Abstract:

When water from the surface, e.g. from a lake, flows through porous carbonate rocks down along some region with high hydraulic conductivity and encounters the water table of a phreatic aquifer both waters mix by diffusion along their boundary. In a carbonate aquifer, where both surface and phreatic waters are saturated with respect to calcite, mixing corrosion causes renewed dissolution capacity ?ceq of the carbonate rock in the diffusion-mixing zone extending from the boundary separating the phreatic water from the surface water encountering it. A numerical model is presented from which the initial change of porosity in such a diffusion-mixing zone is obtained. The initial change of porosity can be calculated from the local distribution of the mixing ratio and the second derivative of ?ceq with respect to m. m(x,y) is the spatial distribution of the mixing ratio m= Vsurf /( Vsuf + Vprh ) , and the V’s assign the corresponding volumes of surface and phreatic water. The second derivative has been calculated for three geochemical scenarios with differing CO2 -concentrations of surface and phreatic water by use of PHREEQC-2. The spatial distribution m(x,y) is obtained by using MODFLOW and MT3DMS in a modeling domain with constant hydraulic conductivity for various ?ow velocities of the phreatic aquifer. From the results the time scale of cave evolution is estimated. Passages of dimensions of about one meter in width and several 10 cm in height, extending in length along the border line where surface and phreatic water meet, can be created in time scales of 10 000 years. These caves are horizontal with blind ending passages and resemble closely the isolated caves observed in Central West Florida.