Radium, thorium and radioactive lead isotopes in groundwaters: application to the in situ determination of adsorption-desorption rate constants and retardation factors

Abstract

Five groundwater samples taken from different Hydrogeologie settings in Connecticut were analyzed for major cation chemistry and the concentration of U and Th decay series nuclides ²³⁸U, ²³⁴Th, ²²⁶Ra, ²²²Rn, ²¹⁰Pb, ²¹⁰Po, ²³²Th, ²²⁸Ra, ²²⁸Th, and ²²⁴Ra. The concentration of ²²²Rn in the waters ranged between 10³ and 10⁴ dpm l^-1 and was three to four orders of magnitude greater than that of the short-lived alpha daughters ²²⁴Ra, ²²⁸Ra, and ²³⁴Th, even though the rates of supply of these four nuclides to solution are expected to be similar. We infer that sorption removes radium and thorium from these groundwaters on a time scale of 3 minutes or less. The (²²⁴Ra/²²⁸Ra) and (²³⁴Th/²²⁸Th) activity ratios in these waters indicate that desorption of these nuclides occurs on a time scale of a week or less and that equilibrium between solution and surface phases is established. In situ retardation factors for radium, thorium, and lead may therefore be calculated directly from the isotopic data; values range from 4,500 to 200,000. Neither sorption time scales nor retardation factors are strongly dependent on the nuclide or on hydrogeology of the aquifer. Since our study includes nuclides with diverse chemical properties, we suggest that other uncomplexed heavy metals and transuranic elements will also behave in a manner similar to those measured here. The approach presented here should therefore find application in developing site-specific models of the transport of radioactive or stable elemental waste through water-saturated media.