Redox and Recharge Controls on Sub-Surface Radium-Isotope Mobility.

Groundwater geochemical systems are known to be highly complex, driven by a wide variety of physical, chemical and biological mechanisms. A north-south transect of wells in the Sheehan well-field (Memphis, TN) that border an aquitard window were used to sample the confined Memphis Aquifer. Short-lived (223Ra and 224Ra) and long-lived isotopes (226Ra and 228Ra) of radium were examined in the colloidal and dissolved groundwater phases to constrain the geochemical mechanisms affecting the Memphis Aquifer. Measured isotope activities increase from the periphery to the center of the measured transect by 0.5 to 1.0 orders of magnitude for all Ra-isotopes. Wells east of the transect show an additional increase of 0.3 orders of magnitude for short-lived Ra-isotopes. Additionally, 223Ra/226Ra and 224Ra/226Ra activity- ratios increased as a function of distance from the local aquitard window. The results suggest 1) low Ra-isotope activities close to the aquitard window by association with the solid-phase, and 2) the increase of short-lived over long-lived Ra-isotope activities further away from the aquitard window due to alpha-recoil.