Water Quality in the Potomac River Basin, Maryland, Pennsylvania, Virginia, West Virginia, and the District of Columbia, 1992-96

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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.


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.


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


See Drainage basin.


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.

Carbonate rocks

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


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


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

Crystalline rocks

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

Degradation products

Compounds resulting from transformation of an organic substance through chemical, photochemical, and(or) biochemical reactions.


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.

Drainage basin

The portion of the surface of the Earth that contributes water to a stream through overland runoff, including tributaries and impoundments.


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


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

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.

Fish community

See Community.

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.


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

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.


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


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


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


An animal having no backbone or spinal column.


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.


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

Maximum contaminant level (MCL)

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


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


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


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


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


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.


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

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.


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


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

Siliciclastic rocks

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

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.


A heavy silvery-white metallic element, highly radioactive and easily oxidized. Of the 14 known isotopes of uranium, 238U is the most abundant in nature.


See Drainage basin.

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.

U.S. Geological Survey Circular 1166

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Suggested citation:
Ator, S.W., Blomquist, J.D., Brakebill, J.W., Denis, J.M., Ferrari, M.J., Miller, C.V., and Zappia, H., 1998, Water Quality in the Potomac River Basin, Maryland, Pennsylvania, Virginia, West Virginia, and the District of Columbia, 1992-96: U.S. Geological Survey Circular 1166, on line at <URL:>, updated June 10, 1998 .

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Last modified: Mon Jun 29 16:00:44 1998