Hydrogeological significance of the Jiulongshan-Xiangyu syncline at Yuquanshan sping

Karst water of the Yuquanshan is one of the main sources of water supply for the city of Beijing. It is commonly accepted that the the Yuquanshan spring was orinigated from the Lujiatan, Hebei Township and Junzhuang areas. The karst water (including the leakage of the Yongding river) of the Junzhuang area crosses the Jiulongshan-Xiangyu syncline before it reaches the Yuquanshan spring. Boreholes drilled in this syncline reveal that the maximum depth of the top of the Ordovician carbonate rock is 1,669 m. In respect of how the karst water of the Junzhuang area traversing the Jiulongshan-Xiangyu syncline and recharging Yuquanshan spring, there are two different points of view. One is the karst aquifer that conducts groundwater at depth, and the other is that the karst water is transmitted by the Junzhuang (Yongding river) fault. Based on analyses of the geological structure, groundwater flow field, the mechanism of karst development and water quality composition, on the one hand, we question the point that the karst water of the Junzhuang area runs through karst aquifers in deep syncline and arrives the Yuquanshan spring. On the other hand, we suggest that there is another possible flow path that firstly the karst water of the Junzhuang area flows through the unconsolidated layers overlying the Yongding river across the synclinal axis. Then in the south via the fault zone the water reenters karst aquifer across the uplift structure in the vicinity of the Hongmiaoling-Badachu anticline axis, which ultimately forms the source of the Yuquanshan spring.