SOURCES, MECHANISMS, AND IMPLICATIONS FOR NATURAL RADIONUCLIDES CONTAMINATION IN GROUNDWATER BASINS IN THE MIDDLE EAST

In groundwater systems natural-occurring radionuclides are typically retained to the aquifer matrix and are not dissolved in associated groundwater. Radium is exceptional since the ratio between adsorbed and dissolved radium depends on the ionic strength, temperature, acidity, and redox conditions of the solution. Here, we report the results of an on-going study that investigates the sources, mechanisms of enrichment, and implementation of extremely high levels of radium isotopes found in deep groundwater from the Paleozoic and Lower Cretaceous Nubian Sandstone, the Upper Cretaceous carbonate, and the Quaternary aquifers in Israel (Negev and Arava Valley), southern Jordan (Disi area), and the Palestinian Authority (Jordan Valley). We measured the Ra isotope quartet (226Ra-half life 1600 y; 228Ra-5.6 y; 224Ra-3.6 d; 223Ra-11.4 d) and were able to discriminate between radioactivity derived from a thorium decay chain (high 228Ra/226Ra and 224Ra/223Ra ratios) found in groundwater flowing in the Nubian Sandstone aquifer and an uranium source (low 228Ra/226Ra and 224Ra/223Ra ratios) in groundwater from the carbonate aquifer. In the southern Arava Valley we show that the radium activity is positively correlated with temperature (and depth). The increase in radioactivity is associated with radium isotopic discrimination; the 228Ra/226Ra and 224Ra/223Ra ratios increase and decrease with temperature in the Nubian sandstone and carbonate aquifers, respectively. This reflects preferential dissolution of thorium and uranium sources in the two aquifers. In the Disi aquifer in southern Jordan we report high levels of radium isotopes although the salinity of the groundwater is low (TDS<1000 mg/l). We found an inverse correlation between pH and radium activity. In contrast, in the brackish groundwater from the Jordan Valley the 226Ra is dominant and is linearly correlated with salinity. These findings indicate that high radioactivity can be found in low-saline groundwater and not only in saline groundwater or brines as previously reported. We show that most of the deep groundwater in southern Israel and Jordan has high radioactivity that far exceeds international drinking levels and is not suitable for human consumption. These findings could have implications for other similar groundwater basins in the Middle East.