Reducing Stormwater Runoff and Pollutant Loading with Biochar Addition to Highway Greenways

This study examines the reduction of nutrient loading and stormwater runoff volume by biochar amendment to the soils of highway greenways. Biochar is a charcoal-like material formed by combusting waste organic matter in an oxygen-limited environment and has high internal porosity and low particle density. Amending biochar to highways soils may increase total porosity and water retention, increase soil hydraulic conductivity, and enhance sorption and transformation of nitrogen compounds. In lab experiments, biochar was amended to three representative Mid-Atlantic soils: silt loam, sandy loam, and loamy sand. Biochar amended at 2 or 6% mass fraction to these soils increased available water content by 20-70% depending on the soil. Biochar amendment almost always increased unsaturated hydraulic conductivity; however, the impact of biochar amendment on saturated hydraulic conductivity was less significant. These findings suggest that biochar amendment to a roadway soil may result in reductions in stormwater runoff, but will depend on the roadway soil and amount of biochar added. To evaluate if biochar-induced changes in soil hydraulic might reduce stormwater runoff, four greenways were constructed in the laboratory: two filled with sandy loam soil and two filled with the mixture of sandy loam and 4% biochar by mass. The biochar amendment increased water retention and saturated hydraulic conductivity. These effects reduced runoff by 13% with respect to biochar -free soil, consistent with what was expected from laboratory experiments. Data collected over multiple wetting and drying cycles showed gradually increasing soil water retention. These data suggest time- dependent formation of soil aggregates will enhance the soil’s ability to retain and treat stormwater as biochar and soil age. In a field experiment, biochar was amended at 4% to a sandy loam in a roadway filter strip along a four -lane divided highway. Over 74 storm events, biochar amendment reduced average stormwater runoff volume and peak flow rate by 84 and 77%, respectively. In comparison, tillage alone of biochar-free soil reduced average stormwater runoff volume and peak flow rate by 54 and 51%, respectively. Thus, biochar amendment increased the ability of the tilled roadway soil to reduce stormwater runoff and peak flow rate by ~ 50% . This effect was 2 to 3 times more significant than what might be expected from lab and pilot -scale experiments in this soil/biochar mixture. The reason for the greater stormwater treatment in the field was soil macropores, which accounted for 84% of the flow under saturated conditions. In summary, biochar amendment to three representative roadway soils indicated that biochar amendment will improve soil hydraulic properties. Experiments demonstrate that these effects enhance stormwater treatment, resulting in reductions in stormwater runoff volume and peak flow rate. A cost analysis indicated that biochar is less expensive than most other best management practices.

  • Record URL:
  • Supplemental Notes:
    • This NCHRP-IDEA investigation was conducted by the University of Delaware.
  • Authors:
    • Imhoff, Paul T
    • Nakhli, Seyyed Ali Akbar
  • Publication Date: 2017-10


  • English

Media Info

  • Media Type: Digital/other
  • Edition: Final Report
  • Features: Appendices; Figures; Photos; References; Tables;
  • Pagination: 51p
  • Serial:
  • Publication flags:

    Open Access (libre)

Subject/Index Terms

Filing Info

  • Accession Number: 01654425
  • Record Type: Publication
  • Report/Paper Numbers: NCHRP IDEA Project 182
  • Files: TRIS, TRB, ATRI
  • Created Date: Dec 14 2017 11:10AM