鄂尔多斯白垩系地下水盆地天然水体环境同位素组成及其水循环意义

Environmental Isotopic Composition of Natural Water in Ordos Cretaceous Groundwater Basin and Its Significance for Hydrological Cycle

鄂尔多斯白垩系地下水盆地天然水体的环境同位素组成表明,区内各种地表水体(河流和湖淖)在δ18O和δD图上主要分布在雨水线右侧,其关系线的斜率明显小于雨水线的蒸发线,集中反映蒸发作用对地表水体的影响。盆地内地下水大致集中沿雨水线分布,反映了白垩系盆地内地下水为大气降水成因。盆地南北两区地下水的环境同位素具有明显差异性特点,集中体现了盆地南北两区水循环条件的差异。盆地北区各含水岩组间地下水垂向水力联系比较密切,垂向运动特点比较明显,现代水积极循环带的深度为200 m;南区地下水分层性明显,以水平径流为主,现代水积极循环带的深度为160 m。区内浅层地下水以富氚和高14C含量为特征,反映为现代水补给;而中、深层地下水则以贫氚和低14C含量为特征,反映为地质历史时期补给。

Environmental isotopic composition of natural water in Ordos Cretaceous groundwater basin（CGB） indicates that the δ^18 O and δD of all the surfaces water lies in the evaporation line with the slope lower than the local meteoric water line （LMWL）, which signifies the effect of evaporation on the surface water. The groundwater is of meteoric origin resulted from the fact that the groundwater in CGB is mainly distributed along LMWL. It is interesting that the δ^18O and δD of groundwater in southern CGB and northern CGB has different distribution features, which signifies that the differences of hydrological cycle in southern CGB and northern CGB. In southern CGB, there exists no or weak hydraulic links between different aquifers and the groundwater flow is dominated by lateral inflow with the depth of up to 160 m involved in modern actively hydrological cycle. But in northern CGB, there is a close hydraulic links between different aquifers and the groundwater flow is characterized by vertical inflow with the depth of up to 200 m involved in modern actively hydrological cycle. The shallow groundwater is characterized by rich tritium concentration and high ^14C activity, which implies that the shallow groundwater is recharged in modern time, but the deep groundwater is characterized by poor tritium concentration and low ^14C activity, which implies that the deep groundwater is recharged in geologic time.