• 标题：Hydro-ecological processes of hyporheic zone in a karst spring-fed pool: effects of groundwater decline and river backflow
• 关键词：Karst spring; Hyporheic zone; River backflow; E. coli; Drinking water source
• 作者：Fang Guo, Guanghui Jiang

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Karst springs constitute an important water resource. Their aquatic environment, however, is susceptible to degradation due to the presence of karst conduit that can deliver pollutants rapidly to the groundwater. This investigation is a case study of a spring-fed pool that is affected by a river. Hydrological, hydrochemical, and biological approaches were used to reveal the characteristics and consequences of the interaction between the groundwater of karst springs and the surface water. During the rainy seasons, the river backflow effect causes the spring-fed pool to form a hyporheic zone that is sensitive to hydrological and hydrochemical processes. Under forward flow conditions, the physicochemical parameters of groundwater at the spring-fed pool exhibited gradient variations in longitudinal flow direction and changed with depth. The threshold value of river backflow into the pool is 99 m  (masl). In 2017, the water level of the pool ranged from 96 to 100 m during the high-flow period, which was 1 to 5 meters higher than those in the low-flow period. One to several times a year of river backflow ultimately leads to the death of submerged plants at the spring-fed pool, followed by phytoplankton blooms. Chlorophyll-a (Chl-a) concentration of groundwater was 16 times higher in regions with the slow flow than in those with the quick flow, and Chl-a concentration was higher in the bottom layer than in the surface layer. E. coli in groundwater was affected by the hyporheic zone and downstream rivers and E. coli counts were associated with hydrodynamic conditions. Due to the reduced upstream discharge and the increased amount of water consumption, the frequency and intensity of river backflow increase, leading to a weakening of the hydrological and biological functions in the hyporheic zone. The presence of the hyporheic zone makes possible the transport of pollutants, microorganisms, and plankton to upstream springs via the reverse flow of downstream rivers. We recommend that the effects of hyporheic zones be considered when classifying sanitary protection zones in the karst water system.