Heavy use draining aquifer

Posted on Dec 06, 2009 by orlanlove.

The Jordan aquifer, a drinking water source for 300,000 Iowans, has lost significant pressure and volume as a result of human usage since the mid-19th century, according to a yearlong study by the Iowa Geological and Water Survey.

The most significant declines have occurred in areas where the aquifer has been most heavily used for the longest time — in Linn, Polk and Webster counties, said State Geologist Bob Libra, who will present findings of the aquifer study at 1 p.m. Thursday at the Linn County Conservation Board’s Wickiup Hill Learning Center.

Libra emphasized that the situation is not critical — that the aquifer, Iowa’s most dependable, productive and widespread source of ground water (see map 14A), still contains bountiful, high-quality supplies. What is critical, he said, is “assuring the long-term sustainability of the aquifer” in the face of likely increased use.

While neither Cedar Rapids nor Iowa City depends on it, drawing their municipal water instead from shallow alluvial wells, many other Eastern Iowa cities and towns, including Marion and North Liberty, do rely on the Jordan. Farms and industries also tap the Jordan, whose water has recently come under increasing demand for ethanol plants and geothermal heating and cooling systems, Libra said.

At current withdrawal rates, he said, a state law prohibiting water-level declines of more than 200 feet from a 1975 baseline would likely be breached within 20 years in parts of Linn and Webster counties.

The Jordan aquifer, which underlies almost all of the state, is not at all analogous to an underground lake, said Libra, who explained that water is stored in pores between sandstone grains and in rock fracture zones.

Because most of the aquifer is relatively deep, it replenishes slowly.

Water pumped today in Marion, where the aquifer is 1,200 feet deep, “probably fell as rain hundreds of years ago,” he said.

With the water pumped from the aquifer greatly exceeding that which recharges it, water levels have declined by as much as 250 to 300 feet since it was first tapped in the 1800s, Libra said.

Those declining levels refer to the distance water rises when a well taps a confined aquifer, whose internal pressure exceeds that of the atmosphere.

Water withdrawn from the aquifer reduces the volume of water in storage and the pressure within the aquifer, Libra said.

From its outcrop belt in northeast Iowa, the aquifer dips southwestward beneath the landscape at about 18 feet per mile. This gradual tilt buries the aquifer to depths greater than 1,200 feet in Linn County and nearly a half mile in south-central Iowa.

As the aquifer deepens, the dissolved mineral content increases, making the water less desirable.