The impact of hydrochemical boundary conditions on the evolution of limestone karst aquifers.

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Publication Date

May 2003

Abstract

The early evolution of karst aquifers depends on a manifold of initial and boundary conditions such as geological setting, hydrologic properties of the initial aquifer, and petrologic properties of the rock. When all water entering at various inputs into the aquifer has equal chemical composition with respect to the system H2O–CO2–CaCO3 early evolution under conditions of constant head exhibits breakthrough (BT) behaviour. If the chemical compositions of the input waters are different, deep in the aquifer where the saturated solutions mix renewed aggressiveness occurs, and additional dissolutional widening of fractures by mixing corrosion (MC) changes the hydrologic properties of the aquifer. To study the impact of MC on the evolution of karst we have modelled a simple karst aquifer consisting of a confined limestone bed, with two symmetrically located inputs at constant head and open flow conditions along the entire width at base level. To calculate dissolutional widening of the fractures the well-known dissolution kinetics of limestone was used, which is linear up to 90% of saturation with respect to calcite and then switches to a nonlinear fourth order rate law. First, two extremes are modelled: (a) Both inputs receive aggressive water of equal chemical composition with [Ca2+]=0.75[Ca2+]eq. In this case two channels migrate downstream with that from one input more competitive and reaching base level first, causing BT. (b) Water at both inputs is saturated with respect to calcite, but in equilibrium with different partial pressures of CO2. Therefore, dissolution widening can occur only where these waters mix. A central channel starts to grow extending down-head until base level is reached. Flow rates through the aquifer first rise and become constant after the channel has reached base level. In the following runs these two extreme modes of karstification are combined. The waters entering have different chemical compositions and therefore different equilibrium concentrations [Ca2+]eq. This allows MC to be active

Keywords

Karst Hydrology, Karstification, Hydrochemistry, Modelling, Mixing Corrosion

Document Type

Article

Notes

Journal of Hydrology, Vol. 276, no. 1,4 (2003-05-13).

Identifier

SFS0055793_00001

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