Distribution and influencing factors of karst underground rivers in the Pearl River Basin

The Pearl River Basin is the area of first-class water resources at the southernmost end of China. Its geographical location is 102°14′-115°57′ E and 21°35′-26°50′N. The main stream of the Pearl River, with a total length of 2,214 km, flows through Yunnan, Guizhou, Guangxi and Guangdong. The total area of the Pearl River Basin is about 438,100 km², including Nanbeipan river, Hongliu river, Yujiang river, Xijiang river, Beijiang river, the Pearl River Delta and the dongjiang river basin. The water-bearing rocks in the Pearl River Basin are divided into karst water of carbonate rock, fissure water of clastic rock, pore water of loose rock and fissure water of magmatic rock. The distribution area of exposed carbonate rocks is about 149,500 km², and that of buried carbonate rocks is about 39,600 km², accounting for 43.16% of the total area. Because there are 1,036 karst underground rivers, and the discharge of karst underground rivers of the Pearl River Basin in the dry season is about 47.39 million m³·d⁻¹. the basin is rich in groundwater resources. Therefore, it is of great significance to explore the distribution and karstification characteristics of underground rivers for the exploitation and utilization of karst water resources in southern China. Based on a large number of field investigations in the southwest karst area, 200,000 hydrogeological survey reports and the 1∶250,000 and 1∶50,000 karst hydrogeological surveys conducted from 2003 to 2016 in the Pearl River Basin, we selected 348 groups of rock samples and 1,036 karst underground rivers as study objects. Then, taking Dalongtan and Sanqiutian underground rivers in Yunnan Province, Sixiaojing, Tianshengqiao and Huachu underground rivers in Guizhou Province, and Disu, Zhaidi and Dizhou underground rivers in Guangxi as examples, we analyzed and summarized the development law, distribution characteristics and influencing factors of underground rivers in the Pearl River Basin from the perspectives of lithology, landform, structure, hydrodynamic conditions and neotectonic movement. According to the analysis, the outlets of 310 underground river are at an elevation between 200-400 m, accounting for the largest proportion. Among them, more than 30 underground rivers, mainly located in the river basins of Hongliu river, Yuhe river, South Panjiang river and north Panjiang river, respectively cover a catchment area greater than 200 km². Their discharge in the dry season is greater than 1,000 L·s⁻¹ and the length of main pipeline is greater than 10 km. The study result shows that the underground river is most developed in fine-grained oolitic bioclastic pure limestone with a specific solubility of 0.84-1.2, moderately developed in dolomite with a specific solubility of 0.62-0.83, and weakly developed in argillaceous limestone with a specific solubility of 0.43-0.61. Landform and surface river network determine the trend and direction of karst underground river movement. The structure controls the spatial pattern of underground river development. The development of underground river is particularly obvious in the reverse composite part of the structure, the fracture zone on both sides of the compressional torsional fault, the contact zone with the non-soluble rock, the part with the largest fold bending, and the fracture zone in the anticline axis and the syncline axis. Hydrodynamic characteristics affect the development scale and depth of underground rivers. The neotectonic movement facilitates the constant changes of cycle and alternation conditions of groundwater, as well as the deep, inherited and new development of underground rivers. Finally, according to the morphology of underground river and the evolution conditions of water circulation, the underground river can be divided into four types: single conduit type at the initial stage of development, multi-stage feather type, neotectonic control network type and mature dendritic type. All in all, this study is expected to provide data support for the exploitation and utilization of karst underground rivers, monitoring and evaluation of water resources, and the selection of backup water sources to meet urban emergency in the Pearl River Basin.