, Available online , doi: 10.11932/karst2025y014
Abstract:
Southwest China constitutes the world's largest karst region with concentrated exposure and distribution of carbonate rocks.Within this area, Chongqing City is a pivotal component of the southwestern karst system, with karst landscapes covering approximately 30,000 km2, primarily concentrated in the northeastern and southeastern of Chongqing City. This region has experienced multiple phases of strong karstification, resulting in well-developed karst landforms such as solution caves, sinkholes, karst springs, and underground rivers. Abundant surface water and groundwater have triggered frequent karst geohazards, including reservoir leakage, water inrush in tunnels, and karst collapses. These events have seriously constrained the safety construction of hydraulic projects and local economic development.The Yangquan River basin in Fengjie County, Chongqing City covers an area of approximately 45.5 km2, with a total river length of about 14 km. The daily average river water level is 895.1 m, while the average annual flow is 0.686 m3∙s−1, corresponding to a average annual runoff of 21.63 million m3. The basin has a subtropical warm and humid monsoon climate, characterized by an average annual precipitation of 1163.1 mm, an average annual evaporation of 1267.8 mm, and an average annual temperature of 18.7 ℃. Limestone and dolomite strata are developed in the basin, with undeveloped and small-scale faults located on the slopes of both banks. In contrast, fold structures are extensively developed, generally trending parallel to the Yangquan River. The left bank of the Yangquan River exhibits multiple gullies controlled by lithology and NW-trending fold structures. The topographic divide is the Jinfeng Mountain with elevations ranging from 1800 m to 1900 m. The right bank constitutes an interstream block between the Yangquan River and the Taoyuan River, characterized by a topographic divide elevation of approximately 1450~1600 m. Therefore, the left and right banks of the Yangquan River have steep topography, forming a typical “V”-shaped longitudinal valley. Overall, the karst hydrogeological conditions are complex, and karst leakage has become a crucial issue in the construction of pumped storage power stations in this area.This paper evaluates the karst hydrogeological conditions of Yangquan River basin by using geological survey and mapping, hydrogeological field experiments, and karst water system analysis. Subsequently, the karst leakage type, karst leakage location, and karst leakage passage are systematically analyzed during the impoundment of the Yangquan River reservoir. Ultimately, the three-dimensional seepage filed of the Yangquan River under natural and impounding conditions are conducted by using MODFLOW software. The karst leakage rates are calculated and validated through numerical modelling and analytical formulas. The results show that in the Yangquan River basin, karst development is mainly concentrated in the high-purity zone of carbonate rocks, structural fracture development zone, and discharge base level, forming five concentrated zones that exhibit a northeast-trending banded distribution. The degree of karst development gradually decreases with the increase of depth, exhibiting significant elevation zoning characteristics. This results in a dual shell-core structure characterized by strong karst development on the surface and weak karst development internally. Moreover, the karst groundwater system in the Yangquan River is a typical type of karstic fissure-tube-underground river system in the anticline mountainous of Southwest China, which can be spatially divided into strong karst anticline zone, strong karst syncline zone, and moderate karst syncline zone. Based on the water balance theory, the karst groundwater system in the site is classified as a secondary hydrogeological unit, namely II-2 interstream block. The recharge, runoff, and discharge modes of karst groundwater in the basin are primarily characterized by rainfall infiltration and concentrated drainage through longitudinal-transverse conduit-fissure flow systems. Packer tests and groundwater tracer tests indicate that the left bank of the Yangquan River develops an interconnected karst conduit system, while the right bank exhibits a dual-media system composed of fissure networks and karst conduits. During the impoundment of the lower reservoir in the Yangquan River, pore leakage may occur at the dam foundation, while karstic tube leakage and karstic tube-fissure coupling leakage may develop on the both left and right banks; however, there will be no leakage in the lower adjacent valley. The analytical solutions for the pore leakage and karstic tube leakage are 9505.13 m3∙d−1 and 117979.20 m3∙d−1, respectively. As a comparison, the numerical result of karst leakage is approximately 12184.60 m3∙d−1, indicating that more efficient and reasonable numerical methods and anti-seepage control measures need to be adopted to address karst groundwater tube system on both banks to achieve the conditions for dam construction and reservoir formation. This study can provide some suggestions for hydraulic projects in Southwest China.
