Groundwater Basics

Aquifers | Water Table | Water Table Aquifers | Bedrock Aquifers | Buried Artesian Aquifers | Aquifer water levels | Groundwater hydrographs | Stratigraphy | Watersheds 

 

Groundwater

Groundwater is the water beneath the land surface that fills the spaces in rock and sediment. It is replenished by precipitation.

Under natural conditions much of that recharge returns to the atmosphere by evapotranspiration from plants and trees or discharges to surface waters. Groundwater discharge to surface waters allows streams to flow beyond rain and snowmelt periods and sustains lake levels during dry spells.

Groundwater supplies about 75 percent of Minnesota's drinking water and nearly 90 percent of the water used for agricultural irrigation. Availability varies throughout the state and may be be limited in some areas for larger withdrawals.

Aquifers

Aquifers are defined as water-bearing porous soil or rock strata that yield significant amounts of water to wells. Inherent in the definition are two points:

  • the aquifer formation must be porous (e.g., sand and gravel or cracks and fractures in more solid rock) and
  • water must be able to flow through and out of the formation in quantities large enough to be significant.

Underground formations like clay do not permit water to flow through them and prevent upward and downward movement of groundwater. These formations are known as aquitards or confining layers.

Aquifers are broadly classified into two categories, unconfined and confined.

Unconfined aquifers are the saturated portions of the upper soil profile located above a confining layer. Their upper surface is in direct contact with the atmosphere through porous materials. This upper surface is known as the "water table."

Confined aquifers are separated from the atmosphere by a very slowly permeable or rock (aquitard) called a confining layer. Water in these aquifers is under pressure and, in a well, will rise above the top of the aquifer.

Water Table

Water beneath the land surface occurs in two principal zones: the unsaturated zone and the saturated zone beneath it. The top of the saturated zone is the water table. Below the water table, the water pressure is great enough to allow water to enter wells, thus permitting groundwater to be withdrawn for use.

The depth to the water table is highly variable. It can range from zero when it is at land surface, such as at a lake or wetland, to hundreds or even thousands of feet deep. In Minnesota, the water table is generally close to the land surface, typically within a few tens of feet in much of the state.

back to top

Water Table Aquifers

Water table aquifers are the saturated portions of the upper soil profile located above a confining layer. The saturated soil must be of such composition (e.g., sand and/or gravel) that the water can be easily withdrawn. (A saturated, clayey soil does not meet the definition of an aquifer.)

Confining layers may be represented by bedrock layers or clay-rich soil layers. These layers severely impede downward movement (percolation) of infiltrated water. As a result, snowmelt or rainfall that enters the soil (infiltrates) accumulates in the soil above the confining layer causing a saturated soil condition. The water surface at the top of the saturated zone is called "the water table."

back to top

Bedrock Aquifers

Bedrock aquifers are, as the name implies, geologic bedrock units that have porosity and permeability such that they meet the definition of an aquifer (able to release water in quantities sufficient to supply reasonable amounts to wells). Water in these units is located in the spaces between the rock grains (such as sand grains) or in the fractures within the more solid rock.

Bedrock aquifers are fairly well defined in terms of their areal extent, and the units are considered to be connected hydrologically throughout their occurrence.

When an aquifer is separated from the ground surface and atmosphere by a material of low permeability, the aquifer is confined. The water in a confined aquifer is under pressure; therefore, when a well is installed in a confined aquifer, the water level in the well casing rises above the top of the aquifer.

back to top

Buried Artesian Aquifers

Buried artesian aquifers are composed of glacially deposited sands and gravels, over which a confining layer of clay or clay till was deposited. Their areal extent and hydraulic connections beneath the ground surface are often unknown.

When an aquifer is separated from the ground surface and atmosphere by a material of low permeability, the aquifer is confined. The water in a confined aquifer is under pressure; therefore, when a well is installed in a confined aquifer, the water level in the well casing rises above the top of the aquifer.

back to top

Aquifer water levels

When a well is placed so that its screen or open end is within an aquifer, water will rise inside the casing to a level that represents the aquifer's "potential energy head." In a water table well, this level would coincide with the top of the surface saturated zone or the "water table." In confined wells such as buried artesian or bedrock situations, where the aquifer water is under pressure, the level in the casing will be above the top of the aquifer.

A record of measurements, gathered over time, of the depths to water in the well is illustrated by a hydrograph and provides information about the relative quantity of water at different times. A long-term record indicates how the aquifer responds to climatic conditions and ground water withdrawals.

back to top

Groundwater hydrographs

Groundwater hydrographs illustrate the historical record of aquifer water levels measured within a well. The example shown below illustrates two features of a hydrograph:

Ground water hydrograph.

1. A long term of record is important when evaluating water level trends. In the above example, an overall, long-term view shows that following a period of steady water levels, conditions changed, causing water levels to decline until again conditions changed, resulting in a water level rise.

2. Seasonal impacts on aquifers are often depicted by a hydrograph. In the above example, water levels drop during summer as demands on the aquifer increase. These demands include irrigation, lawn watering, car washing, and so forth.

Hydrographs and ground water level data for DNR Waters ground water level monitoring wells

back to top

Watersheds

Surface-water watersheds

Surface-water watersheds are generally delineated from topographic maps based on land elevations ("height-of-land" method). The Minnesota Department of Natural Resources (DNR) completed a standard delineation of minor watershed boundaries for Minnesota in 1979. Using U.S. Geological Survey quadrangle maps, the DNR defined minor watershed outlets and delineated height-of-land minor watershed boundaries for all watersheds greater than 5 square miles. However, actual boundaries may be different due to map interpretation assumptions or human-induced changes that have occurred since the map was made. Field inspection of areas in question is required to be certain of actual boundaries.

Groundwater watersheds

Groundwater watersheds are conceptually similar to surface-water watersheds because ground water flows from high points (divides) to low points (outlets, discharge areas). However, the boundaries of surface-water and groundwater watersheds do not always coincide. Ground-water movement occurs in below-ground aquifer systems and is subject to the following:

  1. Hydraulic properties of the aquifer
  2. Input to (recharge) and outflow from (discharge) the aquifer system
  3. Geological factors such as formations that block the flow of water and tilted formations that create a flow gradient

Surficial aquifers (the water table) generally mimic surface-water watersheds, and their flow usually does not cross surface boundaries. Deeper (confined) aquifers, on the other hand, are less likely to conform to surface features and exhibit watersheds (or basins) determined by geologic factors.

More information on watersheds

 

Hydrogeology

back to top

Stratigraphy

Example of stratigraphy: profile of geologic layers.

Stratigraphy is a description of the layering of geologic formations found beneath a point of interest

Over geologic time, many processes occur that result in the formation of the earth's surface and subsurface. During some stages of history, deposition occurs, adding new layers to the already present earth materials. At other times, erosional processes remove materials from the earth's surface. All the while, chemical processes occur that transform the earth materials into different geological types. If one were to drill into the earth, different layers or strata would be encountered, each representing a different formation, such as sand, clay, sandstone, limestone, and granite. The record of such encountered layers at a given point is commonly referred to as the stratigraphy for that point.

 

back to top