Radium in groundwater hosted in porous aquifers: estimation of retardation factor and recoil rate constant by using NAPL
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Radium in groundwater hosted in porous aquifers: estimation of retardation factor and recoil rate constant by using NAPLs Alessandra Briganti1 · Mario Voltaggio2 · Paola Tuccimei1 · Michele Soligo1 Received: 17 February 2020 / Accepted: 29 September 2020 © Springer Nature Switzerland AG 2020
Abstract A new method for estimating retardation factor and recoil constant of radium isotopes in groundwater hosted in porous aquifers is described. The method is based on the evidence that alpha-recoiled radium ions, supplied by thorium parent atoms which occur in phases immersed in NAPL (Non-Aqueous Phase Liquids), are not adsorbed on solid phases. Experimental evidence is given that manganese dioxide, zeolite 4A, natural clay, monazite and weathered volcanic rock, all phases normally adsorbing radium from aqueous solutions, when immersed in NAPL adsorb negligible amounts of radium. This allows using experimental data on rock samples, representative of porous aquifers, for estimating Ra retardation factor and its alpha recoil constant in groundwater, without using Rn data as a comparison term. Unlike estimation of retardation factor between the “NAPL method” and the method based on comparison with radon depends on the different process of entry from aquifer rock into groundwater for radon and radium. Precise estimates of retardation factor and recoil constants of radium allow to apply equations ruling the temporal evolution of radium isotopes in groundwater and to determine its age. Implications, useful for measuring the contamination age of soils by NAPL fluids, are described as well. Keywords Retardation factor · Recoil constant · Radium isotopes · NAPL · Groundwater
1 Introduction The goal of this paper is to provide new insights for determining groundwater ages using radium isotopes. When using radioactive tracers, the tracer age differs from the groundwater age by a factor, which is named tracer retardation factor. This factor, in case of conservative tracers, equals the unity. It should be noted that the groundwater age remains only an estimate of the mean residence time (MRT) of groundwater in aquifer [1]. In fact the shape of the age distribution in real-world groundwater
is unknown. Thus, MRT is a weighted average over many idealized ages and the age of a single sample of groundwater can differ notably from the MRT of groundwater. Ra isotopes are a quartet of radioactive isotopes with half lives extending from some days up to thousands of years (224Ra t1/2 = 3.66 days, 223Ra t1/2 = 11.4 days, 228Ra t1/2 = 5.75 years, and 226Ra t1/2 = 1600 years). They belong to the natural radioactive series of 238U (226Ra), 235U (223Ra) and 232Th (224Ra and 228Ra). The hydrogeochemistry of radium depends mainly on four factors [2]: (1) a low ionic potential that causes strong adsorption of Ra ions on the
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s42452-020-03610-4) contains supplementary material, which is available to authorized users. * Mario Voltaggio, voltaggiomari
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