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• A new advective transport model is used to assess 224Ra/228Ra and 222Rn/226Ra ARs. • Alpha recoil supply and Ra retardation factor control ARs in groundwater. • ARs are used to estimate groundwater transit times in aquifers. • ARs are useful for evaluating discharge pathways and end-members of SGD. 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. [ABSTRACT FROM AUTHOR]