Biochar as a Soil Amendment: Reduction in Mercury Transport from Hydraulic Mine Debris

Mercury mining and its use in gold mine operations left a legacy of contamination in northern California. Contaminated sediments and water continue to affect local and downstream ecosystems. To assess the efficacy of biochar-amended soils on decreasing Hg transport, biochar was used to amend rock and sediment columns and mesocosms to decrease suspended sediment and associated mercury (Hg) in storm water runoff from Sierra Nevada hydraulic mines. Mercury-contaminated storm water runoff and hydraulic mine debris were collected from two hydraulic mine sites in the Yuba River, California watershed. Mercury concentrations and turbidity were analyzed from storm water samples and hydraulic mine debris in three simulated storm runoff experiments using decomposed granite columns, sediment columns, and sediment mesocosms amended at 0%, 2%, or 5% biochar by weight. Columns containing hydraulic mine debris and mixed with 5% biochar had a significant ( p < 0.05) reduction in filter-passed mercury (FHg) in the outflow as compared to control columns. To simulate saturated hydraulic mine debris runoff, mesocosms were filled with mine sediment and saturated with deionized water to generate runoff. Five percent biochar in mesocosm trays decreased FHg significantly ( p < 0.001), but, because of the angle of the tray, sediment also moved out of the trays. Biochar was effective at reducing FHg from hydraulic mine discharge. Biochar in laboratory columns with decomposed granite or mine sediments was more effective at removing Hg than mesocosms.