Fluid – Any material or substance that changes shape or direction uniformly in response to an external force imposed on it; the term applies to liquids and gases.
Fluidization – A technique in which a finely divided solid is caused to behave like a fluid by suspending it in a moving gas or liquid.
Fluoridation – The addition of fluoride to public water supplies to prevent or delay the onset of dental decay.
Fractional distillation – A distillation process in which countercurrent distillation is employed to obtain a product as nearly pure as possible; it is also any distillation process in which the product collected may be a series of separate components of similar boiling range.
Freeze-drying – A method of dehydration or separating water from materials; the material is first frozen and placed in a high vacuum so that the water (ice) vaporizes (sublimes) in the vacuum without melting and the non-watery components are left behind in an undamaged state.
Freshwater – The water that generally contains less than 1,000 mg/L of dissolved solids.
Froth – A foamy mass of bubbles that exists on a body of water.
Gas permeation – The movement of gas from the high pressure side of a membrane to the low pressure side.
Geohydrology – The branch of hydrology involving the study of groundwater and its physical and chemical interactions with the physical environment.
Geology – The science that deals with the origin, history, and structure of the Earth, as recorded in rocks, along with forces and processes that modify rocks.
Geothermal energy – The energy derived from the superheated water and steam trapped in underground reservoirs.
Geothermal gradient – The change of temperature in the Earth with depth, usually expressed in degrees per unit of depth.
Glacial drift – The rock material transported and deposited directly by glaciers or indirectly by ice or water emanating from a glacier.
Glacier – A large body of ice originating on land by the compaction and recrystallization of snow, and showing evidence of present or past movement.
Grab sample – A sample that is collected at such a time and place so that it is ideally most representative of a total discharge.
Gray water – Domestic wastewater composed of water from kitchen sinks, bathroom sinks and tubs, clothes washers, and laundry tubs.
Greenhouse effect – The gradual rise in the average global temperature due to the absorption of infrared radiation reflected by the Earth’s surface by increasing amounts of carbon dioxide, methane, nitrous oxide, and other greenhouse gases in the air.
Groundwater – Water that exists underground in saturated zones beneath the land surface.
Groundwater basin – A porous formation with sides and bottom of relatively impervious material in which groundwater is held or retained.
Groundwater hydrology – The branch of hydrology dealing with groundwater sources, movement, recharge, depletion, etc.
Groundwater reservoir – An area below the ground surface in which groundwater is stored, also called an aquifer.
Groundwater recharge – The input of surface water to resupply groundwater reservoirs through infiltration at the soil surface or from the bottom of rivers and streams; the use of reclaimed wastewater, by surface spreading or direct injection, to replenish freshwater aquifers; to resupply freshwater aquifers to prevent saltwater intrusion, to control or prevent ground subsidence, and to augment non-potable or potable groundwater aquifers.
Groundwater system – A groundwater reservoir and its contained water; also, the collective hydrodynamic and geochemical processes at work in the reservoir.
Habitat – A dwelling place of a species or community, providing a particular set of environmental conditions (e.g. forest floor, seashore, etc.).
Hard water – Any water containing high levels of dissolved divalent cations, primarily calcium and magnesium, that interfere with some industrial processes and prevent soap from lathering.
Hardness (water) – A property of water causing formation of an insoluble residue when the water is used with soap and forming a scale in vessels in which water has been allowed to evaporate; it is due primarily to the presence of divalent ions of calcium and magnesium.
Heat exchanger – A unit or vessel in which a hot fluid stream transfers part of its energy to a cooler fluid stream.
Heat sink – A structure designed to absorb heat.
Heterogenous – A term used to describe a mixture of different phases (e.g. liquid–vapor, liquid–vapor–solid).
High dam – A dam that is taller than 165 feet; the first high dam was Hoover Dam in the United Sates; four other notable high dams are Grand Coulee Dam in the United States, Aswan High Dam in Egypt, Three Gorges Dam in China, and the Sardar Sarovar Dam in India.
Holding pond – A pond or reservoir, usually made of earth, that is built to store polluted runoff.
Holdup – A volume of material held or contained in a process vessel or line.
Homogenous – A term used to describe a mixture or solution comprised of two or more compounds or elements that are uniformly dispersed in each other.
Hot brine – A slightly salty subterranean water, the temperature of which is markedly higher than that dictated by the normal geothermal gradient; it can be employed as a source of geothermal energy.
Hot rock – A subterranean rock, the temperature of which is higher than would be dictated by the normal geothermal gradient; it can be employed as a source of geothermal energy.
Humidifier – A device for increasing the water content of air; it is usually incorporated into an air conditioning system.
Hydration – The chemical process of combination or union of water with other substances.
Hydraulic fill – An earth structure or grading operation in which the fill material is transported and deposited by means of water being pumped through a flexible or rigid pipe.
Hydrogeology – The geology of groundwater, with particular emphasis on the chemistry and movement of water.
Hydrologic cycle – The constant movement and cycling of water by evaporation, precipitation, and condensation in the Earth-atmosphere system.
Hydrology – The science dealing with the properties, distribution, and circulation of water in relation to the land surface.
Hydrolysis – The reaction of a salt with water to form an acid and a base.
Hydrophilic – A term used to describe a substance with an affinity for water.
Hydrophobic
