A Field Study of Biochar Amended Soils: Water Retention and Nutrient Removal from Stormwater Runoff

Practical and economic treatment of stormwater runoff is of vital importance in the cleanup of the Chesapeake Bay and for Delaware municipalities as they try to meet the nutrient loading requirements of the Bay’s TMDL reduction program. Roadway greenways represent a marvelous opportunity for the infiltration and treatment of urban stormwater runoff through the enhancement of existing roadside filter strips and swales without the high costs of purchasing additional highway right-of-way or constructing new stormwater treatment facilities. This research project evaluates the effects of biochar amendment in roadside soils on the soils’ water retention and infiltration characteristics, and ability to remove nitrogen from stormwater runoff. Previous research at the University of Delaware indicates that the addition of as little as 2% biochar, by mass, to the existing soil increases porosity and permeability in the soil which significantly increases water retention and infiltration. Benefits of this include reduced stormwater runoff rates and volumes and increased groundwater recharge. Biochar also promotes microbial activity while increasing the residence time of water in soils, greatly enhancing denitrification and the nitrogen removal capacity of the soils. In this study, a commercial wood biochar, pyrolyzed at 550℃, was amended into existing roadside filter strips and a swale located along a major highway. Roadside filter strips amended with or without biochar were carefully instrumented to measure infiltrating water quality, soil moisture, and surface runoff quantity and quality entering and exiting the filter strips. Results for 20 storm events show that the addition of biochar to the roadside soils decreased the stormwater peak runoff rates by an average of 84% and runoff volumes by an average of 78%. The effect of biochar amendment on reducing stormwater quantity and improving stormwater quality will be presented.