Groundwater System And Water Circulation Pattern In Ordos Cretaceous Groundwater Basin

Ordos basin contains a wealth of natural resources including coal, petroleum and natural gas and it is one of the most important bases for China's energy and heavy chemical industries in the 21st century. Unfortunately because of arid and semi-arid climate and rare precipitation and strong evapotranspiration, there is a severe shortage of water resources in Ordos bsin that has hampered industrial development and eco-environmental improvement and adversely affected people's livelihood. Ordos Cretaceous basin is a independent large scale groundwater basin overlapping Gansu, Shaanxi, Ningxia and Inner Mongolia provinces or autonomous regions, where the large amount of groundwater resources is the major supply water sources for local energy exploration and energy base construction. In order to develop groundwater resources sustainably, prohibit the occurrence of geo-environmental problems and meet the targets of sustainable development of resources, environment and society, this thesis has carried out the research on the groundwater system in Ordos Cretaceous basin based on the groundwater theory and the local hydrogeological settings. The principles and methods of aquifer system and groundwater flow system division have been put forward and the regional groundwater circulation pattern by means of groundwater dynamics and hydrogeochemical and isotopic data and the strategies of groundwater resources development have also been suggested.Ordos Cretaceous basin is dominantly a large scale synclinal sedimentary basin composting of different aquifer formations. Topography, geological structure and litho-paleogeography conditions control the formation of groundwater and distribution.Bordered by Baiyu Mountain in the central basin, the north and south parts of Ordos Cretaceous basin have different attributes in groundwater hydraulics. Cretaceous aquifer system in north basin is dominated by sandstone with a super-thick water-bearing unity of single hydrogeological structure and the north basin is typical of phreatic groundwater basin as the result of no continuous aquitard. South basin has clear rhythmicity and obvious aquifer stratifying. Because there is a weak hydraulic connection among aquifers, the south basin is typical of artisan basin. Based on the theory of groundwater system and methods of groundwater system division, the research results of Cretaceous litho-paleo-geographical features and interpretation results of aquifer media from 436 petrol boreholes, the aquifer system of Ordos Cretaceous basin can be divided into two subsystems and seven aquifer formations. The principles and methods of division of aquifer systems, subsystems, aquifer formations have also been suggested. Based on the characteristics of groundwater flow regime and hydrogeochemistry, groundwater flow system in Ordos Cretaceous basin have been divided into five flow systems and nine subsystems.Controlled by litho-paleo-geography, hydrogeological structure and groundwater circulation conditions, the hydrogeochemical features in Ordos Cretaceous basin are different in north and south part of the basin bordered by Baiyu Mountain. The groundwater in north part is typical of fresh water with low TDS while in south part typical of saline water with high TDS. In addition, the oxygen and hydrogen isotope ratios of groundwater are also different in north and south basin. Heavy oxygen and hydrogen isotopes are enriched in groundwater in north basin while depleted in south basin. The 18O and D in groundwater is characterized by obvious stratifying in vertical direction in south basin and becomes more negative with the increase of the groundwater buried depth, while the 18O and D in groundwater is homogeneous in north basin.The shallow groundwater is typical of high tritium concentration and high 14C activity. The average of tritium concentration in shallow groundwater in north and south basin is about 22TU and 9TU respectively, very close to that of modern precipitation, which signifies that the shallow groundwater is recharged by modern rainwater. However, middle and deep groundwater is typical of low tritium concentration and 14C activity and the tritium concentration is below the detected limit of tritium, which shows that middle and deep groundwater is recharged in geologic periods. The 14C age of middle is mostly less than 5000a and deep groundwater mostly ranges from 5000 to 30000a and older than shallow groundwater and becomes older and older towards the discharge zone.Groundwater in Ordos Cretaceous basin is of meteoric origin and recharged directly and indirectly by precipitation infiltration. The groundwater flow direction is mainly controlled by topography, recharge condition and drainage system. The groundwater circulation patterns are different in north and south basins bordered by Baiyu Mountain. In north basin, groundwater is recharged in surface divide and move in horizontal and vertical direction, then converges into the drainage system such as Dusitu river(western basin),Wuding-Wulanmulun river( eastern basin) and Muling river(northern basin),while in south basin, the groundwater flow regime is not controlled by topography. Groundwater is discharged towards the erosion base plane such as Jing river and Luo river. In north basin, groundwater is discharged dominantly by evaporation and discharge into the surface water bodies after recharge in the east and west margins of the south basin. While in south basin, groundwater is dominantly discharged into river drainage systems.On the basis of the groundwater circulation depth, groundwater flow regime and hydrogeochemical and isotopic features of groundwater, the groundwater flow system can be divided into three patterns, that is, shallow circulation pattern, middle circulation pattern and deep circulation pattern. The shallow circulation pattern is characterized by fast circulation, high renewal capability and good water quality with the circulation depth of no more than 150~200m and the groundwater age of less than 20~50a. The middle and small scale lakes are often the discharging base planes for shallow circulation groundwater and the range of shallow groundwater circulation should be served as the firstly groundwater development sites. Middle circulation pattern is characterized by the circulation depth of 550m and groundwater age of 5000a. The surface water and large scale lakes are the discharging base planes for the middle circulating groundwater, which should be developed no less than the available amount. Deep circulation pattern is characterized by low flow velocity, long flow distance and large circulation depth of 800~1000m and groundwater age of 5000~30000a.Deep circulating groundwater should only be developed as the urgent water source.In Ordos Cretaceous basin, the amount of total natural resources are 70.03×10~8m~3/a, and the amount of available resources is 42.41×10~8m~3/a. The current mining rate is 7.29×10~8m~3/a and the potential is 35.12×10~8m~3/a. Groundwater development should be based on the principles of environmental protection and surface water resources and groundwater resources should be considered together. The advantages of wide and steady distribution, good water quality and ease to be developed must be paid more attention to and the local groundwater resources should be developed sustainably. Because of the semiarid and arid climate and fragile ecological environment in Ordos Cretaceous basin, some measures should be taken, such as monitoring groundwater, managing the major aquifers and prohibiting the occurrence of geo-environmental problems, to meet the needs of the sustainable development of resources, environment and society.