As early as the 1860s, wells were being drilled into what was to become
the most heavily developed aquifer system of the region, the deep bedrock
Cambrian-Ordovician aquifers. Despite the proximity of Lake Michigan, the
presence of this regional aquifer system beneath the whole of northeastern
Illinois contributed greatly to the industrial and population expansion
of the area. Wells reaching this important aquifer system are typically
800 to 1,500 feet deep. Shallower wells tap and use overlying formations
that are not so widespread.
As the industry and population grew in the Chicago region, withdrawals
exceeded the region’s 65 million gallon per day (mgd) estimated sustained
yield for the Cambrian-Ordovician aquifer system. By 1979, pumpage from
this deep bedrock aquifer in the eight-county area in northeastern Illinois
reached an all-time high of 182.9 mgd, nearly triple the estimated sustained
yield of the aquifer system. By 1980, continued heavy pumpage had caused
the potentiometric level of the deep bedrock aquifers at Chicago to decline
more than 850 feet. Until Lake Michigan water became available to the collar
counties in the early 1980s, pumpage in excess of the sustained yield
had occurred every year since the late 1950s. Withdrawals from the deep
bedrock aquifers are currently very near the estimated sustainable yield.
As a result of Lake Michigan diversions and consequent reductions in withdrawals
from the deep bedrock, recovery of water levels has been observed in some
areas. However, continued heavy pumpage persists in the Joliet area corresponds
with the deepest groundwater levels.
Groundwater Quality As a general
rule, the water in the deep bedrock aquifers tends to be more mineralized
than shallow aquifers, but it varies from location to location. The potential
for contamination by vertical migration of chemicals from the land surface
is very low; however, other contaminant pathways, such as abandoned wells,
can pose a threat to the groundwater quality in the deep bedrock. The
Cambrian-Ordovician is also known to contain high concentrations of naturally
occurring barium and radium.
Because
of increases in water demand due to rapid growth in the region, the shallow bedrock and
overlying sand and gravel aquifers will need to be utilized more. In a study funded by the
Illinois Groundwater Consortium (IGC), ISWS researchers
have examined historical water quality data to determine if there are temporal changes in water
quality in these shallow unconfined aquifers. The data indicate that concentrations of several
major ions, especially chloride, and total dissolved solids have increased in the shallow aquifers
in the last 20 years, especially in the shallowest wells (< 100 ft). A poster on this study was
recently presented at the Geological Society of America annual meeting in Seattle.
Proceedings papers from IGC meetings in 2001 - 2003 are also available for viewing:
Lake Michigan Diversion U.S. Supreme
Court decrees limit Illinois to diverting an average of 3,200 cubic feet
per second (cfs) from Lake Michigan. Illinois exceeded this limit during
11 of the 15 years from 1981 through 1995; the State diversion averaged
3,197 cfs during 1995. Essentially, Lake Michigan water is already fully
allocated.
Increasing Demands The Northeastern
Illinois Planning Commission projects the population of the six northeastern
counties to grow by 25 percent by 2020 and that of the outer collar counties
to increase by 70 to 100 percent by 2020. Deregulation of the power industry,
low natural gas prices, and the ever-increasing need for electrical power
have combined in an explosion of proposals for peaking power plants in
northeastern Illinois, placing yet additional demands for water on already
scarce resources.
Science for Water Resources Management
As a result of aggressive resource evaluation and management, Illinois
has reduced Cambrian-Ordovician pumpage in the Chicago region to near the
practical sustained yield of the aquifer. However, this aquifer system
and Lake Michigan are now at or near their sustainable or legally mandated
limits and cannot be relied upon as significant sources of additional water
for the region. The most viable water source alternatives for northeastern
Illinois are the shallow aquifers and the Fox, Des Plaines, and Kankakee
Rivers. Shallow groundwater resources and surface streams are hydraulically
connected and withdrawal of water from the shallower systems is likely
to have an effect on stream flow and wetlands. Consequently, conjunctive
use scenarios must be examined through the collection of field data and
computer modeling. In addition, shallow aquifers are vulnerable to surface-derived
contaminants, and the intensity of urban development in the region has
contributed to shallow groundwater quality degradation (e.g., Lake Calumet).
The Water Resources Task Force of the Northeastern Illinois Planning Commission
also recognized this need to quantify surface water and groundwater resource
use, capacity, and quality in northeastern Illinois in recommending additional
resources be provided to the Water Survey to support research and data
collection.
