Variation in the instream dissolved inorganic nitrogen response between and within rainstorm events in an urban watershed

Abstract

Urban streams play a significant role in the transport of dissolved inorganic nitrogen (DIN) from uplands to sensitive coastal receiving waters. In this study, we investigate the timing of DIN export through monitoring conducted during several storm events of different magnitude and with different antecedent conditions in an urban catchment. Our monitoring was conducted at a first-order stream site in a highly urban catchment in the northeastern United States during four rainstorms. Nitrate made up the dominant portion of the DIN load during all four events and the response of nitrate and ammonia were very different. Discharge, rather than concentration, was the most important factor in determining nitrate export with hot moments of nitrate export coinciding with peaks in flow. However, the highest nitrogen concentrations were observed after the streamflow peak during all of the events. During extreme rainstorms, this delayed response may constitute an important secondary hot-moment of nitrate export. These results may be significant for the development of nitrogen management plans for urban watersheds, especially since many water quality improvement BMPs (best management practices) are being designed to treat the first-flush of stormwater and would miss much of the DIN load.

Keywords:

• Nitrate export
• dissolved inorganic nitrogen (DIN)
• urban watershed
• first-flush
• nutrients

Acknowledgments

Funding was provided by the National Science Foundation, Grant #BES-0607036. This work was also supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2005-214-D00310).

Notes

^a Sampling interrupted 10/12/05 17:20–22:50.

^b Sampling interrupted 10/26/05 18:00–23:00.

^a Discharge (m³) is normalized by catchment area and expressed as a depth for convenience.

^b Event mean concentration.