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

Abstract

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.