CONTRIBUTIONS OF NUTRIENT POLLUTION FROM SEWAGE AND ATMOSPHERIC DEPOSITION IN URBAN WATERSHEDS DETERMINED THROUGH THE USE OF STABLE ISOTOPES

Streams draining urban watersheds export significant amounts of human-sourced nutrient pollutants such as nitrate (NO₃^-). However, the sources and dynamics of nutrients in these systems are poorly understood. Two significant nitrate sources to urban streams are atmospherically deposited nitrogen and human-sourced sewage. Cities are often dominated by impervious cover that collect atmospheric deposition and subsequently routes it, through storm sewers, directly to waterways. 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. The isotopes of nitrogen and oxygen in nitrate (δ¹⁵N, δ¹⁸O, Δ¹⁷O) 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. Isotopic analysis of base-flow water samples suggests that low-flow nitrate export from NMR is dominated by human waste, however nitrate sourced from atmospheric deposition is also present during baseflow. During storm events, isotopic analysis of samples indicates that ~40% of nitrate in the stream is sourced from atmospheric deposition washed in from the land surface. Further, atmospherically deposited nitrate is present in streamwater before the storm begins. The results from isotopic analysis of nitrate provide insight into how sources vary with flow regime, therefore contributing significantly to our understanding of nitrate sources in urban systems.