Leaching of uranyl–silica complexes from the host metapelite rock favoring high radon activity of subsoil air: case of Castañar cave (Spain)
Castañar cave is a subterranean site with an outstanding natural environmental radioactivity. The maintaining of high radon activity of cave air and the detection of spatially anomalies of this gas in some cave emplacements, suggests that some natural geochemical processes are involved on the mobilization of radioactivity sources to cave environment, other than a simple exhalation of radon from the host-rock. The host rocks are interlaid dolostone–metapelite beds with radioactive nuclides of the three actinium, thorium and uranium decay series. In situ measurements on the spatial distribution of radioactivity, uranyl group’s luminescence and radon gas concentrations inside cave were main focus of this work to model lixiviation and deposition mechanisms of radioactive elements from the host rock to the karstic system. In addition, collected micro-samples were also analyzed by a multi-approach suite of analytical techniques: inductively coupled plasma mass spectrometry, environmental scanning electron microscopy with an attached X-ray energy dispersive system and spectral cathodoluminescence detector, thermoluminescence, Raman spectroscopy, X-ray diffraction, differential-thermal and thermogravimetry analysis, Alpha-spectrometry and Gamma-spectrometry techniques. The host metapelitic beds contain Zr(Hf)–Th(U)–Ti–P–REE phases such as zircon, xenotime-(Y), monazite-(Ce, La) and poly-metallic mineralization veins of hydrothermal origin. Carbonated host beds and speleothems show frequently chemical elements leaked from the upper host rock masses. The weathering leakage processes are favored by the existence of pyrite and limonite in the dolostone masses. The cave exhibits under UV lamps abundant hydrous silica–uranyl coatings covering carbonated speleothems with radionuclides of 238U natural decay series. The long-lived radio-nuclides of the radium radioactive decay chain are responsible of the continuous regeneration of radon gas inside cave. The experimental work was focused to identify origin and remobilization processes of radio-nuclides and their latte settlement into the cave environment associated to mineral phases of speleothems and cave deposits.
KeywordsUranyl–silica Luminescence Radionuclide Uranium Radon Leaching Speleothem Cave
This research was supported by the Spanish Ministry of Science and Innovation; project CGL2010-17108/BTE. M.A-G is supported by a MEC FPI-Predoctoral. A. F.-C. was funded by a postdoctoral fellowship the JAE-Doc Program (CSIC). S.C. benefits of a postdoctoral fellowship from the Spanish Ministry of Science and Innovation, research program Juan de la Cierva. We are sincerely grateful for all the help given by the cave guide Ana Blazquez and the cave managers from the Regional Government of Extremadura (Spain) for their valuable collaboration throughout the entire investigation.
- 17.Halk J, Csige I, Hunyadi I, Várhegyi A, Géczy G (1997) Radon transport in fractured porous media—experimental study in caves. Environ Int 22:S433–S437Google Scholar
- 18.Lario J, Sanchez-Moral S, Cuezva S, Taborda M, Soler V (2006) High 222Rn levels in a show cave (Castañar de Íbor, Spain): proposal and application on management measures to minimize the effects on guides and visitors. Atmos Environ 40:7395–7400Google Scholar
- 23.Fernandez-Cortes A, Cuezva S, Garcia-Anton E, Alvarez-Gallego M, Canaveras JC, Porca E, Jurado V, Saiz-Jimenez C, Sanchez-Moral S (2012) Assessment of aerodynamic processes on subsurface in karst terrains by multi-parametric surveys (case of Castañar cave). Geophys Res Abstr. 14:EGU2012-861-2Google Scholar
- 31.Alonso-Zarza AM, Martín-Pérez A, Gil-Peña I, Martínez-Flores EY, Muñoz-Barco P (2005) Formación de dolomita y huntita en depósitos de moonmilk en la Cueva de Castañar de Ibor (Cáceres). Geogaceta 38:247–250Google Scholar
- 37.Oliveira JM, Carvalho FP (2006) Sequential extraction procedure for determination of uranium, thorium, radium, lead and polonium radionuclides by alpha spectrometry in environmental samples. Czech J Phys 56:545–555Google Scholar
- 39.Sanchez-Moral S, Cuezva S, Lario J, Taborda-Duarte M (2006) In: Duran JJ, Andreo B, Carretero J (eds) Karst, cambio climático y aguas subterráneas, Hidrogeología y Aguas Subterráneas n° 18. IGME, Madrid, pp 339–347Google Scholar
- 40.Sanchez-Moral S, Fernandez-Cortes A, Cuezva S, Garcia-Anton E, Benavente D (2012) Balance of pCO2 in the carbonate-bearing unsaturated zone controlling mineral precipitation: implications for geo-environmental analyses in caves. Geophys. Res. Abstr. 14: EGU2012-874-1Google Scholar
- 45.Chalcraft D, Pye K (1984) Humid tropical weathering of quartzite in southeastern Venezuela. Z Geomorphol 28:321–332Google Scholar
- 51.Stohl FV, Smith DK (1981) The crystal chemistry of the uranyl silicate minerals. Am Mineral 66:610–625Google Scholar