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Lanzhou University (2014)

Isotopic Characteristics and Dating of the Groundwater in the Badain Jaran Desert

吴月;

Titre : Isotopic Characteristics and Dating of the Groundwater in the Badain Jaran Desert

Auteur : 吴月;

Grade : Doctoral Dissertation 2014

Université : Lanzhou University

Résumé
Groundwater recharge sources and age determining in the Badain Jaran Desert which are a topic of great national and international research interest, because it is very difficult to obtain accurate hydrogeological conditions, transport conditions, sampling difficulty, testing accuracy and other restrictions due to lacks relevant information in the study area. Recharge source, recharge, formation age of lake water and groundwater are inconclusive. In the paper, we analyze the concentrations of major ions and environmental isotope composition of the typical groundwater near lake which tracing recharge sources and age of groundwater. This paper will fill the research blank and provide scientific guidance for the rational utilization water resources in the study area.For research issues about recharge sources of groundwater in the Badan Jaran Desert, we analyze chemical characteristics and hydrogen-oxygen isotopes characteristics of typical lakes and groundwater of the region in January, April, July and October from2009to2014. Firstly, we analyze chemical characteristics of groundwater in the desert, according to the major ions such as sodium, potassium, calcium, magnesium, chloride, sulfate, carbonate, bicarbonate and other ions characteristics of the groundwater in the Desert, imply that the groundwater ion gradual succession from alkali metals-weak acid was the dominant component in southeast to alkali metals-strong acid in northwest, that is, mainly ion is Na+, but Ca2+and Mg2+concentration decreased, while the anion succession from HCO3-to Cl-, that annual and seasonal variations of the chemical characteristics of groundwater are smaller. The hydrochemical types of the groundwater evolved from HCO3—type in southeast to Cl—type in northwest, indicating that the groundwater flow direction.Secondly, according to the characteristics of hydrogen and oxygen isotopes composition of the typical lakes and groundwater from2009to2013in the desert,δD-δ18O diagram shows that isotope values of groundwater and lake water are distributed along the local evaporation line (EL) slope, which is less than the slope of the global meteoric water line. The comparatively small slope shows the characteristic strong evaporation in the study area which leads to enrichment of heavy isotopes, while groundwater isotopes are lower of heavy isotopes than lake water, indicating that the recharge source of lake water and groundwater are uniformly, and the main.recharge source of lake water is groundwater, and the process of groundwater recharge lakes, lake secondary evaporation clearly, more enrichment heavy isotope, and meteoric recharge is less. Utilizing the δ18O-t relationship of Zheng Shuhui to calculate supply temperature that is2.6~8℃, δ18O is three intersection value of the local evaporation line and global atmospheric precipitation line, and average annual temperature is8℃in Badan Jaran Desert, shows that the temperatures of initial precipitation recharge groundwater is lower than the present. The d-value of groundwater reduce gradually from recharge area to drainage area, show that groundwater replenishment from southeast to northwest ; Abnormal negative d-value shows that groundwater formed in the low temperature than now, and the high relative humidity of ancient climate environment.Recently, there are many methods for groundwater dating. Based on the specific hydrogeological conditions, consider safe and convenient while collection and transportation of samples in the desert, we selected tritium and14C to determined groundwater age, and they were also widely-used and more mature methods in the past. Tritium was mainly dating for younger groundwater. We selected two spring sample from April2012and measured a tritium concentration of<0.8TU, compared some tritium concentration of groundwater (Tritium<3TU) from predecessors in the area and its surrounding areas, we think that tritium dating method is not confined for confined groundwater cross the regions. Then, we used14C (dissolved inorganic carbon) of confined groundwater to dating in the study area. Because of dissolved inorganic carbon source of the groundwater are complex, restore the initial14C concentration become a critical issue of groundwater14C-age-adjusted. Selected variety14C-age-adjusted model to solve this problem, and calculated groundwater age in Badan Jaran Desert. By comparing the advantages and disadvantages of various methods, deduced that14C-adjusted-age of the confined groundwater are between1.77~6.87ka BP, and the groundwater samples does not have (14C>60pmc) modern water feature, shows that deep groundwater formed in the humid climate of the Holocene in the desert, precipitation infiltrated formed the ancient groundwater.14C-adjusted-age are significant differences between with Nuoertu, Naogunnuoer, Zhaorigetu and Sumujilin, Taosijilin, indicating that the two sets of data may come from different recharge periods, namely in the humid climate of the middle Holocene, or different hydrogeological tectonic zone or buried conditions lead to different age, or reasons from a combination of DIC with CO2in the atmosphere when groundwater exposed.14C-age change of the groundwater is not only reflected in the changing with depth and the flow direction(namely the flow path), groundwater14C-age is gradually increase with groundwater flow paths, and due to the different geological conditions formed two kinds of exposed form that tufa or no tufa springs.Combined with the δD,δ18O, d-value and Tritium,14C dating results, we basic determined that groundwater in Badan Jaran Desert region formed the middle Holocene, low temperature and wet of ancient climate environment, it is likely to come from high altitude mountainous area of southeast in the desert. According to groundwater tritium concentrations of confined groundwater estimated the refresh rate in Badan Jaran Desert, is <0.1%, combined with14C concentrations to calculated its circulation rate <30m/a, indicating that circulation rate of deep groundwater is slower in runoff process in Badan Jaran Desert, the update rate is very slow, deep groundwater ages are older

Mots clés : Tritium; 14C age; δD-δ18O; Groundwater; Badain Jaran Desert;

Présentation (CNKI)

Page publiée le 30 septembre 2017