Modeling three-dimensional karst aquifer evolution using different matrix-flow contributions

Abstract

Summary We have developed the program package KARSTAQUIFER, which simulates flow and transport in a three-dimensional transient karst aquifer consisting of fractures and matrix elements. The fractures can be enlarged by chemical dissolution of calcite, increasing the secondary permeability of the karst aquifer. Boundary conditions for our model setup are guided by published benchmark models for karst aquifer evolution in two dimensions (length and width), with a 100 m high hydraulic head difference across the model domain responsible for relatively fast evolution of the aquifer. We are interested in the effect of the third dimension on the evolution of the karst aquifer, especially the effect of matrix flow, with a matrix defined either as porous medium or as fine fracture system, or both. We first discuss our model results using a pseudo-3D setup to be able to directly compare results to the 2D benchmark scenarios published. We then discuss real-3D models with a prominent fault located in the central part of the aquifer, connecting input and output regions. Finally, we replace the prominent fault with a statistical fracture diameter distribution. Our results compare well with existing 2D scenarios, and the additional third dimension offers new insights into the evolution of karst aquifers. Especially the role of matrix flow under fixed-head boundary conditions in their early phases can be studied in more detail.