Tritium tracer, injected below the surface soil, is used to follow the movement of soil moisture in the unsaturated zone at several geo-hydrologically different sites in Sri Lanka. Depth profiles of tritium activity and moisture contents have been measured after rainfall. For two well-established aquifers, Vanathavillu and Mannar, the recharge to groundwater is close to 30% of the rainfall. Infiltration is much higher. The moisture movement is apparently like piston flow. In the central highland regions, where the surface cover is composed of an overburden of fractured rocks overlain by a few metres of soil cover, the infiltration rate is very high.

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