Investigating the Sources and Transport of Benzotriazole UV Stabilizers during Rainfall and Snowmelt across an Urbanization Gradient.

Benzotriazole UV stabilizers (BT-UVs) have attracted increasing attention due to their bioaccumulative nature and ubiquitous presence in surface waters. We apply high-frequency sampling in paired watersheds to describe, for the first time, the behavior of BT-UVs in stream channels during snowmelt and rainfall. Relative to a largely agricultural watershed, concentrations of BT-UVs in an urban watershed were 4-90 times greater during rainfall and 3-21 times greater during snowmelt. During rainfall, a decrease in BT-UV concentrations on particles with increasing suspended sediments and streamflow occurred at all urban sites due to input of relatively clean sediments, while both decreases and increases were observed at rural sites. Where increases occurred in the rural watershed, road sediments were consistently suggested as the source. Contrasts between the urban and rural sites were also observed during snowmelt. While BT-UV concentrations on particles peaked with peak suspended sediment levels at urban stream sites, the opposite was true at rural stream sites. This appeared to be driven partially by different snowpack melt rates in the two watersheds, with earlier melt and presumably higher streamflow facilitating suspension or erosion of more contaminated sediment in the urban stream. In general, it appears that relatively high, consistent emissions in the form of informal (plastic) debris disposal by consumers or industrial releases have likely led to more homogeneous BT-UV profiles and temporal behavior in the urban watershed. In the rural watershed, low emissions instead entail that emissions variability is more likely to translate to variability in chemical profiles and temporal behavior.