New perspectives on the use of Ra-224/Ra-228 and Rn-222/Ra-226 activity ratios in groundwater studies

The naturally occurring Ra isotopes (²²³Ra; T_1/2 = 11.4 d, ²²⁴Ra; T_1/2 = 3.66 d, ²²⁶Ra; T_1/2 = 1,600 y, and ²²⁸Ra; T_1/2 = 5.7 y) and Rn (²²²Rn; T_1/2 = 3.82 d) have been widely applied as environmental tracers. The application of these radioactive tracers has mainly been restricted to the evaluation of oceanographic and land-ocean interaction processes, although in recent years their use has also been extended to the study of groundwater systems. In this context, the activity ratios of ²²⁴Ra/²²⁸Ra and ²²²Rn/²²⁶Ra can be instrumental in providing key information on groundwater transit times in aquifers and those processes governing groundwater discharge into the coastal sea (often referred to as Submarine Groundwater Discharge or SGD). This work evaluates the potential use of these activity ratios as proxies for investigating groundwater systems through an advective transport model that integrates the radionuclides involved in these activity ratios (²²⁴Ra, ²²⁸Ra, ²²⁶Ra, and ²²²Rn) and their immediate parents into a single formulation. The results provided by the transport model indicate that the main factors controlling the ²²⁴Ra/²²⁸Ra and ²²²Rn/²²⁶Ra activity ratios are the alpha recoil supply, the retardation factor of Ra, and the groundwater transit times. The advective transport model and the activity ratios are used to present novel applications that interrelate the disciplines of hydrogeology and coastal oceanography. The main applications include the determination of groundwater transit times and the assessment of pathways and end-members related to submarine groundwater discharge processes. These applications were tested in a Mediterranean coastal aquifer.