Hydrogeological Functioning of a Karst Underground River Basin in Southwest China

The Maocun underground karst river system in the peak cluster depression is an important source of groundwater in southwest China. Multitracers and high resolution water-level-monitoring technology were used to assess and evaluate the hydrogeological structure and flow dynamics. The results showed that the spatial geological structures of the sites had high heterogeneity. Scatter plots of environmental tracers divided the sampling points into groups under different water flow patterns. The karstification was found to increase from sites XLB and LLS to sites BY, SGY and BDP to sites CY and DYQ, where the main water flow patterns at these site groups were diffuse water, both diffuse water and conduit water, and conduit water, respectively. The response times of the subsystems were found to be influenced by the spatial structure, the degree of karstification, and the volume of precipitation and frequency. The average response times of SGY, BDP, ZK, and Outlet in the selected precipitation scenarios were 5.17, 4.08, 16.42, and 5.83 h, respectively. In addition, the EC, δ¹³C_DIC, ²²²Rn, and δ¹⁸O exhibited both linear or exponential relationships. Overall, three hydrogeological conceptual models were constructed showing: (1) high precipitation driving the deep water, resulting in a concentrated flow regime and regional groundwater flow field; (2) both concentrated and diffuse water flows existing under moderate precipitation, resulting in mixed water flow field; (3) the water cycle in the shallow karst aquifer system under low precipitation causing the local groundwater flow field to be dominated by diffuse water flow.