Changes in Molecular Chemistry of Stream Organic Matter and Low Molecular Weight Disinfection Byproduct Precursors Following Initial Post-Fire Rainstorms

Forested ecosystems are vulnerable to wildfires which have short- and long-term effects on downstream water quality, including changes to dissolved organic carbon (DOC) transport and disinfection byproduct (DBP) formation downstream, which can be exacerbated by high-intensity rainstorms. To evaluate short-term changes to streamwater composition and characteristics, we sampled three streams feeding into the Cache la Poudre River within the perimeter of the 2020 Cameron Peak Fire before, during, and after the first postfire summer storm season. Streams were distinguished by the proportion of their upstream watershed that burned at moderate to high severity (<25%, 25–50%, or >50%). DOC concentration and turbidity demonstrated storm-driven responses that were greatest in the 25–50% burn area stream. Nitrogen-containing species were enriched in streams during the storm and poststorm periods, depending on burn area. Chlorination experiments measured low molecular weight (C1 – C2) DBPs, where streams from subcatchments with more extensive burns were greater in DOC and DBPs formed across all events. However, DOC released during the storms formed less DBPs on a DOC-normalized basis in all streams. These results demonstrate that DBP formation following wildfire disturbance is primarily driven by storm-mediated increases in DOC loading rather than enrichment in highly reactive pyrogenic material.