Radium quartet (226Ra, 228Ra, 224Ra and 223Ra) is largely used in coastal karstic systems as tracer of fresh- and sea-water interactions. Ra isotopes have also been applied to the study of waterrock interactions, residence times and mixing of different waters in geothermal systems or groundwaters. The aims of this work were i) to study the relationship between the (228Ra/226Ra) activity ratios and the lithologies through which the water is flowing (especially their Th/U ratios) in the Vidourle River, because it drains terranes from the crystalline basement of the Cévennes to the Mesozoic marl and calcareous series with sinkholes resurgences and karstic tributaries. ii) to monitor the time variation of Ra quartet in karstic springs emerging through the Mesozoic limestones near the Lez spring, and to decipher how the activities of these isotopes are linked to mixing of different waters, and to the hydrodynamic conditions. Ra quartet was measured simultaneously by gamma spectrometry, using a new system for in situ sampling of large amounts of water with low Ra activities (typically 1 to 4 mBq/L). A general decrease of the (228Ra 226Ra) ratios (from 1.4 to 0.9) and of the (226Ra) activities along the Vidourle River is in agreement with the transition from U -rich rocks of the crystalline basement with high Th U ratios, to mainly calcareous Mesozoic rocks with lower U contents and low Th/U ratios. The four karstic springs of the Lez area display lower (228Ra/226Ra) ratios (0.5 to 0.7), but (226Ra) activities similar to those found in the Vidourle. These lower ratios are probably related to the low Th U ratios of the Upper-Jurassic Lower Cretaceous limestones through which the karst system has developed. Significant temporal variations of this ratio at the Lez spring show the participation of different water masses according to the hydrodynamics. Short-lived Ra isotopes are particularly influenced by water-rock interactions (with alpha-recoil processes) in the epikarstic zone.