Uranium contamination of the Aral Sea

Located in an endorrheic basin, the Aral Sea is mainly fed by water from two large rivers, the Syrdarya and the Amudarya. As a result, contaminants in dissolved and suspended form discharged by the rivers are accumulating in the lake. The northern Small Aral water contained 37 µg l− 1uranium and water in the western basin of the Large Aral up to 141 µg l− 1uranium in 2002, 2004 and 2006. The present day uranium concentrations in Aral Sea water mainly originate from the Syrdarya River due to uranium mining and tailings in the river watershed, and have been elevated up to 5 times compared to the pre-desiccation times by the ongoing desiccation in the western basin of the Large Aral. Current data indicate that groundwater does not seem to contribute much to the uranium budget. The uranium concentration in the lake is controlled by internal lake processes. Due to the high ionic strength of the Aral Sea water uranium is kept soluble. 238U/Cl−mass ratios range from 5.88 to 6.15 µg g− 1in the Small Aral and from 3.00 to 3.32 µg g− 1in the Large Aral. Based on the238U/Cl−mass ratios, a removal rate of 8% uranium from the water column inventory to the sediments has been estimated for anoxic waters, and it ranges between 2% and 5% in oxic waters, over periods of time without mixing. Most of the uranium removal seems to occur by co-precipitation with calcite and gypsum both in anoxic and oxic waters. According to simulations with PHREEQC, uraninite precipitation contributes little to the removal from anoxic Aral Sea water. In most of the sampled locations, water column removal of uranium matches the sediment inventory. Based on budget calculations, the future development of uranium load in the Aral Sea has been estimated for different scenarios. If the Syrdarya River discharge is below or in balance with the loss by evaporation, the uranium concentration in the Small Aral will increase from 37 µg l–1to 55 µg l− 1in 20 years time. When the river discharge is larger than loss by evaporation, present-day uranium concentration in the lake may be kept at the current level or even decrease slightly. From the ecotoxicological point of view, an increase in Syrdarya River discharge as the major water source will be crucial for the water quality of the Small Aral, despite its high uranium load. However, as it is intended to restore fishery in the Small Aral, accumulation of uranium in fish has to be monitored. Since the western basin of the Large Aral received no Syrdarya River water since 2005, and may become disconnected from the eastern basin, the slightly higher observed uranium removal from anoxic waters may result in a decrease in uranium concentrations in the western basin by 20% in 20 years time.