Dissolution of calcium carbonate in the saltwater-freshwater
mixing zone of coastal carbonate aquifers up to now has been
treated by coupling geochemical equilibrium codes to a reactive-
transport model. The result is a complex nonlinear coupled set
of differential transport-advection equations, which need high
computational efforts. However, if dissolution rates of calcite are
sufficiently fast, such that one can assume the solution to be in
equilibrium with respect to calcite a highly simplified modelling
approach can be used. To calculate initial changes of porosity in
the rock matrix one only needs to solve the advection-transport
equation for salinity s in the freshwater lens and its transition
zone below the island. Current codes on density driven flow
such as SEAWAT can be used. To obtain the dissolution capacity
of the mixed saltwater-freshwater solutions the calcium equilibrium
concentration ceq(s) is obtained as a function of salinity by
PHREEQC-2. Initial porosity changes can then be calculated by
a simple analytical expression of the gradient of the spatial distribution
s(x, y) of salinity, the distribution of flow fluxes q(x,y)
and the second derivative of the calcium equilibrium concentration
ceq(s) with respect to salinity s.

This modelling approach is employed to porosity evolution in homogeneous and heterogeneous carbonate islands and coastal aquifers. The geometrical patterns of porosity changes and the reasons of their origin will be discussed in detail. The results reveal initial changes of porosity in the order of several 10-6 per year. This places the time scale of cavern evolution to orders from several tens of thousands to a hundred thousand years.

Download pdfThis modelling approach is employed to porosity evolution in homogeneous and heterogeneous carbonate islands and coastal aquifers. The geometrical patterns of porosity changes and the reasons of their origin will be discussed in detail. The results reveal initial changes of porosity in the order of several 10-6 per year. This places the time scale of cavern evolution to orders from several tens of thousands to a hundred thousand years.

Volume: 36

Issue: 1

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