Characterization and spatial distribution of particulate and soluble carbon and nitrogen from wildfire-impacted sediments
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The heavily forested Cache la Poudre (CLP) watershed in northern Colorado, USA, was impacted by the High Park wildfire in 2012. The wildfire burned land and vegetation immediately adjacent to the CLP River where blackened, ashy sediment samples were collected from five sites upstream of the City of Fort Collins drinking water intake to evaluate the spatial distribution and characteristics of burned sediments, along with quantifying and characterizing soluble compounds following a leaching experiment.
Materials and methods
At each site, samples were collected from three locations: (1) the edge of the bank adjacent to the water edge (downbank), (2) 1 m upslope of location 1 (midbank), and (3) 2 m upslope of location 1 (upperbank). All solid sediment samples were analyzed for elemental composition, and a subset of solid sediment samples were analyzed with 13C solid-state nuclear magnetic resonance spectroscopy. Sediments were mixed with the background CLP River water collected from upstream of the wildfire and allowed to leach for 6 and 24 h to determine the quantity and quality of water-soluble constituents. Filtered samples were analyzed for dissolved organic carbon (DOC), iron, manganese, and inorganic nutrient concentrations, by optical properties, and for disinfection byproduct (DBP) formation.
Results and discussion
Percent carbon and nitrogen content of the solid sediments were good predictors of leachate DOC concentration. The mean fluorescence index was higher for wildfire-impacted sediment leachates (1.50) compared to the background CLP River water (1.37), which may be due to changes in DOM molecular weight and oxidation of organic matter. All sediment leachates showed consistently higher haloacetonitrile and chloropicrin yields (DBP concentration/DOC concentration) compared to background CLP River water, whereas carbonaceous DBPs did not.
The collected sediments showed that burned material accumulated downstream near the river and was composed of inputs from burned soil and biomass along with the mobilization of unburned terrestrial material. The leachates of these sediments have different characteristics compared to the background CLP River water, indicating that DOM leached from sediments following a wildfire may increase aquatic DOC concentrations and N-DBP formation.
KeywordsDBP Disinfection byproduct formation Dissolved organic matter DOM Fluorescence NMR Sediments
The authors thank three anonymous reviewers. The authors acknowledge support from the Water Research Foundation (project no. 4524). We would like to thank Ms. Clare Steninger and Mr. Brent Morgensen for their help with collecting sediment samples; Leigh Gilmore and John Meyer for processing samples (filtering, turbidity, UV, etc); Dorothy Noble for running the DBP analysis; and Jesse Nestler for helping with the SEC analysis. Thanks to Deborah Martin, USGS, Boulder, CO, for helpful comments. Funding for the NMR work was provided by the USDA NIFA AFRI postdoctoral fellowship no. 2012-01330. A portion of the research was performed using EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. We would like to thank Dr. Sarah Burton and Dr. Andrew Lipton for their assistance with NMR setup and acquisition.
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