Modeling a Large-Scale Historic Aquifer Test: Insight into the Hydrogeology of a Regional Fault Zone

Area of Science:

• Hydrogeology
• Geology
• Environmental Science

Background:

• Fault zones significantly impact groundwater flow by creating permeability heterogeneity.
• The Sandwich Fault Zone's deep hydrogeology is poorly understood due to burial and fault orientation.
• Understanding fault behavior is crucial for managing regional water resources.

Purpose of the Study:

• To investigate the hydrogeology of the Sandwich Fault Zone.
• To determine the role of the fault zone as a barrier or conduit for groundwater flow.
• To assess the impact of the fault zone on regional groundwater availability.

Main Methods:

• Utilized a large-scale historic aquifer test conducted in 1942.
• Employed a transient MODFLOW-USG model with the CLN package for simulations.
• Calibrated the model to drawdown data from production and observation wells, testing various fault conceptualizations.

Main Results:

• The Sandwich Fault Zone acts as a low-permeability barrier, inhibiting lateral groundwater movement.
• Horizontal hydraulic conductivity within the fault core is at least an order of magnitude lower than in surrounding sandstone.
• Simulations indicate significant regional drawdown north of the fault zone, sensitive to fault parameters.

Conclusions:

• The Sandwich Fault Zone functions as a substantial groundwater flow barrier.
• The fault's low permeability has critical implications for future groundwater availability in the region.
• Effective groundwater management strategies must account for the hydrogeologic influence of this fault zone.