Incorporation of discrete turbulent conduit flow in a karst Equivalent Porous Media model

Sullivan Timothy P.1, Gao Yongli1, Thomas Reimann2
1 Department of Geological Sciences Center for Water Research University of Texas at San Antonio
2 TU Dresden, Institut für Grundwasserwirtschaf

P 8.6 in Karstgrundwasserleiter – Neue Entwicklungen und Methoden in Charakterisierung und Modellierung dualer Fließsysteme

Numerical modeling of groundwater flow is of worldwide importance because karst aquifers contain 25% of worldwide available groundwater.  Historically, karst features have been simulated through Equivalent Porous Media (EPM) models in MODFLOW where areas of very high hydraulic conductivity are used to represent karst conduits.  A limitation of this approach is that flow in karst conduits if often turbulent, while MODFLOW is designed to simulate laminar flow.  This introduces inaccuracies in the simulation especially at the local scale.  These models are further limited by their inability to accurately simulate contaminant fate and transport.  The introduction of MODFLOW-CFP resolved the first limitation by allowing a dual continuum approach where both turbulent flow through karst conduits and laminar flow through the aquifer matrix are simulated.  A further refinement occurred with the introduction of CFPv2, which couples an enhanced version of MODFLOW-CFP with the Three-Dimensional Underground Mine Transport Model (UMT3D) to incorporate advective contaminant transport through karst conduits.

            A promising application of these new modeling tools is the incorporation of conduits into existing EPM models to more accurately simulate turbulent flow as well as contaminant fate and transport.  The addition of conduits to existing EPM models is not completely straightforward, however, because the conduits must be calibrated for their geometry (diameter, roughness, tortuosity), conductance, and recharge.  Furthermore, the hydraulic conductivity of the EPM cells previously used to simulate karst conduits must be recalibrated to represent matrix flow.  The present effort converted an existing EPM for the Edwards Aquifer (Texas), a highly productive, regional karst aquifer in a semi-arid environment to a dual-continuum model.  The calibration progressed sequentially from hydraulic calibration of a steady-state model to hydraulic calibration of a transient model to contaminant transport calibration of a transient model.  Calibration parameters were adjusted using the parallel Parameter ESTimation (BeoPEST) program.  The poster presents the methodology and challenges associated with conducting this calibration including parameter choice, parameter ranges, and improving calibration efficiency.  The information presented will be useful to groundwater modelers looking to incorporate karst features into groundwater models as well as those utilizing automated calibration software.