A Conceptual Model of the Inorganic Carbon Transport Within a Karst Massif

Abstract

The overall conceptual model of the inorganic carbon transport within a karst massif starts from decomposition of organic matter in the soil and epikarst which produces carbon dioxide. The biological activity gives rise to a gas phase in the porous soil with high CO2 partial pressure, up to several tens of thousands ppm. Most of the carbon dioxide is released to the atmosphere, but a fraction is dissolved in the percolating water. When water enriched with soil CO2 comes into contact with limestone in the soil (pebbles) or the epikarst, the calcium carbonate starts to be dissolved. In this process, carbon dioxide in water reacts with carbonate ions to form bicarbonate ions. Two end-members can be considered: (a) dissolution of carbonates occurs without presence of a gas phase (closed system); (b) dissolution process occurs in the presence of a carbon dioxide rich gas phase (open system). In this last case additional CO2 is dissolved in the water enhancing the amount of calcium carbonate that can be kept into solution (and therefore the total amount of inorganic carbon). Water percolating through the vadose zone may reach the cave atmosphere with lower carbon dioxide content and therefore excess of CO2 is released until a new equilibrium condition is achieved. The decrease of carbon dioxide content in the solution creates an oversaturation with respect to calcium carbonate, and consequently calcite precipitates. Direct diffusion from soil and epikarst voids and direct human release may be considered as supplementary sources of carbon dioxide in the cave atmosphere. Other potential sources such as decomposition of large quantity of organic matter in the cave and from deep seated sources have not been considered in this conceptual model. Carbon dioxide in cave air is transported between underground passages by advection and then released to the external atmosphere. Part of the inorganic carbon stays in the water and eventually reaches the phreatic zone, where it is stored until it ultimately exits at springs.