Downpour dynamics: Nitrate export during storm events

Abstract:

Intense precipitation events can alter the amount and source(s) of nitrate (NO₃) exported from watersheds. This topic is poorly understood, however, which impedes projections of NO₃ export dynamics in response to projected increases in storm-event frequency. To assess storm NO₃ export dynamics, high-frequency samples were collected during eight storm events, in addition to monthly baseflow samples, in 2018-2019 from two watersheds, Gwynns Falls (GWN) and Gunpowder Falls (GUN), in Baltimore County, Maryland. Samples were analyzed for concentrations and isotopes (δ¹⁵N and Δ¹⁷O) of NO₃ and oxygen isotopes of water (δ¹⁸O-H₂O). Concentration data indicate dilution of NO₃ during storm events across watersheds. Preliminary results show greater ranges ofδ¹⁵N and Δ¹⁷O in GWN than GUN during storms, which suggests relatively greater variation in NO₃ sources in GWN. Additionally, the proportion of event water (inferred fromδ¹⁸O-H₂O) in total storm runoff was relatively greater in GWN and, across both watersheds, was positively correlated with the proportion of unprocessed atmospheric NO₃ in total NO₃ loads. These initial results suggest that the more development and impervious surface impacts both water and atmospheric NO₃ transport to streams during storm events. The complete dataset will be presented to more fully assess NO₃ source and water flow path dynamics during these events.