– The part of the Earth that is composed of water, including oceans, seas, lakes, rivers, icecaps, etc.
Hydrothermal – A term used to describe any geological process involving heated or superheated water.
Ice – The allotropic, crystalline form of water.
Imported water – Water supply that is conveyed from one watershed to be used in another.
Impurity – The presence of one substance in another, often in such low concentration that it cannot be measured quantitatively by ordinary analytical methods.
In situ – A term used to describe any reaction occurring in place, and a term used to describe a fossil, mineral, or rock found in its original place of deposition, growth, or formation.
Induced draft – The negative pressure created by the action of a fan, blower, or other gas-moving device.
Industrial wastewater – The wastewater generated from industrial processes.
Industrial water – The water that is withdrawn from a source for sole use in an industrial process.
Infiltration – The penetration of water through the ground surface into sub-surface soil or the penetration of water from the soil into sewer, or other pipes through defective joints, connections, or other leaks in a system, or a land application technique where large volumes of wastewater are applied to land, allowed to penetrate the surface, and percolate through the underlying soil.
Inflow – An entry of extraneous stormwater into a sewer system from sources above the ground surface, typically through manhole covers or direct pumping, i.e. via sump pumps.
Influent – Any untreated wastewater stream flowing into a wastewater treatment plant or non-potable water entering a water treatment plant.
Injection well – A well into which fluids are injected for purposes such as waste disposal, remediation, hydraulic control, etc.
Injection zone – A geological formation, group of formations, or part of a formation receiving fluids through a well.
Inland waters – The waters of the United States in the inland zone, waters of the Great Lakes, and specified ports and harbors on inland rivers.
Inland zone – Land inside the coastal zone, excluding the Great Lakes and specified ports and harbors on inland rivers.
Insoluble – A term used to describe a substance that is incapable of being dissolved in a liquid.
Instantaneous sampling – The collecting of a sample in a very short period of time so that the sampling time is insignificant in comparison with the duration of the operation or the period being studied.
Instream use – The water use taking place within a stream channel, hydroelectric power generation, navigation, water quality improvement, fish propagation, or recreation.
Interstitial water – The water contained in the interstices of rocks, where the origin of the water is unknown or unspecified.
Ion exchange – A mass transfer process that involves the interchange of ions between a liquid and a solid material; it can be employed to concentrate and recover desired materials or to remove undesired ions from a water supply.
Irrigation – A technique for applying water or wastewater to land areas to supply the water and nutrient needs of plants.
Irrigation districts – Special units of local government that control the bulk of surface water supplies, primarily in the Western states in the United States.
Irrigation field practices – Techniques that keep water in the field, more efficiently distribute water across the field, or encourage the retention of soil moisture.
Irrigation withdrawals – Withdrawal of water for application on land to assist in the growing of crops and pastures.
Isothermal – A term used to describe a process that exhibits no change in temperature, i.e. constant temperature.
Lagooning – The placement of solid or fluid material in a basin, reservoir, or artificial impoundment for purposes of treatment, storage, or disposal.
Lake – An inland body of fresh or saltwater of considerable size, occupying a basin on the Earth’s surface.
Land application – The discharge of wastewater onto the ground surface for treatment or disposal.
Leaching – A process by which something is removed by percolating, or trickling liquid; it usually refers to the removal of components in soil by water.
Leachate – Any liquid, including any suspended components in the liquid, that has percolated through or drained from a solid, such as liquid generated from waste in a landfill.
Leakage – An undesired and gradual escape or entry of a fluid into or out of a tank or vessel.
Levee – A dike or ridge at the side of a river, intended to protect the land side from flood waters or to confine the stream flow to its regular channel.
Limnetic zone – The well-lit, open-water surface region of a lake away from shore.
Limnology – The scientific study of the physical, chemical, and biological components of freshwater with emphasis on plants and animal life.
Liquid – An amorphous, noncrystalline state of matter; the molecules of which are much more highly concentrated than in gases and usually less concentrated than in solids.
Liquid permeation – The movement of liquid components from one side of a membrane to the other.
Liquid-liquid extraction – The separation of a solute based on its relative solubility in two immiscible liquids, usually water and an organic solvent.
Low-flow showerhead – A showerhead that produces 2.5 gallons per minute or less, as compared to the 4.5 gallons per minute produced by most older standard showerheads.
Low-flush toilet – A toilet that requires 1.6 gallons of water per flush or less, as compared to the 3.5 to 5 gallons of water required to flush most older, standard toilets.
Magma – The molten material within the Earth’s crust that is composed of silicates and volatiles (water and gases) in complex solution.
Make-up water – The water that is employed to replenish a system that loses water through leakage, evaporation, etc.
Manifold – A pipe fitting with numerous branches to convey fluids between a large pipe and several smaller pipes or to permit the choice of diverting flow from one of the several sources or to one of the many discharge points.
Mantle – The part of the interior of the Earth between the crust and the core.
Maximum contaminant level (MCL) – A maximum allowable level of a compound within a treated drinking water deemed safe for human
