A Semi-Structured MODFLOW-USG Model to Evaluate Local Water Sources to Wells for Decision Support.

Area of Science:

• Hydrogeology
• Environmental Modeling
• Water Resource Management

Background:

• Regional groundwater-flow models often use coarse grids, limiting their ability to simulate local surface water interactions.
• Addressing local management issues, such as surface water depletion by wells, typically requires separate, complex inset models.
• The U.S. Geological Survey developed a regional groundwater-flow model for the Lake Michigan Basin (LMB).

Purpose of the Study:

• To adapt a regional groundwater-flow model for improved local surface water-groundwater interaction simulations.
• To test a "semi-structured" grid approach within the MODFLOW-USG framework.
• To enhance the representation of the stream network and shallow hydrological processes.

Main Methods:

• Applied a version of MODFLOW with refined spacing in the topmost layer to the LMB regional model.
• Converted the parent 20-layer model (5000-foot spacing) to a 4-layer model with 500-foot spacing in the top Quaternary layer.
• Utilized MODFLOW-USG's unstructured formulation to allow for variable grid spacing between layers.

Main Results:

• The semi-structured LMB model accurately reproduces regional flow conditions.
• Finer resolution in the top layer significantly improves the simulation of surface water response to shallow wells.
• The model provides statistical measures correlating inputs with simulated surface water depletion by wells.

Conclusions:

• A semi-structured grid approach enhances the utility of regional models for local-scale groundwater-surface water management.
• The refined model facilitates the development of metamodels for predicting surface water depletion due to pumping.
• This methodology offers a more efficient alternative to labor-intensive inset models for local impact assessments.