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Accurate Simulation of Flow through Dipping Aquifers with MODFLOW 6 Using Enhanced Cell Connectivity
Alden M Provost, Kerry Bardot1, Christian D Langevin2
1School of Earth Sciences, University of Western Australia, Perth, Australia.
Ground Water
|January 17, 2025
Summary
The XT3D formulation in MODFLOW 6 improves groundwater flow simulations in dipping aquifers. Accurate results require adequate cell connectivity, achieved with unstructured grids and multiple vertical layers.
Area of Science:
- Hydrogeology
- Computational modeling
- Geosciences
Background:
- Simulating groundwater flow in dipping aquifers often involves deforming model cells, leading to vertical offsets between adjacent cells within the same layer.
- The standard two-point flow formulation in MODFLOW does not accurately account for these vertical offsets.
- The XT3D multi-point flow formulation in MODFLOW 6 was developed to address grid irregularities like vertical offsets.
Purpose of the Study:
- To investigate the accuracy of the XT3D formulation in MODFLOW 6 for simulating groundwater flow in steeply dipping aquifers with significant vertical offsets.
- To identify the reasons for inaccuracies observed in previous studies using XT3D in such scenarios.
- To demonstrate how to achieve accurate simulations using XT3D under challenging aquifer geometries.
Main Methods:
- Evaluation of the standard conductance-based and XT3D multi-point flow formulations in MODFLOW 6.
- Utilizing a synthetic benchmark model of a steeply dipping aquifer with varying vertical offsets.
- Implementing XT3D with MODFLOW's unstructured grid type and varying vertical discretization (number of model layers).
Main Results:
- XT3D generally improved simulation accuracy compared to the standard formulation.
- Neither formulation provided accurate flows when large vertical offsets were present with standard discretization.
- XT3D achieved expected accuracy when using unstructured grids and at least two vertical model layers, indicating sufficient cell connectivity is crucial.
Conclusions:
- The limitations of XT3D in previous benchmark studies were due to insufficient cell connectivity in common MODFLOW 6 discretization packages, not the formulation itself.
- Adequate cell connectivity, facilitated by unstructured grids and vertical discretization, is essential for accurate groundwater flow simulations with XT3D in complex dipping aquifers.
- The findings highlight the importance of grid design and discretization choices for reliable numerical modeling of hydrogeological systems.
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