Spiralling curve characterization of nitrate-nitrogen absorption in a channel at a rural-urban ecotone in Northeast China.

To determine the dynamic change characteristics of NO3-N in a rural-urban ecotone channel, five tracer tests were conducted in the Dongda Channel in the suburbs of Changchuan city, Jilin province, China, from October 2016 to April 2017. NaBr was used as conservative tracer and KNO3 served as an added nutritive salt. The kinetic features of NO3-N were simulated via the addition of tracers and by employing the spiralling curve characterization approach and the Michaelis-Menten (M-M) equation. The average absorption length of NO3--N background concentration (Sw-amb) is 199 m, which is much less than the discharge channel length (2.5 km), thereby suggesting that the channel has a strong NO3-N retention potential. Moreover, the M-M equation fits well the kinetic features of NO3-N adsorption. The average maximum absorption rate and subsaturation constant are 631 μg (m2 s)-1 and 1.46 mg L-1, respectively. The correlation analysis reveals that Sw-amb and NO3--N absorption rates (NO3-Namb) are significantly negatively correlated whereas the absorption rates of NO3--N background concentration (Uamb) and NO3-Namb are significantly positively correlated. The other spiralling indices show faint correlations with the background concentration of NO3-N. Meanwhile, the hydrological factors slightly influence NO3-N retention, but the geomorphic features of the channel, including (width residual) Фw and (cross-sectional area residual) ФA, have significant correlations with most spiralling indices, thereby highlighting the relatively important roles of geomorphic features in NO3-N retention.