Fig. 4: In low-rainfall, arid conditions, infiltrating rainwater moves intermittently, and may remain in the unsaturated zone for decades. The "peak" in tritium concentrations of precipitation worldwide may still be found in soil profiles, such as this one from Senegal, which show how far down moisture has moved from the surface since it was deposited by rain in the early 1960's.
Fig. 5: In moist climates, water moves downwards through the soil fairly steadily. When tritium is injected, just below the surface, it moves with the water. The artificial "peak" can later be traced, and the infiltration rate measured.
