Surprisingly, heavy rain actually cleans nitrogen pollution out of city streams instead of washing road gunk into them.
March 25, 2026
Original Paper
Recreational Forest Preserve Drives Nitrogen Dynamics in Heterogeneous Urban Watershed—Stable Isotope Study
SSRN · 6462623
The Takeaway
The common policy assumption is that rain washes nitrogen from city roads and roofs into local waterways. However, this study found that pollution levels are actually highest during dry periods and that the nitrogen originates from natural soil processes rather than urban pavement.
From the abstract
Understanding spatiotemporal influences of in-stream nutrient sources is imperative for managing contaminants in urban streams, yet seasonal controls on nitrogen transport and sources across landscape types in the Southeastern U.S. are not well understood. This study assesses seasonal and geospatial variation in NO3- sources and concentrations across a heterogenous urban watershed spanning a forest-to-urban land use gradient. Water quality and stable isotope compositions of nitrogen and oxygen i