The Spatial-temporal Variation Of Water Quality In Han River Under The Operation Of The Middle Route Of South-to-North Water Transfer Project

The Han River Basin is the largest tributary of the Yangtze River,with complex natural conditions,unbalanced spatial and temporal distribution of water resources,and numerous water conservancy projects.The Middle Route Project of the South-to-North Water Diversion(MSNWP)has transferred water from Danjiangkou Reservoir in the middle reaches of the Han River since 2014.It has played a significant role in alleviating the shortage of water resources in the Beijing,Tianjin,and North China regions,improving the ecological environment in the receiving areas,and promoting the sustained and stable economic and social development in North China.However,the operation of the MSNWP Project will also change the hydrological conditions in the middle and lower reaches of the Han River,which will greatly reduce the amount of water resources that can be used in the Han River Basin and create a series of water environmental problems.In order to mitigate the impact of water diversion project on the water ecological environment in the middle and lower reaches of the Han River,the Yangtze-Han River Water diversion Project(YHWP)was built to supplement the water requirements in the lower reaches of the Han River and to mitigate the adverse effects of the eutrophication of the Han River.In addition,there are several serious tributaries in the middle and lower reaches of the Han River that will affect the water quality of the mainstream Han River.Therefore,the study on the Han River water environment and ecological health impacts at different time and space scales can provide a theoretical basis for watershed water environment planning and management.This paper objects to study the temporal and spatial variability of the water environment in the middle and lower reaches of the Han River under the implementation of water conservancy projects;to evaluate the ecosystem health of heavy polluted variables in the middle and lower reaches of the Han River.The main water quality indexes and pollution types affected by water conservancy projects and heavily polluted tributaries were analyzed for typical sections of the Han River,and the ecosystem health of the middle and lower reaches of the han river was evaluated according to the typical water quality indexes.The effects of internal pollution on water quality in the Han River were clarified,and the spatial-temporal variation characteristics of water environment pollution in the middle and lower reaches of the han river were comprehensively understood.The main research contents and results include the following aspects:(1)By collecting data on the hydrology and water quality of the middle and lower reaches of the Han River and the natural environment of the region over the years,the Range of Variability Approach(RVA)based on the Index of Hydrodynamic Alteration(IHA)system is used to compare the flow rate variation before and after the operation of MSNWP Project.The Mann-Kendall trend analysis was used to analyze the trend of runoff at hydrological stations,and the impact of water conservancy projects on the hydrological conditions of the basin was clearly defined.The results showed that the average monthly and annual average flow and water level in the middle and lower reaches of the Han River after the Middle Route Project of the South-to-North Water Transfer have significantly decreased;the frequency of flow and water level in the middle and lower reaches of the Han River in the high-threshold conditions(upper quartiles)decreased significantly,while the low threshold showed the similar trend.The decreasing trend for the first year(2014)after the operation of SNWTP was the most obvious,and the regulation of water projects since 2016 reduce the influence of MSNWP.The flow rate in the middle and lower reaches of the Han River in 2015 was much lower than in 2014.(2)The Middle Route of China's South-to-North Water Transfer(MSNW)and Yangtze-Han River Water Diversion(YHWD)Projects have been operated since 2014,which may deteriorate Han River water quality.The 11 water sampling sites distributed from the middle and down streams of Han River were monitored monthly between July 2014 and April 2017.Factor analysis and cluster analysis were applied to investigate the major pollution types and main variables influencing water quality in Han River.The factor analysis distinguishes three main pollution types(agricultural nonpoint source,organic,and phosphorus point source pollution)in Han River.The sites located at manufacturing district received point source pollution in both Dry and Wet seasons.The water quality in downstream Han River(excluding manufacturing district sites)was mainly influenced by nonpoint source pollution from Jianghan Plain.Although the MSNW Project diverts water since December 2014,water volume discharged into Han River in 2015 is slight higher than that in 2014,which is well controlled to maintain the water quality in Han River.In Wet season,high flow volume,which can dilute the pollutants and strengthen self-purification ability of water body,can improve the water quality of Han River.In Dry season,elevating the flow discharges from MSNW and YHWD Projects to Han River can avoid degrading water quality and may decrease the probability of algal bloom occurrence in Han River.The investigation of relationships between water quality and flow rate in both projects can provide management strategies of water quality for various flow conditions.(3)The middle and down streams of Han River are complex river systems influenced by hydrologic variations,population distributions,and the engineering projects.The Middle Route of China's South-to-North Water Transfer(MSNW)project planned to transfer 95 billion m3 annually from Han River to north China.The operation of the MSNW project may alter the flow rate and further influence the water quality of Han River.This study uses min/max autocorrelation factor analysis(MAFA)and dynamic factor analysis(DFA)to analyze spatial and temporal variations of the water quality variables in three typical tributary-mainstream intersection zones in Han River from June 2014 to April 2017.MAFA results show that chlorophyll-a(Chl-a),chemical oxygen demand(COD),suspend solid(SS)and phosphate(PO43-)(represented as trophic dynamics)are main concerned water quality variables in densely populated zones(Zones 1 and 3),and total nitrogen(TN),nitrate nitrogen(NO3-),COD,and PO43-(regarded as nutrient formations dynamics)represent the underlying water quality variations in agricultural cultivation zone(Zone 2).DFA results indicate that domestic and municipal effluent pollutants influence the organic concentrations and nutrient formations in the mainstream in Zones 1 and 2.The non-point source nitrogen and phosphorus discharged from the tributaries Tangbai and Hanbei Rivers elevate the nutrient concentrations and increase Chl-a concentration(i.e.promote the algal growth)in densely populated zones.In addition,controlling the flow rates in low and middle flow rate conditions can avoid degrading water quality.The flow rate should be elevated to more than 700 m3/s(cubic meters per second)in the middle stream and to more than 800 m3/s in the downstream of Han River for preventing water quality deterioration from high loadings of organic pollutants and nutrients.The integrated MAFA and DFA method establishes an efficient analysis distinguishing spatio-temporal variation of water quality variables and provides useful site-specific management to control water quality in various flow conditions.(4)Accurately estimating the temporal-spatial variation of water quality is critical and difficult in complex river systems with various sources of pollutants.Water quality of Han River was influenced by polluted tributaries,engineering projects,and point and non-point pollutants.This study determined the temporal-spatial distributions of three typical water quality variables(total nitrogen(TN),total phosphorus(TP),and chlorophyll-a(Chl-a))in the middle and down streams of Han River by using a modified indicator-ordinary kriging(IK-OK)approach with multiple thresholds.The water quality variables were first estimated using indicator kriging(IK)and ordinary kriging(OK),respectively.To reduce the underestimation of high values and the saltation of water quality estimation,a combination of IK and OK was adopted to probabilistically categorize water quality of Han River.IK determines suitable category in advance and provides accurate spatial estimation(except extreme values)of water quality variables,and OK determines a more suitable estimation of extreme values and its influence to the downstream of Han River.The water quality level of each variable estimated using IK were amended according to the estimation of extreme values determined by OK when they lead to the deterioration of downstream Han River.The research results reveal that the water quality in downstream Han River which was mainly influenced by municipal pollutants is worse than that in middle stream.The intense precipitation increase runoff and carries high amounts of non-point phosphorus pollutants(TP)into Han River,and consequently increase the risk of the algal growth.The modified IK-OK approach with multiple thresholds can reduce the underestimation of high values and the saltation of water quality estimation,and finally obtain an accurate estimation results of Han River water quality.The approach facilitates constructing a numerical model for more effectively evaluating water quality variation which can also provide useful site-specific managements to control water quality.(5)Through the analysis of water quality and sediment samples of mainstream Han River(Xiantao section)and Tongshun River,we found that the water quality and nitrogen and phosphorus content in the sediment of the Tongshun River are much higher than that of the main stream of Han River.From the upstream to the downstream of Tongshun river,there is no obvious increase or decrease trend,indicating that the pollution of Tongshun River is mostly caused by peripheral non-point source pollution.The order of heavy metal concentration in the sediment at the sampling points of Xiantao section and Tongshun section of the main stream the Han River is:Zn>Pb>Cu>As>Cd>Hg,and all heavy metal elements are obviously in excess of their corresponding background values.According to the Hankanson ecological hazard coefficient,the order of potential ecological hazards to heavy metals in the sediment of Tongshun River is:Cd>Hg>As>Pb>Cu>Zn,among which the contribution of Cd is the most obvious.According to the analytical analysis of adsorption characteristics of different sediment samples,the adsorption of phosphorus by sediment increased with the increase of phosphorus level in solution.Elovich equation has the best adsorption kinetics effect on phosphorus from sediment at each sampling point.The fitting results show that the adsorption process of phosphorus by sediment can be divided into two stages:fast adsorption and slow adsorption.During the rapid adsorption phase,the adsorption amount of phosphorus from the bottom sediment reached 80%to 90%of the total adsorption amount within 4 hours.After considering the sediment of TP concentration and disturbance intensity,the phosphorus release under the condition of different fitting equation integration,get equation(P sed=(-0.048POsedP0sed × D1/3-0.012)x ln t+1.1 × P0sed).Compared with the disturbance intensity,the TP concentration of sediment has a more obvious influence on the phosphorus release of sediment.With the increase of sediment concentration,the influence of sediment on phosphorus release tends to increase exponentially.Based on the study of different pollution characteristics in the middle and lower reaches of Han River,various point and non-point source pollution characteristics in the middle and lower reaches of Han River are clarified.Based on the study of the influence mechanism of typical pollution tributaries and water conservancy projects on the main stream of Han River,the influence of major water conservancy projects and tributaries is preliminarily understood,and the chemical circulation mechanism of typical pollutants in the main stream of Han River is understood.The effects of endogenous pollution on water quality were investigated by studying the release characteristics of nitrogen and phosphorus from sediment.A comprehensive understanding of the water environment and hydrochemical cycling mechanism in the middle and lower reaches of Han River can help managers put forward specific measures to control pollution from different internal and external sources,which can provide theoretical basis for water quality planning and management in the middle and lower reaches of Han River.