Great Lakes Basin Pilot

Ground-Water Flows and Storage

The major tasks include:

• Description of ground-water divides for aquifers on the U.S. side of the Great Lakes Basin including discussion of the changes in these divides in response to pumping and the uncertainty in the location of these divides for much of the region (see Compilation of Regional Ground-Water Divides for Principal Aquifers Corresponding to the Great Lakes Basin, United States).
• Estimates of recharge to ground water. Initial estimates are presented in Estimation of Shallow Ground-Water Recharge in the Great Lakes Basin. Current work includes estimates of the changes in recharge with time, increased spatial resolution of recharge estimates, and development of techniques to project recharge under changing climatic conditions.
• Estimation of the amount of potable ground water in storage in the aquifer systems on the U.S. side of the Great Lakes Basin (see Estimate of Ground Water in Storage in the Great lakes Basin, United States, 2006).
• Development of a ground-water-flow model for the contributing area to Lake Michigan to analyze and illustrate the source of water to wells and the dynamics of the ground-water system.

Status

Ground-water availability assessment requires an understanding of the response of the ground-water system to changes in either recharge to the system, surface-water levels, or imposed pumping on the system. Ground-water-flow models typically are used to quantify these changes (for example, see Chapter 7 and Box L from Ground-Water Resources of the Middle Rio Grande Basin, USGS Circular 1222.)

The ground-water-flow model for the contributing area to Lake Michigan has been developed and is currently being calibrated to observed water levels and flows in the system. The model is meant to serve as a template for other studies across the U.S. by producing indicators of water availability. It is meant to support Annex 2001 decision makers (more information regarding the Great Lakes Charter Annex of 2001) and other organizations dedicated to protecting and managing the waters of the Great Lakes Basin. The model will serve to develop and apply innovative methodologies for assessing the ground-water resource and its connection to surface water. The model simulates regional ground-water flow from 1864 through 2005. Current testing includes the use of soil-water-balance techniques to estimate recharge to the model through time and the use of SEAWAT to explore the impact of saline water within the Michigan Basin on ground-water availability.

Representation of grid for ground-water-flow model of contributing area to Lake Michigan. Inner area of model (hatched area) has grid size set to 5000 ft by 5000 ft. Grid size increases in far field of the model (solid yellow).

Average recharge over LMB from 1970 to 2005 is 7.65 inch/yr. Anticipated results from the model include water budget information for each stress period illustrating the dynamics of the system as water use changes. This chart shows an example from the uncalibrated model summarizing the outflow from the ground-water system as pumping, discharge to streams, direct discharge to Lake Michigan, discharge to inland lakes, and lateral flow outside the modeled area for one of the stress periods in the simulation.