Recharge mechanisms of deep soil water revealed by water isotopes in deep loess deposits

Isotopes in soil water are useful tracers of flow and transport processes in the subsurface. The combined utility of stable (2H, 18O) and radioactive (3H) isotopes in soil water, however, is seldom explored in vadose zone studies, particularly in identifying deep soil water recharge mechanisms (sources, fluxes, and ages). Here we show that the combined use of 2H, 18O, and 3H can identify the sources and recharge mechanisms of water to thick deep soils of the Loess Plateau, China. We found that high soil water tritium concentrations were limited between 6 and 7 m from the surface, suggesting that soils deeper than 7 m were comprised of water older than ~50 years, while soils shallower than 6 m represented water younger than ~50 years. We also found that rainfall intensity played an important role in deep soil water recharge. Stable isotopes suggest that only a few extreme rainfall events (≥30 mm/day) during the rainy season infiltrated to deep soil layers, but the age of water that recharged the soils at these depths were older than ~50 years as inferred from tritium. These combined insights from stable and radioactive isotopes in soil water suggest that (1) waters in these deep soils are much older than the apple orchards that cover these landscapes, and (2) very few high-intensity rainfall events have reached the deeper parts of the soil profile since the apple orchards were established