Characterizing the hierarchical groundwater flow systems in Karstic Xiangxi River Basin, West Hubei, Central China

Abstract

Karstification degree is a key factor in controlling the characteristics of karst GFP. In this study, hydrogeology, karst development, hydrograph, artificial and natural tracers and hydrochemistry were integrated to characterize the groundwater flow patterns (GFP) in the Cambrian-Ordovician karst aquifer in Xiangxi River Basin. Groundwater samples were collected from three karst groundwater points: Wulongdong spring (WLD), Shuimoxi spring (SMX), and groundwater exposed by Borehole ZK03 (ZK03), which have different discharge variations. The hydrogeological analysis and the artificial tracer tests showed that groundwater runoff channel for WLD, SMX and ZK03 are dominated by conduits, fracture networks and micro fissures-matrix, respectively. The natural responses (discharge, electrical conductivity and temperature) of WLD to rainfall indicated that flow in this system is discharged by conduit, fracture and matrix, respectively. Stable isotopes (2H, 18O) of precipitation and groundwater suggested that the WLD and SMX recharge at the adjacent zone with mean elevations of 878 m and 676 m, respectively; while ZK03 recharge at mean elevations of 1338 m from thousands of meters away. Mean residence times calculated by artificial and natural tracers (18O, CFC) of conduit flow in WLD (7–19 h), fracture flow in WLD and SMX (243 and 600 days), and matrix flow in ZK03 (41 years) were obviously increased gradually. Hydro-chemical analysis shows that ZK03 has higher concentrations of ions that related to carbonate dissolution, indicating groundwater in ZK03 has longer time to interact with rocks. The results show that GFP in study aquifer are characterized by local fast conduit flow, local intermediate fracture networks flow and regional slow matrix flow with progressively slower groundwater refresh rate. The conceptual model was also proposed to enable a better understanding of groundwater circulation, water resources and vulnerability of the karst system in Central China.