Concept of karst cave hyporheic zone and its significance in water resource management

Groundwater recharge, runoff, and discharge modes in karst area dominated by conduit flow usually causes rapid and frequent interaction and transformation between groundwater and surface water, and therefore triggers the water environmental degradation. Single management of either groundwater or surface water cannot handle and resolve these problems. Take Wuming basin in South China as the case, the mechanism of karst groundwater interaction with other types of water and its environmental function was studied. The paper aims to understand the reason of karst water environment degeneration, and to provide recommendations for effective management of groundwater and surface river. Wuming basin is a typical peak forest area, with plenty of buried karst water. There are 148 groundwater systems in the basin, in which Lingshui spring is the largest and the most important one. Another four springs in the basin were selected too as the studied points. Online monitoring for water level and selected physicochemical parameters, in-situ measurements, and sampling analysis for water chemistry and plankton were used to study the characteristic of hyporheic zone in karst water system where are dominated by conduit flow. The concept of Hyporheic Zone (HZ) originated from hydrology and hydro-ecology is referenced and developed. Karst Cave Hyporheic Zone (CHZ) is defined as the site where conduit flow and other types of water interaction in South China. According to the water flow types and interaction styles, cave hyporheic zone is classified as spring outlet HZ, karst window HZ, sinkhole HZ, and karst conduit HZ. Focusing on spring outlet HZ and karst window HZ, the hydrology process, hydrochemistry characters, and aquatic organism community structure and its relationship with water environment are discussed. The results show the hydrology function of CHZ has a tendency of weakening due to the decrease of groundwater discharge. From the spring outlet to the CHZ, NO3-deceased by 6.5% to 90.9% and SO42- decreased by 2.1% to 18.1%. Degradation capacity of pollutant in spring outlet HZ is distinct, indicating that hydrochemistry function still plays a certain role in pollutant attenuation. Comparison of major community structure of aquatic organism in groundwater and HZ shows the biological function is degraded, mainly caused by weakening of hydrology function. Therefore, weakening of hydrology function is considered as the main reason for environmental function degradation in CHZ. It is suggested that CHZ should receive more attention in hydrogeology and environmental geology work in karst area. More techniques on exploration and monitoring for CHZ need to be discovered, and the environment function of CHZ should be improved through effective water management.