confined by thick tills. The water table in the
LIRB is in the till and in direct connection with streams and lakes. The shallow aquifers (less than
200-ft deep) are recharged from precipitation, but the deeper confined aquifers, such as in the
Sankoty and Mahomet Sands, also have recharge from the bedrock. Bedrock aquifers are in the
Mississippian- and Pennsylvanian-agedformations that yield small amounts of water to wells
from interconnected fractures, joints, and bedding planes.
Forty-eight percent of the public-supply or municipal water in the LIRB is drawn from
ground water and 52 percent from surface water. Mostground-water withdrawals are from glacial
sand and gravel, but in the southern part of the LIRB, private well owners rely primarily on ground
water fromcisterns or large-diameter dug or bored wells. In small basins with thermoelectric facil-
ities, over 50 percent of the surface water withdrawn is used for thermoelectric power.
The water budget of the LIRB was estimated based on precipitation, surface-water discharge,
and evaporation data. The inflow and outflow budget forthe LIRB is difficult to estimate because
many factors affect the flow of water in and out of the basin. The input of water from the upper
Illinois River Basin to the LIRB is about 20 percent of the total inflow to the basin. Total outflow
from the LIRB is mostly by evaporation, but surface-water flow is a substantial output.
There are five natural environmental divisions in the LIRB (Grand Prairie, Western Forest-
Prairie,Illinois River Bottomlands, Illinois River Sand Area, and Middle Mississippi Border). The
two natural environmental divisions that cover the most area are the Grand Prairie in the northeast-
ern half of the basin and the Western Forest-Prairie in the southwestern half of the basin. The Grand
Prairie had the largest percentage of original undisturbed prairie vegetation, and theWestern For-
est-Prairie is a dissected till plain withforests and prairie on the uplands. The fish in theIllinois
River include a number of pollution-sensitive species. The improvement in water quality in the
Chicago waterways have allowed nonnative species, such as the zebra mussel, to invade the Illinois
River from Lake Michigan.
Natural and human factors affect the physical, chemical, and biological characteristics in the
LIRB, which then affect the water quality. For example,diversity and balance in fish population
in the LIRB compared with the upper Illinois River Basin indicate improved water quality in the
LIRB. Soils and wetlands help prevent or filter contaminants moving to the ground water. Bedrock
structural features also affect the water quality of deep glacial aquifers. Bedrock and surface topog-
raphy, type of glacial material, and land use most directly affect water quality in the LIRB.
use with remote sensor data: U.S. GeologicalSurvey Professional Paper 964, 28 p.
Avery, Charles, 1995, Estimated water withdrawals and use in Illinois, 1988: U.S. Geological Survey Open-File Report
95–309, 52 p.
Bell, H.E., III, 1981, Illinois wetlands—their value andmanagement: Chicago, Illinois Institute of Natural Resources,
Doc. No. 81/33, October, 1981, 133 p.
Berg, R.C., Keefer, D.A., Demissie, Misganaw, andRamamurthy, Gana, 1997, Regional evaluation of ground water
and surface water interaction— preliminary method development and analysis:Illinois State Water Survey Mis-
cellaneousPublication 181, 37 p.
Berg, R.C., Kempton, J.P., and Cartwright, Keros, 1984, Potential for contamination of shallow aquifers inIllinois:
Illinois State Geological Survey,Circular 532, 30 p.
