Fractured Porous Media and Karst Systems – Shared and Distinguishing Features in Conceptualization and Assessment of Flow and Transport Processes

Fractured porous media and karst systems are two heavily studied families of geological formations. Both types of systems tend to display highly heterogeneous hydrogeological properties over a wide range of pore-to-field length scales, and they both typically display irregular or non-uniform flow and transport behaviors in both space and time. While these features suggest commonalities in the mechanisms that control flow and transport dynamics, several key features distinguish these two systems and demand different perspectives on assessment and quantification. More specifically, for example, quantifying “just” the volumetric water fluxes in fractured systems – for which the host rock permeability can vary over orders of magnitude, e.g., considering sandstone to carbonate to granitic formations – first faces the obstacle of obtaining a realistic, site-specific delineation of the (3D) fracture geometry. In addition to this difficulty, karst systems can contain large cave-like features (which are not “fractures”) that strongly influence flow and even induce turbulent flow; and at the catchment scale, a critical problem is to assess total system response subject to time-dependent input (precipitation, flooding) conditions and/or output (pumping) conditions. Assessment of chemical and radionuclide transport in both fractured and karst systems, and then chemical reactions and interactions such as precipitation and dissolution, requires even more complex considerations and increased degrees of uncertainty and variability over pore-to-field spatial and temporal scales. Addressing relevant questions requires field measurements, laboratory experiments, and appropriate model formulation. Significantly, data acquisition at field scales is highly restricted and extensive field studies conducted already three to four decades ago in fractured formations – notably at Aspö and Stripa (Sweden), Yucca Mountain (USA), and Chalk River (Canada) – illustrated the severe limitations to detailed characterization, which lead to serious limitations in realistic mimicking and prediction of flow and transport behaviors. Moreover, while laboratory studies provide important insights at pore, column (generally 1D) and flow cell (generally 2D) scales, they are limited in terms of spatial and temporal scales, and realistic representation of fracture network and karst complexities. In terms of model conceptualization and development, the question of how to model flow, transport, and reactive chemical transport should thus be based, first and foremost, on the specific phenomenon or problem of interest (e.g., flow, transport, chemical reactions, local dynamics or overall system behavior, spatial and temporal scale). This then directs conceptualization and the choice of a model that focuses, principally, either on “exact” dynamics or on phenomenological or overall ”functional” dynamics. We will address both similarities and differences in fractured porous media and karst systems, suggesting methods of characterization and quantification that can be common to both, and other methods that are tailored to address the distinct features and real hydrogeological relevance between these systems.