惰性气体含量对深层地下水形成温度的示踪意义分析

惰性气体在地下水中的溶解量主要取决于水温和气体分压 ,而且溶解度在常见的水温范围内是水温的单调函数 .因此 ,当深层地下水系统处于封闭状态 ,系统中不存在蜕变成因或变质成因的惰性气体源时 ,可将溶解的惰性气体作为指示地下水形成温度的天然示踪剂 .

On Tracing Function of Noble Gases Dissolved in Deep Groundwater to Paleotemperature

The paleotemperature of groundwater reflects the climatical background in the zone of aeration at the moment when water infiltrates from atmosphere into underground. If the paleotemperature of the groundwater in a deep storage system can be estimated correctly, based on the well researched vicissitude of the regional paleoclimate, the age of the groundwater and the hydrocirculating conditions can be also determined. However, the paleo? temperature of groundwater can not be measured directly with physical methods yet. So it must be evaluated indirectly by measuring some tracing elements dissolved in groundwater. The concentration of the noble gases dissolved in groundwater depends mainly on the water temperature and the partial pressure of the gases. In normal temperatural range, it changes monotonously with the temperature of the water moving through the zone of aeration. Therefore, the dissolved noble gases can be used as natural tracing elements to estimate the paleotemperature of deep groundwater, if the storage system is confined and meanwhile there is no radiogenic or metamorphic noble gases in it. In this paper the factors influencing the concentrations of the five gases in groundwater and the applicable prerequisite for evaluating the paleotemperature according to the concentrations of noble gases are discussed and a practical method for the calculation of the paleotemperature is explained. The total concentration of the salts dissolved in the groundwater with normal mineralization has only slight influence on the solubility of noble gases which can be ignored in practice. The concentrations of noble gases in deep groundwater are independent of the seasonal changes of the current climate. Of the five noble gases Ar, Ke, Xe, He and Ne, the solubility of the former three heavier gases is much more remarkably influenced by water temperature than that of the latter two. For this reason, the paleotemperature of the groundwater from a confined storage system should be calculated on the basis of the measuring values of Ar, Ke and Xe concentrations. The relative enrichment of He may indicate the existence of radiogenic noble gases in the storage system. The measuring value of Ne can be used for the estimation of sampling errors. Ultimately, a case given in this paper demonstrates it is reasonable and applicable to evaluate the circulating conditions of deep groundwater by means of its paleotemperature derived from the concentrations of noble gases.