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 Absolute humidity – The amount of water vapor present in a unit mass of air, which is usually expressed as kilograms of water vapor per kilogram of dry air or pounds of water vapor per pound of dry air.

 Absolute pressure – The actual pressure exerted on a surface that is measured relative to zero pressure; it equals the gauge pressure plus the atmospheric pressure.

 Absolute pressure gauge – A device that measures the pressure exerted by a fluid relative to a perfect vacuum.

 Absolute scale – A temperature scale that is based on absolute zero and that uses units of measurement equivalent to Centigrade degrees on the Kelvin scale or to Fahrenheit degrees on the Rankine scale.

 Absolute temperature – The temperature expressed in degrees Kelvin (K) or degrees Rankine (°R).

 Absolute temperature scale – A scale (e.g. Kelvin, Rankine) in which temperatures are measured relative to absolute zero.

 Absolute vacuum – A void that is completely empty of matter.

 Absolute zero temperature – The temperature of zero degrees on either the Kelvin or Rankine scale at which molecular motion is thought to cease.

 Absorbate – A substance that is taken up and retained by an absorbent.

 Absorbent – Any substance that takes in or absorbs other substances.

 Absorber – A device in which a gas is absorbed by contact with a liquid.

 Absorption – The process in which one material (the absorbent) takes up and retains another (the absorbate) to form a homogenous solution; it often involves the use of a liquid to remove certain gas components from a gaseous mixture.

 Absorption tower – A vertical tube or pipe in which a rising gas is partially absorbed by a liquid, usually in the form of falling droplets.

 Acid – A material containing hydrogen that produces at least one hydrogen ion when dissolved in a water solution; it can react with and neutralize a base to form a salt.

 Acre-foot – A measure of volume; 43,560 cubic feet, 325,900 gallons, or the volume of water covering 1 acre, 1 foot deep.

 Action level – The level of a contaminant that, if exceeded, triggers treatment or other requirements that a water system must follow.

 Acute health effect – An immediate effect (i.e. within seconds, minutes, hours, or days) that may result from exposure to certain drinking water contaminants (e.g. pathogens).

 Adiabatic – A term used to describe a system in which no gain or loss of heat occurs.

 Adjudication – A process performed by the courts that determines ownership of groundwater and assigns a Watermaster to manage or enforce pumping rights and sometimes water quality; this is often a multiyear process.

 Advanced wastewater treatment – Any process that is employed for the treatment of wastewater that follows secondary treatment and serves to improve the quality of effluent prior to reuse or discharge; this may include the removal of phosphorus, nitrogen, suspended solids, dissolved organic compounds, dissolved solids, etc.

 Aerobic – A term used to describe a system that requires oxygen to sustain itself.

 Agricultural irrigation – Water distribution systems and practices in agriculture.

 Air stripping tower – A tower used to remove volatile organic chemicals (such as solvents) from contaminated water by causing them to evaporate; polluted water is sprayed downward through a tower filled with packing materials while air is blown upward through the tower.

 Algae – A large group of aquatic plants that contain chlorophyll; they vary from a single cell to multicellular organisms; algae exist in saltwater and freshwater, and can adversely affect water quality when excessive growth and death occurs (eutrophication), resulting in the lowering of the oxygen content in water.

 Algae bloom – A sudden proliferation of algae in water bodies, usually stimulated by the excessive input of nutrients (e.g. nitrogen and phosphorous); this can adversely affect water quality by causing a lowering of the oxygen content in the water.

 Ambient – A term used to describe the surrounding area or environment.

 Anaerobic – A chemical/biochemical reaction, process, or microorganism that occurs in the absence of oxygen.

 Appropriative water rights – Holders can use available water that is not taken by anyone else.

 Aquatic – A term used to describe any organism growing in, living in, or frequenting water.

 Aquatic growth – Any floating, drifting, or attached organism in a body of water.

 Aqueduct – A conduit or channel employed to convey water from one location to another.

 Aqueous solution – A solution with water as the liquid phase.

 Aquifer – A geologic formation, group of formations, or part of a formation that is capable of yielding a significant amount of groundwater to wells or springs.

 Artesian well – A well tapping a confined aquifer in which the static water level is above the bottom of the upper confining unit; a flowing artesian well is a well in which the water level is above the land surface.

 Audit – The examination of something with the intent to check, verify, or inspect.

 Available oxygen – The quantity of dissolved oxygen available for oxidation of organic matter in a water body.

 Backwash – Water used in a filtration system to separate clogging material from a filter media to clean the filter so it may be put back into service.

 Backflow – A flow condition induced by a differential in pressure that causes the flow of water or other fluid into the distribution pipes of a reservoir from any source other than its intended primary source.

 Base – Any compound that dissociates in aqueous solution to yield hydroxyl ions; it is employed to neutralize acids.

 Batch process – An unsteady-state process that is not continuous; its operations are carried out with discrete quantities of material.

 Bedrock – The solid rock mass located below the loose material on the Earth’s crust such as soil, glacial drift, or alluvium.

 Biota – All of the living organisms that exist in an area.

 Blackwater – A water that contains human wastes.

 Blowdown – The cyclic or constant removal of a portion of any process flow to maintain the constituents of the flow at a desired level.

 Boiler feedwater – The water fed into a boiler to replace that evaporated in the generation of steam.

 Boiling point – The temperature of a liquid at which its vapor pressure is equal to that of the atmospheric pressure of the environment; it is 100°C (212°F) for water at sea level.

 Bound water – The water molecules that are tightly held by various chemical groups in a larger molecule.

 Brackish water – A water with a salt content in the range between that of freshwater and seawater.

 Brine – A concentrated solution of salt and water that remains after the removal of a distilled product.

 By-pass – The avoiding of a particular portion of a process or system.

 By-product – A material that is not one of the primary products and is not solely or separately produced by the production process.

 Calibration – The determination, checking, or adjustment of the accuracy of any instrument that gives quantitative measurements.

 Carcinogen – Any substance that can cause cancer.

 Carryover – The entrainment of liquid or solid particles in the vapor evolved by a boiling liquid or from a process unit.

 Catch basin – A chamber or well, usually built at the curbline of a street, that admits surface water for discharge into a storm water drain.

 Chemical analysis – The analysis by chemical methods that provides the composition and concentration of a substance.

 Chemical sludge – The sludge obtained by treatment of wastewater with chemicals.

 Chemical treatment – Any one of a variety of technologies that use chemicals or a variety of chemical processes to treat a system or waste.

 Chlorine demand – The amount of chlorine necessary to produce a free chlorine residual in a water sample.

 Chlorine water – A clear, yellowish liquid that degrades on exposure to air and light; it is employed as a deodorizer, disinfectant, and antiseptic.

 Clarification – The removal of suspended solids from wastewater by gravity settling; this process is often accelerated through coagulation of and flocculation of small solids with chemicals.

 Closed loop – A term used to describe an enclosed process.

 Closed loop cooling tower – Water-conserving cooling tower system in which water used for cooling is recycled through a piping system that cools the water.

 Coastal waters – The waters of the coastal zone, except for the Great Lakes and specified ports and harbors on inland rivers.

 Coastal zone – The lands and water adjacent to the coast that exert an influence on the uses of the sea and its ecology, or whose uses and ecology are affected by the sea.

 Coastline – The line separating the land surface and the water surface of the sea.

 Coliform – A group of related bacteria whose presence in water may indicate contamination by disease-causing microorganisms; indicators organisms of recent fecal contamination.

 Communicable disease – An illness that is caused by a specific infectious agent or its toxic products, and that arises through transmission from a reservoir to a susceptible host.

 Community water system – A public water system that serves at least 15 service connections employed by year-round residents or regularly serves at least 25 year-round residents.

 Condenser – Any device that cools gases or vapors to liquid.

 Conduit – Any artificial or natural duct, either opened or closed, employed for conveying fluids.

 Confined aquifer – An aquifer in which groundwater is confined under pressure.

 Confined groundwater – Water in an aquifer that is bounded by confining beds and is under pressure significantly greater than atmospheric pressure.

 Confining bed – A layer or mass of rock having very low hydraulic conductivity that hampers the movement of water into and out of an adjoining aquifer.

 Connate water – Water entrapped in the interstices of sedimentary rock at the time of its deposition.

 Conservative pollutant – A pollutant that does not decay, does not react, is persistent, and is not biodegradable.

 Contaminant – Any physical, chemical, biological, or radiological substance that has a harmful effect on human health or the environment when contained in air, water, or soil.

 Continuous sampling – The continuous withdrawal for analysis of a sample from some larger quantity of liquid, air, or solid.

 Cooling tower – A hollow, vertical structure, perhaps with internal baffles, to disperse water so it is cooled by flowing air and by evaporation at ambient temperature.

 Cooling tower makeup – Water added to a recirculating cooling tower water stream to compensate for water evaporation losses.

 Cooling water – Water typically used to cool heat-generating equipment or to condense gases in a thermodynamic cycle.

 Cooling water blowdown – The procedure used to reduce total dissolved solids by removing a portion of poor-quality recirculating water.

 Cooling water drift – Unevaporated water carried out of a cooling tower by the airflow; it has the same composition as the recirculating water.

 Cooling water evaporation – Cooling water recycling approach in which water loses heat when a portion of it is evaporated.

 Creep – The movement of water under or around a structure built on permeable foundations.

 Cryogenics – The production and utilization of extremely low temperatures.

 Crystallization – The change of state of a substance from a liquid to a solid by the phenomenon of crystal formation by nucleation and accretion (e.g. the freezing of water into ice).

 Deaeration – A process by which dissolved air and oxygen are removed from water.

 Decantation – The separation of a liquid from a solid or a higher density liquid with which it is immiscible by drawing off the fluid.

 Dechlorination – The removal of chlorine from a substance by chemically replacing it with hydrogen or hydroxide ions in order to detoxify the substances.

 Decontamination/detoxification – A process that converts toxic wastes into nontoxic compounds.

 Deep-well injection – A method of ultimate disposal that involves depositing liquid waste into a deep well beneath the surface of the Earth for permanent storage.

 Dehumidifier – A device incorporated into many air conditioning systems to dry incoming air by passing it across a bed of a hygroscopic substance or through a spray of very cold water.

 Dehydration – The chemical process where water in a chemical or material is removed.

 Deionized water – Common industrial water devoid of dissolved salts and organics, used to remove contaminants from products and equipment.

 Demineralization – The process of removing dissolved minerals from water by physical, chemical, or biological means.

 Demister – A device composed of plastic threads, wire mesh, or glass fibers employed to remove liquid droplets entrained in a gas stream.

 Desalination – The extraction of freshwater from sea or other salt water by the removal of salts, usually by evaporation, reverse osmosis, or crystallization.

 Desert – A terrestrial environment where evaporation exceeds precipitation, with consequent lack of vegetation.

 Detoxification – The destruction of the toxic aspects of a substance.

 Dew point – The temperature at which the first droplet of water forms on the progressive cooling of a mixture of air and water vapor; at the dew point, the air becomes saturated with water.

 Dialysis – The separation of smaller molecules from larger ones in a solution by means of the diffusion from a concentrated solution to a dilute solution across a semipermeable membrane.

 Discharge – The volume of water that flows past a given area in a given time.

 Dissolved solids (TDS) – Total dissolved solids, minerals, or salts in water.

 Disinfectant – Any substance that destroys harmful microorganisms or inhibits their activity.

 Disposal well – A well employed for the disposal of waste into a subsurface stratum.

 Dissolved oxygen (DO) – The oxygen freely available in water; it is one of the most important indicators of the quality of a water supply since oxygen is necessary for the life of aquatic organisms.

 Distillation – A process of separating the constituents of a liquid mixture by means of partial vaporization of the mixture and separate recovery of vapor and residue due to a difference of vapor pressure.

 Distilled water – A water of high purity, prepared by repeated distillation.

 Domestic wastewater – Wastewater generated from household activities that includes sanitary waste, liquid waste from food preparation and laundering, bathing and showering waste, and other liquid cleaning wastes generated from households.

 Downstream – The regions of a river system located in a hydraulically lower location than a given position along a stream or river. A section of the river system is hydraulically lower if gravity transports the water in the direction of or nearer to the mouth of the stream or river.

 Downtime – A term used to describe the periods when a system is unavailable or not operating.

 Drain – Any channel that carries off surface water.

 Drawdown – The difference between the water level in a well before pumping and the water level in the well during pumping.

 Drilled well – A well constructed by either percussion or rotary hydraulic drilling.

 Drinking water supply – Any raw water source that is or may be employed by a public water system or as drinking water by one or more individuals.

 Dry cooling – Cooling system using air instead of water as the cooling fluid to eliminate the use and evaporative loss of water.

 Dry well – A shallow well used in storm water systems for the infiltration of collected storm water for shallow groundwater recharge; the well is dry during dry weather conditions.

 Duct – A round or rectangular conduit, usually metal or fiberglass, employed to transport fluids, usually air.

 Dystrophic lake – An acidic, shallow body of water that contains high concentrations of humic substances and organic acids that are often brown in color; it contains many plants but few fish.

 Ebb tide – The tide occurring at the ebb period of tidal flow; it is sometimes referred to as falling tide to describe the direction of the current.

 Ecosphere – The global sum of all ecosystems on Earth.

 Ecosystem – The interacting system of a biological community and its nonliving surroundings.

 Eddy – A circular movement occurring in flowing water caused by currents in the water induced by obstructions or changes and irregularities in the banks or bottom of the channel, or by differences in temperature.

 Effluent – Any fluid emerging from a pipe or similar outlet that enters the environment; it usually refers to treated wastewater from municipal or industrial treatment plants.

 Electrolysis – The use of a direct electric current to drive an otherwise non-spontaneous chemical reaction.

 Elution – The process of moving a substance through a bed by means of a slow-moving stream.

 Elutriation – A process for separating particles based on their size, shape, and density, using a stream of gas or liquid flowing in a direction usually opposite to the direction of sedimentation. This method is mainly used for particles smaller than 1 μm.

 Empirical – Anything derived from experimentation or observation and not from fundamentals or theory.

 Endothermic – A term used to describe a process or change that occurs with absorption of heat.

 Enrichment – The addition of constituents, generally nutrients, from wastewater treatment plant effluent or agricultural runoff to surface water.

 Environment – The system of external conditions affecting the existence and development of an individual or organism.

 Environmental audit – An independent assessment of the current status of a company’s compliance with applicable environmental requirements.

 Epidemiology – The study of diseases as they affect population, including the distribution of disease, or other health-related states and events in human populations, the factors that influence this distribution, and the application of this study to control health problems.

 Erosion – The breakdown of solid rock into smaller particles and its removal by wind, water, or ice; the removal of surface soil by wind, water, or ice.

 Estuary – A region of interaction between rivers and near-shore ocean water where tidal action and river flow create a mixing of fresh and saltwater.

 Euphotic zone – The upper zone of a sea or lake into which sufficient light can penetrate for active photosynthesis to take place.

 Eutrophic lake – A body of water that is characterized by an abundant accumulation of nutrients that support a dense growth of algae and other organisms, the decay of which depletes the shallow waters of oxygen in summer.

 Eutrophication – The process by which a body of water becomes enriched in dissolved nutrients (such as phosphates) that stimulate the growth of aquatic plant life usually resulting in the depletion of dissolved oxygen.

 Evaporation – The conversion of a liquid into a vapor.

 Evaporation pond – A shallow pond or impoundment with a large surface area that is designed to contain wastewater and allow it to evaporate with no discharge to the environment.

 Excursion – An unintentional occurrence, such as a discharge of pollutants above the permitted amount, due to unplanned reasons.

 Exempted aquifer – An underground body of water defined in the underground injection control program as an aquifer that is “not reasonably expected” to be a source of drinking water and that is exempted from regulations barring underground injection activities.

 Exothermic – A term used to describe a reaction or process during which heat is released.

 Faucet aerator – Device that can be installed in a sink to reduce water use.

 Feed – The material supplied to a processing unit for treatment or processing.

 Feedforward control system – A system in which changes are detected at the process input and an anticipated correction signal is applied before process output is affected.

 Feedstock – The raw materials supplied to manufacturing or processing plants for use in the production of goods or materials.

 Film-type condensation – The process by which a saturated vapor comes into contact with a surface whose temperature is below the saturation temperature and uniformly condenses on the surface.

 Filter membrane – A thin film containing many fine pores that is employed to filter a liquid or solid stream.

 Filter press – A mechanical device that forces sludge, usually conditioned with coagulant addition, between moving filter membranes to squeeze water out of it and produce a dry sludge cake of between 15 and 25% solids.

 Finished water – Water that has been treated and is ready to be delivered to customers.

 Flash drum – A unit in which volatile components are vaporized and separated from a liquid stream for further fractionation.

 Flash evaporator – A distillation device in which saline water is injected in a superheated state into a vessel under vacuum and in which boiling occurs without the usual heat source.

 Flocculation – The process by which solids in water or sewage are made to increase in size by biological or chemical action so that they can be more easily separated from the water by sedimentation.

 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 – A term used to describe a substance that separates from water or surfaces that repel water.

 Hydrosphere – 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 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, trade, transportation, or recreation, as indicated under the Clean Water Act.

 Nitrates – Oxidized nitrogen that forms a monovalent anion; is the form of nitrogen that is produced from the aerobic oxidation of ammonia (nitrification); and that serves as an electron acceptor for the anaerobic oxidation of organic matter (denitrification); it is a nutrient that can result in eutrophication of lakes and ponds and is the cause of “blue baby” syndrome in humans if in high concentrations in drinking water: its MCL is 10 mg/L in drinking water.

 Nonpoint source – A source of pollutant discharge that is not traceable to a discrete location.

 Non-potable water – Water considered unsafe and/or unpalatable for drinking (see also potable water).

 Nutrient – Any element or compound that is essential to the life and growth of plants or animals, either as such or as transformed by chemical or enzymatic reactions; nitrogen and phosphorous are the major nutrients of concern in polluted waters.

 Ocean water (seawater) – A uniform solution contained approximately 96.5% water and 3.5% ionized salts; ingestion of substantial amounts will create a bodily chloride imbalance with harmful effects.

 Oceanic – A term used to describe the parts of the oceans deeper than 200 meters.

 Oceanography – The science dealing with oceans, including their form, physical and chemical features, and related phenomena.

 Open channel – Any natural or artificial waterway or conduit in which a fluid flows with a free surface exposed to atmospheric pressure.

 Osmosis – The passage of a pure liquid (usually water) through a semipermeable membrane from a solution of low concentration into a solution of a higher concentration (e.g. the flow of pure water into a solution of salt and water); see also reverse osmosis.

 Osmotic pressure – The pressure that results from osmosis.

 Outfall – The place where an effluent is discharged into a receiving water.

 Outfall sewer – A pipe or conduit that transports wastewater effluent to a final point of discharge.

 Oxygenation – The increase of the dissolved oxygen content within water through aeration.

 Ozonation – The addition of ozone to a water supply to reduce taste and odor problems and for disinfection.

 Ozonator – A device that creates ozone from oxygen and adds it to water.

 Parts per billion (ppb) – The fraction (ppbm for mass fraction and ppbv for volume fraction) multiplied by 109; it is a unit used to measure extremely small concentrations of a substance; equivalent to units of µg/L in water.

 Parts per million (ppm) –The fraction (ppmm for mass fraction and ppmv for volume fraction) multiplied by 106; it is a unit used to measure small concentrations of a substance; equivalent to units of mg/L in water.

 Pathogenic waste – A discarded waste that contains organisms capable of causing disease.

 Percolation – The flow of a liquid downward through a filtering medium or soil layer.

 Permafrost – The portion of the Earth which is permanently frozen, such as the Artic regions and portions of Alaska.

 Permeability – The degree to which a liquid can move freely through soils.

 Physical quality – The physical characteristics possessed by a material; it includes temperature, color, odor, and turbidity.

 Physical treatment – A water or wastewater treatment process that utilizes physical means for pollutant removal; processes include screening, grinding, settling, filtration, and centrifugation.

 Physicochemical – A term used to describe processes that involve both physical and chemical characteristics; adsorption is one such process that involves both physical and chemical attraction of an adsorbate to an adsorption site.

 Point source of pollution – Pollution originating from a discrete source, such as the outflow from a pipe, ditch, tunnel, concentrated animal-feeding operation, or floating craft.

 Pollutant – Any harmful substance present in air, water, or soil.

 Pollution – The direct or indirect alteration of the physical, thermal, biological, or radioactive properties of any part of the environment in such a way as to create a hazard or potential hazard to the health, safety, or welfare of any living species.

 Potable water – Water that is safe for human consumption.

 POTW – Publicly owned treatment works, or municipal wastewater treatment plants.

 Pre-aeration – A water treatment process in which the removal of dissolved gases, and the addition of oxygen are performed; typically, in aeration towers or aeration tanks.

 Precipitate – A solid that separates out from a liquid due to some physical or chemical change in the liquid.

 Pretreatment – Any process employed to partially remove pollutants from water prior to any subsequent treatment process.

 Psychrometric chart – A chart employed to determine the properties of moist air as a function of temperature.

 Public water system (PWS) – Any system that provides piped water for human consumption to at least 15 service connections or regularly serves 25 individuals.

 Purging – A cleansing or removal of impurities, foreign matter, or undesirable contaminants from a process through periodic withdrawals of liquids or solids.

 Purification – The removal of undesirable constituents from a substance by one or more separation techniques.

 Rank – The stage reached by coal in the course of its carbonation; the chief ranks, in order of increasing carbon content, are lignite, subbituminous coal, bituminous coal, and anthracite.

 Raw water – The untreated water that enters the first treatment unit of a water treatment plant.

 Receiving water – Any body of water (e.g. river, lake, ocean, stream, etc.) into which treated wastewater is discharged to.

 Recirculating cooling water – The recycling of cooling water to greatly reduce water use by reusing the water to perform several cooling operations.

 Reclaimed water – Treated wastewater that is reused for generally non-potable uses to supplement or replace other raw water supplies.

 Recycled water – Wastewater that has been treated for reuse and is recycled, generally for non-potable uses within a home or industrial facility.

 Red tide – A proliferation and accumulation of certain microscopic algae, predominantly dinoflagellates, in coastal waters; some species produce toxins that are labeled harmful algae blooms, or Harmful Algal Blooms (HABs), that pose a serious and recurring threat to human health, wildlife, marine ecosystems, fisheries, and coastal aesthetics.

 Red water – A rust-colored water, usually resulting from the presence of precipitated ferric iron salts.

 Release – Any spilling, leaking, pumping, pouring, emitting, emptying, discharging, injecting, escaping, leaching, dumping, or disposing into the environment.

 Renewable water supply – The rate of supply of water (volume per unit time) potentially or theoretically available for use in a region on an essentially permanent basis.

 Reproducibility – The ability to repeat an experiment, reaction, measurement, or process and produce the same results.

 Reservoir – Any body of water employed for the storage, control, or regulation of water.

 Reverse osmosis – A water treatment process employed to separate water from pollutants by the application of pressure to force the water through a semipermeable membrane.

 Rinse – The removal of foreign materials from a surface by using a flow of liquid.

 Rinse water – Water used to remove debris and contaminants from products and equipment.

 Riptide – A strong surface current of short duration flowing outward from the shore.

 River basin – The land area drained by a river and its tributaries.

 River bed – The bottom of a river.

 Runoff – The water from precipitation that exceeds an areas infiltration and storage that flows over the ground into a surface water body.

 Rural area – The area outside the limits of any city, town, village, or other designated residential or commercial area.

 Saline water – Water that generally is considered unsuitable for human consumption or for irrigation because of its high content of dissolved solids, generally greater than 10,000 mg/L of dissolved solids; 35,000 mg/L dissolved solids is normally assigned to seawater.

 Salinity – The amount of salts or minerals dissolved in water.

 Salinization – A process in which a soluble salt accumulates in soils.

 Salt – A chemical compound formed when the hydrogen ion of an acid is replaced by a metal, or when an acid reacts with a base in an aqueous solution.

 Salting out – A reduction in the water solubility of certain molecules in a solution of very high ionic strength.

 Saltwater intrusion – The displacement of fresh groundwater by higher density salt water near coastal regions.

 Sample – A representative specimen of a liquid, solid, or gas collected for the purpose of determining its composition.

 Sampling – A method employed to obtain representative test samples; it consists of the collection, isolation, and the possible concentration of a small fractional part of a larger volume of a media.

 Sand bar – A ridge of sand built up by deposition to the surface or near the surface of a river or along a beach.

 Sand dune – A mound or ridge of loose sand blown by prevailing winds.

 Sanitary survey – An on-site review of water sources, facilities, equipment, operation, and maintenance of a public drinking water system to evaluate the adequacy of those components for producing and distributing safe drinking water.

 Saturated rock – A rock that has all of its void spaces filled with fluid.

 Saturated soil – A soil that has all of its void spaces filled with fluid.

 Saturated zone – A subsurface soil or rock zone in which all the interstices or voids are filled with water.

 Saturation temperature – The minimum temperature at which air is saturated with water vapor; the boiling point of water.

 Scour – The action of a flowing liquid as it erodes and carries away material on the sides or bottom of a channel.

 Screening – The use of screens to separate and remove coarse floating and suspended solids from water.

 Sea – A large body of saltwater, second in rank to an ocean, that is generally part of, or connected to an ocean at some point.

 Sea level – The surface of the sea that is employed as a reference for elevation.

 Seawall – A coastal wall built to provide protection against erosion and/or flooding from the ocean.

 Seawater intrusion – The movement of seawater into freshwater aquifers near the coast when these freshwater aquifers are over pumped.

 Secondary drinking water regulation – A regulation that sets a maximum acceptable level for contaminants that adversely affect the taste, odor or appearance of water or otherwise adversely affect the public welfare.

 Sediment – The solid material or deposits that have settled from a fluid.

 Sedimentation tank – A tank in which water or wastewater containing settleable solids is retained for a period of hours to allow these solids to move to the bottom of the tank by gravity; the settled solids are removed from the bottom and the floating solids are skimmed off the top for further treatment and disposal; also called clarifiers. See also Settling tank.

 Semiconfined aquifer – An aquifer that is partially confined by a layer (or layers) of low permeability soil or rock through which recharge and discharge nevertheless may occur.

 Semipermeable membrane – A membrane that allows substances of a certain size to pass through it while preventing the passage of larger ones.

 Settleable solids – The materials that are of sufficient size and density to sink to the bottom of a sedimentation tank.

 Settling tank – A tank used in water and wastewater treatment to hold water for a period of hours, during which heavier particles sink to the bottom for removal, treatment, and disposal; also called a clarifier. See also Sedimentation tank.

 Sewage – The wastewater produced by residential, commercial, institutional, and industrial facilities.

 Sewer – The system of pipes or conduits employed to collect and deliver wastewater to treatment plants or storm water surface water bodies.

 Shore – The land bordering any body of water.

 Slow sand filtration – A treatment process that involves the passage of raw water through a bed of sand at low velocity which results in the substantial removal of chemical and biological contaminants through the development of a biolayer (schmutzdecke) at the sand surface.

 Sludge – The thick, semisolid waste that accumulates as a result of the chemical coagulation, flocculation, and settling which occurs during drinking water treatment; the thick, semisolid biomass that is produced in the biological treatment of wastewater.

 Sludge cake – The dewatered sludge from a treatment plant that has a solids content of 18 to 30% solids.

 Sludge dewatering – The process of removing water from sludge using methods such as air drying, pressure filtration, vacuum filtration, centrifugation, or belt presses.

 Sludge dryer – A mechanical device for the removal of a large percentage of moisture from sludge by heat.

 Sludge filter – Mechanical devices in which wet sludge, usually conditioned by a coagulant, is dewatered by means of vacuum (vacuum filter) or elevated pressure (pressure filter).

 Slurry –A high solids content mixture of particulate matter and liquid.

 Sodium chloride (NaCl) – A colorless, transparent, crystalline solid or white, crystalline powder; it is noncombustible.

 Soft water – A water with a low concentration of divalent, primarily calcium and magnesium, cations.

 Softening – The chemical precipitation of divalent cations which cause the hardness of water; chemical coagulants, alum or ferric chloride, are normally used as coagulants, with flocculation and sedimentation following coagulant addition.

 Soil drainage – The removal of excess water from a soil by gravity.

 Soil moisture – Water content in the soil, generally given as volume or weight percent.

 Solubility – The ability of one substance to be dissolved by another.

 Spray irrigation – The application of water to a land surface via spray droplet application.

 Steam drum – A vessel in a boiler in which the saturated steam is separated from the steam–water mixture and into which the feedwater is introduced.

 Still – An apparatus to purify liquids through heating to selectively boil and then cool to condense the vapor (e.g. to prepare alcoholic beverages, distilled water, etc.).

 Storm drain – A drain employed for conveying storm water runoff to a sewer.

 Storm sewer – A piping system employed exclusively for the transport of stormwater from streets, buildings, and surface runoff.

 Stormwater runoff – The portion of the volume of a rainfall event that exceeds the infiltration and storage capacity of a watershed.

 Subterranean water – The water that occurs in open spaces within rock materials of the Earth’s crust.

 Superheated steam – Steam at a temperature above its boiling point at a given pressure.

 Supersaturation – An unstable condition in which a solvent contains more dissolved matter or gas than is present in a saturated solution of the same components at an equivalent temperature.

 Surface water – All water that is above the surface of the ground and is naturally open to the atmosphere.

 Surge irrigation – The intermittent application of water to irrigation pathways; this method pulses water down the furrow and creates more uniform irrigation water distribution.

 Suspension – A system in which very small particles are uniformly dispersed in a liquid or gaseous medium.

 Tank – A stationary device that is essentially a container, e.g. designed to contain an accumulation of waste or liquid, that is constructed primarily of non-earthen materials which provide structural support.

 Temperature gradient – The change in temperature with distance or position.

 Tertiary treatment – The advanced treatment of wastewater beyond secondary treatment; it may involve combinations of physical, chemical, and biological treatment processes to remove solids, nutrients, metals, salts, nonbiodegradable organics, etc. to prepare the waste for disposal into highly sensitive environments or for reuse.

 Thickener – A small circular or rectangular sedimentation tank, designed to increase the concentration of solids in a suspension.

 Thickening agent – Any of a variety of substances employed to increase the viscosity of liquid mixtures and solutions without changing its other properties.

 Tidal wave – An exceptionally large wave, tsunami, or increase in the water level along a shore due to strong winds, volcanic eruption, or earthquake.

 Tide – The periodic rising and falling of water that results from the gravitational attraction of the moon and sun acting on the rotating Earth.

 Total organic carbon (TOC) – The total amount of organic carbon present in water as organic compounds (e.g. amino acids, hydrocarbons, proteins, etc.).

 Toilet displacement device – Object placed in a toilet tank to reduce the amount of water used per flush; for example, weighted plastic jugs filled with water or toilet dams that hold back a reservoir of water when the toilet is flushing.

 Toxic – A term used to describe a poisonous substance that has a harmful effect on an organism by ingestion, inhalation, or skin absorption.

 Trace – A very small quantity of a constituent, the amount of which cannot be determined precisely because of its low concentration.

 Transpiration – The process by which water passes through living organisms, primarily plants, and into the atmosphere.

 Treatment – Any method, technique, or process that is designed to change the physical, chemical, and/or biological composition of a waste so as to neutralize it, recover energy or material resources from it, render it nonhazardous or less hazardous, or make it safer to transport, store, reuse, or dispose of.

 Tributary – A stream or river that flows into a larger stream or main stem (or parent) river or a lake.

 Trough – A structure employed to hold or transport fluids.

 Tsunami – A sea wave caused by an underwater seismic disturbance such as sudden faulting, a landslide, or volcanic activity.

 Turbid water – Water that is cloudy due to fine particles in suspension.

 Turbidity – A measure of the fine particles suspended in a fluid; measured as the proportion of light passed through a sample that is refracted by suspended particles in a water column.

 Ultrafiltration – The separation of a solute with a specific molecular size and shape from a solution by applying pressure to force the solvent to flow through a membrane.

 Underground sources of drinking water – The aquifers that are currently being employed as a source of drinking water, and those that are capable of supplying a public water system.

 Unconfined aquifer – An aquifer whose upper surface is free of a confining layer and thus is able to fluctuate under atmospheric pressure.

 Upstream – The regions of a river system located in the direction opposite to the flow of a stream from a given position; a section of a river system that is hydraulically higher, if gravity transports the water away from the given location.

 Uptake – The act of taking up, drawing up, or absorbing.

 Urban runoff – The storm water from city streets and adjacent domestic or commercial properties.

 Utility – Public water service provider.

 Vapor pressure – The pressure exerted by a vapor in equilibrium with a liquid at a given temperature.

 Virgin material – A raw, unused material.

 Wake – The visible trail of turbulence left behind a moving stream.

 Wash solvent – A liquid added to a liquid–liquid extraction to wash or enrich the purity of the solute used in the extraction process.

 Wash water – The water employed to wash equipment.

 Wastewater – The water used to carry liquid waste material, consisting of dissolved and suspended solids, organics, and nutrients from homes, businesses, institutions, and industries to wastewater treatment plants for contaminant removal prior to release to surface water.

 Wastewater operations and maintenance – The actions taken after construction of wastewater treatment facilities to assure that the facilities will be properly operated and maintained.

 Wastewater treatment – A series of processes in which wastewater is treated in order to remove or alter its objectional constituents to a degree that renders it less harmful or dangerous.

 Wastewater treatment plant – A series of unit operations including screening, sedimentation, digestion, stabilization, dewatering, disinfection, and other processes for removing pollutants from wastewater before it discharges into the environment.

 Wastewater treatment unit – A device that is part of a wastewater treatment facility which is subject to regulation.

 Water – A colorless, odorless, tasteless liquid composed of the elements hydrogen and oxygen.

 Water audit – Program involving sending trained water auditors to participating family homes, free of charge, to identify water conservation opportunities such as repairing leaks and installing low-flow plumbing and to recommend changes in water use practices to reduce home water use.

 Water conditioning – The treatments, excluding disinfection, that are intended to produce a water which is free of taste, odor, and undesirable contaminants.

 Water conservation – Activities designed to reduce the demand for water, improve efficiency of use, and reduce losses and waste of water in a potable water system.

 Water consumption – The quantity of water supplied in a municipality or district for a variety of uses during a given period.

 Water main – The water pipe, located in the street, from which domestic water supply is delivered to specific premises.

 Water pollution – The contamination of fresh or saltwater with materials that are toxic, noxious, or otherwise harmful to fish, man, or other animals.

 Water purification – Any process in which water is treated in such a way as to remove or reduce undesirable impurities.

 Water quality – The chemical, physical, or biological characteristics of water with respect to its suitability for a particular purpose.

 Water quality criteria – The specific levels of water quality that, if reached, are expected to render a body of water suitable for its designated beneficial use.

 Water quality standards – The state-adopted, EPA-approved, allowable numeric ambient water quality concentrations for surface water bodies defined for each beneficial use.

 Water recycling – Reuse of water for the same application for which it was originally used.

 Water reuse – Using treated wastewater (reclaimed water) for some beneficial purpose rather than discharging it to surface water; the deliberate use of reclaimed wastewater must be in compliance with applicable rules for a beneficial purpose (landscape irrigation, agricultural irrigation, aesthetic uses, ground water recharge, industrial uses, or fire protection).

 Water rights – The rights acquired under the law to use surface or groundwater for a specified purpose, in a given manner, and usually within the limits of a given period.

 Water softening – A treatment process designed to completely or partially remove hardness-producing ions, Ca2+ or Mg2+, from a potable water.

 Water solubility – A measure of the maximum concentration of a chemical compound that can result when it is dissolved in water.

 Water supplier – A person who owns or operates a water supply system; they can be public or private.

 Water supply system – The collection, treatment, storage, and distribution of potable water from source to consumer.

 Water surcharge – Imposition of an increased cost due to excessive water use.

 Water table – The upper level of the groundwater below which the ground is saturated with water.

 Water treatment – The purification of water to make it suitable for drinking or other beneficial uses.

 Waterlog – Occurs when water is added to land faster than it can drain.

 Watershed – The area surrounding a stream that supplies it with runoff.

 Waterway – Any body of water, other than the open sea, that is or can be employed by boats as a means of travel.

 Well – A bored, drilled, or driven shaft, or a dug hole, whose depth is greater than the largest surface dimension and whose purpose is to reach underground water supplies, or to store or bury fluids below ground.

 Wetland – An area covered or saturated permanently, occasionally, or periodically by fresh or salt water.

 WHO – World Health Organization of the United Nations, based in Geneva (www.who.int).