The Effect Of Drainage Ditches And Ponds On Water Quality Improvement And The Optimization Approach In Yanyun Irrigation Area Of Yangzhou,China

In the process of agricultural production,the large-scale use of fertilizers and pesticides has made it one of the main reasons why agricultural non-point source pollution has become increasingly prominent.Among the various international treatment measures in the treatment of rural water pollution problems,the application of artificial wetland has become a more respected method in the world.As a natural buffer zone connecting farmland and receiving water bodies,agricultural drainage ditches and ponds system in the river network area of the southern plains of my country have the function of purifying the water quality of wetlands and play an important role in the removal of nitrogen and phosphorus pollutants in agricultural drainage.This article takes two typical drainage ditches and ponds system along the Yangzhou section of the Beijing-Hangzhou Grand Canal.According to the water quality monitoring data of farmland drainage ditches and ponds during the rice planting period from 2018 to 2019,the migration laws of nitrogen and phosphorus pollutants in the drainage ditches and ponds system of farmland were studied;in addition,QUAL2K water quality model was used to simulate the reduction of ammonia nitrogen and total phosphorus in drainage by farmland drainage ditches;and the effect of adjusting hydraulic connection and adopting some key treatment measures on improving the overall water purification effect was analyzed;finally,this paper combines the relevant water quality evaluation methods to carry out the water quality of the large ditches and ponds in the study area in two years Overview.The main research results obtained in the thesis are as follows:(1)The water quality analysis results of farmland drainage ditches and ponds show that during the rice growth period,the average nitrogen and phosphorus concentrations in the drainage ditches are significantly higher than the drainage branch ditches/ponds;the concentration of ammonia nitrogen in the ditches and ponds does not change much.At the end of the period,there is a tendency to increase,but it will quickly drop to a very low concentration value;the change of nitrate nitrogen concentration is highly correlated with the water level of the drainage ditches,and nitrate nitrogen is easy to lose under high water level,but it decreases during the entire monitoring process.Small trend;the concentration of total phosphorus has a tendency to increase in field fertilization but is generally at a low level,with an average value of 0.14 mg/L during the monitoring period;the concentration of total nitrogen during monitoring has a large variation,with an average value of 2.60 mg/L.Statistical analysis of the monitoring data in 2018 found that NO3-N showed moderate variability,TP showed strong variability in drainage agricultural ditches,and showed moderate variability in branch ditches and ponds,while NH3-N showed strong variability in most ditches and ponds.(2)The QUAL2K model was used to analyze the water quality indexes of 6-7 monitoring points in the main section of farmland drainage ditch,the relative error of simulation results was about 20%,and the calculated values of correlation coefficient(R)and Nash Sutcliffe coefficient(NSE)were greater than 0.5 in the periodic rate and validation period,which met the accuracy requirements of the model.The simulation results of using QUAL2K model to optimize the water purification effect of farmland drainage ditch and pond in the study area show that the optimization effect is sewage interception>reducing drainage flow>adjusting of drainage ditches flow net system,and the optimization effect of total phosphorus is higher than that of ammonia nitrogen.(3)According to the classification of three-level functional areas,in order to achieve the goal of class III water quality of large ditches and ponds in the study area,the single factor water quality identification index of TN is inferior to the target water quality by three grades,TP and NO3-N are close to the target water quality,and NH3-N is superior to the target water quality by one grade.Comprehensive water quality identification index and Nemerow pollution index were used to evaluate the spatial variation characteristics of water quality in the study area.The evaluation result is close to one grade of the target water quality and meets the requirements of the target water quality on the whole.The water quality of large-scale ditches and ponds in the study area showed significant seasonal changes.The water quality gradually improved in spring,and the water quality category was below class III,with the increase of summer precipitation,the water quality situation became worse,which was class III or class IV.the water quality category returned to below class III in autumn and winter,which basically met the target water quality requirements.