CHARACTERIZATION OF REACTIVE NITROGEN TRANSPORT IN PITTSBURGH’S RIVERS

Human activity has greatly altered the global nitrogen cycle where excess nitrogen is primarily in the form of nitrate (NO₃^-) in aquatic systems. This study uses nitrate isotopes (δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, Δ¹⁷O-NO₃^-) to examine the sources and sinks of nitrate in Pittsburgh’s Rivers. Dual nitrate stable isotopes (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- ) can trace nitrate from wastewater, fertilizer, and atmospheric deposition to aquatic environments, while Δ¹⁷O-NO₃^- precisely quantifies atmospheric nitrate contributions and indicates biological processing. Two sampling campaigns were conducted in August and October 2017 in the Pittsburgh pools of the Allegheny, Monongahela, and Ohio Rivers. Grab samples were analyzed for nitrate isotopes at the University of Pittsburgh’s Regional Stable Isotope Laboratory for Earth and Environmental Science Research. During the latter sampling campaign, a Submersible Ultraviolet Nitrate Analyzer (SUNA) was coupled with a flow cell for continuous measurements of nitrate concentrations at highly resolved five minute intervals. Nitrate concentrations and corresponding GPS locations are used to create high resolution maps of nitrate variability in the Pittsburgh pool of the three rivers.

The study area is a unique, urban region that lies at the confluence of the Allegheny and the Monongahela Rivers to form the Ohio River, a major tributary to the Mississippi River. Pittsburgh also has a high density of combined sewer overflows (CSOs) that directly deliver raw sewage to surface water during wet weather. When precipitation exceeds the collection system’s capacity, CSOs are designed to overflow and discharge directly to surface waters. Quantifying nitrate contributions from CSOs will aid in future remediation efforts. Preliminary results indicate that sewage is the largest contributor of nitrate in the study region. Nitrate concentrations are highest in teh Monongahela River and exceed those in the Allegheny and Ohio Rivers.