Uranium and radium activities measurements and calculation of effective doses in some drinking water samples in Morocco

Purpose: As a way of prevention, we have measured the activities of uranium and radium isotopes (234U, 238U, 226Ra, 228Ra) for 30 drinking water samples collected from 11 wells, 9 springs (6 hot and 3 cold), 3 commercialised mineral water, and 7 tap water samples. Methods: Activities of the Ra isotopes were measured by ultra-gamma spectrometry using a low background and high efficiency well type germanium detector. The U isotopes were counted in an alpha spectrometer. Results: The measured Uranium and radium activities are similar to those published for other non-polluting regions of the world. Except in one commercialised gaseous water sample, and in two hot spring water samples, the calculated effective doses during one year are inferior to the reference level of 0.1 mSv/year recommended by the International Commission on Radiological Protection. Conclusion: These activities don't present any risk for public health in Morocco. The sparkling water of Oulmes is occasionally consumed as table water and waters of warm springs are not used as main sources of drinking water.


Introduction
The natural radioactivity comes mainly from the primordial radio-nuclides in soils ( 40 K and the 4n and 4n+2 series). Further, some cosmogenic radionuclides and/or radionuclides originated from pollution can be found in natural waters used for drinking. The natural polluting radio-elements can rise from industrial wastes, geological erosion of U-bearing rocks or excessive utilization of agricultural fertilizers. Concentrations of natural radio-elements in water depend on the physico-chemical conditions and on the geological, geographical and socio-economical environment.
The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has estimated that exposure to natural sources contributes more than 98% of the radiation dose to the population (excluding medical exposure). There is only a very small contribution from nuclear power production and nuclear weapons testing. The global average human exposure from natural sources is 2.4 mSv/year. There are large local variations in this exposure depending on a number of factors, such as height above sea level, the amount and type of radionuclides in the soil, and the amount taken into the body in air, food, and water. The contribution of drinking-water to the total exposure is very small and is due largely to naturally occurring radionuclides in the uranium and thorium decay series. Levels of natural radionuclides in drinking-water may be increased by a number of human activities.
Because different types of radiation have different biological effectiveness and different organs and tissues in the body have different sensitivities to radiation, the ICRP has introduced radiation and tissue-weighting factors to provide a measure of equal effect. The sum of the doubly weighted dose received by all the tissues and organs of the body gives a measure of the total harm and is referred to as the effective dose. Moreover, radionuclides taken into the body may persist, and, in some cases, the resulting exposure may extend over many months or years. The effective dose is a measure of the total dose incurred over a lifetime following the intake of a radionuclide.
The recommended reference level of the effective dose is 0.1 mSv from 1 year's consumption of drinking-water. This reference level of dose represents less than 5% of the average effective dose attributable annually to natural background radiation. Below this reference level of dose, the drinking-water is acceptable for human consumption, and any action to reduce the radioactivity is not necessary. This recommendation does not differentiate between natural and man-made radionuclides.
The natural radioactivity of groundwater varies greatly. [1][2][3][4][5][6] High concentrations of natural radioactive elements have been found not only in water from U mines, but also in mineral water and in some drilled wells. The knowledge of U and Ra concentrations in drinking water is important because an appreciable fraction of the absorbed U and Ra is deposited in human bone, with the corresponding contribution to the internal dose.
In this work, we describe the techniques applied for measuring U (alpha spectrometry) and Ra (gamma spectrometry) isotopes in natural water samples. 7-8 Some results obtained from drinking water samples collected from different sources (springs, wells and tap water) are given and equivalent effective doses are calculated and compared to the recommended reference level for adult persons.

Sampling sites
The analyzed water samples belong to 3 different water sources: wells, springs (hot springs, cold springs, commercialised mineral waters), and Tap waters. Figure 1 shows the sites of sampling. This study was undertaken in order to establish a radioactivity distribution map in various natural waters in Morocco. The regions of the cities of Fes and Errachidia are rich in warm thermal springs. Regions situated inward by the country are often affected by the drought, what explains the considerable number of wells in these regions (Khouribga, Beni Mellal and Fkih Ben Salah)

Sample preparation
Twenty liters of water are collected in a polyethylene tank and immediately slightly acidified to pH 2-3 with concentrated HNO3. After filtration, an iron carrier (FeCl3) (50-80 mg) and a known amount of 232 U-228 Th equilibrated spike solution are added. U and Th are coprecipitated with Fe (OH)3 by addition of NH4OH to pH 8 to 9 as described by Choukri and Hakam. 7-8 A known quantity of barium in BaCl2 form is then added. Ra isotopes are co-precipitated with BaSO4 formed by addition of H2SO4 drops in the medium without significant changing of pH. Precipitation of hydroxides is achieved before the Ba and H2SO4 addition step to avoid co-precipitation of Th with H2SO4. The two precipitates are recovered by centrifugation and/or filtration.
Hydroxides containing U and Th are soluble in acid medium whereas BaSO4 coprecipitated with Ra is not soluble in this medium. The two fractions are separated by filtration or centrifugation.

Measure of uranium
Complete mixing and oxidation are achieved by evaporating the solution to dryness. The dry residue is dissolved in 8N HCl and the hydroxides are precipitated at pH 8 to 9 with ammonium hydroxide. In this step, Th and U are precipitated together with iron hydroxides, aluminium and other impurities. After dissolution in 8N HCl, the sample is filtered and is ready for the ion exchange procedure. [7][8] A single resin exchange column is used twice: (1) In the first step, U and Fe are fixed on the resin in a 8N HCl medium while Th and other impurities are collected with the effluent. Fe is then eluted with 15 ml of 8N HNO3. U is eluted with 30 ml of 0.1N HCl and evaporated to dryness, and (2) In the second step, Th is loaded on the resin bed in 30 to 40 ml of 8N HNO3 to ensure solely the fixation of Th. The latter is eluted with 8N HCl and evaporated to dryness. The ion exchange procedure is identical to that used by Choukri et al. 7  After these anionic exchange steps, Th is ready for deposition while U must be separated from residual iron. Its purification is performed by di-isopropyl ether extraction, followed by evaporation of the aqueous fraction containing U. In the last step, U and Th are extracted separately with TTA (1-(2-thenoyl)-3, 3, 3-trifluoroacetone) in toluene in a nitric media at pH 3 and 1, respectively. The two organic phases containing U and Th are evaporated onto aluminium foils and flamed to remove any trace of carbon. The thin sources are, at the end, ready for alpha spectrometry.

Determination of radium
Radium is extracted from the water samples by precipitation with BaSO4 after removal of U and Th as described above. Schmidt and Reyss reported that this method is essentially quantitative for Ra removal. 10 The (Ba-Ra) SO4 is dried at room temperature. The obtained powder is sealed into a plastic ferrule whose form is compatible with the dimensions of the used well Germanium detector. The gamma spectrum is measured after 20 days which is necessary to ensure radioactive equilibrium between Ra isotopes and their daughters. The efficiency of Ra extraction by BaSO4 precipitation is obtained by calculating the weight of recovered BaSO4 to introduced BaCl2 (corrected by a mass factor of 1.16). Extraction efficiencies are, in most cases, in the range 75-100%. Ra activity is measured by gamma spectrometry using a 220 cm 3 low-background well type gamma ray detector in the Laboratoire Souterrain de Modane (LSM-Centre National de la Recherche Scientifique (CNRS-France)/Commissariat à l'Energie Atomique).

Results and Discussion
The activities of uranium isotopes ( 234 U, 238 U) and radium isotopes ( 226 Ra, 228 Ra) and the calculated committed effective doses during one year for all analyzed samples, are given in  In cold springs, the activities are low than in hot springs. In springs, both Ra and U distributions are affected by temperature, salinity, and redox condition. High temperature tends to decrease U but to increase Ra concentrations. The aqueous chemistry of Ra may also be greatly influenced by redox state of the environment, but the effect is opposite to that for U. In geothermal sources where the temperature and the concentration of the chlorine are very elevated and the reducing potential is very weak, the 226 Ra / 238 U activity ratios are often very large. Zukin et al. and Herczeg et al. have found in samples of geothermal origin, the ratios in the order of 10 3 to 10 4 . [11][12] Indeed, in a reducing medium where U is much less mobile than Ra, the disappearance of sulphates allows more important concentrations of Ra in solution. 13 In the 3 analysed commercialised mineral water samples, the measured activities are low except in the gaseous mineral water of Oulmes where a high value of 1248 mBqL -1 was reached.
The risks due to the contamination of the water are essentially risks of internal irradiation as a result of ingestion of natural radioelements. To demonstrate that the annual limit of 0.1 mSv recommended for the drinking water is respected, we calculated the engaged doses which would receive the consumers from different water sources in Morocco. The dose supplied for one person via the water consumption depends not only on the quantity of radioelements ingested during one year, but also the coefficients of effective dose engaged by unity of incorporation (DPUI) recommended by the ICRP. 14 ( Table 2). These coefficients allow converting the activities expressed in becquerels (Bq) to dose expressed in sieverts (Sv). These values of DPUI were also taken back by the European directive. [15][16][17] The calculation of the effective dose was made in our work supposing that the total quantity of ingested radioactive substances results from the consumption about 2 liters of water by a day, that is approximately 730 liters by year. The formula used for the calculation of the engaged effective dose is the following one: where, ej is the dose per unit intake by ingestion for adult members of the public (DPUI in Sv/Bq) by ingestion of the radioelement j; Ij is the incorporation by ingestion of the radioelement j during the year (that is the volumic activities in becquerels by liter multiplied by 730 L).
These calculations do not consider 40 K, because its content in the human body is regulated and unchanging whatever is the contribution by ingestion. 18 The activities of isotopes 210 Pb and 210 Po were not calculated, but estimated at a maximal equivalent activity to that of the 226 Ra. Considering its strong affinity for the solid phases, the thorium was not retained in the calculation of dose. [19][20] The basic formula is finally the following shape: ET = 730 (4.5.10 -8 x 238 U + 4.9.10 -8 x 234 U + 2.8.10 -7 x 226 Ra + 6.9.10 -7 x 228 Ra + 1.2.10 -6 x 210 Po + 6.9.10 -7 x 210 Pb) In radioprotection, the effect of the ingestion of radioelements is not the same as it is about a child or about an adult. 21 The annual water consumption as well as the coefficients of effective dose engaged by unity of incorporation (DPUI) recommended by the ICRP differ according to the age of the public considered ( Table 2). The calculated total effective doses during one year are lower than the recommended value of 0.1 mSv by the WHO for drinking water except in two hot springs and in gaseous commercialised mineral water. 22 In spite of that, these waters do not present any sanitary danger for the population. The sparkling water of Oulmes is occasionally consumed as table water and waters of warm springs are not used as main sources of drinking water. Higher values were measured in China, in France, in Tunisia, in Germany and in Italy. [23][24][25] Conclusion Radiochemical analyses of 30 drinking water samples taken from 11 wells, 6 hot springs, 3 cold springs, 3 commercialised mineral water and 7 tap water show, for each source, comparable results ( 234 U. 238 U. 226 Ra and 228 Ra activities) to those reported in previous works for different regions in the world. The measured activities covered wide ranges. The calculated total effective doses during one year are lower than the recommended value of 0.1 mSv by the WHO for drinking water except in two hot springs and in gaseous commercialised mineral water. In spite of that, these waters do not present any sanitary danger for the population. The sparkling water of Oulmes is occasionally consumed table water and waters of warm springs are not used as main sources of drinking water.
We cannot generalize the obtained results for all drinking waters in Morocco because of the limited analyzed samples number. Although, they could give a general idea of U and Ra