Distribution and mobilization of U, Th and 226Ra in the plant-soil compartments of a mineralized uranium area in south-west Spain

The activity concentrations of natural uranium isotopes (238U and 234U), thorium isotopes (232Th, 230Th and 225Th) and 226Ra were studied in soil and vegetation samples from a disused uranium mine located in the Extremadura region in the south-west of Spain. The results allowed us to characterize radiologically the area close to the installation and one affected zone was clearly manifest as being dependent on the direction of the surface water flow from the mine. The activity concentration mean values (Bq/kg) in this zone were: 10,924, 10,900, 10,075 and 5,289 for 238U, 234U, 230Th and 226Ra, respectively, in soil samples and 1,050, 1,060, 768 and 1,141 for the same radionuclides in plant samples. In an unaffected zone, the activity concentration mean values (Bq/kg) were: 184, 190, 234 and 7251 for 235U, 234U, 230Th and 226Ra, respectively, in soil samples and 28. 29, 31 and 80 in plant samples. The activity concentrations obtained for 232Th and 228Th showed that the influence of the mine was only important for the uranium series radionuclides. The relative radionuclide mobilities were determined from the activity ratios. Correlations between radionuclide activity concentrations and stable element concentrations in the soil samples helped to understand the possible distribution paths for the natural radionuclides.     

Occurrence of 222Rn, 226Ra, 228Ra and U in groundwater in Fujian Province, China

222Rn, 226Ra, 228Ra and U were determined in a total of 552 groundwater samples collected throughout Fujian Province of China. The geometric mean concentrations of 222Rn, 226Ra, 228Ra and total U in the groundwater were 147.8 kBq m-3, 12.7 Bq m-3, 30.2 Bq m-3 and 0.54 microgram kg-1, respectively. High groundwater 222Rn was explained by the predominantly granitic rock aquifers in Fujian. A lifetime risk of 1.7 x 10(-3) was estimated for the ingestion of groundwater 222Rn. High ratios of 228Ra to 226Ra contents (geometric mean of 2.4) and their disproportion suggest that 228Ra should also be measured in the assessment of population doses from drinking water in the regions of high rock or soil 232Th. No significant correlation between the 222Rn concentrations in groundwater and air was found.     

Radium and radium-daughter nuclides in carbonates: a brief overview of strategies for determining chronologies

Radium isotopes have been used extensively to trace the movement of groundwater as well as oceanic water masses, but these radionuclides (and their daughters) are also useful chronometers for the determination of the time scales of other Earth and environmental processes. The purpose of this overview is to present the application of Ra and Ra daughters in the dating of carbonates. We show that the choice of dating method (decay of excess radionuclide or ingrowth of daughter) depends strongly on the parent/daughter activity ratios in the water in which the carbonate was precipitated. Thus freshly precipitated carbonates uniformly show excesses of ²²⁶Ra relative to its parent ²³⁰Th, and ²²⁶Ra decay can provide ages of carbonates over Holocene time scales. In contrast, carbonates are precipitated in waters of greatly varying ²¹⁰Pb/²²⁶Ra. Corals, deep-sea hydrothermal vent clams and the shelled cephalopod Nautilus live in waters with significant dissolved ²¹⁰Pb and all show excesses of ²¹⁰Pb in their carbonate. Bivalve molluscs from nearshore and coastal waters, and carbonates deposited from groundwater environments (e.g. travertines) in which ²¹⁰Pb is efficiently scavenged from solution, show deficiencies of ²¹⁰Pb relative to ²²⁶Ra. In contrast, fish otoliths strongly discriminate against ²¹⁰Pb regardless of the environment in which the fish lives. Deficiencies of ²²⁸Th relative to ²²⁸Ra are common in all carbonates. Useful time ranges for the ²¹⁰Pb/²²⁶Ra and ²²⁸Th/²²⁸Ra chronometers are ∼100 y and ∼10 y, respectively.     

Distribution of long-lived radionuclides of the 238U series in the sediments of a small river in a uranium mineralized region of Spain

A study is presented on the distribution and mobilization of the natural U isotopes (238U and 234U), 230Th, and 226Ra in the sediments of a small river crossing an uranium mineralized zone where a disused uranium mine is located. Due to the preferential directions for surface run-off waters and to the mine's situation, one sampling point along the river bed was identified as a point of accumulation of radionuclides. The average values of the activity concentrations (Bq/kg) in this sediment sample were 5,025, 5,055, 5,915 and 1,694 for 238U, 234U, 230Th and 226Ra, respectively, while the respective average values of the activity concentrations (Bq/kg) for the sediment sample considered to give the background level were 125, 124, 131 and 370. Isotopic ratios between the descendants of 238U served to clarify some paths of distribution, involving the soils nearest to the sampling points and the location of these points with respect to the disused mine. The differences in behaviour found between the uranium, thorium and radium isotopes were associated to the mobility of these radionuclides in the fluvial system studied. Correlations between the radionuclide activity concentration ratios and stable element concentrations in the sediment samples were also investigated.     

Measuring the radium quartet (Ra, Ra, Ra, Ra) in seawater samples using gamma spectrometry

Radium isotopes are widely used in marine studies (eg. to trace water masses, to quantify mixing processes or to study submarine groundwater discharge). While ²²⁸Ra and ²²⁶Ra are usually measured using gamma spectrometry, short-lived Ra isotopes (²²⁴Ra and ²²³Ra) are usually measured using a Radium Delayed Coincidence Counter (RaDeCC). Here we show that the four radium isotopes can be analyzed using gamma spectrometry. We report ²²⁶Ra, ²²⁸Ra, ²²⁴Ra, ²²³Ra activities measured using low-background gamma spectrometry in standard samples, in water samples collected in the vicinity of our laboratory (La Palme and Vaccarès lagoons, France) but also in seawater samples collected in the plume of the Amazon river, off French Guyana (AMANDES project). The ²²³Ra and ²²⁴Ra activities determined in these samples using gamma spectrometry were compared to the activities determined using RaDeCC. Activities determined using the two techniques are in good agreement. Uncertainties associated with the ²²⁴Ra activities are similar for the two techniques. RaDeCC is more sensitive for the detection of low ²²³Ra activities. Gamma spectrometry thus constitutes an alternate method for the determination of short-lived Ra isotopes.     

Radiological impacts of natural radioactivity in Abu-Tartor phosphate deposits, Egypt.

Phosphate and environmental samples were collected from Abu Tartor phosphate mine and the surrounding region. The activity concentration of 226Ra (238U) series, 232Th series and 40K were measured using a gamma-ray spectrometer. The activities of uranium isotopes (238U, 235U and 234U) and 210Pb were measured using an alpha spectrometer and a low-background proportional gas counting system, respectively, after radiochemical separation. The results are discussed and compared with the levels in phosphate rocks from different countries. It seems that the Abu Tartor phosphate deposit has the lowest radioactivity level of exploited phosphate of sedimentary origin. 226Ra/238U, 210Pb/226Ra, 234U/238U and 226Ra/228Ra activity ratios were calculated and are discussed. The radioactivity levels in the surrounding region and the calculated exposure dose (nGy/h) will be considered as a pre-operational baseline to estimate the possible radiological impacts due to mining, processing and future phosphate industrial activities. To minimize these impacts, the processing wastes should be recycled to the greatest possible extent.     

Characterization of the TE-NORM waste associated with oil and natural gas production in Abu Rudeis, Egypt

The present study was conducted to characterize the Technically Enhanced Naturally Occurring Radioactive Materials (TE-NORM) waste generated from oil and gas production. The waste was characterized by means of dry screening solid fractionation, X-ray analysis (XRF and XRD) and gamma-ray spectrometry. Sediment of the TE-NORM waste was fractionated into ten fractions with particle sizes varying from less than 100 microm to more than 3 mm. The results showed that the TE-NORM waste contains mainly radionuclides of the 238U, 235U and 232Th series. The mean activity concentrations of 226Ra (of U-series), 228Ra (of Th-series) and 40K in the waste samples before fractionation (i.e. 3 mm) were found to amount to 68.9, 24 and 1.3 Bq/g (dry weight), respectively. After dry fractionation, the activity concentrations were widely distributed and enriched in certain fractions. This represented a 1.48 and 1.82-fold enrichment of 226Ra and 228Ra, respectively, in fraction F8 (2.0-2.5 mm) over those in bulk TE-NORM waste samples. The activity ratios of 238U/226Ra, 210Pb/226Ra, 223Ra/226Ra and 228Ra/224Ra were calculated and evaluated. Activity of the most hazardous radionuclide 226Ra was found to be higher than the exemption levels established by IAEA [International Atomic Energy Agency, 1994. International Basic Safety Standards for the Protection against Ionizing Radiation and for the Safety of Radiation Sources. GOV/2715/94, Vienna]. The radium equivalent activity (Ra-eq), radon (222Rn) emanation coefficient (EC) and absorbed dose rate (Dgammar) were estimated and these are further discussed.     

Natural radioactivity in winter wheat from organic and conventional agricultural systems

The distribution of natural radionuclides was studied in winter wheat plants collected from three sites in Belgium during 2004-2007. Activity concentrations of ⁴⁰K, ²²⁶Ra, ²²⁸Ra and ²²⁸Th in organically and conventionally grown wheat, and in the corresponding soil samples, were determined using ultra low-level gamma-ray spectrometry. The observed soil-to-wheat concentration ratios were calculated for the different parts of the wheat plant (root, stem and grain) in the two agricultural systems (organic and conventional). There were large variations in radionuclide activity concentrations between the sites and fields, but no significant difference between conventionally and organically grown wheat plants was observed.     

The importance of physico-chemical parameters on the speciation of natural radionuclides in riverine ecosystems

External gamma radiation levels were measured in the catchment areas of the Sharavathi River and the dose rates in air were found to be in the range 26.0-61.0 nGy h⁻¹. Soil and sediment samples of the riverine environment were analysed for natural radionuclides such as ²²⁶Ra, ²³²Th and ⁴⁰K using a NaI(Tl) gamma spectrometer. The activity concentration of ²¹⁰Pb and ²¹⁰Po in soil and sediment samples was determined by radiochemical separation techniques. Evaluation of the activity concentration of radionuclides with grain size revealed an increase in the activity of ²²⁶Ra, ²³²Th and ⁴⁰K towards fine grain size. The activity concentrations for all isotopes in all samples were not significantly correlated with pH. However, the activity of ²¹⁰Po and ²¹⁰Pb in sediment showed a moderate positive correlation with organic matter content and a good correlation with clay content of sediment.     

Soil radium, soil gas radon and indoor radon empirical relationships to assist in post-closure impact assessment related to near-surface radioactive waste disposal

Least squares (LS), Theil’s (TS) and weighted total least squares (WTLS) regression analysis methods are used to develop empirical relationships between radium in the ground, radon in soil and radon in dwellings to assist in the post-closure assessment of indoor radon related to near-surface radioactive waste disposal at the Low Level Waste Repository in England. The data sets used are (i) estimated ²²⁶Ra in the <2 mm fraction of topsoils (eRa226) derived from equivalent uranium (eU) from airborne gamma spectrometry data, (ii) eRa226 derived from measurements of uranium in soil geochemical samples, (iii) soil gas radon and (iv) indoor radon data. For models comparing indoor radon and (i) eRa226 derived from airborne eU data and (ii) soil gas radon data, some of the geological groupings have significant slopes. For these groupings there is reasonable agreement in slope and intercept between the three regression analysis methods (LS, TS and WTLS). Relationships between radon in dwellings and radium in the ground or radon in soil differ depending on the characteristics of the underlying geological units, with more permeable units having steeper slopes and higher indoor radon concentrations for a given radium or soil gas radon concentration in the ground. The regression models comparing indoor radon with soil gas radon have intercepts close to 5 Bq m⁻³ whilst the intercepts for those comparing indoor radon with eRa226 from airborne eU vary from about 20 Bq m⁻³ for a moderately permeable geological unit to about 40 Bq m⁻³ for highly permeable limestone, implying unrealistically high contributions to indoor radon from sources other than the ground. An intercept value of 5 Bq m⁻³ is assumed as an appropriate mean value for the UK for sources of indoor radon other than radon from the ground, based on examination of UK data. Comparison with published data used to derive an average indoor radon: soil ²²⁶Ra ratio shows that whereas the published data are generally clustered with no obvious correlation, the data from this study have substantially different relationships depending largely on the permeability of the underlying geology. Models for the relatively impermeable geological units plot parallel to the average indoor radon: soil ²²⁶Ra model but with lower indoor radon: soil ²²⁶Ra ratios, whilst the models for the permeable geological units plot parallel to the average indoor radon: soil ²²⁶Ra model but with higher than average indoor radon: soil ²²⁶Ra ratios.     

Soil to plant transfer of 238U, 226Ra and 232Th on a uranium mining-impacted soil from southeastern China

Both soil and plant samples of nine different plant species grown in soils from southeastern China contaminated with uranium mine tailings were analyzed for the plant uptake and translocation of 238U, 226Ra and 232Th. Substantial differences were observed in the soil-plant transfer factor (TF) among these radionuclides and plant species. Lupine (Lupinus albus) exhibited the highest uptake of 238U (TF value of 3.7x10(-2)), while Chinese mustard (Brassica chinensis) had the least (0.5x10(-2)). However, in the case of 226Ra and 232Th, the highest TFs were observed for white clover (Trifolium pratense) (3.4x10(-2)) and ryegrass (Lolium perenne) (2.1x10(-3)), respectively. 232Th in the tailings/soil mixture was less available for plant uptake than 226Ra or 238U, and this was especially evident for Chinese mustard and corn (Zea mays). The root/shoot (R/S) ratios obtained for different plants and radionuclides shown that Indian mustard had the smallest R/S ratios for both 226Ra (5.3+/-1.2) and 232Th (5.3+/-1.7), while the smallest R/S ratio for 238U was observed in clover (2.8+/-0.9).     

Determination and comparison of uranium and radium isotopes activities and activity ratios in samples from some natural water sources in Morocco.

Radiochemical results (238U, 226Ra and 228Ra activities; 234U/238U, 228Ra/226Ra and 226Ra/238U activity ratios) are reported for 42 natural water samples collected from wells, hot mineral springs, rivers, tap water, lakes and irrigation water in 15 Moroccan locations. Results show that 238U activity varies between 4.5 and about 309 mBq l(-1) in wells, 0.6 and 8.5 mBq l(-1) in hot springs, 9.7 and 28 mBq l(-1) in rivers, 2.5 and 16 mBq l(-1) in tap waters and between 6 and 24 mBq l(-1) in lakes. The 234U/238U activity ratio varies in the range 0.87-3.35 in all analyzed water samples except for hot springs where it reaches values higher than 7. Unlike well water, mineral water samples present low 238U activities and high 234U/238U activity ratios and 226Ra activities. The highest activity of radium in mineral water is 150 times higher than the highest activity of 226 Ra found in well water. 226Ra/238U activity ratios are in the ranges 0.07-1.14 in wells, 0.04-0.38 in rivers, 0.04-2.48 in lakes, and 1.79-2115 in springs. The calculated equivalent doses to all the measured activities are inferior to the maximum contaminant levels recommended by the International Commission of Radioprotection and they do not present any risk for public health in Morocco.     

Invariance of isotope ratios of lithogenic radionuclides: more evidence for their use as sediment source tracers

Activities of radionuclides in the 238U (230Th, 226Ra, 210Pb) and 232Th (232Th, 228Th, 228Ra) decay series were determined in sediments from an east Texas watershed and examined with isotope ratios and compared to particulate organic carbon (POC), % fines (<63 microm) and total concentrations of Al, Fe and Mn. The objective was to elucidate the presence or absence of relationships affecting the effectiveness of these radionuclides in modeling sediment transport. Strong positive correlations were observed between radionuclides and Mn (Th) and % fines (Ra and Th). Isotope ratios effectively reduce these influences, supporting the contention that isotope ratios offset extrinsic variability in terrestrial sediments. Strong associations of 210Pbxs (excess 210Pb) and 226Ra/230Th with POC agree with data from marine and terrestrial settings, indicating that the role of POC in isotope fractionation, transport and sequestration merits further investigation.     

Natural radioactivity levels in topsoil from the Pearl River Delta Zone, Guangdong, China

Concentrations of the natural radionuclides ²³⁸U, ²²⁶Ra, ²³²Th and ⁴⁰K have been measured by γ-ray spectrometry in 796 topsoil samples from the Pearl River Delta Zone (PRDZ) of Guangdong, China. The mean concentrations for ²³⁸U, ²²⁶Ra, ²³²Th and ⁴⁰K were found to be 140 ± 37 Bq kg⁻¹, 134 ± 41 Bq kg⁻¹, 187 ± 80 Bq kg⁻¹ and 680 ± 203 Bq kg⁻¹ dry mass, respectively. These values were all higher than the mean values in soil for China and the world. Outdoor air-absorbed dose rates, calculated from activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K, ranged from 86 to 237 nGy h⁻¹, with a mean value of 165 ± 46 nGy h⁻¹. The corresponding annual outdoor effective dose rate per person was estimated to be between 0.11 and 0.29 mSv y⁻¹, with a mean value of 0.20 ± 0.06 mSv y⁻¹, which was also higher than the world mean value of 0.07 mSv y⁻¹. The radium equivalent activity (Ra_eq) and the external hazard index (I_r) resulted from the natural radionuclides in soil, were also calculated and found to vary from 230 to 676 Bq kg⁻¹ and from 0.6 to 1.8, respectively. The Ra_eq and the I_r in all the investigated regions were up to 75% higher than the set limits of 370 Bq kg⁻¹ and 1.0, respectively.     

Uranium isotopes in groundwater from the continental intercalaire aquifer in Algerian Tunisian Sahara (Northern Africa)

The disequilibrium between ²³⁴U and ²³⁸U is commonly used as a tracer of groundwater flow. This paper aims to identify uranium contents and uranium isotopic disequilibria variation in groundwater sampled from deep Continental Intercalaire aquifer (southern Algeria and Tunisia). Large variations in both U contents (0.006–3.39 ppb) and ²³⁴U/²³⁸U activity ratios (0.4–15.38) are observed. We conduct a first assessment in order to verify whether the results of our investigation support and complete previous hydrogeological and isotopic studies. The dissolved U content and ²³⁴U/²³⁸U activity ratio data were plotted on a two-dimensional diagram that was successfully utilized on sharing the CI aquifer into different compartments submitted to different oxidising/reducing conditions and leads also to distinguished two preferential flow paths in the Nefzaoua/Chott Fejej discharge area. Uranium isotopes disequilibrium indicate that ranium chemistry is mainly controlled by water–rock interaction enhanced by long residence time recognised for this aquifer.     

Environmental characterization and radio-ecological impacts of non-nuclear industries on the Red Sea coast

The Red Sea is a deep semi-enclosed and narrow basin connected to the Indian Ocean by a narrow sill in the south and to the Suez Canal in the north. Oil industries in the Gulf of Suez, phosphate ore mining activities in Safaga-Quseir region and intensified navigation activities are non-nuclear pollution sources that could have serious radiological impacts on the marine environment and the coastal ecosystems of the Red Sea. It is essential to establish the radiological base-line data, which does not exist yet, and to investigate the present radio-ecological impact of the non-nuclear industries to preserve and protect the coastal environment of the Red Sea. Some natural and man-made radionuclides have been measured in shore sediment samples collected from the Egyptian coast of the Red Sea. The specific activities of 226Ra and 210Pb (238U) series, 232Th series, 40K and 137Cs (Bq/kg dry weight) were measured using gamma ray spectrometers based on hyper-pure germanium detectors. The specific activities of 210Po (210Pb) and uranium isotopes (238U, 235U and 234U) (Bq/kg dry weight) were measured using alpha spectrometers based on surface barrier (PIPS) detectors after radiochemical separation. The absorbed radiation dose rates in air (nGy/h) due to natural radionuclides in shore sediment and radium equivalent activity index (Bq/kg) were calculated. The specific activity ratios of 228Ra/226Ra, 210Pb/226Ra, 226Ra/238U and 234U/238U were calculated for evaluation of the geo-chemical behaviour of these radionuclides. The average specific activity of 226Ra (238U) series, 232Th series, 40K and 210Pb were 24.7, 31.4, 427.5 and 25.6 Bq/kg, respectively. The concentration of 137Cs in the sediment samples was less than the lower limit of detection. The Red Sea coast is an arid region with very low rainfall and the sediment is mainly composed of sand. The specific activity of 238U, 235U and 234U were 25.3, 2.9 and 25.0 Bq/kg. The average specific activity ratios of 226Ra/228Ra, 210Pb/226Ra and 234U/238U were 1.67, 1.22 and 1.0, respectively. The relationship between 226Ra/228Ra activity ratio and sample locations along the coastal shoreline indicates the increase of this ratio in the direction of the Shuqeir in the north and Safaga in the south where the oil exploration and phosphate mining activities are located. These activities may contribute a high flux of 226Ra. The concentration and distribution pattern of 226Ra in sediment can be used to trace the radiological impact of the non-nuclear industries on the Red Sea coast.     

Fractionation of natural radionuclides in soils from the vicinity of a former uranium mine ?irovski vrh, Slovenia

As a result of former uranium mining and milling activities at ?irovski vrh, Slovenia, 0.6 million tons of uranium mill tailings (UMT) were deposited onto a nearby waste pile Boršt. Resulting enhanced levels of natural radionuclides in UMT could pose threat for the surrounding environment. Therefore, sequential extraction protocol was performed to assess mobility and bioavailability of ²³⁸U, ²³⁴U, ²³⁰Th and ²²⁶Ra in soils from the waste pile and its surrounding. The radionuclides associated with exchangeable, organic, carbonate, Fe/Mn oxides and residual fraction, respectively, were determined. Results showed that the highest activity concentrations for the studied radionuclides were on the bottom of the waste pile. In non-contaminated locations, about 80% of all radionuclides were in the residual fraction. Considering activity concentrations in the UMT, ²³⁸U and ²³⁴U are the most mobile. Mobility of ²²⁶Ra is suppressed by high sulphate concentrations and is similar to mobility of ²³⁰Th.     

Escaping radioactivity from coal-fired power plants (CPPs) due to coal burning and the associated hazards: a review

Coal, like most materials found in nature, contains trace quantities of the naturally occurring primordial radionuclides, i.e. of ⁴⁰K and of ²³⁸U, ²³²Th and their decay products. Therefore, the combustion of coal results in the released into the environment of some natural radioactivity (1.48 TBq y⁻¹), the major part of which (99 %) escapes as very fine particles, while the rest in fly ash. The activity concentrations of natural radionuclides measured in coals originated from coal mines in Greece varied from 117 to 435 Bq kg⁻¹ for ²³⁸U, from 44 to 255 Bq kg⁻¹ for ²²⁶Ra, from 59 to 205 Bq kg⁻¹ for ²¹⁰Pb, from 9 to 41 Bq kg⁻¹ for ²²⁸Ra (²³²Th) and from 59 to 227 Bq kg⁻¹ for ⁴⁰K. Fly ash escapes from the stacks of coal-fired power plants in a percentage of 3-1% of the total fly ash, in the better case. The natural radionuclide concentrations measured in fly ash produced and retained or escaped from coal-fired power plants in Greece varied from 263 to 950 Bq kg⁻¹ for ²³⁸U, from 142 to 605 Bq kg⁻¹ for ²²⁶Ra, from 133 to 428 Bq kg⁻¹ for ²¹⁰Pb, from 27 to 68 Bq kg⁻¹ for ²²⁸Ra (²³²Th) and from 204 to 382 Bq kg⁻¹ for ⁴⁰K. About 5% of the total ash produced in the coal-fired power plants is used as substitute of cement in concrete for the construction of dwellings, and may affect indoor radiation doses from external irradiation and the inhalation of radon decay products (internal irradiation) is the most significant. The resulting normalized collective effective doses were 6 and 0.5 man-Sv (GW a)⁻¹ for typical old and modern coal-fired power plants, respectively.     

Ra–Po–Pb isotope systematics in waters of Sambhar Salt Lake,Rajasthan (India): geochemical characterization and particulate reactivity

The Sambhar Salt Lake hydrological system, including river waters, groundwaters, evaporating pans and sub-surface brines, has been analyzed for the salt content (TDS) and naturally occurring radionuclides (²¹⁰Po, ²¹⁰Pb and ^226,228Ra). The abundance of these radionuclides and their activity ratios show a wide variation in different hydrological regimes, which helps to geochemically characterize the lake system. A significantly lower Ra to total dissolved solids (TDS) ratio in the brines (by two to three orders of magnitude), when compared to the groundwaters and river waters, suggests removal of dissolved Ra by co-precipitation with Ca–Mg minerals at an early stage of the brine evolution. The concentration of Ra in evaporating lake/pan waters saturates at a value of about 35 Bq L⁻¹ over the salinity range of 100–370 g L⁻¹; attributable to its equilibration with the clay minerals. The two distinct regimes, saline lake system (lake water, evaporating pans and sub-surface brines) and groundwaters have been identified based on their differences in the distribution of ^226,228Ra isotopes. This observation points to the conclusion that the groundwaters and the lake brines are not intimately coupled in terms of their origin and evolution. The abundances of ²¹⁰Po and ²¹⁰Pb along with their activity ratios (²¹⁰Po/²¹⁰Pb) are markedly different among the surface lake waters/evaporating pans, sub-surface lake brines and groundwaters. These differences are explained in terms of different geochemical behaviour of these nuclides in presence of algae and organic matter present in these water regimes.     

Distribution of uranium and radium isotopes in an aquifer of a semi-arid region (Manouba-Essijoumi, Northern Tunisia)

Groundwaters from the Sebkhet Essijoumi drainage basin, situated in northern Tunisia, West of the city of Tunis, were sampled and analyzed for uranium and radium isotopes. Low (234)U/(238)U activity ratios coupled with relatively high (228)Ra and (238)U concentrations were found in the Manouba plain phreatic aquifer, at the northern part of the basin, where remote sensing has indicated that this plain corresponds to the main humid zone of the area. Low (234)U/(238)U ratios probably reflected short residence time for waters in the Manouba plain, and high ratios longer residence time in the south, where water reaching the phreatic aquifer seems to have previously circulated in rocks constituting the southern hills. Assuming that, in the Manouba plain aquifer, the groundwater flows downstream from the Oued Lill pass area to the South-West of the Sebkha, the difference in the (228)Ra/(226)Ra activity ratio suggests that the residence time of water has been 2.8 years longer near the Sebkha than upstream.