Change Rule Along The Way And Pollution Cause Analysis Of Water Quality Of The Middle And Lower Reaches Of Hanjiang River

As the largest tributary of the Yangtze River,the abundant water resources of the Hanjiang River is the foundation of the sustainable development of the social economy in the middle and lower reaches of the Hanjiang River.In recent years,with the rapid development of economy in the middle and lower reaches of the Hanjiang River,a large number of industrial and agricultural wastewater and domestic sewage are discharged into the Hanjiang River,and the water pollution problem in the middle and lower reaches of the Hanjiang River is becoming more and more serious.At the same time,with the completion and operation of water conservancy projects such as the middle route of the South-to-North Water Diversion Project,Hubei-to-North Water Diversion Project and the Cascading Water Conservancy Project,the water volume in the middle and lower reaches of the Hanjiang River decreases,the flow velocity decreases,and the water self-purification capacity weakens,which further aggravates the water pollution trend in the middle and lower reaches of the Hanjiang River.This paper takes the middle and lower reaches of the Hanjiang River as the research object.The water quality status of the middle and lower reaches of the Hanjiang River is investigated through field sampling,and the single factor evaluation method,comprehensive pollution index method and nutrient state index method are used to analyze the water quality status.Combined with the historical water quality data of the Hanjiang River,the variation law of water quality of the Hanjiang River is analyzed by using graphic method and seasonal Kendall test method.The causes of water quality change along the middle and lower reaches of the Hanjiang River are explored based on the current situation of water quality and the trend of water quality change.The main research results are as follows:（1）Nitrogen and phosphorus are the main pollution factors in the middle and lower reaches of Hanjiang River.The TN concentration values of all sampling sections in the main stream exceeds the standard ofⅡclass for surface water,and over 90%of the sampling sections exceed the standard ofⅢclass.The exceeding rate of the standard ofⅤclass is the wet season（22.22%）>the normal season（18.75%）>the dry season（5.56%）.The TP concentration values do not exceed the standard ofⅢclass,and the exceedance rate of the standard ofⅡclass is the wet season（33.33%）>the normal water season（18.75%）>the dry season（16.67%）.Among the three tributaries of Xiaoqing River,Tangbai Piver and Zhupi River,TN and TP are the most seriously polluted in Zhupi River.The annual concentration of TN exceeds the standard ofⅤclass,while the concentration of TP exceeds the standard ofⅤclass only in the wet season.The pollution of TN and TP in Xiangyang section,Hanchuan section and Wuhan section are more obvious.（2）The data of conventional water quality indexes(p H,DO,COD_Mn,NH₃-N and TP)in Xiangyang Hydrological Station,Xiantao Hydrological Station and Hankou Hydrological Station in recent 10 years are analyzed by graphical method and seasonal Kendall test method.The results show that the COD_Mnhas a decreasing trend,and it is speculated that the organic pollution characterized by COD_Mnin the middle and lower reaches of Han River has been effectively controlled.Only the TP concentration of Hankou Hydrological Station has increased,which is presumed to be due to the fact that Hankou Hydrological Station is located in the downstream of Wuhan urban area and is affected by waste water from Wuhan urban area,leading to the increase of TP concentration.（3）Fe and Mn are the main heavy metal pollution factors in the middle and lower reaches of Hanjiang River.The over-standard rate of Fe concentration is the dry season（35.3%）>the wet season（17.6%）>the normal season（0%）;and the concentration of Mn do not exceed the standard,the overall concentration is the dry season>the wet season>the normal season.Among the three tributaries of Xiaoqing River,Tangbai Piver and Zhupi River,the pollution of Fe and Mn is the most serious in the tributary Zhupi River.The concentration of Fe exceeds the standard rate in wet season and dry season which is 4.63 times and 0.74 times,respectively,and the concentration of Mn exceeds the standard rate by 0.13 times only in wet season.The pollution of Fe and Mn in Zhongxiang section,Jingmen section,Qianjiang section and Hanchuan section are more obvious.（4）The data of heavy metals（Cu,As and Fe）in Xiangyang,Xiantao and Hankou hydrological stations in recent 10 years are analyzed by graphic method and seasonal Kendall test method.The results show that Cu has no significant change trend,As presents a general downward trend,and Fe presents a significant upward trend.It is speculated that the concentration of Fe should be paid attention to.（5）Analysis on the causes of water quality pollution in the middle and lower reaches of Hanjiang River shows that the inflow from the upper reaches of the Hanjiang River has a certain influence on the concentration of TN in the middle and lower reaches of Hanjiang River.The water conservancy project such as Danjiangkou,Wangfuzhou and Cuijiaying have no obvious influence on the TN index in the water body,but lead to a certain accumulation of TP,Fe and Mn in the water body at the bottom in front of the dam.In the middle and lower reaches of the Hanjiang River from Xiangyang to Hanchuan,the concentration of TN and TP are generally high,which is mainly from non-point source pollution;in the middle and lower reaches of the Hanjiang River from Jingmen to Hanchuan,the concentration of Fe and Mn are generally high,which is mainly from point source pollution.