GLOSSARY

Action level—A concentration that, when reached, triggers public water systems to take treatment steps if the action level is exceeded in more than 10 percent of tap-water samples.

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.

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

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

Biological indicator—A quantitative measure of biological conditions that may reflect habitat disturbance, chemical contamination, or naturally harsh conditions. The status of algae, invertebrates (insects, worms, and clams), or fish provides a record of water quality and stream conditions that water-chemistry indicators might not reveal. Algal status focuses on changes in the percentage of certain algae in response to increasing siltation and higher nutrient concentrations in many regions. Invertebrate status averages 11 metrics that summarize changes in richness (number of taxa), tolerance, trophic conditions, and dominance associated with water-quality degradation. Fish status sums the scores of four metrics (percentages of tolerant, omnivorous, and non-native individuals, and percentage of individuals with anomalies) that increase in response to water-quality degradation. Indicator scores increase as habitat disturbance, chemical contamination, or harsh conditions increase.

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.

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

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

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 unit—A layer of sediment or lithologic unit of low permeability that bounds an aquifer.

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

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.

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

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

Lifetime health advisory—An advisory guideline for drinking-water exposure over a 70-year lifetime, considering noncarcinogenic adverse health effects.

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.

Minimum reporting level—The smallest measured concentration of a constituent that may be reliably reported using a given analytical method.

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

Outcrop—That part of a geologic formation that is exposed at land surface.

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 gamma-HCH). Most organochlorine insecticides were banned because of their carcinogenicity, tendency to bioaccumulate, and toxicity to wildlife.

Permeability—A measure of the relative ease of fluid flow in porous rocks.

Picocurie (pCi)—One trillionth (1 x 10^-12) of the amount of a radioactive nuclide represented by a curie (Ci). A curie is the quantity of any radioactive nuclide that yields 3.7 x 10¹⁰ radioactive disintegrations per second (dps). A picocurie yields 0.037 dps, or 2.22 disintegrations per minute.

Polychlorinated biphenyls (PCB)—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.

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

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.

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

Subcrop—That part of a geologic formation that is buried; that is, not exposed at land surface.

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

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.

Volatile organic compound (VOC)—An organic chemical that has a high vapor pressure relative to its 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-quality guideline—Specific level of water quality which, if reached, might adversely affect human health or aquatic life. Water-quality guidelines are nonenforceable and are issued by a governmental agency or other institution.

Water-quality standard—State-adopted and U.S. Environmental Protection Agency-approved ambient standard 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.