ENVIRONMENTAL TRITIUM IN THE UNSATURATED ZONE: ESTIMATION OF RECHARGE TO AN UNCONFINED AQUIFER.

Extraction of water from 11 soil cores taken vertically has allowed determination of the natural tritium profiles in the unsaturated zone at three different sites. These sites have the same geographic location and a mean annual rainfall of about 750 mm annually. Comparison of observed and model tritium profiles shows that piston-type flow of soil-water dominates below the root zone (1. 5 m), but above this, direct input to beneath the surface of the soil assumes importance. Estimation of recharge to groundwater from the tritium profiles, using both profile shape and the total quantity of tritium held in the soil profile, shows that mean annual recharge varies between 40 and 140 mm annually for the three sites. Multiple coring at two of the three sites has established that a recharge estimate made using a single core has a coefficient of variation of 15-20%.