Using multiple environmental tracers to unravel surface water and groundwater interactions in a mountainous river basin fragmented by large reservoirs

Abstract

River fragmentation resulting from large reservoirs is rapidly increasing worldwide, but its impacts on surface water (SW)-groundwater (GW) interactions at upper and lower reaches of reservoirs are still poorly understood. Here, multiple water isotopes (δ¹⁸O, δD, and ²²²Rn) and hydrochemistry were jointly applied to identify the SW-GW interactions in the Upper Zhanghe River Basin impacted by three large reservoirs, a headwater basin of North China Plain. A total of 162 water samples were collected during three sampling campaigns for chemical and isotopic analysis. Results showed that SW and GW were predominated by Ca-HCO₃ type, with highly spatial variability and insignificant seasonal changes in chemical signatures. Most of groundwater samples presented higher d-excess and lower δ¹⁸O than those of adjacent river water samples, suggesting that river water was largely recharged by isotopically depleted groundwater. The upstream river of Guanhe and Houwan Reservoirs was recharged by riparian GW in both pre-monsoon and post-monsoon seasons, while their downstream river was a mixture of reservoir water (8 % to 88 %) and riparian GW (12 % to 92 %) with various mixing fractions. The Zhangze Reservoir’s upper reaches varied from a pre-monsoon gaining reach to a post-monsoon losing river segment (∼32 %). Its lower reach was a losing river segment in pre-monsoon, whereas the post-monsoon SW-GW exchange patterns evolved from a river stretch dominated by reservoir water through a gaining segment (GW: ∼11 %) to a losing reach (50 % ± 7 %). Furthermore, the shorter residence time (0.45 day) of groundwater downstream of Zhangze Reservoir than that (2.83 days) at the upper losing reach, also corroborated a stronger influence of river leakage and higher aquifer permeability at the lower reach. These findings will advance the knowledge of SW-GW interactions in ungauged mountainous river basins fragmented by reservoirs, with important implications for future integrated management of multiple water sources in similar river basins worldwide.