The hydrogeologic framework for the Lake Michigan Basin model was developed by grouping the bedrock geology of the study area into hydrogeologic units on the basis of the functioning of each unit as an aquifer or confining layer within the basin. Available data were evaluated based on the areal extent of coverage within the study area, and procedures were established to characterize areas with sparse data coverage. Top and bottom altitudes for each hydrogeologic unit were interpolated in a geographic information system for input to the model and compared with existing maps of subsurface formations. Fourteen bedrock hydrogeologic units, making up 17 bedrock model layers, were defined, and they range in age from the Jurassic Period red beds of central Michigan to the Cambrian Period Mount Simon Sandstone.

Each hydrogeologic unit is referred to as its model layer number as represented in the report U.S. Geological Survey Scientific Report 2009-5060 (SIR2009-5060). They are listed below for reference as to the model layer number, and the hydrogeoloigc unit name. Dataset values represent the bottom of the layer. LSD Land surface L1_3 Quaternary unit (Bottom of Quaternary unit is Layer 3 in the model) L4 Jurassic unit L5 Upper Pennsylvanian unit L6 Lower Pennsylvanian unit L7 Michigan Formation unit L8 Marshall Formation unit L9 Devonian-Mississippian unit L10_12 Silurian-Devonian unit (Bottom of Silurian-Devonian unit is Layer 12 in the model) L13 Maquoketa Formation unit L14 Sinnipee Formation unit L15 St. Peter Formation unit L16 Prairie du Chien-Franconia unit L17 Ironton-Galesville unit L18 Eau Claire unit L19_20 Mt Simon Formation unit (Bottom of Mt Simon Formation unit is Layer 20 in the model)

The Lake Michigan Basin groundwater model is discretized into a grid of 391 by 261 cells. The model has 20 layers: 3 that simulate the glacial and unconsolidated sediments and 17 that simulate the bedrock units. The model provides additional detail in the area of greatest interest, in this case, the Lake Michigan Basin, by use of smaller grid spacing in the innermost model domain compared with the grid spacing at the model boundaries. The smallest interior grid cells are 5,000 by 5,000 ft. At the model boundaries, the size of grid cells reaches approximately 68,930 ft (13 mi) from north to south by 116,490 ft (22 mi) from east to west. The grid cells each have values for the altitude to the bottom of each layer. The layer numbers are from top to bottom of the aquifer system. Three hydrogeologic units are represented by the multiple layers