δ15N and δ18O Reveal the Sources of Nitrate-Nitrogen in Urban Residential Stormwater Runoff

Nitrogen (N) sources are widely distributed in the complex urban environment. High-resolution data elucidating N sources in the residential catchments are not available. We used stable isotopes of N and oxygen (0) of nitrate (delta O-18-NO3- and delta N-15-NO3-) along with delta O-18 and hydrogen (delta D) of water (H2O) to understand the sources and transformations of N in residential stormwater runoff. Stormwater runoff samples were collected over 25 stormwater events at 5 min intervals using an autosampler installed at the residential catchment outlet pipe that drained 31 low-density homes with, a total drainage area of 0.11 km(2). Bayesian mixing model results indicated that atmospheric deposition (range 43-71%) and chemical N fertilizers (range <1-49%) were the dominant NO3-N sources in the stormwater runoff and that there was a continuum of source changes during the stormwater events. Further, the NO3-N transport in the stormwater runoff from the residential catchment was driven by mixing of multiple sources and biotic (i.e., nitrification) processes. This work suggests that a better understanding of N transport and sources is needed to reduce N export and improve Water quality in urban water systems.