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GLOSSARY 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 par- ticulate 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 unconsoli- dated material (sand and gravel) left behind by a river or other flowing water. Ammonia-A compound of nitrogen and hydrogen (NH3) that is a common by-product of animal waste. Ammo- nia readily converts to nitrate in soils and streams. Aquatic-life guideline- Water-quality guidelines for pro- tection of aquatic life. Often refers to U.S. Environmen- tal Protection Agency water-quality criteria for protection of aquatic organisms. See also Water-quality guidelines. Basin- See Drainage basin. 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. Bioaccumulation- The biological sequestering of a sub- stance 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. 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 persis- tent or less persistent compound. Chlorofluorocarbons- A class of volatile compounds con- sisting 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. Community- In ecology, the species that interact in a com- mon 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). 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. Cubic foot per second (ft3ts, or cfs)- Rate of water dis- charge representing a volume of I cubic foot passing a given point during I second, equivalent to approxi- mately 7 A8 gallons per second or 448.8 gallons per minute or 0.02832 cubic meter per second. Detect- To determine the presence of a compound. DDT - Dichloro-diphenyl-trichloroethane. An organochlo- rine insecticide no longer registered for use in the United States. 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 constitu- ent that passes through a OAS-micrometer filter. Drainage basin- The portion of the surface of the Earth that contributes water to a stream through overland run- off, including tributaries and impoundments. Eutrophication- The process by which water becomes enriched with plant nutrients, most commonly phospho- rus and nitrogen. Health advisory- Nonregulatory levels of contaminants in drinking water that may be used as guidance in the absence of regulatory limits. Advisories consist of esti- mates of concentrations that would result in no known or anticipated health effects (for carcinogens, a speci- fied cancer risk) determined for a child or for an adult for various exposure periods. Instream standards- See Water-quality standards. Invertebrate- An animal having no backbone or spinal column. See also Benthic invertebrates. 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. Maximum contaminant level (MCL)- Maximum permis- sible level of a contaminant in water that is delivered to any user of a public water system. MCLs are enforce- able standards established by the U.S. Environmental Protection Agency. 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. Micrograms per liter (!..tgIL)- A unit expressing the con- centration of constituents in solution as weight (micro- grams) of solute per unit volume (liter) of water; equivalent to one part per billion in most stream water and ground water. One thousand micrograms per liter equals I mglL. Midge- A small fly in the family Chironomidae. The larval Uuvenile) life stages are aquatic. Glossary 23