RESUMO
Faults can fundamentally change a groundwater flow regime and represent a major source of uncertainty in groundwater studies. Much research has been devoted to uncertainty around their location and their barrier-conduit behavior. However, fault timing is one aspect of fault uncertainty that appears to be somewhat overlooked. Many faulted models feature consistent layer offsets, thereby presuming that block faulting has occurred recently and almost instantaneously. Additionally, barrier and/or conduit behavior is often shown to extend vertically through all layers when a fault may in fact terminate well below-ground surface. In this study, we create three plausible geological interpretations for a transect in the Perth Basin. Adjacent boreholes show stratigraphic offsets and thickening which indicate faulting; however, fault timing is unknown. Flow modeling demonstrates that the model with the most recent faulting shows profoundly different flow patterns due to aquifer juxtaposition. Additionally, multiple realizations with stochastically generated parameter sets for layer, fault core, and fault damage zone conductivity show that fault timing influences flow more than layer or fault zone conductivity. Finally, fault conduit behavior that penetrates aquitards has significant implications for transport, while fault barrier behavior has surprisingly little. This research advocates for adequate data collection where faults may cause breaches in aquitards due to layer offsets or conduit behavior in the damage zone. It also promotes the use of multiple geological models to address structural uncertainty, and highlights some of the hurdles in doing so such as computational expense and the availability of seamless geological-flow modeling workflows.
RESUMO
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.
