Study On The Change Of Nitrogen And Phosphorus Pollutants In Drainage Ditches And Ponds Of Paddy Field In Yanyun Irrigation Area

With the continuous development of agricultural production activities,the problem of agricultural non-point source pollution has become increasingly prominent.Using constructed wetlands to reduce agricultural non-point source pollution is internationally recognized.The shortage of land resources in southern China limits the application of constructed wetlands.However,the widely distributed drainage ditches and ponds have similar functions as wetlands,which can reduce agricultural non-point source pollution.This paper aims at the drainage ditches and ponds which are distributed in the southern plain with dense river network.The migration and transformation of nitrogen and phosphorus in drainage ditches and ponds during rice cropping period were studied by field investigation,fixed-point monitoring and experimental analysis.At the same time,QUAL2K water quality model was used to test its applicability in this study area,and simulate the effects of different water temperature and meteorological factors on pollution load in drainage ditches and ponds.The main results are as follows:(1)The average concentration of ammonia nitrogen in farm ditches ranged from 0.30 mg/L to 0.40 mg/L,the average concentration of ammonia nitrogen in branch ditches and ponds ranged from 0.20 mg/L to 0.30 mg/L,while the lower level branch ditches ranged from 0.10 mg/L to 0.20 mg/L.The average concentration of ammonia nitrogen in the farm ditches is generally larger than that in the branch ditches.The size of the branch ditches is larger and the corresponding hydraulic retention time is larger,which makes the reduction effect of the branch ditches on ammonia nitrogen better.(2)The average concentration of nitrate nitrogen in farm ditches is mostly between 2.0 and 3.0 mg/L,while the average concentration of nitrate nitrogen in branch ditches and ponds is mostly between 1.5 and 2.5 mg/L,which differs from the farm ditches.The concentration of nitrate nitrogen decreases gradually,while that of ammonia nitrogen decreases to a very low concentration in a short time.These cases indicate that the nitrate nitrogen is more stable compared to the ammonia nitrogen.(3)The average concentration of total phosphorus in the ditches and ponds ranged from 0.10 mg/L to 0.20 mg/L.The fluctuation of the concentration was obvious on the whole,but the concentration fluctuation in different branches or between their upstream and downstream was quite different.(4)QUAL2K model has good simulation results in small-scale drainage ditches and ponds.After the simulation,some data can be supplemented to calculate the reduction rate of pollutants,and the accuracy of the parameters in the reduction rate calculation can be verified as well.(5)QUAL2K water quality simulation results show that the ammonia nitrogen concentration and nitrate nitrogen concentration at the drainage terminal change linearly with the change of water temperature.Adjusting wind speed parameters individually shows a negative correlation reduction law,while the combinations of other meteorological parameters(water temperature,dew point temperature,wind speed)are positive correlation.Among them,the combination of temperature and dew point temperature is the best,and the change of dew point temperature has the most significant effect on the water quality under the single condition.