Ventilation effects in a karstic show cave and in its vadose environment, Nerja, Southern Spain

Abstract

This study deals with the process of CO2 exchange between karst systems and the atmosphere, which is an important issue in the global carbon cycle and in climate change estimations. The study is focused on CO2 measurements in the Nerja Cave (south Spain) and in a number of research boreholes located nearby during the 2006–2008 period. Nerja is an important show cave, with some 500,000 visitors per year. Tourists are only allowed to visit the part of the cavity nearest to the entrance. In 2006, monitoring of environmental variables began in the area closed to visits. Some anthropogenically induced peaks in the CO2 content inside the cavity were used as a tracer to asses the ventilation patterns, which are mainly convection-driven, as in many other Mediterranean show caves. Air circulation is especially active during winter when the inflow of external precedence is important. The record from boreholes allowed identification of CO2 concentrations of some tens of thousands of ppm in the vadose zone (<60 m). The highest CO2 contents are shallower in summer and deeper in winter. This can be explained both by gas dissolution by downward percolation water in winter and by the increase of upward gas diffusion in summer. The overall influence of the external atmosphere, by way of the cave ventilation is presumed to mask incoming flows from the CO2-rich vadose environment.