Characterizing The Karst Groundwater Systems And Simulating The Spring Discharge

The Xiangxi River karst basin,located in the western of Hubei Provience,Central China,has developed both the southern typical underground rivers and the northern karst spring system that recharged from surface and discharged at a point,forming a unique karst feature of the north-south transition zone.With socio-economic development,the importance of karst water resources is increasing.However,the informations about the circulation mechanisms and the structure of karst water systems are insufficient,and the evaluation methods of karst water resources are facing great challenges,due to the complexity of the topography,as well as the multiplicity and uncertainty of the aquifer structure in karst mountainous areas.In this study,guided by the theories of karst groundwater system,multi-method have been conducted in Cambrain-Ordoviation karst aquifer located in Xiangxi river basin to characterizes the karst system from multi-aspects,and revealed the circulation patterns of the karst groundwater flow.The genetic algorithm was used to simulate the conduit paths between sinkholes and springs,quantitatively identifying the recharge areas and aquifer structure（conduit geometries）of the karst water system.According to the identified structure and the flow patterns of karst system,the mathematical models for simulating groundwater flow in karst system were constructed.Then this model was applied in simulating the hrdeograph of karst system,to deeply identify the structure and circulation processes of karst water system and provide a new evolution method of karst water resources.The aquifer structure,circulation depth,average residence time of groundwater,and degree of water-rock interaction for three typical karst water systems with different dynamic variations in the study area were characterized using multi-methods,such as,hydrogeological investigation and monitor,artificial and natural tracer,and hydro-cemical analysis.The results indicate that unlike the flow patterns in a homogeneous aquifer which created mainly by topography,karstification degree is also a key in controlling the karst groundwater flow patterns（GFP）.The GFP in study aquifer were characterized by local fast conduit flow,local intermediate fracture networks flow and regional slow matrix flow with progressively slower groundwater refresh rate.The natural responses（discharge,electrical conductivity and temperature）of underground revier system to rainfall indicated that flow in this system was discharged through conduits,large fractures,medium fractures and micro fracture-pores,respectively.Stable isotopes(D,¹⁸O)of precipitation and groundwater indicated that the circulation depth of local groundwater flow systems（GFS）was about 291m,and those of regional GFS was approximately 1090m.Mean residence times calculated by artificial and natural tracers(¹⁸O,CFC)of local conduit flow,local fracture flow,and regional matrix flow were 13 hours,400 days and 41 years,respectively,which obviously increasing gradually.Hydro-chemical analysis indicated that regional groundwater has strong interaction with the surrounding rocks and has higher concentrations of ions related to carbonate dissolution.The correlation between ions concentration andδ¹⁸O suggested that the ¹⁸O in regional groundwater maybe exchanged with bedrocks,and the rate of(Ca²⁺+Mg²⁺)vs(HCO₃^-+SO₄^2-)shown that the ions maybe exchanged between regional groundwater and carbonate rocks.The pseudo-genetic model that simulating the conduit networks from the recharge to the discharge has been proposed.This model is based on the Anisotropic Fast Marching Algorithm（AFMA）and uses the hydraulic conductivity tensor as an indicator of the potential for karst development.According to the simulation results,the recharge area of the karst water system also can be identified.In this study,the hydraulic conductivity tensor was calculated using a discrete fracture network model（ADFNE）.This model:（i）equivalents the rock with fractures to continuum pores,that avoids the insufficient computational memory caused by simulating a large number of fractures;（ii）follows the basic regulation of karst development,and efficiently accounts for the effects of regional hydrogeology,anisotropy,and regional flow conditions on 3D conduit networks.Compared to other techniques,this methodology substantially improves computational efficiency and extends the applicability.This method was applied in simulating the conduit networks between the sinkholes and two adjacent karst springs-Wulongdong spring（WLD）and Yunlongdong spring（YLD）,to identify their recharge areas and their geometric shape of conduit networks.The results indicated that the No.1-No.7 sinkholes were connected to the WLD,and No.8-No.14 sinkholes were connected to the YLD.The simulation results of the connection between No.1 and No.6sinkholes and the springs are consistent with the results of tracer experiments.The depression catchment area of WLD and YLD are 7.20 km² and 4.84 km²,respectively,the ratio of their depression catchment areas（1.49）are approximately equal their baseflow discharge ratio（1.41）.A model for the simulation of spring discharge based on a data-driven recharge estimation method（DDRE）and a discrete conduit-continuum matrix（CFPv2）has been proposed.According to the master recession curve that fited based on previous observed data and the observed data of individual spring recession curve,the individual spring hydrographs have been reconstructed.Then,the fast and slow flow were also been separated based on the separation identification dertermined by the individual spring discharge and electrical conductivity,and the proporation of concentrated and diffuse recharge to total recharge were estimated.This method has been applied in the simulation of WLD spring discharge and the results have been compared with that of the coupled model of epikarst reservoir（EPI）and CFPv2.The results indicated that the peak flow and spring recession process were much better reproduced by using DDRE＿CFPv2.For individual precipitation events,the range of proporation of concentrated and diffuse range to total precipitation were widely,the values were2.64%-56.4%and 2.37%-47.6%,respectively.Concentrated recharge is the main factor for the variation of spring flow,and its proporation to total recharge can reach 87.7%.A sensitivity analysis of the DDRE＿CFPv2 reveals that the most influential parameters was the specific yield（S_y）,and indicated that the lag function has significant affect on the simulation results.The proposed model effectively calculated the duality（concentrated,diffuse）recharge rate,and provides a new method for the simulation of spring discharge.