Assessing the impact of visitors on airborne particle deposition in the Giant Gypsum Geode of Pulpí (SE Spain)

The Giant Geode of Pulpí is a unique mineralogical phenomenon worldwide, remarkable for its large selenite gypsum crystals. Its recent development as a tourist site and inclusion on UNESCO’s Tentative World Heritage List, highlight the need to assess the impact of visits on its conservation. This study investigates airborne particle dynamics inside the Geode, focusing on tourist activity and natural ventilation. We measured deposition rates and composition of airborne particles using passive traps and a continuous laser-optical particle counter. Microenvironmental variables linked to ventilation, such as temperature and radon gas concentration (²²²Rn), were simultaneously monitored. Results show a predominance of fine particles (μm), which remain suspended longer and penetrate deeper into the cavity. Coarse particles (>5 μm) settle quickly, mainly near the Geode entrance. Chemically, most particles correspond to autochthonous mine minerals (celestine, siderite, quartz, and gypsum), though allochthonous materials such as non-mineral fibers introduced by visitors were also identified. Natural ventilation strongly influences particle behavior. Fine particle concentrations (μm) rise (i.e., up 30×103 particles/L) when the renewal of air with the exterior, characterized by lower suspended particle concentrations, is restricted. Under these conditions, the particle remobilization induced by visitors causes a higher accumulative effect of fine particles in the mine-Geode atmosphere. Autochthonous mining debris and dust is the main source of coarse particles, with concentrations peaking during visiting hours due to resuspension by tourist movement, up to 2,000 particles/Lfor 5-10 mm particles and up to 400 particles/L for >10 mm. These findings provide a foundation for preventive conservation strategies. Adapting visitor pathways and access protocols could reduce particle resuspension and deposition, helping preserve the exceptional crystals of the Geode of Pulpí for future generations.