Effects of Biochar Derived from Coconut Shell on Soil Hydraulic Properties under Salt Stress in Roadside Bioretention

Performance of roadside bioretention is negatively affected by snow-melting water mixed with deicing salts in winters. This study intends to explore how coconut-shell-biochar changes soil hydraulic properties under drying-wetting cycles (especially using salt solution). Four types of soils were prepared, i.e., bare soil subjected to pure water (BS-W), bare soil subjected to salt solution (BS-S), biochar-amended soil subjected to pure water (AS-W) and biochar-amended soil subjected salt solution (AS-S). During drying-wetting cycles, crack propagation, soil water characteristic curve and unsaturated permeability were measured. After cyclic drying-wetting, crack intensity factors of BS-W, BS-S, AS-W and AS-S are 4.3%, 8.3%, 2.4% and 3%, respectively. Salts decrease water-holding capacity at low suction, but improve water-retention capability at high suction. Biochar improves soil water-holding capacity, especially for salt-treated soils. After cyclic drying-wetting, saturated water contents of BS-W, BS-S, AS-W and AS-S are 0.502, 0.464, 0.505 and 0.491, respectively. Due to presence of desiccation crack, saturated permeability is less than maximal unsaturated permeability. Maximal hydraulic conductivities of BS-W, BS-S, AS-W and AS-S are 1.2 × 10^–3, 2.9 × 10^–3, 1.5 × 10^–3 and 1.9 × 10^–3 cm/s, respectively. Based on diffuse double layer theory, increased salt concentration reduces distance between clay particles, which causes contraction of clay particle aggregate (CPA). Further, individual CPA separates itself from surrounding sand particles and CPAs, therefore micro-cracks form. Biochar absorbs salts and thus mitigates salt-induced harms on soil structure. In order to improve serviceability of bioretention subjected to snow-melting salt stress, contributions of coconut-shell-biochar are highlighted in this study. Graphical Abstract: [Figure not available: see fulltext.]