鄂尔多斯白垩系盆地地下水的形成与演化:来自Cl及其同位素^(36)Cl的证据

Formation and Evolution of Groundwater in the Ordos Cretaceous Basin:Evidence from Chlorine and Its Isotope

鄂尔多斯白垩系地下水盆地地处中国西部干旱—半干旱地区,其南部和北部分别被黄土和沙漠覆盖,地下水水化学组成在区域上差异显著,北部以淡水为主,南部以微咸—半咸水为主。在沙漠区和黄土区的深层含水层中(埋深大于400 m)分别采集了5组和3组水化学同位素样品。T和14C的分析结果表明,地下水的滞留时间在20343 a内,并且未受到人工Cl的污染。依据地下水Cl及其同位素36Cl的组成,分析了蒸发作用对研究区降水补给和地下水Cl浓度的影响,结果表明,大气降水的蒸发富集和地层Cl的淋滤溶解对地下水中的Cl均具有重要作用,相对大气降水,沙漠区地下水蒸发浓集了29.9~63.0倍左右,蒸发作用为地下水提供了21.3~45.3 mg/L的Cl,而在黄土区,地下水蒸发浓集了266.0~523.7倍,蒸发作用为地下水提供191.5~377.0 mg/L的Cl。

The Ordos Cretaceous Basin,located at western China,is an arid-semiarid area.Its south section and north section are covered by loess and desert,respectively.Chemical compositions of groundwater are prominently different regionally,with freshwater in the north and brackish water in the south.Eight groups of the samples for hydrochemical and isotopic analysis were collected from deep groundwater（more than 400 m depth） in this basin.Results of T and 14C show that groundwater residence times are within 20343 years,and are not polluted by human being.Based on chlorine concentrations and 36Cl composition of groundwater,this study discussed the influences of evaporation on the recharge of precipitation and the chlorine concentrations of groundwater.The results show that evaporation and enrichment of meteoric water and the dissolution of stratum chlorine have significant effects on the chlorine concentrations of groundwater.Compared with precipitation of meteoric water,the chlorine concentration of groundwater in the desert was evaporated by 29.9~63.0 times,and this provided more than 21.3~45.3 mg/L of Cl with the groundwater.In the loess area,the groundwater was evaporated by 266.0~523.7 times,with 191.5~377.0 mg/L of chlorine derived from evaporation.