Behavior of Radium isotopes in a karstic spring (Lez) and a Mediterranean stream (Vidourle) in the South of France

International audienceRadium quartet (226Ra, 228Ra, 224Ra and 223Ra) is largely used in coastal karstic systems astracer 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 orgroundwaters. 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 theVidourle River, because it drains terranes from the crystalline basement of the Cévennes to theMesozoic marl and calcareous series with sinkholes resurgences and karstic tributaries. ii) to monitorthe time variation of Ra quartet in karstic springs emerging through the Mesozoic limestones near theLez 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 gammaspectrometry, 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 thecrystalline basement with high Th U ratios, to mainly calcareous Mesozoic rocks with lower U contentsand low Th/U ratios. The four karstic springs of the Lez area display lower (228Ra/226Ra) ratios (0.5to 0.7), but (226Ra) activities similar to those found in the Vidourle. These lower ratios are probablyrelated to the low Th U ratios of the Upper-Jurassic Lower Cretaceous limestones through which thekarst system has developed. Significant temporal variations of this ratio at the Lez spring show theparticipation of different water masses according to the hydrodynamics. Short-lived Ra isotopes areparticularly influenced by water-rock interactions (with alpha-recoil processes) in the epikarstic zone