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Radiocarbon values of groundwater in main aquifers of the extremely arid Negev Desert and the Arava Valley, southern Israel, are used for studying the underground flow regime, particularly the complex connections between different aquifers and mixing of water bodies. The study shows that C-14 can serve as a hydrological tracer in arid environments and that groundwater dating may be possible (although not very accurate) even in this extremely arid environment (precipitation, <50 mm/yr), where there is almost no vegetation. There are several aquifers in this region, some of which are deep (deeper than 500 m) and regional and contain mainly fossil water, while others are local and restricted to the Arava, much shallower (50-200 m) and are thought to contain historical to recent waters. Most of the current recharge to these shallow unconsolidated aquifers comes from flash floods that flow from the mountains rising on both sides of the valley. The groundwater in the deep aquifers has low C-14 values (usually <5 pMC), implying old ages (preliminary ages >26,000 yr). Groundwater in the shallow aquifers characterized by higher C-14 values (up to 60-70 pMC) imply younger ages and faster groundwater flow (recent recharge). This is also supported by the presence of tritium in some of the samples. A few exceptional values are explained by the unique mixing of water from different sources; another is due to a technical failure in the well.