漳州盆地水热系统氚同位素研究

STUDY ON TRITIUM ISOTOPE IN HYDROTHERMAL SYSTEM OF ZHANGZHOU BASIN, SE CHINA

本文依据26个天然水样品的氚同位素测试数据,分析了漳州盆地地下热水及其它天然水的氚值特征及其形成条件;利用“活塞模型”方法计算了漳州盆地地下水和地下热水的年龄;为弄清全盆地地下水的补给、迳流和排泄的总体格局和揭示地下热水的成因提供了依据。

In this paper, the authors have selected a hydrothermal system in a typical fault basin on the southeastern coast of China, which is developed on a granitic basement. The basin is considered as an independent hydrogeological unit, and the different types of natural waters in the basin are considered to be different components of a unified water circulation system. Systematical sampling of the waters for tritium determination has been carried out in the basin, which may provide a good example for study on origin and ages of geothermal water of similar systems in other areas.Zhangzhou Basin is located in south part of Fujian Province on the southeastern coast of China which has been developed on a granite-granodiorite basement of late Yanshanian (late Mesozoic), within which a hydrothermal system with its highest temperature of 121.5℃ at 90 m depth has been formed. Tritium data of 26 water samples from the basin were obtained. The Tritium contents of different types of natural waters were determined, and their genetic conditions interpretated. Age of the groundwater is calculated by means of a 'piston model' and results are discussed. The results show that by using the tritium dating method the overall framework of the groundwater flow system of a hydrothermal basin can be effectively determined as well as the relationship between different types of natural waters. The difference in origin of the natural waters in the Zhangzhou Basin is clearly reflected on their tritium contents. Geothermal water is usually low in tritium coctent, but the rain and river waters are always high in tritium content; the nearby seawater is also low in tritium; however, the tritium content in groundwater varies with the location of the water sample along the flow path in the system. The age (residence time) of groundwater has been calculated by means of a 'piston model' and reliable results are obtained in most cases. The groundwater around the geothermal field has an age of more than 30 years. The age of the thermal water is judged to be older than 30 years, which can be used as a primary reference age of the thermal water. Distribution of the tritium contents in different types of natural waters in the basin indicates that the recharge of the Zhangzhou Geothermal Field comes from the periphery of the basin rather than from local vertical penetration of precipitation within the basin.