Isotopic Tracing Of Hydrogeochemical Evolution Of Groundwater In The Northern Part Of Ordos Cretaceous Basin

As China's important base for energy resources and chemical industry, Ordos Basin played an important role in western development strategy. However, effected by arid and semi-arid climate conditions, the contradictions between water resources supply and demand are outstanding. North region of Ordos cretaceous basin, which is rich in groundwater resources, is the major water supply sources in this region in the future. As drinking water source, groundwater should not only satisfy quantity demand, but also quality demand. In order to guide water resources development in North region of Ordos basin, avoid and delay water quality deterioration in the process of water resources development, Isotopic tracing investigation of groundwater hydrochemistry evolution in North region of Ordos cretaceous basin is developed.Based on hydrogeology condition and groundwater cycle pattern in North region of Ordos cretaceous basin, this paper analyzed spatial distribution law of TDS , major ions and hydrochemistry type of groundwater in different aquifers, and preliminary determined main functions of hydrochemistry formation by further combining with mineral saturation index, ion-combination and ratio methods. In order to determine the accuracy of the conclusion, this project revealed the sources of carbonate, Sulfate and Sr by utilizing efficient tracer ability of Carbon, Sulfur and Strontium isotope. By summarizing and analyzing above results, this paper got the main hydrochemistry formation of groundwater in North region of Ordos cretaceous basin. The following conclusions were given: The groundwater of surface divide and its east area in AnBian- Sishililiang-Dongshengliang beam was low in TDS and major ions, and the hydrochemical type was mainly HCO3, while TDS and major ions of the groundwater in the west area of AnBian-Sishililiang-Dongshengliang continually increase toward the boundary, hydrochemical type gradually transisted from HCO3 to HCO3?SO4?Cl or SO4?Cl, and water quality gradually deteriorated.TDS and major ions of Groundwater in west and north area in Etuoke- Sishililiang manifest the trend of increasing with depth. Whereas, the east area manifested homogenicity characteristics.Groundwater hydrochemistry formation effects in North region of Ordos cretaceous basin included lixiviation, evaporation function, cation alternative adhesive action and mixing action and so on. Among them, lixiviation was the major formation function, which was major in carbonate dissolution such as calcite, dolomite and silicate mineral lixiviation, then gypsum dissolution.The difference of geomorphology, vegetation types and coverage made a great difference in spatial distribution of shallow groundwater dissolved inorganic carbonδ13C. Variation of middle and deep groundwater dissolved inorganic carbonδ13C was mainly effected by matrix exchange of carbonate mineral in water-bearing media and carbonate dissolution. According to analysis of groundwater dissolved inorganic carbonδ13C, it could be found that the evolution ofδ13C in groundwater was mainly effected by calcite dissolution, then dolomite dissolution.Different solubility of silicate, carbonate, sulfate and influence of meteoric water, both the content of Sr and isotopic composition in groundwater had a large spatial difference. In the depth range of Modern water active circulation, Sr in groundwater was mainly from precipitation, and reflected isotope ratio of meteoric water; whereas groundwater in lower part is mainly effected by water-rock interactions, strontium mainly originated from carbonate and sulfate dissolution, some areas might be effected by silicate mineral dissolution such as feldspar. According to analysis ofδ³⁴S-SO4 andδ18O-SO4 value and distribution characteristics of the content of SO₄^2- , it could be found that sulfate in groundwater in the depth range of Modern water active circulation originated from meteoric water, while groundwater hydrochemical components within influence region of the lower relic water originated from soluble salts dissolution such as silicate mineral in water-bearing media. Sulphate in groundwater in local areas originated from biological sulfur oxidation.