Abstract
When water from the surface, e.g. from a lake, fl ows down through porous carbonate rocks, through a 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 Δc eq 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 Δc eq with respect to m . m(x,y) is the spatial distribution of the mixing ratio m= V surf /(V suf + V prh ) , where the V ’s assign the corresponding volumes of surface and phreatic water. The second derivative has been calculated for three geochemical scenarios with differing CO 2 - concentrations of surface and phreatic water by the use of PHREEQC-2. The spatial distribution m(x,y) is obtained by using MODFLOW and MT3DMS in a modelling domain with constant hydraulic conductivity for various fl ow velocities of the phreatic aquifer. The time scale of cave evolution is estimated from the results. Passages of dimensions of about one metre in width and several 10 cm in height, extending in length along the boundary line, where surface and phreatic waters meet, can be created in time scales of 10 000 years. These caves are horizontal with blind ending passages and closely resemble the isolated caves observed in Central West Florida.
Highlights
Caves in young porous rocks are remarkably different from those in ancient fractured rocks
Due to the asymmetry of the second derivatives the maximum of porosity evolution shifts upwards into the surface water region. These results indicate that caves with dimensions of about 1m width and 0.2 m in height can be created along the border where surface water with higher PCO2 encounters the water table of the phreatic base flow
When the two water bodies meet along a one dimensional border, isolated caves with blind ending passages can develop in the diffusion-mixing zone extending along such boundary lines
Summary
Caves in young porous rocks are remarkably different from those in ancient fractured rocks. Caves in porous rocks where the flow is not guided along fissures but where water is transported by Darcy flow through the matrix of the rock are different They are known from young carbonate islands. Initial changes of porosity are restricted to a region close to the boundary line where surface water and phreatic water meet and voids directed vertical to the flow of the phreatic water originate Since both bodies of water exhibit Darcy flow, mixing of the phreatic water with the surface water can only be affected by molecular diffusion and transversal dispersion across the dividing streamline (DS). Mixing corrosion is active in this region and causes increasing porosity along the z-axis where the surface water enters, and down the y-direction along the dividing streamline This way an isolated cave passage could originate, similar to those passages observed in porous rocks in Florida. We will investigate how and over what time scale, caves can originate in such a hydrological setup
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