Distribution and Transport of Pyrogenic Black Carbon in Soils Affected
by Wildfires, Valles Caldera, New Mexico, with Implications for
Daniel Cadol, Amy Galanter, Phoebe Nicholls, Department of Earth and Environmental Science, New Mexico Institute of Mining
Wildland fires produce pyrogenic black carbon (PyC) on a continuum from soot to char. Post-fire runoff floods and debris flows subsequently transport and redeposit this material throughout the watershed. Our aim was to identify geomorphic settings where PyC accumulates and to study its impact on contaminant transport. Based on the capabilities of our PyC quantification method, this report focuses on the distribution of black carbon that combusts between 340-950°C, which includes most soot and some char. Sorption analyses utilized field soil samples in which the PyC was quantified in this combustion range, as well as soils that were augmented with char collected from the field site. This report presents the results of three related analyses: 1) a post-fire soils sampling effort, 2) a pre- and post-fire soil PyC content comparison, and 3) an analysis of the ability of soils with varying PyC contents to sorb arsenic from solution.
In the first study, three sites were sampled near Valles Caldera, NM: Thompson Ridge fire (TR; burned in 2013), Las Conchas fire (burned in 2011), and Cebolla Creek (control, unburned for >70 years). Sampling was stratified by geomorphic feature: floodplain, alluvial fan, and north- and south-facing hillslopes. Sites were sampled before and after the 2013 monsoon. Soot content as a fraction of total organic carbon decreased with time after the burn event, both for whole watersheds and for specific geomorphic settings. Floodplain soot content downstream of TR was higher than the other sites where recent flood evidence was observed, but lower beyond the limit of flooding. Soot content on floodplains downstream of both fires increased during the monsoon where flooding occurred. Hillslope soot redistribution on the north-facing TR hillslope during monsoon season showed decreases on the upper slope and increases near the toe of the slope, consistent with downslope transport by runoff. Other hillslopes analyzed did not show this movement. From this analysis, we conclude that soot is relatively mobile, and is predominantly transported by runoff and floods over short time scales following fire.
In the second study, soil samples that were collected at various locations on Redondo Peak in 2009 were obtained from a collaborator. We resampled the soil within 10 m of these locations in 2014. All sampling sites were within the burn perimeter of the Thompson Ridge fire, predominantly in moderate- to low-severity burn areas. The upper 5 cm of soil showed greater PyC content change between the two sampling times, while the soil between 5-15 cm depth was relatively stable in PyC content. Upslope locations tended to have reduced PyC concentrations in the post-fire sampling, while downslope locations tented to have increased PyC concentrations. We again attribute this to erosion and deposition of material by post-fire monsoon events that initiated overland flow.
In the final study presented here, field soils were mixed with sodium arsenate solutions varying from 0.001 to 0.1 mg/L. The low concentration solutions increased in As content over time, presumably as As was leached from the soil. The higher concentration solutions lost As through time, presumably via sorption onto the solids. Soils with added PyC content sorbed As more rapidly than the unchanged field soil samples, but all soils reached roughly the same equilibrium concentration. This suggests that PyC may be important in sorbing out heavy metals from mobile plumes, but is unlikely to change the long-term capacity of these soils to sequester heavy metals. Ongoing work with polycyclic aromatic hydrocarbons will evaluate whether this conclusion also holds for organic contaminants.
Keywords: pyrogenic black carbon, soil carbon, carbon cycle, erosion, sedimentation, soot