Radium isotopes, radon and 210Pb in karstic waters: Example of the Lez system (South of France)
Introduction
The use of radium isotopes (226Ra, 228Ra, 224Ra and 223Ra) and radon (222Rn) in water as tracers of geochemical and hydrological processes is well known. Their behavior in aquatic systems and most applications have been reviewed in recent publications (Porcelli, 2011, Porcelli and Swarzenski, 2003). For example, the long-lived Ra isotopes, 226Ra (238U-series, t1/2 = 1600 y) and 228Ra (232Th-series, t1/2 = 5.75 y), can give useful information on the water origin, mixing proportions of water masses, and rock/sediment interactions. These processes have been studied in thermal waters (e.g. Sturchio et al., 1993), as well as in fresh groundwaters with contrasted 226Ra activities or/and (228Ra/226Ra) isotopic ratios (e.g. Vinson et al., 2012). Meanwhile, the short-lived Ra isotopes, 224Ra (Th-series, t1/2 = 3.6 d) and 223Ra (235U–series, t1/2 = 11.43 d), as well as 222Rn (238U–series, t1/2 = 3.82 d), are more related to dynamic processes like water residence or transfer times, even though they are also largely applied in rock/sediment-water interaction studies. One of the main applications of Ra isotopes and Rn concerns their use as tracers of submarine groundwater discharge (SGD) and related processes in coastal environments (e.g. Rama and Moore, 1996, Rodellas et al., 2017). Additionally, the solubility and mobility of radon in water, and the contrasted activities between surface- and ground-waters, make radon a suitable tracer for studies related to groundwater flow paths and ground- and surface-water interactions (e.g. Hoehn and Von Gunten, 1989, Khadka et al., 2017).
However, the behavior of Ra isotopes in continental carbonate aquifers and in karst systems has been less studied (Guerrero et al., 2016). This is partly due to the much lower Ra activities compared to those of thermal waters, brines, and even other groundwaters interacting with rocks having higher U and Th contents than limestone (Vinson et al., 2012). There are also relatively few studies of Rn in karst systems, although its potential to infer the hydrodynamics in karst has been demonstrated by the pioneering works of Surbeck and Medici (1991), Monnin et al. (1994), Pane (1995), Eisenlohr and Surbeck (1995), and more recently by Savoy and Surbeck (2003); Savoy et al. (2011).
The aim of the present work was to test the possible use of Ra isotopes and Rn as geochemical and hydrodynamic tracers, applied in the well-known Lez karst system (South of France), that has been continuously monitored for several decades. We have also analyzed U-series in representative samples of rocks, karst-filling alterites and soils of the Lez watershed.
Section snippets
Geological and hydrogeological context
The Lez karst system or Lez aquifer (Fig. 1) is located 15 km north of the city of Montpellier (Hérault, France). With a watershed surface of 380 km2 (Thiéry and Bérard, 1983), it consists of one main perennial outlet, the Lez spring, and several seasonal springs like Lirou, Restinclières, Fleurette and Lauret (Fig. 1a). This karstic system has developed in the 650 to 1100 m-thick massive limestones and marly-limestones of the Argovian and Kimmeridgian (Upper-Jurassic), and of the Berriasian
Sampling and analytical methods
The Lez spring and the Lirou, Restinclières and Fleurette seasonal springs have been constantly monitored since 2006 and 2008, respectively, as part of a monitoring program supported by the Multiscale observatory of flood dynamics and hydrodynamics in karst (MEDYCYSS-OSU OREME, www.medycyss.org). This monitoring program includes hydrological parameters (piezometric level and flow rates) for the Lez and Lirou springs, as well as physico-chemical parameters (electrical conductivity (EC),
Chemical composition of waters
Physicochemical parameters and water compositions in major and some selected trace elements of samples that were also analyzed for Ra isotopes, are reported in Table 1. In order to take into account both the mineralization and the hydrological conditions of the Lez spring (LZ), as described in Section 2, water samples in Table 1 were classified, as suggested by Caetano Bicalho et al. (2012), in five general types of water: i) “diluted waters” (DW): with EC < 700 μS/cm; ii) “high-waters” (HW): with
Conclusions
New data were obtained on the behavior of Ra isotopes, 222Rn and 210Pb in waters of one of the main karst systems of the South of France, the Lez aquifer.
In spite of its low activities in the Lez spring waters, 226Ra has a very coherent behavior and is well correlated to other stable major and trace elements, especially chloride. The (228Ra/226Ra) activity ratios are not systematically related to the Th/U ratios of the reservoir rocks, as shown by the waters draining the Jurassic karst. We
Acknowledgements
This work was supported by the Institutional Program for Scholarships Abroad for University Officials of the Office of International Affairs and External Cooperation (OAICE) of the University of Costa Rica (Grant number OAICE-02-CAB-045-2014) and the Institut français d'Amérique centrale (IFAC) of the Ministère français des affaires étrangères et du développement international (MAE-DI, grant number 789478C). We thank Sandra Van Exter for her help with major element analyses. We are grateful to
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