Variation in the uranium isotopic ratios ²³⁴U/²³⁸U, ²³⁸U/total-U and ²³⁴U/total-U in Indian soil samples: Application to environmental monitoring

¹ Health Physics Unit, NFC, Environmental Assessment Division, Bhabha Atomic Research Center, P.O. ECIL, Hyderabad - 500062, India
² Environmental Assessment Division, Bhabha Atomic Research Centre, Bhabha Atomic Research Center, Trombay, Mumbai - 400085, India

^* e-mail: sksnfc@rediffmail.com

Received: 11 June 2012
Accepted: 16 October 2012

Abstract

The uranium isotopes ²³⁸U, ²³⁵U and ²³⁴U are found naturally in the environment. ²³⁸U and ²³⁵U are parent nuclides of two independent decay series of isotopes, while ²³⁴U is a member of the ²³⁸U decay chain. When decay series occur in a closed system the series tends to reach, with time, the state of secular equilibrium in which the activities of all series members are equal to the activity of its first nuclide. The activity ratio ²³⁴U/²³⁸U in natural uranium may vary as a consequence of decay chain disequilibrium due to alpha recoil and biogeochemical processes. A study based on measurement of uranium concentration and ²³⁴U/²³⁸U activity ratios in soil samples collected from Nalgonda district, Andhra Pradesh, India, a proposed mining site, was carried out to find the spatial distribution of uranium and the state of secular equilibrium of ²³⁴U/²³⁸U to examine the possibility of applying uranium concentration and uranium isotopic activity ratios to detect any hydrogeochemical changes in the environment during/post-operation. Soil samples were collected and analyzed for uranium concentration using the conventional UV fluorimetric method, showing a uranium concentration in the range of 0.7 ± 0.2 ppm to 7.9 ± 0.4 ppm with an average of 3.4 ppm, and ²³⁴U/²³⁸U activity ratios were estimated using the alpha spectrometry technique, showing an activity ratio in the range of 0.92 ± 0.11 to 1.02 ± 0.11. The ²³⁴U/ ²³⁸U activity ratio obtained indicated that these two uranium isotopes are in the state of secular radioactive equilibrium. The percent activity ratio of ²³⁸U/total U and ²³⁴U /total U is observed to vary from 47.94 ± 4.83 to 50.76 ± 4.87 and 45.80 ± 3.83 to 49.14 ± 3.99, respectively.