Combustion Temperature and Molecular Weight Fractionation Provide Insights into Soil-Derived Wildfire-impacted Disinfection Byproduct Precursors

Limited information exists on the molecular characteristics of disinfection byproduct (DBP) precursors derived from pyrogenic dissolved organic matter. Forest soils from an unburned site within a watershed impacted by the 2021 Caldor Fire were heated in the laboratory to 250 °C, 350 °C, and 450 °C to simulate increasing fire intensity. Leachates from these soils were compared to those from high-burn-severity soils collected within the same watershed, fractionated into hydrophilic (HPI) and hydrophobic (HPO) fractions, and analyzed by size exclusion chromatography (SEC) with absorbance, fluorescence, and dissolved organic carbon (DOC) detection. In laboratory-heated soils, leached carbon increased at 350 °C relative to unheated soils but declined at 450 °C, while apparent molecular weight shifted toward smaller compounds. DOC-normalized yields of carbonaceous DBPs decreased with increasing temperature, whereas nitrogenous DBPs (HAN6, HAM4) increased up to 3-fold relative to unheated soils. Similar trends were observed for Caldor Fire soils. Differences between HPI and HPO fractions were generally insignificant for THM4, HAA9, HAN6, and HAM4 yields and for most optical indices. In contrast, heating temperature significantly affected DBP yields (p < 0.001). Wildfire transformation of soil organic matter enhances nitrogenous DBP precursor reactivity, and should be considered in postfire water quality prediction and treatment.