Cristina Liñán

Carbon dioxide concentration in air within the Nerja Cave (Malaga, Andalusia, Spain)

INTRODUCTION In caves adapted for tourism, the control of carbon dioxide levels is crucial for the cave’s conservation as well as for public health, so that an adequate air quality is maintained for the visitors. On one hand, the CO2 concentration in the air in karstic caves conditions the development of different speleogenetic processes within these caves given that it directly influences the precipitation/dissolution of carbonates (White, 1988, 1997; Dreybrodt, 2000; Dreybodt & Eisenlohr, 2000). On the other hand, the CO2 concentration determines the cave’s air quality, as elevated CO2 concentrations above 5000 ppmv are noxious to human health (Halbert, 1982). Numerous authors have studied CO2 from diverse standpoints. Pioneering studies have been made on the presence and dynamics of CO2 in caves with respect to the exterior environment, for example Renault (1968), Ek (1968, 1979, 1981), James (1977) and Troester & White (1984). Other authors

Isotopic (13C) Signature of CO2 Sources in the Vadose Zone of a Mediterranean Karst (Nerja Cave Site, Southern Spain)

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 South Spain). CO2 concentrations in depth were registered in boreholes drilled in experimental site. The CO2 content generally increases with depth. Measurements indicate average vadose air CO2 concentrations of nearly 40,000 ppm, with a maximum of nearly 60,000 ppm. In this context, the cave itself appears to be a vadose subsystem above the groundwater level, with significantly lower CO2 concentrations (a few thousands of ppm maximum) due to its ventilation. The δ13C–CO2 data of the vadose air point to an origin of the gas mainly related to microbiological processes associated to the consumption of dissolved organic matter in the groundwater surface. 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.

Role of an extension of pre-Quaternary age for the evolution of the carbonate massifs in the occidental Betic Cordillera: The case of the Yunquera-Nieves massif (southern Spain)

A simultaneous analysis of the fracture geometry and paleo-stress fields of the karstic Yunquera-Nieves massif i n southern Spain (Malaga Province) has been carried out with microtectonic stations. It reveals polyphased t ec -tonics linked to the structural position of this carbonate domain in the western Betic Cordillera. Among the tectonic regimes described in this domain appears a distensive stage with a radial trend probably of post-Tortonian to Quaternary age. Todate, it has seldom been described and is absent from geodynamic models though it seems to have had a regional importance. Furthermore, it played a major role for the acquisition of the hydrodynamic properties of t he aquifer and its karstogenesis. This stage opened all fractures and allowed t he development of karstic drains with NW-SE and N-S directions. Finally, the karstic network was shaped by more recent climatic and tectonic events.

A Field Analog of CO2-Closed Conditions in a Karstified Carbonate Aquifer (Nerja Cave Experimental Site, South Spain)

We present new data that illustrate the hydrochemical evolution of groundwater along a flow line in the Triassic marbles around the Nerja Cave, South Spain. Water dissolves calcite and dolomite, and then CaSO4. The environment is locally rich in CO2 (up to near 60,000 ppmv) and consequently the water increases significantly its content in Ca2+, Mg2+, HCO3 − and SO4 2− along the flow, with EC values between 500 and 900 μS/cm. The pH values are typically in the 7–8 range, and the equilibrium PCO2 of the water varies between 10−1.5 and 10−2.5 atm. In the considered flow line there is a relatively deep borehole (S2: 380 m; 280 m saturated) that shows pH values around 10 and equilibrium PCO2 of 10−6 atm, with EC values generally in the 150–200 μS/cm range. Most of its solutes derive from rainwater concentration, together with the dissolution of carbonate minerals in a system closed to CO2. For this reason we consider S2 to be a field analog of such conditions. The nearly stagnant water of this well also shows evidence of sulphate reduction. Unlike its solute contents, isotopically (δ2H and δ18O) the water of S2 does not show any modification with respect to the other points along the flow line.

Identification of Vadose Karst Voids and Ventilation Patterns Coupling Hydrochemical and Geophysical Methods (Maro Spring, Near Nerja, Southern Spain)

The Maro Spring, near Nerja, southern Spain, drains a Triassic carbonate aquifer. It presents a typical karstic behaviour. We have compiled more than 220 pH values of water from this spring and a nearby well. The spatial variation of this variable suggests a CO2 degassing effect as far as the groundwater flow gets near the spring. The average pH seasonal variation indicates a maximum during autumn and a minimum in spring. This can be explained by water degassing related to the start of the period of convective ventilation in vadose voids, as happens in the important Nerja Cave located nearby. There also seems to be an effect of downward gas diffusion when the soil respiration is at its peak and the ventilation of the vadose voids is nearly inhibited. These circumstances suggest that Maro Spring water can be spatially linked with unknown cavities. The second phase of this study is the identification of some of these voids by the way of combination of two gravity profiles of 200 m, one designed as a test of the method and the other for prospecting the voids. The link between hydrochemical and geophysical methods is the main aim of this study as such a transdisciplinary approach is not frequently used in hydrogeology.