| Dongda area located in the piedmont of Qinling Mountain is typical recharge area of geothermal water in Guanzhong basin. In recent years, blind extraction results in significant attenuation of geothermal water yield of 20%~32%, study on the origin and genetic model of Dongda geothermal water is vital to the supply mode and sustainable development and utilization of Guanzhong basin geothermal resources.Base on the features of geothermal geological structure, environmental isotope and hydrogeochemical methods are used to study the origin, circulating, genetic model and updating of geothermal water of Dongda area; Analyzing the evolution of isotope characteristics of geothermal water from south Qinling Mountain to north Wei River to classify geothermal water according to ability of updating will provide a scientific basis for the sustainable development and utilization of geothermal water in the piedmont Qinling Mountain and Guanzhong Basin.Geothermal water of Dongda belongs to SO4·HCO3-Na type, which has different chemical composition with cold water(HCO3·SO4-Ca type), indicating that geothermal water in the process of flow is influence by mineral dissolution, ions exchange and cold water mixing effect, the average cold water mixed quantity was 41.8%; Element proportional coefficient shows the environment of geothermal water is relative open; Na-K-Mg triangular diagram indicates that ion exchange between water and rock is still under equilibrium, geothermal water may mixed with cold water or mineral is still dissolving; Combined with quartz geothermometer and multi-mineral balance method, the reservoir temperature is 67℃~110℃, the largest hydrothermal circulation depth is between 1687 m and 3120 m.The δ values of geothermal water are distributed near meteoric water line, indicating the origin of precipitation. Recharge elevation of geothermal water should be from the southwestern Qinling Mountain with 679.90m~1445.65 m high; δ18O of geothermal water in Dongda is not relevant to temperature and depth, which is different from that of Guanzhong Basin, indicating that faulted structure is the key factor affecting the enrichment of δ18O. 3H of geothermal water is from 2.4 to 10.3TU, which has no significant relationship with depth, indicating that geothermal water is mixed with cold water and water conducted zone is the dominant factor; the average residence time of tritium model is 71 a. Helium isotope indicates helium gas in geothermal water came from crustal source, which ruled out the possibility of mantle source.Through analysis of typical section of environmental isotope and hydrogeochemical characteristics of geothermal water and cold water in the study area, we know the circulation and conversion relationship between geothermal water and cold water, genetic mechanism conceptual model of geothermal water of Dongda is created. According to the evolution of isotope characteristics, thermal environment, water-rock interaction degree and mean retention time, deep geothermal water from Qinling Mountain to Wei River can be divided into: a. good update ability of Dongda from recharge area in the piedmont of Qinling Mountain; b. general update ability from runoff area in the south of Xi’an depression; c. poor update ability from discharge area in the north of Xi’an depression. |
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