Uranium-Thorium Series Geochemical Tracers

Radium

There are four naturally occurring isotopes of Ra: ²²⁶Ra (t_1/2 1600 years) present in the ²³⁸U series, ²²⁸Ra (5.75 years) and ²²⁴Ra (3.66 days) in the ²³²Th series, and ²²³Ra (11.4 days) in the ²³⁵U series. The differences in half-lives and unique parent-daughter relationships across different decay series have been utilized to study a variety of groundwater processes and water-rock interactions. For the two short-lived Ra isotopes, the principal input term to groundwater is recoil, rather than weathering processes. Steady-state activities of ²²³,²²⁴Ra are thus often achieved in groundwater. Under low-salinity conditions, Ra occurs as Ra²⁺, whereas sulfate, carbonate, and Cl- complexes will occur in saline groundwater with high respective ligand concentrations. Organic Ra complexes are generally not considered to be significant in fresh groundwater, yet colloid transport via clays and iron oxides may influence subsurface Ra transport. The solubility limit of Ra is generally not reached in fresh groundwater, but Ra can be precipitated in solid solution within Ca and Ba minerals.

Adsorption onto aquifer solids exerts a strong control on the behavior of Ra in coastal groundwater, and adsorption rate constants are dependent on substrate type and the chemical composition of the groundwater. Decreases in adsorption efficiency have been observed under conditions of increasing salinity due to such processes as cation competition and displacement, increases in mineral surface charge and increases in the stability of inorganic complexes. In a coastal aquifer, fluctuating groundwater salinities at the freshwater-saltwater interface may cause Ra to adsorb onto aquifer surfaces, thereby becoming a localized source of ²²²Rn.

Read more in USGS Fact Sheet 65-99: Examining Freshwater-Saltwater Interface Processes with Four Radium Isotopes.