The terms in this glossary were compiled from numerous
resources. Some definitions have been modified and may not be the only
valid ones for these terms.
- The dissolving or blending of a metal (commonly
gold and silver) in mercury to separate it from its parent material.
As related to fish, externally visible skin or subcutaneous
disorders, including deformities, eroded fins, lesions, and tumors.
- 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.
- 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.
- Background concentration
- A concentration of a substance in
a particular environment that is indicative of minimal influence by human
- Basic Fixed Sites
- Sites on streams at which streamflow is measured
and samples are collected for temperature, salinity, suspended sediment,
major ions and trace elements, 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 area
- Areas that do not receive adequate precipitation
for ground-water recharge and streamflow. These areas can receive recharge
and streamflow from upstream headwater areas.
- Basin and Range physiography
- A region characterized by a series
of generally north-trending mountain ranges separated by alluvial valleys.
- General term for consolidated (solid) rock that underlies
soils or other unconsolidated material.
- 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.
- Bottom (bed) sediment
- The material that temporarily
is stationary in the bottom of a stream or other watercourse.
- Bottom 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
- Carbonate rocks
- Rocks (such as limestone or dolostone) that
are composed primarily of minerals (such as calcite and dolomite) containing
the carbonate ion (CO32-).
- 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.
- Chlorinated solvent
- A volatile organic compound containing
chlorine. Some common solvents are trichloroethylene, tetrachloroethylene,
and carbon tetrachloride.
- Confining layer
- A layer of sediment or lithologic unit of low
permeability that bounds an aquifer.
- The flowing together of two or more streams; the
place where a tributary joins the main stream.
- Consolidated rock
- Metamorphic, sedimentary, or igneous (granitic
or volcanic) rock that typically form mountainous uplands and underlie
unconsolidated deposits in valleys.
- A chemical or biological substance in water, sediment,
or biota that can be measured by an analytical method.
- Degradation of water quality compared to original
or natural conditions due to human activity.
- Degradation products
- Compounds resulting from transformation of an organic substance through
chemical, photochemical, and (or) biochemical reactions.
- Dissolved solids
- Amount of minerals, such as salt, that are dissolved in water; amount
of dissolved solids is an indicator of salinity.
- Drainage basin
- The portion of the surface of the Earth that contributes water to a
stream through overland runoff, including tributaries and impoundments.
- Drinking water advisory
- A non-regulatory document that analyzes the available cancer and
non-cancer data on a contaminant and recommends acceptable levels.
- Endocrine system
- The collection of ductless glands in animals
that secrete hormones, which influence growth, gender, and sexual maturity.
- The addition of an element or chemical compound so that its total
concentration is at least 10 percent greater than the concentration
at a background site.
- Evaporite minerals (deposits)
- Minerals or deposits of minerals formed by evaporation of water
containing salts. These deposits are common in arid climates.
- A collective term that includes water lost through evaporation from
the soil and surface-water bodies and by plant transpiration.
- 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
- Fish species that interact in a common area.
- Relating to the Earth's internal heat; commonly
applied to springs or vents discharging hot water or steam.
- Granitic rocks
- Coarse-grained igneous rock.
- Headwater area
- Mountains and adjacent areas where precipitation
is adequate to provide ground-water recharge and streamflow.
- Human health advisory (HA)
- 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.
- Graph showing variation of water elevation, velocity,
streamflow, or other property of water with respect to time.
- 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.
- Irrigation drainage
- 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.
- A material or constituent in solution, in suspension,
or in transport; usually expressed in terms of mass or volume.
- Main stem
- The principal course of a river or a stream.
- 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.
- The average of a set of observations, unless otherwise specified.
- The middle or central value in a distribution of data ranked in order of
magnitude. The median is known also as the 50th percentile.
- Metamorphic rocks
- Rocks that have formed in the solid state in response to pronounced
changes of temperature, pressure, and chemical environment.
- Element or compound essential for animal and plant
growth including nitrogen and phosphorus.
- 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 material
- Natural material derived from plants and animals.
- Organochlorine compound
- Synthetic organic compounds containing chlorine. As generally used,
refers to compounds containing mostly or exclusively carbon, hydrogen,
and chlorine. Examples include organochlorine insecticides and
- Soil or rock overlying a valuable mineral deposit.
- A chemical applied to crops, rights of way, lawns, or residences to
control weeds, insects, fungi, nematodes, rodents or other "pests."
- A class of organic compounds containing an aromatic ring and hydroxyl
groups. Phenols are used in the production of phenolic resins, germicides,
herbicides, fungicides, pharmaceuticals, dyes, plastics, and explosives.
- Phthalate esters
- A class of organic compounds containing an
aromatic ring, oxygen, and other organic groups. Pthtalates are used as
plasticizers, to manufacture products from polymers of vinyl chloride,
propylene, ethylene, and styrene.
- 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, fossil fuels,
and natural or anthropogenic introduction of uncombusted coal and oil.
PAHs include benzo(a)pyrene, fluoranthene, and pyrene.
- Principal aquifer
- Unconsolidated deposits with more than 100
feet of saturated thickness and dissolved solids less than 1,000 mg/L.
The primary sources of ground water for public supply and irrigation.
- 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.
- Particles, derived from rocks or biological materials,
that have been transported by a fluid or other natural process, and are
suspended or settled in water.
- Sedimentary rocks
- Rocks formed by the accumulation of sediment in water or from air.
- 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  procedures.
- Semipermeable membrane device
- Passive sampling devices for
organochlorine and semivolatile organic compounds. They are low-density
polyethylene tubes filled with fish lipid. The fish lipid sequesters dissolved
compounds from water.
- 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, phthalate esters, and polycyclic aromatic
- Shallow water-table aquifer
- Unconsolidated deposits containing water whose surface is at atmospheric
pressure. In the Study Unit, these aquifers are not typically used as public
- Populations of organisms that may interbreed and produce
fertile offspring having similar structure, habits, and functions.
- Stream reach
- A continuous part of a stream between two specified points.
- 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.
- Compression of soft aquifer materials in a confined
aquifer due to pumping of water from the aquifer. Subsidence has occurred
in Las Vegas Valley within the Study Unit.
- Sulfide mineral
- A mineral with reduced forms of sulfur usually combined with another element
such as iron (pyrite, FeS2). Sulfide minerals often produce sulfuric
acid when exposed to the atmosphere and water.
- Synoptic study
- A short-term investigation of specific water-chemistry
or ecological conditions during selected seasonal or hydrologic conditions
to provide improved resolution for critical water-quality conditions.
- Synthetic organic compounds
- Compounds that are manmade and contain carbon. Common synthetic organic
compounds are pesticides, volatile organic compounds such as solvents, gasoline
components, and trihalomethanes, phenols, polychlorinated biphenyls, polycyclic
aromatic hydrocarbons, and pthtalate esters.
- Rock that remains after processing ore to remove the valuable minerals.
- Tertiary-treated sewage
- The third phase of treating sewage that removes nitrogen and phosphorus
before it is discharged.
- Tolerant species
- Those species that are adaptable to (tolerant of) human effects on the
- Trace element
- An element found in only minor amounts (concentrations
less than 1.0 microgram per liter) in water or sediment; includes arsenic,
cadmium, chromium, copper, lead, mercury, nickel, and zinc.
- 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,
prometon, and simazine.
- A river or stream flowing into a larger river, stream, or lake.
- Trihalomethanes (THM)
- A group of volatile organic compounds containing one carbon atom, one
hydrogen atom, and three halide (fluorine, chlorine, bromine, and iodine)
atoms. Common THMs include chloroform, bromoform, and dichlorofluoromethane.
These compounds can be a result of chlorination processes in water-supply wells.
- Unconsolidated deposit
- Deposit of loosely bound sediment that typically fills topographically
low areas. These deposits are common in valleys throughout the Study Unit and
typically form principal aquifers when saturated.
- Un-ionized ammonia
- The neutral form of ammonia in water.
The un-ionized ammonia is present as NH4OH in an amount dependent on ammonia
concentration, temperature, and pH. As water temperature and pH are increased,
the proportion of un-ionized ammonia increases.
- Urban drainage
- Water derived from runoff or shallow ground-water
discharge from urban areas.
- 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.
- Water table
- The point below the land surface where ground water is first found and
below which the earth is saturated. Depth to the water table varies widely
across the country.