Wildfire-specific optical surrogates detect pyrogenic organic matter fluxes in freshwater mixing scenarios
Sarah J. Fscher1,2, Tim Fegel3 and Paul J. Wilkerson2, Leah Rivera2, Charles C. Rhoades3 and Fernando L. Rosario-Ortiz1,2
1Department of Civil, Environmental and Architectural Engineering, University of Colorado Boulder, Colorado 80309
2 Environmental Engineering Program, University of Colorado Boulder, Colorado 80309
3 United States Department of Agriculture, USDA
Wildfires significantly impact watersheds by charring and heating soils, depositing ash, and increasing sediments loads to surrounding streams. Colorado wildfires that impact source water pose risks to treatment efficiency and disinfection byproduct precursor (DBP) formation. Due to the impacts of wildfires on soil and water quality, forest and water utility managers require strategies to assess water quality impacts in degraded watersheds.
Benchtop UV-visible absorbance and fluorescence spectroscopy offers insight into dissolved organic matter (DOM) quality via 2D and 3D spectrum. To date, handheld sensors or online probes only assess limited excitation and emission couples. To advance optical spectroscopy for wildfire specific monitoring, we evaluated a suite of optical quantification strategies such as specific UV-254nm (SUVA254), humification index (HIX), fluorescence index (FI), and fluorescence Quantum Yield for DOM leached from wildfire-impacted soils. Soils were collected from two Colorado wildfires: the Ryan Fire and Cameron Peak fire for a range of burn severities and forest types and leached with dilute 0.01 M CaCl2 to generate water soluble organic matter. Results from both fires indicate that optical properties of DOM from impacted soil horizon and forest type could also be characterized by a range of significantly higher optical intensities than unburned control DOM, making optical properties suitable surrogates for pyrogenic carbon contamination. For the Ryan Fire, DOM optical properties were greatest for material leached from o-horizon forest ash, with SUVA254= 3.25 (L/mgCM) and quantum yield at 350 nm=4.1 %. These values were statistically greater than commonly reported values for control soil DOM from this study and unimpacted natural waters, where with typical SUVA254 range = 1–3 L/mgCM and quantum yield at 350 nm = 1-1.7%. Mixing experiments with riverine and wildfire DOM are currently underway to assess the robustness of optical detection at different concentrations of pyrogenic organic matter. This work presents a systematic evaluation of optical properties for pyrogenic DOM to better assess fire severity and water quality impacts.