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Glossary of Water Terms

Learn With Us

Glossary of Water Terms

Glossary of Water Terms 


Use this glossary to find definitions, examples, and context for EMWD water, recycled and wastewater terms. 


Acre Foot (AF)
Covers one acre to a depth of one foot (football field). An acre foot is 325,900 gallons which is enough to meet the needs of two average southern California households a year.

The Act  
The Metropolitan Water District Act. State legislation signed into law by the governor on May 10, 1927, effective July 29, 1927. Metropolitan incorporated December 6, 1928.

Acre-foot per year.

A naturally occurring element in the environment. Arsenic in drinking water commonly comes from natural sources in the ground, but some can come from industrial pollution. At high concentrations it can cause cancer.

Natural underground layer of porous, water-bearing materials usually capable of yielding a large amount or water supply.


Best Management Practice . Methods that have been determined to be the most effective, practical means of preventing or reducing pollution.

A reverse flow condition, created by a difference in water pressures.

The solids, or sludge. That remain after wastewater treatment. This material is separated from the cleaned water, treated and composted into fertilizer. Biosolids are often referred to as sludge.

Brackish Water
Water containing dissolved minerals in amounts that exceed normally acceptable standard municipal, domestic, and irrigation uses. Considerably less saline than seawater.

Brown Act
Ralph M. Brown Act enacted by the State legislature governing all meetings of legislative bodies known as the Open Meeting requirements.


California Plan  
Officially “California’s draft Colorado River Water Use Plan,” also sometimes called the “4.4 Plan.” A planning document designed to reduce California’s reliance on surplus Colorado River water over the next 15 years through conservation, water transfers, and conjunctive use measures.

Any substance that can cause or aggravate cancer.

Community-based organization. Local organization with which Metropolitan works on mutually beneficial programs.

A naturally occurring element found in air, soil, water and food.

Chromium VI
One of the most common species of chromium, chromium VI can be carcinogenic and can constitute anywhere from 7 to 80% of the total chromium in drinking water.

California Environmental Quality Act that requires an assessment of the possible environmental impacts of public projects.

The application of chlorine to water, sewage, or industrial wastes, generally to disinfect or to oxidize undesirable compounds.

Clean Water Act (CWA)
The primary federal law that protects our nation’s waters, including lakes, rivers, aquifers, and coastal areas. The CWA provides a comprehensive framework of standards, technical tools, and financial assistance to address the causes of pollution and improve water quality.

Type of bacteria – main habitat being human intestinal tract.

Conjunctive use
The planned use of groundwater in conjunction with surface water in overall management to optimize total water resources.

Conservation Factor
The Conservation Factor (CF) is based on a landscape’s water use efficiency. A landscape with an efficient irrigation system and more low-water using plants will have a lower CF and require less water to maintain.

Cubic foot
A frequent water industry term of measurement, as in cubic feet per second. One cubic foot (cf) equals 7.48 gallons. A cubic foot per second is 450 gallons per minute.

Colorado River Aqueduct, built 1933-1941 and owned and operated by the Metropolitan Water District of Southern California.


Removing salts from ocean or brackish water by using various technologies.


Recognizable, relatively homogeneous units, including the organisms they contain, their environment and all the interactions among them.

Wastewater–treated or untreated–that flows out of a treatment plant, sewer, or industrial outfall.

Environmental impact report; a study and report on the possible effects of a proposed project, and what can be done to avoid or mitigate them.


A program to generate water by paying farmers to fallow land, i.e., not grow crops. The water not used for irrigation is then transferred to urban areas or stored for future use.


Gray water reuse
Reuse, generally without treatment, of domestic type wastewater for toilet flushing, garden irrigation and other nonpotable uses. Excludes water from toilets, kitchen sinks, dishwashers, or water used for washing diapers.

The supply of fresh water found beneath the Earth’s surface, usually in aquifers, which supply wells and springs. Because groundwater is a major source of drinking water, there is growing concern over contamination from leaching agricultural or industrial pollutants or leaking underground storage tanks.

Groundwater basin
A groundwater reservoir defined by all the overlying land surface and the underlying aquifers that contain water stored in the reservoir. Boundaries of successively deeper aquifers may differ and make it difficult to define the limits of the basin.

Groundwater mining
The withdrawal of water from an aquifer in excess of recharge over a period of time. If continued, the underground supply would eventually be exhausted or the water table could drop below economically feasible pumping lifts.

Groundwater overdraft
The condition of a groundwater basin in which the amount of water withdrawn by pumping exceeds the amount of water that recharges the basin over a period of years during which water supply continues approximate average.

Groundwater recharge
The action of increasing groundwater storage by natural conditions or by human activity.

Groundwater table
The upper surface of the zone of saturation (all pores of subsoil filled with water), except where the surface is formed by an impermeable body.

Gallons per minute.


Imported water
Water that has originated from one hydrologic region and is transferred to another hydrologic region.

Water, wastewater, or other liquid flowing into a reservoir, basin, or treatment plant.

Interceptor Sewer
A sanitary sewer which collects flow from a number of trunk sewers and conveys wastewater to a treatment plant, lift station, or another interceptor sewer.

IRP (Integrated Resources Plan)
The Metropolitan Water District (MWD) program is a coordinated effort of interested stakeholders to determine how best to meet the region’s water supply reliability objectives.


Lift Station
A facility within a sanitary sewer system which pumps sewage from a lower elevation to a higher elevation.


Million gallons per day.

Milligrams per liter.

A physical separation process where tiny, hollow filament members separate particles from water.


NPDES (National Pollutant Discharge Elimination System)
A federal permit authorized by the Clean Water Act which is required for discharge of pollutants to navigable waters of the United States which includes any discharge to surface waters - lakes, streams, rivers, bays, the ocean, wetlands, and storm sewer tributary to any surface water body.


A gas that is bubbled through water during the treatment processes to kill bacteria.


A chemical used in manufacturing rocket fuel that has contaminated some Southern California groundwater basins.

The downward movement of water through the soil or alluvium to the groundwater table.

Potable Water
Suitable and safe for drinking.

Purified Water Replenishment

The proposed Purified Water Replenishment (PWR) project is part of EMWD’s Groundwater Reliability Plus (GRP) program to improve the quality and quantity of water in local groundwater basins. Purified Water Replenishment includes recharging a blend of advanced treated water (purified water) and recycled water into spreading ponds along the San Jacinto River corridor. Proposed facilities include an advanced treatment plant adjacent to the San Jacinto Valley Regional Water Reclamation Facility, brine evaporation ponds, a recycled water pipeline, and use of a raw water pipeline for water delivery.

Primary treated water
First major treatment in a wastewater treatment facility, usually sedimentation but not biological oxidation.

Public Water System
A system that provides piped water for human consumption to at least 15 service connections or regularly serves 25 individuals.


Refers to Quantification Settlement Agreement, a proposed agreement among Metropolitan Water District, Coachella Valley Water District and Imperial Irrigation District to settle a variety of long-standing disputes regarding the priority, use and transfer of Colorado River water within California.


Replenishing an aquifer with storm water or imported water.

Recycled Water
Wastewater that becomes suitable for a specific beneficial use as a result of treatment.

Reverse Osmosis
The application of pressure to a concentrated solution which causes the passage of a liquid from the concentrated solution to a weaker solution across a semipermeable membrane. The membrane allows the passage of the water but not the dissolved solids. The liquid produced is a demineralized water.


The scaling or white deposits that accumulate on coffee pots, water heaters and plumbing fixtures resulting from dissolved mineral salts in the water.

Secondary Treatment
A wastewater treatment process used to convert dissolved or suspended materials into a form more readily separated from the water being treated.

The settleable solids separated from liquids during processing or the deposits of foreign materials on the bottoms of streams or other bodies of water.


Tertiary Treatment
A process of water renovation that upgrades treated wastewater to meet specific reuse requirements. Also called Advanced Waste Treatment.

Byproduct of chlorinating water that contains natural organics. Recent U.S. Environmental Protection Agency changes in national drinking water quality standards now require that water treatment systems begin to reduce TTHM.


The region or land area that contributes to the drainage or catchment area above a specific point on a stream or river.

Water table
The top level of water stored underground.

An area that is saturated by surface or groundwater with vegetation adapted for life under those soil conditions, as swamps, bogs, fens, marshes, and estuaries.