Groundwater Concepts

Hydrologic Cycle

Only 3 percent of the world's water supply is available to humans as fresh water. Of this small amount, 98 percent exists as groundwater. To understand the occurrence and movement of groundwater in a particular locality, it is first necessary to understand the interactions between water, the land and the atmosphere, called the hydrologic cycle. This cycle begins as water evaporates from waterbodies, land surfaces and vegetation on a continental scale. The water vapor moves through the atmosphere until it condenses into droplets, which then fall onto the land or sea as precipitation.

Precipitation that falls on land continues through the hydrologic cycle via a number of different paths. If the ground is impermeable or already saturated, some water will flow across the land surface to streams and lakes as overland flow. If the land surface is permeable and not fully saturated, water will filter into the ground. Some of that water will be taken up by growing plants and transpired back into the atmosphere. Excess water will continue to move downward under the influence of gravity. At some depth, the downward moving water will reach the water table where the formation is saturated. Groundwater is water found in saturated rock and soil formations. These formations may be consolidated bedrock, such as limestone or sandstone, or they may be loose unconsolidated deposits of sand, gravel or silt and clay. Water is stored in areas between rock and soil particles ("interstitial spaces"). The relative volume of these spaces is known as porosity.

Groundwater moves via gravity through porous materials and discharges to lakes, streams, wetlands or pumping wells. The size and interconnection of the spaces within the rock and soil controls the rate water flows through the material and is known as permeability. The layer where useable water is stored is called the aquifer. "Unconfined" aquifers occur where unsaturated porous material overlies saturated material. Where this occurs, the upper surface of the saturated zone is called the water table. The water table generally follows the contour of the ground surface terrain and can be mapped by examining well depth in these contoured areas. Aquifers may also be bounded at the top and bottom by impermeable layers called aquitards or confining beds. These beds are typically of clay or shale, but may be composed of granite or other impermeable rock. Under confining conditions the water table does not determine flow directions. A map of the piezometric surface can be made by comparing the levels to which the water rises in wells, reflecting the pressure the water is under in that portion of the aquifer.

Groundwater is made up of rainfall or snowmelt that percolates down through the soil until it reaches the saturated zone of an aquifer. This process is known as aquifer recharge. Recharge may take place throughout the aquifer, but occurs principally in aquifer recharge areas. These are usually located in the uplands. Lowlands tend to be discharge areas where groundwater is discharged to surface waters.

Surface watersheds can usually be used to approximate shallow groundwater hydrologic boundaries when specific information is not available. Surface watershed boundaries and groundwater divides do not, however, always coincide. Specific site investigations require the use of detailed hydrologic maps or field data to properly identify groundwater flow patterns.

Regional groundwater flow in Wisconsin is similar to surface topography. Groundwater usually enters the aquifer in upland areas and flows toward low points in a drainage basin. Sometimes it reaches the surface in the form of springs or artesian wells or seeps into swamps or rivers and lakes. These are called discharge areas. That portion of stream flow comprised of groundwater contributions is called base flow. Streams in the Lower Rock River Basin are comprised of base flow and overland flow.

For more information on groundwater, visit the WDNR - Drinking Water and Groundwater Program