Study on the influence of external recharge of Yufu river on karst groundwater

DING Guantao; LI Changsuo; WEI Shanming; WANG Shaojuan; LI Yan; LU Qianqian; SUN Bin; Liu Haoran

doi:10.11932/karst20230504

Abstract：Jinan is famous for its abundant spring water resources,but with the advancement of urbanization,the ground continues to harden,resulting in a significant reduction of the area effectively supplying karst water,and hence a sudden decrease in water supply.The decrease has seriously threatened the continuous gushing of spring water in Jinan.At the same time,the urban population is constantly rising,so is the demand for water supply.The contradiction between spring water protection and urban water supply is becoming increasingly serious.In order to alleviate the contradiction in Yufu river,the projects in which groundwater was recharged by surface water were completed in different phases.In these projects such as the water discharge project of the Wohushan reservoir,the project of converting surface water into groundwater,and the Jinan City project of the East Line of the South-to-North Water Diversion Project,the surface water from the Wohushan reservoir,the Yuqing lake reservoir,and the Yangtze River has been respectively used for the recharging.They play an important role in maintaining the continuous gushing of spring water for the normal use of groundwater in cities. In the South-to-North Water Diversion Project,water from the Yangtze River is significantly different from local karst water in quality.Therefore,long term recharging will have a certain impact on karst water.In response to this issue,taking the recharging source of the Yufu river from 2021 to 2022 as an example,we studied the impact of external water supply on karst water in the Yufu river basin,and analyzed the hydrochemical characteristics and the change of hydrochemical composition of groundwater in the affected area.In order to fulfill the research purpose,we used isotope testing and comprehensive hydrochemical analysis.The results show that the recharging of the Yufu river has a certain replenishment effect on karst water.According to its degree of replenishment,the research area is divided into three zones.After two months of replenishment,Zone Ⅰ is distributed on the east bank of the Yufu river,with the highest mixing ratio of river water in karst water,ranging from 50％to 70％,and is most affected by river water replenishment.Within a certain range of the periphery of Zone Ⅰ,the proportion of river water in Zone Ⅱ is 10％to 50％,and the degree of influence of infiltration river water on karst water isotopes and hydrochemistry decreases.ZoneⅢ is far away from the Yufu river,with river water accounting for less than 10％and less affected by the river water.After 11 months of replenishment,the proportion of river water in the karst water monitoring points in Zone Ⅰ has decreased,but the proportion is still around 50％,indicating that Zone 1 is still the main affected area for river water replenishment.The proportion of river water in monitoring points in Zone Ⅱ has significantly increased,with J108 reaching 37％,indicating that the impact range of river water has expanded to Zone Ⅱ.The proportion of river water in monitoring points in Zone Ⅲ has also increased,but it is not significant with the percentage of around 10％,indicating the limited impact from river water.The hydrochemical characteristics of river water from the external source are significantly different from those of karst water and spring water.The hydrochemical type of river water is SO4 C1-Na Mg type,while the karst water is mainly HCO3-Ca·Mg type and HCO3-Ca type.Spring water is influenced by the replenishment of Xingji river with HCO3·SO4-Ca·Mg type.Two months later,the hydrochemical type of groundwater has gradually transitioned from SO4·Cl-Na·Mg type to HCO3-Ca·Mg type in the water source area of the western suburb along Yufu river.Due to the water diversion and replenishment of the Xingji river in the direction of the spring group,the groundwater type first changed from SO4·Cl-Na·Mg type to HCO3-Ca type,and then transitioned to HCO3·SO4-Ca·Mg.The recharging of water from the external source has a certain impact on the quality of the local karst water.The changes in the ion content of karst water in the affected area are mainly caused by the physical mixing of river water and karst water,accompanied by changes in water-rock balance.After a hydrological year,each ion gradually approaches its original content level after supplementing the source.The research results provide a basis for the scientific implementation of recharging and replenishment of the Yufu river,and have certain significance in promoting the organic unity of spring protection and water supply.This article does not consider the impact of seasonal atmospheric precipitation on groundwater hydrochemistry and isotopes.The atmospheric waterline used for isotope analysis is not the local atmospheric waterline,and these issues may have a certain impact on the results obtained,which is a topic that we need to further study.