Identifying hydraulic connectivity among the vadose zone, unconfined and confined aquifers in the thick loess deposits using multiple tracers

[Display omitted] •Piston flow dominates the water flux from thick vadose zone to loess aquifer.•Sulfate isotopes are useful to identify connectivity among different aquifers.•Unconfined and confined aquifers are not vertically hydraulically connected.•Multiple tracers can advance the conceptualization of water cycle processes. Low water flux and a thick vadose zone increase the likelihood that water bearing units are hydraulically disconnected in shallow groundwater systems. Particularly for the thick loess deposit of China, hydraulic connectivity among the vadose zone, unconfined and confined aquifers is still unclear since it is controversial whether the unconfined groundwater is recharged by precipitation or confined groundwater. Taking several loess tablelands as example, we collected precipitation, soil water, unconfined and confined groundwater to measure the contents of multiple tracers (e.g., hydrochemistry, δD, δ18Owater, δ17Owater, δ34S, δ18Osulfate, δ13C and 14C) to evaluate hydraulic connectivity among the vadose zone, unconfined and confined aquifers. Deep soil water can be recharged by precipitation with intensities ≥ 30–40 mm/day, and then vertically discharge to unconfined water primarily through matrix flow with an infiltration velocity of 0.15 m/year. Matrix flow dominates the water flux from the vadose zone to the unconfined aquifer with the rates of 85.4 ± 39.6 mm/year, reflecting the regional hydraulic equilibrium. The unconfined and confined aquifers exhibit negligible vertical hydraulic connectivity because of different recharge periods, flow patterns, water − rock interaction, water residence time and sulfate evolution behaviors. The identified connectivity supports local precipitation as the recharge source, which conflicts with the external source hypothesis. The utilization of multiple tracers in this study can complement each other to provide more comprehensive results, and the results greatly improve the conceptualization of vertical water exchange processes in the loess deposits.