Pore-scale simulations of solute transport in urban soil substrates hosting roots

The transport of nutrients in urban soil substrates, such as green roofs, is largely dictated by the need of the plants themselves, via adsorption by their root networks. This process pulls nutrients such as fertilizer from the soil substrate where biochar can act as a reservoir, holding the fertilizer for longer periods. Biochar is sometimes added to soils to store excess fertilizer resulting in reduced loss of fertilizer and leaching of contaminants into the groundwater. The biochar then desorbs the fertilizer (solute) into the surrounding soil. Due to limited experimental insights on the adsorption-desorption system of biochar and plants roots; we outline a complete methodology by which root structures are generated and incorporated into a fully 3D geometry. The lattice Boltzmann method is used for pore-scale simulations to compare the tap and fibrous (grass) roots’ behavior. The results show a difference in the mean contaminant breakthrough as well as in homogeneity, with the tap root having a lower mean but higher variation of concentration at the outlet. This is due to its lower surface area, which is directly linked to its adsorption capacity and the consequent quantity of biochar needed in the substrate. The lower biochar presence is quantified by a large ℓ ξ , the average distance between desorbing particles. This distance dictates the mixing strength within the system. At higher values, we see a lower mean contaminant breakthrough and a higher statistical variation within the breakthrough, engendering higher contaminant peak values during rainfall events. The opposite is true for lower values of ℓ ξ , represented by the fibrous case, which has a higher mean breakthrough but less variation. This methodology allows for testing varying plant and fertilizer deployment methods in order to create some best practices for minimizing contaminant breakthrough without the need for in situ tuning.