Where It Comes From


The Mahomet Aquifer is the remnant of the prehistoric Mahomet river valley. Approximately 1.6 million years ago, when our evolutionary ancestors were beginning to walk upright and use stone tools, the valley in which the aquifer lies was a large stream valley that meandered across what is now east-central Illinois. Meltwater from the glaciers approaching from the north inundated the valley with sand and gravel, until the region was overrun by glaciers. When the glaciers finally retreated some 13,000 years ago, the sand and gravel filling the former stream valley had been buried beneath hundreds of feet of clay-rich materials that had been deposited by the glaciers. The porous, water-saturated sand and gravel that makes up the aquifer was effectively trapped on the bottom and sides by the bedrock, and on the top by the cap of nearly water-tight, clay-rich glacial till.
The only known location where water enters the aquifer at a relatively rapid rate is in Champaign County, where glaciers deposited thin layers of sand and gravel within the overlying clay. Still, water movement within the aquifer is slow. Rain and snow that falls on the surface in Champaign County begins a roughly 3,000-year journey downwards to the Mahomet Aquifer, traveling at an average rate of less than an inch a year. Once it reaches the aquifer, it travels laterally in every compass direction but south. After about 7,000 years, water that journeyed westward seeps into the Illinois River along the river bottom near Havana, Illinois.
As it turned out, the aquifer the company had tapped was actually the Glasford Aquifer, a smaller but similar sand and gravel reservoir that overlies the Mahomet Aquifer. In the 1940s, deeper wells drilled by the Illinois State Geological Survey (ISGS) revealed the seemingly limitless water resources of the Mahomet Aquifer. Wells were yielding water at a rate of 3,000 gallons a minute.
Aquifers are like sponges. Although you can't see the water, it is there, filling the spaces between particles. Materials made up of particles that are tinier than sand, such as clay, hold water molecules tightly


Aquifers are like sponges. Although you can't see the water, it is there, filling the spaces between particles. Materials made up of particles that are tinier than sand, such as clay, hold water molecules tightly within their many pores, making the water difficult to extract. Aquifers made up of relatively large particles, like sand and gravel, create sizable voids between the particles so that water has plenty of space to fill and can flow easily. The Mahomet Aquifer is made up of these latter materials.

Protecting the Aquifer


The notion of protecting groundwater is as recent as the 1960s. Until then, aquifers had been largely out of sight and out of mind for most people. What's more, data about these resources--even their size and location--were scant. Surface-water resources could be mapped from a few good aerial photos. Documenting the depth and breadth of water locked within the subsurface required geologists to drill wells and extrapolate from the data collected at these scattered points and with other data about the geologic structures. More sophisticated technologies are providing more data at lower cost, but geologic mapping in the flat, glaciated terrain of Illinois is still a labor-intensive, costly endeavor. With limited data, though, the potential for error is great; until the 1980s, scientists believed that the Mahomet Aquifer was part of a single system of aquifers called the Teays-Mahomet system, which stretched all the way to West Virginia.
Protecting groundwater resources from possible contamination has yet to capture the public's imagination in much of Illinois; however, it is a major concern for public health agencies. Nearly half of the state's, for that matter the nation's, population depends on groundwater for its drinking water; in rural areas, that figure climbs to more than 90%.
Deep aquifers, like the Mahomet Aquifer, are not immune from contamination, but they are better shielded than most. Water quality problems that now exist are naturally occurring. For instance, water pumped from the Mahomet Aquifer is "hard" due to dissolved minerals, especially calcite and dolomite. They pose no health risk, but the minerals cause a scaly buildup in pots and water pipes if they are not removed with commercially available softening systems. The iron that is abundant in Mahomet Aquifer water is also harmless, but it is a nuisance because it discolors ceramic fixtures if it is untreated. One naturally occurring contaminant that may pose a health risk in some areas is arsenic. It appears to
Clay-rich tills tens to hundreds of feet thick were depositied beneath the glaciers of the Pleistocene Epoch (top). The result was the entrapment of the sand and gravel bodies between bedrock below and clay-rich till above--perfect conditions for the formation of aquifers (bottom). Zoom in to improve image clarity (try the right mouse button).


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