Did you know ......?
- More than 70% of Minnesotans rely on groundwater for drinking water.
- As of 1990, an estimated 483,000 Minnesota residences used private wells to obtain water for their homes.
- As of 1990, there were 2,388 active community public water supply wells in Minnesota.
- In 1995, an estimated 700 million gallons of groundwater per day were withdrawn from Minnesota's aquifers (550 million gallons per day were permitted).
- As of 1989, contaminated groundwater cost 17 Minnesota cities and 18 Minnesota companies a total of $67,072,000.
- As of 1994, there were an estimated 700,000 to 1.175 million unsealed, abandoned wells in Minnesota that could potentially serve as contamination pathways to harm Minnesota groundwater.
- As of May 1998, 100,000 unused wells have been sealed to protect Minnesota groundwater.
What is groundwater?
Groundwater refers to water beneath the land surface. Groundwater may reside in the spaces between earth materials such as sand, silt, or clay particles. Where there is rock, groundwater can reside in rock openings such as fractures in granite.
Technical note: Groundwater scientists typically restrict the use of the term "groundwater" to underground water that can flow freely into a well, tunnel, spring, etc. This definition excludes underground water in the unsaturated zone. The unsaturated zone is the area between the land surface and the top of the groundwater. The unsaturated zone is made up of earth materials and open spaces that contain some moisture but, for the most part, this zone is not saturated with water. Groundwater is found beneath the unsaturated zone where all the open spaces between sedimentary materials or in fractured rocks are filled with water and the water has a pressure greater than atmospheric pressure.
What is the source of groundwater? Where goes it go?
The Hydrologic Cycle
For the most part, groundwater comes directly from precipitation or surface water that infiltrates into the subsurface (below the land surface). In turn, groundwater flows into many streams and lakes. Groundwater can be seen exiting from the subsurface as springs. But most commonly, we obtain groundwater from wells.
A spring at the Lanesboro Fish Hatchery
How fast does groundwater move?
Most groundwater is constantly moving, but it normally moves very slowly as it winds its way through tiny pathways around grains of sediment or through small openings in rock. Some rock materials have large enough fractures or openings that groundwater can move faster; however, often, the openings do not continue very far. In certain restricted areas, such as portions of southeastern Minnesota, carbonate rock formations have developed cave-like openings that allow groundwater to move much faster like an underground river.
Groundwater can move rapidly
through some of the fractures and
openings seen in this rock
Caves allow water to move quickly.
What exactly is a well? How are they constructed?
Typically, a well (a specially-designed tube) is constructed by drilling a hole (boring) in the ground and placing a long tube inside the hole. However, in practice, the process of drilling the boring and properly installing the well can be a very complicated task. First, an aquifer must be located. An aquifer is a groundwater bearing formation sufficiently permeable (allows water to pass through easily enough) to transmit and yield water in usable quantities. Sometimes the drillers find satisfactory groundwater very close to the land surface; at other locations, they may need to drill hundreds of feet deep to find a ground water supply that meets their customer's needs.
When a water-supply well is desired, the drillers are usually looking for a subsurface zone that is highly permeable and has good quality water. However, specially-designed wells called monitoring wells may be installed in a variety of subsurface zones soley to measure water levels or to test for contamination. In either case, the well must have openings set at just the right depth to only let groundwater from the desired subsurface zone into the well. In most cases, a pump is needed to lift the water in the well up to the land surface or even higher so we can use it for drinking, washing, industrial processes, agricultural irrigation and many other uses.
What Is all the concern about groundwater contamination?
Although most Minnesota groundwater is naturally potable (suitable for human consumption), nature does produce groundwater with a chemical make up that is not potable in some areas. In addition, many human activities such as urban development, industrial processing, agriculture, chemical spills and even individual household septic systems have caused significant groundwater contamination in areas that previously had clean, potable groundwater.
Groundwater contamination can disperse over a wide area or migrate very deep underground. Often, many tons of overlying soil, sediment or rock hide the exact location of the contamination and present a substantial physical barrier to clean up efforts. As the groundwater moves, it often contaminates the earth materials it passes through which increases the volume of material that needs to be cleaned. The cost and technical difficulty of removing the contamination often multiplies over time as the contamination spreads out or migrates deeper.
Under favorable conditions, certain contaminants tend to degrade or clean up naturally in a reasonable amount of time in ground water. However, in other cases, contamination can persist for long times because groundwater typically moves very slowly and often lacks the range of purifying organisms and processes that tend to cleanse streams and lakes much quicker. As a matter of fact, some of Minnesota's groundwater entered the subsurface more than 30,000 years ago and is still slowly traveling deep underground.
For more information, see our groundwater webpage.