Conceptual models of karst aquifers. Left: a conduit having finite volume embedded in a porous matrix; the flow in the matrix is modeled by the Darcy model whereas the free flow in the conduit is modeled by the Stokes or Navier-Stokes or Brinkman or some other 2D or 3D flow model. Right: a one-dimensional pipe embedded in a porous matrix; the flow in the matrix is again modeled by the Darcy model whereas now the flow in the 1D conduit is modeled by a 1D pipe flow model. Of crucial importance is to correctly model the exchange of fluid and solutes, e.g., contaminants, across the matrix-conduit interface. Exchange laws for the pipe flow/Darcy and, e.g., Stokes/Darcy models invariably contain parameters that may be tuned to improve the accuracy of the flow exchange between conduit and matrix.