Wildfire Temperature and Oxygen Availability Alter Disinfection Byproduct Precursors in Laboratory and Weathered Forest Soils

Wildfires alter forested watershed soils and can increase disinfection byproduct (DBP) precursor loading in drinking water sources. Varying wildfire conditions produce differences in thermal severity and oxygen availability, shifting organic matter transformation between combustion and pyrolysis (oxygen-limited). However, the influence of these regimes on DBP precursor formation is not well understood. To investigate, we thermally altered soils at 250 °C, 350 °C, and 500 °C under oxygen concentrations of 0%, 10%, and 20% and evaluated dissolved organic matter properties and DBP formation. Oxygen concentration altered fluorescence-based optical properties but did not significantly affect dissolved organic carbon-normalized DBP yields; temperature was the dominant factor for DBP precursor reactivity. Trihalomethane (THM) yields decreased 17× at 500 °C relative to unaltered soils. Haloacetic acid (HAA) and haloacetonitrile (HAN) yields decreased at 350 °C, but HAN yields increased at 500 °C. Bromine incorporation into THMs and HAAs increased with temperature, and nitrogenous DBP yields were elevated at 500 °C, indicating a shift toward more toxicologically relevant DBP speciation. Field-weathered soils from a wildfire site sampled six years postfire had DBP yield trends that were consistent with laboratory results, suggesting that temperature-driven organic matter alterations can persist and continue to affect downstream drinking water systems for years.