Mechanism of groundwater recharge in the thick loess deposits by multiple environmental tracers.

The loess-covered region accounts for ~10 % of global land surface. Because of dry climate and thick vadose zones, the subsurface water flux is low but the water storage is relatively large . As a result, the groundwater recharge mechanism is complicated and currently controversial (e.g., piston flow or dual mode with piston and preferential flow). Taking typical tablelands in China's Loess Plateau as example study area, this study aims to qualitatively and quantitively evaluate the forms/rates and controls of groundwater recharge considering space and time. We collected 498 precipitation, soil water and groundwater samples in 2014-2021 for hydrochemical and isotopic analysis (Cl, NO, δO, δH, H and C). A graphical method was employed to determine appropriate model to correct C age. Dual model exhibited in the recharge: regional-scale piston flow and local-scale preferential flow. Piston flow dominated groundwater recharge with a proportion of 77 %-89 %. Preferential flow gradually declined with increasing water table depths, and the upper depth limit may be <40 m. The dynamics of tracers proved that mixing and dispersion effects of aquifers limited the ability of tracers to capture preferential flow at short-time scales. Long-term average potential recharge (79 ± 49 mm/year) was close to actual recharge (85 ± 41 mm/year) at the regional scale, indicating the hydraulic equilibrium between unsaturated and saturated zones. The thickness of vadose zone controlled recharge forms, and precipitation dominated the potential and actual recharge rates. Land-use change can also affect the potential recharge rates at point and field scales but maintain the dominance of piston flow. The revealed spatially-varied recharge mechanism is useful for groundwater modeling and the method can be referred for studying recharge mechanism in thick aquifers.