Study Of Age Structure And Patterns Of Groundwater Recharge And Groundwater Flow In Hohhot Basin

Groundwater is the main source of water supply in Hohhot basin.After about 30 years'high-intensity extraction,the environment of underground water is damaged seriously,such as groundwater depression cone,shrinked area of artesian water,groundwater pollution,etc.Sustainable utilization of groundwater is facing with a severe challenge.However,as a foundation to identify underground water cyclic process and analyze underground water distribution rules,the study of groundwater recharge and groundwater flow pattern is theoretically and practically important to ensure the sustainable utilization of groundwater in region.The aim of this study is to discuss the groundwater recharge source and flow in Hohhot basin by using the hydrogeochemical characteristics and isotopic features both in the unsaturated zone and aquifer.The sources of groundwater recharge were identified by using hydrogen-oxygen stable isotope and hydrochemical characteristics from different water.Groundwater age structure was established and groundwater flow pattern was determined by comparing the results of multiple dating methods.Then groundwater flow model will be described with traditional hydrogeology methods together to assess on the hydraulic connection and renewability between aquifers.The study mainly emphasizes on three aspects:(1)Construction of groundwater age structure.(2)Identification of groundwater supply and flow pattern.(3)Analyzation of hydraulic connection between aquifers.The main progresses were obtained as below:(1)Groundwater ages were determined.In the part of young groundwater age dating,the methods of 3H-3He,CFCs,3H and 85Kr were used to estimate groundwater age.Discussions have been conducted on the reasons of age difference caused by using different methods,so it reduces errors by using single method and improved the correction of results.In the part of old groundwater age dating,the traditional chemical correction method and netpath method were both used to verify the 14C age.Results from different methods are compared and analyzed to obtain a more reasonable 14C age and improve the correction precision.Then the age structures are set up for the shallow and deep ground water in the basin.(2)Groundwater recharge source and mechanism in the basin are identified by distribution characteristics of hydrogen-oxygen stable isotope.The results show that the main supply sources are rainfall in the mountain area,local rainfall,surface water,return flow from irrigation and part from the Yellow Water,etc in the basin.There is a little different between different regions.In eastern part,local rainfall,surface water and return flow from irrigation is the main supply,while in the northern part,rainfall in the mountain area and surface water is the main source.The main recharge source in the central is local rainfall and return flow from irrigation.The supply system is based on runoff,but some areas are with vertical supply and return flow from irrigation.The recharge mechanism is mainly lateral recharge and shallow groundwater overflow recharge.(3)The natural groundwater recharge rate and integrated groundwater recharge rate were estimated.The natural groundwater re charge rate in the study area is estimated with methods of artificial 3H tracing method and chlorine mass balance method in the unsaturated zone.The groundwater recharge rate in aquifer is estimated with groundwater age.On the basis of comprehensive analysis of these recharge rates,different recharge patterns and their contributions were distinguished,which can provide theoretical supports for reasonable groundwater exploitation.(4)It was found that the shallow groundwater near the northern part of Hohhot city near the boundary of silty layers has leakage recharge to deep groundwater.There are both regional groundwater flow system and local groundwater flow system in Hohhot basin.The regional groundwater flow system can be divided into two groundwater flow systems along the boundary of the plain with lacustrine sediments.Local water flow system mainly exists in the region of groundwater depression cone and leakage flow occurred area.(5)The renewability and aquifer sensitity of groundater system was evaluated.The results show that most of the regional aquifers in the flood plain and alluvial lacustrine plain of the Hohhot basin have medium pollution resistance.Most of the areas in front of Daqingshan and Manhan(except the area next to the Haqinying-Bayan Town in the north of Hohhot)and the regional aquifers near the black urban and rural areas have low antifouling properties.