Human Activities and Extreme Precipitation Boost Groundwater Renewal Capacity in the Beijing Plain

Groundwater plays a crucial role in the global water cycle. Unlike surface water, which exhibits immediate and observable reactions to environmental changes, groundwater is stored in aquifers beneath the Earth’s surface and demonstrates a more gradual response to external influences. The impact of environmental changes on the driving factors and interaction mechanisms of the water cycle introduces uncertainties regarding the renewal and response status of groundwater. This study aims to establish a comprehensive evaluation system to scientifically assess groundwater renewal capacity (GRC) and systematically elucidate the interaction processes between groundwater and various water sources during the recharge phase.

To meet urban water supply demands and mitigate the impacts of prolonged drought periods, Beijing has extensively relied on groundwater since the 1980s. However, since 2015, groundwater levels have exhibited an upward trend due to the implementation of the South-to-North Water Diversion Project, ecological replenishment initiatives, and several significant rainfall events. This region, which has undergone natural fluctuations characterized by periods of decline followed by recovery, serves as a pertinent case study for examining the responses of groundwater to both climatic influences and human activities.

The study elucidates the concept of GRC and identifies essential interaction and evaluation indicators between groundwater and external hydrological cycles. By considering factors such as groundwater sources, age, ion sensitivity, flow dynamics, and variations in burial depth, we evaluate GRC from multiple perspectives. Furthermore, we examine the responsiveness and adaptability of groundwater to external drivers, as well as the characteristics of recharge areas, pathways, and the overall mobility and openness of the system.

The following results are observed: (1) the system exhibits a high degree of openness and rapid responsiveness, characterized by swift infiltration and mixing processes, as well as a strong correlation between groundwater levels and precipitation. The heavy rainfall experienced in 2023 resulted in significant replenishment and mixing, leading to a reduction in burial depth from 16.97m to 15.22m. Following the flood season, the young water fraction of groundwater in the Chaobai River and Yongding River basins was 11.2% and 30.7%, respectively. (2) In recent years, GRC have undergone significant changes due to intensive environmental events, which are reflected in variations in sources, age, water levels. While precipitation remains the primary source of replenishment, the proportion of direct precipitation infiltration has decreased. Conversely, river water recharge, particularly following flooding events, has emerged as a significant contributor near the river channel, accounting for 25%. (3) Spatial variations in GRC have been identified, and areas with high potential for utilization, as well as groundwater circulation depths, have been preliminarily determined. The hydraulic gradient and runoff velocity exhibit a gradual decrease from the top of the alluvial fan to the middle aquifer, which corresponds with a decline in GRC. Furthermore, as aquifer depth increases in the plain area, GRC also diminishes.

This study elucidates the dynamics of groundwater circulation and renewal within the context of changing hydrological conditions, thereby contributing to a better understanding and management of groundwater resources.