MULTIPLE ASSAYS OF URANIUM TOXICITY IN SOIL

The literature offers conflicting information on the phytotoxicity of uranium (U) in soil. Levels as low as 5 mg U/kg soil, well within the normal background range, have been cited as toxic, whereas many studies report no toxicity at levels 100- to 1000-fold higher. Uranium in soil does not often present a radiological hazard to humans, but toxicity to plants could lead to prescribed cleanup and assessment criteria for industrial activities. This study was designed to identify the toxic threshold of soil U. Nine levels of U were added to background-level soils. Beans were grown to maturity in an outdoor lysimeter study with 2 soils. Germination tests in the growth chamber were done with 11 soils and with 5 plant species. A rapid cycling Brassica rapa cultivar, proposed as a new toxicity test subject, was grown to maturity on 2 soils in the growth chamber, and germination of the second-generation seeds was investigated. Earthworm survival and soil phosphatase activity were measured. Measures of soil-extractable and tissue U concentrations were obtained. None of the measurements indicated detrimental effects of U below 300 mg/kg. In one of the soils, B. rapa produced seed at concentrations as high as 10,000 mg U/kg. The phytotoxicity of U occurred at concentrations 8- and 3-fold higher than the concentrations of As and Zn, respectively, which gave the same effect.