Hundreds of feet below southern Minnesota, the Mt. Simon aquifer provides drinking water for more than 1 million Minnesotans. It was once so robust the water forced its way to the surface in some spots, a phenomenon known as a "flowing well."
Those days, however, are long gone. Mt. Simon water levels have dropped by as much as 200 feet in some areas since pioneer settlement times.
While there's no danger of the aquifer going dry, and a recent analysis showed water levels have stabilized, officials worry pressures on the aquifer are intensifying, from above ground and below.
Mankato relies on water from Mt. Simon to mix with river water that is contaminated with nitrates from nearby cropland, said City Manager Pat Hentges.
Blending the aquifer water with river sources keeps city drinking water within safe nitrate limits, he said. The nitrate problems could lead the city to drill deep into the Mt. Simon aquifer for more water.
"Should we just dig another deep well? That's always an option. Probably the cheapest option. But not, I think, consistent with what our council feels," said Hentges.
Mankato is trying to avoid depending too much on Mt. Simon. One reason the City Council put in the river wells was to relieve pressure on the aquifer.
"They have a strong spirit of stewardship," said Hentges.
But nitrate pollution may force the city to act. Hentges said the most likely alternative to a new Mt. Simon well is to construct a nitrate removal facility, but that's at least three times the cost of a well.
Regulators will be closely watching new demands for Mt. Simon water and tracking an extensive network of wells as they monitor the aquifer's health, said Jim Berg, a research scientist with the Minnesota Department of Natural Resources.
A 1989 law restricting new Mt. Simon wells in the Mankato area has helped, he said, although there's no data showing exactly how much.
Heavier rainfall in recent years also played a role, but not because it's filtering down to the aquifer and boosting water levels. Berg said it's more likely that increased precipitation helps by reducing demand for aquifer water for lawns and other summer uses.
The reality is a single drop of water can take decades, or even centuries, to reach Mt. Simon.
The aquifer's big test will come during the region's next multi-year drought, Berg said.
"If we can make it through a period of lower than normal precipitation," said Berg, "and the aquifer levels can remain relatively stable, no dramatic drops, that will mean that our conservation efforts are working."
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