Distribution and Transport of Pyrogenic Black Carbon in Soils Affected by Wildfires, Valles Caldera, New Mexico, with Implications for Contaminant Transport.

摘要

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 study, 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.