U-236/U-238 and Pu-240/Pu-239 isotopic ratios in small (2 L) sea and river water samples

Accelerator Mass Spectrometry (AMS) and alpha spectrometry were used to determine uranium (²³⁶U, ²³⁸U, ²³⁴U) and plutonium isotopes (²³⁹Pu, ²⁴⁰Pu) in sea and river water samples. Plutonium was separated by Dowex^® 1 × 8 resin and UTEVA^® resin was used for uranium purification. The measured ²³⁶U/²³⁸U isotopic ratios for surface water from the Atlantic Ocean, the Pacific Ocean and the Black Sea were in the order of 10⁻⁹, while values for river water were in the order of 10⁻⁸. These contaminations may be attributed to global fallout. A sample of the reference material IAEA-443, collected from the Irish Sea, showed, in accordance to the reference value, a ratio that was 10³ times higher due to effluents from the reprocessing plant at Sellafield. These results underline the good suitability of ²³⁶U/²³⁸U as a tracer for hydrology and oceanography, and show that relatively small water samples are sufficient for the determination of ²³⁶U by AMS, which is not the case for plutonium with present techniques. The plutonium concentrations in our water samples could only be measured with large uncertainties and were in the order of 10⁻³ mBq/L (with the exception of the Irish Sea sample).