Retardation of ²³⁸U and ²³²Th decay chain radionuclides in Long Island and Connecticut aquifers

Abstract

Knowledge of the ability of an aquifer to retard the groundwater transport of toxic or radioactive ions can be inferred from the analysis of groundwater for the radionuclides of the ²³⁸U- and ²³²Th-decay chains. Groundwaters of varying chemical composition were analyzed from wells in Long Island, New York, and Connecticut. Aquifer lithologies are arkose, quartz sandstone, granite, and glacial drift. Adsorption (k₁) and desorption (k₂) coefficients for Ra and Pb were calculated using ²²²Rn activity as a measure of the supply of other α-recoil nuclides. Laboratory tests of the validity of this assumption were made by measuring the flux of ²²⁴Ra and ²²²Rn from aquifer solids. The ratio k₁/k₂ is the distribution coefficient, K, which is effectively equal to R_ƒ, the retardation factor. The average value of K for Ra is 6 × 10² in Long Island aquifers and 5 × 10⁴ in Connecticut. The distribution coefficient for Pb is 10⁴ in Long Island and 10⁵ in Connecticut. Results from this and other studies reveal a strong dependence of retardation on pH, ƒ_O2, and ionic strength that tends to overwhelm any dependence of retardation on lithology.