GIS-Based Potential Recharge- and Discharge-Area Mapping from Three-Dimensional Hydrogeologic Modeling in Glacial Terrains of the Midwestern United States

Detailed three-dimensional geologic maps have been developed and discretized for use as input to a fully three-dimensional ground-water model for variably saturated porous media. To date, two models have been completed for areas of complex glacial stratigraphy in Michigan and Ohio. The top of each geologic model is the surface digital elevation model, which is also used to derive the drainage network that is an important boundary condition in the ground-water flow model. The bottom of each geologic model is the bedrock surface.

Transient flow simulations were undertaken for the study areas. Three-dimensional flow vectors were generated as part of the ground-water modeling results, from which a subset was extracted for near-surface portions of the model. The vectors were processed to resolve the magnitude and direction of the flow for each finite-difference cell, and decision rules were applied to identify vectors representing significant ground-water recharge and discharge. The ratio of vertical to horizontal flow was calculated and classified as recharge or discharge only if v_z/v_xy was greater than 1. Vectors meeting this criterion were classified as recharge (downward flow) or discharge (upward flow), then grouped into weak (5 to 25 cm/day), medium (25 to 50 cm/day), and strong (>50 cm/day) flow categories. The resulting classified potential recharge/discharge area raster data layers were then mapped on a georeferenced base for interpretation.

The maps show that locations of potential recharge and discharge are controlled by a combination of geologic materials and topographic variables. For example, although some recharge areas occur through highly conductive geologic units, the entire unit is not a recharge area; in addition, many recharge areas are located in areas of transitional relief, such as breaks in slopes near drainage features. Some potential recharge and discharge areas are almost coincident, reflecting dynamic flow in areas with high infiltration rates.