Simulation On Water Quality Improvement Of Guishui River Surface Flow Wetland

Surface flow constructed wetland（SFCW）are widely used in ecological restoration and water quality improvement in rivers and lakes due to its low construction cost and its natural approximating characteristics.There are a lot of experimental studies on the water quality improvement of SFCW.However,it is difficult to restore the large-scale site in the laboratory,the numerical simulation of large-scale SFCW has great advantages.In this study,the water quality simulation software is used to build a water quality module suitable for wetland plants based on the Ecolab module in MIKE.A series of research and analysis are carried out around the key index of wetland water purification.The causes of water pollution of study area and the assessment of comprehensive environmental measures are brought out by the model tool.The main results of this study are summarized as follows:（1）Based on the WQ with nutrients and chlorophyll-a module in MIKE 21,considering the weak mobility of plants,the wetland water quality module is constructed to of reflect the function of plants on water body.The effects of wetland plants on water are mainly reflected in the following four aspects:the physical process of wetland plants hindering the flow of water body;the oxygen secretion of wetland plants into the reaction process of dissolved oxygen;the uptake of nitrogen and phosphorus by microorganisms after planting wetland plants;the adsorption or absorption of nitrogen and phosphorus by plants.The spatial distribution of wetland plants is modified,and the water purification effect under the change of wetland area is successfully simulated.The results of simulation calculation are in good agreement with the measured results.（2）The constructed wetland water quality module is applied to quantify the water quality purification of Guishui River surface flow wetland.The simulation results show that there is a nonlinear relationship between river water purification efficiency and wetland plant coverage.Considering that the river needs flood control and discharge in flood season,the plant coverage rate of Guishui River riparian wetland is between 20%and 30%.Affected by the upstream reservoir and water consumption,the water quantity of the river is small and the water quality is poor in the study area.The downstream water is transferred to the upstream through the circulating pipeline to build a circulating connection system.The results show that the circulating water system can improve the shrinking status of riparian wetland plants affected by water shortage and improve the quality of river outlet water.After water diversion,the effective wetland area of Sanli River and Guishui River increased by 144.44%and 13.16%respectively.Compared with that before water diversion,do content in rubber dam increased by 9.52%,and NH₄⁺-N,TP and TN decreased by 35.71%,50.00%and 46.67%respectively.（3）The water quality responses by pollution sources,self-purification ability,wetland vegetation effects and flow rates change in flood and dry season are the main factors affecting the inflow of GTR.The contribution rate of GSR inflow is the largest,and ammonia nitrogen,total phosphorus and total nitrogen contribution rates from GSR inflow to GTR are 18.32%-45.76%,9.31%-31.17%and 29.34%-67.56%respectively.The reduction rates of ammonia nitrogen,total phosphorus,and total nitrogen are 44.85%-61.29%,51.40%-77.92%,and8.40%-23.06%,respectively in flood season including wetland vegetation action and dry season.Considering the serious water quality problems in flood season,the construction of riparian wetland and the implementation of sewage interception measures are integrated.The simulation results show that dissolved oxygen,ammonia nitrogen,total phosphorus,and total nitrogen of GSR entering GTR are 7.23 mg/L,0.27 mg/L,0.03 mg/L and 0.62 mg/L respectively,reaching the class III water quality standard of surface water environment.