REDISTRIBUTION OF PYROGENIC DEBRIS FOLLOWING POST-FIRE MONSOONS: IMPLICATIONS FOR CARBON BUDGETS AND CONTAMINANT SEQUESTRATION

Generation of pyrogenic black carbon, in the form of char, soot, and ash, is a ubiquitous result of wildland fires. When these black carbon byproducts are incorporated into soils some fraction may be retained for centuries due to the relative chemical stability of the carbon structure. Recent research has elucidated the residence time in stable soils, proposing the presence of both labile and recalcitrant pools. For the recalcitrant pool, however, physical erosion and fluvial transport may significantly change the average residence time of the initial black carbon deposit. In this project, we sampled black carbon in a range of geomorphic settings immediately before and after a post-fire monsoon season, as well as leveraging soil samples that were fortuitously collected prior to the occurrence of a moderate severity fire. A general down-slope redistribution of black carbon was common, but not universally observed. Sample heterogeneity likely contributed to this result. Modified residence time estimates were generated for the hillslope and floodplain locations, taking this transport into account. Additionally, batch sorption experiments were conducted in the lab to observe the impact of increased black carbon content on arsenic dynamics in the study site soils. At low arsenic concentrations, total sequestration was only minimally increased by black carbon, but the rate at which sequestration occurred over short (hourly) time scales was significantly increased. Based on the soil black carbon concentration data and the sorption results, we estimate the impact of a moderate intensity wildfire on the transport of chronic (e.g., natural load or slow release from tailings) and acute (e.g., mine waste spill) arsenic releases through hillslope and floodplain soils.