GROUND-WATER STORAGE IN THE GREAT LAKES BASIN, UNITED STATES

As part of the USGS Great Lakes Water Use and Availability Pilot Program, a series of studies are examining different aspects of the regional water balance, including ground-water recharge, divides and storage. Vast quantities of water are available in aquifer systems within the Great Lakes Basin, and they play a critical role in the overall water supply of the area. The major regional aquifer systems are the Cambrian-Ordovician, predominantly sandstone, aquifer system in Wisconsin, Illinois, and Indiana; the Silurian-Devonian carbonate aquifers in Wisconsin, Michigan, Illinois, Indiana, and Ohio; and the surficial aquifer system (aquifers of alluvial and glacial origin) that blankets the bedrock aquifers throughout most of the Great Lakes Basin. Minor aquifer systems are the Mississippian sandstone aquifers and the Pennsylvanian sandstone and carbonate-rock aquifers in the Lower Peninsula of Michigan.

An estimated total aquifer storage of approximately 1,300 mi³ is based on aquifer properties compiled from previous regional ground-water studies. Approximately 73 percent of the water in ground-water storage for the Great Lakes Basin is freshwater. The surficial aquifer system is the largest in terms of freshwater storage, containing about 580 mi³. Another 190 mi³ of freshwater is stored in the Cambrian-Ordovician aquifer system in the western part of the Great Lakes Basin; however, withdrawals in southeast Wisconsin and northern Illinois have shifted the position of the regional ground-water divide and affected ground-water storage. Relocation of the regional ground-water divide has increased the volume of storage in the Cambrian-Ordovician aquifer system by about 100 mi³.

Subsequent earth-tide analyses of long-term well hydrographs yield specific storage estimates for aquifers that are similar to those from previous studies, ranging from 1x10^-6 to 1x10^-7/ft. The analyses also give porosities of 0.15 to 0.25 and barometric efficiencies of 20 to 90 percent for Great Lakes Basin aquifers.

This ground-water storage information will help an ongoing evaluation of the existing ground-water level network within the Great Lakes Basin.