应用环境同位素研究地热水起源和补给机制  

Application of Environmental Isotopes to Studies of the Origin and Recharge Mechanism of Geothermal Water

作　　者：王坤[1] 王东升[2] 

机构地区：[1]北京大学地球与空间科学学院,100871 [2]中国地质科学院水文地质环境地质研究所,石家庄050061

出　　处：《地质论评》2002年第S1期203-209,共7页Geological Review

摘　　要：研究地下水补给机制,须确定补给水来源、补给区位置、补给期、补给通道和补给速率等。应用环境同位素研究地下水补给,是以水中同位素组成随空间和时间的变化为基础。一方面,同位素组成在从热源向采样点运移途中不受物理化学过程影响,基本保持不变,因此,适于作标记或示踪剂;另一方面,它们对于温度变化、水-岩相互作用、蒸汽散发、不同来源水的混合稀释和表面蒸发十分敏感,因此,适于作这些过程的地质指示剂。在地热调查中用得最多的放射性同位素是氚和^(14)C,它们可用来测定地热水年龄,或查明近期大气降水所占份额。在循环型地热水中,在δD值和δ^(18)O值之间具有类似于现代大气降水的线性关系,其δ^(18)O值略高于当地大气降水,其δD值低于当地大气降水,δD和δ^(18)O值随纬度和高程变化。据20世纪80年代末检测,其氚比值一般少于1T.R.。由此可知,循环型地热水是老水(前核试爆水)。事实上,华北平原深层循环型地热水的^(14)C年龄逾20000a。因此,其贫氘特征则指示古大气降水的补给发生于逾20000a BP的冷湿期。在封存型热卤水中,Cl-Na型热卤水的^(40)Ar/^(36)Ar值为大陆型值域,δD值域接近于在硬石膏化过程中脱出的石膏结晶水的值域,说明在地层剖面中巨厚硬石膏形成时由石膏脱出的结晶水参与了热卤水的形成;而Cl-Ca型烃矿床卤水贫氘富钙,其氘和^(18)O接近初生水值域,^(40)Ar/^(36)Ar值接近海岭型值域,反映水和烃二者均来自上地幔。The study of recharge mechanism includes the source and location of the recharge water, as well as the period, passage and rate of recharge. The application of environmental isotopes to problems of groundwater recharge is based on the spatial and temporal variations of the isotope compositions of water. On the one hand, some isotope compositions remain largely unaffected by physical and chemical processes during their migration from the source to the sampling site, which enables them to serve as markers or tracers. On the other hand, their sensitivity to variations in temperature and the effects of rock-water interaction, steam dispersion, dilution and mixing of water from different origins, and surface evaporation renders them suitable to be geoindicators for these processes. The radioactive isotopes most commonly used in geothermal investigations are tritium and ^(14)C. They may be used to date geothermal waters or detect the contributions from recent precipitation. In the cycling geothermal water,there is a linear correlation between the δD values and the δ^(18)O values similar to that of meteoric water. The δ^(18)O and δD values also change with the latitude and the altitude,in which the δ`(18)O values are higher than those of the local meteoric water, and the latter are lower than those of the local meteoric water. The tritium ratios of the cycling geothermal water are less than 1 as measured in the end of the 1980s. It follows that the cycling geothermal water is the age-old water (pre-bombing water). In fact, the cycling geothermal water stored in the deep aquifer in the North China plain has a ^(14)C age of over 20 ka BP. In that case, the ~2H-poor character of the cycling geothermal water denotes that the palaeometeoric water recharged at the coldwet period of over 20 ka BP. The geothermal water is a hot brine, which may either be a Cl-Ca type hot brine with hydrocarbon deposits, or Cl-Na type hot brine with salt deposits. In the Cl-Na type hot brine, the ^(40)Ar/ ^(36)Ar ratios are part of the cont

关 键 词：环境同位素 地热水 补给机制 

分 类 号：P314.1[天文地球—固体地球物理学]