Sankoty and Mahomet Sands, receive approximately 9.5 percent of the recharge from the bedrock
(Panno and others, 1994). The potential for aquifer recharge is a function of depth to the aquifer,
occurrence of adjacent major aquifers, and potential infiltration rate of soil. The potential for aqui-
fer recharge is highest along the Illinois River, especially in Mason and Tazewell Counties where
there is sandy aquifer material at the surface (Keefer and Berg, 1990).
The geometry of the buried bedrock valleys affects the configuration of the embedded glacial
drift aquifers and flow patterns. Near the Illinois River, in eastern Mason and Tazewell Counties,
is the intersection of two major buried bedrock
valleys—theMahomet and Mackinaw (fig. 13).
Based on thechloride concentrations and mass-balance calculations of selected constituents, only
about 17 percent of the ground water at the area of intersection is derived from the Mahomet Bed-
rock Valley (Panno and others, 1994). Most of the water at the confluence of the buriedMahomet
and Mackinaw Valleys is from recharge because it is a ground-water divide (Wilson and others,
1994). At the confluence, the water in the SankotySand of the buried Mackinaw Valley flows north
and discharges to the Mackinaw River, and the Mahomet Sand of the buried Mahomet Valley flows
southwest and discharges to the Illinois River.
Discharge to streams from shallow aquifersis affected by precipitation. Annual ground-
waterdischarge to streams for a year of normal precipitation is from 0.09 to 0.36 (ft3/s)/mi2 for
most of the basin, except for a small portion of the eastern LIRB where ground-water discharge is
0–0.1 (ft3/s)/mi2(Walton, 1965) (table 3).
The bedrock aquifers are in Mississippian- and Pennsylvanian-aged formations. The Penn-
sylvanian-age rocks, which underlie the glacial drift, are mostly shale with alternating carbonate
beds. These rockshave a low permeability and porosity, and yield small amounts of water to wells
from interconnectedfractures, joints, and bedding planes. The relativelylow permeability restricts
deep percolation. In the southern and western part of the basin, the bedrocksurface lies within 50
ft of land surface (fig. 21). Regional
flow in the deeper bedrock aquifers is toward the southeast in
the direction of the axis of the structural Illinois Basin. Recharge to the bedrock aquifers is limited
by low permeability loess, clayey silts, and glacial till. Hydraulic properties of the bedrock aquifers
vary less than the sand and gravel aquifers.
More than half of the drinking water, that water supplied for domestic use, in the LIRB is
from ground water. Forty-eight percent of the public-supply or municipal water is drawn from
ground water and52 percent from surface water. In 1988, the totalpublic-supply deliveries was
190 Mgal/d of which108 Mgal/d, or 57 percent, was for domestic use (Avery, 1995). In addition
to public supplies,25 percent of the population in counties in the LIRBare self-supplied. Fourteen
percent of the total domestic water is estimated to be self-supplied. If the self- supplied population
obtains the water from wells, then the percentage of water for domestic use is 55 percent from
ground water and 45 percent from surface water. In large parts of western and southern Illinois,
where shallow aquifers are not present or the ground water in aquifers is naturally of poor quality,
homeowners rely primarily on cisterns or
large-diameter dug or bored wells (fig. 22) (McKenna
and Keefer, 1991). These large-diameter wells are highly susceptible to contamination from sur-
face-water runoff and shallow ground water.
Ground-water withdrawals in the LIRBare mostly from sand and gravel aquifers andMis-
sissippian-Pennsylvanian-aged aquifers (Kirk, 1987). The primary uses of water from sand and
gravel aquifers is public supply, industry and mining, and domestic and commercial. The primary
