Changes In The Hydrological Situation Of The Middle And Lower Reaches Of The Han River And Evaluation Of The Water Environment

The Han River is the largest tributary of the Yangtze River,and its abundant water resources are the basis for the economic development of the coastal areas.In recent years,with the rapid growth of population and the rapid development of industry and agriculture in the middle and lower reaches of the Han River,humans have discharged large amounts of pollutants into the Han River,leading to increasingly serious water pollution problems in the middle and lower reaches.At the same time,there are multi-stage hydropower hubs and water transfer projects in the middle and lower reaches of the Han River,and the completion and operation of these facilities will greatly change the hydrological situation in the middle and lower reaches,and may aggravate the water pollution problem in the region.This paper takes the middle and lower reaches of the Han River as the research object,and collects hydrological data of the region over the years to determine the impact of the Danjiangkou Reservoir,the South-North Water Transfer Project and the Yangtze-Han River Water Transfer Project on the hydrological situation of the region.And a comprehensive evaluation of river water quality conditions using 2016 water quality data from the middle and lower reaches of the Han River.Finally,the ecological risk was assessed and the possible sources were analyzed in the middle and lower reaches of the Han River by the measured heavy metal concentration data.The main findings of the study are as follows:（1）By collecting hydrological series data of the middle and lower reaches of the Han River over the years,the impact of the Danjiangkou Reservoir,the South-North Water Transfer Project and the Yangtze-Han River Water Transfer Project on the hydrological situation of the region was investigated based on the IHA/RVA（indicators of hydrologic alteration,IHA;range of variability approach,RVA）method.The establishment of Danjiangkou Reservoir has played a good role in controlling the hydrological situation in the middle and lower reaches of the Han River basin,as evidenced by the decrease in the flow in the downstream during the abundant water period,the increase in the flow in the dry water period,and the trend of decreasing the extreme flow and the duration of high and low pulses.After the operation of the South-North Water Transfer Project,the average monthly flow and water level at all stations in the middle and lower reaches of the Han River have been reduced to a certain extent.It shows that the operation of the South-North Water Transfer Project has played a good role in regulating the hydrological situation in the middle and lower reaches of the Han River.The monthly average flow rate,annual extreme flow rate,flow rate change and frequency of Xiantao station after the operation of the Yangtze-Han River Water Transfer Project are moderately changed.Annual extreme flow indicators and the time of occurrence are highly variable,with a high degree of overall variability.Each gradient hub assumes different roles in the middle and lower reaches of the Han River,and has a better role in regulating the flow of each station in the region,especially in the abundant water period.（2）By collecting the water quality data of the middle and lower reaches of the Han River in 2016,we analyzed the temporal and spatial changes of water quality and applied three water quality evaluation methods to evaluate the water quality situation of the region.Comparing with the Environmental Quality Standard for Surface Water,all the indicators of the stations meet the corresponding standards,except for the total phosphorus（TP）indicator of Zongguan station,which does not meet the corresponding standards;The single-factor evaluation results show that phosphorus pollution exists in Yuekou and Zongguan stations;The evaluation results of the integrated pollution index method show that the water quality of all monitoring points in the middle and lower reaches of the Han River reached the qualified status throughout 2016;Fuzzy mathematical integrated evaluation method calculation results show that the water quality of the Zongguan station is rated as Class V.Calculating the water environment capacity of each section of the middle and lower reaches of the Han River in 2016,there is still some space for the three indicators of five-day biochemical oxygen demand（BOD₅）,ammonia nitrogen（NH₃-N）and TP in each section of the region.（3）In this paper,the risk evaluation of heavy metals in sediments was carried out using the Nemerow integrated risk index（NIRI）and biological unfavorability evaluation.The potential sources of heavy metals in sediments were also analyzed by Spearman correlation analysis and PMF method to clarify the distribution characteristics,ecological risks and pollution sources of heavy metals in sediments in the middle and lower reaches of Han River.The average concentration of the six heavy metals in the sediments of the middle and lower reaches of the Han River was ranked as Zn>Pb>Cu>As>Cd>Hg.Compared with the background values of soil heavy metals in Hubei Province,the mean concentration of all six heavy metals significantly exceeded their corresponding background values.All sampling sites in the middle and lower reaches of the Han River have NIRI values greater than 320 and are at extreme risk.The sediment Mean PEC Quotients（m PEC-Q）values at all sampling sites ranged from 1 to 5,showing a higher risk of biotoxic effects.Two sources lead to the high concentrations and risks in the middle and lower reaches of the Han River:One is a combination of traffic and agricultural pollution sources consisting of five heavy metals,Pb,Hg,Zn,Cu and As;The other is industrial pollution sources,mainly Cd and As.