Atmospheric Contributions of Nitrate to Stormwater Runoff from Two Urban Watersheds

Two independent methods were used to assess if atmospheric deposition was a major source of nitrate nitrogen (NO3-N) to stormwater in two small urban watersheds located in Lincoln, NE. First, event mean concentrations (EMCs) of NO3-N in stormwater during approximately 13 storms were compared with corresponding NO3-N concentration in regional wet deposition. Second, NO3-N contributions to stormwater were estimated using Bayesian inference based on isotopic composition (δ15N and δ18O in NO3-). EMC values indicated that roughly half of the total N in storm runoff was from NO3-N. Average mass flux per hectare between the watersheds was significantly different for organic nitrogen (N) but not for inorganic N (nitrate and ammonia N), suggesting inorganic N originates from similar sources. Both methods showed that atmospheric deposition was a major source of NO3-N in stormwater for smaller storms (<32 mm in this study), when most runoff is likely to be derived from impervious cover. In larger storms, the relative contribution from atmospheric deposition was smaller, with nitrified soil and fertilizer being the largest proportional contributors.