The southwest China is covered by large areas of carbonate rock strata, in which the movement of many terranes results in strong folds and rock fractures. The spatial structure diversity of strata and tectonics has formed various complex and distinctive karst water systems. Therefore, tunnels for traffic lines and water diversion projects are often built in southwest China, but when tunnels cross karst water systems, the tunnel inflow conditions are difficult to be identified due to the complexity of water inflow, which has been a challenge in tunnel construction projects. In the Xiaopu tunnel area of the central Yunnan Province, linear extension and closely interbedded soluble and non-soluble rock strata are developed, forming a typical tight-narrow monoclinic karst water system. Since the tunnel construction, the water inrush has respectively occurred in Section 1 and Section 2 with large and long-lasting water inflow. The complexity of the water inflow brings a great challenge for the tunnel construction. Therefore, it is urgent to find out the source and mode of water inflow in Xiaopu tunnel. Focusing on specific engineering problems, this study carefully sorts out the controlling factors of the characteristics of the tight-narrow monoclinic karst water system in Xiaopu tunnel. The division of the karst water system is reduced to a small scale, and the water inflow conditions of the tunnel are identified according to the groundwater monitoring data as well as the inflow conditions such as the hydrochemical characteristics and hydrogen and oxygen isotopes of water inflow.

The results show that the tight-narrow monoclinic structure affects the spatial pattern of karst development, the runoff and the transformation mode of groundwater. This structure also controls the characteristics of aquifer media development and groundwater recharge, runoff and discharge distribution. Hence, the study area can be subdivided into four small-scale karst water systems with different characteristics, i.e., the bare-covered fissure flow and conduit flow with concentrated discharge, bare conduit flow with concentrated discharge, the bare-covered fissure flow with multiplex discharge, and the bare fissure flow with concentrated discharge. The hydrochemical type of groundwater in the Duimen-Daliyuan-Niuliancun karst water system, which is crossed by the Section 1 of Xiaopu tunnel, is mainly HCO₃·SO₄-Ca·Mg. And the concentration of Ba²⁺ in groundwater is higher than that in other karst water systems due to the dissolution of barite minerals in the strata of the Weining group of the middle Carboniferous and the upper Devonian. The hydrochemical type of groundwater in the Piantoushan-Lunacun-Jinxiandong karst water system crossed by Section 2 is HCO₃-Ca·Mg, indicating relatively pure karst water. This shows that dolomite dissolution is the main hydrochemical process for controlling water quality. Due to the slow flow of the karstic fracture network, the groundwater has a long residence time in the runoff path, and the water-rock interaction and evaporation are relatively strong, thus the heavy hydrogen and oxygen isotopes are more enriched. Huge differences in hydrogen and oxygen isotope composition also rule out the possibility of surface water flooding into the tunnel. It is concluded that the water inflow in Section 1 is caused by the uncovering of the underground conduit and the interception of the discharge spring of the shallow karst downstream. The karst conduit developed in the contact zone of igneous rock constitutes the main water inflow channel, in which the groundwater in the conduit is the water source with a recharge elevation of 2,165.4 m, mainly located in the karst depression in the north of Wangjiawan. The water inflow in Section 2 is caused by the exposure of karst fissures. The fissure network constitutes the main water flow passage, and the groundwater in Dengying formation is the water source, with a recharge elevation of 2,234.6 m, mainly located in the slope area on the north ridge of Piantou mountain.

The groundwater in different karst water systems has experienced different water-rock interaction and evaporation processes. Therefore, the hydrochemistry and stable hydrogen and oxygen isotopes of groundwater are good natural tracers for tracing the source of water inflows in tunnels crossing different karst water systems. Comprehensive utilization of multiple approaches and information verification can improve the accuracy of identifying tunnel water inflow conditions, which may provide a basis for the subsequent design of tunnel drainage projects and the prevention of water inrush disasters in the tunnel.