Modelling of hydrodynamic behaviour in the Bossea aquifer system through natural radon concentrations: results and applications

Bossea Cave (Western Ligurian Alps) is affected by the presence of natural radon, originating from the radioactive decay of uranium and thorium isotopes contained in the Permian meta-volcanic basement of the karst aquifer. The non-chemically reactive Rn-222 concentrations have been investigated since 2003 by Bossea Underground Karst Laboratory: the diffusive and advective processes allow the reconstruction of the paths related to the inflowing water from the surface into the fractured network. Hence, radon fluctuations are cross-correlated with hydrodynamics: the transfer of gas from underground water flows is lagged behind the discharge peaks. The 45 hour delay in the upper meta-carbonate canyon and the 66 hour delay in the collapse chambers (located at the contact with the meta-volcanics) exclude the influence of external meteorological factors and confirm the double convective cell model describing the hypogeal atmospheric circulation. Different radon concentrations in water can also be detected between the main creek (Torrente Mora) and a secondary inflow (Polla delle Anatre). Although both are subjected to a piston flow hydrodynamic response, the former shows an increase in Rn during floods (with a lag of 46 hours), due to the mobilization of long-resident water volumes in karst fractures. In the latter case, there is a dilution of water in contact with radioactive elements of the impermeable basement (delayed by 6 hours and caused by volumes of neo-infiltration). In agreement with these results, radon proves to be an innovative tool to study the hydrodynamics of a karst aquifer and suggests a wide range of further cross-disciplinary applications, such as in the field of seismology.