Research On Groundwater Circulation Evolution And Rational Exploitation In Valley Plain Of Lhasa City

The valley plain of Lhasa City is one of the regions with the most intensive human activities in the Qinghai-Tibet Plateau.Groundwater is the main source of water supply,and groundwater exploitation is increasing year by year.With the development of social economy and the acceleration of urbanization process,human intervention in the groundwater environment has become more intense.Therefore,research on groundwater cycle evolution and reasonable exploitation has important practical significance for strengthening the rational development and sustainable use of groundwater resources.Based on the systematic analysis of hydrogeological conditions,this paper comprehensively applies hydrochemical,isotope,mathematical analysis,and numerical simulation methods to analyze the dynamic evolution of groundwater and its influencing factors,recharge sources and circulation patterns in valley plain of Lhasa City,and carries out the laboratory experiment of groundwater recharged by river.Finally,the reasonable exploitation of groundwater is discussed based on groundwater flow model.The main understanding and conclusions is as follows:(1)From 1995 to 2000,the average water level of groundwater in valley plain of Lhasa City was relatively stable,but it declined year by year after 2000.In addition,based on the long-term monitoring data of groundwater level,the dynamic changes of groundwater level at each monitoring point can be divided into 7 modes as a whole.The content of main chemical components of groundwater in the study area is relatively low and water quality is excellent.The formation of groundwater hydrochemistry is mainly controlled by water-rock interaction and cation exchange.In addition,human activities also have a certain impact on the chemical characteristics of groundwater.From 1991 to 2015,the chemical composition of groundwater changed obviously and showed a certain stage,especially the content of Mg2+and SO42-increased rapidly after 2013.The groundwater hydrochemical type evolved from HCO3-Ca type to SO4·HCO3-Ca·Mg-based mixed water.(2)The natural and human factors affecting the dynamic evolution of groundwater in the study area were analyzed,and based on the grey correlation analysis,the degree of correlation between the main factors affecting the dynamic evolution of groundwater and the quality and level of groundwater was determined.On the whole,the correlation degree between population precipitation and groundwater dynamic evolution is the highest.In addition,the average correlation degree of groundwater and other influencing factors is more than 0.6,indicating that the dynamic evolution of groundwater in valley plain of Lhasa City is affected by both human and natural factors.(3)The main recharge source of surface water and groundwater in the region is atmospheric precipitation,and there is direct supply of ice and snow melt water.In addition,there are close hydraulic and transformation relations between surface water and groundwater.At different depths,the chemical composition of groundwater and the characteristics of hydrogen and oxygen isotopes are obviously different,and the main sources of recharge are also different.Shallow groundwater is mainly recharged by atmospheric precipitation and surface water infiltration,while deep groundwater is mainly recharged by lateral runoff of groundwater.In addition,reverse hydrogeochemical simulation indicates that the hydrogeochemical reactions in different depths of groundwater flow path are different.Based on the established two-dimensional numerical model of groundwater flow in a typical profile,the groundwater system in the study area is divided into three modes:shallow groundwater circulation mode,middle groundwater circulation mode,and deep groundwater circulation mode.After comprehensive analysis,the circulation model of groundwater in valley plain of Lhasa City is obtained.(4)Based on the laboratory experiment of river recharge groundwater,some results similar to previous studies were obtained and new knowledge and findings were also obtained.After the disconnection of river and groundwater,the thickness of the inverted saturated zone under the riverbed is not only related to the depth of river water,but also to the physical characteristics of the water bearing medium.Based on the obtained experimental data,an estimation formula for the inverted saturated zone is given.In addition,it is also concluded that although capillary water in the unsaturated zone cannot spontaneously flow out of the aqueous medium,it can continuously flow freely from the aqueous medium under a certain pressure condition,and this pressure is defined as "discharge water pressure".Under the condition of river-groundwater disconnection,with the increase of river water depth,the distribution pattern of areas with discharge water pressure and continuous free water flow in unsaturated zones gradually evolved from "continuous" type to "disconnected" type.(5)Under the condition of strong exploitation,the main source of groundwater recharge in valley plain of Lhasa City is the river water infiltration recharge of Lhasa River.It is estimated that the maximum seepage recharge of Lhasa River at the lowest water level in the study area is much larger than the current groundwater exploitation,that is,the current groundwater exploitation will not make Lhasa River lose the saturated hydraulic connection with groundwater.Based on the actual situation,adjusting the amount of groundwater exploitation in each water source area is the best plan in the near future.If the groundwater exploitation continues to increase,in addition to adjusting the amount of groundwater exploitation in the later period,additional water sources can be added in the near-river area.