Abstract: A study of uranium disequilibrium in groundwater from the Cambrian-Ordovician(∈-O)carbonate aquifer system,the Mouthspring,southwest of Datong has been undertaken to answer questions pertaining to the development of this aquifer system.An adjacent ∈-O aquifer system,the Deityhead Spring,is differentiated from the Mouthspring using activity ratio of ²³⁴U/²³⁸U,third kinds of ∈-O groundwater related to strong to weak circulation patterns and aquifer environments are interpreted,too.²³⁴U excess is used for identification of many ∈-O subsystems,their mutual relations and the lateral recharge and discharge as well. The source waters of these subsystems are deduced,contributions of these waters are evaluated quantitatively by mixing ratios.The concentration and activity ratio of uranium in ∈-O groundwater are varied greatly,it is seldom seen that activity ratios less than that of the secular equilibrium are found from ∈-O groundwater of the Deityhead-spring.