A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Gage Height
The height of the water surface above a measuring gage datum (zero point). Gage height in rivers and lakes is often recorded on a regular basis as part of routine hydrological data. Gage height is often used interchangeably with the term river stage.

Gaging Station
Principally used to designate a site on a stream, lake or reservoir where hydrologic data are obtained. Water level gages are also used on wells. The U.S. Geological Survey, Water Resources Division is the principal organization involved with hydrologic measurement and data collection in the US. The monitoring of ground water levels is an important part of the USGS data collection became it provides information about changes in the amount of water stored in aquifers.

Gaining Stream
A flow of water (river or stream) that is receiving water because of discharge from ground water. (See losing stream)

Geophysical Well Logs
A generic name for a suite of technologies that reveal absolute or relative properties of geologic formations, aquifers and wells. Technologies include electrical resistivity, gamma-log, acoustic etc.

Geyser
A hydrologic spring-like feature from which hot water and steam reach the earth's surface. "Old Faithful" in Yellowstone Park, Wyoming is probably the world's most famous geyser. Virtually all the world's geysers occur where circulating ground water come in contact with rock formations in the earth's crust that are hot because of proximity to crustal magma.

Giardia
A protozoan parasite (Giardia Intestinalis) commonly found in surface water. Drinking giardia-affected water that is either not filtered or not chlorinated may result in sickness. The disease is more likely to affect children than adults and is characterized by abdominal discomfort, nausea, and diarrhea.

Glacial Drift
A general term for unconsolidated sediments transported by glaciers.

Glacial Outwash
Clay, sand and gravel deposits that were washed out by glacial melt-water streams. These deposits may form extensive plains or fans. Thick deposits of glacial outwash may provide excellent aquifers.

Glacial Till
An unsorted, and often compacted mixture of clays, sands, rocks, and boulders deposited by melting glaciers.

Glacier
A mass of ice, formed by the compaction and recrystalization of snow, that moves very slowly down slope (valley glacier) or outward (ice sheet) due to its own weight. 75% of the world's fresh water is ice, and almost 25% is groundwater. The last major glaciation that affected North America ended only 10,000 years ago.

Gneiss
A type of metamorphic rock.

Grain Size
It is very common in ground water science and engineering to use the size of the grains in sedimentary rocks as a method of description and as the basis for designing well screens.
See also well screens, sediments.

Grains per Gallon
A unit of measurement still used in some North American water analyses. One grain per US gallon is equivalent to 17.12 milligrams per liter.

Gravel Packed Well
A well in which a sand or gravel material is placed in the annular space between the drilled hole and the well screen. A gravel pack changes the hydraulics of water flow to a well. The main purpose of the gravel pack is to slow the entrance velocity of water from the aquifer to the well so that fine material is not drawn into the well when the pup runs.

Graywater
A word coined recently to describe domestic wastewater from washing machines, showers, and baths. In some communities gray water may be recycled for lawn and garden watering. Homeowners should check local building code regulations before diverting graywater to their yards or gardens.

Ground Water (confined)
Ground water under pressure that is greater than atmospheric pressure. Confined ground water is separated from direct contact with atmospheric pressure because of overlying impermeable layers of rock.

Ground Water (groundwater)
Ground water is that part of the hydrologic system that occurs in a geologic environment. Water that is found in fully saturated soils, sediments and rocks below the surface of the ground. The water table is the upper surface of the ground water system. Aquifers contain ground water but not all ground water occurs in aquifers.
Ground water may be spelled as one word or may be hyphenated.
See also aquifer.

Ground Water (unconfined)
Water in an aquifer that has a water table that is at atmospheric pressure.

Ground Water Banking
A water management strategy whereby an agency "sells or rents" excess storage space in aquifers. The surface water is used to recharge aquifers. There are aquifers in Arizona that "bank" excess or "unused" Colorado River water for late use.
See also ASR.

Ground Water Discharge
The fluid output from a ground water system. Natural ground water discharge may occur in the form of springs or seepages. Ground water also discharges into rivers and lakes via bank seepage or by upward flow in river and lake beds. Ground water at the coast may reach the oceans via coastal discharge at or beyond the shore zone. Ground water discharge and recharge are commonly linked in water budget calculations.

Ground Water Flow Rates
Rates of flow are typically very slow when compared with surface flow in streams. Most ground water flow is laminar in nature and does not have any excess energy to transport particulate matter. Ground water at different depths may be moving at different rates of flow. By use of tests such a tritium content, or carbon 14, it has been possible to age date some ground water and hence determine rates of sub-surface flow.

Ground Water Mining
Long term pumping water from an aquifer system at a rate greater than natural recharge. In some cases, such as in parts of Libya, the mining of ground water is a deliberate and planned resource use strategy.

Ground Water Recharge
The process of adding water to ground water storage. In most cases ground water recharge occurs from the infiltration of precipitation. There are projects in the US and elsewhere that use injection wells or spreading basins to artificially recharge aquifers as a technique of ground water management.
See also ASR.

Grout
A fluid sealing mixture usually comprising bentonite and or cement that is used to seal well casing. Once emplaced, grout forms an impermeable seal.