Experiment And Simulation Study Of Multistage-Ecological Pond On Nitrogen And Phosphorus Removal In The Rainfall Runoff Of A Paddy Rice Field

China has applied the most chemical fertilizer to the farmland.A large amount of nutrients containing nitrogen and phosphorus run into the ditches and lakes directly when rainfall or drainage occurs,and it will bring a great risk to the environment.The natural pond,which is adjacent to the paddy field and the river,because of its perennial bare soil,has a limited interception ability on nitrogen and phosphorus in the rainfall runoff.Therefore,it is significant to study the interception effects of multi-ecological pond on nitrogen and phosphorus in the rainfall runoff of rice field and the removal effect on nitrogen and phosphorus in water by experiment and simulation.A typical area of farmland in the upstream of Hejia Creek basin in Yixing city was chosen.A natural pond was transformed into a multi-ecological pond in this region.The interception effects of multi-ecological pond on nitrogen and phosphorus in rainfall runoff events happened in rice growth periods was studied,as well as the distribution of nitrogen and phosphorus in different layers of the multi-ecological pond.What's more,the removal effects of multi-ecological pond on nitrogen and phosphorus under stationary state and the effect of the releases of nitrogen and phosphorus by the sediment of the ecological pond was studied.At last,the transport and diffusion of N and P under dynamic condition in the multi-ecological pond was simulated by EFDC,a three dimensional water quality model,and the simulation results and the experimental data were compared.The results suggested that:?1?In the process of three different rainfall events,the average removal rate of total nitrogen?TN?and total phosphorus?TP?by ecological ditch were 34.7%and 34.8%respectively,the TN concentration in inflow water had little effect on removal rate of TN,while the higher TP concentration in inflow wate,the higher removal rate of TP in the multi-ecological pond;?2?The sort order of nitrogen and phosphorus removal rate by ecological ditch was ammonia nitrogen?NH₄⁺-N?>grain nitrogen?PN?>nitrate nitrogen?NO₃^--N?and particles phosphorus?PP?>dissolved phosphorus?DP?.Nitrogen and phosphorus were mainly in granular form at the initial stage of fertilization and runoff,so they were easier to be intercepted.And the concentration of nitrogen and phosphorus in different water body vertical stratifications varied during rainfall runoff,but the concentration of nitrogen and phosphorus in the underlying water body was mostly greater than that in the surface water body;?3?After the rainfall runoff,the removal rate of TN and TP in ecological pond were more than 40%;?4?The concentration of nitrogen and phosphorus in the released water was higher than that in the corresponding overlying water.The concentrations of TN in the released water and the overlying water were both in the range of Grade?of Earth's surface water?TN?1.5mg/L?,and the TP concentrations were both in the range of Grade?of Earth's surface water?TP?0.1mg/L?.The plant played a certain role in inhibiting the release of TN and TP from sediment;?5?The simulation results of EFDC model well fitted with the test results,and average relative errors of EFDC model simulated results were all less than 20%.