Physiography
It is necessary to identify the physiographic sub-sections, as this shows the dominant glacial depositional environment, morainal topography, and variations in depositional history. These factors affect soil permeability, and transmissivity of unconsolidated materials, and consequently the discharge rates to the underlying bedrock aquifers. The majority of the Lower Illinois River Basin (LIRB) is in the Tills Plains Section of the Central Lowland physiographic province. A small area near the confluence of the Illinois and Mississippi Rivers is in the Lincoln Hills Section, which is part of the Ozark Plateau Province.

The Till Plains Section is composed of four subsections; the Bloomington Ridged Plain (38%), the Galesburg Plain (26%), the Springfield Plain (35%), and the Kankakee Plain (.5%). Landforms in each of the subsections are the result of glaciation. The site selection process was narrowed to the first three subsections, as they make up 99% of the Till Plains Section, which is the majority of the basin (98%). The Bloomington Ridged Plain Subsection includes the Wisconsinan glacial moraines and the associated glacial topography. The Galesburg Plain Subsection and the Springfield Plain Subsection were grouped together because they are both distinguished by the older Illinoian drift.

Bedrock Valleys
In our study, bedrock valleys are defined as buried bedrock that is less than 500 feet above mean sea level with large topographic relief on the bedrock surface. The major buried bedrock valleys within the LIRB are the Mackinaw Valley, the Illinois Valley, and the Mahomet Valley. The Mahomet Valley is differentiated from the other bedrock valleys because it contains some of the most highly productive, non-alluvial sand and gravel aquifers in the southern three-fourths of Illinois. The aquifers associated with the buried Mahomet Valley provide the only large source of irrigation, industrial, and municipal supplies of ground water in east-central Illinois. Five sections of interest exist at this stage of the stratification. The sections are the Bloomington Ridged Plain with no bedrock valley, the Bloomington Ridged Plain with bedrock valley, the Bloomington Ridged Plain and Mahomet bedrock valley, the Springfield/Galesburg Plain with no bedrock valley, and the Springfield/Galesburg Plain with bedrock valley.

Land Use
Agriculture is the predominant land use (87%) in the LIRB. The major crops grown are corn and soybeans. The rest of the land use in the LIRB is forest (8%), urban (2%), water and wetlands (2%), and a remainder of miscellaneous land-use areas (1%). The land use category, agriculture, is important because it affects water-quality issues. Urban land use also affects water-quality, but because land use is such a small percentage of the LIRB, it was best suited to locate sites of study to areas representative of agriculture.

Equal Area Distribution Program
Computer software was written to randomly select sites for a ground water quality sampling network. It is desirable to collect ground-water-quality samples from various areas in a study region that have different values of a spatial characteristic, such as land use or hydrogeologic setting. The software subdivides the study region into areal subsets that have a common spatial characteristic to stratify the population into categories from which sampling sites are selected. Sites are selected from each category of areal subsets.

A population of potential sampling sites may be defined by either specifying a fixed population of sites, or by preparing an equally spaced population of potential sites. In either case, each site is identified with a single category, depending on the value of the spatial characteristic of the areal subset in which the site is located. Sites are selected from one category at a time. One of two approaches are used to select sites. Sites may be selected randomly, or the areal subsets in the category can be grouped into cells and sites selected randomly from each cell (Scott, 1990).