Isotopic Constraints On Hydrochemical Evolution Of Groundwater In The Ordos Cretaceous Basin

The rich groundwater resources in the Ordos Cretaceous Basin are the important water sources, which have made great contribution to the energy exploration and development of the region. As an important part of the water circulation, complex exchange of material, energy and information takes place between the environmental media and the groundwater in the cretaceous aquifer, and lots of physical, chemical, and biological functions happen during its long circulation, which leads to its water chemical evolution both in time and space constantly. The further research on the hydrochemical evolution of groundwater in the Ordos Cretaceous Basin is benefit for the determination of the exact groundwater quality, the clarification of the utilization value of groundwater, and the revealation of the groundwater circulation regularity and the ecological environment protection.Considering the lithofacies and paleogeographic, geologic and hydrogeologic conditions of the basin, the main influential factors of the groundwater chemical evolution are analyzed with the integrated use of factor analysis, and the water chemical analysis and isotopic (carbon, sulfur, strontium) tracer technology. The paper also analyzes the main sources of the dissolved sulfate and strontium ions, which reveals the main hydrogeochemical processes that affects the evolution of the dissolved inorganic carbon, sulfate and sr isotopes in the groundwater. Besides, the groundwater hydrochemical evolution is also involved. The main results of the paper are stated as follows:1. Based on the summary of the groundwater hydrochemical distribution characteristics of the research area, the hydrogeochemical processes which influence the groundwater hydrochemical characteristics are studied with the utilization of the factor analysis,The main hydrogeochemistry processes include the salt leaching process, the carbonate mineral dissolution, the sulfate mineral dissolution, the silicate mineral dissolution and the cation exchange. The precipitation dilution only happens in the groundwater of Luo Han-dong and Huan river group aquifer.2. Based on summary of the main evolution paths of the dissolved inorganic carbon in the groundwater, combining the carbon isotope characteristics of the aquifer medium, the evolution paths of the dissolved inorganic carbon in the three aquifers groundwater are analyzed quantitatively by the calculation DIC content, the actualδ13CDIC value and pH value.The main surface vegetation coverage of the study area is the C3 vegetation coverage area, with the existence of C3 and C4 vegetation mixed coverage area. The groundwater of the Luo Han-dong, Huan river group and mid-eastern Luo river group aquifer in the north area are in the better open system, with the comparison with the north part.The main evolution path of the dissolved inorganic carbon includes the cation alternate adsorption, the gypsum dissolution and the calcite dissolution in the closed system. The soil CO2 evolution in the open system only occurs in the groundwater of the Huan river group in the north area, and the dedolomitisation only occurs in the groundwater of the Luo river group in south area.3. The analysis on the major sources of the dissolved sulfate are carried out, using the sulfur and oxygen isotopic characteristics of the hydrous media and groundwater in the Ordos Cretaceous Basin. The bacterial and abiotic hydrogeochemical processes which affect the evolution of the dissolved sulfate are revealed.The dissolved SO42- mainly derives from the atmospheric precipitation, the sulfate minerals dissolution and the sulfide mineral oxidation. In addition to the evaporation, the main hydrogeochemical processes which affect the evolution of the dissolved sulfate in the groundwater include the sulfate minerals dissolution, the microbial sulfate reduction and the sulfide mineral oxidation. The influences of the both are different. The sulphate dissolution exists widely, and causes more influences; and the microbial sulfate reduction only occurs in the Luo river group groundwater in the east of the north area and west of the south area, the enrichment coefficientεis 11.5‰and -17.4%o. respectively.4. the primarily sources of Sr in the groundwater are captured by Sr content and 87Sr/86Sr ratios of hydrous media and groundwater. The water-rock interaction which affects the evolution of the Sr isotope in the groundwater are also revealed.Sr in the groundwater is primarily derived from the carbonate mineral, the sulfate mineral and the silicate mineral dissolution. Furthermore, the contribution of the plagioclase is much more than the potassium feldspar in the silicate mineral.The evaporation and the water-rock interaction effect the hydrochemical evolution of the groundwater on some degree. The main water-rock interaction not only includes the carbonate mineral and the sulfate mineral dissolution which were revealed formerly, but also the silicate mineral dissolution, among which the plagioclase dissolution effects obviously.