How does precipitation recharge groundwater in loess aquifers? Evidence from multiple environmental tracers

Characterization of the rainfall infiltration mechanism, whether piston or preferential flow, through an unsaturated zone is necessary for groundwater resource assessment and for analysis of environmental geological hazards. However, several limitations and difficulties remain in determining the infiltration mechanism in arid and semiarid areas. This study combines the evaluations of soil water infiltration characteristics, recharge rate, and determination of groundwater age at a water table in a semiarid loess tableland to examine the recharge processes. The case study is the Xifeng loess tableland in the Loess Plateau of China, which has an unsaturated zone depth of 40-75 m and an obvious 1963-3H peak at a depth of 7.5 m. According to the piston flow, the soil water infiltration velocity is 0.12-0.14 m/yr, corresponding to a recharge rate of 37-41 mm/yr, based on the 1963-3H peak and chloride mass balance method. All of the groundwater from the water table and wells is old (tritium-free), with ages ranging from hundreds of years to 20,000 years, which suggests that no rapid modern recharge has occurred since the 1950s. The chloride content of soil water in the deep unsaturated zone, with an average value of 10.8 mg/L in the unsaturated zone below 30 m, overlaps with that in groundwater, at 4.0-10.9 mg/L. The comprehensive isotopic and chemical compositions for soil water and groundwater showed that piston flow was dominant during soil water infiltration at the field scale, resulting in the delay of modern water and related solutes entering the groundwater. Combined evaluations of the unsaturated zone with those of the saturated zone and the use of multiple environmental tracers, such as Cl, Br, H-2, O-18, C-13, H-3 and C-14, can be regarded as a generic framework for studying the recharge processes in arid and semiarid areas with thick unsaturated zones at macroscopic and field scales.