A map of the Mahomet Aquifer is shown at top. Below a cross-sectional view is drawn along the axis of the aquifer at its thickest point (red line). Sand deposits in the clay-rich till serve as productive aquifers and provide avenues for replenishing the Mahomet Aquifer with surface water. (Zoom in to improve image clarity.)

The Ogallala, the nation's largest aquifer, which underlies portions of eight High Plains states, is dropping by as much as 3 feet a year. The causes are low recharge rates and high water demands. A similar imbalance could occur in Illinois. The cities of Bloomington, Normal, and Danville, with a combined population of 135,000, have expressed interest in tapping the Mahomet Aquifer. Further, the city of Decatur installed a 25-million-gallon-per-day well field within the Mahomet Aquifer as a backup supply, which it uses periodically.
be leaching from oxidized pyrite--or iron sulfide, commonly referred to as fool's gold--within the aquifer, the source of which was coal-bearing bedrock beneath portions of the aquifer west of the Piatt-Champaign county line. Within the aquifer above the shale bedrock, arsenic concentrations range from 20 to 70 parts per billion (ppb), which may exceed the new lower limit of 20 ppb under consideration by the U.S. Environmental Protection Agency (the current limit is 50 ppb). Fortunately, arsenic can be effectively removed with commercially available water-treatment systems.
As with other deep aquifers, the greatest threat to the continued viability of the Mahomet Aquifer comes not from contamination but overpumping; that is, removing water faster than it is replaced. Water consumption from the aquifer now averages 84 million gallons a day. Of that amount, approximately 38 million gallons is consumed by the region's 800,000 residents, 24 million by industry, 15 million by commercial enterprises, and the remaining 7 million gallons by miscellaneous uses, such as irrigation. (These estimates are extrapolated from water consumption data for Champaign-Urbana, the only data readily available).
The rate at which water is recharging the aquifer has been thought to be hundreds of millions of gallons per day, but that estimate may be overly optimistic. Recent studies of the aquifer by scientists from the ISGS and the Illinois State Water Survey indicate that well-water levels around Champaign and Urbana are dropping. If the original estimates are incorrect, the surplus could vanish with the addition of a few high-demand users
  To prevent future problems with water supply or contamination, the Mahomet Aquifer Consortium, consisting of 70 representatives from water companies; local, county, and state governments; and the public are developing a study and management plan for the aquifer. If they obtain the necessary funding and complete the study, they'll be able to use computer models of the aquifer system to predict water levels in the aquifer under an array of variables for 20 to 50 years in the future. One of the things the research will determine is the location of recharge areas. Recent carbon-14 analyses of water by ISGS geochemists indicate that the water below Champaign County is 5,000 to 7,000 years younger than water 50 miles to the west, which told them that the aquifer was mostly recharging-refilling in the northern part of Champaign County.
Not surprisingly, water conservation and protection measures are a tough sell. Planning for the long term is not one of the state's nor the nation's strengths, says former Senator Paul Simon, who has written extensively about water-related issues. Warning Illinoisans against complacency, he says that east-central Illinois is not immune from water scarcities. Even if water usage here remains within the aquifer's recharge rate, he says, "Southwestern states, which already face severe shortages, are likely to demand the right to purge the water here. There is no escaping."
Planning now will provide the state with the information it needs to combat such requests and prevent overpumping, as well as potential contamination. Given that it takes many thousands of years for water to flow through the Mahomet Aquifer, the development of a management plan for the next 20 to 50 years is a mere drop in the bucket.

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