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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Recent studies in several karstic systems in France have highlighted that Total Organic Carbon (TOC) was a relevant parameter in order to characterise the behaviour of aquifers, to differentiate the water types that participate in karstic flow (fast infiltration, unsaturated zone, saturated zone) and to evaluate their vulnerability. This study on TOC dynamics at the experimental site of the Nerja Cave has been performed to test the potential of this tracer in a particular climatic (Mediterranean climate), lithologic (dolomitic marbles) and hydrodynamic (diffuse flow behaviour) context. TOC evolution, compared to those of classical tracers used in hydrogeology, permits the understanding of the hydrodynamical behaviour of the unsaturated zone of this dolomitic aquifer, whose response to precipitation is slower than that commonly obtained in karstic aquifers of calcareous nature.

the CO 2 concentration of the air in the interior and exterior of the Nerja Cave was studied and its relation with the air temperature and visitor number. The average annual CO 2 concentration outside of the cave is 320 ppmv, whilst inside, the mean concentration increases to 525 ppmv during autumn and winter, and in the order of 750 ppmv during spring and summer. The temporal variation of CO 2 content in the air of the cave is strongly influenced by its degree of natural ventilation which is, in turn, determined by the difference between external and internal air temperatures. During autumn, winter and spring, a positive correlation between the CO 2 content of the air inside the cave and the temperature difference between the external and internal air was observed, such that when this difference increased, there was a higher level of CO 2 within the cave. Then, the ventilation is high and CO 2 levels are mainly of human origin. During summer, there was a negative correlation between CO 2 and the temperature difference between the air outside and that inside the cave: when the temperature difference increases, the CO 2 content within the cave is lower. At this time of the year, the renovation of the air is much slower due to the lower ventilation. A positive correlation between CO 2 concentration of the air in the cave and the visitor number can only be observed during August, the month that receives the most visits throughout the year averaging 100,000.

We analysed the stable isotopes ( 18 O and 2 H) of rainwater and drip water within a cave (Nerja Cave) located in the unsaturated zone of a carbonate aquifer. Rainfall is more abundant and presents lower isotopic content in winter, while the volume of drip water is greater and its isotopic content is lower in summer. The flow analysis of 18 O through the unsaturated zone confirms the seasonal lag between rainfall and the appearance of drip water in the cave and reveals that the unsaturated zone of the aquifer, in the sector of the cave, behaves like an inertial system with a strong capability to modulate the input signal.