Benzene Sulfonic Acids as Unique Aquatic Contaminants from Urban Runoff at the Wildland–Urban Interface (WUI)

Abstract Climate change has driven warmer fall and early winter temperatures in the Mountain States of the United States, creating dry and windy conditions that contributed to the rapid spread of the Marshall Fire, which burned approximately 2500 hectares in Boulder County, Colorado, and destroyed around 1100 homes in under 2 days at the end of 2021. Ash and smoke entered nearby streams, posing potential risks to drinking water quality. Characterization of over 50 unknown compounds, including isomers, revealed a suite of benzene sulfonic acids (BSAs) in stream water and runoff from burned urban areas. These compounds have never been reported in wildfires or fires at the wildland–urban interface (WUI). Thus, BSAs may serve as unique indicators of WUI fire impacts on drinking water sources. A second urban fire, the Nederland Fire (September 2025 in Colorado), replicated the occurrence of these BSAs as tracers of WUI fire impact on drinking water sources. Finally, laboratory experiments indicate that drywall (gypsum board) is one source of BSAs, as confirmed by LC/QTOF-MS analysis.