Modeling spring flow of an Irish karst catchment using Modflow-USG with CLN

Abstract

Karst systems present multiple challenges for the application of physically based numerical models, due to the heterogeneity and high spatial variability of their hydrogeological parameters. Some models can face conceptual limitations specific to the karst environment (like equivalent porous medium models). A new version of MODFLOW called MODFLOW-USG (UnStructured Grid) was developed and released (Panday et al., 2013) implementing unstructured grids and finite volume numerical solutions, to enable a more physically accurate modelling of karst aquifers. In this study, we tested for the first time the abilities of MODFLOW –USG with CLN within on a small Irish catchment feeding the spring of Manorhamilton, located in county Leitrim, Ireland. In order to create a numerical model of this karst system, we developed a methodology combining 3D geological models built using the software MOVE and Leapfrog Geo to characterise accurately the aquifer and set up its hydrogeological properties. The model is then exported to MODFLOW-USG with CLN using the GUI Groundwater Vistas 7. The flow model is calibrated against the measured spring discharge, with a qualitative assessment of the variations of the potentiometric surfaces. The Manning equation is used to simulate the non-laminar groundwater flow in the conduit system, while the Darcy equation is used for laminar groundwater flow in the continuum porous medium. Sensitivity analysis indicates that matrix hydraulic conductivity, specific yield, conduit drain conductance and conduit radius are important parameters in the model. The dynamics of diffuse flow in areas of the aquifer between conduits are investigated, which reveals that the most influential conduit in terms of groundwater flow through such areas can change over time depending on prevailing hydrological conditions. Overall, whilst such a modelling approach does require a good understanding a the system, large datasets and a relatively long calibration process, to achieve a satisfying performance (in the current configuration of the code and GUI and available data at the site), MODFLOW-USG with CLN proves to be a powerful tool which can be used to further understand the dynamics of the karst aquifer involved and can help inform the management of water resources both quantitatively and qualitatively.