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Pacific Coastal and Marine Science Center

Submarine Groundwater Discharge

Uranium-Thorium Series Geochemical Tracers


There are four naturally occurring isotopes of Ra: 226Ra (t1/2 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.

Diagram of Uranium-Thorium series decay chain: Radium.
Figure A. Uranium-Thorium Series decay chain for Radium. To view a larger image, click on the image above.
Distance from shoreline versus AR
Figure B. Distance from shoreline (m) versus ex 224Ra/223Ra activity ratios (AR) in two groundwater and five surface water samples.
Distance from shoreline versus AR
Figure C. Distance from shoreline (m) versus apparent water mass ages. Average observed velocity (0.06 >cm s-1) is bracketed by 0.01 cm s-1 and 0.1 cm s-1 lines.


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