Fracture Network Structure and Groundwater Seepage Characteristics in the Karst of Northern China

ABSTRACT

The high heterogeneity and anisotropy of karst aquifer media are key scientific issues that constrain accurate simulation of groundwater flow in karst regions. Taking the typical Cambrian–Ordovician karst water system in northern China as an example, this study systematically reveals the multi‐scale structural characteristics of karst fractures and their controlling mechanisms on seepage behavior using high‐precision micro‐focus CT scanning, three‐dimensional digital core reconstruction, pore‐network model extraction, and seepage simulation. The results show that karst fracture development in northern China exhibits significant spatial heterogeneity. The effective quantification range of karst fracture aperture is 284–35,916 μm, fracture length is 284–36,113 μm, roughness is 0.16–1.77 mm, and tortuosity is 1.03–1.68; throat radius ranges from 89 to 7840 μm, and coordination number ranges from 0 to 12, which jointly control the development of preferential flow paths. Based on Reynolds number (Re) discrimination and Re– r log–log regression, the critical throat radius corresponding to the Reynolds number of throats reaching the turbulence criterion (Re = 200) is 231.7 μm (95% confidence interval 212.1–243.8 μm), i.e., a critical diameter of about 463 μm (424–488 μm); Darcy‐flow throats with Re  < 10 are distributed over a broad radius range of 89–2912 μm, and the onset of deviation from Darcy's law cannot be defined by a single throat‐size threshold, but is mainly controlled by throat connectivity and local flow rate; in effective flow throats, inertial effects increase with increasing aperture. In karst media, Darcy and non‐Darcy flow coexist; pressure decreases hierarchically along the path, and flow velocity concentratedly rises at throat junctions and flow‐direction turns, exhibiting channel dependence and local acceleration characteristics. A cross‐scale seepage motion equation integrating porosity, tortuosity, and roughness was established, achieving coupling between microstructural parameters and macroscopic nonlinear seepage, and revealing the response characteristics of spring water level dynamics to seepage in voids of different scales. The results can enrich the theory of karst water seepage in northern China and provide a basis for karst water pollution prevention and control and water resource management.