Reducing Stormwater Runoff through Biochar Addition to Roadway Soils

Roadway greenways, the soils adjacent to roads, may be amended with biochar to increase stormwater infiltration and retention, thus decreasing runoff requiring treatment in bioretention facilities, bioswales, etc. In this project we evaluate the addition of a commercial wood biochar pyrolyzed at 550° on roadway soil hydraulic properties in the laboratory and the field. The effect of biochar addition at 2% and 6% mass fraction on water retention and saturated and unsaturated hydraulic conductivity was evaluated in the laboratory for loamy sand, sandy loam, and silt loam soils typically found adjacent to roadways in the MidAtlantic region of the US. Biochar increased hydraulic conductivities and field capacity for sandy loam and loamy sand - up to 320% and 308%, respectively, for loamy sand when added at 6% mass fraction, while the effect on silt loam was much less. The impact of biochar addition on water retention was predicted reasonably well using information on the intraparticle pore volume of biochar (mercury porosimetry) and the particle size distribution of the soil/biochar mixture. The wood biochar was added to a sandy loam along a roadway in Delaware to evaluate its effect on reducing stormwater runoff. Using a control section without biochar and an adjacent section amended with biochar at 4% mass fraction, trench drains quantified the impact of biochar on stormwater runoff. In twenty five storm events over a five month period, biochar addition reduced stormwater runoff volume by an average of 75% and peak flow rate by an average of 54%. These measured reductions are compared with independent predictions using information from laboratory tests.