Groundwater in the Great Lakes Basin

INTRODUCTION
Groundwater, a major natural resource in the Great
Lakes Basin, supplies drinking water for 8.2 million
people in the basin, and many manufacturing processes
and other industrial and agricultural applications use
large amounts of groundwater. In addition to human
uses, one of the most important functions of groundwater
is to help maintain flow in streams, lakes and wetlands
by slowly and consistently discharging water during
periods of little or no rainfall. Discharge to streams
during periods of no surface runoff is essential to support
aquatic organisms, especially during periods of drought.
Although groundwater forms a large subsurface reservoir,
the groundwater systems in the basin are not a
vast pool of contiguous water. Rather, groundwater is
contained in many different geologic units, each with
its own distinct hydraulic properties. Some units are
widely used aquifers that readily transmit water and
yield large quantities of water to wells, while others
are not commonly used but do yield moderate amounts
of water. In some limited parts of the basin, little or
no aquifer material exists. Issues generally involve the
amount of groundwater available, the quality and/or
the connection to an ecosystem (Grannemann, Hunt,
Nicholas, Reilly and Winter, 2000).
The amount of groundwater available depends on many
climatic and hydrogeologic factors. In confined aquifers,
the effects of pumping are manifested rapidly. However,
in unconfined or some semi-confined aquifer systems,
because of its relatively slow movement, the effects of
pumping groundwater in large quantities are manifested
slowly. Years may pass before there are measurable
effects in either the surface or groundwater systems.
Threats to both groundwater quality and quantity can
come from human activities on the land surface, the
effects of over-pumping or natural conditions underground.
The quality can be altered by either point or
non-point sources of contamination that enter from the
land surface and infiltrate to the groundwater system.
Some of the more common contaminants are hydrocarbons,
solvents, pathogens, pesticides, herbicides and
fertilizers. Groundwater quality also may be diminished
by over-pumping, which may induce natural, but generally
unwanted, chemical constituents into the fresh
groundwater system. These constituents may include
brine, arsenic and radium. Large-scale groundwater
withdrawal also can redirect, or significantly reduce,
the discharge of groundwater to streams, lakes and
wetlands, thus depriving the surface water of a generally
high-quality, constant temperature source of water. The
resulting changes can alter the amount of surface water
and also ecosystems that rely on groundwater discharge.
The flow of many streams in the Great Lakes Basin,
especially those in watersheds with highly porous soils,
consists largely of groundwater discharge to the streams.
Hence, a high percentage of water flowing to the Great
Lakes consists of water that infiltrates the land surface,
enters the groundwater system, flows underground for
varying distances, discharges to a stream or lake and
then continues its path to the Great Lakes as streamflow.
Therefore, to be comprehensive, management
strategies for protecting the quality of Great Lakes water
must incorporate the groundwater flow component.
In short, the Great Lakes cannot be protected without
protecting the groundwater resources in the basin.
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