Retention Characteristics And Coupling Uptake Effect Simulation Of Nitrogen And Phosphorus In A Suburban Drainage Ditch

As part of the river water system,ravine stream ditches both have a drainage function and are important to reduce the nonpoint source nitrogen and phosphorus loading.Thus,it is important for the environment and ecology to play the nitrogen and phosphorus retention function of ravine stream ditches.This paper selects the source stream on the suburb of Hefei as research objective,KBr,and NaCl as conservative tracers and KNO3,NH4Cl and KH2PO4 as added nutrients.From March 2018 to October 2019,a number of field tracer tests were carried out in the field.On this basis,depending on the OTIS model,analyze and evaluate the transient storage characteristics of ravine stream ditches and the retention potential of nitrogen and phosphorus;by TASCC technology method and Michaelis-Menten(M-M)equation,analyze the retention characteristics of soluble inorganic nitrogen(DIN)and soluble reactive phosphorus(SRP)and simulate the DIN and SRP absorption kinetics.Supported by the M-M equation and the 3D dynamic Megee model,quantitatively characterize the coupling absorption effect of DIN and SRP.The main results of this study are as follows:(1)Based on the OTIS model,the mean values of hydrological parameters D,A,As and a are 0.147m2/s,0.077m2,0.031m2 and 2.45 × 10-31/s,respectively;The transient storage indexes As/A,TC,Ts,Rh,Ls and Fmed of the test ditches are 0.41,458s,186s,4.22s/m,26.55m,and 0.38%,respectively,with the similar results of small rivers with similar spatial scales all the world.The results of Tc?Ts indicates that the retentivity of solutes in the ravine stream ditches mainly resources from the mainstream area.The nutrient spiral indicators of Sw-NH4+and Sw-SRP?Sw-NO3-,shows that the test ditch has weaker retention potential for NH4+ and SRP than NO3-.(2)According to the TASCC simulation.the mean values of the DIN and SRP background absorption lengths of the streams are 222 and 162m,which is obviously lower than the total stream length of the test ditch.The above results indicate that stream water body has good retentivity on both of DIN and SRP and stronger retentivity on DIN than SRP.(3)According to the simulation results of the M-M equation,the mean values of the maximum absorption rates Umax-X of the streams DIN and SRP are 1.297 and 0.39 mg/(m2·s),respectively;The mean values of half-saturation constants Km-x are 1.726 and 0.58 mg/L,respectively.Umanx-DIN?Umax-SRP indicates that DIN absorption rate is stronger than SRP.(4)According to the coupling absorption simulation,the maximum absorption rate of DIN and SRP Umax-X increases range from 7%to 31%(average 14%),and 5%?37%(average 16%),compared with calculation value with less consideration of coupling effect.The results indicate that in the ravine stream,the increase of DIN(or SRP)enhances the absorption effect of SRP(or DIN),this is,the concentration increase of a type of nutrients is helpful for the absorption of another type of nutrients.(5)The 3D coupling response curved surface model is able to intuitively and quantitatively simulate the coupling absorption kinetic process of DIN and SRP,and the DIN maximum absorption rate Umax-X mean value is 1.62mg/(m2·s),with 0.53 mg/(m2·s)for SPR.Comparing the calculation result with the M-M equation fitting result,he absorption rate of the coupling condition is relatively higher.