A model of karstification in extended limestone plains by mixing corrosion

Abstract

When water from the surface of a limestone plain seeps through the vadose zone in fractured rock down to the water table of an unconfined aquifer containing water saturated with respect to calcite, mixing of these waters causes renewed dissolution of limestone. A model coupling flow in the fractures with dissolution rates describes the evolution of karstification by dissolutional widening of the fractures. At the beginning of karstification a domain of surface water saturated with respect to calcite at a pCO2of 0.05 atm floats on the phreatic water saturated with respect to calcite at a pCO2 of 0.001 atm. At the border of these domains their waters mix and create dissolutional widening of the fractures by mixing corrosion. A channel evolves along the border migrating downstream by about 100m in 100,000 years. Below this channel a zone of fractures with aperture widths up to one cm has originated. The change of the hydraulic conductivity in the mixing zone shifts the border of the domains, allowing the channel to migrate downstream. Below it the zone of widened fractures is invaded by saturated phreatic water and dissolution stops. This process continues at the downstream part of the conduit. In summary, this modeling reveals an effective mechanism of intense karstification on large karst plains.