THE TRANSPORT AND ACCUMULATION OF PYROGENIC BLACK CARBON IN RECENTLY BURNED WATERSHEDS

Post-wildfire debris flows and flooding deliver high loads of sediment and black carbon (BC) to downstream waterways. Generated by incomplete combustion of organic matter, BC in the form of soot and char is transported and re-deposited throughout the watershed. Due to the effects of BC on water quality, its potential to sequester contaminants and its role in storing carbon over short and long timescales, the accumulation of BC is a complex issue for ecosystems and human populations. The impacts of two recent wildfires in Northern New Mexico are being studied with the goal of understanding the accumulation, fluxes, and residence times of BC in post-wildfire, semi-arid mountainous watersheds.

Utilizing burn severity maps and geospatial data, three sites were selected to collect soil and water samples to characterize BC: a control, an area impacted by the 2011 burn, and an area impacted by the 2013 burn. In order to gather representative data, sampling locations were stratified by burn severity, aspect, and slope, and then randomly selected via GIS. Semi-monthly water samples were collected during June and July and analyzed for total dissolved organic carbon (DOC) and total dissolved inorganic carbon (DIC). Over 200 soil samples were collected before the monsoon season.

Two black carbon quantification processes are being performed: the Benzene Polycarboxylic Acids (BPCA) method for water samples and the chemo-thermal oxidation (CTO-375) method for soil samples. Precipitation data is being correlated with soil and water data in order to characterize the timing of flooding events and the relationship to BC fluxes in the soil and water. Using the results of the BC analyses, a conceptual model of flow and transport of pyrogenic carbon through a fire-impacted watershed is being created. The model assumes BC to be a particulate that is transported and deposited, as well as a solute that is dissolved, degraded, and transported. Preliminary DOC water data shows that streams in the area impacted by the 2013 burn contain 1.8 times the concentration of DOC as streams impacted by the 2011 burn and 1.3 times the concentration of DOC as the control area. This data suggests short residence times for elevated levels of DOC after wildfire.