Groundwater
12
Water is the lifeblood of every living creature on earth. Approximately 70 percent of the earth's surface is covered with water. Through the wonders of nature, water can take on many different forms, from the water we drink, to the ice we use to chill a glass of lemonade, to the water vapor used to steam clean equipment. It is easy to understand the significance water plays in our lives. It may be much more difficult to understand the groundwater that exists below the Earth's surface.
When it comes to water, there is no place like Wisconsin. We are water rich. Between the mighty Mississippi River and the Great Lakes of Michigan and Superior, there are more than 15,000 lakes, 7,000 streams and five million acres of wetland. And that just scratches the surface. Below our feet Wisconsin has a buried treasure – 1.2 quadrillion gallons of ground-water. It is hard to grasp just how much water is stored underground unless you look at how much we use every day.
Each year about 29 trillion gallons of water fall as rain or snow on Wisconsin’s 36 million acres. Plants and animals consume some, some is returned to the atmosphere by evaporation, liquid changing into gas on the surface, or by transpiration, moisture given off from plants to the atmosphere. Some water becomes runoff, flowing into rivers, lakes and streams. The rest becomes groundwater by infiltrating or percolating, soaking into and through the soil, into groundwater aquifers. Aquifers are large areas of sand, gravel and rock that store water for later use.
If you could somehow pour all the water below ground on top, you'd need to trade in your ranch house for a houseboat: Wisconsin's groundwater could cover the whole state to a depth of 100 feet. Despite this abundance of groundwater, there is a growing concern in certain areas of the state about the quantity of good quality groundwater available for municipal (city, village, town), industrial, agricultural and domestic use. There is also concern about adequate base flow; the groundwater that sustains our lakes, streams and wetlands.
Getting a clean glass of water isn’t as easy as turning on the tap!
In Wisconsin, the quality and quantity of groundwater varies from place to place. The difference is caused by a combination of geology, varying precipitation and use. Cities and towns in the North Central and Northeastern third of Wisconsin receive the most precipitation in the state, but they are underlain by crystalline bedrock; a type of rock formation notorious for yielding only small quantities of water. Even though there may be plenty of rain, finding enough groundwater to supply municipalities in these regions can be difficult. Groundwater levels have been going down by hundreds of feet around some of Wisconsin’s growing metropolitan areas.
At last estimate, there were more than 850,000 private wells in Wisconsin. In areas where water moves through aquifers very slowly, private wells can still yield enough water for residential use. You can drill a hole just about anywhere in Wisconsin and find water. But is this water drinkable? Not necessarily. Ground-water can be contaminated in many ways. Clean groundwater is Wisconsin’s buried treasure that needs to be cared for and protected.
Hydrologic Cycle
Water might be called our most recycled resource. The water you showered in this morning may have contained the same water molecules that gave a dinosaur a cool drink during prehistoric times or carried the early explorers across our country. The distribution of the Earth's total supply of water changes but the quantity has remained constant. Remember, the water we use today is the water we will need in the future too.
Surface water and groundwater are part of the hydrologic cycle. The hydrologic cycle is the constant movement of water above, on and below the Earth’s surface. The cycle has no beginning and no end. You can understand it best by tracing it from precipitation.
Precipitation occurs in several forms, including: rain, snow, sleet, dew, fog and hail. Wisconsin receives an average 30 to 32 inches of precipitation per year. Rain can do three things once it reaches the Earth’s surface. It can filter into the ground, runoff into water bodies or evaporate. Rain, for example, wets the ground surface. As more rain falls, water begins to filter into the ground (infiltration). Approximately 70 to 90 percent of the water that falls to the earth's surface enters the soil. This water can become groundwater but most of it evaporates from the soil surface or is used by vegetation. How fast water soaks into, or infiltrates the soil depends on soil type, land use, slope of the land and the intensity and length of the storm. Water infiltrates faster into soils that are mostly sand rather than those made mostly of clay or silt. Almost no water filters into paved areas. Rain that cannot be absorbed into the ground flows across the surface forming runoff streams.
When the soil is completely saturated additional water moves slowly down through the unsaturated zone (drier, upper soil layers) to the saturated zone (wet lower layers or aquifer) replenishing or recharging the groundwater. The distance water has to travel to reach groundwater can range from a few feet to hundreds of feet. Water movement toward groundwater may take hours or years depending on the depth to the aquifer and the characteristics of the unsaturated zone. Water then moves through the saturated zone to groundwater discharge areas such as springs or artesian wells.
Evaporation occurs when water from such surfaces as oceans, rivers and ice is converted to water vapor. Evaporation, together with transpiration from plants, rises above the Earth’s surface, condenses, and forms clouds. Transpiration is the process by which plants release water to the atmosphere. Water from both runoff and from groundwater discharge moves toward streams and rivers and may eventually reach the ocean. Oceans are the largest surface water bodies that contribute to evaporation. It is estimated that 39 inches of water annually evaporates from each acre of ocean.
After water vapor is drawn into the atmosphere it can be transported over hundreds of miles by large air masses. When water vapor cools, it condenses to form clouds. As water condenses within clouds, water droplets increase in size until gravity pulls them to the Earth's surface as precipitation.
What is Groundwater?
Groundwater is fresh water (from rain or melting ice or snow) that soaks into the soil and is stored in the pores (tiny spaces) between rocks and particles of soil. Approximately 70% of Wisconsin’s residents and 97% of Wisconsin’s communities rely on groundwater to meet their water supply needs. Groundwater can stay underground for hundreds of thousands of years or it can come to the surface and help fill rivers, streams, lakes, ponds and wetlands. Groundwater can also come to the surface as a spring, artesian well or be pumped from a well.
How Does the Ground Store Water?
Groundwater is stored in the tiny open spaces between rock, sand, soil and gravel. How well loosely arranged rock (such as sand and gravel) holds water depends on the size of the rock particles. Layers of loosely arranged particles of uniform size (such as sand) tend to hold more water than layers of rock with materials of different sizes. This is because smaller rock materials settle in the spaces between larger rock materials, decreasing the amount of open space that can hold water. Porosity is how well rock material holds water. Porosity is dependent on the shape of the rock particles and the amount of pore space available. Round particles will pack more tightly than particles with sharp edges. Material with angular-shaped edges has more open space and can hold more water.
Groundwater is found in two zones. The unsaturated zone, also called the vadose zone, is immediately below the land surface. It contains water and air in the open spaces or pores. The saturated zone, a zone in which all the pores and rock fractures are filled with water, is below the unsaturated zone. The boundary between the unsaturated and saturated zones is called the water table. The water table is the top of the saturated zone and may vary in depth from place to place and year to year. It may be a few feet below the surface or hundreds of feet below the land surface. The water table may rise or fall depending on many factors. Heavy rains or melting snow may cause the water table to rise, or an extended period of dry weather may cause the water table to fall. Extensive pumping of water by high capacity wells can lower the water table.
How Groundwater Moves
What is an Aquifer?
An aquifer can form where groundwater moves rapidly, such as through gravel and sandy deposits. An aquifer has enough groundwater so that it can be pumped to the surface and used for drinking water, irrigation, industry or other uses. An aquifer is the area where groundwater is stored naturally before being used or discharged to the surface.
For water to move through underground rock, pores or fractures in the rock must be connected. If rocks have good connections between pores or fractures and water can move freely through them. These rocks are referred to as being permeable. Permeability refers to how well a material transmits water. If the pores or fractures are not connected, the rock material cannot produce water and is therefore not considered an aquifer. When water does not readily move through material, the material is considered impermeable. A layer of such material would be considered a confining layer. Confining layers are often made of clay or shale. The amount of water an aquifer can hold depends on the permeability and porosity of the underground materials present.
An aquifer may be a few feet to several thousand feet thick, less than a square mile or hundreds of thousands of square miles in area. For example, the High Plains Aquifer underlies about 280,000 square miles in 8 states including: Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming.
How Does Water Fill an Aquifer?
Aquifers get water from precipitation (rain, snow, etc…) that filters through the unsaturated zone. Aquifers can also receive water from surface waters such as lakes and rivers. This taking in of water is called recharge. Recharge areas are where aquifers take in water. When the aquifer is full, and the water table meets the surface of the ground, the water stored in the aquifer can appear at the surface as a spring or artesian well. These are known as discharge areas. Discharge areas are where groundwater flows to the land surface. Water moves from higher-elevation areas of recharge to lower-elevation areas of discharge through the saturated zone.
How do we get water from an Aquifer?
Wells are used to remove water from aquifers. Basically, a well is a hole drilled into an aquifer. A pipe and a pump are used to pull water out of the ground. A screen filters out unwanted particles that could clog the pipe. Wells come in different shapes and sizes. It depends on the type of material the well is drilled into and how much water is being pumped out. Cities often store this water for later use in water towers to provide water pressure.
Removing water lowers the water level in the well. The difference between the initial water table depth or static water level (elevation of the water table above sea level) and the pumping water level causes water to move in the aquifer. Since the pumped well water level is lowest the water from the surrounding aquifer flows toward the well to replace the water being removed. Shallow wells may go dry if the water table falls below the bottom of the well. Some wells, called artesian wells, do not need a pump because of natural pressures from the slope of the aquifer and/or confining layers that force the water up and out of the ground naturally under pressure.
Rate of Groundwater vs. Surface Water Movement
Surface water generally flows in rivers or streams at velocities of 2-8 miles per hour. Rate of surface water flow is determined by slope of the land, amount and rate of precipitation, rate of infiltration and evaporation, amount and type of vegetation and land use.
Groundwater moves through the spaces (pores) between particles of a saturated material anywhere from 0.1 foot per day to 3 feet per day. That translates into movement of 35 to 1,100 feet per year for groundwater. Ground-water moves only if sufficient pressure is available to force water through the spaces between porous aquifer materials. Geology helps to controls the rate of groundwater movement. The size of the cracks in rocks, the size of the pores between soil and rock particles, and whether the pores are connected determine the rate at which water moves into, through and out of the aquifer. Water generally moves quickly in coarse sand, sometimes as much as several feet per day. Openings between the grains are large and interconnected, resulting in high permeability. Very fine-grained material like clay has many pores where water can be stored, but the pores are so small that moving water through or out is difficult. Clay formations are relatively impermeable -- water may move only a few inches a year. Permeability in limestone, on the other hand, primarily depends not on pore spaces, but on the size, frequency and distribution of fractures and cracks.