Deep aquifers, deep questions

Sealing the well
An employee of a well drilling company works on sealing a Mankato city well near the Minnesota River. Monitoring equipment will be sealed inside the casing to help monitor the health of the Mt. Simon aquifer.
MPR Photo/Mark Steil

One of the deepest and oldest aquifers is the state lies under the farmland of south central Minnesota and extends into the Twin Cities metropolitan area.

Hundreds of thousands of people rely on the Mt. Simon aquifer for water. It's basically saturated sandstone, anywhere from 600 to more than a 1,000 feet underground. Tests shows some of the aquifer's water is 30,000 years old, but there are isolated problems of supply in the formation. Research is underway to determine the health of the Mt. Simon aquifer.

In Mankato, near the Minnesota River, a worker is shoveling sand and concrete into an old city well to seal it up. Before it's completely closed, the defunct well is also being loaded with monitoring equipment. The devices at the bottom of the old well casing will keep track of water levels in the Mt. Simon aquifer long after the well is sealed.

Leo Getsfried
Leo Getsfried works for the Minnesota Department of Natural Resources Waters Division in the Mankato area.
MPR Photo/Mark Steil

Standing nearby, Leo Getsfried with the Minnesota Department of Natural Resources, said the Mt. Simon aquifer is the most productive underground water source the region has. He said to insure its long term health, the amount of groundwater reaching the aquifer must match what's being drawn out.

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"We know that there's approximately 3.2 billion gallons per year that's being pumped out of the aquifer in this Mankato vicinity," Getsfried said. "What we don't know is whether the recharge, or the infiltration into this aquifer from up above, is occuring at a rate comparable to that 3.2 billion."

It's common to think of an aquifer as an underground lake, but that's inaccurate. Think instead of sand soaked with water. The water doesn't flow, but rather trickles slowly down between the grains of sand. Pump too much water out of an aquifer and it may take years for a similar quantity of water to return.

In the Mount Simon aquifer there are so-called 'cones of depression' where excessive pumping has left dry spots. Getsfried said Mankato-area city and industrial users -- which depend on the Mt. Simon aquifer -- are paying for the monitoring site. Eventually, they hope to have three monitoring locations.

Getsfried said if the monitors show water is being pumped too fast, some drastic steps might be taken.

"It's a very complicated system."

"If it looks like it's continuing on a pretty steady, longer-term basis that will obligate us to probably look at some means of reducing that level of withdrawal from the aquifer," Getsfried said.

The Mankato-area monitoring is just one part of an effort to learn more about the Mt. Simon aquifer. Last spring the state legislature approved more than a $1.5 million for another Mt. Simon monitoring effort to start later this summer. The Legislative-Citizen Commission on Minnesota Resources, the LCCMR, will oversee the three year project.

Geologist Jeff Broberg serves on the commission. He said monitoring wells will go in at 21 locations. He said besides measuring water levels, the study will also test the age of the water. Broberg said there's rarely a uniform age for the moisture held in an aquifer.

"It's a very complicated system because of the landscape and the flow of the water and the ability of the water to percolate down through the various types of rocks with their various porosities," he said.

Broberg said neighboring pockets of water in an aquifer may differ in how old they are by thousands of years. He said age is an indicator of supply because it demonstrate how fast water is reaching the aquifer.

"Is it sitting there for tens of thousands of years, is it sitting there for 50 years, or is it in there for a day or two?" Broberg said.

Water planners in the Twin Cities are also interested in what the monitoring efforts turn up.

After a drought 20 years ago the state legislature became so concerned about the Mt. Simon aquifer that they banned industrial pumping from the formation in the seven-county metro area.

Household use of the aquifer there is allowed only if it's the sole practical option. Metropolitan Council water supply planning manager Chris Elvrum says the demands on the aquifer are likely to grow, especially in the west and north metro.

"In parts of those areas the choices they have are the Mt. Simon or one or two other aquifers that are often less productive," Elvrum said. "So, those communities as they develop would probably be most interested in using Mt. Simon water."

Before that happens it's likely there will be several years of study. Part one are the monitoring wells. They'll be out there every day, digging deep, scooping up important information, filling in the deep gaps of knowledge about the Mt. Simon aquifer.