Groundwater Hydrochemical Characters And Hydrogeochemical Processes Of Ulan Buh Desert

As an unique landscape of arid and semi-arid area in China, the formation of desert is affected by climate change and human activities. It is vital to research groundwater recharge and evolution as well as the response mechanisms between groundwater and climate for investigating climate change and hydrological cycle in arid areas in Northwestern China. Ulan Buh Desert is located in the western Alashan Plateau.The study investigates regional hydrogeochemical spatiotemporal variability and evolution, to show interrelationship between groundwater and environment, and then analysis possible recharge sources of groundwater, according to hydrogeochemical analyses, isotopic evidence and reactive path simulation of inverse geochemistry method. The research results provide a theoretical basis for the sustainable use of groundwater resources in arid area.We concluded:(1) Based on hydrogeochemical analysis, it is revealed that from recharge zone to runoff zone, and then to discharge zone, chemical types evolve from HCO3-Ca-Na to HCO3-Na and Cl-Na. Along the flow path, groundwater chemical type has vertical stratification: shallow and middle groundwater chemical type mainly is Cl-Na, while HCO3-Ca-Na, HCO3-Ca, Cl-Na in deep groundwater. According to analysis, shallow groundwater chemical composition is controlled by evaporation/inspissation.However, deep ground water is controlled by water-rock interaction and ion exchange. By analyzing the geochemical characteristics of groundwater hydrology, we can conclude the groundwater recharge sources and mechanisms in Ulan Buh Desert groundwater is infiltration of bedrock fissure water from Bayan Ulan Mountains and Helan Mountains, lateral seepage recharge from the Yellow River, leaking recharge between different aquifers.(2) The δ2H and δ18O of Ulan Buh Desert groundwater has layered characteristics: average value of δ2H and δ18O decrease with increasing depth. Ulan Buh Desert Local Meteoric Water Line isδ2H=6.93δ18O-8.68(‰, VSMOW, R2=0.95), Evaporation Line is δ2H=5.00δ18O-35.14. Groundwater is recharged by precipitation in low humidity conditions. During recharge process, groundwater evaporate intensively. The isotope research providesanother recharge sources: indirect recharge of precipitation and mixing recharge in colder climate.(3)We use Netpath to calculate groundwater 14 C age, and correct groundwater age by Vogel, Tamers, Pearson, Fontes and Garnier, Evans, Eichinger,Gonfiantini model. Based on Pearson, Fontes and Garnier modle, study area 14 C age range from 22000 years to now, which is Quaternary Period on the geological chronology, including early Holocene and late Pleistocene. Groundwater is recharged in a colder environment.(4) We choose Ca, Mg, Na, K, S, Cl, C as constraints and dolomite, calcite, illite, gypsum, ion exchange, Na Cl as phases in reactive path simulation of inverse geochemistry method. 5 flow paths were chosen when we ues Netpath to simulate. The reaction mode of Ulan Buh Deasert groundwater is: initial+gypsum+Na Cl+ion exchange+evaporation=final+dolomite+calcite+illite. Reactive path simulation of inverse geochemistry method indicates that the formation of chemicals in groundwater is affected by lixiviation, ion exchange, evaporation and mixing between groundwater of different depth.