Occurrences and Mobility of Uranium in Soil Profile Due to Groundwater–Soil Interaction

Abstract

Uranium (U) is a naturally occurring radioactive element. It has atomic number of 92 and relative atomic mass of 238.03 g/mol. Naturally occurring uranium is a mixture of three isotopes and contains about 99.27% 238 U, 0.72% 235 U, and 0.0054% 234 U. This is the longest known radioactive decay series, ending with the stable daughter isotope of 206 Pb. Considering shallow crustal level heat production, radioactive decay of U, Th, and 40 K are the main sources of heat. Uranium is very mobile and soluble under oxidizing condition. Therefore, uranium can be a major pollutant in the groundwater, occurring UO 2 2+ as dominant aquatic species. Natural occurrence of uranium can be of 4 + , 5 + , and 6 + oxidation states. However, under natural environmental conditions, only the tetravalent (4+) and hexavalent (6+) oxidation states are stable. Uranium solubility in aqueous system is controlled mainly by three factors. Most important one is oxidation-reduction potential (Eh), followed by pH, and lastly dissolved carbonate content. Under oxidizing condition, hexavalent uranium (6+) is considered more soluble than tetravalent uranium (4+). Under reducing condition, uranium 4+ forms aquatic complexes with hydroxides or fluorides. Normally at very low pH (<5), uranyl species occur. However, under normal groundwater pH conditions (6–9), the U 6+ hydroxyl form of (UO 2 ) 3 (OH) 5- is the dominant aquatic species. Uranyl carbonates are considered most stable among the complex uranium ions. However, this species is also predominant in bicarbonate, neutral sulfate, or chloride type weakly alkaline or acidic groundwater. Uranium is transported in soil profile as emulsoids, as colloids, or by diffusion in the groundwater. Mobility of uranium in soil profile is controlled by the following factors: Uranium concentration in source rocks, sediments, and soils along with the ease with which they can be leached by groundwater. Groundwater proximity from the uranium source. Presence of highly sorptive compounds in the soil profile, such as organic matter, petroleum/hydrocarbon, oxyhydroxides of Fe 3+ , Mn, Ti, and clays. Temperature, salinity, pressure, and Eh-pH of the groundwater. Change in climatic effects/conditions, such as seasonal variations, precipitation, and water evaporation rate. Presence of suitable complexing agent concentration in soil and water to form/precipitate insoluble uranium minerals. To the degree of which the groundwater involved in uranium mobilization is isolated hydro-dynamically from areas of groundwater mixing. Mobilization of uranium and getting contaminated with groundwater have huge health hazards. Uranium toxicity is of more concern than the risk of radiological cancer. Toxic effects of uranium exposure include nephritis/kidney disease and other health effects. However, other organs may be at risk from chronic radiological toxicity including the lymph nodes and bone. Presence of radiogenic He 4+ in groundwater is a strong evidence of uranium pollution of groundwater. Uranium mobilization in soil profile has huge implication as passive geochemical method of hydrocarbon exploration. Not only that uranium disequilibrium series, especially the 234 U/ 238 U method has application in dating very old groundwater. Groundwater age dating may be helpful in decision making of sustainable groundwater management. Natural radioactive logging (U and Th) is also a common practice for petrophysical data acquisition (natural hydrocarbon logging) in hydrocarbon industry at the time of drilling and production stage. Efforts have also been made to correlate natural radioactive emission with earthquake. This chapter includes discussion on uranium mineralogy and mode of occurrence, its geochemical behavior particularly in the form of metal complex formation, and mobility due to soil–water interaction. Not only that, the application of radiometric methods for geochemical hydrocarbon exploration and dating very old groundwater has also been incorporated for discussion. Uranium health hazard has been addressed as a matter of concern. Lastly, the chapter has been enriched with published case studies where uranium pollution of groundwater has been considered as a subject matter of the research.