Thursday, December 09, 2004

The Mahomet Aquifer, an Underappreciated Resource

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Do you know where your water comes from? Most of us don’t think twice about that, as long as the water flows when we turn on the faucet. But here in east-central Illinois many of us draw our water from the Mahomet aquifer, a largely under appreciated natural resource.

The aquifer is a layer of sand and gravel whose top lies between one and two hundred feet beneath the ground we walk on. It was deposited there in the valley carved into bedrock by the prehistoric Mahomet river more than five hundred thousand years ago. The clay-rich sediments that now cover the aquifer and constitute the ground as we know it were left by glaciers that later came down from the north and trapped the sand and gravel in the Mahomet bedrock valley. The aquifer holds more water than the finer soils above it because the coarser grains of sand and gravel leave more in-between space for water to fill.

Its great size is one of the remarkable characteristics of the Mahomet aquifer. It stretches about a hundred and twenty-five miles long, from its eastern edge north of Danville to its western edge near Peoria, and it’s typically from four to fourteen miles wide. The aquifer lies beneath 1.26 million acres and crosses the boundaries of fifteen counties, as well as many other political entities. Geologists calculate that the Mahomet aquifer holds about four trillion gallons of water, about the volume of water that flows past St. Louis in the Mississippi river every two and a half weeks.

The water in the Mahomet aquifer is of remarkably high quality. Most of it is between one thousand and twelve thousand years old, which means it fell as rain at a time before it could pick up the industrial pollutants and synthetic pesticides that plague our surface water today. Water from the aquifer is also free of bacteria harmful to humans, having been isolated from the sources of such organisms for so long.

The only contaminant that poses an immediate concern for users of the aquifer is arsenic, which occurs naturally at concentrations high enough to pose a risk to humans at various sites throughout the region. Municipal systems remove arsenic, but well users must test for it.

The most pressing question concerning the Mahomet aquifer is how much water we can pump from this remarkable resource without depleting it. The frustrating non-answer to that question is, we don’t know—not yet, anyway. In order to find that answer, we’ve got to fund more science.

Beyond the issue of identifying sustainable levels of withdrawal from the Mahomet aquifer, we will eventually need to grapple with the question of how that water is allocated. That’s a complicated question since aquifers don’t conform to political boundaries. But it’s also one best addressed before anyone’s well runs dry.