Revisiting MODFLOW's Capability to Model Flow Through Sedimentary Structures

Abstract

Sedimentary structures have unique geometries and anisotropic hydraulic conductivity, both of which control groundwater flow. Traditional finite-difference simulators (e.g., MODFLOW) have not been able to correctly represent irregular, dipping and anisotropic structures due their use of a simplified conductivity tensor, causing many modelers to turn toward finite-element codes with their sophisticated meshing capabilities. However, the release of MODFLOW 6 with its flexible discretization and multipoint flux approximation scheme prompts us to revisit its capability to compute flow through complex sedimentary structures. Through the use of a novel benchmark and case study, we show that when versions previous to MODFLOW 6 are applied to dipping structures, modeled fluxes and hence flow through the system, can be significantly over or underestimated. For example, effective conductivity for a 30° dipping layer with a 100:1 conductivity ratio is reduced to only 2% of its inputted value. We show that MODFLOW 6, with its XT3D capability and flexible discretization options is far superior to its predecessors, allowing flow through complex sedimentary structures to be simulated more accurately. However, on vertically offset grids, which have been available in all versions of MODFLOW and are often used in practice, loss of accuracy is still a concern when the vertical offset is large, that is, the dip of the sedimentary layer is steep, particularly if the layer is much more conductive than the surrounding material. The hypothesis that vertically offset grids lack sufficient hydraulic connectivity between adjacent model layers to accurately simulate the steeply dipping, highly heterogeneous case is a topic for further investigation.