Study On Hydrogeochemical Evolution And Circulation In North Of Ordos Cretaceous Groundwater Basin

The Ordos Energy Base that distribute in the north of the Ordos Basin is the key area of construction of China's Energy Base. The current construction of Ordos Energy Base is beginning to take shape. However, the demand of ground water resources development has become more and more urgent as the lack of local surface water resources for expanding scale of the Energy Base. Therefore, the effective and rational exploitation of groundwater have become the most critical issue for the groundwater resources development and utilization of the Energy Base. Through the study of the chemical constituents exist in groundwater and the characteristics of the evolution of the environmental isotopes, which base on the system analysis of meteorological, hydrological, geological and hydrogeologic conditions in the northern part of the Ordos Basin, to reveal the genetic mechanism, occurrence form, hydrogeochemical evolution and recycling characteristics of the Cretaceous groundwater from the hydregeochemical point of view. The study not only has profound theoretical significance, but also has important practical value for the construction of the Ordos Energy Base.The northern Ordos Basin is a typical temperate continental climate with low but concentrated precipitation, intense evaporation and a few more days wind. The aquifer system in the study area is constituted by the Quaternary water-bearing rock series and Cretaceous water-bearing rock series, and it can be divided into Salawusu, shallow, intermediate and deep aquifers in the vertical according to the aquifer system attributes and characteristics. Groundwater flow in the study area is controlled by the terrain elevation, and surface and groundwater watershed is almost consistent. Under the control of the regional surface watershed in Anbian-Sishililiang-Etuokeliang, Xinzhao and so on, three different patterns of groundwater flow systems in the study area are identified, including Wulanmulun river-Wuding river in the east, Molin river-Yanhaizi in the north and Situ river-Beidachi in the west. The study of groundwater hydyogeochemical evolution characteristics in the study area in this paper mainly focuses on four aquifer systems and three run-off areas.The study of hydrochemical spatial distribution characteristics and cluster analysis results in the study area show that most of the total mineralization degree of groundwater in the Salawusu groundwater system and Wulanmulun river-Wuding river groundwater system that lies to the east of the Anbian-Sishililiang-Etuokeliang watershed is less than 1g/L, and the chemical type of groundwater mainly is HCO3 type. The groundwater in groundwater system of Molin river-Yanhaizi in the north and Situ river-Beidachi in the west which lies to the west of the watershed mentioned above, whose total mineralization degree increase up to 5g/L or more along the watershed to the southwest and northwest, and the chemical type transit from HCO3 type to HCO3·Cl type, HCO3·SO4·Cl type, SO4·Cl type and Cl·SO4 type.The isotopes exist in groundwater provides good evidence for the hydrogeochemical evolution. Through the study of the isotopes ofδ18O,δD,14C,δ34S and Sr in the groundwater have shown that groundwater of different depth in the study area mainly recharge from the precipitation, and effect by evaporation to some extent. The overall spatial performance of middle and deep groundwater age have illustrated that the age of groundwater become older and older with the gradually increasing depth of groundwater, and groundwater age that to the west of the Anbian-Sishililiang-Etuokeliang watershed is apparent older than that to the east. The mineral dissolution and precipitation intensity is weak in the Wulanmulun river-Wuding river system as the high velocity of the groundwater flow, however, the lixiviation enhance with the depth increasing in Molin river-Yanhaizi aquifer system and Situ river-Beidachi Aquifer system.Hydrogeochemical research of the typical profile in the study area is proposed for the wider study area of the issue, and the performance of its advantage in further found different aquifer systems and different hydrogeochemical evolution characteristics. The results have shown that the hydrochemical characteristics of the profile are consistent with the overall research characteristics that mentioned above. Groundwater are subjected to the surface watershed of Anbian-Sishililiang-Etuokeliang in the two sections, and its characteristics of the hydrochemical and isotopes distribution are obvious different in the both sides of the watershed. To the west of the watershed, groundwater show clear zonation as the different of the total mineralization degree of the shallow, middle, deep groundwater, hydrochemical type and isotopes characteristics, while to the east of the watershed, the zonation of the groundwater is not obvious as the similar of the hydrochemical and isotopes characteristics. Through the reverse hydrogeochemical modeling, to the east and west sides of the Anbian-Sishililiang-Etuokeliang watershed, because of the different intensity of water-rock interaction, to the west of the watershed, the mineral mainly dissolve into the water, while mineral mainly precipitation from the groundwater to the east of the watershed.Through research the hydrogeochemical evolution of groundwater in study area in order to validate the groundwater cycle rule in the Ordos Cretaceous aquifer and the validated results show that the groundwater cycle rule derived from the hydrodynamic field is consistent with the rule from the water chemical field, reaching unity between the hydrodynamic field and the water chemical field. Precipitation is the main supply source reached to the aquifer in this area. In the runoff process, groundwater in the shallow, middle and deep aquifers have all been affected by the surface watersheds which are named as Anbian-Sishili girder-Otogs girder watershed and Xinzhao watershed. In the two groundwater systems in the west of Anbian-Sishili girder-Otogs girder watershed, groundwater accepts recharge around the watershed. In the Molin river-Yanhaizi groundwater system, however, the groundwater is discharged from the northwest of Molin river and Yanhaizi. And, in the groundwater system called Dousitu river-Beidachi, groundwater flows into the downstream of Dousitu river and Beidachi Inner flow area, both of which are in the southwest of this groundwater system. Based on the rule of water chemistry which has been researched, the hydraulic connection among the shallow, middle and deep aquifers is comparatively close and the update rate of groundwater is relatively quick and groundwater flow towards the southeast along the watershed in the east of Anbian-Sishili girder-Otogs girder watershed. Evaporation of groundwater, rivers, lakes and mud are all the ways of groundwater discharge in our study area.Finally, groundwater exploitation and planning projects are given according to the research results of hydrogeologic conditions, hydrogeochemical evolution and groundwater circulation characteristics. Salawusu aquifer, shallow and middle aquifers serve as the major exploitation aquifers, there are 8 water sources in Salawusu aquifer,24 water sources in shallow and middle aquifer, while the deep aquifer serves as the deposit exploitation aquifer. Meanwhile, the area that located in the vicinity of Anbian-Sishililiang-Etuokeliang and Xinzhao watershed, where the aquifer always have a large quantities of groundwater with good quality, and is the ideal area for construction of the water source.