Hydrochemical evolution and groundwater-river interaction in a headwater catchment in Miyun Reservoir, China

Water resource shortage is a serious problem that hinders the development of metropolis, especially in arid and semiarid area. As an important drinking water source of Beijing, Miyun Reservoir has experienced water supply decline in recent decades. This decline was caused not only by insufficient water quantity but also by reduced water quality in both groundwater and surface water. It is urgent to enhance our knowledge of hydrochemical evolution and groundwater-river interaction in this area. Here we conducted a field work in a headwater catchment of Miyun Reservoir from July 2013 to November 2014. The study area has an area of 20.6 km2 and is characterized by a thin soil layer and an underlying thick zone of weathered bedrock. We found the solute constituents of river water and groundwater in the study area are mainly of Ca-HCO3 or Ca-SO4 type. Atmospheric input and rock weathering are the major processes that account for the water chemistry. From upstream to downstream, the study area can be divided into three zones, i.e., groundwater discharge, river seepage, and human activity impact zones based on the conceptual model of hydrochemical evolution. The interaction between river water and groundwater changed from intensive to insignificant from the upstream to downstream, and the influence of anthropogenic activity on groundwater quality slightly increased. The results of this study can improve our understanding of the mechanism of runoff generation and solute transport in the Miyun Reservoir catchment and facilitate local water resource management.