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Gas Chromatograph/Mass Spectrometer: Instrument that identifies the molecular composition and concentrations of various chemicals in water and soil samples.

Gasoline Volatility: The property of gasoline whereby it evaporates into a vapor. Gasoline vapor is a mixture of volatile organic compounds.

Generator: 1. A facility or mobile source that emits pollutants into the air or releases hazardous waste into water or soil. 2. Any person, by site, whose act or process produces regulated medical waste or whose act first causes such waste to become subject to regulation. Where more than one person (e.g. doctors with separate medical practices) are located in the same building, each business entity is a separate generator.

Geology: The geologic features of an area. The science that deals with the history of the earth and its life, especially as recorded in the rocks.

Geological Log: A detailed description of all underground features (depth, thickness, type of formation) discovered during the drilling of a well.

Grab Sample: A single sample collected at a particular time and place that represents the composition of the water, air, or soil only at that time and place.

Granular Activated Carbon Treatment: A filtering system often used in small water systems and individual homes to remove organics. Also used by municipal water treatment plants. GAC can be highly effective in lowering elevated levels of radon in water.

Groundwater: Water located below the water table that fills the pore spaces between sand, gravel, and fractures in rock, etc., which may supply water to wells or springs.

Ground Water Under the Direct Influence (GUDI) of Surface Water: Any water beneath the surface of the ground with direct connection to surface water infiltration.

Ground-Penetrating Radar: A geophysical method that uses high frequency electromagnetic waves to obtain subsurface information.

Ground-Water Discharge: Point where groundwater exits aquifer and discharges at the earths surface. An example would be an artesian spring.

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