Dynamic Analysis And Quantification Of Water Environment Capacity For The Non-point Source Pollution Dominated River

Water pollution is a fundamental problem that effects China's economic development and people's lives.Watershed eutrophication phenomenon is not uncommon in the southern region.The water environmental pollution caused by non-point sources is becoming more and more prominent with the control of point-source pollution.How to quantify the pollution process and control target of non-point source pollution is the most basic and critical step in non-point source pollution control practice,and it is also the research hotspot and difficulty in the field.In this paper,the water quality monitoring of 6 sampling points,including Nanshan,Tanyuan,Shihuangjiang,Ganlin,Chongren and Yazhi,was carried out from January 2007 to December 2012 in the typical small watershed-Changle River Basin in eastern Zhejiang Province.The annual average daily load for TN was simulated and calculated by using the LOADEST model based on the comprehensive evalution of the water environment.The residual water environmental capacity was calculated and its variation was analysed.The reduction load was allocated according to the characteristics of non-point source pollution in the basin.The emission reduction load was calculated based on the different coefficients of pollutants into the river for all pollution sources.The main research conclusions are as follows:1.The synthetical water quality of Changle river gradually deteriorated from upstream to downstream during the monitoring period.The spatial variation was relatively uniform and gentle while the time variation was extremely large.It was characterized by typical non-point source pollution and the variation was mainly affected by climate and human activities.The concentrations of ammonia nitrogen in mainstream and COD in Changle watershed generally met Class ? water standards.The concentration of ammonia nitrogen at four creeks met Class ? standards.The concentration of TP generally met Class ? standards,but TN concentrations throughout the year exceeded 2.0 mg·L^-1.Therefore,the key to the environmental problems of Changle watershed was non-point source pollution control of TN eutrophication.2.Changle watershed was recharged by the rain and run-off.There was a positive significant linear correlation between the monthly mean flow and the rainfall at the end of Changle River?R²=0.7362,p<0.01?;in the space,the flow gradually increased with the increase of the area of the river basin from upstream to downstream,and the average daily flow varied from 1.98m³/s in the first sampling point to 13.08m³/s in the outfall;from the time,the flow from March to August was under the high flow conditions.The average daily flow in June reached the maximum?49.5m³/s?;the monthly average flow from September to February of the following year was under the mid-range and the low flow conditions.The minimum value of daily flow usually occured in January and the minimum value was 8.61 m³/s.3.In theory,water environmental capacity is the function of water flow.However,the runoff along the river is not only the main body of the flow changes but also the main source of pollution for the river characterized by typical non-point source pollution.The average annual load of TN was 1469.6 t·a^-1 in the watershed.There was a linear correlation between TN load and flow?r=0.92**?.However,the actual water environmental capacity does not necessarily increase with the increase of river flow for the river characterized by typical non-point source pollution.This paper proposed the residual water environmental capacity?the difference between the theoretical water environmental capacity and the actual load?to characterize the actual water environmental capacity change of river characterized by typical non-point source pollution.The water environmental capacity for TN in Changle river was 826.92 t·a^-1 with a significant variation in time.The residual water environmental capacity was negatively correlated with river flow?y =-0.0015x²-0.0804x-0.1211;R²=0.564?,which indicated that the water environmental capacity of Changle River decreased with the increase of runoff.4.The characteristics of seasonal distribution of annual average load for TN in Changle River was constructed:The proportion of TN load in summer?June-August?was 30%?40%,which was the peak period of nitrogen loss in the whole year,followed by the spring?March-May?and winter?December-February next year?with a proportion of 21.4%?29.0%and 23.0%?30.4%respectively.The proportion of annual average TN load in autumn was 16.5%?18.8%.5.According to the calculated water environment capacity and the characteristics of the pollution source in Changle watershed,the reduction and control schemes of pollutants were obtained.The annual reduction load of TN was 642.68 t ·a^-1 in the watershed.The emission reduction load of fertilizer and pesticide application?rural living and livestock and poultry breeding were 1481.8 t·a^-1?318.16 t·a^-1 and 417.32 t·a^-1 respectively in the watershed.