Groundwater Cycle Pattern And Renewability Evaluation In The Jinghe Watershed Of Ordos Cretaceous Basin

Due to the arid and semiarid climate, shortage of water resource has been the uppermost obstacle to the construction of energy source base.Because groundwater is the principle source of the water resource in Jinghe basin, so it's significant to evaluate the groundwater scientifically and develop it reasonably. Therefor this study has very important significance in practice and theory.Study on the groundwater circulation and renewability is the important basis of evaluating groundwater resources reasonablely and managing it scientificly.Environmental isotope can trace water cycle effectively because isotope can sign and dating water cycle.Futhermore using hydrogeochemistry combine with isotope can make characteristics of water cycle more clear.According to the ideas above,we analysis relationship of different type of water synthetically,cause of formation of Cretaceous groundwater and condition of groundwater recharge,runoff and discharge,also determine the relationship of Cretaceous groundwater inside the basin and karst groundwater in the west boundary based on the isotope and chemistry analysis result of water samples in the Jinghe basin.Base on research above,set up the groundwater circulation pattern in representative section,then calculate the age and flow velocity of Cretaceous groundwater,finally evaluate the renewability of groundwater. The main achievements of this paper are as follows:(1)The distribution law of surface water and groundwater chemistry are analyzed qualitatively.Most of the TDS of surface water is less than 1g/L,except Malianhe river. Transformation of surface water type is complex,except Malianhe river,most of water type of surface water is HCO₃ type water or HCO₃·SO₄ one such as Puhe river,Jinghe river and Heihe river.With the direction of the Malianhe river, the transformation of water thpe is that Cl type water→SO₄·Cl one→Cl·HCO₃·SO₄ one→Cl·HCO₃ one→HCO₃ one。From the south of the Huan city of Malianhe watershed, Creataceous groundwater recharge Malianhe river from east and west limes to axis of the Tianhuan synclinal in cential section,and in the catchment area of Jinghe river(130-200m segment), groundwater recharge surface water.The values of TDS and other six different kinds of ions in three different aquifers are generally high in the northwest while low in the southeast. The distribution of the groundwater chemistry in Luohandong aquifer is in reverse with the direction of groundwater flow, the transformation of groundwater types is that SO₄?Cl thpe water→HCO₃·SO₄·Cl one→HCO₃?SO₄ one→HCO₃ one. Groundwater types of Huanhe aquifer and Luohe aquifer are characterized with the Huan Country-Qingyang Country-Xunyi Country and Malianhe River as the axis. The groundwater types in the west of the axis change from the southwest to the northeast in the order HCO₃ type water→HCO₃·SO₄ one→SO₄ one→HCO₃·SO₄·Cl one→SO₄·Cl one, and in the east of the axis they change from the northeast to the southwest in the order HCO₃·SO₄ type water(partly distribute in Huanhe aquifer)→HCO₃·SO₄·Cl one→SO₄ one(partly distribute in Huanhe aquifer)→SO₄·Cl one.The vertical distribution of the groundwater chemistry shows the delaminating characteristics of the Cretaceous groundwater. (2)Analysis the characteristic of isotope in different kind of water.Because of the arid and semiarid climate,the slope and intercept of the local meteoric line (LMWL) in the studying area are lower than that of global meteoric water line (GMWL).The values ofδ¹⁸O andδD of all type of water distribute along the local meteoric line, this indicates that both the surface water and groundwater in the basin are recharged by precipitation.The values ofδ¹⁸O,δD along Malianhe river are increasing and ³H is decreasing.It shows that to the north of Qingyang City, precipitacion is the main origin of the surface water,while to the south of Qingyang city, groundwater charge surface water intensely and it becomes groundwater drainage area. The values ofδ¹⁸O,δD and 3H along Jinghe river is decreasing as a whole.It shows that downriver of Jinghe is groundwater drainage basin.Theδ¹⁸O andδD values of groundwater in each aquifer cover obviously different scope,which shows the delaminating characteristics of theδ¹⁸O andδD isotopes in some degree, and the vertical hydraulic contact is weak, and the lateral flow is the dominate runoff method.(3) Determine the direction of groundwater flow between the west boundary- Liupanshan Donglu karst area and inside the Cretaceous basin by the isotopic and hydrogeochemistry method and investigate the groundwater cycle pattern of representative section.The values ofδ¹⁸O,δD and 3H is higher in the Karst groundwater area and is decreasing along with the groundwater flow to the inside of basin.The research of the isotope and hydrogeochemistry indicate that Cretaceous groundwater in Jinghe groundwater system has accepted the recharge from the karst groundwater in the west boundary- Liupanshan Donglu karst area. The direction is from northwest to southeast, and concentrate in middle east of the basin to Jinghe river. From the level of different aquiferous groundwater in the representative section, combine with geological and hydrogeological condition and the differences of vertical distribution law of isotope and hydrogeochemistry of B5,B10 and B12 bores can prove that the characteristics of vertical groundwater flow in this section is delaminated piston flow, horizontal characteristics is such as the Centripetal pattern that the flow is from the east and west boundary to Malianhe river region and throuth the direction of groundwater flow the groundwater is older and the velocity is slower. Groundwater cycle system can divide into local cycle system,middle cycle system and regional cycle system.The flow in Local cycle system is short and the velocity of the flow is fast.The middle cycle system is developed in Luohandong and Huanhe aquifer and the depth of the circulation system is not so big.The regional cycle system is mainly developed in Luohe aquifer and the direction of the flow is from east,north and west to Jinghe and Malianhe river.(4)Calculate the age of 3H Cenozoic groundwater,corrected the 14C age of Cretaceous groundwater and calculate velocity of the groundwater cycle along the different flow path.Two reference curve method have been employed to recovery of the tritium concentration in the precipitation, then using grey series forecasting method to calculate the tritium concentration both before 1960 and after 1986. Employing the piston flow model, protract the chart of mutative trend of tritium concentration in different years, according to the chart estimate the age of Cenozoic groundwater. The result is that: to the north of Qingyang City, age of of Cenozoic groundwater is 23 years,while to the south of Qingyang City is 46 years.It shows that renewability of sallow groundwater to the north of Qingyang City is stronger that to the south of Qingyang City.Corrected the 14C age of middle and deep groundwater employing different correcting models, then according to equipotential line choosing different flow path, finally calculate the velocity of the groundwater circulation along the different flow path. The result is that: the velocity of the groundwater cycle in Luohe aquifer is about 5.10～10.78m/a; the velocity of the groundwater cycle in Huanhe aquifer is about 14.41～23.18m/a; the velocity of the groundwater cycle in Luohandong aquifer is about 26.96m/a. The velocity of the groundwater in the same aquifer is maximal in Malianhe river watershed.(5)Using the calculated age of groundwater,residence time of groundwater and the velocity of the groundwater cycle,evaluate the renewability of the Cretaceous groundwater.We finally get the conclusion that the renewability of groundwater distribute in this order: the renewability in the Luohandong aquifer is the best, then the Huanhe aquifer,finally the Luohe aquifer.And the renewability of Cretaceous groundwater in the same aqufer is that in Malianhe river watershed is stronger than the other area.Luohandong aquifer has the best renewability, which has the shallowest embedding, heigher conductivity and better recharge condition of groundwater; Huanhe aquifer has the smaller renewability than Luohandong aquifer, where granularity is smaller and penetrability is poorer; Luohe aquifer has the worst renewability, which has the deeper embedding, the older groundwater and poorer cycle of modern water. Due to the stronger cycle and alternation of groundwater, Malianhe area has the better renewability than anywhere else, which is located in the axis of Tianhuan syncline as the discharge area of groundwater in the studying area.