Apport du radon et des isotopes du radium à la caractérisation des circulations souterraines en domaine karstique : application à l'aquifère du Lez (Hérault, France)

Abstract : Understanding recharge processes and hydrodynamics of karstic systems is an essential rule to assess their vulnerability and their groundwater resource availability. Numerous studies use the behavior of natural geochemical elements and isotopes in groundwaters to improve this understanding. The radium quartet (223Ra, 224Ra, 226Ra and 228Ra) and the radon (222Rn), natural radioactive elements, have been commonly applied for studies devoted to the identification of water mass origins and to their mixing and transfer processes in different type of aquifers. Nevertheless, the applications to karstic systems are mainly concerned to coastal and thermal systems, because of their uncommon Ra enrichment. Just a few works have been undertaken in continental karstic aquifers, probably because of these radionuclide low activities in such environments. This statement makes their quantification not easy and requires large water sampling. The first part of our research work has been devoted to the development of a new sampling system able to pre-concentrate in situ the radium present in several tens of liters of water (20-300 L) with MnO2-fibers and to improve the determination of Ra quartet activities by means of gamma spectrometry. This method gave us the ability of measuring, for the first time, the very low activities of the four radium isotopes (0.4 to < 7.0 mBq/L for 226Ra, 228Ra et 224Ra and from 0.05 to 0.3 mBq/L for 223Ra) in the waters of two Mediterranean karstic hydrosystems, the Haut Vidourle and the Lez systems (both located in South of France). The radon measuring method has also been optimized and improved. The study of the Ra isotope and Rn behavior over the survey area put in evidence the multiple applications of measuring these radionuclides in karstic aquifers. 226Ra activities and (228Ra/ 226Ra) ratios in water are in agreement with the existing lithologies into which the waters flow or are stored. The effective participation of Vidourle river water to the allogenic recharge of the Sauve aquifer has been specified. We showed that sometimes, the Ra isotope activities correspond to characteristic values of the aquifer compartments instead of the bedrock. These data also allow us to estimate the mixing proportions of the three geochemical endmembers composing the waters flowing at the Lez karstic spring in relation with hydrodynamic conditions. Coupling the long-lived radium isotopes (226Ra and 228Ra) together with the short life isotopes (223Ra and 224Ra) potentially led us, at the same time, to determine the mixing rates and the transfer time of the groundwaters. Moreover, the short life isotope excess would express groundwater venues at surface, 223Ra and 224Ra becoming more reliable tracers of superficial exchanges than radon because insensitive to degassing processes towards the atmosphere. On the other end, the study of the radon time evolution at the Lez spring confirmed that this natural radioactive gas is a good tracer of the diffuse recharge processes of karstic aquifers. The relations between 223Ra and 222Rn as well as between 210Pb and 222Rn, could give access to the transfer time of groundwater. However, more exhaustive researches are needed.
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Arnold Molina Porras. Apport du radon et des isotopes du radium à la caractérisation des circulations souterraines en domaine karstique : application à l'aquifère du Lez (Hérault, France). Hydrologie. Université Montpellier, 2017. Français. ⟨NNT : 2017MONTT112⟩. ⟨tel-01737009⟩

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