NUMERICAL SIMULATIONS EVALUATING THE EFFECT OF FINE GRAIN SEDIMENT TRANSPORT IN DISCRETE FRACTURES ON THE DEVELOPMENT OF PROTO-CONDUITS IN LIMESTONE AQUIFERS

A discrete random aperture, variably-saturated, three-dimensional numerical model has been developed to evaluate the geochemical evolution of proto-conduits in limestone aquifers. Kinetic and equilibrium mass action relationships are integrated into the reactive chemistry solution to account for calcite and dolomite dissolution along vertical and horizontal fracture planes and atmospheric and soil enhanced PCO2 boundary conditions. In addition to the solute transport of aqueous species, sediment transport is also considered with equations adopted from surface water observations and theory.

Simulations are presented which illustrate the effects of sediment supply and grain size distribution on the development of proto-conduits ranging from mean particle diameters of very fine sands to medium diameter clays with grain size distributions ranging between 1.0 and 10.0 PHI units. Results illustrate that depending upon the average grain sizes, sediment settling and transport velocities and original fracture aperture field and spatial distribution, branchwork, rectilinear and spongework proto-conduit networks may evolve.