The 1990 population in the LIRB was 1.3 million (U.S. Department of Commerce, 1992).
Over 50 percent of the population lives in the Counties of Macon, McLean, Peoria, Sangamon, and
Tazewell. The four most populated cities in the basin in 1990 are as follows:Peoria (113,504),
Springfield (105,227), Decatur (83,885), and Bloomington (51,972). The basin population
increased approximately 5 percent from 1970 to
1980 and peaked at 1.4 million in 1980 (fig. 4).
Several counties had increases in population from 1980 to 1990, but the total basin population
decreased approximately 7 percent from 1980 to
1990 (fig.
5).
The population distribution reflects the extent of rural areas in the LIRB. Agriculture is the
predominant land use (fig. 6).
In 1990, about 87 percent of the land use in the LIRB was agricultural
(Hitt, 1994). The major agriculture use is cropland, typically corn and soybean. The sandy soils in
Mason and Tazewell Counties are used for specialty crops, such as pumpkin, watermelon, sun-
flower, pea, cucumber, sweet corn, lima and green beans, and popcorn (Walker, Bergstrom, and
Walton, 1965). Mason County has the largest harvested acreage of watermelon (925 acres); Cass
County has the largest harvested acreage of sunflower (2,244 acres); Tazewell County has the larg-
est harvested acreage of pumpkin (5,320 acres); and McLean County has the largest harvested acre-
age of lima bean (2,536 acres) (Neely and Heister, 1987). This area in Mason and Tazewell
Counties commonly is irrigated with shallow ground water (less than 200-ft deep) to keep the crops
healthy in the extremely well-drained soil. The remaining 13 percent of the land area in the basin
is forests (8 percent), urban areas (2 percent), water and wetlands (2 percent), and miscellaneous
land-use areas (1 percent). Miscellaneous land use includes at least 39 nature areas that are mostly
tallgrass prairie, but less than 0.01 percent of the original prairie remains (Illinois Department of
Conservation, 1991) (table 1). Illinois ranks
49th among states remaining in its original vegetation
type—only Iowa ranks lower (Iverson and others, 1991).
The second largest land use is forest. Forest land prevents deterioration of the quality of water
by reducing soil erosion, sustaining natural plant and animal communities, and maintaining bio-
logical diversity. Many counties have forest lands that have the capability and are potentially avail-
able to produce commercially valuable trees (Iverson and others, 1991). Forty percent of the land
use in Calhoun County is forest land. Oak-hickory forests account for about half of the acres of
Illinois forests and a majority of these are older than 60 years (Iverson and others, 1991). Maple-
beech forests also are found in western Illinois, but white pine is most common in the western part
of the State where it has been planted extensively (Iverson and others, 1989). The number of tree
species varies from 38 in Warren County to 88 in
Peoria County (fig. 7). Rare plants are found
in
abundance in forests. Forty-six percent of the threatened and endangered plants of Illinois are
found in forests (Iverson and others, 1989).
The development of soils is determined by parent materials, climate, plants and animals,
topographic relief, and time. The soils in the LIRB developed mostly in thick loess with some loess
thicknesses greater than 60 in. Thin loess (10–40 in.) soil is found in the northeastern part of the
LIRB (Neely and Heister, 1987). Soils developed on sandy to clayey alluvial sediments are found
near major streams. A multicounty area south of Peoria, along the Illinois River, has sandy soil with
a high permeability (fig. 8);
therefore,
the aquifer is vulnerable to contamination (Berg and others,
1984). The potential for contamination is affected by the infiltration or attenuation rate of the soil.
