Predicting impact of biochar on saturated hydraulic conductivity of natural and engineered media

Abstract

Biochar application to agricultural soil may improve soil physical and chemical properties and promote sustainable management of natural resources. Biochar has also been proposed as an amendment to stormwater treatment media, including both bioretention media and roadway soils. In both applications, the effect of biochar on the saturated hydraulic conductivity (K) of the amended media is a critical factor affecting soil or stormwater treatment performance. The objective of this research was to advance models for predicting K of natural soil and bioretention media amended with biochar of different particle size and mass. Experiments were conducted using three natural soils, silt loam, loamy sand, and sandy loam, amended with unsieved biochar (2% and 6% by w/w); a uniform sand amended with six different sizes of biochar (4% and 6% w/w); and a bioretention medium amended with three different sizes of biochar (4% w/w). Experimental measurements of K in these media showed a strong positive correlation between the interporosity of each sediment/biochar medium and K. The classical Kozeny-Carman equation was able to describe the data better than a more recently proposed model for biochar-amended soil, the Lim model, as long as the total porosity in the Kozeny-Carman model is replaced by the interporosity. If the objective is instead to predict the relative change in K with biochar-amendment, the Lim model performed better overall. These results can be used to guide selection of biochar to improve the hydraulic properties of natural soils and bioretention media.