Mathematical Methods In Isotope Hydrogeology And Its Applications To The Inland River Basins In Northwest China

In spite of sparse precipitation,a dry climate and widely distributed desert regions across northwest China,the high mountains such as Qilan constitute "wet islands" in a desert region.The many large inland rivers that are fed by glacier melt-water and disappear into the desert nourish its many oases.Unlike many other arid zones in the world,the dry lands in Northwest China exhibit an alternatively distributed pattern of high mountains and depressions in the geomorphologic features. Water resources mainly come from mountain area and disappear in the desert plains. A river basin can be regarded as a total hydrological system in which meteoric water; surface water and groundwater transfer between each other to form a complete water cycle.This cycle controls the regional geochemical characteristics of the water environment.Furthermore,as water volume and quality in an ecosystem are closely related,the exploitation and study of water resources are generally carried out with a river basin or watershed as an ecological unit.In recent years,the arid regions in NW China have faced increasingly serious eco-environmental challenges with the population growth and economy development. To utilize water resources effectively and achieve sustainable development,it is necessary to understand the interaction between surface water and groundwater, groundwater renewal rate,and the renewability of groundwater in different aquifers. Environmental isotope extensively exists in various kinds of water.Every kind of water has distinct isotope composition because of isotopic fractionation in the transfer and circulation among precipitation,surface water,groundwater,soil moisture and plant water.The difference of isotopic content in different water can provide some important information to investigate the exchange of surface water with groundwater and the leakage between aquifers.Radioisotopic dating(³H and ¹⁴C)of groundwater can provide us with significant information about the circulation time and renewability of groundwater.In fact,the application of these relatively new techniques has proved to be an attractive tool for the quantitative evaluation of groundwater systems,assessment of the availability of water resource,as well as to protect the fragile eco-environmental system.Based on the field investigation and experiment and other data from the former study,a case study of the Heihe River and Shiyang River is presented by using the hydrological isotope mathematical models to illustrate the interactions of surface water with groundwater,the mean residence time and renewal rate of the groundwater. Taken groundwater as information carriers of ancient geological environment changes, another purpose of present work is to establish aquifer archive,reconstruct the process of groundwater resource formation and evolution.The following results are obtained from the present study:1.Based on the data of annual runoff from the meteorological stations of mountains watershed,the wavelet transform is applied to the tendency characteristics of total runoff in the recent 50 years in the Heihe River.The results show that there are 3 main period of the runoff of mountainous watershed which are 3 years,5-9 years, and 21-29 years;Applying empirical mode decomposition(EMD)to runoff data of the Heihe River,different scale signal is seprated.By analyzing the residual variable R,it shows that the runoff in Heihe River has a trend to increase,especially after the 1980s.This may be due to the climate shift from warm day pattern to warm humid pattern in the study area,which results in the precipitation and snowpack in spring and autumn as well as the runoff of mountainous watershed have an increasing tendency.2.With the addition of the isotope mass balance equation,the composite proportion of different mixing components namely precipitation,melt water and bedrock fissure water can be calculated.In despite of at different sample sites,the composite proportion of different mixing components is different.Averagely,the summer surface runoff is mainly coming from melt water,the next is bedrock fissure water,and the least is precipitation.3.The mean residence time of the shallow groundwater in the Ejina Basin was calculated,according to the exponential-piston model(EPM).The results shows that the oldest shallow groundwater has an age of 58 years located in the Gurinai region, and the youngest is 13 years in Dingxin-Laoximao.The shallow groundwater in Ejina city has an age of 22 years,and 20 years in Bantanjing.4.It is vital for sustainable development of groundwater resource to understand the groundwater renewal rate in the Ejina basin.The groundwater renewal rates of the shallow and deep aquifer are estimated by using ³H and ¹⁴C,respectively.The results suggest that the renewal rate of the shallow groundwater range from 0.76%·a^-1to3.3 %a^-1.These waters are renewable resources.The deep confined groundwater has low renewal rate,ranging from 0.003-0.01%·a^-1.These waters could be regarded as poorly renewable resources.5.The characteristics of the groundwater system in the Minqin Basin are:a.the carbon source of the groundwater in the Minqin Basin predominantly come from C4 species;b.Carbonatite dissolved under closed conditions.According to the most commonly used ¹⁴C correction models,the radiocarbon groundwater ages were calculated which yield ages of approximately 4 ka B.P.to 36 ka B.P.6.By using the stable isotope(δ¹⁸O andδ²H)in the deep groundwater in the Shiyang River and the Heihe River Basins,the changes of ancient geological environment has been reconstructed.36 ka B.P.- 10 ka B.P.,δ¹⁸O andδ²H values are relatively light,implied that this period was the last glacial period and the climate was cold.The mean surface air temperature is 7.3℃;10ka B.P.-1ka B.P.,δ¹⁸O andδ²H values increased with fluctuation,indicating the unstable climate in the transition from the last glacial period to the Holocene.1 ka B.P.-present,theδ¹⁸O andδ²H values were significant heavier than the average value,reflecting that climate in this period was warm.