Water flow characteristics of Maocun underground river basin based on environmental isotopes

Underground river basins in peak-cluster depressions are main water supply sources of villages in Guangxi. But there exist problems of groundwater exploitation, groundwater pollution and natural disasters in these areas due to a special geological structure. Besides, the karst hydrological process is critical driving force for the sustainable development of ecological environment in karst areas; therefore, understanding mechanism of hydrological process is the key to the problem of ecological environment. Hydrochemical characteristics are closely related to hydrodynamics in a karst groundwater system, and hydrochemical tracers have been successfully used to indicate recharging sources, flow paths and water flow velocities.Taking Maocun underground river basin in peak-cluster depression in Guilin, Guangxi as an example, we interpret water flow characteristics in the study area based on the hydrodynamic processes indicated by hydrochemical environmental tracers. Temporal and spatial variations of isotopes and their influence factors, especially those of stable environmental isotopes as natural tracers of water cycle, are suitable for indicating water flow characteristics and hydrodynamic processes in karst areas. Variation ranges of δD and δ18O of karst water samples fall in the ranges of local precipitation, indicating that precipitation is the main recharge source in the study area. Dissolved inorganic carbon (DIC) in karst water is approximately equal to \begin{document}${\rm{HCO}}_3^{-}$\end{document}, and the concentration of \begin{document}${\rm{HCO}}_3^{-}$\end{document} is mainly affected by dissolution of carbonates and soil CO2. Based on isotopes of δ13CDIC and the linear mass conservation, the average value of DIC from carbonates dissolution is 52.13‰, which can be used to interpret the water-carbonate interaction.In southwestern karst areas, karst is well developed with the characteristics of special dual structure. Surface water and shallow groundwater is well connected, the retention time of shallow groundwater is short, and hydrological processes are sensitive to precipitation or artificial influences. 222Rn with the half-time of 3.82 d is the decay product of radioactive uranium and radium, which can be used to determine the characteristics of karst hydrological processes on a short-time scale. Different types of water bodies are of significant differences in the values of 222Rn, which can indicate the seasonal variation characteristics of groundwater levels, fractured karst networks, karst water cycles and water flow patterns.Electrical conductivity (EC) in the karst groundwater system is determined by interaction time of water and carbonate, which could be used to interpret ratios of recharing sources, groundwater flow patterns and structure characteristics of aquifer system. The complex spatial structures of karst aquifers interpreted by the responding characteristics of 222Rn and EC values to the precipitation show that karst water is recharged by different sources and water flow paths under different meteorological and hydrological conditions.Precipitation is driving force of water flow in karst areas. Accumulated precipitation can drive fractured water of deep karst flow to the downstream with strong dilution. The larger the amount of precipitation is, the faster the velocity of karst water flow is. The dispersed fissure flow is the main pattern in the karst aquifer system under the situation of less precipitation in August. Because the flow time in the conduit is longer than the half-time of 222Rn (3.82 d), decay characteristics of 222Rn (3.82 d) cannot be used to calculate the flow velocity of karst water. Accumulated precipitation could drive fractured water of the deep karst and conduit water to flow to the downstream under the situation of heavy precipitation in June. The flow time of conduit water from Dayanqian to outlet is 0.82 d (<3.82 d). The influence of water-carbonate interaction on 222Rn can be ignored, and the decay characteristics of 222Rn can be used to interpret flow patterns of conduit water in the rainy season. Based on the decay model of 222Rn in June, the effective water flow velocity in the underground conduit is 2,427.49 m·d−1, which is in the same order of magnitude as the results calculated by artificial tracers. Therefore, the decay characteristics of 222Rn can effectively reflect water flow patterns of conduits in the rainy season.Water flow patterns in the underground river of southwest China are similar to those of surface water, and the underground river responds sensitively to precipitation. Due to the limitations of spatial characteristics indicated by artificial tracers, they cannot be used to interpret spatial structure characteristics of a karst aquifer system. However, hydraulic connection of the karst aquifer system can be interpreted by the interrelationships among SI, 222Rn, δ18O and δ13CDIC of water samples located in the transition zone from non-karst areas to karst areas. Karst water in Xiaolongbei, Laolongshui, Bianyan and Shanwan are well connected hydraulicly. The good linear relationships among Beidiping-Shegengyan-Outlet and Laolongshui-Dayanqian-Outlet by environmental tracers indicate that there are complex water flow paths recharging the outlet in the underground river basin. Environmental isotopes can divide karst water samples into several groups, and better interpret water flow paths and spatial structure characteristics of karst aquifer system. Therefore, hydrochemical environmental tracers can provide important information of a karst aquifer system, interpret multiple flow characteristics and compensate for drawbacks of hydrodynamic methods.