222Rn and CO2 monitoring in soil and indoor atmosphere to understand changes in the gaseous dynamics of Rull cave (Spain)

Rull cave is a karstic cave located in Vall d’Ebo (Alicante, Southeast of Spain) developed in massive Miocene conglomerates and Cretaceous limestones. Processes in soil above Rull cave and outdoor atmosphere directly influence the underground environment of the cave. Continuous and discrete monitoring of cave atmosphere and soil (from 2012 to 2022) allows to characterise the spatial distribution and temporal variations of the gaseous concentration (222Rn and CO2) and understand the relationship between the processes which occurred in the 3-component system (soil-cave-atmosphere). Besides the presence of visitors, Rull cave maintains stable values of mean temperature (16.2 °C) and relative humidity (97.6%). In an annual cycle the cave presents two different gaseous stages (stagnation and ventilation). Maximum average values of CO2 and 222Rn concentration are reached within the stagnation stage, in the warmest period of the year. On the contrary, in the ventilation stage (in the coldest months) the cave reaches the lowest concentrations in its inner atmosphere. For the study period, daily average CO2 and 222Rn concentrations are 2008 ppm and 1745 Bq/m3, respectively. 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 from this study provide substantial information about the environmental situation of the cave atmosphere in terms of air quality for visitors and workers.