Uranium contamination of the Aral Sea

Abstract

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.

Introduction

The Aral Sea is an inland water body that experienced dramatic shrinkage in water volume since the 1960's due to water diversion for intensive irrigation from the two main inflowing rivers, the Syrdarya (Jaxartes) and the Amudarya (Oxus). All input of surface water into the lake comes via those two rivers. The Syrdarya originates in the Tien Shan mountains, the Amudarya in the Pamir Mountains. The watersheds of both rivers cover together about 530,000 km². River discharge to the Aral Sea decreased by a factor of 13 since the 1960s; from a total of 56 km³per year to about 4.2 km³per year in the 1980s. Before the decrease, the Amudarya contributed 5 times more water than the Syrdarya, whereas in the 1980s, both rivers contributed an equal share [Bortnik, 1996]. Today, only the Syrdarya River effectively discharges into the lake. The Amudarya almost ceased to contribute to the water balance. There is considerably uncertainty about today's river water input. According to the Uzbek Hydrometeorological Service river input increased in the 1990's to 9 km³annually (Zavialov, 2005). Other estimates on recent water budgets assign 4 km³annually to river discharge (Syrdarya River), 1–4 km³to precipitation and loss of 15–25 km³annually to evaporation (Zavialov, 2005). The total groundwater flow into the pre-desiccation Aral Sea in the 1960s was estimated as 3.2 km³year^− 1(Chernenko, 1983), which had been about 12% of the total river discharge. More recent estimates of the groundwater input into the lake vary from zero to about 8 km³annually (Veselov et al., 2002) and (Benduhn and Renard, 2004), respectively. The Aral Sea has no outflows.

The surface and volume shrinkage is intensified by the arid climate in the region. The lake's surface decreased by a factor of 3.5 to 19,000 km²in 2003, and the volume shrank by a factor of 9 to 118 km³(Zavialov, 2005). Considering the differing estimates of today's river input, assuming an equal contribution of Syrdarya River water to the different parts of the Aral Sea and average precipitation of 2.5 km³year^− 1, the water residence time ranges between 10 and 18 years. However, as loss due to evaporation is larger than water input, the water balance is negative in reality. The northern part, the Small Aral, disconnected from the Large Aral around 1990. However, a part of the Syrdarya River water that discharged into the Small Aral continued to flow south into the Large Aral through the Berg Strait. This outflow has now been interrupted by construction of a 13 km long dam in 2005. Inflowing Syrdarya River water does not divert to the Large Aral anymore, and as a result the water level in the Small Aral has started to rise again, while parts of the Large Aral are subject to accelerated desiccation. This has changed water residence times and balances considerably.

The desiccation leads to environmental problems in the Aral Sea basin such as decrease in ground water table, decreasing water availability for irrigation, salinization of groundwater and soils, and blow-off of salts from the dry sea bottom. The desiccation also leads to a continuous change in the chemical composition of the Aral Sea water itself. Dissolved substances increase in concentration as desiccation proceeds until they start to precipitate or co-precipitate. As a result, the mineralization of the Aral Sea water increased from 10 g kg^− 1in the 1960's (Mirabdullayev et al., 2004) to 82 g kg^− 1in the western basin in 2002 (Zavialov, 2003, Friedrich and Oberhänsli, 2004) and 130 g kg^− 1in the eastern basin of the Large Aral in 2005 (Zavialov et al., 2009-this issue). The concentration of contaminants in Aral Sea water which enter the lake by windblown salts and river input increase by the same process. Heavy metal contamination of the lake water has its source in the rivers. The head watershed of the Syrdarya and Amudarya Rivers has a history of metal and uranium ore mining, dating back to the early 20th century. A comprehensive monitoring study for radionuclides in Central Asian transboundary rivers (NAVRUZ experiment, Barber et al., 2005) identified sources of natural radionuclides and metals in the watershed of the Aral Sea and analysed their distribution in water, soil and biomass along the river courses. Uranium ores and related mining and ore processing activities at Karatau ridge and Mailuu-Suu (Kyrgyzstan), and Shyili (Kazakhstan) are the major sources of the radionuclide load in the Syrdarya river (Kadyrzhanov et al., 2005). Of particular concern are tailings and dumps near Mailuu-Suu Town and River. They contain 2.7 million m³of toxic material containing uranium and its decay products with a total radioactivity equivalent to about 1.1 × 10¹⁵Bq (UNECE, 2001). In addition, radioactive waste was accumulated from other countries such as former Czechoslovakia and German Democratic Republic (Vasiliev et al., 2005). The Mailuu-Suu River is a tributary to the Syrdarya near the Ferghana Valley. High seismic activity, mud flows and runoff in the region cause infiltration of toxic material into the Mailuu-Suu River. Another hotspot, the uranium deposits near Shyily, is located further downstream the Syrdarya River. As the mineralization and the pH in the lower course of the Syrdarya and Amudarya rivers is rather high (ca. 1–2 g kg^− 1with a pH of about 8 (Passell et al., 2003), uranium is largely kept in dissolved form and is discharged into the Aral Sea. In the 1960's, at the onset of desiccation, rivers are reported to have discharged annually 387 tons of dissolved uranium and 258 tons of uranium in the suspended matter into the Aral Sea (Kochenov and Baturin, 1967).

The uranium concentration in the Syrdarya River near Kazalinsk, located close to the Aral Sea, is about 16.07 µg l^− 1(Kadyrzhanov et al., 2005). This is well above the proposed WHO guideline for drinking water of about 2 µg l^− 1. The elevated concentrations of uranium in Syrdarya River water and its fate in the Aral Sea should be of environmental concern.

The work hypothesis of this study is that the main source of uranium in the Aral Sea is the inflowing river water and that the desiccation leads to accumulation of uranium in the water column reaching ecotoxicologically critical levels. To my knowledge, very few studies are available on the contamination of the Aral Sea itself with radionuclides apart from Kochenov and Baturin (1967), dating from Soviet times.

The aim of my paper is to identify the sources of uranium for the Aral Sea and to look at its pathways within the Aral Sea. In this way our study ties in with the already mentioned radionuclide monitoring study by Barber et al. (2005) in Central Asian Rivers.