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 y...