Study of Rull Cave Dynamics to Understand the Complex Relationships Between Soil, Cave and External Atmosphere

Rull Cave is a karstic cavity located in Vall d’Ebo (Alicante, Southeast of Spain) developed in massive Miocene conglomerates and Cretaceous limestones which consists of a single room with a rounded shape. It is a tourist cave with an average of 15,091 visitors per year. The environmental conditions in the cave’s interior and exterior, as well as the physical properties above the cave, were measured from 2012 to 2021 to understand the gaseous dynamics and their relationships in this three-component system (soil-cave-atmosphere). Besides the presence of visitors, Rull Cave maintains stable values of mean temperature (16.1 °C) and relative humidity (97.6%). The cave presents two different gaseous stages in an annual cycle. In the study period, maximum average values of CO2 and 222Rn were reached in the hottest months (3966 ppm of CO2 and 4185 Bq/m3, respectively), while coldest months are related to the lowest gaseous concentrations (478 ppm of CO2 and 368 Bq/m3, respectively). Discrete sampling of δ13C of CO2 in cave, outdoor atmosphere and soil allows to characterize the spatial distribution and temporal variations of the gaseous concentration in background atmosphere, cave and soil air as well as the relationship between them. Results show that the dynamics of 222Rn and CO2 in the cave air follow different patterns defined by the complex relationships between external and internal factors. Findings permit lately to preserve the quality of the cave indoors and to understand the possible risk derived from the presence of indoors hazardous substances.