[en] The characteristics of the alluvial-diluvial aquifer of the Beijing area were studied by means of environmental isotopes, especially tritium, which may be considered as a useful natural tracer to demonstrate the pollutant behavior in groundwater aquifer. The results of tritium monitoring indicate that the natural regime of the groundwater system of the Quaternary aquifer has been destroyed due to intensive exploitation. Two subsystems could be divided based on tritium data. Subsystem A with active circulation was formed in the course of exploitation. Subsystem B is of slow circulation. The δ²H and δ¹⁸O values of the groundwater are higher in the western suburb than that in eastern suburb. The sketch maps os δ²H and δ¹⁸O isolines reflect a mixing between ground waters from the base rock and from the local vertical recharge. The trace elements Sr, Ru and Rh have a very special distribution in groundwater system with very high concentrations in the north-eastern part of the old Beijing city. The results of ⁸⁷Sr/⁸⁶Sr measurement show that Sr in groundwater of the Quaternary aquifer is from the groundwater of the basement rock. High concentrations of Sr in groundwater of the Quaternary aquifer are not related to any special pollution source. (author)