Uranium in groundwater of the Al-Batin Alluvial Fan aquifer, south Iraq

Abstract

This study focuses on identifying concentrations, species, spatial distribution and potential sources of uranium (U) in groundwater in Al-Batin area south of Iraq. Although few studies reported elevated concentration of U in groundwater in some parts of the Al-Batin Alluvial Fan, a comprehensive investigation of U in the groundwater of the entire fan is still missing so far. To achieve this goal, 43 groundwater samples are collected for major cations and anions, trace elements, total organic carbon and total inorganic carbon analyses. U concentrations range between 0.1 and 98 μg/L with an arithmetic average of 16 μg/L. The spatial distribution of U in groundwater shows an increase eastward and northeastward with some anomalies. UO2(CO3)22−, UO2(CO3)34− and UO2(HPO4)22− are the dominant U species according to PHREEQC simulations. Spearman correlation analysis (n = 43, p < 0.01) shows a positive correlation between U and K, HCO3−, Ca, Mg, Al, Si and rare earth elements (∑REE). Well depth, on the other hand, had a negative correlation (r = −0.65 at p < 0.01) with U, indicating that U is dominant in more shallow groundwater. This study assumes a geogenic source of U from the Arabian Shield in Saudi Arabia, derived by flood plain and trapped in the east and northeast of the study area under reduced conditions of the neighbor marshes. The release of uranium into the groundwater can be explained by three hypotheses: The first hypothesis is desorption of U sorbed onto ferric minerals (e.g., hematite, magnetite and goethite). The second hypothesis is desorption of U from clay minerals in mudstone lenses within the aquifer. The third hypothesis is the release of U from the carbonate lattice structure due to the replacement with Ca. The relatively high Eh and the elevated concentrations of SO42−, Ca2+ and Na+ in groundwater are appropriate conditions to release U load of the supposed sources into groundwater.