Study of groundwater recharge using combined unsaturated-and saturated-zone chloride mass balance methods

Understanding groundwater recharge mechanisms is essential for reliable groundwater recharge estimation for sustainable groundwater resource assessment and management, especially in arid and semiarid areas. In this study, chloride, together with stable isotopes (O-18 and H-2), in 1-year rainfall, soil water from representative profiles, and groundwater samples, collected in the Ordos Plateau, northern China were analysed. Groundwater recharge was estimated using unsaturated- and saturated-zone chloride mass balance (CMB). An equation that describes the relationship between chloride concentrations in saturated zone water (C-sz) and soil water residence time (tau) was derived. With the help of the relationship, chloride concentrations in unsaturated zone water (C-uz) and in groundwater (C-sz) can be used to uncover water flow and chloride transport in the unsaturated zone. The relationship between C-sz and C-uz indicates that, the groundwater was recharged mainly by intense rainfall events. Small rainfall events did not lead to groundwater recharge but contributed to the flux of chloride to soil surface. Stable isotopic compositions of O-18 and H-2 provided corroborative evidence of the recharge processes. The relationship between C-sz and tau indicates further that there is no source of chloride in the groundwater other than that from precipitation. Thus, C-sz can give more reliable recharge estimates. C-uz was influenced by heavy rainfall-induced runoff, run-on and bypass flow events and cannot give reliable recharge estimates. However, if used jointly with C-sz, C-uz can help to gain insights into recharge processes and yield groundwater recharge estimates with higher certainty. The outcomes of this study can assist for groundwater recharge investigation and assessment in regions where the assumptions and boundary conditions necessary for the correct application of the CMB method may not be met.

Automated summary

Understanding groundwater recharge mechanisms is crucial for sustainable groundwater resource assessment and management, especially in arid and semiarid areas. This study utilized chloride and stable isotopes to estimate groundwater recharge in the Ordos Plateau, northern China. The relationship between chloride concentrations in saturated zone water and soil water residence time was derived, providing insights into water flow and chloride transport. The study concluded that intense rainfall events were the main contributors to groundwater recharge, while small rainfall events only contributed to chloride flux on the soil surface.