Hydrogeology of the Lower Illinois Region

Groundwater along with surface water are the major sources of water supply to communities in the Lower Illinois Water Supply Planning Region (WSPR). The occurrence of abundant groundwater is highly dependent on geology.

Groundwater is stored in rocks within fractures or pores between mineral grains. Rock units that readily transmit water to wells and springs are referred to as aquifers. The greater the capacity of an aquifer to store and conduct water, the greater the transmissivity. The greater the transmissivity, the more likely wells are to be productive. Drilling a well is costly so understanding transmissivity of an area enables groundwater specialists to select locations for wells that will be good investments for water supply.

The Physiographic Regions of Illinois (Figure 2, left) help refine area of different geologic history in the state. In the Lower Illinois Region, four physiographic regions are relevant. Two of these regions, the Galesburg Plain and the Springfield Plain, are landscapes formed by a glaciation event that covered and is named after the state of Illinois. These regions are largely composed of till, a glacial deposit with low transmissivity. The Lincoln Hills is a more weathered landscape defined by bluffs overlooking the two rivers. The Salem Plateau, directly south of Lincoln Hills, is separated from the other regions by an ancient geologic fault.

The most transmissive areas occur along the Illinois and Mississippi River Valleys (Figure 2, right). This is because those valleys store water in highly transmissive materials such as sand and gravel. The transmissivity of the river valleys is more than 10 to 100 times greater than that of the till plains that host the lowest transmissivity values. These are the most productive sources of water in the region (Figure 2) and are up to  100 feet thick in Calhoun County . A  1957 Illinois State Geological Survey report  describes the sand and gravel deposits in the Illinois River Valley as "the best source of groundwater supplies for industrial and municipal use in the county." Areas of intermediate transmissivity are transitional areas between the till plains and river valleys, in the tributaries feeding into the rivers, and along the limestone bluffs in the Lincoln Hills.

The second primary groundwater supply unit in the Lower Illinois WSPR is the limestone bedrock aquifer. The transmissivity of this unit is not considered in Figure 2, because unlike the sands and gravels, it is usually at some depth. Generally, wells are drilled in this aquifer when an area does not have access to river sands and gravels, especially in Morgan and Macoupin counties. Productivity in these units is dependent on fracture flow, and the limestones is more weathered and transmissive where not overlain by glacial sediments such as in the Lincoln Hills and Salem Plateau.

This story map is part of a series about Water Supply Planning throughout the state of Illinois. Water Supply Planning is an ongoing research effort conducted by the Illinois State Water Survey and is funded by the   Illinois Department of Natural Resources Office of Water Resources.