Evaluation of the use of manure-based biochar for trace metal retention in mining-impacted waters

Marginal water sources have become increasingly important in fulfilling human and environmental needs. In situ contaminant removal is frequently used as a low-cost method of improving water quality in areas impacted by industries such as mining, which can cause complex adverse effects in surrounding watersheds. Biochar has been used as a soil amendment and considered as a sorbent for aqueous contaminants at a benchtop scale, but studies examining its use to improve surface waters at the field scale have been limited. This study observed the effects of manure-based biochar on two mining-influenced water bodies. Lead and zinc concentrations in an intermittent stream were minimally affected, and exchangeable ions from the biochar increased electrical conductivity and nutrient concentrations to undesirable levels in water downstream from the installation. Lead and zinc concentrations decreased downstream from biochar installed at a small seep in a mine waste repository, but further analysis of the solids showed that lead and zinc were sorbed to iron (oxyhydr)oxide solids precipitating from this water before encountering the biochar. When compared with optimal controlled conditions observed in benchtop studies, field conditions, such as contact time, did not allow for effective use of biochar and resulted in substantial environmental degradation of downstream waters.