A model of the early evolution of karst aquifers in limestone in the dimensions of length and depth

A new model of the early evolution of Limestone karst aquifers in the dimen sions of length and depth is presented. In its initial state the aquifer co nsists of a rock massive with evenly spaced fractures of about 50 mum apert ure widths with an hydraulic conductivity of 10(-7) ms(-1). In addition to this a coarser network of prominent fractures with aperture widths of sever al 100 mum is also present. Boundary conditions of constant recharge 450 mm /year, or constant head from the input of allogenic streams are imposed. Fi rst the position of the water table in the aquifer is calculated, then diss olutional widening during a time step in all the fractures below the water table is found by use of the well-known nonlinear dissolution kinetics of l imestone. This is iterated and the position of the water table as well as t he fracture widths are found as a function of time. In the case of constant recharge to a karat plateau, the water table in any case drops to base lev el and conduits there propagate from the spring headwards. If constant head conditions are valid the position of the water table remains almost stable and conduits propagate along the water table from the input towards the sp ring. There is competition between conduit evolution along prominent fractu res and along tight fissures close to the water table. In any case under co nstant head conditions one of these pathways wins, and early karst evolutio n is terminated by a breakthrough event with an explosive increase of the f low through the aquifer until constant head conditions break down. Dependin g on the boundary conditions of constant head or constant recharge or a com bination of both it is possible to describe models of cave genesis, which h ave been derived from field evidence, such as the water table models of Swi nnerton (Geol. Soc. Am. Bull., 34 (1932) 662) and Rhoades and Sinacori (J. Geol., 49 (1941) 785) as well as the four-state model by Ford and Ewers (Ca n. J. Earth Sci., 15 (1978) 1783). (C) 2001 Elsevier Science B.V. All right s reserved.