Application of high-resolution tracer technique in identifying the source of water gushing and the structure of aquifer medium in karst tunnel

The study area is located in the Longyan river basin, a tributary of Wujiang river, and is in Sanquan town, Nanchuan district, Chongqing. The strata of this area comprise the upper Cambrian dolomite. The terrain is dominated by medium mountain and mid-low mountains, with topographic high in the northeast and low in the southwest. Dissolution karst depressions and dolines are well developed in the area, below which is the Changtan underground river basically recharged by the source from Shangwan depression and Maiyincao depression. The Sanquan tunnel is located at 60 m under the Changtan underground river, where water inrush disaster occurred during the rainstorm on 15 and 16 April 2016. This study focuses on the verification of the connection between gushing water and Changtan river, identification of the source of gushing water and the discussion on the properties of the karst aquifer media. A highresolution tracer test using sodium fluorescein and Tinopal CBS-X was applied to the localities between the tunnel water bursting point and Shangwan and Maiyincao depressions . The results show that,(1) The receiving point received the Tinopal CBS-X released at Maiyincao depression , but no sodium fluorescein from Shangwan depression was received. The recovery rate of the Tinopal CBS-X at the left and right caves were 28.7% and 36.6%, respectively;(2) The maximum flow velocity of groundwater is 246.1 m?h-1 , with an average value of 118.5 m?h-1 . Two breakthrough curves (BTCs) presented a double-peaked shape, with the main peak in the front and the second peak in the back;(3) Based on the application of Qtracer2 software, the geometric and hydrogeological parameters such as flow channel volume, flow channel surface area, average diameter, distance of underground river, the friction coefficient, Sherwood number, Schmidt number, hydraulic depth and molecular diffusion boundary layer thickness are 1,148.4 m3 , 1.30×106 m2, 1.37 m, 780 m, 0.51, 1，055.1, 1，140, 1.08 m and 1.3 mm, respectively. From the analytical results, it is realized that there is a hydraulic connection between Maiyincao sinkhole and the tunnel gushing point, affected by Changtan underground river. The low recovery rate of Tinopal CBS-X is perhaps attributed to the presence of other discharge points and the properties of the flow systems. The groundwater flow field is a typical turbulent flow pattern. Two conduits are developed in this karst aquifer, controlled by parallel fissures without underground lakes. The hydraulic connectivity between Shangwan depression and water gushing section needs a further study. The accuracy of the study depend on flow condition, test period, density of monitoring points, and a contrastive test is necessary. Tunnel excavation disturbance and continuous precipitation are the main causes of water inrush to the tunnel, which is a type of karst pipeline inrush disaster. Due to the water inrush point of the Sanquan tunnel has direct hydraulic connection with surface water, and the amount of water gushing is large, it is suggested to drain the water from the depression, fill the Maiyincao sinkhole above the tunnel, or build a drainage ditch at the bottom of the tunnel.