Locating Your Well
and the Illinois State Geological Survey can provide advice on developing new wells and solving
problems with old wells.
High-Quality Water
Are you planning a new home on a site that
is not served by a community water sysem? Or
are you thinking of replacing your water well
with a new well?
If so, the importance of having a dependable supply of safe, high-quality water cannot
be overstated. Family health is at stake, and the
market value of a home will reflect any serious
shortcomings in the water supply.
The first step in planning a new well is to familiarize yourself with the geologic conditions of
your site. If you plan to install a new well, these
conditions will influence the kind of well you put
in, how deep you will have to drill to obtain a
dependable supply, and even the potential for
future water contamination.
To learn about the condition of your site, you
can contact local well drillers, geologists, soil
scientists, agricultural and civil engineers, and
University of Illinois Extension staff. In addition,
specialists at the Illinois State Water Survey
How Geology Affects Groundwater
Although groundwater occurs everywhere, its
quality and the amount available varies from one
location to the next. The variation is due to geologic formations underlying different areas, which
affect impurities found in the water and the rate
of water withdrawal. Some of the geologic factors that affect groundwater are: permeability,
depth to aquifer, and type of aquifer.
Permeability
Clay and Shale. Certain geologic materials,
such as clay and shale, may contain abundant
supplies of water in their minute pores. However,
the movement of water between pores in clay
and shale is so restricted that water cannot flow
into a well in usable quantities. These formations
Figure 1. Groundwater
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When water fills all of the spaces between soil particles or rock, it is called groundwater.
The upper level of the groundwater is the water table.
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Figure 2. Aquifers
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An unconfined aquifer
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water moves very slowly. Because of pressure inside a confined aquifer, the water level
sometimes naturally rises in the well. This is called the “pressure head.” With an unconfined aquifer, however, water in the well remains at the same level as the water table—unless it is being pumped upward. The water table is the upper level of the groundwater.
have low water-yielding capability, or what is
called low permeability.
Sand and Gravel. Other geologic materials
capable of transmitting and storing significant
quantities of water, such as sand and gravel, are
referred to as aquifers. In Illinois, aquifers are
commonly found in bedrock (such as sandstone,
dolomite, and limestone) and in overlying unconsolidated deposits (such as sand and gravel).
These materials have varying degrees of
permeability.
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If you drill into a deeper aquifer, however,
there are no assurances that the quality of
water will be good. Some deep aquifers are too
highly mineralized or salty for most domestic
uses.
The best way to determine the aquifer’s
depth is to obtain a well log. If there is an existing well on the property, there should be a well
log. If you do not have one, check with the previous property owners or the company that dug
the well. Otherwise, contact the state geological
survey, state water survey, local well drillers, or
your nearest Extension office.
Depth to Aquifer
If the aquifer from which you draw water is
within 50 feet of the surface, it is generally considered to be a shallow aquifer.
Wells finished in very shallow aquifers may
cost less to drill and may require a smaller
pump, but they are subject to seasonal variations in precipitation that can cause the water
table to rise or drop significantly. They are also
more susceptible to surface sources of contamination than wells drawing from deeper aquifers.
Type of Aquifer
There is a strong association between the
detection of agrichemicals in rural wells and
the type of aquifer from which water is drawn.
The potential for contamination depends a lot
on whether the aquifer is “confined” or “unconfined.”
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Figure 3. Minimum Setbacks
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In the illustration above, the well is placed out of reach from
several sources of contamination. The well is more than 25
feet from the stream, more than 50 feet from the barnyard,
more than 100 feet from the manure pile, and more than 200
feet from the sand/salt storage pile and the underground fuel
storage tank.
Confined Aquifers. An aquifer is confined
when it is bounded on the top by what is called
an “aquitard”—geologic materials through which
water moves very slowly. This material, such as
clay, shales, dense crystalline, and sedimentary
bedrock, helps prevent contaminants from entering the aquifer.
Unconfined Aquifers. An unconfined aquifer
is bounded by permeable layers—geologic material through which water moves rapidly.
Generally, confined aquifers offer cleaner,
safer water; but if chemicals should enter them,
it takes a long time to decontaminate them. Confined aquifers may also be harder to find and can
be expensive to tap.
Boundaries. The well must be within your
property lines. Its exact distance from property
lines is governed by local codes.
Maintenance. The well must be accessible for maintenance, inspection, and pump
replacement. For example, wells should not be
located directly under trees or power lines or
inside a house or basement. Wells also must
be at least 2 feet clear of an overhang.
Groundwater Flow. Ideally, the well
should be placed on the side of the contaminant source opposite the flow of groundwater.
For example, if you know groundwater flows
to the south, place the well as far north of the
contaminant as possible. To find out the direction of groundwater flow in your area, contact
a well driller, an engineering firm, or the Illinois
State Water Survey.
Contamination Sources. The well must
be as far removed as possible from potential
contamination sources and routes, such as
septic tanks, road salt, underground gas storage tanks, manure piles, fertilized cropland,
and solid waste disposal sites.
The Illinois Department of Public Health
has established the following minimum setbacks for common sources and routes of
contamination:
• At least 200 feet from potential sources
and routes of contamination. However, if
What About Surface Water
as Drinking Water?
Location Requirements
When locating a new well, most people do not
have a site large enough to offer a wide range of
options. This is because a well’s location is expected to meet several requirements.
The following specifications are described in
the Illinois Water Well Construction Code. Any
variances must be approved in writing by the local
health department or the Illinois Department of
Public Health.
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Surface waters (lakes, steams,
ponds) generally are undesirable sources of drinking water for private water
systems because they require a high
degree of treatment and are easily contaminated.
However, if groundwater is unsuitable for a domestic supply in your area,
then you may need to consider this option. For technical assistance and advice
on the potential costs and effectiveness
of a surface water supply, contact the
nearest office of the USDA Natural Resources Conservation Service or University of Illinois Extension.
These lateral distances apply to clay- and
loam-textured soils; they should be increased for
more permeable soils. For example, these distances should be doubled for highly permeable,
coarse-textured soils, such as loamy sand.
As you evaluate a site for your well, also
consider future land uses near the site, as well
as pesticides that will be applied nearby. Some
pesticides, such as atrazine, require setback
zones around wells where the chemical cannot
be applied.
the owner of the private well is the same as
the owner of the potential source or route
of contamination, the setback is 75 feet.
(For information on sources and routes
of contamination, see the sidebar below.)
150 feet from existing cesspools. New
installations of cesspools are not approved
in Illinois.
100 feet from leaching pits.
75 feet from a pit privy or a septic seepage
tile, which should be downslope from a
well.
50 feet from a barnyard, animal confinement lot, or septic tank.
25 feet from lakes, ponds, and streams.
10 feet from a sewer with watertight joints.
2 feet from a pump house floor drain
100 feet from a temporary manure stack.
(This is an Illinois EPA setback requirement.)
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Sinkholes
Regions with shallow limestone or dolomite
can be particularly susceptible to groundwater
contamination because water may move rapidly
through dissolved caverns or sinkholes.
Once water enters a sinkhole, it receives
little filtration or chance for degaradation of the
chemical. Fractured rock systems also allow
rapid movement of contaminant-bearing water.
Potential Sources of Contamination
Potential Routes of Contamination
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Landfills and land disposal areas for municipal, special, and hazardous wastes
Underground storage tanks containing
more than 500 gallons and aboveground
tanks containing more than 25,000 gallons of petroleum products
Storage and handling areas for de-icers,
hazardous substances, road oils, agricultural chemicals, and animal waste
September 20072 • Number
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Abandoned and improperly plugged wells
Injection wells (most commonly used to
drain surface water and dispose of industrial and municipal waste water)
Excavations related to the production of
stone, sand, and gravel
Sources
Bob Frazee, University of Illinois Extension natural resources educator
Jerry Dalsin, Illinois Department of Public Health
Brian Kaiser, Illinois State Water Survey
7 & Water Coordinator
Land
Michael C. Hirschi, U of I Extension soil and water specialist
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