Discussion on the characteristics of karst water systems in the Xishan area of Beijing

Beijing is located in the north of the North China platform, where 3 second-order tectonic units meet, i.e. the Shanxi uplift, North China faultdepression, and Yanshan fold belt. In geomorphology, it lies at the conjunction of the Taihang Mountains, Yanshan Mountains and the North China plain. The study area is Xishan area of Beijing, part of the Taihangshan Mountains. The terrain is generally high in the northwest and low in the southeast, and the main rivers are the Dashi river and the Yongding river. The karst groundwater is dominated by cycles of the relatively independent karst groundwater system in the Xishan region of Beijing. The hydrogeological conditions, the system structure model and the importance of water resources of the karst groundwater in this area have certain representativeness in the north. It has abundant groundwater resources and good water quality and is one of the important groundwater supply sources for Beijing City. The karst water in the area receives atmospheric precipitation，and seepage from the Yongding river and the Dashi river. At the same time, there is a transformation relationship between the partial area and the piedmont covering loose pore groundwater and coal measure strata fissure groundwater. The resource elements of constitute are many and the transformation relationship is complex. The carbonate aquifers of the Xishan area include the thick layer of upper Proterozoic karst aquifer group, carbonate intercalation of the middle upper Cambrian-lower Cambrian karst water-bearing rock group and the huge thick layer of Cambrian-Ordovician carbonate water-bearing rock group, which cover most of karst aquifers in North China. According to the definition of karst water systems, the karst water bearing system, surface hydrology system, topography and geomorphology, geological structure and aquifer medium structure, previous study divided the karst water system in the Xishan area of Beijing into 6 systems, which are the Yuquanshan, Gaozhuang-Ganchi, Yugudong, Heilongguan, Mapao and Shangqingshui karst water systems. This paper takes the Yugudong, Heilongguan and Yuquanshan karst systems as the research objects. From the karst aquifer structure, groundwater circulation and geologic structure, we summarize the characteristics of karst water system as follows, double-layer aquifer structure, multiple subsystems and multiple points discharge, complicated and various karst water transformation relationships, and the concentration of karst water and system boundaries in thrust faults.