[en] The movement of the moisture in an unsaturated zone can be monitored using both environmental and artificial tritium. Artificially tritiated water is used as a tracer in this study for determining groundwater recharge. Groundwater recharge is the influx of rain into the saturated zone and is essential for urban water supply, agriculture, and resource evaluation. The technique was used on a large scale in Rio Grande do Norte state in northeastern Brazil, covering an area of ∼1500 km². The region of the study has a semiarid climate, and most of the soil was very sandy, except one site that was pure clay. This study was part of a larger water resource evaluation project