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The Aral Sea has been shrinking since 1963 due to extensive irrigation and the corresponding decline in the river water inflow. Understanding of the current hydrological situation demands an improved understanding of the surface water/groundwater dynamics in the region. Rn-222 and Ra-226 measurements can be used to trace groundwater discharge into surface waters. Data of these radiometric parameters were not previously available for the study region. We determined Rn-222 activities after liquid phase extraction using Liquid Scintillation Counting (LSC) with peak-length discrimination and analyzed Ra-226 concentrations in different water compartments of the Amu Darya Delta (surface waters, unconfined groundwater, artesian water, and water profiles from the closed Large Aral Sea (western basin).

The water samples comprise a salinity range between 1 and 263 g/l. The seasonal dynamics of solid/water interaction under an arid climate regime force the hydrochemical evolution of the unconfined groundwater in the Amu Darya Delta to high-salinity Na(Mg)Cl(SO4) water types. The dissolved radium concentrations in the waters were mostly very low due to mineral over-saturation, extensive co-precipitation of radium and adsorption of radium on coexisting solid substrates.

The analysis of very low Ra-226 concentrations (<10 ppq) at remote study sites is a challenge. We used the water samples to test and improve different analytical methods. In particular, we modified a procedure developed for the alpha-spectrometric determination of Ra-226 after solid phase extraction of radium using 3M Empore(TM) High Performance Extraction Disks (Purkl, 2002) for the analysis of the radionuclide using an ICP sector field mass spectrometer. The Ra-226 concentration of 17 unconfined groundwater samples ranged between 0.2 and 5 ppq, and that of 28 artesian waters between <0.2 and 13 ppq. The ICP-MS results conformed satisfactorily to analytical results based on gamma-measurements of the Rn-222 ingrowth after purging and trapping on super-cooled charcoal. The Ra-226 concentrations were positively correlated with the salinity and the dissolved NaCl concentrations. The occurrence of unusually high Ra-226 activities is explained by radium release from adsorption sites with increasing salinity. The inferred spatial variability of Rn-222 in the Aral Sea and of Rn-222 and Ra-226 in the groundwater of the Amu Darya Delta is discussed in the context of our own previous hydrochemical studies in the study sites. Relatively low Rn-222 activities in the unconfined GW (1-9.5 Bq/l) indicate the alluvial sediments hosting the GW to be a low-U-238(Ra-226) substrate. Positive correlations between U and Ra-226, and U and Rn-222 are likely related to locally deposited Fe(Mn)OOH precipitates. The Rn-222 activity of the GW, however, distinctly exceeds the Rn-222 concentration in the Aral Sea (10 mBq/l), in principle, making Rn-222 a sensitive tracer for the inflow of GW. The high water volume of the Large Aral Sea and wind induced mixing of its water body, however, hamper the detection of local groundwater inflow. (C) 2015 Elsevier Ltd. All rights reserved.