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Groundwater and aquifers in Minnesota

Groundwater is held in underground aquifers, which are geologic formations (sediment or rock layers) or groups of formations capable of yielding water to a well. An aquifer can be defined using a combination of geologic and practical features.

The geologic definition of an aquifer is a layer full of water and permeable geologic material. The practical description of an aquifer is that it can release significant quantities of water when pumped.

Contrary to popular belief, aquifers are not large underground lakes. Instead, they’re usually made up of large amounts of water flowing in between pore spaces and fractures. Think of a jar of marbles or cracks in underground rock filled with water.

Types of aquifers

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The makeup of aquifers in Minnesota

Three main kinds of geologic features hold water in aquifers in Minnesota.

Glacial sands and gravel aquifers

In Minnesota, where the state has been covered in ice at one time or another, sand and gravel deposited primarily by glacial meltwater streams is a common aquifer type. These may be at the surface or buried, of limited extent, or covering hundreds of square miles.  They are mostly located in Central Minnesota.

Map of glacial sand and gravel representation in Minnesota.

Sedimentary rock aquifers

Long before Minnesota was covered in glaciers, much of southeastern and extreme northwestern areas of the state were covered by an ocean that deposited extensive and thick layers of sand, mud and lime to form sandstone, shale and limestone (or dolostone) aquifers. These are high-yielding aquifers and are regional in extent, covering and shared by much of the midcontinent.

Open cliff face from a road cut in Southeast Minnesota made up of sedimentary rock deposited in layers with many fractures between the layers.
Sedimentary rock in southeast Minnesota
Waterfall over a large cliff of igneous rock in Northern Minnesota.
Igneous rock with waterfall in northern Minnesota

Igneous and metamorphic rock aquifers

The bedrock underneath all of Minnesota was cooled from a molten state to form igneous rocks (like granite) or was under pressure and heat to form metamorphic rocks (like gneiss, schist or quartzite). These rock types generally do not have primary permeability (pores) but can have significant secondary permeability (fractures). The northeastern part of Minnesota has very little glacial sediment and therefore relies heavily on this type of aquifer.

Groundwater contaminant susceptibility

Many factors can affect how quickly contaminants move from the soil at the surface to groundwater, including soil type, geological features, and depth to groundwater.

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Nitrates in groundwater and drinking water supply management areas (DWSMAs)

Nitrates are a form of nitrogen naturally occurring in soils. Plants use nitrogen as a source of energy. It helps give plants their green color and creates food for the plant through photosynthesis. 

A common background level of nitrate in water is around 3 parts per million or lower. But several activities can increase the concentrations in water, one being the addition of nitrogen fertilizer to the soil. If concentrations are above 10 parts per million the water can have adverse effects on the health of infants. 

DWSMAs are the protected areas surrounding a public well with elevated nitrate concentrations. 

  • A DWSMA is determined using the wellhead protection area extended to recognizable boundaries such as roads.
  • The wellhead protection area is defined by the surface and subsurface area containing water that will reach the well within 10 years.
  • DWSMA outlines are determined by the Minnesota Department of Health and can be found on the Minnesota Department of Agriculture (MDA) interactive map
  • DWSMAs are sorted into mitigation level designations determined by: 
    • Nitrate concentration level in the well.
    • The trend on nitrate concentrations over time.
    • Level of nitrogen fertilizer best management practice (BMP) adoption on farmland within the DWSMA. 
  • Map of the Cold Spring DWSMA

More information on mitigation level designation can be found on the MDA groundwater protection rule web page. The DWSMA outlines and mitigation levels are updated every year on January 15.

Several factors can influence increased nitrate in public wells including:

  • Groundwater susceptibility (see above).
  • Direct sources of nitrate to ground or surface water such as septic systems.
  • Agricultural fertilizers. 

The MDA and the University of Minnesota work with farmers in these areas to adopt best management practices that will help reduce nitrate concentrations entering the groundwater in these areas. Nitrogen management has a direct impact on crop yields as well as nitrate leaching to groundwater.

Author: Anne Nelson, Extension educator, water resource management and policy and Taylor Becker, Extension educator, agricultural water quality protection

Reviewed in 2021

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