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The carbonate Dammam Formation of Eocene age and the unconformably overlying clastic Kuwait Group of rocks of Mio-Pliocene age are the two aquifers that provide useable groundwater (salinity 5000 mg/l or less) in Kuwait. These aquifers extend beyond the political limits of the country to Saudi Arabia and Iraq. A detailed environmental isotopic (H-2/1, O-18/16, C-13/12, H-3, C-14) characterization of the groundwater system of southern Kuwait (a regional aquifer extending beyond the political limits of the country to Saudi Arabia and Iraq) has been carried out to understand its hydrodynamics so as to evolve a better conceptual flow model of the aquifer system. From the study, it was observed that the groundwater salinity of the Kuwait Group aquifer increases generally from south-west to north-east, although locally a few of them show low values. Groundwater samples from the Dammam Formation aquifer are characterized by a relatively low mineralization, compared with those of Kuwait Group aquifer. The water in the Kuwait Group aquifer in the south-western part of the country is Na-Cl type and in the Dammam Formation aquifer is Na-Cl to SO4-Na (Cl > SO4) type. A wide range of d-excess of the groundwater samples was seen (Kuwait Group aquifer ? 16.0 to -20.6 parts per thousand; Dammam Formation aquifer: +22.8 to -19.6 parts per thousand), indicating that the groundwater system contains more than one type of water with different origin (recharge area) and history (time, evaporation, climate). In both the aquifers, the groundwater samples from south and central parts and along the coast showed negative d-excess values with heavier O-18 and H-2, indicating older recharge and greater degree of evaporation prior to recharge. The delta O-18 and delta H-2 of the initial recharge waters, respectively, are -8.8 and -40 parts per thousand for the Kuwait Group aquifer and -7.2 and -32 parts per thousand for the Dammam Formation aquifer. The corrected C-14 ages (Tamer’s model) of groundwater samples of Kuwait Group aquifer ranged from similar to 5000 to 24,000 a, B.P, and that of Dammam Formation aquifer ranged from similar to 6000 to 20,000 a, B.P. The intermediate groundwater in the southwest and central parts of Kuwait Group aquifer is recharged during humid period (similar to 5000 to 10,000 a, B.P), whereas deeper groundwater in the south is recharged during arid period (similar to 9000-16,000 a, B.P). The recharge to the Dammam Formation aquifer mostly occurred during a cooler (humid) phase in the past. The Dammam Formation aquifer water is hydraulically well mixed below 350 m depth. Interconnection between the two aquifers established near the coast and south-west border of the country. The results of the study were used to refine the existing conceptual groundwater flow model for the aquifer system.