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 consumption; concentrations above this level are deemed unsafe and unacceptable for human consumption.
Membrane – A thin sheet of material through which a gaseous or liquid solution may pass.
Membrane hydrolysis – A hydrolysis process that occurs when a colloidal electrolyte is separated from pure water by a membrane.
Membrane selectivity – The ability of a membrane to allow passage of only anions or cations.
Microorganism – An organism of microscopic size generally considered to include bacteria, algae, protozoa, fungi, and rotifers but excluding viruses; they (primarily bacteria) are employed in biological wastewater treatment processes to remove biodegradable organic matter and suspended solids. Some species are human pathogens and are removed by water and wastewater treatment to prevent disease transmission and protect public health.
Mine water – The water encountered in mining operations that, if discharged into surface streams or other bodies of water, often contaminates them and makes them unfit for beneficial use.
Mineral spring – A spring that contains high levels of mineral salts.
Miscibility – The ability of a liquid or gas to dissolve uniformly in another liquid or gas.
Monitoring – A periodic or continuous surveillance or testing to determine the level of compliance with statutory requirements and pollutant levels in various media, or in living things.
Monitoring well – A well drilled to collect groundwater samples for analysis to determine the amounts, types, and distribution of contaminants in the groundwater.
Mother liquor – A concentrated solution that is substantially freed from undissolved matter by filtration, centrifuging, or decantation; the product can be obtained by evaporation and/or crystallization.
Municipality – A city, town, borough, county, parish, district, or other public body.
Natural resources – The land, fish, wildlife, biota, air, water, groundwater, drinking water supplies, and other such resources belonging to, managed by, held in trust by, pertaining to, or otherwise controlled by the United States.
Navigable water – Any stream, lake, arm of the sea, or other natural body of water that is navigable and of sufficient capacity to float watercraft for the purposes of commerce,