Understanding nitrogen dynamics in urban riparian systems through the use of stable isotopes

Background/Question/Methods

Urban streams export significant amounts of nutrient pollutants, such as nitrate (NO₃^-), and thus can compound downstream eutrophication problems including low dissolved oxygen, loss of species diversity, and increased mortality of aquatic species.    However, concentrations, dynamics and sources of nitrate pollution in urban systems are poorly understood.  Two significant nitrate sources to urban streams are atmospherically deposited nitrogen and human-sourced sewage.   In urban watersheds, interactions between hydrologic flow paths and sewer systems can route nitrate away from zones where nitrate processing may occur and moreover contribute additional nitrate to surface waters.  Cities are often dominated by impervious cover that collect atmospheric deposition and subsequently routes it, through storm sewers, directly to waterways.  The isotopes of nitrogen and oxygen in nitrate (δ¹⁵N and δ¹⁸O) are effective in distinguishing contamination sources to ground and surface waters.  In this study, we sampled Nine Mile Run (NMR), an urban stream draining a 1,600 hectare watershed covered by 38% impervious surface cover.  Our objective was to characterize the influence of these two specific sources of nutrient pollution, atmospheric deposition and sewage, on an urban watershed using these isotopic constraints.  

Results/Conclusions

In this presentation, we examine spatial and temporal patterns in nitrate isotopic composition in Nine Mile Run.  Nitrate export from NMR watershed is consistently high throughout the year (~19 kg NO₃^-ha^-1yr^-1).  Potential nitrate sources in the watershed include combined sewer overflows directly connected to the stream, stream/sewer cross-connections, as well as atmospheric deposition.    Isotopic analysis of base-flow water samples suggests that low-flow nitrate export from NMR is primarily influenced by human waste.   However, storm flow water samples indicate that ~40% of nitrate in the stream is sourced from atmospheric deposition.   Our conclusions suggest that in order to minimize the impact of nutrient loading in complex urban systems, mitigation efforts should consider how nitrate sources vary in conjunction with flow regime.