Enrichment of Disinfection Byproduct Precursors in Fire-Affected Soils from the 2025 Eaton Fire in Los Angeles, California

Wildland–urban interface (WUI) fires combust both natural vegetation and anthropogenic materials, potentially enriching soils with disinfection byproduct (DBP) precursors that may threaten downstream drinking water quality. To improve understanding of this enrichment, we sampled surface soils (0–5 cm) from 29 residential parcels classified by CalFire as destroyed ( n = 11), partially damaged ( n = 5), or undamaged ( n = 13) following the 2025 Eaton Fire in Los Angeles, California. Soils were leached with simulated stormwater and chlorinated under uniform formation conditions to quantify trihalomethane (THM4) and haloacetic acid (HAA5) formation potential. Destroyed-parcel leachates produced significantly higher dissolved organic carbon (DOC)-normalized THM4 and HAA5 yields than partially damaged or undamaged parcels, despite having leachable DOC within similar ranges. Bromide (Br – ) concentrations were elevated in fire-affected leachates and strongly correlated with brominated DBP formation (ρ > 0.75, p < 0.05). Using representative runoff factors and measured DBP precursor loadings, we estimated that a WUI fire may produce ∼3–60× more DBP precursors compared to a wildland fire of the same size. These results provide a field-based characterization of DBP precursor reactivity from WUI fire-impacted soils, demonstrating that anthropogenic combustion residues substantially increase DBP-forming potential and brominated DBP speciation.