Natural Groundwater Radioactivity of Selected Saudi Arabian Aquifers

Nils Michelsen1, Michael Schubert2, Alexander Bassis1, Christoph Schüth1, Randolf Rausch3, Mohammed Al-Saud4
1 TU Darmstadt, Institute for Applied Geosciences
2 Helmholtz-Centre for Environmental Research – UFZ, Leipzig
3 GIZ International Services, Riyadh, Kingdom of Saudi Arabia
4 Ministry of Water & Electricity, Riyadh, Kingdom of Saudi Arabia

P 10.6 in Hydrogeologie arider Gebiete

Water supply in semi-arid and arid regions often relies on non-renewable, fossil groundwater from deep aquifers. Aiming at an efficient “mining” of the limited resources, a sound understanding of the aquifer systems is crucial, referring to both available water quantities and qualities. With respect to the latter, standard analyses usually focus on major ions and selected trace elements, but do not comprise radionuclides, which however might be present in significant concentrations (compare Vengosh et al. 2009).

In order to find out whether Saudi Arabian groundwaters are subject to this phenomenon as well, samples were obtained from several aquifers. As the sandstones of the Kingdom are directly or indirectly derived from granites, which are generally known to feature appreciable amounts of incompatible elements such as uranium and thorium, these strata are probably more prone to the issue. Hence, emphasis was placed upon sandstone aquifers. For reference, however, also selected limestone aquifers were studied. All water samples were analyzed for uranium-234, uranium-238, radium-226, and radium-228.

Although uranium is known to occur in the aquifer matrix, the corresponding radionuclide activity concentrations are for the most part low to moderate and range well below their corresponding nuclide-specific WHO guideline values. From a radio-ecological perspective, the uranium isotopes are thus only of minor importance. With a few exceptions also radium-226 shows activities that do not exceed its threshold value. Radium-228, by contrast, frequently occurs in appreciable concentrations. Partly, also the guideline value of 100 mBq/l is exceeded, particularly in samples from sandstone aquifers. Distribution maps created for the studied aquifers do mostly not reveal clear spatial trends, i.e., radium-228 appears erratically. An inter-aquifer comparison however shows that elevated activities are more common in the older clastic strata cropping out in the eastern part of the country. This suggests that the depositional setting of the aquifer, particularly its distance to the Arabian-Nubian Shield, plays a significant role. Considering these findings, thorium-bearing heavy minerals in the sandstone aquifers, originating from the crystalline basement complex, seem to be a plausible source for the encountered radium‑228. Due to the fact that radium isotope ratios differ between sandstone and limestone aquifers, they might provide considerable fingerprinting potential.

Vengosh, A., Hirschfeld, D., Vinson, D., Dwyer, G., Raanan, H., Rimawi, O., Al-Zoubi, A., Akkawi, E., Marie, A., Haquin, G., Zaarur, S. & Ganor, J. (2009): High Naturally Occurring Radioactivity in Fossil Groundwater from the Middle East. - Environ. Sci. Technol., 43: 1769-1775.

Letzte Änderung 31.10.2013