[en] Full text: The tertiary aquifer system is the most significant system in Kuwait. This aquifer system contains a compatible quality of brackish water with a TDS value of less than 7500 mg/l. Two small locally shallow aquifers of the Neogene, with fresh water (having a TDS value of less than 1500 mg/l), also exist in this aquifer system in the north of the country. The early studies during the 1970s and 1980s, which were based on drilling data, geophysical logs and chemical analysis, showed the importance of the upper clastic sediment (Kuwait Group) and the lower Dammam Limestone of the Tertiary aquifer system, in developing the ground water resources in Kuwait state. Consequently, the activities of ground water exploration and exploitation in Kuwait have been limited to these aquifers. The flow direction has been recognized since the 1950s to be from the main recharge area in south west Saudi Arabia to the main discharge area in the Arabian Gulf and Shatt Al-Arab in a north easterly direction, in all aquifer systems. The recharge areas of the aquifer systems in Kuwait are located at the outcrop of the aquifer formations in Saudi Arabia in the south and southwest of Kuwait. These recharge areas provide the aquifers in Kuwait by lateral flow from areas receiving infiltrating rainwater. The quantity of this lateral flow depends on the hydraulic properties of these aquifers Environmental isotope studies are important tools in the investigations of the complex aquifer systems in different areas of the world. The isotopic characteristics of different groundwater in any system, has often provided powerful evidence for the resolution of hydrological problems, such as groundwater aging, sources of recharge water to the system, and estimated time of recharge. In the current study the environmental isotope investigations were applied to confirm the recharge area of the aquifer system in Kuwait. Environmental isotopic studies including oxygen-18 (δ¹⁸O), deuterium (δD), and carbon (¹³C and ¹⁴C), at the recharge areas of the Kuwait main groundwater aquifers in Saudi Arabia were carried out. The results of these studies can be summarised as follows: 1. All the isotopes confirm that effective recharge is currently taking place, though the rate and conditions are highly variable. 2. The rainfall and near surface aquifer data indicate that moisture derived from the Arabian Gulf is less likely to produce recharge than moisture derived from the eastern Mediterranean or the Indian Ocean. 3. The carbon , δ¹⁸O, and δD isotopes indicate that a large proportion of the groundwater in the system was probably recharged during a more humid, cool climatic period, 20,000 to 30,000 years ago. 4. Most of the water in the Tertiary aquifer system near the outcrop is a mixture, both in terms of age and origin, due to the vertical flow between the aquifers where the separating layers are discrete or relatively thin. However, in the present study several samples were collected from the observation sites representing the Kuwait Group and Dammam Limestone aquifers in Kuwait, in order to determine the δ¹⁸O and δD values. The results reflect almost the same range of values as in the recharge area (recharge area δ¹⁸O is range from -4.4 to -2.47 per mille and δD range from -18 to -30 per mille). This suggests that the study area has the same stable isotopic properties, and subsequently, they have the same origin and source of recharge under the same conditions. However in the northern part of Kuwait the environmental isotope investigations indicate that the fresh groundwater lenses belong to different regime, which confirm the occurrence of the local recharge in this area only. (author)