Analysis of characteristics and controlling factors of underground river pipeline based on tracer tests and fracture measurement:Take the Luohandu underground river as an example

LI Tengfang; HUANG Qibo; SHEN Lina; WU Qingpeng; MA Junfei; LIAO Hongwei; WU Huaying; ZOU Changpei; PU Zhenggong; LENG Rubing

doi:10.11932/karst20250505

Abstract：The karst development in Southwest China is very remarkable.At present,3,066 karst underground rivers have been identified.As the main reservoir of groundwater resources,studying the distribution characteristics and controlling factors of karst pipelines is of great importance.In this study,the Luohandu underground river in Gongcheng county was selected as the research object,and a 1∶50,000-scale hydrogeological survey was carried out.It was found that an upper karst cave exists at the outlet of the Luohandu Underground River,which is supposed to be the original discharge outlet.The karst cave measures 567.9 m in length,and investigations revealed that the pipeline of the lower underground river extends about 1.6 km.Based on regional geological data,the Luohandu underground river is located near the structural belt formed by Limu-Heping fault and the Jiahui translational fault.The Limu-Heping fault trends north-south,while the Jiahui translational fault trends east-west direction.The Limu-Heping fault intersects the study area,and its structure controls the development of underground river pipelines. To identify the controlling factors of pipeline development,groundwater tracer tests and fracture measurement were conducted.The tracer was introduced into the skylight of the underground river in its upper reaches,and the tracer was detected after only 1.5 hours.The tracer concentration peaked at 1.7 hours and then began to decline.It was calculated that the maximum velocity of groundwater between the skylight and the outlet of the underground river was 550.4 m·h-1,with an average velocity of 447.7 m·h-1.These results showed that the groundwater flow between the skylight and the outlet of the underground river is closely connected,the migration path is relatively smooth,and the karst has been developed in the form of a single pipe.At the same time,four groups of fractures were measured by statistical window method,including three groups of structural fractures and one group of bedding fractures.To investigate the influence of fractures on the development of karst pipelines,the statistical analysis of fractures measurements revealed that the direction of the first and fourth fracture groups are generally consistent with the development direction of the underground river.Further analysis showed that high-angle structural fractures provide important channels for groundwater recharge into underground rivers.By calculating the principal values of permeability tensor for the structural fractures,it was found that the dip directions of the third principal permeability tensor(K3)at five measurement points ranged from 254° to 292°,with dip angles mostly less than 25°.The direction of maximum permeability governs the seepage direction of karst groundwater,and long-term erosion and dissolution along this direction ultimately control the development of the underground river pipilines. Based on a 1∶50,000-scale hydrogeological survey and regional geological data,this study examines the structural patterns and provides a detailed description of the distribution and spatial characteristics of underground river pipelines using high-precision tracer tests.It also analyzes the influence of structural fractures on these underground river pipelines by combining fracture measurements with permeability tensor calculations.Through various technical approaches,this study not only systematically characterizes the distribution characteristics of underground river pipelines but also reveals the controlling factors in their formation,thereby providing an important scientific foundation for subsequent investigations of underground rivers and water resource evaluation.