Integrating hydrochemical and biological approaches to investigate the surface water and groundwater interactions in the hyporheic zone of the Liuxi River basin, southern China

The characterization of groundwater (GW) and surface water (SW) interaction in riparian zones is of decisive importance for purifying and maintaining the water quality, and for protecting the riverine ecosystem. Through intensive monitoring using a combination of hydrochemical and biological analyses of the riparian GW and SW in the Liuxi River basin, China, the spatial-temporal patterns and extent of GW-SW interactions were qualitatively and quantitatively identified. The similarity of dynamic changes in the river water and GW levels reflected the hydrological connectivity between the river water and riparian GW. The discontinuity of the concentration in the majority of water chemistry parameters delineated the GW-SW interaction chemical boundaries as being within a distance of 2 and 0.5 m from the river in the wet and dry seasons, respectively. The GW in the GW-SW interaction zone was characterized as having lower levels of dissolved oxygen and nitrate, moderate levels of electrical conductivity and major ions, and higher total organic carbon values. The percentage of river water in riparian GW increased with the proximity to the river from 9.0 to 68.8% in the wet season (WS) to 6.0-23.1% in the dry season (DS). The extent of GW-SW interactions was quantitatively identified by the chloride mass balance to be within 2 and 0.5 m from the river in the WS and DS, respectively. Based on the one-dimensional numerical particle tracking method, the exchange width of river water and GW was calculated to be 0.52 m in the DS as well. The analyses of microbial data demonstrated that GW samples within a distance of 2 m in the WS and 0.5 m in the DS exhibited a unique microbial community structure that contained a mixture of microbial components in the river water and background GW, indicative of the GW-SW mixing. The GW-SW interaction microbial boundaries were consistent with the chemical boundaries in both seasons. This study demonstrated the effectiveness of an integrated approach combining hydrochemical data and microbial community structure for the determination of GW-SW interactions.