The Riverine Export of Particulate and Dissolved Black Carbon Following a Colorado Wildfire

Abstract:

Wildfires are one of the primary sources of combustion-derived, polycondensed aromatic molecules known as black carbon (BC) in the environment. BC can be mobilized from soils and char in fire-affected aquatic systems, potentially impacting downstream water quality. The High Park Fire burned over 87,000 acres of the Cache La Poudre River watershed in Larimer County, Colorado during June of 2012. Since the Cache La Poudre River serves as one of the main sources of drinking water for the city of Fort Collins, the implications of pyrogenic organic matter inputs to such waterways must be assessed. The export of BC in both the dissolved (DBC) and particulate (PBC) phases from the Cache La Poudre River was measured biweekly at two downstream fire-affected sites and one upstream unburned reference site during the year following the High Park Fire. Both DBC and DOC concentrations were low during base flow and increased with spring melt and increasing water discharge. Seasonal changes in DBC composition indicated a shift in hydrology and associated DOM source between base and peak flow conditions. However, there was little difference observed in overall DBC or DOC concentration between the burned and reference sites. This suggested that most dissolved organic matter within the river is sourced upstream. Although the mobilization of DBC did not notably increase with recent fire activity, PBC export was substantially greater at fire affected sites when compared with the unburned site. This indicates that BC export in riverine ecosystems occurs mainly in the particulate phase during the years immediately following a significant wildfire event, while the mobilization of the DBC may be uncoupled from the PBC due to mobilization processes acting on different time scales.