Process-based flow and transport models for karst aquifers -

Abstract

Karst aquifers are characterized by the presence of highly developed subterranean conduits embedded in a low porosity matrix. The conduits are hydraulically connected to the matrix and act either as a source or drain depending on the recharge conditions. Distributive models generally face several difficulties in simulating karst aquifers due to the high data requirements and often unknown location and geometry of the conduits. In the present study, simplified process-based flow and transport models are proposed using a one-dimensional conduit system embedded in a two-dimensional matrix domain. The flow in the conduit can be under pressurized or free-surface flow conditions, and it is driven by a diffuse aquifer recharge as well as a concentrated recharge applied at the conduit entrance. The governing equation is a coupled system of nonlinear differential equations that is solved numerically using the method of finite differences. Analytical and Laplace transform solutions are also obtained for given initial and boundary conditions using linearizing assumptions. The linear solutions simulate the typical shape of a spring hydrograph using physical parameters rather than empirical ones. They are computationally advantageous and reproduce the response of more complex numerical models while requiring less data than two- or three-dimensional dual-hydraulic models. The proposed models are successfully applied to real karst aquifer systems thus demonstrating their effectiveness in simulating observed spring hydrographs. A transport model is also proposed using the same conceptual framework as the flow models. It takes into account the physical representation of the matrix and its parameters, and serves to simulate tracer breakthrough curves. The transport model is applied to actual karst systems and effectively reproduced measured tracer breakthrough curves using physically meaningful parameters.