Biochar as soil amendment in flow-through planters - for increased treatment of zinc roof runoff

Abstract: In times of increased flooding, enhanced by climate change, polluted stormwater poses an increased threat to the environment through contaminated water entering waterways. Bioretention utilizes natural processes in soil and vegetation to treat pollutants and combat this threat. Biochar produced through pyrolysis, has a high cation exchange capacity (CEC) and could therefore increase treatment in bioretention systems. This research applies a literature review, interview, and a model to explore the benefits and disadvantages of biochar in order to specify a soil-mix through an understanding of the production process and preferred application rate. High purification through CEC, increased water holding capacity, and carbon sequestration being the benefits discussed. Biochar application can however, cause clogging due to weathering, which decreases the performance of bioretention systems. A scenario consisting of a zinc roof discharging runoff into a flow-through planter is set in Alnarp, Sweden. The model presents pollution load and treatment capabilities of substrates to then design four soil-mixes to allow maximum hydraulic conductivity, maximized treatment through CEC, stability over time, and enhanced plant habitat. The theoretically optimal soil-mix consists of 50% sand, 30% biochar, 10% loam, and 10% compost, accommodating these factors and providing the best solution for a substrate in a flow-through planter for the removal of zinc pollution from stormwater.