Determination of uranium concentration in ground water samples of Northern Greece

The activity concentration of U and U has been determined in groundwater samples of hot springs and deep wells from the region of Northern Greece. The analysis was performed by alpha spectroscopy after pre-concentration and separation of uranium by cation exchange (Chelex 100 resin) and finally its electro-deposition on stainless steel discs. The uranium concentration in deep wells and springs varies strongly between 0.15 and 7.66μg l−1. Generally the springs present higher uranium concentration than the deep wells, except of the Apollonia spring, which has shown the lowest value of 0.15 mg l−1. U and U activity concentration ranged between 1.8-95.3 mBq l−1 and 1.7-160.1 mBq l−1, respectively. The obtained isotopic ratio U/U varies between 0.95 and 1.74 which means that the two isotopes are not in radioactive equilibrium. The highest U/U activity ratio values correspond to the Langada springs, indicating most probably old-type waters. On the other hand, ground waters from wells with relatively low uranium activity concentration and low U/U isotopic ratios, point to the presence of younger waters with a stronger contribution of a local recharge component to the groundwater. ∗E-mail: anta@physics.auth.gr EPJ Web of Conferences DOI: 10.1051/ C © Owned by the authors, published by EDP Sciences , 2012 , epjconf 20122 / 24 03005 (2012) 403005 SIF This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20122403005


Introduction
Uranium is a ubiquitous, primordial radionuclide, the concentration of which in the environment strongly depends on the geological matrix and varies between 0.1 and 500 ppm Uranium in water results from the weathering of rocks and soil.In groundwater, the nuclide activities are governed by a number of processes, including recoil supply and absorption-desorption [1].This results in isotopic fractionation of this element, which is unique among the heavy elements, and has been useful in identifying waters, tracing them in the hydrologic cycle and estimating mixing ratios when surface waters merge.Accurate knowledge of uranium isotopic ratios in natural systems is of particular interest regarding geochronology, paleothermometry and pollution budgets.
Furthermore, knowledge of the uranium concentration in ground and surface waters is important in performing radiological impact assessment of various anthropogenic activities and aims to secure the increased standard of life in modern societies.
In the present work activity concentration of 238 U and 234 U has been determined in two different types of groundwater systems i.e. deep wells and hot springs of the region of Northern Greece, in order to define the levels of their concentrations.The analysis was performed by alpha spectroscopy after pre-concentration and separation of uranium by cation exchange and finally its electro-deposition on stainless steel discs.

Instrumentation and analysis
In the present work a relatively simple and effective method for the preconcentration and separation of uranium by cation exchange (Chelex 100 resin) and its alpha radiometric determination after electrodeposition on stainless steel discs was used.The employment of high-resolution alphaspectroscopy allows an accurate determination of the activity of the 238 U and 234 U radioisotopes, even though they appear at very low concentrations.The measurements have been carried out in the Laboratory of Radioanalytical Chemistry of the University of Cyprus.
From each sampling position 1.5 l of groundwater samples were obtained.500 ml of each groundwater sample was pre-treated by cation-exchange using Chelex 100 as described elsewhere [2].This pre-analytical procedure is selective for uranium and was carried out in parallel for every sample.
Alpha-spectroscopic analysis of selected samples was performed using a high-resolution alpha-spectrometer (STC Amplituda, Doza) equipped with semiconductor detectors.Prior to the sample measurement, the background was carefully measured under identical conditions and was found to be about 50 counts per day within the energy range of 3 to 8 MeV.
Method calibration using cation exchange separation shows an efficiency of 85%.The electrodeposition of uranium on stainless steel discs resulted in excellent yields, generally over 99% [3].The uranium activities were measured by alpha spectrometry with a detection counting efficiency of 8%.The Minimum Detectable Activity (MDA) reached in the measurements is about 1 mBq l −1 at the 95% confidence limit.
A typical α-spectrum of a groundwater sample is presented in fig. 1.The two main alpha peaks due to the 238 U and 234 U radioisotopes with energies of 4.198 MeV and of 4.776 MeV, respectively, are clearly shown.The small contribution of the 235 U radioisotope (below the experimental detection limit) does not give a statistically significant alpha peak.

Uranium activity concentration
Sample collection included groundwater samples of springs and deep wells from ten different areas of the region of Northern Greece.All samples were collected at the end of the spring period and the beginning of the summer period.The activity concentration of the different uranium radioisotopes of the analyzed samples are shown in table 1.The ratio and the sum of the radioisotope activity concentration are also given for each measured sample.For 238 U, the activity concentration varies between 4.08 and 95.32 mBq l −1 .For 234 U the activity concentration varies between 3.88 and 160.13 mBq l −1 .The 234 U/ 238 U ratio varies between 0.95 and 1.74.Generally the springs present higher uranium concentration than the deep wells, except of the Apollonia spring, which has shown the lowest value of 0.15 mg l −1 .Our results are in agreement with other investigators.Labidi et al. [4], measured uranium isotopes in thermo-mineral water springs in Tunisia and they found that the 234 U activity concentration varies between 1.1 and 82.2 mBq l −1 and that the 238 U concentration varies between 1.5 and 42.7 mBq l −1 .However, values are general higher than the concentrations of 238 U and 234 U in drinking waters in Greece.Kehagia et al. [5] found that 238 U and 234 U in the drinking water samples in Greece ranges from 0.91 to 17.27 mBq l −1 and from 2.13 to 22.01 mBq l −1 , respectively.There is a correlation between the 234 U activity concentration values and those corresponding to 238 U (fig. 2).This observed correlation is related to the leaching of uranium isotopes with a high rate to the underground water flowing through the faults and fissures between the grains of reservoir rocks [6].
In naturally occurring uranium radioisotopes, the activity ratio 234 U/ EPJ Web of Conferences 03005-p.4 238U can be about 1.05.In the present investigation, all the analyzed groundwater samples (except Thermi hot spring) show a radioactive disequilibrium between the two isotopes 234 U and 238 U.This disequilibrium occurs, because 234 U is more soluble than 238 U.In fact, it is established that 234 U recoil, crystal damage and leaching are the main mechanisms for 234 U/ 238 U disequilibrium in ground water [7,8].The determination of the 234 U/ 238 U ratio in disequilibrium is of importance in understanding hydrological, geochemical and chemical processes in terrestrial studies.The observed disequilibrium in samples of this investigation might be correlated with the different geological formations and water flow paths in the studied areas.The highest 234 U/ 238 U activity ratio values correspond to Langada springs, indicating most probably old-type waters.On the other hand, groundwater from wells with relatively low uranium activity concentration and small 234 U/ 238 U isotopic ratios, point to the presence of younger waters with a stronger contribution of a local recharge component to the groundwater.However, further studies are required to establish the relationship between the disequilibrium activity ratio and the source of uranium in the water.
From the radiological point of view, using ground waters for human consumption would result in significantly increased radiation doses and their utilization for irrigation purposes would not imply an excessive radiation exposure to population, in cases of high uranium concentration.However, the studied groundwater samples do not exhibit uranium content, above the limits of 15 μg l −1 suggested by WHO (2004) for uranium concentration in waters.

Conclusion
234 U and 238 U uranium isotopes concentration was measured using alpha spectrometry in ten different groundwater samples from Northern Greece.In general, rather low activity concentration of the 238 U and 234 U radioisotopes exists, exhibiting values of 95.32 mBq l −1 and 160.13 mBq l −1 , respectively.In only one sample the activity ratio of 234 U/ 238 U was found to be consistent with the value of 1.05, which is expected from naturally occurring uranium.In order to define the reasons for this disequilibrium, further investigation is needed, including at least the lithostratigraphic formation of the points of investigation.From the radiological point of view the studied groundwater does not exhibit an uranium content above the limits suggested by [9] for uranium concentration in waters, which is 15 μg l −1

Figure 1 :
Figure 1: Typical alpha-spectrum of a groundwater sample displaying the alpha peaks of the corresponding uranium isotope 238 U and its daughter nuclide 234 U.

Figure 2 :
Figure 2: 234 U activity concentration values correlated with those corresponding to 238 U.

Table 1 :
Concentration of 238 U and its daughter nuclide 234 U in groundwater samples collected from the region of Northern Greece.The uncertainties of 238 U and 234 U activity values is about 10%.