Home > Glossary: Water Resources

Glossary: Water Resources

Following is a definition of terms frequently used in the instrumentation, industrial automation and test & measurement communities. Click on the first letter of the term you wish to look up.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

abandoned water right: a water right which was not put to beneficial use for a number of years, generally five to seven years.

abandoned well: a well which is no longer used. In many places, abandoned wells must be filled with cement or concrete grout to prevent pollution of ground water bodies.

absorb: to take in.

accretion: a gradual increase in land area adjacent to a river.

acid rain: the acidic rainfall which results when rain combines with sulfur oxides emissions from combustion of fossil fuels.

acre-foot: the amount of water required to cover one acre to a depth of one foot. An acre-foot equals 325,851 gallons, or 43,560 cubic feet. A flow of 1 cubic feet per second produce 1.98 acre-feet per day.

activated carbon adsorption: the process of pollutants moving out of water and attaching on to activated carbon.

adhesion: the molecular attraction asserted between the surfaces of bodies in contact. Compare cohesion.

adjudication: a court proceeding to determine all rights to the use of water on a particular stream system or ground water basin.

adsorption: the adhesion of a substance to the surface of a solid or liquid. Adsorption is often used to extract pollutants by causing them to be attached to such adsorbents as activated carbon or silica gel. Hydrophobic, or water-repulsing adsorbents, are used to extract oil from waterways in oil spills.

aeration: the mixing or turbulent exposure of water to air and oxgen to dissipate volatile contaminants and other pollutants into the air.

aggressive water: water which is soft and acidic and can corrode plumbing, piping, and appliances.

alkalinity: the measurement of constituents in a water supply which determine alkaline conditions. The alkalinity of water is a measure of its capacity to neutralize acids. See pH.

algal bloom: a phenomenon whereby excessive nutrients within a river, stream or lake cause an explosion of plant life which results in the depletion of the oxygen in the water needed by fish and other aquatic life. Algae bloom is usually the result of urban runoff (of lawn fertilizers, etc.). The potential tragedy is that of a "fish kill," where the stream life dies in one mass extinction.

alluvium: sediments deposited by erosional processes, usually by streams.

alvusion: a sudden or perceptible change in a river's margin, such as a change in course or loss of banks due to flooding.

annular space: the space between two concentric cylindrical objects, one of which surrounds the other, such as the space between the walls of a drilled hole and a casing.

aquatic: growing in, living in, or frequenting water.

aquiclude: a formation which, although porous and capable of absorbing water slowly, will not transmit water fast enough to furnish an appreciable supply for a well or a spring.

alluvium: sediments deposited by erosional processes, usually by streams .

aquiculture: the raising or fattening of fish in enclosed ponds. Compare mariculture.

aquifer: a geologic formation that will yield water to a well in sufficient quantities to make the production of water from this formation feasible for beneficial use; permeable layers of underground rock or sand that hold or transmit groundwater below the water table.

artesian aquifer: a geologic formation in which water is under sufficient hydrostatic pressure to be discharged to the surface without pumping.

artesian well: a water well drilled into a confined aquifer where enough hydraulic pressure exists for the water to flow to the surface without pumping.

artesian zone: a zone where water is confined in an aquifer under pressure so that the water will rise in the well casing or drilled hole above the bottom of the confining layer overlying the aquifer.

average annual recharge: amount of water entering the aquifer on an average annual basis. Averages mean very little for the Edwards because the climate of the region and structure of the aquifer produce a situation in which the area is usually water rich or water poor.

B

backsiphonage: reverse seepage of water in a distribution system.

backwashing: reversing the flow of water through a home treatment device filter or membrane to clean and remove deposits.

barrage: any artificial obstruction placed in water to increase water level or divert it. Usually the idea is to control peak flow for later release.

beneficial use: the amount of water necessary when reasonable intelligence and diligence are used for a stated purpose; Texas law recognizes the following uses as beneficial: (1) domestic and municipal uses, (2) industrial uses, (3) irrigation, (4) mining, (5) hydroelectric power, (6) navigation, (7) recreation, (8) stock raising, (9) public parks, and (10) game preserves.

bioaccumulation: uptake and retention of substances by an organism from its surrounding medium (usually water) and from food.

biomonitoring: a test used to evaluate the relative potency of a chemical by comparing its effect on a living organism with the effect of a standard population on the same type of organism.

bioremediation: a process that uses living organisms to remove pollutants.

biosolids: a nutrient-rich organic material resulting from the treatment of wastewater. Biosolids contain nitrogen and phosphorus along with other supplementary nutrients in smaller doses, such as potassium, sulfur, magnesium, calcium, copper and zinc. Soil that is lacking in these substances can be reclaimed with biosolids use. The application of biosolids to land improves soil properties and plant productivity, and reduces dependence on inorganic fertilizers.

biosphere: the earth and all its ecosystems

blackwater: wastewater from toilet, latrine, and agua privy flushing and sinks used for food preparation or disposal of chemical or chemical-biological ingredients.

blinds: water samples containing a chemical of known concentration given a fictitious company name and slipped into the sample flow of the lab to test the impartiality of the lab staff.

blowdown: the water drawn from boiler systems and cold water basins of cooling towers to prevent the buildup of solids.

bog: a type of wetland that accumulates appreciable peat deposits. They depend primarily on precipitation for their water source, and are usually acidic and rich in plant matter with a conspicuous mat or living green moss.

boiling point: the temperature at which a liquid boils. It is the temperature at which the vapor pressure of a liquid equals the pressure on its surface. If the pressure of the liquid varies, the actual boiling point varies. For water it is 212 degrees Fahrenheit or 100 degrees Celsius.

BOD: Biochemical Oxygen Demand. A measure of the amount of oxygen required to neutralize organic wastes.

brine: highly salty and heavily mineralized water containing heavy metal and organic contaminants.

buoyancy: the tendency of a body to float or rise when immersed in a fluid; the power of a fluid to exert an upward force on a body placed in it.

C

calcium carbonate: CACO3 - a white precipitate that forms in water lines, water heaters and boilers in hard water areas; also known as scale.

calorie: amount of energy required to raise the temperature of 1 gram of water 1 degree Celsius.

capillary zone: soil area above the water table where water can rise up slightly through the cohesive force of capillary action. See phreatophytes.

carbamates: a class of new-age pesticides that attack the nervous system of organisms.

carbonates: the collective term for the natural inorganic chemical compounds related to carbon dioxide that exist in natural waterways.

casing: a tubular structure intended to be watertight installed in the excavated or drilled hole to maintain the well opening and, along with cementing, to confine the ground waters to their zones of origin and prevent the entrance of surface pollutants.

cavern: a large underground opening in rock (usually limestone) which occurred when some of the rock was dissolved by water. In some igneous rocks, caverns can be formed by large gas bubbles.

cement grout: a mixture of water and cement in the ratio of not more than 5-6 gallons of water to a 94 pound sack of portland cement which is fluid enough to be pumped through a small diameter pipe.

CERCLA: Comprehensive Environment Response, Compensation and Liability Act. Also known as SUPERFUND. The Act gave EPA the authority to clean up abandoned, leaky hazardous waste sites.

certificate of water right: an official document which serves as court evidence of a perfected water right.

CFU: colony forming units.

check dam: a small dam constructed in a gully or other small water course to decrease the streamflow velocity, minimize channel erosion, promote deposition of sediment and to divert water from a channel.

chemical weathering: attack and dissolving of parent rock by exposure to rainwater, surface water, oxygen, and other gases in the atmosphere, and compounds secreted by organisms. Contrast physical weathering.

chlorination: the adding of chlorine to water or sewage for the purpose of disinfection or other biological or chemical results.

chlorine demand: the difference between the amount of chlorine added to water, sewage, or industrial wastes and the amount of residual chlorine remaining at the end of a specific contact period. Compare residual chlorine.

chute spillway: the overall structure which allows water to drop rapidly through an open channel without causing erosion. Usually constructed near the edge of dams.

circulate: to move in a circle, circuit or orbit; to flow without obstruction; to follow a course that returns to the starting point.

cistern: a tank used to collect rainwater runoff from the roof of a house or building.

climatic cycle: the periodic changes climate displays, such as a series of dry years following a series of years with heavy rainfall.

climatic year: a period used in meteorological measurements. The climatic year in the U.S. begins on October 1.

climate: generalized weather at a given place on earth over a fairly long period; a long term average of weather. Compare weather.

cloudburst: a torrential downpour of rain, which by it spottiness and relatively high intensity suggests the bursting and discharge of water from a cloud all at once.

coagulation: in water treatment, the use of chemicals to make suspended solids gather or group together into small flocs.

cohesion: a molecular attraction by which the particles of a body are united throughout the mass whether like or unlike. Compare adhesion.

cold vapor: method to test water for the presence of mercury.

coliform bacteria: non-pathogenic microorganisms used in testing water to indicate the presence of pathogenic bacteria.

collector well: a well located near a surface water supply used to lower the water table and thereby induce infiltration of surface water through the bed of the water body to the well..

colloids: finely divided solids which will not settle but which may be removed by coagulation or biochemical action.

combined sewer: a sewer system that carries both sanitary sewage and stormwater runoff. When sewers are constructed this way, wastewater treatment plants have to be sized to deal with stormwater flows and oftentimes some of the water receives little or no treatment. Compare separate sewer.

completion: sealing off access of undesireable water to the well bore by proper casing and/or cementing procedures.

composite sample, weighted: a sample composed of two or more portions collected at specific times and added together in volumes related to the flow at time of collection. Compare grab sample.

concentration: amount of a chemical or pollutant in a particular volume or weight of air, water, soil, or other medium.

condensation: the change of state from a gas to a liquid. Compare evaporation, sublimation.

conduit: a natural or artificial channel through which fluids may be conveyed.

cone of depression: natural depression in the water table around a well during pumping.

confined aquifer: an aquifer that lies between two relatively impermeable rock layers.

confining bed or unit: a body of impermeable or distinctly less permeable material stratigraphically adjacent to one or more aquifers.

confluent growth: in coliform testing, abundant or overflowing bacterial growth which makes accurate measurement difficult or impossible.

conjunctive management: integrated management and use of two or more water resources, such as an aquifer and a surface water body.

connate growth: water trapped in the pore spaces of a sedimentary rock at the time it was deposited. It is usually highly mineralized.

conservation: to protect from loss and waste. Conservation of water may mean to save or store water for later use.

consolidated formation: naturally occurring geologic formations that have been lithified (turned to stone). The term is sometimes used interchangeably with the term "bedrock." Commonly, these formations will stand at the edges of a bore hole without caving.

consumptive use: the quantity of water not available for reuse. Evapotranspiration, evaporation, incorporation into plant tissue, and infiltration into groundwater are some of the reasons water may not be available for reuse. Compare nonconsumptive use.

contact recreation: activities involving a significant risk of ingestion of water, such as wading by children, swimming, water skiing, diving and surfing. Compare noncontact recreation..

contamination: the introduction into water of sewage or other foreign matter that will render the water unfit for its intended use.

cooling tower: large tower used to transfer the heat in cooling water from a power or industrial plant to the atmosphere either by direct evaporation or by convection and conduction.

correlative rights: rights that are coequal or that relate to one another, so that any one owner cannot take more than his share.

creek: a small stream of water which serves as the natural drainage course for a drainage basin. The term is relative according to size. Some creeks in a humid region would be called rivers if they occurred in an arid area.

crest: the top of a dam, dike, or spillway, which water must reach before passing over the structure; the summit or highest point of a wave; the highest elevation reached by flood waters flowing in a channel.

critical low flow: low flow conditions below which some standards do not apply. The impacts of permitted discharges are analyzed at critical low-flow.

cubic foot per second (CFS): the rate of discharge representing a volume of one cubic foot passing a given point during 1 second. This rate is equivalent to approximately 7.48 gallons per second, or 1.98 acre-feet per day.

current: the portion of a stream or body of water which is moving with a velocity much greater than the average of the rest of the water. The progress of the water is principally concentrated in the current. See thalweg.

D

dam: a structure of earth, rock, or concrete designed to form a basin and hold water back to make a pond, lake, or reservoir.

deionized water: water free of inorganic chemicals.

delta: an alluvial deposit made of rock particles (sediment, and debris) dropped by a stream as it enters a body of water.

demand: the number of units of something that will be purchased at various prices at a point in time. Compare supply.

dental fluorosis: disorder caused by excessive absorption of fluorine and characterized by brown staining of teeth.

deposit: something dropped or left behind by moving water, as sand or mud.

desalination: the process of salt removal from sea or brackish water.

detection limit: the lowest level that can be determined by a specific analytical procedure or test method.

diatomaceous: consisting of or abounding in diatoms, a class of unicellular or colonial algae having a silicified cell wall that persists as a skeleton after death.

diluting water: distilled water that has been stabilized, buffered, and aerated. Used in the BOD test.

discharge: the volume of water that passes a given point within a given period of time. It is an all-inclusive outflow term, describing a variety of flows such as from a pipe to a stream, or from a stream to a lake or ocean.

discharge permit: a permit issued by a state or the federal government to discharge effluent into waters of the state or the United States. In many states both State and federal permits are required.

disinfection: the killing of the larger portion of the harmful and objectionable bacteria in the sewage. Usually accomplished by introduction of chlorine, but more and more facilities are using exposure to ultraviolet radiation, which renders the bacteria sterile.

disinfection byproducts: halogenated organic chemicals formed when water is disinfected.

dispersion: the movement and spreading of contaminants out and down in an aquifer.

displacement: distance by which portions of the same geological layer are offset from each other by a fault.

dissolve: the process by which solid particles mix molecule by molecule with a liquid and appear to become part of the liquid.

dissolved oxygen (DO): amount of oxygen gas dissolved in a given quantity of water at a given temperature and atmospheric pressure. It is usually expressed as a concentration in parts per million or as a percentage of saturation.

dissolved solids: inorganic material contained in water or wastes. Excessive dissolved solids make water unsuitable for drinking or industrial uses. See TDS.

distillation: water treatment method where water is boiled to steam and condensd in a separate reservoir. Contaminants with higher boiling points than water do not vaporize and remain in the boiling flask.

distilled water: water that has been treated by boiling and condensation to remove solids, inorganics, and some organic chemicals.

diversion: to remove water from a water body. Diversions may be used to protect bottomland from hillside runoff, divert water away from active gullies, or protect buildings from runoff.

drainage area: of a stream at a specified location is that area, measured in a horizontal plane, enclosed by a topographic divide from which direct surface runoff from precipitation normally drains by gravity into the stream above the specified location.

driller's well log: a log kept at the time of drilling showing the depth, thickness, character of the different strata penetrated, location of water-bearing strata, depth, size, and character of casing installed.

dripstone: deposits of calcium carbonate that include stalactites, stalagmites, columns, and cave pearls.

drought: although there is no universally accepted definition of drought, it is generally the term applied to periods of less than average precipitation over a certain period of time. In south Texas ranchers say drought begins as soon as it stops raining.

duplicates: two separate samples with separate containers taken at the same time at the same place.

E

ecosphere: total of all the ecosystems on the planet, along with their interactions; the sphere of air, water, and land in which all life is found.

Edwards Aquifer: an arch-shaped belt of porous, water bearing limestones composed of the Comanche Peak, Edwards, and Georgetown formations trending from west to east to northeast through Kinney, Uvalde, Medina, Bexar, Comal, Hays, Travis, and Williamson counties.

Edwards outcrop: where the Edwards and associated limestone formations are found at the surface. This area is also referred to as the Recharge Zone.

effective porosity: the portion of pore space in saturated permeable material where the movement of water takes place.

effective precipitation: the part of precipitation which produces runoff; a weighted average of current and antecedent precipitation "effective" in correlating with runoff. It is also that part of the precipitation falling on an irrigated area which is effective in meeting the requirements of consumptive use.

effluent: any substance, particularly a liquid, that enters the environment from a point source. Generally refers to wastewater from a sewage treatment or industrial plant.

electrodialysis: a process which uses an electrical current and an arrangement of permeable membranes to separate soluble minerals from water. It is often used to desalinate salt or brackish water.

endangered species: one having so few individual survivors that the species could soon become extinct in all or part of its region.

enteric viruses: a category of viruses related to human excreta found in waterways.

environment: aggregate of external conditions that influence the life of an individual organism or population.

EPA: Environmental Protection Agency

epilimnion: warm, less dense top layer in a stratified lake. Compare hypolimnion.

erosion: the wearing away of the land surface by wind, water, ice or other geologic agents. Erosion occurs naturally from weather or runoff but is often intensified by human land use practices.

escarpment: the topographic expression of a fault.

estuarine waters: deepwater tidal habitats and tidal wetlands that are usually enclosed by land but have access to the ocean and are at least occasionally diluted by freshwater runoff from the land (such as bays, mouths of rivers, salt marshes, lagoons).

estuarine zone: area near the coastline that consists of estuaries and coastal saltwater wetlands.

estuary: thin zone along a coastline where freshwater system(s) and river(s) meet and mix with a salty ocean (such as a bay, mouth of a river, salt marsh, lagoon).

euphotic zone: surface layer of an ocean, lake, or other body of water through which light can penetrate. Also known as the zone of photosynthesis.

eutrophic: having a large or excessive supply of plant nutrients (nitrates and phosphates). Compare oligotrophic.

eutrophication (natural): an excess of plant nutrients from natural erosion and runoff from the land in an aquatic ecosystem supporting a large amount of aquatic life that can deplete the oxygen supply.

evaporation: the change by which any substance is converted from a liquid state and carried of in vapor. Compare condensation, sublimation.

evapotranspiration: combination of evaporation and transpiration of water into the atmosphere from living plants and soil. Distinguish transpiration.

external cost: cost of production or consumption that must be borne by society; not by the producer.

extinction: complete disappearance of a species because of failure to adapt to environmental change.

F

fecal coliform: the portion of the coliform bacteria group which is present in the intestinal tracts and feces of warm-blooded animals. A common pollutant in water.

fen: a type of wetland that accumulates peat deposits, but not as much as a bog. Fens are less acidic than bogs, deriving most of their water from groundwater rich in calcium and magnesium.

fermentation, anaerobic: process in which carbohydrates are converted in the absence of oxygen to hydrocarbons (such as methane).

field capacity: the amount of water held in soil against the pull of gravity.

filter: a device used to remove solids from a mixture or to separate materials. Materials are frequently separated from water using filters.

filtration: the mechanical process which removes particulate matter by separating water from solid material, usually by passing it through sand.

"first in time, first in right": phrase indicating that older water rights have priority over more recent rights if there is not enough water to satisfy all rights.

fixed ground water: water held in saturated material that it is not available as a source of water for pumping.

flocculation: large scale treatment process involving gentle stirring whereby small particles in flocs are collected into larger particles so their weight causes them to settle to the bottom of the treatment tank.

flood: an overflow or inundation that comes from a river or other body of water and causes or threatens damage. It can be any relatively high streamflow overtopping the natural or artificial banks in any reach of a stream. It is also a relatively high flow as measured by either gage height or discharge quantity.

floodplain: land next to a river that becomes covered by water when the river overflows its banks .

flora: plant population of a region.

flow: the rate of water discharged from a source expressed in volume with respect to time.

flow augmentation: the addition of water to meet flow needs.

forbay: the water behind a dam.

forfeited water right: a water right canceled because of several consecutive years of nonuse.

free ground water: water in interconnected pore spaces in the zone of saturation down to the first impervious barrier, moving under the control of the water table slope.

freezing: the change of a liquid into a solid as temperature decreases. For water, the freezing point is 32 F or 0 C.

fresh water: water containing less than 1,000 parts per million (ppm) of dissolved solids of any type. Compare saline water.

fresh:salt water interface: the region where fresh water and salt water meet. In the Edwards region, it is commonly referred to as the "bad water line", although it is zone and not a line.

frost: a covering of minute ice crystals on a cold surface.

G

gaging station: the site on a stream, lake or canal where hydrologic data is collected.

gallon: A unit of volume. A U.S. gallon contains 231 cubic inches, 0.133 cubic feet, or 3.785 liters. One U.S. gallon of water weighs 8.3 lbs.

geohydrology: a term which denotes the branch of hydrology relating to subsurface or subterranean waters; that is, to all waters below the surface.

geologic erosion: normal or natural erosion caused by geological processes acting over long geologic periods and resulting in the wearing away of mountains, the building up of floodplains, coastal plains, etc.

geopressured reservoir: a geothermal reservoir consisting of porous sands containing water or brine at high temperature or pressure.

geyser: a periodic thermal spring that results from the expansive force of super heated steam..

glacier: a huge mass of land ice that consists of recrystallized snow and moves slowly downslope or outward.

grab sample: a sample taken at a given place and time. Compare composite sample.

granular activated carbon: pure carbon heated to promote "active" sites which can adsorb pollutants. Used in some home water treatment systems to remove certain organic chemicals and radon.

greywater: wastewater from clothes washing machines, showers, bathtubs, handwashing, lavatories and sinks that are not used for disposal of chemical or chemical-biological ingredients.

groundwater: water within the earth that supplies wells and springs; water in the zone of saturation where all openings in rocks and soil are filled, the upper surface of which forms the water table.

groundwater hydrology: the branch of hydrology that deals with groundwater; its occurrence and movements, its replenishment and depletion, the properties of rocks that control groundwater movement and storage, and the methods of investigation and utilization of ground water.

groundwater law: the common law doctrine of riparian rights and the doctrine of prior appropriation as applied to ground water.

groundwater recharge: the inflow to a ground water reservoir.

groundwater reservoir: an aquifer or aquifer system in which ground water is stored. The water may be placed in the aquifer by artificial or natural means.

groundwater runoff: the portion of runoff which has passed into the ground, has become ground water, and has been discharged into a stream channel as spring or seepage water.

groundwater storage: the storage of water in groundwater reservoirs.

gully: a deeply eroded channel caused by the concentrated flow of water.

gully reclamation: use of small dams of manure and straw; earth, stone,or concrete to collect silt and gradually fill in channels of eroded soil.

H

hail: a form of precipitation which forms into balls or lumps of ice over 0.2 inch in diameter. Hail is formed by alternate freezing and melting as precipitation is carried up and down in highly turbulent air currents.

hardpan: a shallow layer of earth material which has become relatively hard and impermeable, usually through the deposition of minerals. In the Edwards region hardpans of clay are common.

hard water: water containing a high level of calcium, magnesium, and other minerals. Hard water reduces the cleansing power of soap and produces scale in hot water lines and appliances.

hardness (water): condition caused by dissolved salts of calcium, magnesium, and iron, such as bicarbonates, carbonates, sulfates, chlorides,and nitrates.

head: the pressure of a fluid owing to its elevation, usually expressed in feet of head or in pounds per square inch, since a measure of fluid pressure is the height of a fluid column above a given or known point.

headgate: the gate that controls water flow into irrigation canals and ditches. A watermaster regulates the headgates during water distribution and posts headgate notices declaring official regulations.

heat of vaporization: the amount of heat necessary to convert a liquid (water) into vapor.

heavy water: water in which all the hydrogen atoms have been replaced by deuterium.

holding pond: a small basin or pond designed to hold sediment laden or contaminated water until it can be treated to meet water quality standards or be used in some other way.

hydroelectric plant: electric power plant in which the energy of falling water is used to spin a turbine generator to produce electricity.

hydrograph: a chart that measures the amount of water flowing past a point as a function of time.

hydrologic cycle: natural pathway water follows as it changes between liquid, solid, and gaseous states; biogeochemical cycle that moves and recycles water in various forms through the ecosphere. Also called the water cycle.

hydrologic unit: is a geographic area representing part or all of a surface drainage basin or distinct hydrologic feature.

hydrometer: an instrument used to measure the density of a liquid.

hydropower: electrical energy produced by falling water.

hygroscopic nuclei: piece of dust or other particle around which water condenses in the atmophere. These tiny droplets then collide and coalesce, with as many as 10,000 nuclei contributing to formation of a raindrop.

hydrosphere: region that includes all the earth's liquid water, frozen water, floating ice, frozen upper layer of soil, and the small amounts of water vapor in the Earth's atmosphere.

hydrostatic head: a measure of pressure at a given point in a liquid in terms of the vertical height of a column of the same liquid which would produce the same pressure.

hydrostatic pressure: pressure exerted by or existing within a liquid at rest with respect to adjacent bodies.

hypolimnion: bottom layer of cold water in a lake. Compare epilimnion.

I

ice: a solid form of water.

impermeable: material that does not permit fluids to pass through.

impervious: the quality or state of being impermeable; resisting penetration by water or plant roots. Impervious ground cover like concrete and asphalt affects quantity and quality of runoff.

impoundment: a body of water such as a pond, confined by a dam, dike, floodgate or other barrier. It is used to collect and store water for future use.

inchoate water right: an unperfected water right.

indicator organisms: microorganisms, such as coliforms, whose presence is indicative of pollution or of more harmful microorganism.

indicator tests: tests for a specific contaminant, group of contaminants, or constituent which signals the presence of something else (ex., coliforms indicate the presence of pathogenic bacteria).

inland freshwater wetlands: swamps, marshes, and bogs found inland beyond the coastal saltwater wetlands.

instream use: use of water that does not require withdrawal or diversion from its natural watercourse; for example, the use of water for navigation, recreation, and support of fish and wildlife.

interbasin transfer: the physical transfer of water from one watershed to another; regulated by the Texas Water Code.

intermittent stream: one that flows periodically. Compare perennial stream.

interstate water: according to law, interstate waters are defined as (1) rivers, lakes and other waters that flow across or form a part of state or international boundaries; (2) waters of the Great Lakes; (3) coastal waters whose scope has been defined to include ocean waters seaward to the territorial limits and waters along the coastline (including inland streams) influenced by the tide.

interstices: the void or empty portion of rock or soil occupied by air or water.

irrigation efficiency: the percentage of water applied, and which can be accounted for, in the soil moisture increase for consumptive use.

irrigation return flow: water which is not consumptively used by plants and returns to a surface or ground water supply. Under conditions of water right litigation, the definition may be restricted to measurable water returning to the stream from which it was diverted.

irrigation water: water which is applied to assist crops in areas or during times where rainfall is inadequate.

isotherm: line that connects points of equal temperature.

isohyet: line that connects points of equal rainfall.

J

jet stream: a long narrow meandering current of high-speed winds near the tropopause blowing from a generally westerly direction and often exceeding a speed of 250 miles per hour.

jetteau: a jet of water.

jetter: one (as a geyser) that sends out a jet.

jetty: a structure (as a pier or mole of wood or stone) extending into a sea, lake, or river to influence the current or tide or to protect a harbor.

K

kalema: a violent surf that occurs on the coast of the Guinea region, West Africa.

kame: a short ridge, hill, or mound of stratified drift deposited by glacial meltwater.

kame terrace: a terrace of stratified sand and and gravel deposited by streams between a glacier and an adjacent valley wall.

L

laboratory water: purified water used in the laboratory as a basis for making up solutions or making dilutions. Water devoid of interfering substances.

lag time: the time from the center of a unit storm to the peak discharge or center of volume of the corresponding unit hydrograph.

lagoon: a shallow pond where sunlight, bacterial action, and oxygen work to purify wastewater. Lagoons are typically used for the storage of wastewaters, sludges, liquid wastes, or spent nuclear fuel.

lake: an inland body of water, usually fresh water, formed by glaciers, river drainage etc. Usually larger than a pool or pond.

landscape impoundment: body of reclaimed water which is used for aesthetic enjoyment or which otherwise serves a function not intended to include contact recreation.

leachate: water containing contaminants which leaks from a disposal site such as a landfill or dump.

leaching: extraction or flushing out of dissolved or suspended materials from the soil, solid waste, or another medium by water or other liquids as they percolate down through the medium to groundwater.

lentic system: a nonflowing or standing body of fresh water, such as a lake or pond. Compare lotic system.

levee: a natural or man-made earthen obstruction along the edge of a stream, lake, or river. Usually used to restrain the flow of water out of a river bank.

limestone: rock that consists mainly of calcium carbonate and is chiefly formed by accumulation of organic remains.

limiting factor: factor such as temperature, light, water, or a chemical that limits the existence, growth, abundance, or distribution of an organism.

limnology: scientific study of physical, chemical, and biological conditions in lakes, ponds, and streams.

liquid: a state of matter, neither gas nor solid, that flows and takes the shape of its container.

littoral zone: area on or near the shore of a body of water.

lotic system: a flowing body of fresh water, such as a river or stream. Compare lentic system.

M

mariculture: cultivation of fish and shellfish in estuarine and coastal areas. Compare aquiculture.

marsh: an area periodically inundated and treeless and often characterized by grasses, cattails, and other monocotyledons

MCL - Maximum Contaminant Level: the maximum level of a contaminant allowed in water by federal law. Based on health effects and currently available treatment methods.

median streamflow: the rate of discharge of a stream for which there are equal numbers of greater and lesser flow occurrences during a specified period.

melting: the changing of a solid into a liquid.

meltwater: water that comes from the melting ice of a glacier or a snowbank.

meteoric water: new water derived from the atmosphere.

mermaid: a fabled marine creature usually represented as having the head, trunk, and arms of a woman and a lower part like the tail of a fish.

method blank: laboratory grade water taken through the entire analytical procedure to determine if samples are being accidentally contaminated by chemicals in the lab

micrograms per liter - Ug/L: micrograms per liter of water. One thousands micrograms per liter is equivalent to 1 milligram per liter. This measure is equivalent to parts per billion (ppb)

migration: the movement of oil, gas, contaminants, water, or other liquids through porous and permeable rock.

milligrams per liter - mg/L: milligrams per liter of water. This measure is equivalent to parts per million (ppm).

minimum streamflow: the specific amount of water reserved to support aquatic life, to minimize pollution, or for recreation. It is subject to the priority system and does not affect water rights established prior to its institution.

municipal sewage: sewage from a community which may be composed of domestic sewage, industrial wastes or both.

N

natural flow: the rate of water movement past a specified point on a natural stream. The flow comes from a drainage area in which there has been no stream diversion caused by storage, import, export, return flow, or change in consumptive use caused by man-controlled modifications to land use. Natural flow rarely occurs in a developed country.

natural resource: any form of matter or energy obtained from the environment that meets human needs.

NIPDWR: National Interim Primary Drinking Water Regulations.

nitrogen: a plant nutrient that can cause an overabundance of bacteria and algae when high amounts are present, leading to a depletion of oxygen and fish kills. Several forms occur in water, including ammonia, nitrate, nitrite or elemental nitrogen. High levels of nitrogen in water are usually caused by agricultural runoff or improperly operating wastewater treatment plants. Also see phosphorous.

nonconsumptive use: using water in a way that does not reduce the supply. Examples include hunting, fishing, boating, water-skiing, swimming, and some power production. Compare consumptive use.

noncontact recreation: recreational pursuits not involving a significant risk of water ingestion, including fishing, commercial and recreational boating, and limited body contact incidental to shoreline activity. Compare contact recreation.

nonporous: something which does not allow water to pass through it. Compare porous.

nonpoint source: source of pollution in which wastes are not released at one specific, identifiable point but from a number of points that are spread out and difficult to identify and control. Compare point source.

nonpotable: not suitable for drinking. Compare potable.

nonthreshold pollutant: substance or condition harmful to a particular organism at any level or concentration.

NPDES permit: permit issued under the National Pollutant Discharge Elimination System for companies discharging pollutants directly into the waters of the United States.

NTU: nephlometric turbidity units.

nutrient: as a pollutant, any element or compound, such as phosphorous or nitrogen, that fuels abnormally high organic growth in aquatic ecosystems. Also see eutrophic.

O

oligotrophic: having a low supply of plant nutrients. Compare eutrophic.

open system: system in which energy and matter are exchanged between the system and its environment, for example, a living organism.

organic chemicals: chemicals containing carbon.

orogeny: period of mountain-building.

orographic precipitation: rainfall that occurs as a result of warm, humid air being forced to rise by topographic features such as mountains. Precipitation on the Edwards Plateau is slightly higher because of the orographic effect of the escarpment and hills.

outcrop: exposed at the surface. The Edwards limestone outcrops in its recharge zone.

outfall: the place where a wastewater treatment plant discharges treated water into the environment.

outwash: a deposit of sand and gravel formed by streams of meltwater flowing from a glacier.

oxygen demanding waste: organic water pollutants that are usually degraded by bacteria if there is sufficient dissolved oxygen (DO) in the water.

P

pathogen: microorganisms which can cause disease.

peak flow: in a wastewater treatment plant, the highest flow expected to be encoutered under any operational conditions, including periods of high rainfall and prolonged periods of wet weather.

perched water table: groundwater standing unprotected over a confined zone.

percolation: the movement of water through the subsurface soil layers, usually continuing downward to the groundwater or water table reservoirs.

percolating waters: waters passing through the ground beneath the Earth's surface without a definite channel.

perfected water right: a water right which indicates that the uses anticipated by an applicant, and made under permit, were made for beneficial use. Usually it is irrevocable unless voluntarily canceled or forfeited due to several consecutive years of nonuse.

perennial stream: one that flows all year round. Compare intermittent stream.

permeability: the ability of a water bearing material to transmit water. It is measured by the quantity of water passing through a unit cross section, in a unit time, under 100 percent hydraulic gradient.

pH: numeric value that describes the intensity of the acid or basic (alkaline) conditions of a solution. The pH scale is from 0 to 14, with the neutral point at 7.0. Values lower than 7 indicate the presence of acids and greater than 7.0 the presence of alkalis (bases). Technically speaking, pH is the logarithm of the reciprocal (negative log) of the hydrogen ion concentration (hydrogen ion activity) in moles per liter.

phosphorous: a plant nutrient that can cause an overabundance of bacteria and algae when high amounts are present, leading to a depletion of oxygen and fish kills. High levels of phosphorous in water are usually caused by agricultural runoff or improperly operating wastewater treatment plants. Also see nitrogen.

phreatophytes: plants that send their roots into or below the capillary zone to use ground water.

physical weathering: breaking down of parent rock into bits and pieces by exposure to temperature and changes and the physical action of moving ice and water, growing roots, and human activities such as farming and construction. Compare chemical weathering.

phytoplankton: free-floating, mostly microscopic aquatic plants.

piezometroc surface: the imaginary surface to which groundwater rises under hydrostatic pressure in wells or springs.

plankton: microscopic floating plant and animal organisms of lakes, rivers, and oceans.

plate tectonics: refers to the folding and faulting of rock and flow of molten lava involving lithospheric plates in the earth's crust and upper mantle.

plug: cement, grout, or other material used to fill and seal a hole drilled for a water well.

plume: the area taken up by contaminant(s) in an aquifer.

pluvial: pertaining to precipitation.

point source: source of pollution that involves discharge of wastes from an identifiable point, such as a smokestack or sewage treatment plant. Compare nonpoint source.

pollution: undesireable change in the physical, chemical, or biological characteristics of the air, water, or land that can harmfully affect the health, survival, or activities of human or other living organisms.

pond: a body of water usually smaller than a lake and larger than a pool either naturally or artificially confined.

porous: something which allows water to pass through it. Compare nonporous.

potable: suitable, safe, or prepared for drinking. Compare non-potable.

ppb - parts per billion: number of parts of a chemical found in one billion parts of a solid, liquid, or gaseous mixture. Equivalent to micrograms per liter (Ug/L).

ppm - parts per million: number of parts of a chemical found in one million parts of a solid, liquid, or gaseous mixture. Equivalent to milligrams per liter (mg/L).

precipitate: a solid which has come out of an aqueous solution. (ex., iron from groundwater precipitates to a rust colored solid when exposed to air).

preservative: a chemical added to a water sample to keep it stable and prevent compounds in it from changing to other forms or to prevent microorganism densities from changing prior to analysis.

price at equilibrium: where supply and demand curves intersect. The price at equilibrium is what allocates resources.

primary treatment: mechanical treatment in which large solids are screened out and suspended solids in the sewage settle out as sludge. Compare secondary treatment, tertiary treatment.

priority date: the date of establishment of a water right. It is determined by adjudication of rights established before the passage of the Water Code. The rights established by application have the application date as the date of priority.

profundal zone: a lake's deep-water region that is not penetrated by sunlight.

puddle: a small pool of water, usually a few inches in depth and from several inches to several feet in its greatest dimension.

pump: a device which moves, compresses, or alters the pressure of a fluid, such as water or air, being conveyed through a natural or artificial channel.

pumped hydroelectric storage: storing water for future use in generating electricity. Excess electrical energy produced during a period of low demand is used to pump water up to a reservoir. When demand is high, the water is released to operate a hydroelectric generator.

purge: to force a gas through a water sample to liberate volatile chemicals or other gases from the water so their level can be measured.

purgeable organics: volatile organic chemicals which can be forced out of the water sample with relative ease through purging.

Q

quarry water: the moisture content of freshly quarried stone, esp. if porous.

quicksilver water: a solution of mercury nitrate used in gilding.

quickwater: the part of a stream that has a strong current; an artificial current or bubbling patch of water just astern of a moving boat.

R

rain: water drops which fall to the earth from the air.

rain gage: any instrument used for recording and measuring time, distribution, and the amount of rainfall.

RCRA: Resource Conservation and Recovery Act - federal legislation requiring that hazardous waste be tracked from "cradle" (generation) to "grave" (disposal).

receiving waters: a river, ocean, stream, or other watercourse into which wastewater or treated effluent is discharged.

recharge: refers to water entering an underground aquifer through faults, fractures, or direct absorption.

recharge zone: the area where a formation allows available water to enter the aquifer. Generally, that area where the Edwards Aquifer and associated limestones crop out in Kinney, Uvalde, Medina, Bexar, Comal, Hays, Travis, and Williamson counties and the outcrops of other formations in proximity to the Edwards limestone, where faulting and fracturing may allow recharge of the surface waters to the Edwards Aquifer.

reclaimed water: domestic wastewater that is under the direct control of a treatment plant owner/operator which has been treated to a quality suitable for a beneficial use.

recurrence interval: average amount of time between events of a given magnitude. For example, there is a 1% chance that a 100-year flood will occur in any given year.

reserves: amount of a particular resource in known locations that can be extracted at a profit with present technology and prices.

reservoir: a pond, lake, tank, or basin (natural or human made) where water is collected and used for storage. Large bodies of groundwater are called groundwater reservoirs; water behind a dam is also called a reservoir of water.

residual chlorine: the available chlorine which remains in solution after the demand has been satisfied. Compare chlorine demand.

reverse osmosis: a water treatment method whereby water is forced through a semipermeable membrane which filters out impurities.

right of free capture: the idea that the water under a person's land belongs to that person and they are free to capture and use as much as they want. Also called the "law of the biggest pump".

riparian water right: the legal right held by an owner of land contiguous to or bordering on a natural stream or lake, to take water from the source for use on the contiguous land.

riparian zone: a stream and all the vegetation on its banks.

river: a natural stream of water of considerable volume.

river basin: the area drained by a river and its tributaries.

runoff: surface water entering rivers, freshwater lakes, or reservoirs.

S

saline water: water containing more than 1,000 parts per million (ppm) of dissolved solids of any type. Compare fresh water.

salinity: amount of dissolved salts in a given volume of water.

sanitary landfill: landfill that is lined with plastic or concrete or located in clay-rich soils to prevent hazardous substances from leaking into the environment.

saturation: the condition of a liquid when it has taken into solution the maximum possible quantity of a given substance at a given temperature and pressure.

seal: the impermeable material, such as cement grout bentonite, or puddling clay placed in the annular space between the borehole wall and the casing of a water well to prevent the downhole movement of surface water or the vertical mixing of artestian waters.

secondary treatment: second step in most waste treatment systems, in which bacteria break down the organic parts of sewage wastes; usually accomplished by bringing the sewage and bacteria together in trickling filters or in the activated sludge process. Compare primary treatment, tertiary treatment. Compare primary treatment, tertiary treatment.

sediment: soil particles, sand, and minerals washed from the land into aquatic systems as a result of natural and human activities.

sedimentary cycle: biogeochemical cycle in which materials primarily are moved from land to sea and back again.

sedimentation: a large scale water treatment process where heavy solids settle out to the bottom of the treatment tank after flocculation.

seep: a spot where water contained in the ground oozes slowly to the surface and often forms a pool; a small spring.

separate sewer: a sewer system that carries only sanitary sewage, not stormwater runoff. When a sewer is constructed this way, wastewater treatment plants can be sized to treat sanitary wastes only and all of the water entering the plant receives complete treatment at all times. Compare combined sewer.

septic tank: underground receptacle for wastewater from a home. The bacteria in the sewage decopose the organic wastes, and the sludge settles to the bottom of the tank. The effluent flows out of the tank into the ground through drains.

settleable solids: in sewage, suspended solids that will settle when the sewage is brought to a quiet state for a reasonable length of time, usually two hours.

siltation: the deposition of finely divided soil and rock particles upon the bottom of stream and river beds and reservoirs.

sleet: precipitation which is a mixture of rain and ice.

slickensides: a smooth striated polished surface produced on rock by movement along a fault.

sludge: solid matter that settles to the bottom of sedimentation tanks in a sewage treatment plant and must be disposed of by digestion or other methods or recycled to the land.

snow: precipitation in the form of branched hexagonal crystals, often mixed with simple ice crystals, which fall more or less continuously from a solid cloud sheet. These crystals may fall either separately or in cohesive clusters forming snowflakes.

solute: any substance derived from the atmosphere, vegetation, soil, or rock that is dissolved in water.

soil erosion: the processes by which soil is removed from one place by forces such as wind, water, waves, glaciers, and construction activity and eventually deposited at some new place.

specific conductance: a measure of the ability of a water to conduct an electrical current. Specific conductance is related to the type and concentration of ions in solution and can be used for approximating the dissolved solids concentration in water. In general, for the San Antonio River basin, conductivity * .6 approximates TDS. People monitoring water quality can measure electrical conductivity quickly in the field and estimate TDS without doing any lab tests at all. See TDS.

specific heat: the amount of heat required to raise the temperature of a kilogram of a substance (water) by 1 degree Celsius.

spillway: the channel or passageway around or over a dam through which excess water is diverted.

spray irrigation: application of finely divided water droplets to crops using artificial means.

spring: an issue of water from the earth; a natural fountain; a source of a body or reservoir of water.

standard solution: any solution in which the concentration is known.

stormwater discharge: precipitation that does not infiltrate into the ground or evaporate due to impervious land surfaces but instead flows onto adjacent land or water areas and is routed into drain/sewer systems.

stream: a general term for a body of flowing water.

stream segment: refers to the surface waters of an approved planning area exhibiting common biological, chemical, hydrological, natural, and physical characteristics and processes. Segments will normally exhibit common reactions to external stress such as discharge or pollutants.

streamflow: the discharge that occurs in a natural channel.

sublimation: the transition of water directly from the solid state to the gaseous state, without passing through the liquid state; or vice versa. Compare condensation, evaporation.

subsidence: sinking down of part of the earth's crust due to underground excavation, such as removal groundwater.

supply: a schedule that shows the various quantities of things offered for sale at various prices at a point in time. Compare demand.

surface impoundment: an indented area in the land's surface, such a pit, pond, or lagoon.

surface irrigation: application of water by means other than spraying such that contact between the edible portion of any food crop and the irrigation water is prevented.

surface water: water that flows in streams and rivers and in natural lakes, in wetlands, and in reservoirs constructed by humans.

sustainable management: method of exploiting a resource that can be carried on indefinitely. Removal of water from an aquifer in excess of recharge is, in the long term, not a sustainable management method.

sustained overdraft: long term withdrawal from the aquifer of more water than is being recharged.

T

technology-based treatment requirements: NPDES permit requirements based on the application of pollution treatment or control technologies including BTP (best practicable technology), BCT (best conventional technology), BAT (best available technology economically achievable), and NSPS (new source performance standards).

tertiary treatment: removal from wastewater of traces or organic chemicals and dissolved solids that remain after primary treatment and secondary treatment.

thalweg: the line of maximum depth in a stream. The thalweg is the part that has the maximum velocity and causes cutbanks and channel migration.

thermal gradient: temperature difference between two areas.

thermal pollution: an increase in air or water temperature that disturbs the climate or ecology of an area.

thermocline: fairly thin zone in a lake that separates an upper warmer zone (epilimnion) from a lower colder zone (hypolimnion).

threshold pollutant: substance that is harmful to a particular organism only above a certain concentration, or threshold level.

TDS - total dissolved solids: the sum or all inorganic and organic particulate material. TDS is an indicator test used for wastewater analysis and is also a measure of the mineral content of bottled water and groundwater. There is a relationship between TDS and conductivity. In general, for the San Antonio River basin, TDS/.6 approximates conductivity. Or, conductivity * .6 approximates TDS. People monitoring water quality can measure electrical conductivity quickly in the field and estimate TDS without doing any lab tests at all. See specific conductance.

Toxicity Reduction Evaluation (TRE): a study conducted to determine the source(s) of toxicity in a discharge effluent so that these sources can be controlled sufficiently to allow a discharger to comply with their permit limits.

toxicity test: the means to determine the toxicity of a chemical or an effluent using living organisms. A toxicity test measures the degree of response of an exposed test organism to a specified chemical or effluent.

Tragedy of the Commons: the idea that no one takes responsibility for things that everybody owns.

transmissivity: refers to the rate at which limestone allows the transmission of water. Limestone can be highly porous, but not very transmissive if the pores are not connected to each other. Technically speaking, it is the rate at which water is transmitted through a unit width of aquifer under unit hydraulic gradient. Transmissivity is directly proportional to aquifer thickness, thus it is high where the Edwards is thick and low where it is thin, given the same hydraulic conductivity.

transpiration: direct transfer of water from the leaves of living plants to the atmosphere. Distinguish evapotranspiration.

tributary: a stream that contributes its water to another stream or body of water.

turbid: thick or opaque with matter in suspension. Rivers and lakes may become turbid after a rainfall.

troposphere: the layer of atmosphere closest to the Earth, extending seven to ten miles above the surface, containing most of the clouds and moisture.

U

USGS: United States Geological Survey

unclassified waters: those waters for which no classification has been assigned and which have not been identified in Appendix A of 31 Texas Administrative Code, Chapter 307.10 of Title 31 (relating to definitions).

unconsolidated formations: naturally occurring earth formations that have not been lithified. Alluvium, soil, gravel, clay, and overburden are some of the terms used to describe this type of formation.

undercurrent: a current below the upper currents or surface of a fluid body.

underdrain: a concealed drain with openings through which the water enters when the water table reaches the level of the drain.

underflow: movement of water through subsurface material.

undertow: the current beneath the surface that sets seaward or along the beach when waves are breaking on the shore.

underwater: under the surface of the water; lying, growing, performed, worn, or operating below the surface of the water, as underwater caverns, underwater operation of a submarine.

upflow: an upward flow.

V

vested water right: the right granted by a state water agency to use either surface or ground water.

virgin flow: the streamflow which exists or would exist if man had not modified the conditions on or along the stream or in the drainage basin.

void: the pore space or other openings in rock. The openings can be very small to cave size and are filled with water below the water table.

W

wastewater: water containing waste including greywater, blackwater or water contaminated by waste contact, including process-generated and contaminated rainfall runoff.

water: the liquid that descends from the clouds as rain; forms streams, lakes, and seas, and is a major constituent of all living matter. It is an odorless, tasteless, colorless, very slightly compressible liquid.

water cycle: natural pathway water follows as it changes between liquid, solid, and gaseous states; biogeochemical cycle that moves and recycles water in various forms through the ecosphere. Also called the hydrologic cycle.

water pollution: degradation of a body of water by a substance or condition to such a degree that the water fails to meet specified standards or cannot be used for a specific purpose.

water quality-based toxics control: an integrated strategy used in NPDES permitting to assess and control the discharge of toxic pollutants to surface waters. There are two approaches: the whole-effluent approach involves the use of toxicity tests to measure discharge toxicity; the chemical specific approach involves the use of water quality criteria or State standards to limit specific toxic pollutants directly.

water quality criteria: scientifically derived ambient limits developed and updated by EPA, under section 304(a)(1) of the Clean Water Act, for specific pollutants of concern. Criteria are recommended concentrations, levels, or narrative statements that should not be exceeded in a waterbody in order to protect aquatic life or human health.

water quality standards: laws or regulations, promulgated under Section 303 of the Clean Water Act, that consist of the designated use or uses of a waterbody or a segment of a waterbody and the water quality criteria that are necessary to protect the use or uses of that particular waterbody. Water quality standards also contain an antidegradation statement. Every State is required to develop water quality criteria standards applicable to the various waterbodies within the State and revise them every 3 yeaars.

water table: level below the earth's surface at which the ground becomes saturated with water. The surface of an unconfined aquifer which fluctuates due to seasonal precipitation.

water table aquifer: an aquifer confined only by atmospheric pressure (water levels will not rise in the well above the confining bed).

water well: any artificial excavation constructed for the purpose of exploring for or producing ground water.

water year: The 12-month period, usually October 1 through September 30. The water year is designated by the calendar year in which it ends and which includes 9 of the 12 months. Thus, the year ending September 30, 1998 is called the1998 Water Year.

waterfall: A sudden, nearly vertical drop in a stream, as it flows over rock.

waterlogging: saturation of soil with irrigation water so the water table rises close to the surface.

watermaster: An employee of a water department who distributes available water supply at the request of water right holders and collects hydrographic data.

watershed: land area from which water drains toward a common watercourse in a natural basin.

weather: day to day variation in atmospheric conditions. Compare climate.

wetland: area that is regularly wet or flooded and has a water table that stands at or above the land surface for at least part of the year, such as a bog, pond, fen, estuary, or marsh.

whole-effluent toxicity: the aggregate toxic effect of an effluent measured directly by a toxicity test.

X

xeriscape: creative landscaping for water and energy efficiency and lower maintenance. The seven xeriscape principles are: good planning and design; practical lawn areas; efficient irrigation; soil improvement; use of mulches; low water demand plants; good maintenance.

Y

yield: the quantity of water expressed either as a continuous rate of flow (cubic feet per second, etc.) or as a volume per unit of time. It can be collected for a given use, or uses, from surface or groundwater sources on a watershed.

Z

zone of aeration: a region in the Earth above the water table. Water in the zone of aeration is under atmospheric pressure and will not flow into a well.

zone of saturation: the space below the water table in which all the interstices (pore spaces) are filled with water. Water in the zone of saturation is called groundwater.

zooplankton: tiny aquatic animals eaten by fish.