| Liujiang Basin, with an area of 378 km2, is located in the north of Qinhuangdao, Hebei province, taking Shimenzhai as its center. The main geological structure in the basin is Liujiang syncline, and the main aquifer is the Cambrian and Ordovician limestone aquifer, which is a backup water resource in Qinhuangdao. There is a good knowledge of karst water and the developing features of karst caves, based on the study in regional hydrogeology and environmental geology investigation finished by former people. However, the spatial structure, the groundwater flow field and its hydraulic relationship with surface water for the whole karst water system in this basin still remain some issues. This paper focused on the macrofeature in karst water system and groundwater flow field in Liujiang Basin, so that more basis could be supplied to sustainable use of groundwater and the protection of groundwater environment.According to the outcrop of karst aquifer and hydraulic connection, we can devide karst water system into 4 parts: Dashihe karst water system, Westernwing karst water system, Dongbuluo karst water system and Shahezhai karst water system. Among them, the Dashihe karst water system is the biggest one, and it has direct hydraulic connection with the Westernwing karst water system, so we can take them as one complete karst water system, and it is the key point in this paper. This karst water system covers an area of 122.57 km2,made up of carbonate rocks with clasolite in Cambrian system from Zhangxia formation to Fengshan formation(?2z-?2f), and limestone or dolostone in Ordovician system from Yeli formation to Majiagou formation, with a total thickness of over 700 m. These karst aquifers are confined aquifers in the center of the syncline where they are covered by aquiclude of Carbonic and Jura system. And they become unconfined aquifer in the western or eastern wimg of the syncline where they crop out. For these karst aquifers covered by quaternary system in river valley, they have a mixed structure. Karst caves and karst fissures mainly appear in the strong runoff zone. New hydrogeologic drilling and geophysical prospecting show that there are karst caves or fissures in the limestones of Ordovician system about 200-300 meters below surface in the eastern bank of Dashihe. There are hidden strong karst development zone buried by the quaternary system in Dashihe valley. This indicates that deep karst water also has the condition to participate in water cycle.With the help of GMS, this paper built a groundwater flow model in the key part of Liujiang Basin with a boundary of limestone formation. The model includes 3 layers, covering all the stratum from Zhangxia formation in Ordovician system to quaternary system. And this is a 3-D regional groundwater steady flow with the method of finite difference of MODFLOW. Rivers are dealt as dirichlet boundary condition, and the observed wells are basis for identifying the parameters. In the end, the model showed the groundwater flow direction in karst water system, and the major groundwater resources were estimated. The results show: regional karst groundwater flows from north to sourth on the whole, while there are also a trendancy for groundwater to flow from west to east in the sourth of this basin; And the net water exchange capacity from groundwater to surface rivers are 6.5′104 m3/d,taking up aroud 14% of the mean runoff volume in Dashihe. |
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