| Guanzhong Basin is located at the southern edge of Ordos Block, and is a special geological body between Ordos Plateau and Qinling Mountain. Since the Cenozoic, there forms a complicated geothermal system under the control of geological evolution. Geothermal water is the product of interaction among geothermal energy, groundwater and rock minerals. It can indicate the change of geological environment, and also reflect the scale and intensity of human activities related geothermal water. The exploitation and utilization of geothermal water has a long history in Guanzhong Basin, but owing to lack of the scientific management, which not only changes the natural occurrence and circulation conditions of geothermal water, but also causes a series of environmental problems, such as resource exhaustion, environmental pollution, land subsidence and earthquake. Therefore, "how to exploit and utilize geothermal water effectively, safely and continuously but to minimize its environmental impacts?" is one of the important questions need to be answered urgently. In order to know about the flow, occurrence, geochemistry of geothermal water and its environmental impacts, firstly this paper systematically analyzes geological conditions of geothermal water, secondly studies the evolution of hydrochemical elements and environmental isotopes, and finally evaluates the geothermal temperature, hot water quality and its environmental impacts.The study indicates the hydrochemical elements and environmental isotopes have the spatial evolution characteristics of horizontal sub-band and vertical sub-stratification under the impacts of the regional topography, lithology, geological structure etc. Namely, in the horizontal direction, from the marginal basin to the central, with the increase of depth, TDS,δ18O,δ13C,14C, andδ34S of geothermal water gradually, the hydrochemical type is represented as HCO3-Na type to SO4Na or Cl-Na type. The evolution characteristics indicate in the centre of Guanzhong Basin, the geological environment is closed and deoxidized, the residence time of geothermal water is long, and water-rock interaction is strong with obvious 18O drift; in the vertical direction, the hydrogeochemical evolution characteristics are the same as the horizontal, and simultaneously with sub-stratification evolution characteristics from the shallow to the deep. It indicates different reservoir has relatively independent circulation system. According to the recharge, runoff and discharge of geothermal water, the paper respectively selects the representative flows in the northern and southern Basin to quantitatively research on hydrochemical evolution by mass balance simulation. The research results show:in Guanzhong Basin, the main hydrogeochemical processes are the precipitation of kaolinite, calcite and chalcedony; the dissolution of rock salt, gypsum, albite and fluorite; the cation exchanges in the runoff or discharge zones; the mixture of cold water and geothermal water migrating from the deep to the shallow along the fault zones.On the basis of the hydrochemical data, the paper estimates the reservoir temperature and evaluates geothermal water quality. The results show:in the horizontal direction, from the marginal to the central basin, the reservoir temperature, the medical value, the corrosion to carbon steel, and the degree of exceeding the drinking standard are all increasing gradually; and the same evolution characteristics are shown from the shallow to the deep in the vertical direction. In a word, it is consistent with the hydrogeochemical and Isotopic evolution characteristics.The environmental impacts from exploitation and utilization of geothermal water are the important part for the research. The main conclusions are:the static reserves in geothermal system are over-exploited in Guanzhong Basin, and then caused geothermal water level is dropping continuously, the regeneration ability is weakening, and geothermal water is exhausting; In geothermal tail water, many components such as Temperature, F", Cl", SO42-and TDS all exceed the relative standard for discharge, so the direct discharge of geothermal water already causes some pollutions on the surrounding water, soil and ecology environment.By calculating subsidence caused by exploitation of geothermal water, it can be seen that from 1995 to 2000 in Xi'an geothermal field, the average subsidence rate on the original Xi'an zoo Block is 12.76mm/a, and 7.96mm/a on Small Wild Goose Pagoda Block. The calculation results indicate in Xi'an area, the over-exploitation of geothermal water can cause land subsidence with slow, hidden and unrecoverable. Moreover, the study also shows a positive correlation between earthquake and the exploitation amount of geothermal water, and the shear stress from the fault dislocation is usually released by the small earthquakes M=2-3 or M=1-2.Considering above environment problems, the paper advises:the exploitation and utilization of geothermal water should regard "sustainable development strategic thinking" as the guidance to strengthen the research of the basic theory, to improve the dynamic monitoring of geothermal water and geological environmental monitoring system, to implement the cascade development and comprehensive utilization of geothermal water, to scientifically plan and manage geothermal water, and to ensure the sustainable exploitation and utilization of geothermal water in Guanzhong Basin.
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