THE ROLE OF NATURAL CHARCOAL IN METAL CYCLING IN MINING-IMPACTED WETLAND SEDIMENTS

Charcoal is known for its ability to adsorb contaminants including metals. Natural charcoal from wildfires is often a component of sediments. As such, sedimentary charcoal may act as an important repository for metals and other contaminants in sediments affected by mining or industrial processes. However, to our knowledge no systematic study has ever examined the role of natural charcoal in metal storage, deposition and cycling at any contaminated site. Here we examine the role of wildfire charcoal in a mining-impacted wetland in the Coeur d’Alene River valley of north Idaho. The wetland sediments are heavily contaminated with metals including As, Cd, Pb and Zn. Analyses of charcoal in these sediments indicate that it is enriched in these metals with respect to the host soil. Redox cycling at the field site has transported metals upwards to the near-surface and co-precipitated them in Fe and Mn oxyhydroxide aggregates. Charcoal acts both as a deposition site for these precipitates, and as a source of abundant sorption sites for dissolved metals. We are examining metal concentrations in soil and charcoal as a function of depth, to assess the role played by charcoal in this cycling, and to determine the proportions of adsorbed and precipitated metals.