Karstbase Bibliography Database

The early stage of hypogene karstification is investigated using a coupled
thermohydrochemical model of a mountain hydrologic system, in which water enters along a
water table and descends to significant depth (1 km) before ascending through a central
high-permeability fracture. The model incorporates reactive alteration driven by dissolution/
precipitation of limestone in a carbonic acid system, due to both temperature- and pressuredependent
solubility, and kinetics. Simulations were carried out for homogeneous and
heterogeneous initial fracture aperture fields, using the FEHM (Finite Element Heat and Mass
Transfer) code. Initially, retrograde solubility is the dominant mechanism of fracture aperture
growth. As the fracture transmissivity increases, a critical Rayleigh number value is exceeded
at some stage. Buoyant convection is then initiated and controls the evolution of the system
thereafter. For an initially homogeneous fracture aperture field, deep well-organized buoyant
convection rolls form. For initially heterogeneous aperture fields, preferential flow suppresses
large buoyant convection rolls, although a large number of smaller rolls form. Even after the
onset of buoyant convection, dissolution in the fracture is sustained along upward flow paths
by retrograde solubility and by additional ‘‘mixing corrosion’’ effects closer to the surface.
Aperture growth patterns in the fracture are very different from those observed in simulations
of epigenic karst systems, and retain imprints of both buoyant convection and preferential
flow. Both retrograde solubility and buoyant convection contribute to these differences. The
paper demonstrates the potential value of coupled models as tools for understanding the
evolution and behavior of hypogene karst systems.