Characterizing groundwater recharge sources within reduced pumped aquifers in the Heilonggang region, North China Plain

Groundwater exploitation reduction (GWER) and cross-regional water diversion projects are exerting profound effects on the subsurface water cycle in the North China Plain. A detailed characterization of groundwater recharge sources is a prerequisite of groundwater restoration and multiple water sources' integrated management. For such an endeavor, a total of 222 groundwater samples, 35 surface water samples, and 48 precipitation samples were collected from an important agricultural production base (i.e., Heilonggang region). The information from stable isotopes was combined with geochemistry and water levels to identify the recharge sources of different aquifer units. The unconfined groundwater was recharged by the surface water and the semi-confined groundwater, and preferential recharge during heavy rain events. Moreover, the unconfined groundwater showed the most enriched heavy isotopes and the highest salinity, which were attributed to intense evaporation, vegetation transpiration and anthropogenic inputs. The GWER and water transfer projects jointly elevated the unconfined water levels by reducing the pumping amount and increasing the surface water infiltration. The semi-confined aquifer was a mixture of the unconfined aquifer, the confined aquifer, and the lateral flow. The confined groundwater was mainly old water formed in the paleo-climatic wet and cold conditions, and it was recharged by the semi-confined groundwater and lateral flow in localized areas. Additionally, more depleted isotopes and lower salinity in the confined aquifer than the two upper aquifers reflected their weak hydraulic connections. Due to insufficient recharge sources, the restoration of the confined groundwater with poor renewability may require a long period. These findings enhance the knowledge of groundwater recharge sources in such a region where aquifer restoration is in progress, with important implications for the ongoing efforts on the integrated water resources management in other similar watersheds worldwide.