Hydrochemical characteristics and delimitation of sensitive protection area in the exposed area of Baimai spring group in Zhangqiu

Zhangqiu is rich in spring resources, known as the "Little Spring City", and Baimai spring group is among the 72 famous springs in Jinan. In recent years, the flow of Baimai spring group has been cut off for many times, and the competent departments have implemented a series of spring protection measures to basically maintain the continuous gushing of spring water. Against the backdrop of the planned development and construction of the core exposed area of the spring group, it is urgent to investigate the shallow micro-stratigraphic structure of the exposed area and its relationship with the spring water, in order to avoid the negative effect of engineering construction on the gushing of spring water. Based on the data collected from years of exploration for water supply in the exposed area of spring, this study has been conducted to identify the shallow stratigraphic structure and structural development characteristics of the exposed area of spring, with the methods of geophysical exploration, drilling, and water level observation. Meanwhile, the main influencing factors of spring water gushing in the exposed area have been analyzed, and the sensitive protection area has been delimited. The research findings may provide a scientific basis for the development and construction of the core exposed area of the spring group, and also for the spring water protection.

The results show that the fault structures in the exposed area are highly developed, with a total of 5 faults and 1 fracture zone, becoming a channel for karst water upwelling. The distribution of spring water is closely related to structural development. The West Mawan spring group is mainly controlled by the Yanchi fault, with spring water distributed along both sides of the Yanchi fault, while the East Mawan spring group is mainly distributed along the secondary fault F3 of the Mingshui fault. The gravel layer is widely developed on both sides of the fault, showing a characteristic of thinning and gradually pinching out from the middle to both sides. The gravel layer is in direct contact with the underlying Permian Carboniferous bedrock, and the structure cuts the Permian Carboniferous aquifer, forming a connection between karst water and the pore water of the gravel layer. Pressurized karst water flows up along the fault structure and enters the gravel layer for storage. The hydraulic connection between pore water and karst water is very close, and the gravel layer has become the "skylight" of karst water. According to the data on groundwater level measurement of the exposed area in 2018, the elevation of pore water level is 55.68–55.71 m, and the elevation of spring and karst water level is 55.78–55.88 m. The karst water level is very close to the pore water level, slightly higher than the pore water level by about 10 cm, indicating a close hydraulic connection between the two, and the karst water supports the supply of pore water. The gravel layer provides storage space for karst water after upwelling, and springs emerge at shallower burial depths. The main factors affecting the gushing of spring water in the exposed area are the control of fault structures and the connectivity and permeability of gravel layers. Based on the comprehensive development thickness of fault structures and gravel layers, sensitive and relatively sensitive areas are delimited, with the area of 0.78 km² and 0.67 km², respectively. It is generally not suitable for excavation of foundation pits in sensitive protected areas; When conducting engineering construction in these areas, the excavation depth of the foundation pit should not be too large and the gravel layer should not be damaged. In addition to taking protective measures during the development and construction of underground engineering, measures of protecting water quantity and quality should also be taken above the surface of the core exposed area. For example, constructing buildings (structures) that may affect the flow of spring water or damage the appearance of famous springs should be prohibited. The discharge of sewage and wastewater into springs is not allowed. The projects such as construction of new karst water wells, excavation of springs, and interception and diversion of water should be banned. In addition, the protection in the recharge area of spring water is equally important. It is necessary to accelerate tree planting and afforestation, seal off mountains for afforestation and grass cultivation, protect vegetation, and conserve water sources. It is also necessary to restrict the construction of buildings (structures) or development projects unrelated to spring water protection, and avoid hardening the ground in the recharge area to reduce the recharge amount. It should be forbidden to dump, pile up or fill urban domestic waste, construction waste, industrial solid waste and hazardous waste. The research findings will provide reference for the protection of similar exposure areas of spring water.