Biochar alleviates the negative impact of compaction on hydraulic conductivity in roadside stormwater control measures

Abstract

Compaction of urban soil where stormwater infrastructures are built reduces infiltration, vegetation growth, and stormwater treatment capacity. Biochar—a carbonaceous porous material produced by pyrolysis of organic waste – can be used as a soil amendment to improve the function of stormwater infrastructure in addition to the proven benefit of increased pollutant removal. However, the benefits depend on the biochar’s properties such as particle size distribution and concentration. Further, because biochar’s particle size distribution is altered by compaction, the hydraulic functions of compacted biochar amended soil is unknown. Herein, we examined the effect of biochar concentrations (0-6% w/w) and particle sizes (unsieved, sieved to < 2mm, and to < 0.5 mm) on water retention and saturated or unsaturated hydraulic conductivity of compacted stormwater media amended with biochar. Our results show the particle size of biochar plays a critical role in whether or not compaction is alleviated: while increasing concentration of unsieved biochar increased hydraulic conductivity up to 3% biochar, increasing concentration of fine biochar (< 2 mm) resulted in consistent decline in hydraulic conductivity under compaction. The results indicate that large biochar particles can effectively dissipate the compaction energy, while the fine biochar under compaction increased clogging by generating more fines that occupy the pores. Water retention improved regardless of the size distribution of added biochar, indicating that addition of biochar would reduce the irrigation requirement to maintain plant health in dry climate or water-stressed conditions. Overall, the results indicate that biochar addition can be effective in mitigating the negative impacts of compaction on stormwater infrastructures, depending on the proportion of coarse biochar.