Water Quality Standards

Uranium is used mainly as fuel in nuclear power stations. It is present in the environment as a result of leaching from natural deposits, release in mill tailings, emissions from the nuclear industry, the combustion of coal and other fuels, and the use of phosphate fertilizers that contain uranium. The major source of exposure to uranium is food.

There are insufficient data regarding the carcinogenicity of uranium in humans and experimental animals. The guideline value for the chemical toxicity of uranium was therefore derived using a TDI approach. As no adequate long-term study was identified, the TDI was derived using the results of the most extensive short-term study conducted to date, in which uranium was administered in drinking-water to the most sensitive species and sex. In a 91-day study in rats, the LOAEL for degenerative lesions in the proximal convoluted tubule of the kidney in males was 0.96 mg of uranyl nitrate hexahydrate per litre, equivalent to 0.06 mg of uranium per kg of body weight per day.

A TDI of 0.6 µg/kg of body weight per day was derived using the LOAEL of 60 µg/kg of body weight per day and an uncertainty factor of 100 (for intra- and interspecies variation). Application of an additional uncertainty factor to account for the use of a LOAEL instead of a NOAEL is unnecessary because of the minimal severity of the lesions being reported. Moreover, no additional uncertainty factor for the length of the study (91 days) is required because the estimated half-life of uranium in the kidney is 15 days, and there is no indication that the severity of the renal lesions will be exacerbated following continued exposure.

This TDI yields a guideline value of 2 µg/litre (rounded figure), assuming a 60-kg adult consuming 2 litres of drinking-water per day and a 10% allocation of the TDI to drinking-water.

There are several methods for removing uranium from drinking-water, although some have been tested only in the laboratory or on a pilot scale. Coagulation using ferric sulfate or aluminium sulfate at optimal pH and coagulation dosages can achieve 80–95% removal of uranium, whereas at least 99% removal can be achieved using lime softening, anion exchange resin, or reverse osmosis processes. In areas with high natural uranium levels, a value of 2 µg/litre may be difficult to achieve with the treatment technology available. The guideline value is provisional because of these difficulties and because of limitations in the key study. It should be noted that several human studies are under way that may provide helpful additional data. 
This review addresses only the chemical aspects of uranium toxicity. Information pertinent to the derivation of a guideline based on radiological effects is presented in Radiological aspects.