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• Groundwater


  Water that exists in the cracks, crevices, and pore spaces of rock and soil beneath the Earth's surface
• Groundwater is a major source of water worldwide, providing about half of our drinking water and being essential to the vitality of agriculture and industry
• Groundwater plays a crucial role in sustaining streamflow, especially during protracted dry periods, and many ecosystems depend on groundwater discharge into streams, lakes and wetlands
• Only about six-tenths of 1 percent of the total hydrosphere (all of Earth's water) occurs underground, but this small percentage is a vast quantity
• When the oceans are excluded and only sources of fresh water are considered, groundwater represents more than 94 percent of the total
• Groundwater


  • It is a valuable natural resource
  • It is one of our most important and widely available resources
• Groundwater is an erosional agent, slowly removing rock and allowing surface depressions known as sinkholes to form as well as creating subterranean caverns
• Groundwater is an equalizer of streamflow, as much of the water that flows in rivers is not direct runoff from rain and snowmelt, but rather water that has soaked in and moved slowly underground to stream channels
• In the United States, groundwater provides 23 percent of the total fresh water used each day, with the remaining 77 percent coming from surface sources
• More groundwater is used for irrigation than for all other uses combined, accounting for 68.4 percent of groundwater use
• Public and domestic uses, including water for indoor and outdoor household purposes as well as commercial uses, account for 19.3 percent of groundwater use
• The remaining groundwater use is for thermoelectric power, mining, aquaculture, livestock, and industrial purposes
• Water table


  The upper limit of the zone of saturation, where all the open spaces in sediment and rock are completely filled with water
• The water table is rarely level, instead its shape is usually a subdued replica of the surface topography, reaching its highest elevations beneath hills and descending toward valleys
• Lakes and streams generally occupy areas low enough that the water table is above the land surface
• Several factors contribute to variations in the water table, including the quantity, distribution, and timing of precipitation, as well as factors like slope, surface material, and vegetation
• Water table


  The upper surface of the zone of saturation, below which the soil or rock is saturated with water
• Water table


  • Its configuration varies seasonally and from year to year because the addition of water to the groundwater system is closely related to the quantity, distribution, and timing of precipitation
  • Except where the water table is at the surface, we cannot observe it directly
  • Its elevation can be mapped and studied in detail where wells are numerous, because the water level in wells coincides with the water table
• The water table is rarely level, as we might expect a table to be
• Shape of the water table

  A subdued replica of the surface topography, reaching its highest elevations beneath hills and then descending toward valleys
• Where a wetland (swamp) is encountered, the water table is right at the surface
• Factors contributing to the irregular surface of the water table

  • Groundwater moves very slowly and at varying rates under different conditions
  • Variations in rainfall and permeability from place to place
• If rainfall were to cease completely

  The water table "hills" would slowly subside and gradually approach the level of the valleys
• In times of extended drought


  The water table may drop enough to dry up shallow wells
• Gaining stream


  A stream that receives water from the inflow of groundwater through the streambed
• Losing stream


  A stream that loses water to the groundwater system by outflow through the streambed
• Streams may gain in some sections and lose in others
• Losing stream (disconnected)


  The stream is disconnected from the groundwater system by an unsaturated zone, and the water table may have a discernible bulge beneath the stream
• Groundwater contributes to streams in most geologic and climatic settings
• Even where streams are primarily losing water to the groundwater system, certain sections may receive groundwater inflow during some seasons
• 52 percent of the streamflow was contributed by groundwater in a study of 54 streams in the United States
• Groundwater is also a major source of water for lakes and wetlands
• Porosity


  The percentage of the total volume of rock or sediment that consists of pore spaces
• Porosity


  • Voids most often are spaces between sedimentary particles, but also common are joints, faults, cavities formed by the dissolving of soluble rocks, and vesicles (voids left by gases escaping from lava)
  • Porosity can vary greatly, from 10-50% in sediments to negligible in some igneous and metamorphic rocks
• Permeability


  The ability of a material to transmit a fluid
• Aquitard


  An impermeable layer that hinders or prevents water movement
• Aquifer


  A permeable rock stratum or sediment that transmits groundwater freely
• Porosity is not always a reliable guide to the amount of groundwater that can be produced, and permeability is significant in determining the rate of groundwater movement and the quantity of water that might be pumped from a well
• Groundwater movement


  Exceedingly slow, from pore to pore, anywhere from millimeters per year to perhaps a kilometer per year
• Hydraulic gradient


  The slope of the water table, which determines the velocity of groundwater flow