Transport And Retention Of Nitrogen And Phosphate In A Suburban Drainage Ditch

As an important part of a river watershed, drainage ditch have an impact on retention and regulation of nutrient flux to downstream waters. Simultaneous slug additions of a conservative tracer (NaCl) and nutrients (NH4Cl and KH2PO4) were conducted in Guanzhenhe Branch, a drainage ditch in suburban Hefei. To analyze the effect of transient storage on solute transport and the parameter sensitivity, the OTIS model were used to characterize the transport and fate of the water-borne solutes in the study reach. Based on the nutrient spiraling theory, nitrogen and phosphate retention in Guanzhenhe Branch were characterized and the retention capacity were assessed. The main results were summed up as follows:(1) For the hydrological parameters, the mean values of D, A, As, a in Guanzhenhe Branch were 0.145 m2·s-1,0.417 m2,0.125 m2 and 2.30×10-3 s-1, respectively, which were similar to the findings of the small streams in different regions. Analogously, for the transient storage metrics, the values of As/A, Tc, Ls, Rh and Fmed200 in Guanzhenhe Branch were comparable to that of the other streams except Ts was smaller. Regression analysis showed that discharge (Q) was only related to A in the hydrological parameters, Q and velocity (v) showed correlations with most of transient storage metrics.(2) Transient storage exerted a significant influence on the peak values of simulated Cl- breakthrough curves (BTCs), and the relative errors ranged from 24.32% to 117.26%, which were much higher than those of the peak times. The effect of solute transport by transient storage remarkably exceeded that of lateral inflow. Correlation analysis presented that the relative errors of the peak values and peak times had significant correlations with As/A, respectively, but did not related to Q.(3) Based on the parameter sensitivity analysis, the ranking of parameter sensitivity in the OTIS model was A>As>?>D.(4) The analysis of nutrient retention in Guanzhenhe Branch showed that the values of Sw-NH4+ and Sw-SRP reached 934020 and 47518 m, respectively, indicating that Guanzhenhe Branch was incapable of efficiently removing and detaining nutrients. Meanwhile, the negative values of Sw-NH4+ and Sw-SRP presented that the drainage ditch even acted as nutrient sources. On the basis of correlation analysis, Vf-NH4+ were significant related to h, Q and ?(NH4), respectively; VrSRP and 17-SRP had positive correlations with w; most of spiraling metrics were not correlated to ambient nutrient concentrations.(5) The phosphate amount of sediment adsorption increased with the time. The value of EPCo ranged from 0.073 to 0.150 mg·L-1, indicated high risk of phosphate release. The values of sorption parameters (?,?s) in OTIS model were 9.84×10-5±3.95×10-4 and -1.95×10-4±5.13×10-4 s-1, respectively, similar to the findings reported in the published literatures. The fitting degree of the BTCs suggested that sorption was the main mechanism in SRP retention.