Study On Nutritive Salt Export And Modeling In The Polder Area Around Chao Lake

As one of the five freshwater lakes in China,since the 1950 s,with the development of industry and agriculture and rapid increase of population,a large number of production sewage and living sewage into the lake and intensify the degree of eutrophication of lake,so it was listed in the national government project "Three Rivers and Three Lakes".There are many polder areas located around Chao Lake and agricultural activities were conducted in it.Nutritive salt exportation from polder areas is an important factor of non-point source pollution in Chao Lake.In order to understand nutritive salt export features in this area,a typical polder area located on the coast of the lake was chosen for the study,systematic field survey observation,sampling and indoor water quality analysis were conducted in wheat and rice growing period in 2014 and discussed the change of concentration and export characteristic of nutritive salt in polder areas under natural rainfall-runoff condition.The polder area belongs to Xisong Village of Chao city,which is located in the between of Hefei and Chaohu.The total square of the polder is about 253.35 hm².There is no industry in area and mainly dominated by farming,wheat and rice is the main crop of polder.The content of this study include:1)to learn the nutrient’s change feature in the ditch;2)to understand the nutrient export characteristic in the rainfall-drainage event;3)to simulate event nutrient concentrations and total loads,from October,2013 to September,2014.These samples were analyzed for Total Nitrogen（TN）,nitrate（NO₃-N）,ammonium（NH₄-N）,Total Phosphorus（TP）and Dissolved Phosphorus（DP）.At the same time,we also gather the rainfall data and calculation the water drainage outflow by pumping.（1）The results showed that:During the process of delivery,the nutrient concentrations generally present a diminishing regular.But because of the new pollutants entered the ditch during the transportation,so the concentration will be increased.Three ditches connected the different source,the nutrient had different pattern.The ditch which connect to vegetable planting that the concentration of total nitrogen（TN）can attained 12.436mg/L and connect rural living area that the concentration of total phosphorus can also attained 4.34mg/L.（2）In wheat season,The mean concentration of total nitrogen（TN）and total phosphorus（TP）were 4.99、0.239mg/L in the 8 rainfall-runoff events;in rice season,the concentration can also reached 3.42 、 0.22 mg/L in the 9 rainfall-runoff events.Both the average concentration of TN exceeded the level Ⅴof surface water environment quality standard（N,2mg/L）and the average concentration of TN can reached the levelⅣ.Nitrate was the main form of nitrogen export and ammonium represented a small proportion of TN.DP was the important loss form of TP.（3）Volume of runoff and fertilizing were important factors of nutritive salt export amount.With the increase of runoff,nutritive salt load is also greater.The NO₃-N export amount was less affected by fertilizing and was positively correlated with the volume of runoff.The NO₃-N export amount accounted for the larger proportion of the total nitrogen.the export amount of TN and TP in wheat growing period accounted for 1.32% and 0.18% of nitrogen and phosphorus fertilization input and in rice growing period its accounted for 1.73% and 0.16%.（4）Using SPSS software for correlation analysis and regression analysis,we can get nutritive salt export concentration model and export load model.The regression equation include three variables:the runoff,the fertilization and the interval days from fertilization to rainfall-runoff events.The result showed that:The output of the wheat season nutritive salt load and a multivariate linear relationship with the three variables;The rice season’s export load model was different from wheat season.Except NO₃-N export load is similar to the wheat season,the other nutritive salt export load were nonlinear relationship with three variables.