2006 BES Annual Meeting Presentation and Poster Abstracts
Sources and transformations of nitrate in streams draining an urban landscape of the mid-Atlantic U.S.
Co-Authors: Sujay S. Kaushal (UMCES), Peter M. Groffman (IES), Lawrence E. Band (UNC), Emily M. Elliott (USGS), and Carol Kendall (USGS)
Abstract: Nitrate is an abundant pollutant in many surface waters, but little is typically known regarding its sources and transformations in urban watersheds. We examined the dual isotopic composition of nitrate (d 15N and d 18O) in streams within the Baltimore metropolitan area in order to trace sources and cycling of nitrate. Surface water was sampled bi-weekly from eight streams draining forest, suburban, and urban watersheds and 1 storm sewer system over a six-month period. We observed a steady decline in concentrations of nitrate from headwaters to downstream stream reaches, but differing patterns in the isotopic composition of nitrate across streams and land use. Overall, d 15N of nitrate in suburban and urban streams was typically elevated up to 8 –15 parts per mil suggesting a dominant sewage source whereas baseline d 15N of nitrate in a forest reference stream was much lower at < 2 parts per mil. Only streams with low levels of channel incision draining agricultural and low-residential watersheds showed consistent increases in both d 15N and d 18O of nitrate indicating the importance of denitrification. A simple two end-member mixing model suggested that up to 75% of the nitrate in suburban and urban streams was derived from leaky sewer systems. Episodic peaks in d 18O of nitrate also suggested the importance of atmospheric sources of nitrate during rain events in urban watersheds with high levels of impervious surface coverage. Although the capacity for streams to remove nitrogen may be high, leaky sewer systems and atmospheric deposition may be major sources of nitrate in streams draining older metropolitan areas along the Chesapeake Bay.