Abstract

This study is based on in situ measurements of the soil and the vadose zone (<60 m) in a Mediterranean karst experimental site near Nerja Cave (a show cave in dolomite marbles in southern Spain). Air temperature, relative humidity, and CO2 concentrations are the main variables measured, especially their variations with depth in a number of boreholes. The CO2 content generally increases with depth. Our measurements indicate average vadose air CO2 concentrations of nearly 40,000 μL L−1, with a maximum of nearly 60,000 μL L−1. In this context, the cave itself appears to be a vadose subsystem above the groundwater level, with significantly lower CO2 concentrations (a few thousand microliters per liter maximum) due to its ventilation. The vadose air in the lower part of the boreholes also exhibits near-saturation humidity and a quite stable temperature around 21°C, similar to the conditions inside the cave. The measured vadose conditions were simulated by a reaction-path hydrogeochemical model that starts with the local rainwater composition and reproduces the chemistry of the cave drip water, particularly its high Mg2+ content. The soil cover, although very scarce, has a relatively high organic matter content. The δ13C-CO2 data of the vadose air point to an origin of the gas mainly related to biological soil processes. This gas can diffuse or flow laterally, upward, or downward through karst conduits. Interactions between air masses of surface origin (relatively dry, with variable temperature and low CO2 content) and typical vadose attributes (relatively high CO2 content, near-saturated humidity and 21°C temperature) produce clear ascendant or descendant air fluxes inside the boreholes, especially those that cross significant karst voids.

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