Uranium isotopes in rivers, estuaries and adjacent coastal sediments of western India: their weathering, transport and oceanic budget

Abstract

The two major river systems on the west coast of India, Narbada and Tapti, their estuaries and the coastal Arabian sea sediments have been extensively studied for their uranium concentrations and ²³⁸U/²³⁸U activity ratios. The ²³⁸U concentrations in the aqueous phase of these river systems exhibit a strong positive correlation with the sum of the major cations, σNa + K + Mg + Ca, and with the HCO₃^- ion contents. The abundance ratio of dissolved U to the sum of the major cations in these waters is similar to their ratio in typical crustal rocks. These findings lead us to conclude that ²³⁸U is brought into the aqueous phase along with major cations and bicarbonate. The strong positive correlation between ²³⁸U and total dissolved salts for selected rivers of the world yield an annual dissolved ²³⁸U flux of 0.88 × 10¹⁰g/yr to the oceans, a value very similar to its removal rate from the oceans, 1.05 × 10¹⁰g/yr, estimated based on its correlation with HCO₃^- contents of rivers. In the estuaries, both ²³⁸U and its great-grand daughter ²³⁴U behave conservatively beyond chlorosities 0.14 g/l. These data confirm our earlier findings in other Indian estuaries. The behavior of uranium isotopes in the chlorosity zone 0.02-0.14 g/l, was studied in the Narbada estuary in some detail. The results, though not conclusive, seem to indicate a minor removal of these isotopes in this region. Reexamination of the results for the Gironde and Zaire estuaries (Martin et al., 1978a and b) also appear to confirm the conservative behavior of U isotopes in unpolluted estuaries. It is borne out from all the available data that estuaries beyond 0.14 g/l chlorosities act neither as a sink nor as a source for uranium isotopes, the behavior in the low chlorosity zones warrants further detailed investigation. A review of the uranium isotope measurements in river waters yield a discharge weighted-average ²³⁸U concentration of 0.22 μg/l with a ²³⁴U/²³⁸U activity ratio of 1.20 ± 0.06 is missing. The residence time of uranium isotopes in the oceans estimated from the ²³⁸U concentration and the ²³⁴U/²³⁸U A. R. of the rivers yield conflicting results; the material balance of uranium isotopes in the marine environment still remains a paradox. If the disparity between the results is real, then an additional ²³⁴U flux of about 0.25 dpm/cm²·10³ yr into the oceans (about 20% of its river supply) is necessitated.