Influence of the ventilation of a karst show cave on the surrounding vadose CO2 reservoir (Nerja, South Spain)

Abstract

This study continues previous work done in a karst area—the Nerja Cave research site to characterize the vadose CO2 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 CO2 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 CO2 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 CO2 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 CO2 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.