Experiment of monitoring multi-layer groundwater at a karst hill slope and its scientific significance

GUO Fang; JIANG Guanghui; LIU Fan; LI Zhijie

doi:10.11932/karst20240301

Abstract：The heterogeneity of karst aquifers and the complexity of karst hydrogeological conditions pose significant challenges to groundwater observation compared to the groundwater in pore and fissure aquifers.In the areas of springs or underground rivers in which lack natural groundwater outcrops,borehole drilling is crucial for us to reveal aquifer structures and monitor groundwater.Clustered single-hole drilling inadequacies are pronounced in karst regions,because we need numerous times of single-hole drillings to comprehensively reveal different aquifer conditions,which may in turn result in land occupation and high costs.Besides,the difficulty in accurately locating karst conduits or fractures,due to complex hydrogeological conditions,may contribute to potential incompleteness of single-hole representativeness.Finally,drilling for groundwater monitoring may traverse multiple geological layers,causing interconnection of groundwater,thereby making it impossible for us to distinguish water quality conditions in different layers or channels during groundwater quality monitoring. The drilling of one shaft with multi-layer borehole refers to the technique that can achieve segmentation of multiple layers within a single borehole drilling operation.This technique,also known as Continuous Multichannel Tubing(CMT),is an internationally recognized method for layered drilling monitoring,is widely utilized in North America but seldom used in China.Because the technique of one shaft with multi-layer monitoring demonstrates its high ability in differentiating water head or solute distribution,it has been extensively applied in monitoring in pore and fissure aquifers,but seldom in karst media because of its complex structure.Therefore,there are no reported cases of application of this technique in karst regions,and its feasibility needs to be confirmed. The Yaji karst hydrogeological experimental site(hereinafter referred to the Yaji experimental site)represents a typical bared karst aquifer system in Southern China.The aquifer comprises three parts:the upper vadose zone,lower vadose zone,and saturated zone,playing the role of rainfall storage and regulation.Eighteen clustered-distributed boreholes established at the experimental site can reflect the heterogeneity of karst development or water level changes on a planar scale,yet they can hardly reveal the vertical groundwater movement.It is difficult for us to use open boreholes in defining how the differences in karst development vertically affect hydrodynamics.Therefore,this study selected one borehole(ZK6)from the runoff area on the western slope of the Yaji experimental site to implement the one shaft with multi-layer drilling experiment,aiming to explore the application of layered drilling monitoring technique in karst aquifers.The study result can provide conditions for understanding groundwater stratification,vertical distribution of water heads,and layered pumping,and more importantly,provide experience in applying borehole layering techniques and schemes in highly heterogeneous karst water-bearing media. Firstly,based on the data of borehole core,pumping tests,and results from hydrochemical and temperature logging,ZK6 was divided into four layers,with karst development characteristics as the main basis for division.Secondly,hydrological and hydrochemical characteristics of the four layers were compared and analyzed.Significant differences were found in the water level dynamics of the four layers in response to rainfall.The first layer predominantly alternates between being dry and being filled with water throughout a hydrological year,occupying 79％and 21％of the time respectively,which indicates alternating dry and wet conditions in the karst media,with a tendency towards dryness.The second layer exhibits significant lags in water level response.Differences in water heads between the third and fourth layers reflect the vertical distribution characteristics of groundwater discharge zones.Differences in hydrochemical characteristics among the four layers suggest that discrepancies in karst media structures affect both hydrological processes and hydrochemical characteristics.The electrical conductivity in the fourth layer remains relatively stable at 450 μS·cm-1,while in the second layer,due to surface element accumulation and slow water renewal rates,the electrical conductivity is twice that of the fourth layer.The drastic changes in electrical conductivity in the first layer reflect the strong influence of rainfall recharge in the surface karst zone.Finally,based on the hydrological and hydrochemical characteristics of borehole layers,it is concluded that the first to fourth layers respectively represent the epikarst zone,fissure-matrix zone,upper saturated zone and lower saturated zone.Although the technique of one shaft with multi-layer poses operating challenges in karst mountain slopes,once achieved,it will not only facilitate our understanding of the vertical hydrodynamic mechanisms of karst stratification,but will also achieve possible breakthroughs in technology for layering development and utilization of groundwater in heterogeneous karst aquifers.The use of borehole layering monitoring technique enables the measurement of vertical distribution of water heads within boreholes,thereby providing the potential for utilizing layering isolation techniques to facilitate runoff utilization from epikarst zones and vadose zones.