Radon dynamic and air quality in the Rull cave (southeast Spain)

In poor-ventilated caves, indoor gases can seasonally reach concentrations much higher than normal atmospheric values, which may become a critical health risk. In this study, we investigate the cave atmosphere of the Rull cave in terms of assessing the cave air quality and understanding the cave dynamics. Rull cave is located in Vall d’Ebo (Alicante, southeast Spain). It is a karstic cave developed in massive Miocene conglomerates and Cretaceous limestones. Above the cave, the silty-silty loam soil profile has a thickness below 1 m. Inside the cave, calcite speleothems and cave sediments are widely present. The uranium and thorium concentrations are higher for the soil and cave sediments, and minor for host-rock, and lower for speleothems. Moreover, both soil and cave sediments are powder materials, and the emanation is enhanced in comparison to host-rock and speleothems, although their volume in the cave is higher than soil and cave sediments.

At the cave location and for a decade-long monitoring period (November 2012 - July 2022), the prevailing meteorological conditions in terms of temperature and relative humidity were 16.1 ºC and 69.9%, respectively, on daily average values. The average annual precipitation is 553 mm. In the cave interior and besides the presence of visitors (15000 visitors/year), mean temperature (16.2 °C) and relative humidity (97.6%) maintain stable values.

In an annual cycle, the cave presents two different gaseous stages (stagnation and ventilation). Maximum average values of CO₂ and ²²²Rn concentration (3966 ppm and 4185 Bq/m³) are reached within the stagnation stage (April/May to September). On the contrary, in the ventilation stage (October to March/April) the cave reaches the lowest concentrations in its inner atmosphere (478 ppm and 404 Bq/m³).

The spatial distribution of gases in the cave is dependent on the air density gradient between the cave and the outer atmosphere and controls the seasonal variations of both gases. The emanation of ²²²Rn from cave sediments, host-rock, and speleothems, contributes to increasing and maintaining a nearly ²²²Rn cave concentration during the stagnation stage. Dripping water degassing might also contribute to raising gas concentrations in caves. However, in Rull cave dripping waters are not abundant and thus the contribution of CO₂ or ²²²Rn degassing from seepage waters to increase cave gaseous concentration might be low.

The continuous monitoring of Rull cave provides substantial information about the environmental situation of the cave atmosphere in terms of air quality for visitors and cave guides. The maximum average concentrations of CO₂ and ²²²Rn in Rull cave and exposition times and doses comply with the recommendations of the legislation. Considering ²²²Rn, accurate planning of the cave guides and visitors during the stagnation stage is recommended to be less exposed to ionizing radiation due to the presence of radon gas.