Water movement in the unsaturated zone of high and low permeability strata by measuring natural tritium

Measurement of the natural tritium concentration in groundwater samples from a vertical profile in the unsaturated zone of a formation has been used to provide information on the downward movement of water in the strata since the production of thermonuclear tritium in 1954. The tritium profiles are evaluated in terms of hydrological models to examine the mechanism of water recharge. Tritium measurements in the Chalk have shown that downward movement is principally by intergranular seepage with a smaller component of movement through crack systems. Two hydrological models offer alternative interpretation of the water recharge. Tritium measurements in a low-permeability clay show the downward movement of water by intergranular seepage and indicate that recharge occurs by replenishment of the summer soil moisture deficit by autumn rainfall. (author)