Arsenic in groundwater in central Illinois.

              Figure 1. The Mahomet Aquifer region of central Illinois
        	 
  

     
 	   
  
	        
         
          	
	 
  
           
Illinois Department of Natural Resources April 2005
Arsenic in Groundwater
in Central Illinois
The Mahomet and Glasford Aquifers provide nearly
700,000 central Illinois residents with drinking water.
Arsenic occurs naturally in many areas of both these sand-and-
gravel aquifers at concentrations greater than the new
federal drinking water standard of 10 micrograms per
liter (μg/L). When that standard takes effect in 2006,
many public water systems will need to install additional
treatments to meet it. Domestic wells are not subject to
water-quality regulations, but some testing suggests that
those wells may be even more susceptible to arsenic
contamination than public supply wells.
If ingested at high concentrations (above 100 μg/L),
arsenic can cause serious dermatological conditions; affect
the respiratory, gastrointestinal, cardiovascular, and
nervous systems; and also has been linked to various
forms of cancer. Such severe health effects have occurred
in South Asia, Taiwan, and elsewhere in the world where
arsenic levels are very high. These levels are not typically
found in Illinois.
Figure 1 shows the location of the Mahomet
Aquifer, which occupies an ancient buried river valley that
extends from Indiana to the Illinois River. Above most of
the Mahomet Aquifer lies the much thinner, more spatially
variable Glasford Aquifer, an important water source for
domestic well owners and some communities.
Arsenic is a minor constituent of some common
minerals, and dissolved arsenic concentrations commonly
exceed 1 μg/L in groundwater. Most arsenic in aquifers is
associated with iron minerals. Arsenic may enter ground-water
when microorganisms in the subsurface dissolve
those minerals. This is most likely to occur in areas
containing large amounts of buried organic matter.
The Research
Illinois State Water Survey (ISWS) scientists
collected water samples from nearly 200 domestic and
public supply wells in both aquifers for analyses of arsenic
and other substances between 2000 and 2003. Those
results, and previously collected data from approximately
400 domestic and public supply well samples, were used
to characterize the spatial distribution of arsenic and
deter-mine what controls the amount of arsenic in
groundwater.
Elevated arsenic concentrations (Figure 2) are as
likely to be found in the shallow Glasford Aquifer as in
the Mahomet Aquifer. There are two primary areas of
Bloomington-
Normal
Peoria
Champaign-
Urbana
Champaign
Ford
Iroquois
Vermilion
Macon Piatt
DeWitt
Tazewell
McLean
Mason Logan
00 1100 2200 3300
Miles
Illinois River