Abstract

Geochemical models like MEDUSA (make equilibrium diagrams using sophisticated algorithms) and the PRHEEQC versions 2 have been used to assess the uranium speciation under the influence of redox conditions and complexant concentration in ground water. For this study, the groundwater samples (bore well and hand pump) were analysed for different geochemical parameters including uranium. Using a Piper diagram, groundwaters were classified into four categories as (Na+K)−SO₄ type, (Na+K)-HCO₃ type, (Ca+Mg)−SO₄ type and (Ca+Mg)–HCO₃ type reflecting the complicated nature of geology of the study area. Based on the water composition, carbonate weathering appeared to be the major water–rock interaction in the ground water. The measured mean concentration of uranium in groundwater samples was found to be 99.96 µg/L, about 3 times higher than EPA guidelines of 30 μg/L. Chemical speciation, oxidation state, redox reactions and sorption characteristics are necessary in predicting the solubility and mobility of uranium in aqueous systems. The low redox potential of the groundwater minimizes the concentration of dissolved uranium since it is mainly present as U (IV). The simultaneous presence of different oxidation states of uranium in aqueous system complicate the prediction of its behaviour with changes in Eh, pH and ligand concentration. The formation of U₄O₉(c) and UO₂(c) and UO₂(CO₃)₃^4- were observed as the predominant species that control the uranium concentration in most of the ground water. The solubility of hexavalent UO₂²⁺ in ground water is relatively high due to formation of carbonate complexes. A strong correlation of uranium with salinity and TDS observed indicating that the mobility of uranium in the groundwater was very much influenced and controlled by these two parameters. 

Key words: Groundwater, PRHEEQC version 2, (make equilibrium diagrams using sophisticated algorithms) MEDUSA code, saturation indices, salinity, TDS, uranium.