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

The uranium isotopes U-238, U-235 and U-234 are found naturally in the environment. U-238 and U-235 are parent nuclides of two independent decay series of isotopes, while U-234 is a member of the U-238 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 U-234/U-238 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 U-234/U-238 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 U-234/U-238 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 U-234/U-238 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 U-234/U-238 activity ratio obtained indicated that these two uranium isotopes are in the state of secular radioactive equilibrium. The percent activity ratio of U-238/total U and U-234/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.