Determination of groundwater recharge mechanism in the deep loessial unsaturated zone by environmental tracers

Studying the groundwater recharge mechanism in regions with thick unsaturated zone can greatly improve our understanding of hydrological processes since these regions have complex groundwater processes. This study attempted to discuss the groundwater recharge in a region covered by loess over 130 m deep in China's Loess Plateau. The water stable isotope, tritium and chloride in precipitation, groundwater and soil water were determined and used as inputs of mass balance methods. The tracer technique is found to be applicable and effective this region with thick unsaturated zone. The groundwater originates from rapid precipitation infiltration through some fast flow paths. The total recharge is likely to be 107 ± 55 mm yr⁻¹ accounting for 19 ± 10% of average annual precipitation, while the recharge from preferential flow accounts for 87 ± 4% of the total recharge. The identified recharge mechanism has important implication to groundwater management and recharge modeling for regions covered by thick loess. - Highlights: • We determined groundwater recharge mechanism in thick unsaturated zone covered by loess. • Both preferential flow and piston flow contribute to groundwater recharge. • Preferential flow dominates the total recharge. • The finding has great implication to hydrological modeling.