Radium in groundwater hosted in porous aquifers: estimation of retardation factor and recoil rate constant by using NAPLs

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 diferent 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 fuids, are described as well.