A Porous Flow Model for Steady State Transport of Radium in Groundwater

The quasi‐steady variation of uranium and radium isotopes from the decay of ²³⁸U, ²³⁵U, and ²³²Th has been determined along an idealized, one‐dimensional aquifer which is a distributed source of activity and from which the transfer of radionuclides to solution occurs by alpha recoil and chemical exchange. The model includes the effects of dispersive flow and retardation by adsorption. The importance of these effects on the variation of U and Ra activity ratios along the model aquifer is shown. Predicted limiting activity ratios (for large flow times) of ²²⁶Ra/²²³Ra, ²²⁴Ra/²²⁸Ra, and ²²⁶Ra/²²⁸Ra are 21.4, 1.5, and (∼1.7–2.0) (U/Th)_s, respectively (where (U/Th)_s is the activity ratio ²³⁸U/²³²Th in the aquifer rocks), although in many cases, an aquifer may not be long enough for the ratios involving ²²⁶Ra to achieve their limiting values.