Seasonal Dynamics of Gaseous CO[sub.2] Concentrations in a Karst Cave Correspond with Aqueous Concentrations in a Stagnant Water Column

Dissolved CO[sub.2] in karst water is the key driving force of karstification. Replenishment of CO[sub.2] concentrations in karst water occurs by meteoric water that percolates through the vadose zone, where CO[sub.2] produced from microbial activity is dissolved. CO[sub.2] can thus be transported with the percolating water or in the gas phase due to ventilation in karst systems. We measured seasonally fluctuating CO[sub.2] concentrations in the air of a karst cave and their influence on aqueous CO[sub.2] concentrations in different depths of a stagnant water column. The observed data were compared to numerical simulations. The data give evidence that density-driven enhanced dissolution of gaseous CO[sub.2] at the karst water table is the driving force for a fast increase of aqueous CO[sub.2] during periods of high gaseous concentrations in the cave, whereas during periods of lower gaseous concentrations, the decline of aqueous CO[sub.2] is limited to shallow water depths in the order of 1 m. This is significant because density-driven CO[sub.2] dissolution has not been previously considered relevant for karst hydrology in the literature. Attempts at reproducing the measured aqueous CO[sub.2] concentrations with numerical modeling revealed challenges related to computational demands, discretization, and the high sensitivity of the processes to tiny density gradients.