This PowerPoint file contains information about ground water in Minnesota. The presentation includes information on occurrence, quality, and quantity of ground water resources in Minnesota. Minnesota’s Ground Water 1. Abundant supplies of high quality ground water underlie most of Minnesota. 2. Ground water provides drinking water for three-fourths of us. 3. Ground water feeds our lakes and rivers. 4. Ground water is used to irrigate our crops.
Photo United States Geologic Survey Minnesota’s ground water systems are a function of the state’s geology. There are three basic type of aquifer in Minnesota.
1. Igneous and metamorphic rock aquifers 2. Sedimentary rock aquifers 3. Glacial sand and gravel aquifers Hard Rock (igneous and metamorphic) • The entire state is underlain by igneous and metamorphic bedrock. In places, such as northeast Minnesota, Photo Institute on Lake Superior Geology this bedrock is at the land surface, while in other places, such as southeast Minnesota, it is hundreds of feet below the ground. Photo Stearns County, Minnesota Igneous and metamorphic rocks are used as aquifers only when sedimentary rock and glacial aquifers are absent. These areas (pink on map) occur primarily in northeastern Minnesota.
Photo Stearns County, Minnesota
Ground water moves in the spaces between rocks Characteristics of Hard Rock Aquifers
• Water occurs in cracks (fractures) in the rock. • These rocks resist fracturing and hold little ground water. • These aquifers yield low quantities of water. • The water quality of these aquifers depends on the type of rock. Generally they have few dissolved chemicals because the rocks resist weathering, but concentrations of some metals such as iron and manganese can occur at levels of concern. • Wells are expensive to drill because the rock is hard to drill through. Sedimentary Rock
Much of southern Minnesota (areas in green) was once covered by a sea, leading to the formation of sedimentary rocks (sandstone, limestone, shale) Sedimentary rock aquifers, shown in green, occur primarily in southern and extreme western Minnesota. Photo North Dakota State University
St. Peter sandstone in St. Paul. Ground water flows through both cracks in the rock and in small pore spaces in the rock. Characteristics of Sedimentary Rock Aquifers • Useable quantities of ground water occur in fractures and pore spaces of the rocks. • Limestone and sandstone are easily weathered and can hold large amounts of ground water. • These aquifers yield large quantities of water. • Except for Cretaceous bedrock, the water quality of these aquifers is generally good. Aquifers usually have few dissolved chemicals, but if they are close to the land surface they may be vulnerable to contamination. • Wells are expensive to drill because the rock is hard to drill through. Glacial Deposits The last glaciers covered most of Minnesota between 11,000 and 13,000 years ago. Glacial advances, shown by arrows in the diagrams, came from both the northwest and northeast. Glacial melt water formed many large lakes, shown in grey in the diagrams. The map shows shallow glacial aquifers in Minnesota. These occur in outwash plains, along rivers, and in old lake beds. Deeper, buried glacial aquifers cover the entire state except for the Arrowhead region and small areas in central and Southwest Minnesota. Characteristics of Glacial Aquifers • Useable quantities of ground water occur in pore spaces of sand and gravel. • These aquifers can hold large amounts of ground water and yield large quantities of water. • The water quality of these aquifers varies. When aquifers are close to the land surface, they have few dissolved chemicals but may be vulnerable to contamination. When aquifers are far below the land surface they are more protected but may have high concentrations of dissolved chemicals, including some such as arsenic that can represent a health concern. • Wells are not as expensive to drill. This map shows the location of six ground water provinces in Minnesota. A province is an area where the characteristics of the ground water system are similar. In Province 1, sand aquifers are usually >100 feet thick. Sandstone and limestone aquifers yield large quantities of water. Aquifers in agricultural areas often have high concentrations of nitrate and pesticides may be present in low concentration. Low concentrations of fuel oils and industrial chemicals are often found in shallow aquifers from urban areas. In Province 2, small isolated sand and gravel aquifers occur more than 100 feet below the land surface. Deeper sedimentary rock aquifers provide moderate to good quantities of water. Aquifers are generally safe from contamination but may have high concentrations of dissolved chemicals, such as calcium. In Province 3, sedimentary rock aquifers provide large quantities of water. When these aquifers are close to the land surface they are vulnerable to contamination. In these aquifers, nitrate is often present at high concentrations and pesticides are often detected. Sand aquifers generally occur only along rivers. In Province 4, sand aquifers are thick and yield large quantities of water. When these aquifers are near the land surface, they may be vulnerable to contamination. In agricultural areas, shallow ground water often has detectable concentrations of pesticides and high concentrations of nitrate. Bedrock aquifers yield low to moderate quantities of water in areas where sand aquifers do not occur. In Province 5, sand aquifers are isolated and occur more than 100 feet below the land surface. In areas where these are not present, bedrock aquifers provide low to moderate quantities of water. Aquifers are generally not vulnerable to contamination except sand aquifers located along rivers. Ground water often contains high concentrations of dissolved chemicals, such as calcium and sulfate. In Province 6, igneous and metamorphic rocks occur at or near the land surface. Ground water occurs in fractures and faults in this rock. Quantities of available water are small. Water quality varies with type of rock. Concentrations of dissolved solids are usually low but concentrations of iron, manganese and boron can be high. Ground Water Quality in Minnesota is generally good. The primary ground water quality concerns in Minnesota are: • Naturally-occurring – Arsenic – Boron • Human sources – Nitrate – Pesticides – Fuel oils – Industrial chemicals Arsenic in Ground Water
• Exceeds the drinking standard in about 15% of private wells in Minnesota • Primary source is from geologic materials (naturally-occurring) • The map shows areas where concentrations are high in sand aquifers Boron in Ground Water
• Exceeds current drinking standard in about 5% of private wells in Minnesota • Primary source is from geologic materials (naturally-occurring) • The map shows areas where concentrations are high in aquifers (primarily bedrock) Nitrate in Ground Water • Exceeds the drinking standard in about 3% of private wells in Minnesota • Primary sources are fertilizer in agricultural areas and septic systems in some urban areas • The map shows areas where concentrations are high in bedrock (green) and sand (blue) aquifers and often above standards in sand aquifers (red) Pesticides in Ground Water • Limited data exist for occurrence of pesticides in ground water. • In vulnerable aquifers in agricultural areas, pesticides or pesticide breakdown products have been found at low concentrations in more Minnesota Department of Agriculture than 50% of sampled wells. The percentage in all sampled wells is much lower Pesticide (see upper right figure). sampling • There is very little data for locations – MN urban areas. Dept. Agriculture Fuel Oils in Ground Water • Leaking tanks; often
at gasoline and storage Minnesota Pollution Control Agency facilities • More than 15000 throughout Minnesota • New leak detection systems help prevent new sites from
occurring Underground tanks containing gasoline or other liquids often leak contaminants into • These chemicals often soil and ground water. break down in ground water Industrial Chemicals in Ground Water
Minnesota Pollution Control Agency
• Many sources, including paints, dry cleaners, industrial solvents, metals • More than 1000 sites across Minnesota • Chemicals often persist in ground water and many are highly toxic Ground Water Quantity
• As population grows, our water 250 consumption 200 increases 150 • Some areas will experience water 100 shortages 50 0 • Some lakes and 1985 1990 1995 2000 2003 rivers will be Water used for drinking has increased affected by ground over the past 20 years. We now use more water pumping than 225 billion gallons of water each year. In some places in Minnesota, we are seeing water level declines in wells due to pumping at rates that exceed the ability of the aquifer to replenish itself
Minnesota Department of Natural Resources The areas most vulnerable to limited supply are in Southwest Minnesota, in the Moorhead area, and in some locations in the Twin Cities Metro Area. • The area in pink shows the extent of reliable bedrock aquifers. Locations in the Metro Area outside the pink shading may have water supply concerns. • In Southwest Minnesota and in the Moorhead area (blue), localized aquifers are the only reliable sources of ground water. Other Water Quantity Issues • Well interference – pumping one well may interfere with the ability to pump a nearby well • Surface water drawdown – Minnesota Department of Natural Resources pumping of ground water may lower Wetland vegetation and ecosystems, such water levels in lakes, as that found in calcareous fens (above), rivers, and wetlands are particularly sensitive to changes in water levels that may accompany nearby pumping of ground water. Additional Sources of Information • http://www.dnr.state.mn.us/groundwater/index.ht ml • http://www.pca.state.mn.us/water/groundwater/g wmap/index.html • http://www.health.state.mn.us/divs/eh/water/index .html • http://www.mda.state.mn.us/appd/ace/maace.htm • http://www.geo.umn.edu/mgs/lcmr.htm