Groundwater Circulation Patterns And Its Change Trendin Southern Qaidam Basin,Northwest China

Groundwater is not only an important water source for production activities and drinking water supplies but also the safeguard water of the vulnerable ecology in arid basins,northwest China.A full understanding of groundwater cycle patterns and maintaining their good status are essential to maintain proper management and implement sustainable utilization of groundwater resources.Therefore,based on the sufficient analysis of the geological and hydrogeological conditions of Nomhon and Golmud watershed of sourthern Qaidam basin,this study reveals the recharge,cycle and evolution characteristics of groundwater,establishes typical groundwater cycle patterns,and simulates the responses of groundwater to the climate change and human activities in the basin using hydrogeochemistry,isotopes and numerical approaches.Conclusions that be got in this study are presented as follows.Groundwater in the basin originates from meteoric water and glacier melt-water of surrounding mountains under different paleo and modern climatic conditions.The recharge of groundwater is happened by modern water under modern hot and dry climate condition for alluvial fan,and by paleo water after the last glacial period when climate was getting hotter and drier for fine soil plain.Groundwater in the low-lying depression of the central basin is paleo-brines migrated from the western part of the basin due to tectonic uplift in the geological past.The surface brine water of modern salt lake in the basin center is recharged and supported by the modern water.Underground brines in the shallow aquifers of the low-lying depression can receive some infiltrating modern water during flooding periods.Groundwater cycle patterns in the basin have both commonalities and differences due to the joint control of various factors such as geological sediments,aquifer lithologic structure,geological structure.The circulation and hydrogeochemiscal evolution of groundwater in both Nomhon and Golmud watershed are characterized by arid inland basin.Groundwater is recharged by river seepage and mountain lateral inflow in the alluvial fan,and flows towards to the low-lying depression of the basin.Groundwater chemistry along the flow paths evolves from HCO₃·Cl-Ca·Mg·Na type fresh water to Cl-Naor Cl-Mg type brine water with the control of minerals dissolution/precipitation,cation exchange interaction and its reverse process,as well as evaporation.The specific geological structure leads the circulation and hydrochemical evolution of groundwater in Nomhon watershed having some differences.Groundwater flow is blocked by the uplift structure in front of the alluvial fan.Due to the influence of uplift structure,the flow direction of phreatic groundwater is changed to the northwest and high pressure artesian springs are widely formed in the overflow zone.In addition,SO₄^2-bacterial reduction under the reducing environment formed by the closed flow system of the uplift structure leads groundwater chemical evolution lacking SO₄^2-water type.The oasis belt in the basin is the active response zone for groundwater cycle,discharging more than 80%of the total cycling groundwater of the watershed.While the salt lake is a very weak response zone,and only discharges less than 1%of the total cycling groundwater.Global warming can produce active effects on groundwater system.It is predicted that the climate warming of 2.21?would cause the groundwater recharge amount increasing more than 30%,groundwater level rising,the upper position of overflow zone moving towards upstream for about 5 km,springs discharging amount increasing more than 52%,evaporation and lake discharging amount also increased in some degree.While the expanding exploitation of groundwater can produce negative effects on groundwater system,leading to groundwater level drawdown,the upper position of overflow zone moving towards downstream,natural discharge water amount decrease.When the exploiting amount expanded to 2.93×10⁸ m³·a^-1,the effects caused by climate warming of 2.21?and expanding exploitation are basically offset,and no degradation of the oasis belt in the Golmud watershed would be occurred.