Geochemistry of capillary seepage in Mammoth Cave

Low-velocity capillary seepage in the vadose zone is responsible for a variety of geochemical processes in Mammoth Cave. Water that infiltrates through the cap-rock of detrital sandstone and shale is isolated from the high-CO2 of the soil before it encounters the underlying carbonate rocks, so that carbonate dissolution in narrow fissures around the cave takes place under nearly closed conditions with respect to CO2. As a result, the equilibrium PCO2 of the capillary water decreases to nearly zero and the pH can rise to more than 9.0. When the water emerges into the cave it rapidly absorbs CO2 from the cave air and becomes highly aggressive toward carbonate rocks. Where discrete trickles exit from fissures, deep irregular rills are formed. Where the flow is more diffuse, the cave walls are weathered to a chalky white by partial dissolution and recrystallization of the carbonate rock. If the water has acquired sulfate from oxidation of pyrite or dissolution of residual gypsum within the bedrock, the SO4=/CO3 = ratio of the water rises sharply at the cave walls, promoting the replacement of carbonate bedrock by gypsum. The pH decrease caused by the uptake of CO2 enables silica to precipitate in small amounts in the weathering rind. Direct measurement of capillary water chemistry is difficult because of the small quantity and inaccessibility of the water involved, but it can be reliably inferred from the geochemical setting and the effects upon the cave