Analysis of the influence of tunnel engineering on groundwater resources in karst mountain areas:A case study of a tunnel in Wushan county of Chongqing City

WEN Jinmei; WU Tao; LI Xian; JIANG Chen; CHEN Li; LIU Sheng; MENG Li

doi:10.11932/karst20250304

Abstract：Tunnel construction can disrupt the groundwater balance,impacting the drainage of both surface water and groundwater,as well as causing other eco-environmental effects.This study takes a water-diversion tunnel as the research object.Prompted by a pronounced decline in the storage of the adjacent reservoir of the tunnel,it integrates hydrogeological surveying and tracer tests to characterize the altered recharge-flow-discharge conditions,elucidate the hydraulic interactions between the tunnel and the surrounding groundwater system,and quantitatively evaluate the tunnel's impact on the observed reservoir-storage variations. Field investigations indicate that groundwater in the study area can be divided into three categories:karst water from carbonate rock fissures,karst water from fissures characterized by carbonate rock interbedded with clastic rock,and karst water from pores and fissures characterized by mudstone interbedded with silty mudstone.The water bearing formations of karst water from carbonate rock fissures consist of the Lower Triassic Jialingjiang Formation(T1j)and the Daye Formation(T1d3+4)along the axis and wings of an anticline.These units exhibit significant water abundance and are well developed with karst landforms,such as karst depressions,sinkholes,caves,and underground rivers.Karst water from fissures characterized by carbonate rock interbedded with clastic rock occurs within the first and third members of the Middle Triassic Badong Formation(T2b1)along the axis and wings of a syncline.The water abundance of the formation is moderate,and karst features are only locally developed.Karst water from pores and fissures characterized by mudstone interbedded with silty mudstone is primarily stored in the second member of the Middle Triassic Badong Formation(T2b2)and exhibits comparatively low water abundance.Groundwater recharge is derived primarily from atmospheric precipitation and surface water bodies such as reservoirs and fish ponds.Due to the large terrain cutting,the speed of surface water runoff is fast. The catchment of the reservoir is located west of the axis of the water-diversion tunnel on the southeast wing of the anticline.Exposed strata comprise limestone,dolomitic limestone,dolomite,and rock-solution breccia of T 1j and T1d3+4,as well as limestone,argillaceous limestone,argillaceous dolomite,and mudstone of T2b.Within the Jialingjiang and Daye Formations,sinkholes,depressions,and dissolution fissures are well developed,facilitating efficient infiltration of rainwater through karst conduits to the saturated zone.Although karst development is weak in the Badong Formation,secondary structures have intensely fractured the rock mass;rainwater recharge,therefore,occurs dominantly through fissures.Overall,the catchment exhibits favorable groundwater recharge conditions.After infiltration,groundwater migrates southwestward along either longitudinal karst conduits or fissure networks.The deeply cut watercourse in which the reservoir is located,along with the dense network of tributary gullies,ultimately leads to the discharges of groundwater in the form of karst springs within these gullies. The water-diversion tunnel crosses the anticline,traversing limestone and dolomite of T1d and T1j,with T1j serving as the principal aquifer.The anticlinal axis functions as a watershed,resulting in the southern and northern segments of the tunnel each forming an independent groundwater system.The tunnel site is deeply cut,resulting in poor recharge conditions.Scattered precipitation accumulates only within the upper narrow peak-cluster depressions,subsequently infiltrating vertically along sinkholes.Southwest of the tunnel,the reservoir catchment consists of T1j,T1d,and T2b formations,which are characterized by well-developed sinkholes,karst depressions,and corrosion fissures that create favorable discharge conditions.Following rainfall events,groundwater is directed southwestward along longitudinally oriented karst conduits.These conduits are cut by gullies,where groundwater ultimately discharges as karst springs.Before the construction of the water-diversion tunnel,atmospheric precipitation infiltrated through surface karst depressions,sinkholes,dolines,or corrosion fissures to recharge groundwater.Afterward,a portion of the precipitation flowed through the aquifer and discharged in the form of an underground river along the riverbank.Another portion of flowed through the runoff that crossed fissures perpendicular to the structural trend,and discharged in the form of karst springs(including the S01 spring point)or underground rivers within the gully and watercourse that cut the south-eastern wing of the anticline.The spring points exposed in the gully and watercourse were the main source of water supply for the reservoir.Because the water-diversion tunnel intersected the transition zone between the vertical and horizontal circulation cells of the karst aquifer,its excavation repeatedly intersected underground rivers and cave systems.Severe water inflows were encountered,resulting in twelve springs within a 3-km radius becoming either completely or partially dewatered.Springs located to the southwest of the tunnel were less affected than those to the northwest. Analytical calculations indicate that the influence radius of the water-diversion tunnel on surface water and groundwater extends from 649 to 2,073 m,covering 13.7 km2.The overlapping area of the reservoir catchment is 4.02 km2.Tracer tests confirm a strong hydraulic connection between the surface sinkholes located above the tunnel,and the drainage outlet of the water-diversion tunnel and several spring points.Consequently,a portion of the reservoir's recharge has been diverted to runoff at the tunnel exit and subsequently discharged,thereby reducing the storage capacity of the reservoir.The findings demonstrate that the alteration of regional groundwater recharge-flow-discharge conditions induced by the tunnel project is the primary cause of the observed decline in groundwater-derived inflow to the nearby reservoir.