Air Carbon Dioxide Contents in the Vadose Zone of a Mediterranean Karst

Abstract

AU›o®ƒa®EAÝ : OM, organic matter; PVC, polyvinyl chloride. OU®¦®Aƒ½ R›Ý›ƒU‘« 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, relaƟ ve humidity, and CO 2 concentraƟ ons are the main variables measured, especially their variaƟ ons with depth in a number of boreholes. The CO 2 content generally increases with depth. Our measurements indicate average vadose air CO 2 concentraƟ ons 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 signifi cantly lower CO 2 concentraƟ ons (a few thousand microliters per liter maximum) due to its venƟ laƟ on. The vadose air in the lower part of the boreholes also exhibits near-saturaƟ on humidity and a quite stable temperature around 21°C, similar to the condiƟ ons inside the cave. The measured vadose condiƟ ons were simulated by a reacƟ on-path hydrogeochemical model that starts with the local rainwater composiƟ on and reproduces the chemistry of the cave drip water, parƟ cularly its high Mg 2+ content. The soil cover, although very scarce, has a relaƟ vely high organic maƩ er content. The δ 13 C-CO 2 data of the vadose air point to an origin of the gas mainly related to biological soil processes. This gas can diff use or fl ow laterally, upward, or downward through karst conduits. InteracƟ ons between air masses of surface origin (relaƟ vely dry, with variable temperature and low CO 2 content) and typical vadose aƩ ributes (relaƟ vely high CO 2 content, near-saturated humidity and 21°C temperature) produce clear ascendant or descendant air fl uxes inside the boreholes, especially those that cross signifi cant karst voids.