ArchPy and MODFLOW: Toward a General Integration of Heterogeneity into Groundwater Models.

Groundwater models are important and useful tools for answering scientific and technical questions about the quantity and quality of groundwater, as well as for making critical management decisions. However, the heterogeneity of subsurface properties, such as hydraulic conductivity, is known to play a central role in groundwater flow and transport; therefore, its accurate quantification and incorporation into the groundwater workflow are critical. This paper presents a novel tool, ArchPy2Modflow, that efficiently combines a stochastic geological generator, ArchPy, with a groundwater flow software, MODFLOW. ArchPy2Modflow provides a rapid and practical way to convert and link any ArchPy model to a new (or existing) MODFLOW model, where any MODFLOW spatial parameter (such as porosity, hydraulic conductivity, or storativity) can be obtained from an ArchPy property, which is then upscaled according to the MODFLOW grid. ArchPy2Modflow offers several different options for selecting the appropriate MODFLOW grid: using the same grid as in the ArchPy model, defining each ArchPy geological unit as a MODFLOW layer, coarsening the grid by a certain factor, or directly using an existing MODFLOW grid. This flexibility enables users to adapt their models to suit their needs and constraints. The usefulness and practicality of the new tool are demonstrated by a synthetic example considering flow and transport in a heterogeneous aquifer, while the impact of a particular grid selection on the simulations is demonstrated.