Gallionella ferruginea - A genus of stalked, ribbon-like bacteria which utilize iron in their metabolism, and cause staining, plugging and odor problems in water systems. (See iron bacteria.)

gallon - A unit of liquid volume; the U.S. gallon has a volume of 231 cubic inches or 3.78533 liters; the British (Imperial) gallon has a volume of 277.418 cubic inches or 4.54596 liters.

galvanic cell - A cell which generates an electrical current, consisting of dissimilar metals in contact with each other and with an electrolyte.

galvanic corrosion - The form of corrosion which occurs in a galvanic cell, in which one of the metals goes into solution; accelerated by high concentrations of dissolved minerals in water, which increases the electrical conductance; and elevated temperatures. (See corrosion, electrolyte.)

gate valve - A valve with the closing element that is a disc which is moved across the stream, often in a groove or slot, for support against pressure.

gel zeolite- A synthetic sodium aluminoscilicate ion exchanger.

Giardia Lamblia - An intestinal parasite commonly found in water supplies originating in mountainous or wooded watersheds. It exists as a free-swimming protozoan-like organism in warm-blooded animals' intestines, causing chronic diarrhea, cramps, bloating and weight loss. Outside of the intestines, it forms a tough cyst that protects it until it finds a new host. Resistant to chlorine and most oxidizing agents, Giardia can be removed effectively through filtration below 1 micrometer.

globe valve - A valve in which the closing element is a sphere, or a flat or rounded gasket, which is moved into or onto a round port.

gpg - Abbreviation for "grain(s) per gallon".

grab sample - A single sample of material collected at one place and one time.

grain(gr.) - A unit of eight equal to 1/7000th of a pound, or 0.0648 gram.

grain(s) per gallon (gpg) - A common basis for reporting water analyses in the United States and Canada; one grain per U.S. gallon equals 17.12 milligrams per liter (mg/l) or parts per million (ppm). One grain per British (Imperial) gallon equals 14.3 milligrams per liter or parts per million.

gram (g) - The basic unit of weight (mass) of the metric system, originally intended to be the weight of 1 cubic centimeter of water at 4^oC. (One pound is 454 grams.)

gram-milliequivalent- The equivalent weight of a substance in grams, divided by one thousand.

gravimetric- Measurement of matter on the basis of weight.

greensand- A natural mineral, primarily composed of complex silicates, which possess ion exchange properties. (See manganese greensand, zeolite.)