GLOSSARY

Anthropogenic—A condition or occurrence that is the result of, or is influenced by, human activity.

Aquatic-life criteria—Water-quality guidelines for protection of aquatic life. Typically refers to U.S. Environmental Protection Agency water-quality criteria for protection of aquatic organisms.

Aquifer—A water-bearing layer of soil, sand, gravel, or rock that will yield usable quantities of water to a well. Background concentration- A concentration of a substance in a particular environment that is indicative of minimal influence by human (anthropogenic) sources.

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.

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

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, or dietary or other sources.

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

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

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

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

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.

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

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 guideline—Nonenforceable Federal guideline regarding cosmetic (tooth or skin discoloration) or aesthetic effects (such as taste, color, odor).

Drinking-water standard—A threshold concentration in a public drinking-water supply, designed to protect human health or 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.

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

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.

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.

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.

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

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.

Invertebrate—An animal having no backbone or spinal column.

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.

Land subsidence—Compression of soft aquifer materials in a confined aquifer due to pumping of water from the aquifer.

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.

LOWESS smooth—LOcally WEighted Scatterplot Smoothing is a statistical method of defining a smooth curve through the middle of a scatterplot to highlight trends or patterns in the data.

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.

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.

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 stream water and ground water. One thousand micrograms per liter equals 1 mg/L.

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

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

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.

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

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

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

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.

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

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

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), is acceptable 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.

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

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

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.