Effect of Rainfall–runoff Process on Sources and Transformation of Nitrate at the Urban Catchment Scale

Nonpoint source nitrogen pollution in urbanized regions poses a substantial challenge for comprehensive management of lake watersheds. Considerable uncertainty remains in identifying nitrate sources and elucidating nitrate transformation in urban regions coupled with lack of high-resolution sampling campaigns. In this study, high-frequency sampling was conducted in river and different typical functional areas and impervious underlying surfaces during a rainfall event, followed by nitrogen and oxygen isotopes and hydrochemical characteristics. Nitrate and major ions in urban river were attributable to similar sources linked to anthropogenic factors. The delta N-15-NO3- and delta O-18-NO3- values in the river ranged from 1.94 parts per thousand to 8.89 parts per thousand and from -1.16 parts per thousand to 4.22 parts per thousand, respectively. Nitrate originated from the mixing process dominated by nitrification before and during the rainfall event, whereas it was derived mainly from the nitrification process in the river after the rainfall event. The Bayesian model results showed that rainfall runoff in the residential area contributed most to the nitrate in the river (65.4% +/- 20.2%). Additionally, atmospheric deposition was the major source of NO3- in pavement (34.05% +/- 2.3%) and roof (41.2% +/- 4.24%) runoff. Nitrogen input from dry deposition associated with heavy automobile traffic in city should receive greater attention.