Science of Water : Concepts and Applications

Glossary 405
Confl uence The fl owing together of two or more streams; the place where a tributary joins the
main stream.
Conglomerate A coarse-grained sedimentary rock composed of fragments larger than 2 mm
in diameter.
Constituent A chemical or biological substance in water, sediment, or biota that can be measured
by an analytical method.
Consumptive use The quantity of water that is not available for immediate rescue because it has
evaporated, transpired, or been incorporated into products, plant tissue, or animal tissue.
Contact recreation Recreational activities, such as swimming and kayaking, in which contact
with water is prolonged or intimate, and in which there is a likelihood of ingesting
water.
Contaminant A toxic material found as an unwanted residue in or on a substance.
Contamination Degradation of water quality compared to original or natural conditions due to
human activity.
Contributing area The area in a drainage basin that contributes water to streamfl ow or recharge
to an aquifer.
Core sample A sample of rock, soil, or other material obtained by driving a hollow tube into the
undisturbed medium and withdrawing it with its contained sample.
Criterion A standard rule or test on which a judgment or decision can be based.
Cross connection Any connection between safe drinking water and a nonpotable water or
fl uid.
C×T value The product of the residual disinfectant concentration C, in milligrams per liter,
and the corresponding disinfectant contact time T, in minutes. Minimum C×T values are
specifi ed by the Surface Water Treatment Rule as a means of ensuring adequate killing
or inactivation of pathogenic microorganisms in water.
D
Datum plane A horizontal plane to which ground elevations or water surface elevations are
referenced.
Deepwater habitat Permanently fl ooded lands lying below the deepwater boundary of
wetlands.
Degradation products Compounds resulting from transformation of an organic substance
through chemical, photochemical, and/or biochemical reactions.
Denitrifi cation A process by which oxidized forms of nitrogen such as nitrate are reduced to
form nitrites, nitrogen oxides, ammonia, or fee nitrogen: commonly brought about by the
action of denitrifying bacteria and usually resulting in the escape of nitrogen to the air.
Detection limit The concentration of a constituent or analyte below which a particular analytical
method cannot determine, with a high degree of certainty, the concentration.
Diatoms Single-celled, colonial, or fi lamentous algae with siliceous cell walls constructed of
two overlapping parts.
Direct runoff The runoff entering stream channels promptly after rainfall or snowmelt.
Discharge The volume of fl uid passing a point per unit of time, commonly expressed in cubic
feet per second, million gallons per day, gallons per minute, or seconds per minute
per day.
Discharge area (groundwater) Area where subsurface water is discharged to the land surface,
to surface water, or to the atmosphere.
Disinfectants-disinfection by-products (D-DBPs) A term used in connection with state and
federal regulations designed to protect public health by limiting the concentration of
either disinfectants or the by-products formed by the reaction of disinfectants with other
substances in the water (such as trihalomethanes—THMs).