Geochemical and hydrogeological factors influencing high levels of radium contamination in groundwater in arid regions

Abstract

One of the most challenging environmental issues in arid regions is radionuclide groundwater contamination; typically, radionuclide sources, mobility, and spatial distributions are not well understood. The main objectives of this study are to investigate the groundwater hydrochemistry and identify the factors governing the radium occurrences and mobility. Groundwater samples were collected from shallow unconfined zone and deep confined Saq sandstone aquifer in the Hail area, Saudi Arabia. They were analyzed for major, minor, and trace elements as well as radium isotopes (²²⁶Ra and ²²⁸Ra). The hydrochemical relationships, water facies, spatial distribution, and the factor analysis were integrated to elucidate the governing processes in the system. The hydrochemical facies exhibited four water types characterized by earth alkaline and alkaline elements. Most samples contained sulfates and chlorides. The hydrochemical processes affecting groundwater included the dissolution of certain minerals, mixing between modern and fossil water types, and reverse ion exchange. There are high concentrations of nitrate in the unconfined zone, with low concentrations in areas under confining conditions. High radium concentrations were recorded in the groundwater, and the ²²⁶Ra and ²²⁸Ra activity concentrations of the examined samples were 11% and 98% above the World Health Organization (WHO) guidelines, respectively. The spatial distribution of ²²⁶Ra showed high activity concentration in the shallow zone under prevailing oxidizing conditions. High ²²⁸Ra contamination was identified in the confined zone where the redox potential appears to decrease and the temperature increases result in higher mobility or desorption of the radium ions. In the unconfined zone, the oxidation of Fe⁺² in the groundwater and precipitation of Fe⁺³ in the aquifer pore spaces and co-precipitation with barite can accelerate radium adsorption. The ²²⁸Ra/²²⁶Ra ratio classified the radium groundwater enrichment into three main clusters, namely, those depending on the redox potential values, the primary source distribution, and enrichment in ²²⁶Ra relative to ²²⁸Ra. Five major factors influencing groundwater hydrochemistry were identified using factor analysis. The first factor explained the processes resulting in the dissolution of the silicate minerals and thereby increased the uranium mobility. The second factor encompassed processes leading to a rise in the groundwater salinity. The third factor identified thorium minerals as the source of the ²²⁸Ra. The fourth factor was ascribed to the decrease in radium through sorption processes or co-precipitation with barite. The fifth factor referred to by the uneven distribution of Th and U containing minerals in the aquifer.

Keywords: Arid environments; Groundwater; Radionuclides; Radium contamination; Saq aquifer.