National Water-Quality Assessment (NAWQA) Project

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General Information
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The terms in this glossary were compiled from numerous sources. Some definitions have been modified and may not be the only valid ones for these terms.
See also the glossary of study components in Circular 1112.

Acre-foot - A volume of water equal to 1 foot in depth and covering 1 acre; equivalent to 43,560 cubic feet or 325,851 gallons.

Algae - Chlorophyll-bearing nonvascular, primarily aquatic species that have no true roots, stems, or leaves; most algae are microscopic, but some species can be as large as vascular plants.

Alluvium - Deposits of clay, silt, sand, gravel or other particulate rock material left by a river in a streambed, on a flood plain, delta, or at the base of a mountain.

Alluvial aquifer - A water-bearing deposit of unconsolidated material (sand and gravel) left behind by a river or other flowing water.

Amalgamation - The dissolving or blending of a metal (commonly gold and silver) in mercury to separate it from its parent material.

Ammonia - A compound of nitrogen and hydrogen (NH3) that is a common by-product of animal waste. Ammonia readily converts to nitrate in soils and streams.

Anomalies - As related to fish, externally visible skin or subcutaneous disorders, including deformities, eroded fins, lesions, and tumors.

Anthropogenic - Occurring because of, or influenced by, human activity.

Aquatic guidelines - Specific levels of water quality which, if reached, may adversely affect aquatic life. These are nonenforceable guidelines issued by a governmental agency or other institution.

Aquatic-life criteria - Water-quality guidelines for protection of aquatic life. Often refers to U.S. Environmental Protection Agency water-quality criteria for protection of aquatic organisms. See also Water-quality guidelines, Water-quality criteria, and Freshwater chronic criteria.

Aquifer - A water-bearing layer of soil, sand, gravel, or rock that will yield usable quantities of water to a well.

Artificial recharge - Augmentation of natural replenishment of ground-water storage by some method of construction, spreading of water, or by pumping water directly into an aquifer.

Atmospheric deposition - The transfer of substances from the air to the surface of the Earth, either in wet form (rain, fog, snow, dew, frost, hail) or in dry form (gases, aerosols, particles).

Background concentration - A concentration of a substance in a particular environment that is indicative of minimal influence by human (anthropogenic) sources.

Bank - The sloping ground that borders a stream and confines the water in the natural channel when the water level, or flow, is normal.

Base flow - Sustained, low flow in a stream; ground-water discharge is the source of base flow in most places.

Basic Fixed Sites - Sites on streams at which streamflow is measured and samples are collected for temperature, salinity, suspended sediment, major ions and metals, nutrients, and organic carbon to assess the broad-scale spatial and temporal character and transport of inorganic constituents of streamwater in relation to hydrologic conditions and environmental settings.

Basin -See Drainage basin.

Basin and Range physiography - A region characterized by a series of generally north-trending mountain ranges separated by alluvial valleys.

Bedload - Sediment that moves on or near the streambed and is in almost continuous contact with the bed.

Bedrock - General term for consolidated (solid) rock that underlies soils or other unconsolidated material.

Bed sediment - The material that temporarily is stationary in the bottom of a stream or other watercourse.

Bed sediment and tissue studies - Assessment of concentrations and distributions of trace elements and hydrophobic organic contaminants in streambed sediment and tissues of aquatic organisms to identify potential sources and to assess spatial distribution.

Benthic - Refers to plants or animals that live on the bottom of lakes, streams, or oceans.

Benthic invertebrates - Insects, mollusks, crustaceans, worms, and other organisms without a backbone that live in, on, or near the bottom of lakes, streams, or oceans.

Best management practice (BMP) - An agricultural practice that has been determined to be an effective, practical means of preventing or reducing nonpoint source pollution.

Bioaccumulation - The biological sequestering of a substance at a higher concentration than that at which it occurs in the surrounding environment or medium. Also, the process whereby a substance enters organisms through the gills, epithelial tissues, dietary, or other sources.

Bioavailability - The capacity of a chemical constituent to be taken up by living organisms either through physical contact or by ingestion.

Biochemical - Refers to chemical processes that occur inside or are mediated by living organisms.

Biochemical oxygen demand (BOD) - The amount of oxygen, measured in milligrams per liter, that is removed from aquatic environments by the life processes of microorganisms.

Biodegradation - Transformation of a substance into new compounds through biochemical reactions or the actions of microorganisms such as bacteria.

Biomass - The amount of living matter, in the form of organisms, present in a particular habitat, usually expressed as weight per unit area.

Biota - Living organisms.

Blue-baby syndrome - A condition that can be caused by ingestion of high amounts of nitrate resulting in the blood losing its ability to effectively carry oxygen. It is most common in young infants and certain elderly people.

Breakdown product - A compound derived by chemical, biological, or physical action upon a pesticide. The breakdown is a natural process which may result in a more toxic or a less toxic compound and a more persistent or less persistent compound.

Canopy angle - Generally, a measure of the openness of a stream to sunlight. Specifically, the angle formed by an imaginary line from the highest structure (for example, tree, shrub, or bluff) on one bank to eye level at midchannel to the highest structure on the other bank.

Carbonate rocks - Rocks (such as limestone or dolostone) that are composed primarily of minerals (such as calcite and dolomite) containing the carbonate ion (CO32-).

Center pivot irrigation - An automated sprinkler system involving a rotating pipe or boom that supplies water to a circular area of an agricultural field through sprinkler heads or nozzles.

Channelization - Modification of a stream, typically by straightening the channel, to provide more uniform flow; often done for flood control or for improved agricultural drainage or irrigation.

Chlordane - Octachloro-4,7-methanotetrahydroindane. An organochlorine insecticide no longer registered for use in the U.S. Technical chlordane is a mixture in which the primary components are cis- and trans-chlordane, cis- and trans-nonachlor, and heptachlor.

Chlorinated solvent - A volatile organic compound containing chlorine. Some common solvents are trichloroethylene, tetrachloroethylene, and carbon tetrachloride.

Chlorofluorocarbons - A class of volatile compounds consisting of carbon, chlorine, and fluorine. Commonly called freons, which have been used in refrigeration mechanisms, as blowing agents in the fabrication of flexible and rigid foams, and, until several years ago, as propellants in spray cans.

Chrysene - See Polycyclic aromatic hydrocarbon (PAH).

Clastic - Rock or sediment composed principally of broken fragments that are derived from preexisting rocks which have been transported from their place of origin, as in sandstone.

Climate - The sum total of the meteorological elements that characterize the average and extreme conditions of the atmosphere over a long period of time at any one place or region of the Earth's surface.

Combined sewer overflow - A discharge of untreated sewage and stormwater to a stream when the capacity of a combined storm/sanitary sewer system is exceeded by storm runoff.

Community - In ecology, the species that interact in a common area.

Concentration - The amount or mass of a substance present in a given volume or mass of sample. Usually expressed as microgram per liter (water sample) or micrograms per kilogram (sediment or tissue sample).

Confined aquifer (artesian aquifer) - An aquifer that is completely filled with water under pressure and that is overlain by material that restricts the movement of water.

Confining layer - A layer of sediment or lithologic unit of low permeability that bounds an aquifer,

Confluence - The flowing together of two or more streams; the place where a tributary joins the main stream.

Constituent - A chemical or biological substance in water, sediment, or biota that can be measured by an analytical method.

Consumptive use - The quantity of water that is not available for immediate reuse because it has been evaporated, transpired, or incorporated into products, plant tissue, or animal tissue. Also referred to as "water consumption".

Contamination - Degradation of water quality compared to original or natural conditions due to human activity.

Contributing area - The area in a drainage basin that contributes water to streamflow or recharge to an aquifer.

Criterion - A standard rule or test on which a judgment or decision can be based.

Crystalline rocks - Rocks (igneous or metamorphic) consisting wholly of crystals or fragments of crystals.

Cubic foot per second (ft3/s, or cfs) - Rate of water discharge representing a volume of 1 cubic foot passing a given point during 1 second, equivalent to approximately 7.48 gallons per second or 448.8 gallons per minute or 0.02832 cubic meter per second.

Degradation products - Compounds resulting from transformation of an organic substance through chemical, photochemical, and/or biochemical reactions.

Denitrification - A process by which oxidized forms of nitrogen such as nitrate (NO3-) are reduced to form nitrites, nitrogen oxides, ammonia, or free nitrogen: commonly brought about by the action of denitrifying bacteria and usually resulting in the escape of nitrogen to the air.

Detect - To determine the presence of a compound.

Detection limit - The concentration below which a particular analytical method cannot determine, with a high degree of certainty, a concentration.

Diatoms - Single-celled, colonial, or filamentous algae with siliceous cell walls constructed of two overlapping parts.

DDT - Dichloro-diphenyl-trichloroethane. An organochlorine insecticide no longer registered for use in the United States.

Dieldrin - An organochlorine insecticide no longer registered for use in the United States. Also a degradation product of the insecticide aldrin.

Discharge - Rate of fluid flow passing a given point at a given moment in time, expressed as volume per unit of time.

Dissolved constituent - Operationally defined as a constituent that passes through a 0.45-micrometer filter.

Dissolved solids - Amount of minerals, such as salt, that are dissolved in water; amount of dissolved solids is an indicator of salinity or hardness.

Diversion - A turning aside or alteration of the natural course of a flow of water, normally considered physically to leave the natural channel. In some States, this can be a consumptive use direct from another stream, such as by livestock watering. In other States, a diversion must consist of such actions as taking water through a canal, pipe, or conduit.

Drainage area - The drainage area of a stream at a specified location is that area, measured in a horizontal plane, which is enclosed by a drainage divide.

Drainage basin - The portion of the surface of the Earth that contributes water to a stream through overland run-off, including tributaries and impoundments.

Drawdown - The difference between the water level in a well before pumping and the water level in the well during pumping. Also, for flowing wells, the reduction of the pressure head as a result of the discharge of water.

Drinking-water standard or guideline - A threshold concentration in a public drinking-water supply, designed to protect human health. As defined here, standards are U.S. Environmental Protection Agency regulations that specify the maximum contamination levels for public water systems required to protect the public welfare; guidelines have no regulatory status and are issued in an advisory capacity.

Drip irrigation - An irrigation system in which water is applied directly to the root zone of plants by means of applicators (orifices, emitters, porous tubing, perforated pipe, and so forth) operated under low pressure. The applicators can be placed on or below the surface of the ground or can be suspended from supports.

Drought - Commonly defined as being a time of less-than-normal or less-than-expected precipitation.

Ecological studies - Studies of biological communities and habitat characteristics to evaluate the effects of physical and chemical characteristics of water and hydrologic conditions on aquatic biota and to determine how biological and habitat characteristics differ among environmental settings in NAWQA Study Units.

Ecoregion - An area of similar climate, landform, soil, potential natural vegetation, hydrology, or other ecologically relevant variables.

Ecosystem - The interacting populations of plants, animals, and microorganisms occupying an area, plus their physical environment.

Effluent - Outflow from a particular source, such as a stream that flows from a lake or liquid waste that flows from a factory or sewage-treatment plant.

Endocrine system - The collection of ductless glands in animals that secrete hormones, which influence growth, gender and sexual maturity.

Environmental framework - Natural and human-related features of the land and hydrologic system, such as geology, land use, and habitat, that provide a unifying framework for making comparative assessments of the factors that govern water-quality conditions within and among Study Units.

Environmental sample - A water sample collected from an aquifer or stream for the purpose of chemical, physical, or biological characterization of the sampled resource.

Environmental setting - Land area characterized by a unique combination of natural and human-related factors, such as row-crop cultivation or glacial-till soils.

Ephemeral stream - A stream or part of a stream that flows only in direct response to precipitation or snowmelt. Its channel is above the water table at all times.

EPT richness index - An index based on the sum of the number of taxa in three insect orders, Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera (caddisflies), that are composed primarily of species considered to be relatively intolerant to environmental alterations.

Equal-width increment (EWI) sample - A composite sample across a section of stream with equal spacing between verticals and equal transit rates within each vertical that yields a representative sample of stream conditions.

Erosion - The process whereby materials of the Earth's crust are loosened, dissolved, or worn away and simultaneously moved from one place to another.

Eutrophication - The process by which water becomes enriched with plant nutrients, most commonly phosphorus and nitrogen.

Evaporite minerals (deposits) - Minerals or deposits of minerals formed by evaporation of water containing salts. These deposits are common in arid climates.

Evapotranspiration - A collective term that includes water lost through evaporation from the soil and surface-water bodies and by plant transpiration.

Fecal bacteria - Microscopic single-celled organisms (primarily fecal coliforms and fecal streptococci) found in the wastes of warm-blooded animals. Their presence in water is used to assess the sanitary quality of water for body-contact recreation or for consumption. Their presence indicates contamination by the wastes of warm-blooded animals and the possible presence of pathogenic (disease producing) organisms.

Fecal coliform - See Fecal bacteria.

FDA action level - A regulatory level recommended by the U.S. Environmental Protection Agency for enforcement by the FDA when pesticide residues occur in food commodities for reasons other than the direct application of the pesticide. Action levels are set for inadvertent pesticide residues resulting from previous legal use or accidental contamination. Applies to edible portions of fish and shellfish in interstate commerce.

Fertilizer - Any of a large number of natural or synthetic materials, including manure and nitrogen, phosphorus, and potassium compounds, spread on or worked into soil to increase its fertility.

Fish community - See Community.

Fixed Sites - NAWQA's most comprehensive monitoring sites. See also Basic Fixed Sites and Intensive Fixed Sites.

Flood - Any relatively high streamflow that overtops the natural or artificial banks of a stream.

Flood irrigation - The application of irrigation water where the entire surface of the soil is covered by ponded water.

Flood plain - The relatively level area of land bordering a stream channel and inundated during moderate to severe floods.

Flowpath - An underground route for ground-water movement, extending from a recharge (intake) zone to a discharge (output) zone such as a shallow stream.

Flowpath study - Network of clustered wells located along a flowpath extending from a recharge zone to a discharge zone, preferably a shallow stream. The studies examine the relations of land-use practices, ground-water flow, and contaminant occurrence and transport. These studies are located in the area of one of the land-use studies.

Fluvial deposit - A sedimentary deposit consisting of material transported by suspension or laid down by a river or stream.

Freshwater chronic criteria - The highest concentration of a contaminant that freshwater aquatic organisms can be exposed to for an extended period of time (4 days) without adverse effects. See also Water-quality criteria.

Fumigant - A substance or mixture of substances that produces gas, vapor, fume, or smoke intended to destroy insects, bacteria, or rodents.

Furrow irrigation - A type of surface irrigation where water is applied at the upper end of a field and flows in furrows to the lower end.

Gaging station - A particular site on a stream, canal, lake, or reservoir where systematic observations of hydrologic data are obtained.

Geothermal - Relating to the Earth's internal heat; commonly applied to springs or vents discharging hot water or steam.

Granitic rock - A coarse-grained igneous rock.

Ground water - In general, any water that exists beneath the land surface, but more commonly applied to water in fully saturated soils and geologic formations.

Habitat - The part of the physical environment where plants and animals live.

Headwaters - The source and upper part of a stream.

Health advisory - Nonregulatory levels of contaminants in drinking water that may be used as guidance in the absence of regulatory limits. Advisories consist of estimates of concentrations that would result in no known or anticipated health effects (for carcinogens, a specified cancer risk) determined for a child or for an adult for various exposure periods.

Herbicide - A chemical or other agent applied for the purpose of killing undesirable plants. See also Pesticide.

Human health advisory - Guidance provided by U.S. Environmental Protection Agency, State agencies or scientific organizations, in the absence of regulatory limits, to describe acceptable contaminant levels in drinking water or edible fish.

Hydrograph - Graph showing variation of water elevation, velocity, streamflow, or other property of water with respect to time.

Hydrologic cycle - The circulation of water from the sea, through the atmosphere, to the land, and thence back to the sea by overland and subterranean routes.

Index of Biotic Integrity (IBI) - An aggregated number, or index, based on several attributes or metrics of a fish community that provides an assessment of biological conditions.

Indicator sites - Stream sampling sites located at outlets of drainage basins with relatively homogeneous land use and physiographic conditions; most indicator-site basins have drainage areas ranging from 20 to 200 square miles.

Infiltration - Movement of water, typically downward, into soil or porous rock.

Insecticide - A substance or mixture of substances intended to destroy or repel insects.

Instantaneous discharge - The volume of water that passes a point at a particular instant of time.

Instream use - Water use taking place within the stream channel for such purposes as hydroelectric power generation, navigation, water-quality improvement, fish propagation, and recreation. Sometimes called nonwithdrawal use or in-channel use.

Integrator or Mixed-use site - Stream sampling site located at an outlet of a drainage basin that contains multiple environmental settings. Most integrator sites are on major streams with relatively large drainage areas.

Intensive Fixed Sites - Basic Fixed Sites with increased sampling frequency during selected seasonal periods and analysis of dissolved pesticides for 1 year. Most NAWQA Study Units have one to two integrator Intensive Fixed Sites and one to four indicator Intensive Fixed Sites.

Intermittent stream - A stream that flows only when it receives water from rainfall runoff or springs, or from some surface source such as melting snow.

Intolerant organisms - Organisms that are not adaptable to human alterations to the environment and thus decline in numbers where human alterations occur. See also Tolerant species.

Invertebrate - An animal having no backbone or spinal column. See also Benthic invertebrate.

Irrigation return flow - The part of irrigation applied to the surface that is not consumed by evapotranspiration or uptake by plants and that migrates to an aquifer or surface-water body.

Karst - A type of topography that results from dissolution and collapse of carbonate rocks such as limestone and dolomite, and characterized by closed depressions or sinkholes, caves, and underground drainage.

Kill - Dutch term for stream or creek.

Land-use study - A network of existing shallow wells in an area having a relatively uniform land use. These studies are a subset of the Study-Unit Survey and have the goal of relating the quality of shallow ground water to land use. See also Study-Unit Survey.

Leaching - The removal of materials in solution from soil or rock to ground water; refers to movement of pesticides or nutrients from land surface to ground water.

Load - General term that refers to a material or constituent in solution, in suspension, or in transport; usually expressed in terms of mass or volume.

Loess - Homogeneous, fine-grained sediment made up primarily of silt and clay, and deposited over a wide area (probably by wind).

Long-term monitoring - Data collection over a period of years or decades to assess changes in selected hydrologic conditions.

Main stem - The principal course of a river or a stream.

Major ions - Constituents commonly present in concentrations exceeding 1.0 milligram per liter. Dissolved cations generally are calcium, magnesium, sodium, and potassium; the major anions are sulfate, chloride, fluoride, nitrate, and those contributing to alkalinity, most generally assumed to be bicarbonate and carbonate.

Maximum contaminant level (MCL) - Maximum permissible level of a contaminant in water that is delivered to any user of a public water system. MCLs are enforceable standards established by the U.S. Environmental Protection Agency.

Mean - The average of a set of observations, unless otherwise specified.

Mean discharge (MEAN) - The arithmetic mean of individual daily mean discharges during a specific period, usually daily, monthly, or annually.

Median - The middle or central value in a distribution of data ranked in order of magnitude. The median is also known as the 50th percentile.

Metabolite - A substance produced in or by biological processes.

Metamorphic rock - Rock that has formed in the solid state in response to pronounced changes of temperature, pressure, and chemical environment.

Method detection limit - The minimum concentration of a substance that can be accurately identified and measured with present laboratory technologies.

Micrograms per liter (µg/L) - A unit expressing the concentration of constituents in solution as weight (micrograms) of solute per unit volume (liter) of water; equivalent to one part per billion in most streamwater and ground water. One thousand micrograms per liter equals 1 mg/L.

Midge - A small fly in the family Chironomidae. The larval (juvenile) life stages are aquatic.

Milligram (mg) - A mass equal to 10-3 grams.

Milligrams per liter (mg/L) - A unit expressing the concentration of chemical constituents in solution as weight (milligrams) of solute per unit volume (liter) of water; equivalent to one part per million in most streamwater and ground water. One thousand micrograms per liter equals 1 mg/L.

Minimum reporting level (MRL) - The smallest measured concentration of a constituent that may be reliably reported using a given analytical method. In many cases, the MRL is used when documentation for the method detection limit is not available.

Monitoring - Repeated observation or sampling at a site, on a scheduled or event basis, for a particular purpose.

Monitoring well - A well designed for measuring water levels and testing ground-water quality.

Monocyclic aromatic hydrocarbons - Single-ring aromatic compounds. Constituents of lead-free gasoline; also used in the manufacture of monomers and plasticizers in polymers.

Mouth - The place where a stream discharges to a larger stream, a lake, or the sea.

National Academy of Sciences/National Academy of Engineering (NAS/NAE) recommended maximum concentration in water - Numerical guidelines recmmended by two joint NAS/NAE committees for the protection of freshwater and marine aquatic life, respectively. These guidelines were based on available aquatic toxicity studies, and were considered preliminary even at the time (1972). The guidelines used in the summary reports are for freshwater.

Nitrate - An ion consisting of nitrogen and oxygen (NO3-). Nitrate is a plant nutrient and is very mobile in soils.

Noncontact water recreation - Recreational activities, such as fishing or boating, that do not include direct contact with the water.

Nonpoint source - A pollution source that cannot be defined as originating from discrete points such as pipe discharge. Areas of fertilizer and pesticide applications, atmospheric deposition, manure, and natural inputs from plants and trees are types of nonpoint source pollution.

Nonpoint source contaminant - A substance that pollutes or degrades water that comes from lawn or cropland runoff, the atmosphere, roadways, and other diffuse sources.

Nonpoint-source water pollution - Water contamination that originates from a broad area (such as leaching of agricultural chemicals from crop land) and enters the water resource diffusely over a large area.

Nonselective herbicide - Kills or significantly retards growth of most higher plant species.

Nutrient - Element or compound essential for animal and plant growth. Common nutrients in fertilizer include nitrogen, phosphorus, and potassium.

Occurrence and distribution assessment - Characterization of the broad-scale spatial and temporal distributions of water-quality conditions in relation to major contaminant sources and background conditions for surface water and ground water.

Organic detritus - Any loose organic material in streams - such as leaves, bark, or twigs - removed and transported by mechanical means, such as disintegration or abrasion.

Organochlorine compound - Synthetic organic compounds containing chlorine. As generally used, term refers to compounds containing mostly or exclusively carbon, hydrogen, and chlorine. Examples include organochlorine insecticides, polychlorinated biphenyls, and some solvents containing chlorine.

Organochlorine insecticide - A class of organic insecticides containing a high percentage of chlorine. Includes dichlorodiphenylethanes (such as DDT), chlorinated cyclodienes (such as chlordane), and chlorinated benzenes (such as lindane). Most organochlorine insecticides were banned because of their carcinogenicity, tendency to bioaccumulate, and toxicity to wildlife.

Organochlorine pesticide - See Organochlorine insecticide.

Organophosphate insecticides - A class of insecticides derived from phosphoric acid. They tend to have high acute toxicity to vertebrates. Although readily metabolized by vertebrates, some metabolic products are more toxic than the parent compound.

Organonitrogen herbicides - A group of herbicides consisting of a nitrogen ring with associated functional groups and including such classes as triazines and acetanilides. Examples include atrazine, cyanazine, alachlor, and metolachlor.

Organophosphorus insecticides - Insecticides derived from phosphoric acid and are generally the most toxic of all pesticides to vertebrate animals.

Outwash - Soil material washed down a hillside by rainwater and deposited upon more gently sloping land.

Overland flow - The part of surface runoff flowing over land surfaces toward stream channels.

Part per million (ppm) - Unit of concentration equal to one milligram per kilogram or one milligram per liter.

Perennial stream - A stream that normally has water in its channel at all times.

Periphyton - Organisms that grow on underwater surfaces, including algae, bacteria, fungi, protozoa, and other organisms.

Pesticide - A chemical applied to crops, rights of way, lawns, or residences to control weeds, insects, fungi, nematodes, rodents or other "pests."

pH - The logarithm of the reciprocal of the hydrogen ion concentration (activity) of a solution; a measure of the acidity (pH less than 7) or alkalinity (pH greater than 7) of a solution; a pH of 7 is neutral.

Phenols - A class of organic compounds containing phenol (C6H5OH) and its derivatives. Used to make resins, weed killers, and as a solvent, disinfectant, and chemical intermediate. Some phenols occur naturally in the environment.

Phosphorus - A nutrient essential for growth that can play a key role in stimulating aquatic growth in lakes and streams.

Photosynthesis - Synthesis of chemical compounds by organisms with the aid of light. Carbon dioxide is used as raw material for photosynthesis and oxygen is a product.

Phthalates - A class of organic compounds containing phthalic acid esters [C6H4(COOR)2] and derivatives. Used as plasticizers in plastics. Also used in many other products (such as detergents, cosmetics) and industrial processes (such as defoaming agents during paper and paperboard manufacture, and dielectrics in capacitors).

Physiography - A description of the surface features of the Earth, with an emphasis on the origin of landforms.

Phytoplankton - See Plankton.

Picocurie (pCi) - One trillionth (10-12) of the amount of radioactivity represented by a curie (Ci). A curie is the amount of radioactivity that yields 3.7 x 1010 radioactive disintegrations per second (dps). A picocurie yields 2.22 disintegrations per minute (dpm) or 0.037 dps.

Plankton - Floating or weakly swimming organisms at the mercy of the waves and currents. Animals of the group are called zooplankton and the plants are called phytoplankton.

Point source - A source at a discrete location such as a discharge pipe, drainage ditch, tunnel, well, concentrated livestock operation, or floating craft.

Point-source contaminant - Any substance that degrades water quality and originates from discrete locations such as discharge pipes, drainage ditches, wells, concentrated livestock operations, or floating craft.

Pollutant - Any substance that, when present in a hydrologic system at sufficient concentration, degrades water quality in ways that are or could become harmful to human and/or ecological health or that impair the use of water for recreation, agriculture, industry, commerce, or domestic purposes.

Polychlorinated biphenyls (PCBs) - A mixture of chlorinated derivatives of biphenyl, marketed under the trade name Aroclor with a number designating the chlorine content (such as Aroclor 1260). PCBs were used in transformers and capacitors for insulating purposes and in gas pipeline systems as a lubricant. Further sale for new use was banned by law in 1979.

Polycyclic aromatic hydrocarbon (PAH) - A class of organic compounds with a fused-ring aromatic structure. PAHs result from incomplete combustion of organic carbon (including wood), municipal solid waste, and fossil fuels, as well as from natural or anthropogenic introduction of uncombusted coal and oil. PAHs include benzo(a)pyrene, fluoranthene, and pyrene.

Pool - A small part of the stream reach with little velocity, commonly with water deeper than surrounding areas.

Postemergence herbicide - Herbicide applied to foliage after the crop has sprouted to kill or significantly retard the growth of weeds.

Precipitation - Any or all forms of water particles that fall from the atmosphere, such as rain, snow, hail, and sleet.

Preemergence herbicide - Herbicide applied to bare ground after planting the crop but prior to the crop sprouting above ground to kill or significantly retard the growth of weed seedlings.

Public-supply withdrawals - Water withdrawn by public and private water suppliers for use within a general community. Water is used for a variety of purposes such as domestic, commercial, industrial, and public water use.

Quality assurance - Evaluation of quality-control data to allow quantitative determination of the quality of chemical data collected during a study. Techniques used to collect, process, and analyze water samples are evaluated.

Radon - A naturally occurring, colorless, odorless, radioactive gas formed by the disintegration of the element radium; damaging to human lungs when inhaled.

Recharge - Water that infiltrates the ground and reaches the saturated zone.

Reference site - A NAWQA sampling site selected for its relatively undisturbed conditions.

Relative abundance - The number of organisms of a particular kind present in a sample relative to the total number of organisms in the sample.

Retrospective analysis - Review and analysis of existing data in order to address NAWQA objectives, to the extent possible, and to aid in the design of NAWQA studies.

Riffle - A shallow part of the stream where water flows swiftly over completely or partially submerged obstructions to produce surface agitation.

Riparian - Areas adjacent to rivers and streams with a high density, diversity, and productivity of plant and animal species relative to nearby uplands.

Riparian zone - Pertaining to or located on the bank of a body of water, especially a stream.

Runoff - Excess rainwater or snowmelt that is transported to streams by overland flow, tile drains, or ground water.

Secondary maximum contaminant level (SMCL) - The maximum contamination level in public water systems that, in the judgment of the U.S. Environmental Protection Agency (USEPA), are required to protect the public welfare. SMCLs are secondary (nonenforceable) drinking water regulations established by the USEPA for contaminants that may adversely affect the odor or appearance of such water.

Sediment - Particles, derived from rocks or biological materials, that have been transported by a fluid or other natural process, suspended or settled in water.

Sediment guideline - Threshold concentration above which there is a high probability of adverse effects on aquatic life from sediment contamination, determined using modified USEPA (1996) procedures.

Sediment quality guideline - Threshold concentration above which there is a high probability of adverse effects on aquatic life from sediment contamination, determined using modified USEPA (1996) procedures.

Selective herbicide - Kills or significantly retards growth of an unwanted plant species without significantly damaging desired plant species.

Semipermeable membrane device (SPMD) - A long strip of low-density, polyethylene tubing filled with a thin film of purified lipid such as triolein that simulates the exposure to and passive uptake of highly lipid-soluble organic compounds by biological membranes.

Semivolatile organic compound (SVOC) - Operationally defined as a group of synthetic organic compounds that are solvent-extractable and can be determined by gas chromatography/mass spectrometry. SVOCs include phenols, phthalates, and Polycyclic aromatic hydrocarbons (PAHs).

Sideslope gradient - The representative change in elevation in a given horizontal distance (usually about 300 yards) perpendicular to a stream; the valley slope along a line perpendicular to the stream (near the water-quality or biological sampling point).

Siliciclastic rocks - Rocks such as shale and sandstone which are formed by the compaction and cementation of quartz-rich mineral grains.

Sinuosity - The ratio of the channel length between two points on a channel to the straight-line distance between the same two points; a measure of meandering.

Sole-source aquifer - A ground-water system that supplies at least 50 percent of the drinking water to a particular human population; the term is used to denote special protection requirements under the Safe Drinking Water Act and may be used only by approval of the U.S. Environmental Protection Agency.

Solid-phase extraction - A procedure to isolate specific organic compounds onto a bonded silica extraction column.

Solute - See Solution.

Solution - Formed when a solid, gas, or another liquid in contact with a liquid becomes dispersed homogeneously throughout the liquid. The substance, called a solute, is said to dissolve. The liquid is called the solvent.

Solvent - See Solution.

Sorption - General term for the interaction (binding or association) of a solute ion or molecule with a solid.

Source rocks - The rocks from which fragments and other detached pieces have been derived to form a different rock.

Species - Populations of organisms that may interbreed and produce fertile offspring having similar structure, habits, and functions.

Species diversity - An ecological concept that incorporates both the number of species in a particular sampling area and the evenness with which individuals are distributed among the various species.

Species (taxa) richness - The number of species (taxa) present in a defined area or sampling unit.

Specific conductance - A measure of the ability of a liquid to conduct an electrical current.

Split sample - A sample prepared by dividing it into two or more equal volumes, where each volume is considered a separate sample but representative of the entire sample.

Stage - The height of the water surface above an established datum plane, such as in a river above a predetermined point that may (or may not) be near the channel floor.

Statistics - A branch of mathematics dealing with the collection, analysis, interpretation, and presentation of masses of numerical data.

Stratification - Subdivision of the environmental framework. The Study Unit is divided into subareas that exhibit reasonable homogeneous environmental conditions, as determined by both natural and human influences.

Stream-aquifer interactions - Relations of water flow and chemistry between streams and aquifers that are hydraulically connected.

Streamflow - A type of channel flow, applied to that part of surface runoff in a stream whether or not it is affected by diversion or regulation.

Stream mile - A distance of 1 mile along a line connecting the midpoints of the channel of a stream.

Stream order - A ranking of the relative sizes of streams within a watershed based on the nature of their tributaries. The smallest unbranched tributary is called first order, the stream receiving the tributary is called second order, and so on.

Stream reach - A continuous part of a stream between two specified points.

Study Unit - A major hydrologic system of the United States in which NAWQA studies are focused. Study Units are geographically defined by a combination of ground- and surface-water features and generally encompass more than 4,000 square miles of land area.

Study-Unit Survey - Broad assessment of the water-quality conditions of the major aquifer systems of each Study Unit. The Study-Unit Survey relies primarily on sampling existing wells and, wherever possible, on existing data collected by other agencies and programs. Typically, 20 to 30 wells are sampled in each of three to five aquifer subunits.

Subsidence - Compression of soft aquifer materials in a confined aquifer due to pumping of water from the aquifer.

Substrate size - The diameter of streambed particles such as clay, silt, sand, gravel, cobble and boulders.

Subsurface drain - A shallow drain installed in an irrigated field to intercept the rising ground-water level and maintain the water table at an acceptable depth below the land surface.

Surface water - An open body of water, such as a lake, river, or stream.

Survey - Sampling of any number of sites during a given hydrologic condition.

Suspended (as used in tables of chemical analyses) - The amount (concentration) of undissolved material in a water-sediment mixture. It is associated with the material retained on a 0.45- micrometer filter.

Suspended sediment - Particles of rock, sand, soil, and organic detritus carried in suspension in the water column, in contrast to sediment that moves on or near the streambed.

Suspended-sediment concentration - The velocity-weighted concentration of suspended sediment in the sampled zone (from the water surface to a point approximately 0.3 foot above the bed) expressed as milligrams of dry sediment per liter of water-sediment mixture (mg/L).

Suspended solids - Different from suspended sediment only in the way that the sample is collected and analyzed.

Synoptic sites - Sites sampled during a short-term investigation of specific water-quality conditions during selected seasonal or hydrologic conditions to provide improved spatial resolution for critical water-quality conditions.

Tailings - Rock that remains after processing ore to remove the valuable minerals.

Taxon (plural taxa) - Any identifiable group of taxonomically related organisms.

Taxa richness - See Species richness.

Tertiary-treated sewage - The third phase of treating sewage that removes nitrogen and phosphorus before it is discharged.

Tier 1 sediment guideline - Threshold concentration above which there is a high probability of adverse effects on aquatic life from sediment contamination, determined using modified USEPA (1996) procedures.

Tile drain - A buried perforated pipe designed to remove excess water from soils.

Tissue study - The assessment of concentrations and distributions of trace elements and certain organic contaminants in tissues of aquatic organisms.

Tolerant species - Those species that are adaptable to (tolerant of) human alterations to the environment and often increase in number when human alterations occur.

Total concentration - Refers to the concentration of a constituent regardless of its form (dissolved or bound) in a sample.

Total DDT - The sum of DDT and its metabolites (breakdown products), including DDD and DDE.

Trace element - An element found in only minor amounts (concentrations less than 1.0 milligram per liter) in water or sediment; includes arsenic, cadmium, chromium, copper, lead, mercury, nickel, and zinc.

Tracer - A stable, easily detected substance or a radioisotope added to a material to follow the location of the substance in the environment or to detect any physical or chemical changes it undergoes.

Triazine herbicide - A class of herbicides containing a symmetrical triazine ring (a nitrogen-heterocyclic ring composed of three nitrogens and three carbons in an alternating sequence). Examples include atrazine, propazine, and simazine.

Triazine pesticide - See Triazine herbicide.

Tributary - A river or stream flowing into a larger river, stream or lake.

Tritium - A radioactive form of hydrogen with atoms of three times the mass of ordinary hydrogen; used to determine the age of water.

Turbidity - Reduced clarity of surface water because of suspended particles, usually sediment.

Unconfined aquifer - An aquifer whose upper surface is a water table; an aquifer containing unconfined ground water.

Unconsolidated deposit - Deposit of loosely bound sediment that typically fills topographically low areas.

Un-ionized - The neutral form of an ionizable compound (such as an acid or a base).

Un-ionized ammonia - The neutral form of ammonia-nitrogen in water, usually occurring as NH4OH. Un-ionized ammonia is the principal form of ammonia that is toxic to aquatic life. The relative proportion of un-ionized to ionized ammonia (NH4+) is controlled by water temperature and pH. At temperatures and pH values typical of most natural waters, the ionized form is dominant.

Upgradient - Of or pertaining to the place(s) from which ground water originated or traveled through before reaching a given point in an aquifer.

Upland - Elevated land above low areas along a stream or between hills; elevated region from which rivers gather drainage.

Uranium - A heavy silvery-white metallic element, highly radioactive and easily oxidized. Of the 14 known isotopes of uranium, U238 is the most abundant in nature.

Urban site - A site that has greater than 50 percent urbanized and less than 25 percent agricultural area.

Volatile organic compounds (VOCs) - Organic chemicals that have a high vapor pressure relative to their water solubility. VOCs include components of gasoline, fuel oils, and lubricants, as well as organic solvents, fumigants, some inert ingredients in pesticides, and some by-products of chlorine disinfection.

Wasteway - A waterway used to drain excess irrigation water dumped from the irrigation delivery system.

Water budget - An accounting of the inflow, outflow, and storage changes of water in a hydrologic unit.

Water column studies - Investigations of physical and chemical characteristics of surface water, which include suspended sediment, dissolved solids, major ions, and metals, nutrients, organic carbon, and dissolved pesticides, in relation to hydrologic conditions, sources, and transport.

Water-quality criteria - Specific levels of water quality which, if reached, are expected to render a body of water unsuitable for its designated use. Commonly refers to water-quality criteria established by the U.S. Environmental Protection Agency. Water-quality criteria are based on specific levels of pollutants that would make the water harmful if used for drinking, swimming, farming, fish production, or industrial processes.

Water-quality guidelines - Specific levels of water quality which, if reached, may adversely affect human health or aquatic life. These are nonenforceable guidelines issued by a governmental agency or other institution.

Water-quality standards - State-adopted and U.S. Environmental Protection Agency-approved ambient standards for water bodies. Standards include the use of the water body and the water-quality criteria that must be met to protect the designated use or uses.

Watershed - See Drainage basin.

Water table - The point below the land surface where ground water is first encountered and below which the earth is saturated. Depth to the water table varies widely across the country.

Water year - The continuous 12-month period, October 1 through September 30, in U.S. Geological Survey reports dealing with the surface-water supply. 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, 1980, is referred to as the "1980" water year.

Weather - The state of the atmosphere at any particular time and place.

Wetlands - Ecosystems whose soil is saturated for long periods seasonally or continuously, including marshes, swamps, and ephemeral ponds.

Withdrawal - The act or process of removing; such as removing water from a stream for irrigation or public water supply.

Yield - The mass of material or constituent transported by a river in a specified period of time divided by the drainage area of the river basin.

Zooplankton - See Plankton.

Reference cited
U.S. Environmental Protection Agency, 1996, The National Sediment Quality Survey: A report to Congress on the extent and severity of sediment contamination in surface waters of the United States: U.S. Environmental Protection Agency, Office of Science and Technology, Draft Report EPA 823-D-96-002.

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