Developing Potentiometric Surfaces and Flow Fields with a Head-Specified MODFLOW Model

Area of Science:

• Hydrogeology
• Groundwater modeling
• Aquifer system analysis

Background:

• Investigating aquifer changes typically uses potentiometric surfaces from manual or statistical methods.
• Head-specified MODFLOW models offer an alternative by simulating observed heads with constant head cells, enforcing Darcy's Law and mass balance.

Purpose of the Study:

• To develop and analyze a transient head-specified MODFLOW model for the Cambrian-Ordovician sandstone aquifer system in northeastern Illinois.
• To assess the flow dynamics within a 275m deep cone of depression.
• To evaluate the assumption of no vertical head difference in potentiometric surface development.

Main Methods:

• Developed a transient head-specified MODFLOW model for the stratified Cambrian-Ordovician sandstone aquifer.
• Utilized static heads from production wells, initially assuming no vertical head difference.
• Tested this assumption against steady-state, head-specified models for individual sandstone strata in 1980 and 2014.

Main Results:

• The assumption of no vertical head difference was valid in 1980 but not in 2014.
• A significant vertical head difference developed at the center of the cone of depression by 2014.
• The transient head-specified model showed good agreement with a flow-specified model in earlier years but overestimated water storage removal in later years.

Conclusions:

• MODFLOW effectively generates time-series potentiometric surfaces and allows testing of conceptual model assumptions, like vertical head differences.
• For deep confined aquifers, MODFLOW provides advantages over statistical interpolation for generating potentiometric surfaces and flow fields.
• Future research should explore MODFLOW's performance in diverse hydrogeological settings.