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The Aral Sea, located in a semi-arid environment, undergoes substantial annual (1200 mm/yr) and decadal lake-level fluctuations due to extreme seasonality in evaporation and precipitation along with steadily reduced river discharge. To trace the source ofthe lake water and understand the internal dynamics of the lake, we used oxygen and deuterium isotope composition of the lake water collected at different depths during spring and autumn from 2004 to 2006. We collected data from both the western (W) and eastern (E) basins ofthe Large Aral Sea as well as the channel connecting the two basins. The oxygen and hydrogen isotope ratios ofthe lake water vary widely (+4.6 to -5.3%. and -48 to +11%., respectively). We further measured isotopic ratios of groundwater leakage near the shoreline of the W basin of the Large Aral Sea and released in an artesian well on the Kulandy Peninsula. These ratios range from -16 to +3.4%. (delta O-180) and - 120 to +2.2% (delta D). The river water displays ratios of -12% (delta O-18) and -81.3% (delta D). Precipitation from winter and early spring 2006 exhibit delta O-18 values of -14% (snow) and +0.2% (rain) and delta D values of -97% (snow) and +3.6% (rain). The oxygen and hydrogen isotope snapshots show that in addition to evaporation, groundwater effluent flows at different depths are major contributors to the lake in spring and autumn. The d-excess, ranging between -25% (lake water) and +10% (groundwater), further demonstrates the impact of both effluent groundwater and evaporation on the isotopic composition of the lake water. Thus, stable isotope ratios can provide a first insight into seasonally triggered hydrologic interactions in the western part of the endorheic Aral Sea region. Remote sensing studies prove that major groundwater leakage occurs along the entire shoreline, except for the western shore where spatial resolution was too low. (C) 2008 Elsevier B.V. All rights reserved.