Characterizing the groundwater renewability and evolution of the strongly exploited aquifers of the North China Plain by major ions and environmental tracers

The Yongding River Alluvial Fan is a northwestern part of the North China Plain, at which Beijing is located. Since 1950 and especially 1970, excessive groundwater exploitation has resulted in a significant drop of groundwater table, and it's believed that groundwater will be intensively used continuously in the future. It's essential to reveal the renewability and the evolution of the groundwater for reasonable management, which was achieved based on the analyses of the major ions and environmental tracers in groundwater. According to the dating results of H-3, CFCs, and C-14, the age of the shallow and deep groundwater was from <5 to 60 a and from <4,000 to >12,000 a, respectively. The recharge rate determined by the groundwater age generally had a decreasing trend fromtheNW(0.40-1.44 m/a) to the SE (0.07-0.48 m/a). Na and HCO3 accumulated from the NW to the SE and the latter was the dominant anion. Chemical modification occurred progressively along the flow paths. The water type of the shallow and deep groundwater changed from Ca-Mg-HCO3 to Na-HCO3 and from Ca-Mg-HCO3 to Ca-Na-Mg-HCO3 and Na-Ca-Mg-HCO3 from the NW to the SE, respectively. The shallow groundwater was vulnerable and susceptible to contamination scattered throughout the region, which would probably further extend to the deep groundwater because of the vertical groundwater flow especially through the preferential pathways due to their increasing hydraulic connections.