222Rn and CO2 soil–gas geochemical characterization of thermally altered clays at

The physical properties of clay allow argillaceous formations to be considered geological barriers to
radionuclide migration in high-level radioactive-waste isolation systems. As laboratory simulations are
short term and numerical models always involve assumptions and simplifications of the natural system,
natural analogues are extremely attractive surrogates for the study of long-term isolation. The clays of
the Orciatico area (Tuscany, Central Italy), which were thermally altered via the intrusion of an alkali-trachyte
laccolith, represent an interesting natural model of a heat source which acted on argillaceous materials.
The study of this natural analogue was performed through detailed geoelectrical and soil–gas
surveys to define both the geometry of the intrusive body and the gas permeability of a clay unit characterized
by different degrees of thermal alteration. The results of this study show that gas permeability
is increased in the clay sequences subjected to greater heat input from the emplacement of the Orciatico
intrusion, despite the lack of apparent mineral and geotechnical variations. These results, which take into
consideration long time periods in a natural, large-scale geological system, may have important implications
for the long-term safety of underground storage of nuclear waste in clay formations.