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What are the Dimensions of Water Availability in Illinois? (Graphic/Map Detail)
 
More information on major bedrock aquifers. (Gray text is from last page)
 
The Hydrologic Cycle
This map shows the distribution of major bedrock aquifers containing potable water within 300 feet of ground surface in Illinois. Major bedrock aquifers are defined to be geologic units capable of yielding at least 70 gpm from a single well. Potable water is defined to be water containing less than 2,500 milligrams per liter (mg/L) of total dissolved solids. Bedrock aquifers within 300 feet of ground surface cover most of northern and central Illinois and are commonly overlain only by thin layers of less permeable silts and clays. Many are directly overlain by shallow or major sand and gravel aquifers allowing direct hydraulic communication between aquifer systems. These major bedrock aquifers include the Hunton Limestone Megagroup, Ancell Group, Prairie Du Chien Group, Ironton-Galesville Sandstone, and Elmhurst-Mt.Simon Sandstone (see table 1)

Table 1. Summary of major aquifer units in Illinois from youngest to oldest *

(Major aquifers are defined to be units capable of yielding at least 70 gallons per minute of water containing less than 2,500 mg/L total dissolved solids to a single well.)
 
Unit Name Character Description
1. Quaternary sand and gravel Unconsolidated sand and gravel
2. Hunton Limestone Megagroup
    Middle Devonian Series
    Lower Devonian Series
    Cayugan Series (Silurian)
    Niagaran Series (Silurian)
    Alexandrian Series (Silurian)
Consists predominantly of Silurian and
Devonian limestones and dolomites
3. Ancell Group (Ordovician)
    Glenwood Formation (northern IL)
    Joachim Dolomite (southern IL)
    Dutchtown Limestone (southern IL)
    St. Peter Sandstone
Consists of sandstone and argillaceous
limestone and dolomite
4. Prairie du Chien Group (Ordovician)
    Shakopee Dolomite
    New Richmond Sandstone
    Oneota Dolomite
    Gunter Sandstone
Predominantly consists of cherty dolomites
and interbedded sandstones
5. Ironton-Galesville Sandstone (Cambrian)
    Ironton Sandstone
    Galesville Sandstone
Consists of fine to coarse-grained sandstones
6. Elmhurst-Mt. Simon Sandstone (Cambrian)
    Elmhurst Sandstone
    Mt. Simon Sandstone

* Source: Illinois State Geological Survey

Composed of a coarse-grained, partly
conglomeratic sandstone


Source: Illinois State Geological Survey.

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