Identification Of Karst Water Flow Paths And Hydrochemical Formation Mechanisms In The Baotu Spring Watershed

Karst water is a vital groundwater resource worldwide,supplying approximately one-fifth of the global population’s water needs.Due to strong anisotropy and heterogeneity,exploring and interpreting karst and its groundwater flow system is a highly challenging task.The Baotu Spring karst water system,one of the typical karst water systems in northern China,serves as an essential water resource reservoir and plays a significant role in regulating water supply and supporting aquatic ecosystems.However,the Baotu Spring karst water system faces several problems,such as continuous deterioration of water quality and increased water supply risk.The lack of comprehensive and in-depth knowledge about the circulation and evolution of the Baotu Spring karst water system makes it urgent to conduct further research in this area to understand the flow system and water quality formation mechanism.In this study,the research was focused on the Baotu Spring karst water system located in Jinan.Historical monitoring data in the study area were collected,and on the basis of analyzing the water level time series,combined with borehole data and tracing experiments,a three-dimensional geological model of the spring area was established to characterize the features of karst conduits.Further,a coupled model of karst pore-conduite flow was constructed to determine the changing rules of the groundwater hydrodynamic in the study area and the complementation and drainage relationships between different aquifers.Field hydrogeological investigations,hydrochemical analysis diagrams,reverse geochemical simulations,hydrochemical-multi-isotope tracing,and other methods were utilized to elucidate the formation and evolution mechanism of groundwater hydrochemistry in the study area.This article draws the following conclusions and understandings from the above research:1.Analysis of the dynamic characteristics of the karst water level can reveal the trend of changes in groundwater resources and the evolution process of karst water systems.By using historical monitoring data combined with mathematical analysis methods,this article analyzed the fluctuation of karst water,main controlling factors,and different recharge-discharge conditions of groundwater in different regions.Since 1958,the water level in the Baotu Spring area has generally gone through four stages: high water level stage(1958-1967),water level decline stage(1968-1980),water level fluctuation stage(1981-2003),and stable stage(2004-2020).The water level dynamics are primarily affected by rainfall and pumping amounts.The Mann-Kendall test results show that although the karst water level has changed in the past twenty years,the change is not significant.There is a significant correlation in the frequency domain period of about 360 days between groundwater level monitoring points and between groundwater and precipitation.The difference in lag time reflects the different recharge paths of each groundwater monitoring point.2.The chemical and isotopic compositions of groundwater can indicate the source and evolution process of groundwater.This thesis focuses on the spatial distribution characteristics of hydrochemical parameters,D,18 O,87Sr/86 Sr,13C,34S-SO4 to indicate the recharge-discharge features of the karst groundwater flow system in the spring area and identify different karst water flow paths.The results show that the hydrochemical types of fissure water and karst water in the bedrock mountainous areas in the southern part were mainly HCO3-Ca and HCO3-Ca·Mg type waters.The hydrochemical type of karst water in the indirect recharge area in the south was mainly HCO3·SO4-Ca type.The overall hydrochemical types of karst water in the direct recharge area and the discharge area were mainly HCO3·SO4-Ca type.The main runoff paths in the spring area are: northeast overflow recharge,northwest runoff recharge,and north-south runoff recharge(the eastern runoff zone on the west side of the Dongwu fault,the north-south rapid runoff zone of the Qianfo mountain fault,and the western leakage runoff of Yufu River to Spring Group).The karst water of the north-south and eastern runoff on the Qianfo Mountain receives recharge from the karst water in the south,while the karst water in the west not only receives recharge from the karst water in the south but also receives significant leakage recharge from the Yufu River.The amount of leakage from the river is much larger than the recharge from the karst water in the south to the karst water in the west.In the northeastern and northwestern parts of the spring area,there is a lateral karst water flow that receives atmospheric precipitation recharge,which crosses the Dongwu and Mashan Faults and discharges from the suburban spring groups to the urban areas.3.Simulating karst groundwater flow can evaluate karst water resources and quantify recharge-discharge relationships between different aquifers.Based on hydrogeological data collected from the spring area,this article constructs a threedimensional karst groundwater flow model to calculate water resources in the Baotu Spring area and the exchange volume between karst water and fissure water.Furthermore,through tracer experiments,a three-dimensional coupled karst groundwater flow model of pore-conduit flow is constructed to accurately identify and simulate karst conduits.The results show that the dynamic of karst water level in a year is significantly affected by atmospheric precipitation.The amount of Cambrian karst water recharged by Tai Shan intrusive rock fissure water is about21900~82000 m3/d,accounting for 42.6%-63.8% of the total supply amount of the Cambrian limestone aquifer.During the annual rainy season(June-September),the Ordovician karst water is replenished with Cambrian karst water,while in the dry season,Cambrian karst water supplies the Ordovician karst water,with a supply amount of about 3389-47700 m3/d,accounting for 16.4% to 40.1% of the total supply amount.Using tracer experiments,the horizontal and vertical distribution of conduits in the main runoff zone of Qianfo Mountain has been accurately identified.Simulation of karst groundwater using the pore-conduit flow coupling model improves the accuracy of the simulation.4.Analyzing the formation and evolution characteristics of groundwater chemistry is essential to identifying the source of groundwater pollution and related ecological environment protection.This article explores the formation and evolution mechanism of groundwater chemistry using methods such as graphical analysis,isotopic analysis,and reverse geochemical simulation.It further analyzes the formation mechanism of groundwater chemistry from the perspective of runoff paths.The results show that groundwater chemistry in the karst area is mainly controlled by rock weathering.The weathering and dissolution of carbonate minerals(calcite,dolomite)are the main water-rock interactions in the spring area,followed by the dissolution of silicate rocks and evaporite minerals(gypsum).The main source of HCO3-in karst water in the southern part of the spring area is the dissolution of carbonate rocks and recharge of fissure water,while the primary source of SO42-is atmospheric deposition.In the eastern and northern-southern runoff karst water and Qianfo Mountain,HCO3-mainly comes from the dissolution of carbonate rocks,and SO42-comes from atmospheric deposition and sewage leakage.The primary source of HCO3-and SO42-in karst water in the Yufu River leakage zone in the west is surface water leakage recharge.Karst water in suburban areas and near spring groups is polluted with organic matter.The primary source of HCO3-in karst water is the joint action of carbonate rock dissolution and sewage pollution.Organic matter is decomposed by microorganisms and enters karst water,while the source of SO42-is atmospheric deposition and sewage pollution.The major ions in the groundwater of the spring area show an increasing trend over time.In the past 20 years,the increase in TDS,sulfate,nitrate,and chloride concentrations in karst water has been significant.The large-scale increase in sulfate in karst water may be related to local acid rain and the acidic weathering and dissolution of sulfide minerals.Agricultural activities such as fertilizer use can also contribute,while human activities and municipal wastewater may be the primary sources of chloride ions in groundwater.The main innovative points of this thesis are mainly reflected in the following two aspects: firstly,by comprehensively utilizing the historical water level dynamic data,combined with hydrochemistry and multiple isotopic evidence,the multi-flow recharge pattern of the Baotu Spring area is identified,and a threedimensional karst pore-conduite coupling model is constructed based on this to quantitatively analyze the recharge and discharge relationship between different aquifers,providing a paradigm for the study of northern karst water flow systems;secondly,by comprehensively utilizing hydrochemical and multiple isotopic identification technologies,from the perspective of the multi-flow recharge pattern and historical evolution mechanism of the Baotu Spring area,the formation and evolution mechanism of karst water hydrochemistry in the Baotu Spring area are analyzed.