Groundwater in the Great Lakes Basin

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DESCRIPTION OF NATURAL SYSTEMS
Mapping Hydrogeological Units
Recent Research
Groundwater is present throughout the Great Lakes
Basin, but the amount of water available from the
groundwater system depends on the water-bearing
characteristics of the rocks and sediments that contain
the groundwater. Maps of the extent and hydraulic
properties of these hydrogeological units are important
ways to more completely understand the groundwater
system. The largest portion of groundwater stored
in the basin is contained in unconsolidated material
deposited at or near the land surface as a result of
large-scale glacial ice advances and retreats during the
last two million years (Coon and Sheets, 2006). Glacial
debris that was deposited directly from the glacial ice
is composed of mixtures of clay, silt, sand, gravel and
boulders, which generally are poor aquifers. However,
glacial sediments that were deposited in streams whose
flow originated as meltwater from the glacial ice are
composed primarily of sand and gravel. These deposits
generally constitute productive aquifers which form a
near-surface aquifer system that is present in much of
the basin. Although discontinuous, “their ubiquity has a
regional effect on groundwater resources and, therefore,
allows them to collectively be treated as a regional
system” (Coon and Sheets, 2006). As an aquifer system,
the glacial deposits contain far more water in storage
than any other regional aquifer system in the basin.
Because most of the basin’s groundwater is stored in
this aquifer and because more wells are drilled into
it than any other geologic unit, the need to develop
detailed, three-dimensional maps of the glacial deposits
is increasingly important. Such maps have been
compiled for a few areas such as near Toronto, Ontario
(Sharpe, Russell and Logan, 2007), Berrien County,
Michigan (Stone, 2001), and Lake County, Illinois
(Stiff, Barnhardt, Hansel and Larson, 2005). Most of the
mapping on the U.S. side of the border is done by the
Geological Surveys of the eight Great Lakes states. In
addition, the Central Great Lakes Geologic Mapping
Coalition, formed by the state geological surveys of
Illinois, Indiana, Michigan and Ohio and the U.S.
Geological Survey (USGS) to produce detailed, 3-D
geologic maps of surficial materials in the glaciated areas
of these four states, is helping to coordinate consistent
geologic mapping in the basin (USGS, 1999). These
efforts complement other ongoing cooperative efforts
among the state surveys and the USGS to produce
detailed geologic maps of the glaciated portions of the
Great Lakes region. An effort to coordinate mapping in
Canada and the U.S. also has been proposed.
In addition to new geologic maps, other scientists
have improved techniques to use publicly available
well-log information to improve understanding of
geology and hydraulic properties of commonly used
aquifers (Groundwater Conservation Advisory Council,
2006). Because the quality of groundwater determines
whether or not the water can be used for certain
purposes, some work is now being undertaken to
describe the quality of water from glacial deposits as
part of the USGS’s National Water-Quality Assessment
Program (Warner and Arnold, 2005). More work on
groundwater quality has been done at the local level
without placing the results of water-quality studies
into a broader context. As analyses of the effects of
groundwater use on the regional scale become more
common, water-quality analyses at the regional scale
also will become more common.
Consolidated bedrock underlies the unconsolidated
glacial deposits or outcrops at the surface throughout
the basin. However, because not all the bedrock units
store and transmit water readily, they are only considered
aquifers in about half of the basin. The non-aquifer
bedrock is only used for low-volume withdrawals when
water from other sources is insufficient. Some bedrock
aquifers in the region extend far beyond the watershed
boundaries, thus requiring detailed information about
the relation of groundwater flow inside and outside the
basin. Although this relationship has been evaluated for
a number of years, new information was published by
Feinstein, Hart, Eaton, Krohelski and Bradbury (2004),
and additional work is underway by the Southern Lake
Michigan Regional Water Supply Consortium (2007)
and the USGS (Grannemann and Reeves, 2005). A wide
variety of sources, such as new maps and more complex
evaluation of information from well logs, provides input
for this work.
It is estimated that there is about the same volume of
groundwater stored in the aquifers of the U.S. side of
the basin as there is surface water in Lake Michigan
(Coon and Sheets, 2006). This estimate is based on
hydraulic properties of geologic units from existing
studies of six regional aquifer systems in the basin.
Because saline water underlies fresh water nearly
everywhere in the basin, the estimate is delineated into
fresh-water and saline-water components as indicated
by Coon and Sheets: “Summation of the volumes in the
many regional aquifers of the basin indicates that about
1,340 cubic miles of water is in storage; of this, about
984 cubic miles is considered fresh water (water with
dissolved-solids concentration less than 1,000 mg/L).”
This large amount of stored groundwater makes it
important to comprehensively evaluate groundwater as
one of the region’s most valuable resources.