Introduction

Groundater is a very important natural resource related directly to the survival of all living organisms. The concern over exposure of humans to radioactivity is an important driving factor behind the studies of environmental radiation and natural radioactivity levels in groundwaters. There are four naturally-occurring radium isotopes with half-lives ranging from 3.7 days to 1600 years. The ²²⁶Ra and ²²⁸Ra are the decay products of the insoluble Thorium isotopes [1]. The presence of radium in groundwater is influenced by many factors, such as the occurrence of the radium isotope or its parent thorium nuclide in the aquifer  matrix, leaching from the rocks penetrated by groundwater, the alpha recoil, the adsorption/desorption from the surface layer and the water chemical composition [2, 3, 4, 5]. In Saudi Arabia, there are many aquifers within the Saudi Arabian territories differ in lithology, age and extension, and some of them are considered as principal or main aquifers, these aquifers have different concentration of radionuclides. Due to arid to semi-arid conditions prevailed on the Arabian peninsula, the annual groundwater recharge is very low and the stored water within most of these aquifers is old (fossil water), therefore it is in direct contact with rocks and soils for a long time, leading to the change of its hydrochemical characters [6, 7]. The objectives of this study are to determine the activity concentrations of ²²⁶Ra and ²²⁸Ra in the groundwater from different wells in Saudi Arabia and to elucidate the hydrochemical influences, if any, on the observed radium activities under these conditions.

Methodology

Unless otherwise indicated, all references to water refer to deionized water (DDW).

Sampling

The studied wellsare situated at four different areas. Samples were collected from WadiAl-Dawaser, Al Jawf, Tabouk, Al Qasim and Al Sharqiya, as shown in map 2-1.

Figure Click here to View figure

Water samples (105 samples, 5 liters each) were collected from local wells, and the locations of the sites were precisely recorded using global positioning system (GPS).The water was allowed to run in a continuous flow for a short period to eliminate any contamination from the pipes, and have a representative samples. For radioactivity measurements, sampling was often accompanied by samples treatment (acidification and/or filtration) in order to minimize any interference which could potentially affect the required analysis. Water samples were collected from the continuous flow, filtered with 0.45µ membrane filter, acidified with 11 M HCl at the rate of 10 ml per liter of sample immediately after filtration to avoid the adsorption of radionuclides on the walls of the container and the growth of micro-organisms, and transferred to polyethylene bottles.

Forchemical analysis, the water samples were collected in suitable bottles without the acidification step, and the chemical parameters were monitored with a multi parameter water quality meter, Hachsension 156.

Materials and Apparatus

Radium extractions from water samples were carried out using a strong cation exchange resin, Purolite C-100, Na form, supplied by Veolia Water Co. (Riyadh, Saudi Arabia). Standard reference solutions of ²²⁶Ra and ²²⁸Ra were supplied by the National Institute of Standardsand Technology (NIST), (SRM 4967A, SRM 4339B). The ¹³³Ba standard solution was supplied by NorthAmerican Technical Services (NATS) (EZ-83879-767). The cation exchange resin was used in a column mode withBioRad Glass Econo columns of 0.9 cm diameter, together withpolypropylene funnels and Teflon end fittings connected withplastic taps.All gamma radioactivity measurements were carried out using a Canberra HPGe coaxial detector (Model GC4020) with relative photo-peak efficiencies of 40% for the 1332 keV line of ⁶⁰Co. The germanium detector wasconnected to a Digital Spectrum Analysis model DSA-1000. The alpha spectrometric analysis were carried out using a Canberra Alpha Analyst, with a chamber containing a passivated implanted planar silicon (PIPS) detector with an active area of 450 mm². The efficiency of the detector was calibrated against a standard alpha multi-source (67970-121,Analytics Co.) using the certified activity of the measured radionuclides.Diphonix Resin (50-100 and 100-200 mesh) was supplied from Triskem International, 35170  Bruz, France. All other chemicals used in this study, including KMnO₄, isopropanol, ammonium sulphate and different mineral acids were of analytical grade.

Radioanalyses and Measurements

The radium isotopes (²²⁶Ra and ²²⁸Ra) were determined in the water samples using the procedure described by A.El-Sharkawy et al, 2013 [8]. Four liters of each sample were allowed to pass through the purolite resin packed columns, the resin was transferred to standard counting containersand the containers were tightly sealed for four weeks to allow secular equilibrium between ²²⁶Ra, ²²⁸Ra and their decay products.The efficiency calibration of the germanium detector for the radium isotopes (Ra-226 and Ra-228) was carried out using standard samples. Known activity resins were prepared by spiking water (DDW) samples with known amounts of ²²⁶Ra and ²²⁸Ra. The spiked resin samples containing a known amount of the radionuclide of interest were used to provide anidentical matrix with a known activity, and all other conditions were followed typically (flow rate, resin volume, counting time, geometry). The ²²⁶Ra activities were determined via its daughters ²¹⁴Pb and ²¹⁴Bi through the gamma energy lines 295.22, 351.93 and 609.31 keV. The ²²⁸Ra activities were determined through the gamma energy lines of 338.32 and 911.2 keV. The calculated specific activities were basically performed using a comparison method:

where;

A_unkis the calculated activity of the sample;

A_std is the activity of the standard resin;

CR_std is the counting rate for the standard resin; and

CR_unk is the counting rate of the unknown sample.

Errors were propagated due to nuclear counting statistics, tracer and volume.

Quality Assurance

For quality assurance and validation purposes, blank sampleswere prepared in the same manner as the corresponding samples, and measured for background estimation.Reference water samples were determined using the same analysis and measurement protocol, and were compared against their certified values to testthe closeness of the measured samples to its reference values. Also, to evaluate the accuracy of the method, some selected samples were analyzed for ²²⁶Ra following an alpha spectrometric method described by S. Nour et al, 2004 [9]. In this approach, ²²⁶Ra and ¹³³Ba tracer are co-precipitated with MnO₂, dissolved in 2MHCl, loaded into a Diphonix resin column to eliminate other interfering radionuclides and the collected Ra/Ba fraction is then precipitated using BaSO₄ micro-precipitation by adding (NH4)₂SO₄, barium carrier and isopropanol in a tube, mixed well and allowed to stand in an ice bath for 30 min before vacuum filtration[10]. The filter is mounted on a disk, counted by gamma for the ¹³³Ba recovery and the ²²⁶Ra is assessed by alpha spectrometry.

Results and Discussion

Radium Isotopes in Groundwater Samples from Alsharqiya Region

The activity concentrations of ²²⁶Ra and ²²⁸Ra in the groundwater samples from Al Sharqiya region were determined and represented in fig. 3-1.

Figure 3-1 : Activity concentrations of Ra-226 and Ra-228in Al Sharqiya samples Click here to View figure

The mean activities of ²²⁶Ra and ²²⁸Ra were 13.7 and 2.5 pCi/L respectively. The ²²⁸Ra activities were almost within the low limit of detection (2.3 pCi/L). The mean ²²⁸Ra/²²⁶Ra activity ratio was 0.3.The ²²⁸Ra/²²⁶Ra ratio in groundwater may be expected to resemblethe ²³²Th/²³⁸U ratio of the host rock [11].Figures 3-2 and 3-3 present the correlations between the sum of ²²⁶Ra and ²²⁸Ra, reported as the total radium, and the dissolved oxygen (DO) and oxidation-reduction potential (ORP) respectively.

Figure 3-2 : Correlation between total Ra and dissolved oxygen in Sharqiya Click here to View figure

As shown in fig. 3-2, no correlation is observed between total Ra and DO in the investigated samples. A weak correlation between the redox potential and total radium was also observed, as shown in fig. 3-3.

Figure 3-3 : Correlation between total Ra and redox potential in Sharqiya Click here to View figure

Radium Isotopes in Groundwater Samples from Al Qasim and Tabouk Region

The mean activities of ²²⁶Ra and ²²⁸Ra in the Qasim and Tabouk groundwater samples were 10.5 and 15.9 pCi/L respectively, as shown in fig. 3-4

Figure 3-4 : Activity concentrations of Ra-226 and Ra-228in Al Qasim and Tabouk Click here to View figure

A good correlation between the redox potential and total radium was also observed, as shown in fig. 3-5.

Figure 3-5 : Correlation between total Ra and redox potential in Al Qasim and Tabouk Click here to View figure

The total radium in the investigated groundwater samples were found to be strongly associated with the decrease in the dissolved oxygen content of water, as presented in fig. 3-6. It has been reported that more radium is expected in groundwaters with low DO content, where the reducing conditions are more prevailed [5, 12].

Figure 3-6 : Correlation between total Ra and dissolved oxygen inAl Qasim and Tabouk Click here to View figure

Radium Isotopes in Groundwater Samples from Al Jawf Region

The activity concentrations of ²²⁶Ra and ²²⁸Ra in Al Jawf groundwater samples are represented in fig. 3-7. As shown in figure, the mean activity concentration of Ra-226 is 9.7 pCi/L, ranging from 4.0 to 26.5 pCi/L, while the mean activity concentrationof Ra-228 is 12.8pCi/L, ranging from 2.4 to 33.4pCi/L.

Figure 3-7 : Activity concentrations of Ra-226 and Ra-228in Al Jawf region   Click here to View figure

A correlation was observed between total radium and the dissolved oxygen content of the groundwater samples in Al Jawf region, with a correlation coefficient (R²) = 0.46, while a good correlation was found between the combined (²²⁶Ra and ²²⁸Ra) and the redox potential, as shown in fig. 3-8 and fig. 3-9.

Figure 3-8 : Correlation between total Ra and dissolved oxygen in Al Jawf Region Click here to View figure

Figure 3-9 : Correlation between total Ra and redox potential in Al Jawf Region Click here to View figure

Radium Isotopes in Groundwater Samples from WadiAl-Dawaser Region

Combined radium (²²⁶Ra and ²²⁸Ra) activity concentrations in groundwater samples from 21 wells were determined, as presented in fig. 3-10.

Figure 3-10 : Activity concentrations of Ra-226 and Ra-228in WadiAl-Dawaserregion Click here to View figure

The mean activity concentration of Ra-226 is 4.7 pCi/L, ranging from 4.0 to 7.9 pCi/L, while the mean activity concentrationof Ra-228 is 7.9pCi/L, ranging from 2.4 to 12.6pCi/L. From the results of the analyzed groundwater samples, only ²²⁸Ra radioisotope presents higher activity than WHO recommendations (about 99% of all the sampledwells). The mean ²²⁸Ra/²²⁶Ra activity ratio was 1.8. For the relationship between total radium and water chemical constituents, a weak correlation was found between the combined radium and DO, as shown in fig. 3-11.

Figure 3-11 : Correlation between total Ra and dissolved oxygen in WadiAl-Dawaser Click here to View figure

No correlations were observed between total radium and other water chemical constituents (TDS, ORP, pH).

Conclusion

The activity concentrations of ²²⁶Ra and ²²⁸Ra were determined in 107 groundwater samples collected from selected wells in Saudi Arabia. Radioactive isotope ²²⁸Ra shows highest activity in Al Qasim. Good correlations between some chemical constituents and the activities of the combined radium (²²⁶Ra and ²²⁸Ra) in the analyzed groundwater samples were observed.The relationship of combined Ra with DO shows that elevated ²²⁶Ra and ²²⁸Ra activities are strongly inverselyrelated to DO in Al Qasim and Al Jawf. The redox potential has also good correlation with the combined radium in Al Qasim and Al Jawf. No correlations were found between pH, TDS and total radium in the studied groundwater samples. The data obtained may serve as a reference or baseline radiological map for the natural radioactivity levels in these regions in case of any future studies.

Acknowledgment

We express our appreciation to the Deanship of Scientific Research at Princess Nora University for funding this work through the research project 37907/JKD. Also acknowledgment is extended to the chemists and technicians of Technology Experts Co.for their role in the sampling, measurements and analyses of the water samples.

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