Completion of Kr-81 and Kr-85 Analysis Development for Hydrogeology and Testing its Validity by Assessing Aquifer Recharge Rates

Summary The overall objective was to complete the development of a RIMS-based analytical technique to determine the concentration of the rare krypton radioisotopes, 81Kr and 85Kr, in samples of interest to the geoscience and planetary science community The key to RIMS is the use of tunable lasers to selectively and efficiently excite by resonant photon absorption atomic states unique to the chosen element. Ionization of the specified element can then occur while excluding all other constituents of the sample, bringing detection limits down to the single-atom level. Combining RIMS with several steps of isotopic enrichment makes detection of a rare isotope, such as 81Kr, feasible. A complete process for groundwater samples consists of starting with (1) collecting the groundwater sample, (2) degassing the water sample, (3) separating Kr from the recovered gases, (4 & 5) two isotopic enrichments reducing interfering isotopes by >109, and (6) detecting the rare krypton isotope using RIMS in a time-of-flight system. Required water sample size is 20 liters for 81Kr and 10 to 3 liters for 85Kr. Weak links in the above steps were to be identified and rectified. Most of the troublesome issues were resolved, but unfortunately, two key difficulties could not be resolved with the available resources, so the overall, final goal of completing a suite of measurements was not achieved. In spite of this, valuable collaborations were established to demonstrate the value of 81Kr and 85Kr measurements in two critical applications, the future site of a nuclear waste repository, where high efficiency 81Kr measurements would allow groundwater dating of low-yield formations, and a multi-method analysis of potential contamination inflow into a large municipal water system, where the simplicity of interpretation of 85Kr measurements could become a valuable future interpretive tool. Therefore, samples were collected anyway, and processing started using the working parts of the method, so that comparisons to other methods could be made in the futur