Illinois Water Resources
Illinois Water Usage
Data from “A Plan for Scientific
Assessment of Water Supplies in Illinois”,
ISWS ,2001
 20 billion gallons per day (bgd) used for
domestic, municipal, commercial,
agricultural, mining, power generation,
recreation, navigation, and waste dilution
purposes.
 Use does not include water required for
sustainment of healthy ecosystems
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Distribution of Water Sources
Estimated Potential Yields of Fresh Water
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Sources of water
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Lake Michigan – 1350 Mgd
Shallow Aquifers – 200 Mgd
Deep Aquifers – 60 Mgd
Rivers
 Fox River – 50 Mgd
 Kankakee River – 130 Mgd
◦ Streams
◦ Lakes
◦ Reservoirs
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The amount or quantity of water that exists in Illinois depends on
four factors:
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water availability
water demand
supply from existing delivery systems
actual use.
Recharge
Statewide, about 25 percent of the
precipitation runs off the land surface
or infiltrates the soil in an average year.
Statewide, about 75 percent of
precipitation is returned to the
atmosphere through evapotranspiration
in an average year.
During droughts, this percentage
approaches 100 percent, leaving little
precipitation for runoff or aquifer
recharge.
The amount of water that runs off the
land surface or percolates into the soil
also is influenced by temperature, the
nature of the land cover, and drainage
systems.
Surface water Use
Describe the
distribution of
surface water
withdrawal?
 How do they
correspond to
population centers
in the state?

Available Water?
Conversions:
 1 gal = 0.134 ft3
 1 ft3 =7.48 gal
 1 gal per day (gpd) =
1.55 * 10-6 ft3 per
second (cfs)
 1 cfs = ~646,000 gpd
 How do these values
compare to use?
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Reliance on Surface Waters
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About two-thirds of the water supplies in Illinois are from lakes, rivers,
streams, and reservoirs.
In southern Illinois, where groundwater resources are not very
abundant, there is heavy reliance on surface waters.
Surface waters are the source of most public water supplies in
southern and central Illinois, both from direct withdrawals from
streams and, more commonly, through reservoirs formed by stream
impoundment.
Lake Michigan
◦ 201public water supply systems serving approximately 6.8 million northeastern
Illinois residents have a permit to use Lake Michigan water
◦ NE Illinois, about 2 Bgd are withdrawn
◦ Withdrawal of water from Lake Michigan is set by decree of the United States
Supreme Court and by agreement with other states and Canada. The allocation is
almost fully used and is unlikely to be increased in the near future. 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.
Groundwater Use
Groundwater is withdrawn for large public, industrial, and
commercial purposes average about 1 billion gallons per day
and serve about a third of Illinois’ population.
 Three principal categories of aquifers in Illinois
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◦ (1) unconsolidated sand-and-gravel aquifers contained within the
glacial drift
◦ (2) shallow bedrock aquifers
◦ (3) deep bedrock aquifers.
Thousands of private wells tap water in the shallow aquifers.
 Many factors influence the practical renewable yield of
aquifers and water quality,
 High-capacity wells also may have adverse impacts on nearby
wells and the base flows of rivers and streams, regardless of
whether or not the practical renewable yield of the source
aquifer is exceeded.
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Practical Renewable Yield
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What is it?
Practical renewable yield:
“The maximum amount of water that can be continuously
withdrawn from a water source without exceeding recharge
or causing water levels to decline below critical levels.”
Sustained Yield – alternate terminology, “Maximum
amount of water that can be continuously withdrawn
from existing pumping centers without detwatering
the most productive water yielding formation.”
 Safe yield - “The attainment and maintenance of a
long-term balance between the amount of ground
water withdrawn annually and the annual amount of
recharge
 Problems with either definition?
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Major Aquifers of Illinois
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Cambrian-Ordovician System (deep bedrock) – aquifer system of NE
Illinois
The Mahomet aquifer of east-central Illinois from Indiana to the Illinois
River
The shallow sand-and-gravel and bedrock aquifers in northeastern Illinois
The Sankoty aquifer of Lee and Whiteside Counties and from Hennepin to
Washington
The shallow dolomite in Kankakee, Iroquois, Will, and Cook Counties
The Saline Valley in Saline and Gallatin Counties
Alluvial aquifer systems
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Mississippi River
Wabash River Valley
Lower Illinois River from Beardstown to Alton
Rock River from Wisconsin to Sterling
Kaskaskia River Valley
Cache River Valley
Embarras River Valley
Bedrock Aquifers - Deep
Bedrock Aquifer - Shallow
Sand & Gravel Aquifers - Shallow
Shallow Aquifers
Cambrian-Ordovician System
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Most highly developed aquifer
in northern Illinois.
System is confined and under
artesian pressure where it has
not been over utilized.
Complex multiaquifer system
with individual aquifers
separated by leaky confining
units.
Capped by the Maquoketa
confining unit, which confines
them as an aquifer system.
Water transmitted primarily
through intergranular
porosity
Hydrogeology of C-O Aquifer
Unit
K (gpd/ft2)
Thickness (ft)
St. Peter
30
75-650
Galena-Platteville
13
150-350
Ironton-Galesville
10
100-300
Mt. Simon
100
2000+
Average T = 17,000 gpd/ft
 Estimated sustained yield – 46-65 Mgd
 Average well yield - > 700 gpm
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Ground-water flow is to
the south, east, and west,
away from a groundwater divide.
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Recharge to the
Cambrian-Ordovician
aquifer occurs in northcentral and northwest
Illinois. The recharge area
corresponds to that part
of northern Illinois
where the shale deposits
of the Maquoketa Group
and the Devonian,
Mississippian, or
Pennsylvanian System
have been removed by
erosion and the deposits
of the CambrianOrdovician aquifer are at
the bedrock surface.
Water Quality
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Unconfined area (Maquoketa absent)
◦ Ca-Mg-HCO3 type, TDS < 400 mg/L
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Confined area (Maquoketa present)
◦ Evolution to Na-SO4-Cl type, TDS > 500 mg/L
◦ SO4- from evaporative deposits beneath Lake
Michigan (how does this fit with flow?)
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Mt. Simon – not used TDS too high
◦ Used for gas storage
Stress on Cambrian-Ordovician
System
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1864 – oil exploration encountered fresh artesian groundwater, initial head 80 ft above
land surface, flowing at 150 gpm
1900 – wells producing 23 Mgd from system
As the industry and population grew in the Chicago region, withdrawals exceeded the
region's for the Cambrian-Ordovician aquifer system.
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
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.
Wisconsin sues, artifical gradient drawing a modled 9 Mgd from Wisconsin to Illinois
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.
The Cambrian-Ordovician aquifer system is under stress from extensive ground-water
withdrawals in southeastern Wisconsin, much of Iowa, and especially in the Chicago,
Illinois area in the adjoining Segment 10. In all but the deeply buried parts of the aquifer
system, the water is chemically suitable for all uses.
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Water naturally “polluted”
◦ Arsenic, radium, suspended sediment, and
chloride
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Anthropogenic pollutants
◦ Pesticides, metals, nutrients, sediments, and
organic compounds
TDS in Deep Aquifers
Mahomet Aquifer
Most important aquifer in east-central
Illinois.
 Non Ag Use estimated at +30 mgd.
 Irrigation puts usage well over 100 mgd
 Part of the buried Mahomet Bedrock
Valley.
 Underlies 15 counties.
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Mahomet Aquifer
Hydrogeologic setting-Mahomet Aquifer
Sand and gravel deposited by glacial meltwater
flowing westward along the Mahomet Bedrock
Valley during the pre-Illinois glacial episode.
 western part of the Teays-Mahomet Bedrock
Valley System that extends into Illinois from
Indiana and can be found into West Virginia.
 The Mahomet
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◦ 4 to 15 miles wide (6 to 24 km)
◦ 50 to 200 feet (15 to 60 m) thick,
◦ average thickness is 100 feet (30 m).
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Confined except in Mason, Menard, and Tazewell
Counties in the west near the Illinois River.
Mahomet Aquifer
Groundwater use-Mahomet Aquifer
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The Mahomet Aquifer supplies
approximately 850,000 people with water.
An estimated 100,000,000 US gallons
(380,000 m3) per day is pumped from the
aquifer .
Approximately 45% of the water withdrawn
is consumed by the public, 29% by industry,
and 18% by commercial users. The remaining
percentage is for other uses such as
livestock and irrigation.
Mahomet Stresses
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A long-term hydrograph at Champaign shows a decline in artesian head of nearly 50 feet
since 1950, as a result of increasing water demand in the Champaign-Urbana area. The
elevation of the top of the aquifer in this region is at approximately 525 feet, so additional
development of the aquifer can continue in the Champaign area for quite some time. Similar,
perhaps greater, declines can be expected in other areas of development as use of the aquifer
increases.
Increasing Demands Population projections suggest that by 2020, the Mahomet aquifer
region may increase by 100,000 people to a total of 900,000. Additional demands for water
may result as several central Illinois communities currently using surface water reservoirs
look to the Mahomet to alleviate reservoir capacity and water quality problems. The
additional demands of Springfield, Decatur, Bloomington, and Danville could potentially
increase groundwater demands on the Mahomet aquifer by 80 mgd.
Groundwater Quality Water
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generally of excellent quality
areas of high ammonia and total organic carbon
Arsenic approaching or even exceeding drinking standards.
elevated concentrations of dissolved minerals, possibly as a result of upwelling from the underlying
bedrock.
potential for agricultural chemical and nutrient contamination in areas where unconfined(Mason and
Tazewell Counties).
Science for Water Resources Management While the sustained yield of the Mahomet aquifer
has been estimated to be in excess of 400 mgd, over-development of the aquifer can occur in
localized areas.
Water quality-Mahomet Aquifer
The water is high in dissolved minerals
and iron.
 Concern is the naturally occurring
arsenic. The arsenic leaches from
oxidized-pyrite. Concentrations can range
from 20 to 70 parts per billion (ppb)
which exceeds the standard of 10 ppb
adopted by the U.S. Environmental
Protection Agency.
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Predicted Water Availability
Projected Water Needs
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Demand for water is increasing in many parts of the state, primarily as a result of growth
in the population and the economy.
Population in the Chicago metropolitan area is projected to grow by about one million by
2020
The demand for water in some parts of the state already exceeds or soon will exceed
practical renewable yields, for example, from the deep bedrock aquifer of northeastern
Illinois. Water levels in some central Illinois reservoirs were reduced to critical levels in
the moderate 1999–2000 drought.
Researchers from Southern Illinois University’s Department of Geography report water
use statewide is projected to increase almost 28 percent overall by 2025.
Water use will increase in 89 of 102 counties.
Even under normal climatic conditions, NIPC projects water shortages for 11 townships
in the Chicago metropolitan area by 2020
State laws permit reasonable use of water resources, but the courts often determine what
is reasonable and resolve conflicts. Water withdrawals typically are not evaluated based on
cumulative impacts or renewable yields.
The deep 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.