• Aquifers are rocks or sediment that act as storage reservoirs for groundwater
• An aquiclude is rock or sediment that represents a barrier to groundwater flow
• Water infiltrates into open aquifers from above
• Open aquifers contain a saturated zone where pore spaces are filled with water
• The water table is the top of the saturated zone
• Water enters closed (artesian) aquifers from a recharge area

quifers are rock or sediment that act as storage reservoirs for groundwater and are typically characterized by high porosity and permeability. In contrast, an aquiclude is composed of a low permeability rock or sediment that essentially acts as a barrier to groundwater flow. Water has been found in wells that have penetrated as deep as 9 km (over 5 miles) into the earth's crust. Most usuable fresh groundwater is relatively shallow (less than 100 meters [330 feet]).  Deeper waters are more expensive to retrieve and often contain high concentrations of minerals. Aquifers (and aquicludes) are typically in sediments or sedimentary rocks as they sediments and sedimentary rocks are found at the earth's surface more frequently than the relatively impermeable igneous and metamorphic rocks.

Open Aquifers
Aquifers can be divided into open (unconfined) aquifers and closed (artesian or confined) aquifers. In an open aquifer, water infiltrates through permeable soil and rock or sediment that make up the unsaturated zone (where pore spaces are only partially filled with water) into the saturated zone of the aquifer (where all the pore spaces are filled with water). The top of the saturated zone (base of unsaturated zone) is the water table.

Simple animation of water levels in an open aquifer during periods of wet and dry climate. Precipitation recharges the groundwater supply. Wetlands and/or springs are present where the water table lies at the ground surface. Streams gain water during the wet season and may lose water to the groundwater system during the dry season. Some wells may become dry if they do not penetrate far enough into the saturated zone.

The water table mimics the shape of the land surface as it is higher under hills and lower in valleys. The elevation of the water table will fluctuate with variations in precipitation. Wells must be drilled far enough into the saturated zone to ensure a year-round supply of water. Groundwater will flow down the hydraulic gradient from from areas where the water table is high to areas where it is low, however, the flow paths may vary from straight lines to long looping curves. The orientation of the flow path is controlled by geological conditions (e.g. rock type, fractures).

Groundwater flows down the hydraulic gradient (slope) from high elevations to low elevations.The shape of the flow path varies depending upon the local geological characteristics.

Much of the upper Midwest is supplied by groundwater from open aquifers formed in sands and gravels left behind by glaciers at the close of the last ice age.

Closed (Artesian) Aquifers
A closed (artesian) aquifer is confined by an overlying aquiclude that prevents water simply infiltrating down into the aquifer. Instead, water enters the tilted aquifer layer through a recharge area where the aquifer rock is exposed at higher elevations. Flow in an artesian aquifer resembles water flowing through a J-shaped tube (left). Water added on the long limb of the tube (equivalent to recharge) provides the pressure to drive water upward on the shorter right limb of the tube (artesian well). Pressure from the overlying water column in the aquifer (see illustration below) is sufficient to cause groundwater to rise above the level of the aquifer. Water in artesian wells will rise above the aquifer itself to the potentiometric surface, essentially a projection of the water table.

Simple animation of changing water levels in an artesian aquifer and its effect on wells drilled into the aquifer. Water flows at the surface in wells where the projected water table lies above the ground surface.

Settlers on the plains of South Dakota in the early 1900's observed water gushing to 100 feet above the ground surface in artesian wells drilled into the Dakota sandstone.  The aquifer was recharged by rainfall in the Black Hills to the west.  There were over 10,000 wells drilled into the aquifer in South Dakota by 1915. Inevitably, the water table fell requiring many of the wells to add pumps to bring water to the surface.

Natural groundwater budget: recharge vs. outflow
The origin of groundwater is dependent on two current natural sources (infiltration, streams) and on the recent geological history of the aquifer system (storage).

• Infiltration - Rainfall infiltrates through soil, sediment and permeable bedrock to replenish open aquifers or through recharge areas for closed aquifers.  There are relatively small seasonal variations in the elevation of the water table and greater changes during prolonged droughts or periods of sustained precipitation. The volume of precipitation that enters an aquifer is dependent on the temporal and spatial distribution of precipitation and the character of the ground cover. Steady, regional rainfall will replenish an aquifer more than torrential local storms. Likewise, rain falling on farm fields or natural lands is more likely to infiltrate below ground than rain falling in urban areas where it will descend storm sewers en route to a nearby stream.

• Streams - The water table is often far below the ground surface in areas with dry climates. Permanent streams flowing through these areas may lose water to groundwater through the stream banks and bed. Such stream are termed losing (effluent) streams (see open aquifer animation above). Unlined canals and surface reservoirs, built to supply irrigation waters from the Colorado River, act like losing streams in the desert southwest.

• Storage - The climate of North America is warmer and drier today than it was prior to the close of the last great ice age (10,000 years ago). Much of the water in many U.S. aquifers represents this ancient groundwater source. Much of this water will not be replenished by streams or precipitation but instead represents a finite resource that can not be replaced once used.

Water will eventually leave the groundwater system at one of three discharge points defined by streams, springs or wetlands, and the ocean.:

• Streams - Groundwater may flow into streams in areas with relatively high water tables. These streams are termed gaining streams, (see open aquifer animation above). Depending on the region, groundwater can account for most of the base flow of a stream. For example, the Sturgeon River, Michigan, flows over permeable sands and gravels and receives approximately 90% of its minimum discharge from groundwater.

	Representative hydrographs for the Sturgeon River, Michigan, and the Forest River, North Dakota. Base flow in Stugeon River does not approach zero as the river receives much of its discharge from groundwater (gaining stream). In contrast, the Forest River flows over less permeable silt and clays and receives little of its flow from groundwater sources.

• Wetland/Spring - Water may flow out at the ground surface from a spring or wetland located where the water table intersects the ground surface (see open aquifer illustration above). Springs may form where fracture systems or cave systems (see below) reach the ground surface. An oasis represents an equivalent feature in a desert.
  

  	Water flows from the rocks along fractures or bedding planes. Pierys Mill spring (left), West Virginia. Image courtesy of Dr. Ira Sasowsky.
  	Al Khufrah oasis, southeast Libya, (left) with fields watered by center-pivot irrigation. Image courtesy NASA's Earth from Space program.

• Ocean discharge - Groundwater will discharge to the ocean along the coast. A lens of fresh groundwater floats above more dense salt water in coastal regions. Coastal cities extract water from the fresh water lens. Over-pumping may cause salt water to enter water wells (salt-water intrusion) and pollute the water supply. Salt-water intrusion has been a problem for communities in Long Island, New York, and along the southeast coast of Florida.
  

  Salt-water intrusion occurs with coastal wells pump too much water and draw up saltwater below the freshwater lens.