MAHOMET AQUIFER

The Mahomet Aquifer (Figure 1) consists of saturated glacial sand and gravel below fourteen counties, from the Illinois-Indiana state line west to the Illinois River. The aquifer lies on top of the bedrock, and in the eastern three-quarters of the area, it is deeply buried below younger fine-grained sediment.

Figure 1. Extent of the Mahomet Aquifer across Illinois (Roadcap et al., 2011).

Figure 2 is a map showing the major sand and gravel aquifers in Illinois (from ISGS 1996). These aquifers are generally within 300 feet of the ground surface, with bases typically within 500 feet. In Illinois, major sand and gravel aquifers can produce 70 gallons of potable water per minute. Potable water is defined as water containing less than 2,500 mg/l of total dissolved solids. Major sand and gravel aquifers predominantly consist of Quaternary deposits within preglacial bedrock valleys or alongside contemporary streams and rivers. These aquifers are frequently separated from shallower aquifers by layers of less permeable till or fine-grained lacustrine sediments.

Figure 2. Major sand and gravel aquifers in Illinois (from ISGS 1996)

The Mahomet Aquifer is composed of sand and gravel deposited by glacial meltwater, which flowed westward through the Mahomet Bedrock Valley during the Pre-Illinoian glacial episode. This bedrock valley constitutes the western section of the Teays-Mahomet Bedrock Valley System, which extends from Indiana into Illinois (Larson et al., 2003). The Mahomet Bedrock Valley was part of a considerably larger river system known as the Teays-Mahomet River System, which historically facilitated water drainage from the Appalachian Mountains westward toward the Ancient Mississippi River (Figure 3).

Figure 3. The proposed Teays drainage system across Illinois, Indiana, Ohio, and West Virginia (Mehnert, E et al 2004).

On multiple occasions, beginning about 2.6 million years ago, glaciers covered much of the Upper Midwestern United States (Killey, 2007). As glaciers advanced through east-central Illinois, meltwater flowed through the Mahomet Bedrock Valley, carrying significant sediment. This meltwater filled much of the valley with sand and gravel. By the end of the Illinois Episode glaciation, the valley was filled.

According to Locke et al 2018, Unconsolidated sediments from three glaciation events—Wisconsin, Illinois, and pre-Illinois—cover the bedrock surface and fill the preglacial valleys. The lowest unit in the Mahomet Bedrock Valley is sand and gravel, known as the Mahomet Sand Member, deposited by rivers that drain the glaciers. In some areas, this sand and gravel are covered by glacial till from the pre-Illinois glaciation. However, ice and water erosion during the following Illinois glaciation removed much of the older sediment. The eroded landscape was then covered by glacial sand, gravel, and till from the Grigg tongue and Vandalia Member. Similar erosion occurred later in the Illinois glaciation and at the start of the Wisconsin glaciation, when the Pearl Formation and Ashmore Tongue were deposited. In some places, erosion during different events affected the same areas. This repeated erosion and deposition cycle created overlapping sand and gravel deposits. Roadcap et al. (2011) referenced Soller et al. (1999) to identify the Mahomet aquifer as the lowest deposits of preglacial and glacial sand and gravel (Mahomet Sand Member) in the Mahomet Bedrock Valley within the Banner Formation (Figure 4).

Figure 4. Diagrammatic stratigraphic column of glacial deposits (lithostratigraphic units) in east central Illinois (Soller et al., 1999). Hydrogeologic structure depicts the Mahomet aquifer (A), aquifers in the Banner Formation (B), aquifers in the lower Glasford Formation (C1), aquifers in the upper Glasford Formation (C2), and shallow and surficial aquifers (D) recreated from Roadcap et al., 2011. The Mahomet Sand Member and the Sankoty Sand Member of the Banner Formation (lithostratigraphic units) are outlined in blue.

A hydrologic conceptual model of flow in the Mahomet aquifer system (Figure 5) was developed by Roadcap et al. (2011) to represent the different flow processes within the Mahomet aquifer system that are represented in greater detail within the numerical groundwater flow model used actually to calculate flows. Recharge to the aquifer predominantly occurs through the vertical movement of water from precipitation that infiltrates the land surface within the aquifer's boundaries. The eastern portion of the aquifer is under confined conditions, deeply buried by low permeability glacial tills and lake sediments, resulting in low average recharge rates ranging from approximately 6 inches per year to less than 0.5 inches per year. This recharge primarily occurs through limited connections to shallower coarse-grained materials and adjacent streams. The recharge rate cannot be directly measured; it is calculated through mass balance calculations in the groundwater flow model constructed for the Mahomet aquifer system. The western end of the aquifer, near the Illinois River in Mason and Tazewell Counties, has no confining layers. In this area, the land surface elevation decreases significantly, exposing permeable aquifer sands that facilitate rapid recharge exceeding 12 inches per year.

Figure 5. Conceptual flow model in the Mahomet aquifer (Roadcap et al., 2011).

According to Locke et al 2018, Groundwater flow directions and recharge and discharge areas for the eastern segment of the Mahomet aquifer can be determined from a contour map of water level measurements from the 141 wells, known as a potentiometric surface map (Figure 6). Similar to water on the land surface, groundwater flows from high areas (lighter shading) to low areas (darker shading). The map shows groundwater flow is divided into several sub-regional flow systems with two prominent features: a high potentiometric surface near Paxton in Ford County and a large cone of depression at Champaign. Pumpage in Champaign-Urbana averages around 23 million gallons per day (mgd), which has lowered water levels by up to 100 feet and caused a flow reversal in Piatt and western Champaign Counties. All groundwater withdrawn at Champaign is balanced by local precipitation and stream leakage.

Figure 6. Composite potentiometric surface map of the Mahomet aquifer based on measurements from 1990 to 2009.