Study On Seepage Mechanism Of Dominant Flow In Typical Karst Spring In North China

The distribution of carbonate rocks in northern China is very extensive,and the huge carbonate rock aquifers contain abundant underground water resources,which provide an important guarantee for alleviating the contradiction between water supply and demand in the relatively water-scarce northern region and provide strong support for local economic development and social progress.However,with the joint action of nature and human social activities,many karst springs have been severely polluted and even dried up,posing a great threat to the local ecosystem.To effectively protect karst springs,various artificial recharge technologies have been proposed and implemented.However,the water level of the springs during the annual dry period is still close to the alert line,and the problem of spring drying has not been fundamentally solved.The root cause is insufficient understanding of the karst water seepage process and dominant flow mechanism,resulting in a certain degree of blindness in the implementation of artificial recharge measures.Therefore,the study selected the Bao Tu Spring area in Jinan as a typical representative of the northern karst area,and combined fractal geometry,hydrogeology,and other disciplines to conduct a systematic study of the pore structure and topological space composition of the karst medium,established a three-dimensional digital rock core and equivalent pore network model,and conducted an in-depth analysis of the spatial development and heterogeneity characteristics of the northern karst aquifer medium,and analyzed the movement rules of dominant water flow and the impact of dominant channels on spring water dynamics.The study shows that the structure of the karst medium plays an important controlling role in the dominant flow of spring water.The main conclusions are as follows:(1)The pore morphology of the northern karst aquifer medium can be divided into three types: open,semi-open,and closed.Most of the equivalent pore diameters are less than 2mm,and the shape is mostly elliptical,with a small development of long cylindrical and irregular shaped pores.The connectivity of the pore network is poor and it is difficult to form a good water transport channel.The honeycomb-like dissolution pores,caves,and small cracks are important water storage spaces in the karst medium.The degree of development of the fissure medium is uneven,mostly in the form of interlocking distribution,and the main fissures are interconnected,providing good seepage conditions for groundwater.(2)The analysis of the morphological structure and development characteristics of the karst aquifer medium at the microscopic,micrometer,and macroscopic levels reveals the differential characteristics of karst development in different strata.Among them,the rock cores of the Chao Mi Dian,San Shan Zi b,and Bei An Zhuang formations are relatively dense,with only larger pores developed in local areas,poor water storage capacity,high pore development degree,concentrated pore throat distribution,mostly seepage pores,good connectivity,and can provide dominant flow channels for karst water movement.The pore distribution of the San Shan Zi a and Dong Huang Shan formations is uneven,with poor pore development and rough interior,but strong water storage capacity.The spatial topology of the internal cavities of the rock core is fully presented by the constructed skeleton of the pore distribution and the pore network model,and the correlation between the porosity and permeability of the karst aquifer medium can be described by the K-C model and the S-R model.(3)The microscopic structure of the water-containing medium in northern karst regions exhibits strict fractal characteristics.Based on characteristic parameters such as the fractal dimension,pore-throat ratio,and permeability of core samples,a positive correlation was established between the fractal dimension and porosity of pore media.A three-dimensional fractal expression for permeability of pore media was constructed by incorporating the fractal dimension,porosity,and permeability characteristic parameters of core samples.(4)By means of simulation using an equivalent network model,the flow velocity and pressure distribution characteristics of karst media were revealed.The connectivity and topological relationship of pores are important factors affecting fluid movement on the surface and inside the media,and they determine the path of fluid flow.Narrow and short throats or low coordination number cavities are the main causes of sudden changes in fluid pressure.During the process of seepage,when water flows from larger pores inside the rock media to relatively narrow pores,the pressure distribution transitions from high pressure zones to low pressure zones.However,at the narrow throat,due to the sharp decrease in water velocity,there is a sudden increase in pressure.At the crack connection point or water flow channel turning point,the flow rate will sharply increase.(5)Multiple forms of water flow,including Darcy and non-Darcy,exist simultaneously in karst media.The relationship between the pressure gradient J inside the core and the seepage velocity v exhibits a segmented distribution.The relationship between the pressure gradient J and the seepage velocity v inside the dominant channels is expressed by a nonlinear quadratic equation,which can be described by the Forchheimer formula.Based on the Re criterion,the threshold diameter for pore channels to experience seepage is 34.38 μm;the threshold diameter for the transition from laminar flow to turbulent flow inside the pore throats is 187.48 μm;and the threshold diameter for the failure of Darcy’s law is 353.18 μm.(6)Different dominant channels are closely related to environmental p H.Selective dissolution is an important form of dominant channel development.Considering only the effect of dissolution,it takes about 454 years to form a dominant channel with a diameter of 1mm and a length of 1 cm in the muddy limestone of the Beianzhuang Formation in the Majiagou Group.In the siliceous limestone of the Sanshanzi Formation,Jilong Group,it takes about 570 years to form a dominant channel containing concretionary nodules in section a,and about 1367 years in section b.(7)Water flow of different frequencies has a significant difference in its contribution to spring flow.The fast flow has a duration of about 15-34 days in response to atmospheric precipitation,while the slow flow has a duration of about 50-70 days,which can be extended up to 290 days.The dominant flow rate is the first to complete the replenishment,runoff,and excretion processes of water circulation,which is an important reason for the "sharp rise" of spring water level,and also provides the possibility for multiple rapid pulses of spring water level.The medium-speed flow is an important bridge between the fracture pipeline and the pore,playing a crucial role in the fluctuation of the spring water level.The low water level and small flow rate in the dry season mostly come from the slow release of pore flow,maintaining the continuous flow of spring water for a long time in the dry season.