Illinois Groundwater Protection Program Report-January 2002 Page 23
ISGS/ISWS Groundwater Assessments
The Scientific Surveys have been involved with a variety of activities throughout 2000 and 2001.
In 2000 and 2001, staff from the ISWS and ISGS continued to serve the Mahomet Aquifer
Consortium (MAC) as technical advisors. The MAC was formed in November 1998 to
“...further study the Mahomet Aquifer on a regional basis and to develop a plan for the
management of this valuable resource.” The MAC is composed of members from water
authorities, water companies, local, county, and state agencies, and the general public, all with an
interest in maintaining an adequate and good quality water supply from the Mahomet Aquifer.
Over the last two years, the ISWS has made a concerted effort to continue collecting data on the
Mahomet aquifer while the MAC works to secure federal funding for an aquifer-wide study. In
June 2000, the ISWS added two more observation wells to the Mahomet Aquifer Network. These
wells are located near the towns of White Heath and Cisco and are 297 and 268 feet deep,
respectively. The observation well network now contains 97 wells, most of which are measured
on a quarterly basis. In addition, during the summer of 2000, the ISWS collected water samples
from 21 wells (12 private wells, eight observation wells owned by the city of Decatur, and the
new ISWS observation well at White Heath) in an area just west of the Piatt-Champaign counties
line. Based on a 1994 review of historical groundwater quality data by the ISGS, it has been
theorized that the Mahomet Aquifer is being recharged from underlying bedrock containing
groundwaters of relatively high salinity in this area. These recent ISWS sampling results add
support to this hypothesis. Finally, during the spring of 2001, an ISWS review of recently
discovered data with funding from the city of Decatur, has provided evidence of a hydraulic
connection between the Mahomet aquifer, the overlying Glasford aquifer, and the Sangamon
River in the vicinity of Allerton Park in Piatt County. The results of this investigation suggest
that operation of Decatur’s Cisco well field has a direct influence on flow in the Sangamon
River, thereby greatly reducing the effectiveness of pumping the water into the Sangamon River
for delivery to Lake Decatur (Roadcap and Wilson, 2001).
The Illinois DNR Critical Trends Assessment Program (CTAP)-- The primary goal of CTAP is
to conduct statewide and regional assessments of environmental conditions. Under this program,
the geology and water resources of many Illinois watersheds have been described. In 2000 and
2001, the following watersheds have been completed– Lower Des Plaines River; Calumet;
Lower Sangamon River; Vermilion River (Illinois River Basin); Kinkaid; and the Chicago
River/Lakeshore. All of the technical volumes for these areas have been printed and are available
from the DNR clearinghouse, http://dnr.state.il.us/orep/c2000/assessments/.
A major goal of ISGS geologic mapping is to define groundwater resources and to protect
groundwater quality. The ISGS completed mapping in Jo Daviess and Lawrence counties in
2000 and produced a set of geologic maps including maps of bedrock geology and aquifer
With funding from the IDA (1995 - 2001), the ISGS and ISWS have installed 191 monitoring
wells in 57 counties across the state to monitor for pesticides in selected hydrogeologic settings.
Between September 1998 and December 2000, 160 of the 191 wells were sampled as part of a
one-time sampling effort. In addition, from October 1997 through July 2000, a set of six wells
was sampled on a monthly basis with another four samplings between August and December
2000. Atrazine was the most commonly detected pesticide in the one-time sampling effort, being
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