The layer cake beneath Park Rapids

The Straight River Watershed presents a lot of unanswered questions for those working to develop a water management plan for the area. But it also serves as a great example of how in Minnesota using and protecting groundwater is complicated and very local.

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For starters, what lies beneath the surface of the 75-square-mile watershed?  In general, the area was formed by the outwash from two melting glaciers.  From surface to bedrock there are more than 400 feet of alternating layers of sand and clay.

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straight river GMA map

The sandy layers contain water.  Hydrologists have a good idea of the location of the top aquifer which can be tapped by wells 30 to 70 feet below the surface.

But according to DNR Groundwater Specialist Michele Walker , the area has very complex geology.  Walk 10 feet in any direction and what’s invisible under your feet might change dramatically.

For example, under the top aquifer is a layer of clay that is believed to range from 10 to 70 feet thick.

Under the clay is another layer of sand filled with water.  This is the aquifer Park Rapids is tapping for its new city water supply.  And more irrigation wells are being drilled into this second aquifer according to DNR hydrologist Darrin Hoverson, because farmers want to reduce pumping from the shallow aquifer and mitigate the effect of irrigation on the Straight River and wetlands in the area.

What’s known as the first confined aquifer will be mapped in the next few years.  Its size is uncertain, but evidence suggests it’s connected to the surface aquifer, said Walker.  Well levels indicate water levels change at the same rate in both aquifers.  The surface aquifer is connected to the Straight River.  Pumping from that aquifer will lower the river level.  Does that mean too much pumping from the second aquifer will affect the river?  That’s a question yet to be answered.

And does the apparent connection between aquifers mean the second aquifer will eventually be contaminated by nitrates?  Michele Walker said that could happen.  But the protective clay layer will help.  And the iron rich water that needs to be treated so it doesn’t stain sinks toilets or clothing could also protect against nitrate contamination.  .

Research shows iron in water reacts with nitrates, in a process called denitrification.  So that aquifer might just have a natural nitrate treatment system.

There is a third aquifer some 250 feet below the Straight River watershed.  That water is high in ammonia.

Interestingly, a city official says when they drilled in to that bottom aquifer the drill brought up bits of wood and vegetation deposited  10,000 to 12,000 years ago by the melting glaciers that created  the aquifers.