What is groundwater?
Groundwater is water stored in rocks or sediment below the earth's surface.

What is groundwater used for?
Nearly two-thirds of groundwater used in the U.S. is used for irrigation of crops in areas of limited precipitation. Groundwater is often used as a drinking water source in rural regions or in towns without a municipal water supply.

What is the hydrologic cycle?
The hydrologic cycle links the processes of evaporation, condensation, stream run-off, infiltration, percolation, and transpiration as water moves between the ocean, atmosphere and land.

How long does water spend at each stage of the hydrologic cycle?
The residence time (time water remains in a given location) varies from an average of 4,000 years for the water in the oceans to an average of nine days in the atmosphere. Slow-moving groundwater may remain below ground for years or centuries.

What rock properties are most important for groundwater?
Rock type is important as rocks such as limestone may be dissolved by groundwater to form extensive cave systems. However, most rocks do not dissolve in groundwater but may allow water to flow through them (permeability) or store water in spaces (porosity) between grains (pores) or in fractures. These properties are most commonly developed in sediments or sedimentary rocks but can be present in other rock types under specific conditions.

How are porosity and permeability related to the grain size?
Both porosity and permeability increase with increasing grain size. Larger grains result in larger spaces between grains (more porosity) and larger openings connecting the spaces (more permeability). Coarse grained sediment (sand, gravel) and sedimentary rocks (sandstone, conglomerate) typically exhibit high porosity and permeability. Fine grained materials (clay, mud, shale) are characterized by low permeability and porosity.

What is the difference between an aquifer and an aquiclude?
Aquifers are rock or sediment with good porosity and permeability that act as storage reservoirs for groundwater and allow the extraction of water by wells. Aquicludes are barriers to groundwater flow and are composed of rocks with poor permeability.

How do open and closed aquifers differ?
Open aquifers are recharged with water that infiltrates downward from the earth's surface above the aquifer.  There is no overlying aquiclude to prevent water entering the aquifer by infiltration. In contrast, closed aquifers are overlain by aquicludes. A closed aquifer is replenished by recharge where the aquifer is exposed at the earth's surface. Closed aquifers are typically inclined rock units with groundwater flowing downslope from recharge areas of higher elevation.

What is the water table?
The water table marks the top of the zone of saturation in an aquifer. Pore spaces are filled with water below the water table but are only partially filled above the water table (in the unsaturated zone). The water table will fall or rise depending on seasonal and annual variations in recharge.

Is the volume of groundwater constant?
No. First of all, groundwater is affected by climate cycles. Many groundwater systems in the western U.S. were filled when North America's climate was considerably wetter in the recent geologic past. Today, recharge occurs by infiltration of precipitation and outflow from (losing) streams. In wetter areas this is sufficient to maintain groundwater levels but in some areas in the relatively dry western states water tables have declined as recharge has lagged behind groundwater extraction. Natural outflow from groundwater systems occurs at springs, wetlands, (gaining) streams, and by oceanic discharge.

What is the High Plains aquifer?
The High Plains aquifer stretches from South Dakota to west Texas and  covers an area of 480,000 square kilometers, making it the largest area of irrigation-sustained cropland in the world. The unconfined sand and gravel aquifer contains "fossil" water, the product of a wetter ancient climate associated with the end of the last ice age. There is no sufficient contemporary recharge source although substantial recharge does occur in some areas from streams and from irrigation projects supplied with surface waters.

Where and when does groundwater overdraft occur?
Groundwater overdraft occurs when groundwater extraction occurs more rapidly than recharge. Groundwater has been pumped to heavily  in some parts of the Texas panhandle and southwest Kansas that the water table has dropped as much as 70 meters taking groundwater beyond the reach of many wells.

Can the pumping of one well affect water levels in neighboring wells?
Yes. Pumping of a well lowers the local water table surrounding the well to create a cone of depression. With continued pumping, the cone of depression can expand outwards, lowering the water table in adjacent wells.

How is groundwater extraction associated with ground subsidence?
The weight of the overlying material (rocks, trees, cities) is supported by both the mineral grains and the water in the pore spaces in a confined aquifer. If the mineral grains can not bear the load alone they may collapse inward when the water is extracted. This causes a decrease in the volume of the underlying sediment and can result in subsidence of the ground surface.

Does most water pollution come from industrial plants?
No, although this may have been the case several decades ago, water pollution may come from a variety of sources including landfills, septic tanks, underground storage tanks, mines, livestock compounds, and croplands. It is often difficult to determine the source of pollutants given the slow movement of groundwater and the indirect routes of groundwater flow. By the time pollution is detected the source may have been abandoned.

What features can be used to identify the presence of wetlands?
Although the legal definition of wetlands can require a specific association of features, most geologists identify wetlands if they possess hydric (poorly drained) soils, hydrophitic vegetation, and water on or near the ground surface for weeks or months at a time.

Are there any benefits to the presence of wetlands?
The benefits of wetlands include improvements in water quality, creation of ecological habitats, reductions in flooding, shoreline erosion control, recreation sites, and recharge zones for groundwater

Where are most U.S. wetlands?
Most U.S. wetlands are in Alaska. Florida, Minnesota, and Texas have the most wetland acreage in the lower 48 states.

Why were wetlands destroyed?
In the last century and for much of this one, wetlands were thought to be breeding grounds for malaria and to have little value for other purposes. Consequently, wetlands were drained for agriculture or were infilled to allow the expansion of cities. The rate of wetland losses has declined over the last few decades as the potential benefits of wetlands have been recognized but there is still a net loss of wetlands. California and Ohio have lost the most of their original wetlands (90%).

How has development resulted in changes in the wetlands of southern Florida?
Hydraulic engineering projects have considerably altered the interaction between surface and subsurface water systems in southern Florida. Wetlands south of Lake Okeechobee have been replaced by sugar cane fields. Development in southern Florida has resulted in less groundwater recharge, salt-water intrusion alongeast coast, polluted run-off from sugarcane fields, and reduced habitats for fish, birds, and mammals.