Selecting MODFLOW cell sizes for accurate flow fields

Area of Science:

• Hydrogeology
• Environmental Engineering
• Computational Modeling

Background:

• Contaminant transport models often rely on groundwater flow fields generated by MODFLOW.
• The accuracy of these transport predictions is directly linked to MODFLOW's representation of groundwater flow, especially for advective transport.

Purpose of the Study:

• To assess the accuracy of MODFLOW groundwater flow rates and MODPATH particle traces (advective transport) under various conditions.
• To determine the impact of grid spacing on the reliability of groundwater flow and transport simulations.

Main Methods:

• Comparison of MODFLOW and MODPATH results against analytical solutions for flow in confined or semiconfined aquifers.
• Numerical experiments were conducted using diverse conceptual models and varying grid spacings.

Main Results:

• MODFLOW demonstrated robustness in water balance calculations.
• Accurate groundwater flow rates, particle traces, and travel times were achieved only with sufficiently small grid cells.
• Specific cell density requirements were identified for modeling inflows to surface water bodies and zones of differing transmissivities.
• Maximum cell dimensions were related to a characteristic leakage length (lambda) for accurate representation of aquitard leakage.

Conclusions:

• Adequate grid resolution is critical for accurate groundwater flow and advective contaminant transport modeling using MODFLOW.
• Modelers must carefully consider cell size relative to hydrogeological features like narrow tributaries and transmissivity zones.
• Leakage representation necessitates cell dimensions constrained by the characteristic leakage length to ensure reliable transport predictions.