Geological and hydrogeochemical controls on radium isotopes in groundwater of the Sinai Peninsula, Egypt

Abstract

Radium isotopes (226Ra and 228Ra) were analyzed in 18 groundwater samples from the Nubian Sandstone Aquifer System (NSAS) and the shallow alluvial aquifers overlying the basement complex of the Sinai Peninsula, Egypt. Groundwater samples from deep Nubian aquifer wells (total depths 747 to 1250m) have 226Ra and 228Ra activities ranging from 0.168 to 0.802 and 0.056 to 1.032Bq/L, respectively. The shallower Nubian aquifer wells (63 to 366m) have 226Ra and 228Ra activities ranging from 0.033 to 0.191 and 0.029 to 0.312Bq/L, respectively. The basement shallow alluvial aquifers have 226Ra and 228Ra activities ranging from 0.014 to 0.038 and 0.007 to 0.051Bq/L, respectively. Combined Ra activities in most wells were generally in excess of the US Environmental Protection Agency (EPA), the European Union (EU), and the World Health Organization (WHO) maximum contaminant levels (MCL) for drinking water. Radium in groundwater is produced mainly by decay of parent nuclides in the aquifer solids, and observed activities of dissolved Ra isotopes result from a combination of alpha-recoil, adsorption/desorption, co-precipitation/dissolution processes. The observed correlation between Ra activities and salinity indicates that adsorption/desorption processes may be the dominant factor controlling Ra mobility in Sinai groundwater. Radium activities in central and northern Sinai are generally higher than those in southern Sinai, consistent with a gradual increase in salinity and water-rock interaction with increasing groundwater age. Barite is approximately saturated in the groundwater and may limit maximum dissolved Ra concentration. The results of this study indicate that Sinai groundwater should be used with caution, possibly requiring Ra removal from water produced for domestic and agricultural consumption.