daily flood peak The maximum mean daily discharge occuring in a stream during a given flood event.

daily fluid intake The daily intake of water for drinking and culinary use and is defined as two liters.

daily noise dose (D) A descriptor for noise exposure, in percent, expressed by the following relationship:

D = [C1/T1 + C2/T2 + .... + Cn/Tn] x 100

where:

Cn = total time of exposure at a specified noise level

Tn = total time of exposure permitted at that noise level

dam Any artificial barrier which impounds or diverts water. A dam is generally considered hydrologically significant if it is a) one and one quarter feet (0.4 meters) or more in height from the natural bed of the stream and has a storage of at least 15 acre-feet, or b) or has an impounding capacity of 50 acre-feet or more and is at least six feet (2 meters) above the natural bed of the stream.

dam failure Catastrophic event characterized by the sudden, rapid, and uncontrolled release of impounded water.

Darcy’s Law An equation for the computation of the quantity of water flowing through porous media. Darcy's Law assumes that the flow is laminar and that inertia can be neglected. The law states that the rate of viscous flow of homogenous fluids through isotropic porous media is proportional to, and in the direction of, the hydraulic gradient.

data Plural for datum.

data collection Implementation of appropriate procedures for obtaining necessary information to monitor status of water quantity, quality, use or flow.

data collection platform (DCP) An electronic device which connects to a river gage or rainfall gage which records data from the gage. At predetermined times it transmits that data via satellite to a remote computer.

data compilation Procedures used to develop necessary information products about water, including but not limited to, quality assurance, statistical analysis, mathematical manipulations, integration of data from several sources, and formatting for archiving.

data editing A set of procedures for detecting and correcting errors in the data.

data mart A data mart is a subset of data from the data warehouse designed to support the unique business unit requirements of a specific application.

data warehouse A platform with integrated data of improved quality to support many decision support systems and processes within an enterprise. Data warehousing improves the productivity of corporate decision makers through consolidation, conversion, transformation and integration of operational data and provides a consistent view of an enterprise. Data warehouse components include: operational data stores, data conversion/extraction, database management system, and business intelligence tools.

dateometer A small calendar disc attached to motors and equipment to indicate the year in which the last maintenance was performed.

datum 1) See gage datum. 2) For marine applications, a base elevation used as a reference from which to reckon heights or depths. It is called a tidal datum when defined in terms of a certain phase of the tide.

datum level The zero with reference to which the altitudes of land surfaces and the depths of the sea are determined. Also see sea level.

datum of tabulation A permanent base elevation at a tide station to which all tide gauge measurements are referred. The datum is unique to each station and is established at a lower elevation than the water is ever expected to reach. It is referenced to the primary bench mark at the station and is held constant regardless if changes to the tide gauge or tide staff.

day The period of rotation of the Earth. There are several kinds of days depending on whether the sun, moon, or other object or location is used as the reference for the rotation. See constituent day, lunar day, sidereal day, and solar day.

daylight saving time A time used during the summer months, in some localities, in which clocks are advanced 1 hour from the usual standard time.

day-second foot (dsf) A volume equivalent to a flow of one cubic foot per second (cfs) maintained for one day.

day tank A tank used to store a chemical solution of known concentration for feed to a chemical feeder. A day tank usually stores sufficient chemical solution to properly treat the water (being treated) for at least one day. Also called an age tank.

dead end The end of a water main which is not connected to other parts of the distribution system by means of a connecting loop of pipe.

dead storage The volume in a reservoir below the lowest controllable level.

dealloying The selective corrosion of a mettalic constituent from an alloy— usually in the form of ions.

death phase The stage or period of decline of growth and death of bacteria in a culture. Also called declining phase. This phase may occur after approximately 18 hours or up to several days, depending on the type of organism in the culture.

death-to-case ratio The number of deaths attributed to a particular disease during a specified time period divided by the number of new cases of that disease identified during the same time period.

debt obligation A legal obligation or liability to pay something to someone else.

decant To draw off the upper layer of liquid-suspended solids mixture after the heavier material has settled.

decantation

decay The process by which a substance is broken down by either biotic or abiotic processes or through radioactive emission.

decay heat The heat generated by the radioactive decay of fission products.

decay products Degraded radioactive materials, often referred to as daughters or progeny; from a public health standpoint, radon decay products of most concern are polonium-214 and polonium-218.

dechlorination The deliberate removal of chlorine from water. The partial or complete reduction of residual chlorine by any chemical or physical process.

decibar The practical unit for pressure in the ocean, equal to 10 centibars.

decibel (dB) A dimensionless unit used in physics, which is equal to 10 times the logarithm to the base 10 of the ratio of two values:

dB = 10*log(value1/value2)

[For the application of this general equation in acoustics, see sound intensity level and sound pressure level.]

Decibels, A-weighted (dBA) Unit representing the sound level measured with the A-weighting network on a sound level meter. [Refer to Table 4-1 for the characteristics of the weighting networks.]

decibels, C-weighted (dBC) Unit representing the sound level measured with the C-weighting network on a sound level meter. [Refer to Table 4-1 for the characteristics of the weighting networks.]

declaration of emergency Whenever, in the opinion of the Governor, the safety and welfare of the people of the State require the exercise of extreme emergency measures due to a threatened or actual disaster, he may declare a State emergency or State of Emergency to exist.

declination Angular distance north or south of the celestial equator, taken as positive when north of the equator and negative when south.

declinational inequality Same as diurnal inequality.

declinational reduction A processing of observed high and low waters or flood and ebb tidal currents to obtain quantities depending upon changes in the declination of the Moon; such as tropic ranges or speeds, height or speed inequalities, and tropic intervals.

deciduous Falling off or actively shed at maturity or at certain seasons.

decompose To break down a substance into simpler, basic parts such as in bioremediation.

decomposers Those organisms, usually bacteria or fungi, that participate in the breakdown of large molecules associated with organisms. Hence, those organisms that recycle dead organisms.

decomposition The conversion of chemically unstable materials to more stable forms by chemical or biological action. If organic matters decays when there is no oxygen present (anaerobic conditions), undesirable tastes and odors are produced. Decay of organic matter when oxygen is present (aerobic conditions) tends to produce much less objectionable odors.

decontamination Removal of harmful substances such as noxious chemicals, harmful bacteria or other organisms, or radioactive material from exposed individuals, rooms and furnishings in buildings, or the exterior environment.

declining-rate block See decreasing-bock rate.

decreasing-block rate A pricing structure for which the dollar amount charged per unit of water (such as dollars per gallon) decreases with the amount of water usage. Also declining-block rate.

deepening Used in describing the history of a low-pressure system or an area of cyclonic circulation, it means a decrease in the central pressure of the system. Although it usually describes the action of a pressure system on a constant pressure chart, it also means a surface low is increasing in cyclonic circulation and acquiring more energy.

deep percolation Water that moves downward through the soil profile below the root zone and cannot be used by plants.

deep percolation loss Water which percolates downward through the soil beyond the reach of plant roots.

deep seepage Infiltration which reaches the water table.

deep-well injection Deposition of raw or treated, filtered hazardous waste by pumping it into deep wells where it is contained in the pores of permeable subsurface rock.

default 1) Failure to meet a financial obligation such as a loan payment. 2)

defendant The party or parties named in an information, indictment, or compliant for injunction, and against whom the government is proceeding. The article against which a seizure action is brought is also referred to as the defendant.

defense-in-depth The nuclear power plant design basis used to ensure maximum protection of the environment from an inadvertent release of fission products.

deflocculating agent A material added to a suspension to prevent settling.

defluoridation The removal of excess fluoride in drinking water to prevent the mottling of teeth.

defoliant An herbicide that removes leaves from trees and growing plants.

degasification A water treatment process which removes dissolved gases from the water. The gases may be removed by either mechanical or chemical treatment methods or a combination of both.

degassing To free from gases.

degradate The product of the chemical or biological breakdown of a complex compound into simpler compounds. See pesticide degradate.

degradation The transformation over time of a compound into one or more similar chemicals through either a chemical or biological breakdown.

degradation potential The degree to which a substance is likely to be reduced to a simpler form by bacterial activity.

degreasers Solutions that remove grease.

degree days, cooling A value used to estimate the energy requirements for air conditioning of homes and buildings. One cooling degree day is given for each degree the daily mean temperature is above 75 degrees Fahrenheit.

degree days, growing (GDD) Growing degree days measures the day to day accumulation of the difference between the average daily temperature and a threshold temperature for a specific crop. GDD's give an indication of the amount of heat available for crop growth.

degree days, heating A value used to estimate the energy requirements for heating homes and buildings. One heating degree day is given for each degree the daily mean temperature is below 65 degrees Fahrenheit.

degree of hazard The level of health hazard, as derived from an evaluation of the potential risk to health and the adverse effect upon the waterworks.

delegated state A state (or other governmental entity such as a tribal government) that has received authority to administer an environmental regulatory program in lieu of a federal counterpart. As used with NPDES, UIC, and PWS programs, the term does not connote any transfer of federal authority to the state.

delist Use of the petition process to have a facility's toxic designation rescinded.

delivery The amount of water delivered to a point of use.

delivery/release The amount of water delivered to the point of use and the amount released after use; the difference between these amounts is usually the same as the consumptive use. See also consumptive use.

delta The alluvial deposit at the mouth of a river formed where a stream drops its debris load on entering a body of quieter water.

demand forecast A projection of future demand that can be made on a system-wide or customer-class basis.

demand management Measures, practices, and incentives employed by waterworks to reduce permanently the level or change the demand pattern for water service.

demersal Applied to eggs which are heavy and sink to the bottom of a stream or other body of water.

demineralization A treatment process which removes dissolved minerals (salts) from water.

de minimus Not enough to be considered.

de minimus non curat lexis The law will not concern itself with trifles.

demographic Having to do with population or socioeconomic conditions.

demographic information The personal characteristics—age, sex, race, and occupation—of descriptive epidemiology used to characterize the populations at risk.

denaturation Unfolding of a protein molecule into a generally bio-inactive form. Also the disruption of dna duplex into two separate strands.

dendritic The form of the drainage pattern of a stream and its tributaries when it follows a treelike shape, with the main trunk, branches, and twigs corresponding to the main stream, tributaries, and subtributaries, respectively, of the stream.

denominator 1) The lower portion of a fraction used to calculate a rate or ratio. 2) In a rate, the denominator is usually the population (or population experience, as in person-years, etc.) at risk.

denitrification 1) Chemical conversion of nitrates to molecular nitrogen or to nitrous oxide or to ammonia by bacteria or by lightning. 2) The biochemical conversion of nitrate and nitrite nitrogen in the soil or contained in water to gaseous nitrogen.

density A measure of how heavy a substance (solid, liquid, or gas) is for its size. Density is expressed in terms of weight per unit volume, that is, grams per cubic centimeter or pounds per cubic feet. The density of water is 1.0 grams per cubic centimeter or about 62.4 pounds per cubic foot.

density current A flow of water maintained by gravity through a large body of water, such as a reservoir or lake, and retaining its unmixed identity because of a difference in density.

density flow

density of snow The ratio, expressed as a percentage, of the volume which a given quantity of snow would occupy if it were reduced to water, to the volume of the snow. When a snow sampler is used, it is the ratio expressed as percentage of the scale reading on the sampler to the length of the snow core or sample.

deoxyribonucleic acid (dna) The basic biochemical component of the chromosomes and the support of heredity. Dna contains the sugar deoxyribose and is the nucleic acid in which genetic information is stored (apart from some viruses).

Department of Professional and Occupational Regulation(DPOR)The state regulatory agency for the licensure of waterworks and wastewater works operators.

dependent variable In a statistical analysis, the outcome variable(s) or the variable(s) whose values are a function of other variable(s) (called independent variable(s) in the relationship under study).

depletion curve In hydraulics, a graphical representation of water depletion from storage-stream channels, surface soil, and groundwater. A depletion curve can be drawn for base flow, direct runoff, or total flow.

depolarization The elimination or reduction of polarization by physical or chemical means; depolarization results in increased corrosion.

deposit attack Pitting corrosion resulting from deposits on a metal surface that causes concentration cells.

deposition 1) The sworn testimony of a witness obtained outside the courtroom before trial through examination and cross-examination by the attorneys for the parties. 2) Physical settling or placing of radioactive material onto a surface. Fallout may be deposited on surfaces. Material ingested or inhaled by an individual may be deposited in the lungs or other organs.

depression In meteorology, a name for an area of low pressure, a low, or trough. It also applies to a stage of tropical cyclone development and is known as a tropical depression to distinguish it from other synoptic features.

depression storage The volume of water contained in natural depressions in the land surface, such as puddles.

depressurization A condition that occurs when the air pressure inside a structure is lower that the air pressure outside. Depressurization can occur when household appliances such as fireplaces or furnaces, that consume or exhaust house air, are not supplied with enough makeup air. Under depressurized conditions, for example, radon may be drawn into a house more rapidly

depth of floatation

depth of runoff The total runoff from a drainage basin divided by its area. For convenience in comparing runoff with precipitation, the term is usually expressed in inches of depth during a given period of time over the drainage area or acre-feet per square mile.

derate To use a fraction of a hearing protector's noise reduction rating (NRR) to calculate the noise exposure of a worker wearing that hearing protector. [See noise reduction rating.]

dermal exposure Contact between a chemical and the skin.

dermal toxicity The ability of a pesticide or toxic chemical to poison people or animals by contact with the skin. See contact pesticide.

desalinization The removal of dissolved salts from water by natural means (leaching) or by a specific water treatment process, such as distillation, electrodialysis, and reverse osmosis.

descriptive epidemiology The aspect of epidemiology concerned with organizing and summarizing health-related data according to time, place, and person.

desiccant A drying agent which is capable of removing or absorbing moisture from the atmosphere in a small enclosure.

desiccation A process used to thoroughly dry air; removal of virtually all moisture from air.

desiccator A closed container into which heated weighing or drying dishes are placed to cool in a dry environment. The dishes may be empty or they may contain a sample. Desiccators contain a substance, such as anhydrous calcium chloride, which absorbs moisture and keeps the relative humidity near zero so that the dish or sample will not gain weight from absorbed moisture.

designated pollutant An air pollutant that is neither a criteria nor a hazardous pollutant, but for which new source performance standards exist and the Clean Air Act requires states to control these pollutants.

designated uses Those water uses identified in state water quality standards that must be achieved and maintained as required under the Clean Water Act. Uses can include cold water fisheries, public water supply, irrigation, etc.

design capacity The average daily flow and contaminant level (loadings) that a treatment works or other processing facility is designed to accommodate.

design criteria 1) 2) The hypothetical flood used in the sizing of the dam and the associated structures to prevent dam failure by overtopping, especially for the spillway and outlet works.

designer bugs Popular term for microbes, developed through biotechnology, that can degrade specific toxic chemicals at spills, in toxic waste dumps, or in ground water.

design parameters Various criteria used to determine size, shape, quantity, and/or methods in the design of unit operations in a treatment process.

design value The monitored reading used to determine an area's air quality measured over a time period specific for each chemical of concern.

desktop videoconferencing Videoconferencing on a personal computer equipped with a fast Internet connection (at least 28.8 Kbps modem), a microphone, and a video camera . There can be two-way or multi-way video and audio depending upon the hardware and software of participants. Most appropriate for small groups or individuals. Not yet presently available in many parts of the country due to bandwidth and equipment limitations for this application.

destination facility The facility to which regulated medical waste is shipped for treatment and destruction, incineration, and/or disposal.

destratification The development of vertical mixing within a lake or reservoir to eliminate separate layers of temperature, plant or animal life. This vertical mixing can be caused by mechanical means (pumps) or through the use of forced air diffusers which release air into the lower layers of the reservoir.

destroyed medical waste Regulated medical waste that has been ruined, torn apart, or mutilated through thermal treatment, melting, shredding, grinding, tearing, or breaking, so that it is no longer generally recognized as medical waste, but has not yet been treated.

destruction and removal efficiency (DRE) A percentage that represents the number of molecules of a compound removed or destroyed (in an incinerator) relative to the number of molecules that entered the system (e.g., a DRE of 99.99 percent means that 9,999 molecules are destroyed for every 10,000 that enter.

destruction facility A facility that destroys regulated medical waste by mashing or mutilating it.

desulfurization Removal of sulfur from fossil fuels to reduce pollution.

data steward [GIS] The person responsible for the source content of the coverage.

detectable leak rate The smallest leak from a storage tank, expressed in terms of gallons-or liters-per-hour, that a test can reliably discern with a certain probability of detection or false alarm.

detection criterion A predetermined rule to ascertain whether a tank is leaking or not. Most volumetric tests use a threshold value as the detection criterion. See volumetric tank tests.

detection limit Concentration at which the presence of a contaminant can be detected.

detention basins Structures which are built upstream from a populated area so that precipitation flows do not flood and cause the loss of life or property. They are normally dry, but are designed to detain surface water temporarily during and immediately after a runoff event. Their primary function is to attenuate the storm flows by releasing flows at a lower flow rate. There are no gates or valves allowed on the outlet so that water can never be stored on a long-term basis. Typical detention times in such a basin would be on the order of 24 to 72 hours although some are as long as 5 to 10 days.

detention lag The time period between the moment a change is made and the moment when such a change is finally sensed by the associated measuring instrument.

detention reservoir A basin (above a dam) constructed for the temporary storage of streamflow and superficial surface runoff. Also called retarding reservoir.

detention storage The volume of water, other than depression storage, existing on the land surface as flowing water which has not yet reached the channel.

detention time 1) The theoretical (calculated) time required for a fixed amount of water to pass through a tank at a given rate of flow. 2) In storage reservoirs, the length of time water will be held before being used. 3) The actual time in hours, minutes, or seconds that a fixed amount of water will reside in a settling basin, flocculating basin or rapid-mix chamber. Residence time. In storage reservoirs, detention time is the length of time entering water will be held before being drafted for use (several weeks to years, several months being typical).

Detention Time (hr) = Basin Volume (gal)(24 hr/day)/Flow (gal/day)

detergent A synthetic washing agent that helps to remove dirt and oil.

determinant Any factor, whether event, characteristic, or other definable entity, that brings about change in a health condition, or in other defined characteristics.

detritus A non-dissolved product of disintegration or wearing away. The heavy, coarse material carried by wastewater.

development effects Adverse effects such as altered growth, structural abnormality, functional deficiency, or death observed in a developing organism.

deviation 1) The deflection of the needle of a magnetic compass due to masses of magnetic metal within a ship on which the compass is located. This deflection varies with different headings of the ship. The deviation is called easterly and marked plus if the deflection is to the right of magnetic north, and is called westerly and marked minus if it is to the left of magnetic north. A deviation table is a tabular arrangement showing the amount of deviation for different headings of the ship. Each compass requires a separate deviation table.

devolution Process whereby functions performed by the Federal government are turned over to States, local governments, or the private sector.

dewater 1) To remove or separate a portion of the water present in sludge or slurry so it can be handled and disposed of more efficiently. 2) To remove or drain water from a tank or a trench. 3) The draining, pumping, or removal of water that is affecting construction or mining site, or to lower the water table for agriculture.

dewaterable When water readily drains from a material. Generally, dewatering raw sludge is more difficult than water removal from a digested sludge.

dew point The temperature at which air with a given quantity of water vapor must be cooled to cause condensation of the vapor in the air.

dezincification The parting of zinc from an alloy (in some brasses, zinc is lost, leaving a weak, brittle, porous, copper-rich residue behind).

diatom Any class of minute, planktonic or attached unicellular or colonial algae with cases of silica.

diatomaceous earth A fine, siliceous (made of silica) "earth" composed mainly of the skeletal remains of diatoms, a type of free-floating, microscopic plant found in the ocean.

diatomaceous earth filtration A filtration method resulting in substantial particulate removal that uses a process in which: 1) a "precoat" cake of diatomaceous earth filter media is deposited on a support membrane (septum) and 2) while the water is filtered by passing through the cake on the septum, additional filter media, known as "body feed," is continuously added to the feed water to maintain the permeability of the filter cake.

Diazinon An insecticide. Banned in 1986 on open areas such as sod farms and golf courses because it posed a danger to migratory birds. The ban did not apply to agricultural, home lawn or commercial establishment uses.

Dicofol A pesticide used on citrus fruits.

diel Referring to the 24-hour daily period; to avoid the ambiguity of diurnal.

diethylstilbestrol (DES) A synthetic estrogen is used as a growth stimulant in food animals. Thought to be carcinogenic.

diffused air A technique by which air, under pressure, is distributed into wastewater in an aeration tank. The air is passed at a uniform rate (weight per time) through a perforated diffuser at an established rate (cfm per foot).

diffused air aeration A diffused air activated sludge plant takes air, compresses it, and then discharges the air below the water surface of the aerator through some type of air diffusion device.

diffused surfaced water

diffuser 1) A device (porous plate, tube, bag) used to disperse the air stream from a blower system into fine bubbles in a liquid. 2) A device used to spread a confined flow or stream of fluid or light across a larger area.

diffusion 1) The process by which both ionic and molecular species dissolved in water move from areas of higher concentration to areas of lower concentration. 2) The scattering of light.

digester In wastewater treatment, a biochemical oxidation or conversion unit operation in which bacterial action is induced and accelerated in order to break down organic matter forming carbon dioxide and water in the presence of oxygen (aerobic) and forming methane, carbon dioxide, and water in the absence of oxygen (anaerobic).

digestion The process for decomposing organic matter in digesters resulting in the partial gasification, liquefaction, and mineralization of the total solids content.

digital communication A communications format used with both electronic and light-based space systems that transmits audio, video, and data as bits ("1s" and "0s") of information. Codecs are used to convert traditional analog signals to digital format and back again. Digital technology also allows communications signals to be compressed for more efficient transmission. See Analog Communication.

digital readout Use of numbers to indicate the value or measurement of a variable. The readout of an instrument by a direct, numerical reading of the measured value.

digital tide gauge See automatic tide gauge.

dike A low wall that can act as a barrier to prevent a spill from spreading.

diluent A diluting agent.

dilute solution A solution that has been made weaker usually by the addition of water.

dilution To reduce the concentration of a component in a mixture by increasing the amount of one or more other components.

dilution ratio The relationship between the volume of water in a stream and the volume of incoming water. It affects the ability of the stream to assimilate waste.

dilution, serial Successive dilution of a specimen, e.g., 1:10 dilution equals 1 mL specimen plus 9 mL diluent (such as sterile water); 1:100 dilution equals 1 mL of a 1:10 dilution plus 9 mL of diluent.

dimictic Lakes and reservoirs which freeze over and normally go through two stratification and two mixing cycles within a year.

Dinocap A fungicide used primarily by apple growers to control summer diseases.

dioxin Any of a family of compounds known chemically as dibenzo-p-dioxins. Concern about them arises from their potential toxicity.

direct current (DC). Electrical current flowing in one direction only and essentially free from pulsation.

direct discharger A municipal or industrial facility that introduces pollution through a defined conveyance or system such as outlet pipes; a point source.

direct filtration A filtration method of treating water which consists of the addition of coagulant chemicals, flash mixing, coagulation, minimal flocculation, and filtration. The flocculation facilities may be omitted, but the physical-chemical reactions will occur to some extent. The sedimentation process is omitted. Also see conventional filtration and in-line filtration.

direct flood damage The damage done to property, structures, goods, etc., by a flood as measured by the cost of replacement and repairs.

direction of current Same as set.

direction of wind Direction from which the wind is blowing.

direct runoff Water that flows over the ground surface or through the ground directly into streams, rivers, or lakes.

direct transmission The immediate transfer of an agent from a reservoir to a susceptible host by direct contact or droplet spread.

disaccharide A sugar composed of two monosaccharides.

disadvantaged community The service area of a public water system that meets affordability criteria established after public review and comment by the State.

disbursement The transfer of cash from the State Revolving Fund to the assistance recipient.

disbursement schedule A quarterly schedule of estimated disbursements from the SRF.

discharge 1) The rate of flow or volume of water flowing past a specific point within a given period of time. 2) The release of wastewater through a pipe outlet or similar apparatus to a downgradient discharge point such as additional treatment, storage lagoon, or to receiving waters.

discharge curve A curve that expresses the relation between the discharge of a stream or open conduit at a given location and the stage or elevation of the liquid surface at or near that location. Also called rating curve and discharge rating curve.

discharge head The pressure (in pounds per square inch or psi) measured at the centerline of a pump discharge and very close to the discharge flange, converted into feet.

discharge table 1) A table showing the relation between two mutually dependant quantities or variable over a given range of magnitude. 2)A table showing the relation between the gage height and the discharge of a stream or conduit at a given gaging station. Also called a rating table.

disclimax A climax that is the consequence of repeated or continuous disturbance by man, domesticated animals, or natural events.

discount rate The interest rate used in plan formation and evaluation for discounting future benefits and costs to a common time basis; it can be based on the utility’s cost of capital.

discovery The process of obtaining information concerning the other party's case.

disease A deviation from normal health status associated with a characteristic sequence of signs and symptoms and caused by a specific etiologic agent.

dish An earthbound dish-shaped antenna used for receiving satellite signals. Also referred to as downlink dish and ground receiving dish.

disinfectant An agent that kills or inactivates the vegetative cells of microorganisms (thus reducing the possibility of contact with pathogenic microorganisms).

disinfectant contact time ("T" in CT calculations) means the time in minutes that it takes for water to move from the point of disinfectant application to the point where residual disinfectant concentration ("C") is measured.

disinfection A process by which microorganisms are inactivated, removed, or destroyed, by biocidal means such as chemical oxidants or equivalent agents, so that contact with the disinfected medium by the public will not result in transmittal of disease from viable pathogenic microorgansims.

disinfection by-product (dbp) A compound formed by the reaction of a disinfectant, such as chlorine, with organic material in the water supply.

dismissal An order of judgment disposing of an action, suit, motion, etc., by sending it out of court. A dismissal would always be in the defendant's favor.

dispersant A chemical agent used to break up concentrations of organic material such as spilled oil.

dispersion The spreading and mixing of chemical constituents in surface water and groundwater caused by diffusion and mixing due to microscopic and macroscopic variations in velocities.

disposal Final placement or destruction of toxic, radioactive, or other wastes; surplus or banned pesticides or other chemicals; polluted soils; and drums containing hazardous materials from removal actions or accidental releases. Disposal may be accomplished through use of approved secure landfills, surface impoundments, land farming, deep-well injection, ocean dumping, or incineration.

disseminule General term for seeds, spores, resting eggs, pelagic eggs, or larvae, etc.

dissolution Dissolving of a substance in a liquid solvent (e.g., water).

dissolved oxygen (do) The amount of dissolved oxygen, in mg/L, present in water.

dissolved solids A dispersion by very small, nonsettling molecules not removed by filtration practices as defined by the method of measurement.

distance communication Use of telecommunication technology for the implementation of administrative activities such as meetings, focus group, or job interviews when the parties are located at two or more locations.

distance education The term distance education is often used synonymously with distance learning. However, distance education typically refers to distributed learning resources in academic settings. See Distance Learning.

distance learning A system and a process that connects learners with distributed learning resources. While distance learning takes a wide variety of forms, all distance learning is characterized by the following: 1) separation of place and/or time between instructor and learner, among learners, and/or between learners and learning resources, and 2) interaction between the learner and the instructor, among learners, and/or between learners and learning resources conducted through one or more media; use of electronic media is not necessarily required.

distillate In the distillation of a sample, the part that remains after a portion of the sample is evaporated.

distillation 1) A process that boils the water, catches the resulting steam, and condenses the steam on a cold surface (a condenser). Minerals remain behind in the boiling tank. 2) In waste treatment, a process of heating the effluent and then removing the vapor or steam. When the steam is returned to a liquid, it is almost pure water as most of the solid content remains in the concentrated residue.

distribution 1) In epidemiology, the frequency and pattern of health-related characteristics and events in a population. In statistics, the observed or theoretical frequency of values of a variable. 2) The geographic range of a species.

distribution hydrograph A unit hydrograph of direct runoff modified to show the proportions of the volume of runoff which occur during successive equal units of time.

distribution lines Those facilities used to carry water from the transmission lines to the service lines, including water mains, distribution reservoirs, elevated storage tanks, booster stations, and valves.

distribution main A water main whose primary purpose is to provide treated water to service connections.

distribution facilities pipes, treatment, storage, and other facilities used to distribute drinking water to end users.

distributor A mechanical device used to provide a uniform flow over an area or a uniform application to the surface of a contact media. A rotary distributor can be used to dose a media surface at a controlled rate.

disturbance This has several applications. It can apply to a low or cyclone that is small in size and influence. It can also apply to an area that is exhibiting signs of cyclonic development. It may also apply to a stage of tropical cyclone development and is known as a tropical disturbance to distinguish it from other synoptic features.

ditch, oxidation A modification of the suspended growth wastewater treatment process in which the biomass mixture under treatment is circulated in an continuos, circular or oval channel into which influent wastewater is introduced at one or more points with aeration and circulation produced by mechanical devices.

diurnal 1) Pertaining to phenomena of daily occurrence; of or related to that portion of the day in which light occurs. 2) Having a period or cycle of approximately one tidal day. Thus, the tide is said to be diurnal when only one high water and one low water occur during a tidal day, and the tidal current is said to be diurnal when there is a single flood and a single ebb period of a reversing current in the tidal day. A rotary current is diurnal if it changes its direction through all points of the compass once each tidal day. A diurnal constituent is one which has a single period in the constituent day.

diurnal flow Flow that shows marked and regular variations through the course of a day.

diurnal inequality The difference in height of the two high waters or of the two low waters of each tidal day; also, the difference in speed between the two flood tidal currents or the two ebb currents of each tidal day.

diurnal range Same as great diurnal range.

diurnal tide level A tidal datum midway between mean higher high water and mean lower low water.

diversion 1) The taking of water from a stream or other body of water into a canal, pipe, or other conduit. 2) A channel with a supporting ridge on the lower side constructed across a slope to divert water at a non-erosive velocity to sites where it can be used or disposed.

diversion rate The percentage of waste materials diverted from traditional disposal, such as landfilling or incineration, to recycling, composting, or re-use.

diversity See biological diversity.

divide The high ground that forms the boundary of a watershed. A divide is also called a ridge and a drainage divide.

division A primary category of the plant kingdom.

dna See deoxyribonucleic acid.

dna cloning Production of many identical copies of a defined dna fragment. See deoxyribonucleic acid.

dna hybridization Use of a segment of DNA, called a DNA probe, to identify its complementary DNA; used to detect specific genes. See deoxyribonucleic acid.

dna library Set of cloned dna fragments which together represent the entire genome or the transcription of a particular tissue. See deoxyribonucleic acid.

dna polymerase An enzyme which catalyses the synthesis of double-stranded dna from single-stranded dna. See deoxyribonucleic acid.

dna synthesis The formation of dna by the sequential addition of nucleotide bases. See deoxyribonucleic acid.

dnase An enzyme which produces single-stranded nicks in dna. Dnase is used in nick translation. See deoxyribonucleic acid.

DO Dissolved oxygen.

docket A chronological listing of all pleadings and orders filed in each case. It is usually maintained by the clerk of court in the form of a docket sheet.

domains of interest The seven subgroups of the entire population of drinking water wells about which the National Pesticide Survey was particularly interested in obtaining data. The seven domains of interest are: 1) community water systems nationally; 2) community water systems in counties with the highest average groundwater vulnerability; 3) rural domestic wells nationally; 4) rural domestic wells in counties with the highest average pesticide use; 5) rural domestic wells in counties with the highest average groundwater vulnerability; 6) rural domestic wells in counties with the highest average pesticide use and groundwater vulnerability; and 7) rural domestic wells in "cropped and vulnerable” areas of counties.

domestic other nondistribution system plumbing problem A coliform contamination problem in a waterworks with more than one service connection that is limited to the specific service connection from which the coliform positive sample was taken.

domestic use Normal family or household use, including drinking, laundering, bathing, cooking, heating, cleaning, and flushing toilets.

domestic wastewater Wastewater derived principally from dwellings, business buildings, institutions, and other nonindustrial sources.

dominance The degree of influence (usually inferred from the amount of area covered) that a species exerts over a community.

dominant An organism that controls the habitat at any stage of development; in practice, the organism that is most conspicuous and covers the most area.

doppler radar Weather radar that measures direction and speed of a moving object, such as drops of precipitation, by determining whether atmospheric motion is horizontally toward or away from the radar.

dosage/dose 1) The actual quantity of a chemical or energy form administered to an organism or that to which it is exposed. 2) The amount of a chemical added to a process.

dose 1) 2) A general term denoting the quantity of radiation or energy absorbed. Dose may refer to absorbed dose, the amount of energy deposited per unit mass, or to equivalent dose (the absorbed dose adjusted for the relative biological effect of the type of radiation being measured).

dose equivalent The product of the absorbed dose from ionizing radiation and such factors as accounting for biological differences (due to the type of radiation and its distribution in the body) as specified by the International Commission on Radiological Units and Measurements.

dose of radioactivity

dose rate The radiation dose delivered per unit time.

dose response Changes to chemical or biochemical activity following a dose or how a biological organism’s response to a toxic substance quantitatively shifts as its overall exposure to the substance changes.

dose-response assessment Estimating the potency of a chemical.

dose-response relationship The quantitative relationship between the amount of exposure to a substance and the extent of toxic injury or disease produced.

dosimeter A portable device that measures total radiation dose received.

dot plot A visual display of the actual data points of a noncontinuous variable.

DOT reportable quantity The quantity of a substance specified in U.S. Department of Transportation regulation that triggers labeling, packaging and other requirements related to shipping such substances.

double ebb An ebb tidal current having two maxima of speed separated by a smaller ebb speed.

double flood A flood tidal current having two maxima of speed separated by a smaller flood speed.

double gate-double check valve assembly An approved assembly composed of two single independently acting check valves including tightly closing shutoff valves located at each end of the assembly and petcocks and test gauges for testing the water tightness of each check valve.

double tide A double-headed tide, that is, a high water consisting of two maxima of nearly the same height separated by a relatively small depression, or a low water consisting of two minima separated by a relatively small elevation. Sometimes called an agger. See gulder.

downgradient The direction that a fluid moves under the influence of gravity such as groundwater flows; similar to downstream for surface water.

downloading A procedure for transferring or retrieving a file from a distant computer. Opposite of uploading. Many Web sites have links to files such that you can simply click on the link and your browser will handle the downloading of the file(s) to your computer.

downstream slope The slope or face of the dam away from the reservoir water. This slope requires some kind of protection (e.g.; grass) from the erosive effects of rain and surface flow.

DPD

draft 1) The act of drawing or removing water from a tank or reservoir. 2) The water which is drawn or removed.

draft permit A preliminary permit drafted and published by EPA, subject to public review and comment, before final action on the application.

drainage area An area having a common outlet for its surface runoff. Also known as watershed, catchment area, and drainage basin.

drainage basin A part of the surface of the earth that is occupied by a drainage system, which consists of a surface stream or a body of impounded surface water together with all tributary surface streams and bodies of impounded surface water.

drainage coefficient The discharge of an underdrain system expressed in depth of water to be removed from the drainage area in a period of time.

drainage divide The boundary line, along a topographic ridge or along a subsurface formation, separating two ajacent drainage basins.

drainage well A well drilled to carry excess water off agricultural fields. Because they act as a funnel from the surface to the groundwater below, drainage wells can contribute to groundwater contamination.

drains (relief wells) A vertical well or borehole, usually downstream of impervious cores, grout curtains or cutoffs, designed to collect and direct seepage through or under a dam to reduce uplift pressure under or within a dam. A line of such wells forms a drainage curtain.

DRASTIC A classification system that attempts to provide a relative ranking of the vulnerability of groundwater to contamination. The acronym stands for features of the area around the well that may affect the movement of pesticides into groundwater (depth to water, recharge, aquifer media, soil media, topography, impact of the unsaturated aqueous zone between the soil media and the water table, and conductivity of the aquifer). drastic does not consider sources of the contamination or population affected. A drastic score was computed for all of the counties in the US as part of the National Pesticide Survey.

draw A shallow gully or drainage swale.

drawdown The amount that a pumping well lowers the water table or potentiometric surface as water is removed. The amount of drawdown is a function of the discharge rate and the physical properties of the aquifer, such as hydraulic conductivity, storativity, and its boundaries.

drawdown curve

dredging Removal of sediments, such as mud, from the bottom of water bodies. This can disturb the ecosystem.

drift (of current) The speed of the current.

drift current Same as wind drift.

drinking water equivalent level Protective level of exposure related to potentially non-carcinogenic effects of chemicals that are also known to cause cancer.

drinking water well A rural domestic well or a community water system well whose water is used primarily for human consumption (i.e., drinking, cooking, and washing).

drip Procedure that regulates an altering substance into a stream of water; for example, chlorination for drinking water, or the addition of fertilizer, pesticides, and herbicides into irrigation water.

drizzle Very small, numerous, and uniformly dispersed water drops that may appear to float while following air currents. Unlike fog droplets, drizzle falls to the ground.

droplet nuclei The residue of dried droplets that may remain suspended in the air for long periods, may be blown over great distances, and are easily inhaled into the lungs and exhaled.

droplet spread The direct transmission of an infectious agent from a reservoir to a susceptible host by spray with relatively large, short-ranged aerosols produced by sneezing, coughing, or talking.

drop-off The recyclable materials collection method in which individuals bring them to a designated collection site.

drought A period of deficient precipitation or runoff extending over an indefinite number of days. There is no universally accepted quantitative definition of drought.

drought index Computed value which is related to some of the cumulative effects of a prolonged and abnormal moisture deficiency. An index of hydrological drought corresponding to levels below the mean in streams, lakes, and reservoir.

drumlin An elongate or oval hill of glacial drift, molded by moving ice above and at its sides.

dry floodproofing A dry floodproofed building is one which is sealed against flood waters. All areas below the flood protection level are made watertight. Walls are coated with waterproofing compounds or plastic sheeting. Openings like doors windows, sewer lines and vents are closed, whether permanently, with removable shields, or with sandbags. The flood protection level should be no more than 2 or 3 feet (0.5 to 1.0 meter) above the top of the foundation because the buildings walls and floors cannot withstand the pressure of deeper water.

dry suspended solids The weight of the filtrable suspended matter in wastewater or other liquid after drying for one hour at 103°.

dump A site, without environmental controls, used to dispose of solid waste.

dune A hill of drifting sand usually formed on existing or former shores or coasts, but often carried far inland by prevailing winds.

duplex video Two-way video communication capable of simultaneous origination and reception.

duration The period or time increment to which an observed or computed value applies.

duration of flood and duration of ebb Duration of flood is the interval of time in which a tidal current is flooding, and duration of ebb is the interval in which it is ebbing, these intervals being reckoned from the middle of the intervening slack waters or minimum currents.

duration of rise and duration of fall Duration of rise is the interval from low water to high water, and duration of fall is the interval from high water to low water.

Durham tube A small (2 cm to 4 cm) clear, glass tube used to collect CO₂ gas in culture (used in the MPN fermentation procedures).

dustfall jar An open container used to collect large particles from the air for measurement and analysis.

dynamic decimeter See geopotential as preferred term.

dynamic depth (height) See geopotential difference as preferred term.

dynamic depth (height) anomaly See geopotential anomaly as preferred term.

dynamic equilibrium A state of relative balance between forces or processes having opposite effects.

dynamic meter The former practical unit for geopotential difference (dynamic depth), equal to 10 geopotentials (dynamic decimeters). See geopotential (dynamic depth) anomaly.

dynamic topography See geopotential topography as preferred term.

dynamic viscosity A measure of a fluid's resistance to tangential or shear stress.

dysentery A disorder of the gastrointestinal tract result in in the uncontrolled discharge of body fluids. Such infections can result from infection of man by many different types of pathogenic organisms.

dystrophic lakes Acidic, shallow bodies of water that contain much humus and/or other organic matter; contain many plants, but few fish.