Groundwater basics

Wisconsin floats on a vast underground layer of water, known as groundwater. Water from rain and snow melt percolates through the soil and into underground collections of groundwater. The shallow layer of groundwater near the surface that feeds many of our lakes and streams and is accessible to shallow wells is referred to as the water table. Deeper stores of groundwater are called aquafirs.

Less than 30% of Wisconsin's annual rainfall percolates down to recharge the state's aquafirs. The rest of the water is used by plants to grow, enters surface waters that drain into rivers or lakes far away from where the precipitation originally fell, or evaporates back into the atmosphere. The water cycle graphic below illustrates this process.

Not all of Wisconsin's groundwater is accessible or suitable for consumption. Groundwater is truly Wisconsin's buried treasure; it is a finite resource that Wisconsin citizens depend on. Groundwater is pumped for private and municipal drinking water and used for agriculture and other industries.

Groundwater also bubbles up to the surface supplying water to lakes, rivers, wetlands, and springs. These surface water resources support Wisconsin's 11 billion dollar tourism economy, sport fishing industry, waterfront property values, and recreational and commercial navigation.

Maintaining groundwater levels relies on the same amount of water filtering back into aquifers to replace the amount of water that is bubbling or being drawn (by wells) up to the surface. This is called groundwater recharge. Declines in precipitation (drought) or extreme weather events that may result in water quickly running into rivers and streams before it can be absorbed into the ground (flash flooding); this can effect groundwater recharge. Too many hard surfaces (like roads and buildings) that reduce the available area where water can penetrate the ground also effect recharge.

Excessive groundwater demand and use can surpass the rate in which groundwater recharge can occur; when intensive groundwater pumping exceeds sustainable rates, declining groundwater levels in aquifers result.

Groundwater: Wisconsin's Buried Treasure (exits site)

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Groundwater's role in lakes

Lake type     Geography     Weather

There are four main lake types. Groundwater plays a major role in seepage and groundwater drainage lakes, supplying all or most of the lake's water. Other types of lakes may have other sources of water besides groundwater,—including rain and snowmelt, runoff, and streams and rivers (which may be sourced by groundwater themselves)—but groundwater affects all lakes. 

A lake's "water budget" (see graphic below) is the balance between the amount of water coming into the lake (inflow) and the amount of water leaving the lake (outflow). As long as inflows and outflows remain equal the balance of water in the lake (lake level) remains constant.

If water inflows are greater than outflows, flooding may occur. If outflows are greater than inflows, water levels may decline. If both inflows and outflows are reduced, the rate of water flow through a lake can decrease, even as the volume (or level) in the lake remains constant. A change in the rate of flow through a lake, especially over the long term, can also lead to adverse lake impacts.

Lake type

A lake whose main source of water is groundwater, with no inlet or outlet streams, is called a seepage lake. Seepage lakes are the most common lake type in Wisconsin. Water levels on seepage lakes can change substantially from season-to-season, and year-to-year, because their water level is a reflection of the elevation of the water table. If the water table is low levels can be low.

Groundwater is also the primary source of water for spring lakes. Spring lakes are the headwaters of many streams and are a fairly common type of lake in northern Wisconsin. Groundwater can also influence drainage lakes and impoundments, especially if the surface waters (rivers, streams) that flow into these lakes are influenced by groundwater. 

More on lake types

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Geography

Water always flows from high areas to low areas—this fact plays an important role in lake water levels. From a high point in a landscape, (like a hill top) water flows one way or the other, depending on where it lands. This highest point is called a divide because it is the place where water divides and flows in different directions.

Groundwater operates by these same principles. A groundwater divide can be thought of as an underground hilltop, where the groundwater flows in different directions. Lakes close to this high point in the groundwater have less area to draw water from than lakes farther down (the hill). Lakes located close to a groundwater divide can be more susceptible to water level fluctuations when a lack of groundwater lowers the water table.

Groundwater is often close to the surface near shores of lakes and streams; the direction in which the groundwater flows can vary. Groundwater often flows toward the lake on drainage, drained, or spring lakes. On seepage lakes, groundwater may be flowing into the lake on one side, but away from the lake on the other. Groundwater flow direction can also vary seasonally, with water flowing into a lake during some times of the year, but flowing away from the lake at other times.

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Weather

Groundwater levels go up and down based on the amount of water (rain, snow) that falls on the land. There can be significant variations in the annual amounts of rain and snow. Cycles of dry weather can result in lower groundwater tables and lake levels. Global climate change is also playing a role in weather patters, which can ultimately affect water levels.

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Low lake levels in northern Wisconsin and the
relative roles of precipitation and evaporation (PDF 607 KB)

Low lake levels in the Central Sands area (PDF 754 KB)

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Human use of groundwater and impacts on lakes

The amount of water in the ground is dependent on the pace at which it is being extracted (pumped), compared to the rate at which it is being recharged. Cumulatively, human extraction of water (pumping) may be having a long term, regional impact on the water table, and ultimately on lake levels. Our collective demands for groundwater are already outstripping Mother Nature's ability to replenish the supply. Groundwater pumping is one of the factors that affects surface water levels.

High capacity wells can pump 100,000 gallons of groundwater per day or more (with some pumping up to 1 to 2 million gallons per day). Groundwater levels in aquifers in some areas of southeastern Wisconsin have dropped more than 450 feet below original levels due to intensive pumping. Pumping is measurably lowering groundwater and lake levels where density of irrigation and high capacity wells is large. In Dane County, rapid growth and increased demand on groundwater has caused a 60 foot drop in the aquafir, caused many springs to dry up, dropped the base flow of the Yahara River, and is now draining water from lakes Mendota and Monona into the aquafir. The aquafir used to supply these lakes with water.

Declining groundwater levels can affect surface waters. Intense pumping and lower water tables can reduce the flow of groundwater into lakes and streams and reduce critical supplies of surface water for fish and other wildlife, especially during dry periods. Groundwater is used by municipalities for sewage treatment, by farmers to water livestock and irrigate crops, by industries for a variety of processes, and provides drinking water and other residential uses for many citizens.

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Impacts of development on groundwater

As watersheds are developed, the amount area of area covered by impervious surfaces (roads, driveways, rooftops, etc.) increases. An unintended result of development is that impervious surfaces (like roads and buildings) and lawns cause more rain and snow to run off the landscape and less to soak (infiltrate) into the ground to recharge groundwater. Development is frequently also accompanied by increasing water use. The combination of pumping more groundwater while reducing the area available to replenish groundwater supplies can lead to drops in aquifers and  lessen the flow of groundwater into lakes and other surface waters.

Shoreland development density & impervious
surfaces: how much is too much for our lakes and streams? (PDF 863 KB)

The land “behind” the lake: watersheds and
lake water quality (PDF 639 KB)

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