Influence of the ventilation of a karst show cave on the surrounding vadose CO 2 reservoir (Nerja, South Spain)

This study continues previous work done in a karst area—the Nerja Cave research site to characterize the vadose CO 2 reservoir. The Nerja Cave is an important show cave, receiving nearly 500,000 visitors per year. The results of a new monitoring point in the vadose zone are presented: the Pintada Cave man-made shaft. An approximate, semi-quantitative balance of CO 2 inside the Nerja Cave has also been carried out. Finally, data from the network of vadose research boreholes near the cavity have been analyzed. This study suggests that vadose CO 2 is exhaled through the cave into the outside atmosphere during the “winter” period (from October to June), when a relatively strong convective ventilation regime affects the cavity. The net gas flux into the Nerja Cave during this period averages 24 mmol m −2 day −1 . In summer, the cavity ventilation is practically inhibited. The CO 2 concentration measured in boreholes in the relatively less karstified vadose environment around the cavity confirms this, showing a significant increase in the average gas contents: from 15,000 ppmv in “winter” to 27,000 ppmv in summer. The preliminary data in the Pintada Cave shaft show the occurrence of short-term increases of CO 2 levels from the vadose reservoir, although masked by the effect of the significant ventilation affecting this new monitoring point due to its nature: a large, open conduit seemingly connected with the Nerja Cave.