydrology was monitored in laboratory experiments over time for 12 roadway soils collected by DOTs in California, Delaware, Maryland, and North Carolina. For soils with low infiltration without biochar, biochar amendment improved conditions; however, for soils with initially high infiltration, biochar decreased stormwater infiltration. Because the beneficial effect of biochar on soil aggregates observed in pilot- and field-scale experiments was unexpectedly not observed in laboratory column tests, some features of the column experiments did not mimic field conditions and thus underestimate biochar’s benefits. Comparing field tests with laboratory experiments conducted in identical soils, biochar’s benefits for stormwater infiltration were more than double that in the lab. Therefore, laboratory experiments provide a conservative assessment (underestimate) of biochar’s effect on increasing stormwater infiltration and thus reducing stormwater runoff. Future work should include demonstration projects in the field coupled with modified laboratory column experiments. Demonstration projects at selected DOT field sites across the US will evaluate the impact of climate, soil, and regional biochar sources.

The final report is available.