Radium isotopes as submarine groundwater discharge (SGD) tracers: review and recommendations

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Garcia-Orellana, J., Rodellas, V., Tamborski, J., Diego-Feliu, M., van Beek, P., Weinstein, Y., Charette, M., Alorda-Kleinglass, A., Michael, H. A., Stieglitz, T., & Scholten, J. Radium isotopes as submarine groundwater discharge (SGD) tracers: review and recommendations. Earth-Science Reviews, 220, (2021): 103681, https://doi.org/10.1016/j.earscirev.2021.103681.Submarine groundwater discharge (SGD) is now recognized as an important process of the hydrological cycle worldwide and plays a major role as a conveyor of dissolved compounds to the ocean. Naturally occurring radium isotopes (223Ra, 224Ra, 226Ra and 228Ra) are widely employed geochemical tracers in marine environments. Whilst Ra isotopes were initially predominantly applied to study open ocean processes and fluxes across the continental margins, their most common application in the marine environment has undoubtedly become the identification and quantification of SGD. This review focuses on the application of Ra isotopes as tracers of SGD and associated inputs of water and solutes to the coastal ocean. In addition, we review i) the processes controlling Ra enrichment and depletion in coastal groundwater and seawater; ii) the systematics applied to estimate SGD using Ra isotopes and iii) we summarize additional applications of Ra isotopes in groundwater and marine studies. We also provide some considerations that will help refine SGD estimates and identify the critical knowledge gaps and research needs related to the current use of Ra isotopes as SGD tracers.J.Garcia-Orellana acknowledges the financial support of the Spanish Ministry of Science, Innovation and Universities, through the “Maria de Maeztu” programme for Units of Excellence (CEX2019-000940-M), the Generalitat de Catalunya (MERS; 2017 SGR – 1588) and the project OPAL (PID2019-110311RB-C21). V. Rodellas acknowledges financial support from the Beatriu de Pinós postdoctoral program of the Generalitat de Catalunya (2017-BP-00334 and 2019-BP-00241). M. Charette received support from the U.S. National Science Foundation (OCE-1736277). J. Scholten acknowledges the support through the SEAMOUNT BONUS project (art. 185), which is funded jointly by the EU and the Federal Ministry of Education and Research of Germany (BMBF, grant no. 03F0771B). P. van Beek and T. Stieglitz acknowledge support from the French ANR project MED-SGD (ANR-15-01CE-0004) and chair @RAction MED-LOC (ANR-14-ACHN-0007-01). A. Alorda-Kleinglass acknowledges financial support from ICTA “Unit of Excellence” (MinECo, MDM2015-0552-17-1) and PhD fellowship, BES-2017-080740. H. Michael acknowledges support from the U.S. National Science Foundation (EAR-1759879). M. Diego-Feliu acknowledges the financial support from the FI-2017 fellowships of the Generalitat de Catalunya (2017-FIB-00365). Fig. 3, Fig. 4, Fig. 7, Fig. 12 were designed by Gemma Solà (www.gemmasola.com)

Guidelines and Limits for the Quantification of Ra Isotopes and Related Radionuclides With the Radium Delayed Coincidence Counter (RaDeCC)

International audienceThe Radium Delayed Coincidence Counter (RaDeCC) is one of the most extensively used equipment for measuring 223Ra and 224Ra activities in water and sediment samples. Samples are placed in a closed He-circulation system that carries the Rn produced by the decay of Ra to a scintillation cell. Each alpha decay recorded in the cell is routed to an electronic delayed coincidence system which enables the discrimination of 223Ra and 224Ra. In this study, the measurement and quantification methods using the RaDeCC system are assessed through analyses of registered data in different RaDeCC systems worldwide and a set of simulations. Results of this work indicate that the equations used to correct for 223Ra and 224Ra cross-talk interferences are only valid for a given range of activities and ratios between isotopes. Above certain limits that are specified in this study, these corrections may significantly overestimate the quantification of 223Ra and 224Ra activities (up to ~40% and 30%, respectively), as well as the quantification of their parents 227Ac and 228Th. High activities of 226Ra may also produce an overestimation of 224Ra activities due to the buildup of 222Rn, especially when long measurements with low activities of 224Ra are performed. An improved method to quantify 226Ra activities from the buildup of 222Rn with the RaDeCC system is also developed in this study. Wethus provide a new set of guidelines for the appropriate quantification of 223Ra, 224Ra, 227Ac, 228Th, and 226Ra with the RaDeCC system