rack Parallel metal bars or rods evenly spaced and placed at an angle in the influent channel that removes rags, rocks, cans, and other debris from wastewater.
rad An acronym for radiation absorbed dose. A measure of the absorbed dose of radiation following exposure to a source or radioactive energy.
RADAR An acronym for RADIO DETECTION AND RANGING An electronic instrument using ultra high-frequency radio waves to detect distant objects and measure their range by how they scatter or reflect radio energy. Precipitation and clouds are detected by measuring the strength of the electromagnetic signal reflected back.
radial Moving from a center to a periphery of a force or vice-versa.
radial to impeller Perpendicular to the impeller shaft. Material being pumped flows at a right angle to the impeller.
radiation Transmission of energy through space or any medium in the form of waves or particles. Also known as radiant energy.
radiation absorbed dose The basic unit of dose of ionizing radiation. A dose of one rad means the absorption of 100 ergs of radiation energy per gram of absorbing material.
radiation sickness The complex of symptoms resulting from excessive exposure of most of the body to ionizing radiation. The earliest visible symptoms are nausea, fatigue, vomiting, and diarrhea, which may be followed by loss of hair, hemorrhage, inflammation of the mouth and throat, and general loss of energy. In severe cases, death may occur within two to four weeks. Those who survive six weeks after the receipt of a single large dose of radiation will generally recover.
radiation standards Regulations that set maximum exposure limits for protection of the public from radioactive materials.
radiation survey instrument A portable, battery-powered device used to detect and measure the dose rate at the spot where the instrument is held.
radical A group of atoms that is capable of remaining unchanged during a series of chemical reactions. Such combinations (radicals) exist in the molecules of many organic compounds; sulfate (SO42-) is an inorganic radical.
radioactive decay Spontaneous change in an atom by emission of charged particles and/or gamma rays; also known as radioactive disintegration and radioactivity.
radioactive fallout Airborne or deposited radioactive debris, including fission products, from a nuclear detonation.
radioactive material Any material which spontaneously emits particulate or electromagnetic ionizing radiation.
radioactive substances Substances that emit ionizing radiation.
radioactivity Spontaneous emission of alpha or beta particles or gamma radiation by unstable atoms.
radio frequency radiation See Non-ionizing Radiation.
radioisotopes Radioactive chemical variants of an element with potentially oncogenic, teratogenic, and mutagenic effects on the human body.
radionuclide An unstable nuclide. Any man-made or natural element that emits radiation in the form of alpha or beta particles or as gamma rays.
radius of vulnerability zone The maximum distance from the point of release of a hazardous substance in which the airborne concentration could reach the level of concern under specified weather conditions.
radon A colorless naturally occurring, radioactive, inert gas formed by radioactive decay of radium atoms in soil or rocks.
radon daughters/radon progeny Short-lived radioactive decay products of radon that decay into longer-lived lead isotopes, The daughter isotopes can attach themselves to airborne dust and other particles and, if inhaled, damage to lining of the lung. Also known as radon decay products.
radon decay products A term used to refer collectively to the immediate products of the radon decay chain. These include Po-218, Pb-214, Bi-214, and Po-214, which have an average combined half-life of about 30 minutes.
rain Precipitation in the form of liquid water drops which have diameters greater than 0.02 in (0.5 mm). The intensity of rain is based on rate of fall. "Very light" (R--) means that the scattered drops do not completely wet a surface. "Light" (R-) means it is greater than a trace and up to 0.10 inch an hour. "Moderate" (R) means the rate of fall is between 0.11 to 0.30 inch per hour. "Heavy" (R+) means over 0.30 inch per hour.
rain outage Loss of signal at Ku-band satellite frequencies due to absorption and increased sky noise caused by rainfall or ice crystals in the upper atmosphere.
Raman spectroscopy A direct corrosion monitoring method that reflects an infrared beam off a pipe surface and records the change in frequency of the beam as the Raman spectrum. The spectrum, which is different for all compounds, is compared with Raman spectra of known materials to identify constituents of the corrosion film on the pipe system.
random sample A method of statistical sampling that ensures each member of the population has the same chance of being chosen.
range 1) The spread from minimum to maximum values that an instrument is designed to measure. Also see span and effective range. 2)As related to nuclear radiation, the typical distance which a specific type of radiation will travel before all of its energy is absorbed. The range of radiation may vary through different types of material.
ranney collector A water collector constructed as a dug well from twelve to sixteen feet (3.5 to 5 m) in diameter that has been sunk as a caisson near the bank of a river or lake. Screens are driven radially and approximately horizontally from this well into the sand and gravel deposits underlying the river.
rapid deepening A decrease in the minimum sea-level pressure of a tropical cyclone of 1.75 mb/hr or 42 mb for 24 hours.
rapid mix Complete dispersion of a substance throughout a fluid volume in a time period of minutes or fractions of a minute.
raptor Any of several birds of prey (hawks, falcons, eagles, and owls)
rasp A machine that grinds waste into a manageable material and helps prevent odor.
rate An expression of the frequency with which an event occurs in a defined population.
rate ratio A comparison of two groups in terms of incidence rates, person-time rates, or mortality rates.
rating curve A curve showing the relation between gage height (stage) or river elevation and discharge at a specific point or river gaging station.
rating table A table of stage values and the corresponding discharge for a river gaging site.
ratio The value obtained by dividing one quantity by another.
rationing Mandatory water-use restrictions sometimes used under drought or other emergency conditions.
raw sewage Untreated wastewater and its content.
raw wastewater Wastewater before it receives any treatment.
raw water 1) Water in its natural state, prior to any treatment. 2) Usually the water entering the first treatment process of a water treatment plant.
raw water main A water main which conveys untreated water from a source to a treatment facility.
raw water sample A water sample gathered prior to treatment of any kind.
reach The length of a river between two gaging stations. More generally, any length of a river.
reactivation Restoration of the organic absorption or adsorption capacity of a medium by displacing previously absorbed/adsorbed compounds through the addition of energy or mass.
reactor Any vessel in which a chemical, biochemical, or physical reaction takes place.
reactor cooling water The water that circulates through the primary cooling system of a nuclear power plant and provides cooling for the reactor core and core components.
reactor core The heat source of a nuclear power plant consisting of a number of fuel assemblies grouped side by side.
reaeration The introduction of air into a vessel or channel containing water or wastewater to increase the dissolved oxygen level. Compressed air may be transmitted by air diffusers into the lower layers of a body of water such as a reservoir. As the air bubbles form and rise through the water, oxygen from the air dissolves into the water and replenishes the dissolved oxygen. The rising bubbles also cause the lower waters to rise to the surface where oxygen from the atmosphere is transferred to the water. This is sometimes called surface reaeration.
reagent A pure chemical substance that is used to make new products or is used in chemical tests to measure, detect, or examine other substances.
reagent water Water used in the laboratory that is treated to remove all contaminants so it will not be observed at or above the estimated detection limit of any analyte or compound of interest.
real dollars Forecast dollars that are adjusted for inflation.
real time An application in which information is received and immediately responded to without any time delay. See Synchronous.
reasonable further progress Annual incremental reductions in air pollution emissions as reflected in a State Implementation Plan, deemed sufficient to provide for attainment of the applicable national ambient air quality standard by the statutory deadline.
reasonable maximum exposure The maximum exposure reasonably expected to occur in a population.
reasonably available control measures (racm) A broadly defined term referring to technological and other measures for pollution control.
reasonably available control technology (ract) Control technology that is both reasonably available and both technologically and economically feasible.
rebuttal An answer or response to a statement; to refute, oppose, or contradict that which has been stated.
recarbonation A process in which carbon dioxide is bubbled into water or wastewater being treated to lower the pH similar to the addition of acid. Recarbonation is the final stage in the lime-soda ash softening process. This process converts carbonate ions to bicarbonate ions and stabilizes the solution against the precipitation of carbonate compounds.
receiver A device which indicates the result of a measurement. Most receivers in the water utility field either use a fixed-scale and movable indicator (pointer), such as a pressure gage, or a movable scale and movable indicator like those used on a circular-flow recording chart. Also called an indicator.
receiving sites All sites, other than the originating site, participating in a course or meeting that is distributed with technology such as audioconferencing or videoconferencing.
receiving waters All distinct bodies of water that receive runoff or wastewater discharges, such as streams, rivers, ponds, lakes, and estuaries.
receptor 1) In biochemistry, a specialized molecule in a cell that binds a specific chemical with high specificity and high affinity. 2) In exposure assessment, an organism that receives, may receive, or has received environmental exposure to a chemical.
recent (geological) Informal term, usually referring to the period of time from the last glaciation to the present. U.S.G.S. uses it formally, as “Recent,” but has not defined it; most geologists prefer “Holocene.”
recharge 1) Process by which rain water (precipitation) seeps into the groundwater system. 2) Process by which treated wastewater is introduced into groundwater (reuse).
recharge area A land area in which water reaches the zone of saturation from surface infiltration, e.g., where rainwater soaks through the earth to reach an aquifer.
recharge rate The quantity of water per unit time that replenishes or refills an aquifer.
recirculation The return of a portion of treated wastewater to a point upstream in the treatment process such as sending clarified effluent back to combine with reactor influent.
recirculation operation
reclaimed wastewater Wastewater treatment plant effluent that has been diverted for beneficial use before it reaches a natural waterway or aquifer.
reclamation Restoration of materials found in the waste stream to a beneficial use which may be for purposes other than the original use.
recombinant bacteria A microorganism whose genetic makeup has been altered by deliberate introduction of new genetic elements. The offspring of these altered bacteria also contain these new genetic elements.
recombinant dna Dna that contains genes from different sources that have been combined by methods of genetic engineering as opposed to traditional breeding experiments. See deoxyribonucleic acid.
recommended maximum contaminant level (rcml) The maximum level of a contaminant in drinking water at which no known or anticipated adverse effect on human health would occur and it includes an adequate safety margin.
reconstructed source A facility in which components are replaced to such an extent that the fixed capital cost of new components exceed 50 percent of the capital cost of constructing a comparable brand-new facility. New-source performance standards may be applied to reconstructed sources after the proposal of the standard if it is technologically and economically feasible to meet the standard.
recorder A device that creates a permanent record, on a paper or magnetic tape, of the changes of some measured variable.
record of decision (ROD) A public document that ex plains which cleanup alternative(s) will be used at National Priorities List sites.
recovery rate Percentage of usable recycled materials that have been removed from the total amount of municipal solid waste generated in a specific area or by a specific business.
recycle/reuse Minimizing waste generation by recovering and reprocessing usable products that might otherwise become waste (.i.e. recycling of aluminum cans, paper, and bottles, etc.).
recycled water Water that is used more than one time before it passes back into the natural hydrologic system.
recycling The repeated use of a finite body of resources, such as minerals or waste matter that is suitable for other uses.
red bag waste See infectious waste.
red tide A reddish color of near-shore, marine waters due to the presence of extremely large numbers of red-pigmented microorganisms that liberate toxins lethal to fish.
reduced monitoring A schedule that allows a system to monitor with less frequency if the system meets the requirement for a certain number of years.
reduced pressure principle backflow prevention device (RPZ device) A device containing a minimum of two independently acting check valves together with an automatically operated pressure differential relief valve located between the two check valves. During normal flow and at the cessation of normal flow, the pressure between these two checks shall be less than the supply pressure. In case of leakage of either check valve, the differential relief valve, by discharging to the atmosphere, shall operate to maintain the pressure between the check valves at less than the supply pressure. The unit must include tightly closing shut-off valves located at each end of the device, and each device shall be fitted with properly located test cocks. These devices must be of the approved type.
reducer organism Those organisms that have the capability of promoting chemical reductions, as green plants reduce carbon dioxide and sulfate-reducing bacteria reduce sulfate.
reducing agent Any substance, such as a base metal (iron) or sulfide ion (S2), that will readily give up electrons. The opposite is an oxidizing agent.
reduction The addition of hydrogen, removal of oxygen, or the addition of electrons to an element or compound. Under anaerobic conditions, sulfur compounds are reduced to odor-producing hydrogen sulfide (H2S) and other compounds. The opposite of oxidation.
reentry interval The period of time immediately following the application of a pesticide during which unprotected workers should not enter a field.
reference A physical or chemical quantity whose value is known exactly and, thus, is used to calibrate or standardize instruments.
reference dose (RfD) The concentration of a chemical known to cause health problems; also referred to as ADI or acceptable daily intake.
reformulated gasoline Gasoline with a different composition from conventional gasoline (e.g., lower aromatics content) that cuts air pollutants.
refueling emissions Emissions released during vehicle refueling.
refuse Something that is discarded as worthless or useless; rubbish, trash, garbage, or other solid wastes.
refuse reclamation Conversion of solid waste into useful products, e.g., composting organic wastes to make soil conditioners or separating aluminum and other metals for recycling.
regeneration Manipulation of cells to cause them to develop into whole plants.
regimen 1) 2) 3)
regional response team (RRT) Representatives of federal, state, and local agencies who may assist in the coordination of activities at the request of the on-scene coordinator before and during a significant pollution incident, such as an oil spill or major chemical release.
registrant Any manufacturer or formulator who obtains registration for a pesticide active ingredient or product.
registration Under the Federal Insecticide, Fungicide, and Rodenticide Act, a formal listing with EPA of a new pesticide before it can be sold or distributed.
registration standards Published documents which include summary reviews of the data available on a pesticide's active ingredient, data gaps, and existing regulatory position on the pesticide.
regulated asbestos-containing material (RACM) Friable asbestos material or nonfriable ACM that will be or has been subjected to sanding, grinding, cutting, or abrading or has crumbled, or been pulverized or reduced to powder in the course of demolition or renovation operations.
regulated medical waste Any solid waste generated in the diagnosis, treatment, or immunization of human beings or animals, in the research thereof, or in the production or testing of biologicals.
relational analysis Any of the various types of statistical methods used to estimate the correlation or relationship between variables in a given population.
relative humidity A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present.
relative risk A comparison of the risk of some health-related event such as disease or death in two groups.
release Any spilling, leaking, pumping, pouring, emitting, discharging, injecting, escaping, leaching, dumping, or disposing into the environment of a hazardous or toxic chemical or extremely hazardous substance.
reliability The probability that the system will perform satisfactorily for at least a given period of time when used under stated conditions.
reliably and consistently “Reliably and consistently” below the MCL means that although a system detects contaminants in its water supply, the state has sufficient knowledge of the source or extent of the contamination to predict that the MCL would not be exceeded in the future.
relict A species properly belonging to an earlier community type than that in which it is found. A community, or fragment of one, that has survived some important change and now seems to be or is “left behind.”
relief 1) Variations in elevation of the earth's surface. 2) An alternate path provided for fluid or energy to increase capacity.
reliquifaction The return of a gas to a liquid. For example, a condensation of chlorine gas returning to the liquid form.
rem An acronym for Roentgen Equivalent Mammal. The unit of dose equivalent from ionizing radiation to the total body or any internal organ or organ system. A millirem (mrem) is one-one thousandth of a rem.
remedial action The actual construction or implementation phase of a site cleanup that follows remedial design.
remedial design A phase of remedial action that follows the remedial investigation or feasibility study and includes development of engineering drawings and specifications for a site cleanup.
remedial investigation An in-depth study designed to gather data needed to determine the nature and extent of contamination at a Superfund site; establish site cleanup criteria; identify preliminary alternatives for remedial action; and support technical and cost analyses of alternatives. The remedial investigation is usually done with the feasibility study. Together they are usually referred to as the "RI/FS".
remedial project manager (RPM) The government official responsible for overseeing on-site remedial action.
remedial response Long-term action that stops or substantially reduces a release or threat of a release of hazardous substances that is serious but not an immediate threat to public health.
remediation 1) Cleanup or other methods used to remove or contain a toxic spill or hazardous materials from a site. 2) Under the Asbestos Hazard Emergency Response program, abatement methods including evaluation, repair, enclosure, encapsulation, or removal of greater than 3 linear feet or square feet of asbestos-containing materials from a building.
remote sensing A method for determining the characteristics of an object, organism, or community from afar.
removal action Short-term immediate actions taken to address releases of hazardous substances that require expedited response. See cleanup.
renal Pertaining to the kidney.
repeat compliance period Any subsequent compliance period after the initial compliance period.
repeat sampling Subsequent rounds of sampling following the initial base sampling. Generally, repeat base sampling requirements can be reduced if initial sampling results in no detects of a contaminant.
replicate samples Multiple field sample collected at the same time and place under identical circumstances. They are placed in separate sample containers and treated identically throughout field and laboratory procedures. Analysis of field replicates provides a measure of the precision associated with sample collection, preservation, storage, and laboratory procedures.
reportable quantity (RQ) The quantity of a hazardous substance that triggers reports under CERCLA. If a substance exceeds its RQ, the release must be reported to the National Response Center, the SERC, and community emergency coordinators for areas likely to be affected.
report of a disease An official report that notifies an appropriate health authority of the occurrence of a disease in a human, or in an animal. Human diseases usually are reported first to the local health authority, such as a county health department.
repowering Rebuilding and replacing major components of a power plant instead of building a new one.
representative sample A portion of material or water that is as nearly identical in content and consistency as possible to that in the larger body of material or water being sampled.
reptiles One of the major groups of vertebrate animals, including crocodilians, turtles, lizards, and snakes, having scales or horny plates, true lungs, and a three- or four-chambered heart.
reregistration The reevaluation and relicensing of existing pesticides originally registered prior to current scientific and regulatory standards.
res Things, includes real and personal property.
reserve capacity Extra treatment capacity built into solid waste and wastewater treatment plants and interceptor sewers to accommodate flow increases due to future population growth.
reservoir 1) A natural or artificial holding area used to store, regulate, or control water. 2) The supply of any effect or commodity. 3) A reservoir of an infectious agent, such as a virus, is any animal, person, plant, soil, substance—or combination of any of these — in which the infectious agent normally lives. In addition, the infectious agent must primarily depend on the reservoir for its survival, and must be able to multiply there. It is from the reservoir that the infectious substance is transmitted to a human or other susceptible host.
reservoir surface area The surface area of a reservoir when filled to the normal pool or water level.
reservoir volume The volume of a reservoir when filled to normal pool or water level.
resident 1) Normally to be found when looked for at a place or within a habitat. 2) Of birds, nonmigratory.
residential water use See domestic use.
residual 1) The remaining detectable or effective amount following the use, treatment or conversion of an initial level or quantity. 2) Amount of a pollutant remaining in the environment after a natural or technological process has taken place, e.g., the sludge remaining after initial wastewater treatment, or particulates remaining in air after it passes through a scrubbing or other process.
residual chlorine The amount of free and/or available chlorine remaining after a given contact time under specified conditions.
residual disinfectant concentration [“C” in CT calculations] The concentration of disinfectant measured in mg/L in a representative sample of water.
residual nuclear radiation Nuclear radiation emitted from radioactive fallout that persists for some time following a nuclear explosion.
residual risk The extent of health risk from air pollutants remaining after application of the Maximum Achievable Control Technology (MACT).
residue The dry solids remaining after the evaporation of a sample of water or sludge. Also see total dissolved solids.
resilience The ability of any system, e.g., an ecosytem, to resist or recover from stress.
resistance The sum total of host mechanisms that interpose barriers to invasion or multiplication of infectious agents or that prevent damage by the agent's toxic products. Also see immunity.
resistant 1) Characteristic of organisms not overly susceptible to environmental stresses. 2) Opposing conductivity.
resistivity A measure of resistance to conductivity, used as an accurate measure of the ionic purity of water.
res judicata Matters that have been decided between the same parties need not be decided again.
resolution 1) The clarity of the image on video display screen. Three factors influence resolution: lines of resolution (vertical and horizontal), raster scan rate (number of times per second the image on a video screen can be refreshed or "lit up" again), and bandwidth. 2)
resource recovery The process of obtaining matter or energy from materials formerly discarded.
resources, unique Supplies of a commodity not (or not usually ) found elsewhere; for other organisms, most resources are material substances, but for man, many nonmaterial qualities of environment and of society are unique resources.
respiration The process in which an organism uses oxygen for its life processes and gives off carbon dioxide.
respondent One who answers; as in notice of hearing, the response is given by the party to whom notice is given— he then is the respondent.
response action 1) Generic term for actions taken in response to actual or potential health-threatening environmental events such as spills, sudden releases, and asbestos abatement and/ormanagement problems. See cleanup. 2) A CERCLA-authorized action involving either a short-term removal action or a long-term removal response. 3) Any of the following actions taken in school buildings in response to AHERA: to reduce the risk of exposure to asbestos removal, encapsulation, enclosure, repair, and operations and maintenance.
response time The amount of time in which it will take a watershed to react to a given rainfall event.
responsible charge Designation by the owner of any individual to have duty and authority to operate or modify the operation of waterworks or wastewater treatment processes.
responsiveness summary A summary of oral and/or written public comments received by an agency during a comment period on key documents, and the agency’s response to those comments.
restoration Measures taken to return a site to pre-violation conditions.
restraining gland See thrust block.
restricted use 1) Access limitations designed to protect public health. 2) Applies to any pesticide classified (under FIFRA regulations) for restricted use if it requires special handling because of its toxicity and it may be applied only by trained, certified applicators or those under their direct supervision.
restriction enzymes Enzymes that recognize specific regions of a long DNA molecule and cut it at those points.
restriction map The linear arrangement of various restriction enzyme sites.
restriction site Base sequence recognized by an enzyme.
retardation Preferential retention of contaminant movement in the subsurface zone. Retention may be the result of adsorption processes or solubility differences.
retarding reservoir Ungated reservoir for temporary storage of floodwater. Sometimes called a detention reservoir.
retrofit Addition of new equipment or processes within an existing facility without making major changes to the plant.
retrogressing Changing in a reverse order to a simpler or earlier state.
retrospective study An epidemiological study which compares diseased persons with nondiseased persons and works back in time to determine exposures.
retrovirus A family of RNA viruses that have the unique characteristic of producing an enzyme that makes a DNA copy of its genetic information from an RNA template (the opposite of what normally takes place). The most widely recognized of these viruses is HIV, the causative agent in AIDS. Another virus from this family (HTLV-1) has been associated with T cell leukemia. Initial reports of an association of an HTLV-II-like retrovirus with CFS could not be confirmed in subsequent studies.
return flow The water that reaches a ground- or surface-water source after release from the point of use and thus becomes available for further use.
return on assets (ROA) The relationship of annual after tax earnings to total assets (average or ending balance), used as a measure of the productivity of an organization's assets.
return on investment (ROI) The relationship of annual after tax earnings to the book value (average or ending balance) of the asset, business, or profit center generating these earnings. Used as a measure of the productivity of the investment.
return period The average time interval between occurrences of a hydrological event of a given or greater magnitude, usually expressed in years.
reuse Beneficially using a product or residuals, including a component of waste treatment, in some form more than once, e.g., refilling a glass bottle that has been returned or irrigating with treated wastewater.
revenue-producing water Water metered and sold.
reversal The act of reversing a court order or verdict; i.e., an appellate court may reverse a decision, rendered by a district court.
reverse osmosis The application of pressure to a concentrated solution which causes the passage of a liquid from the concentrated solution to a weaker solution across a semipermeable membrane. The membrane allows the passage of the solvent (water), but not the dissolved solids (solute). The liquid produced is a demineralized water. Also see osmosis.
reverse transcriptase An enzyme that catalyzes the synthesis of dna from rna.
reversible effect An effect which is not permanent, especially adverse effects which diminish when exposure to a toxic chemical ceases.
revetment A facing of stone, concrete, etc. built to protect a scarp, embankment, or shore structure against erosion by the wave action or currents.
RFD (Reference dose). The daily exposure level, during an entire lifetime of a human, which appears to be without appreciable risk on the basis of all facts known at the time. Same as ADI.
RGB monitor Video display screen with colors Red, Green, Blue as light waves. The three colors combined in different ways produce all other colors.
rheotropism The behavorial response of an organism, cell, or organ to a current of water.
ribonucleic acid (rna) Basic biochemical component of the chromosome that is found mainly in the nucleolus and ribosomes. Messenger rna transfers genetic information from the nucleus to the ribosomes in the cytoplasm and also acts as a template for the synthesis of polypeptides. Transfer rna transfers activated amino acids from the cytoplasm to messenger rna.
rill A small channel eroded into the soil surface by runoff, rills can be smoothed out easily by normal tillage.
Ringlemann Chart A series of shaded illustrations used to measure the opacity of air pollution emissions, ranging from light grey through black; used to set and enforce emissions standards.
riparian Pertaining to the banks of a stream (e.g., riparian vegetation).
riparian habitat Areas adjacent to rivers and streams with a high density, diversity, and productivity of plant and animal species relative to nearby uplands.
riparian rights A doctrine of state water law under which a land owner is entitled to use the water on or bordering his property, including the right to prevent diversion or misuse of upstream water. Riparian land is land that borders on surface water.
riprap A layer, facing, or protective mound of stones randomly placed to prevent erosion, scour, or sloughing of a structure or embankment.
rising sludge A problem in secondary settling tanks generally attributed to dentrification in the sludge blanket.
risk 1) The potential for realization of unwanted adverse consequences or events. 2) The likelihood that a person having specified characteristics (e.g., age, sex, immune status) will acquire a specific disease. 3) The chance of being exposed to an infectious agent by its specific transmission mechanism. 4) The chance of becoming infected if exposed to an infectious agent by its specific transmission mechanism.
risk analysis
risk assessment A qualitative or quantitative evaluation of the environmental and/or health risk resulting from exposure to a chemical or physical agent (pollutant); combines exposure assessment results with toxicity assessment results to estimate risk.
risk based targeting The direction of resources to those areas that have been identified as having the highest potential or actual adverse effects on human health and/or the environment.
risk characterization Final component of risk assessment that involves integration of the data and analysis involved in hazard evaluation, dose-response evaluation, and human exposure evaluation to determine the likelihood whether humans will experience any of the various forms of toxicity associated with a substance.
risk communication The exchange of information about health or environmental risks among risk assessors and managers, the general public, news media, interest groups, etc.
risk estimate A description of the probability that organisms exposed to a specific dose of chemical will develop an adverse response (e.g., cancer).
risk factor Physical characteristic (e.g., race, sex, age, obesity) or variable (e.g., smoking, occupational exposure level) associated with increased probability of a toxic effect.
risk management Decisions about whether an assessed risk is sufficiently high to present a public health concern and about the appropriate means for the control of a risk judged to be significant.
risk ratio A comparison of the risk of some health-related event such as disease or death in two groups.
risk specific dose The dose associated with a specific risk level.
riverbank overstory Those plants growing along streams whose canopies occupy the greatest heights.
river basin The land area drained by a river and its tributaries.
river flooding The rise of a river to an elevation such that the river overflows its natural banks causing or threatening damage.
river forecast center (RFC) A division of the National Weather Service which provides river forecasts for rivers within its area of responsibility. There are 13 RFCs in the United States. Their areas are demarked by hydrologic bounaries and watersheds rather than political boundaries.
river gage A device for measuring the river stage.
riverine Of or related to rivers.
river observing station An established location along a river designated for observing and measuring river properties.
river system All of the streams and channels draining a river basin.
rna See ribonucleic acid.
rna polymerase An enzyme that catalyzes the synthesis of rna in transcription. See ribonucleic acid.
rodenticide Any substance or chemical used to kill or control rodents.
roentgen (R) A unit of exposure to ionizing gamma radiation in air.
rookery The breeding or nesting place of colonies of birds, seals, etc.
root-mean-square sound pressure (RMS sound pressure) The square root of the sum of the square values of sound pressure over a specified period of time.
root zone The depth of soil penetrated by crop roots. Also called the vadose zone.
rotameter A device used to measure the flow rate of gases and liquids. The gas or liquid being measured flows vertically up a tapered, calibrated tube. Inside the tube is a small ball or bullet-shaped float (it may rotate) that rises or falls depending on the flow rate. The flow rate may be read on a scale behind or on the tube by looking at the middle of the ball or at the widest part of the top of the float.
rotary distributor A movable distributor made up of horizontal arms that extend to the edge of a circular filter bed or contactor that revolves about a central post and distributes liquid over the bed through holes or jets in the arms.
rotary kiln incinerator An incinerator with a rotating combustion chamber that keeps waste moving, thereby allowing it to vaporize for easier burning.
rotifer A small, multi-celled animal that gets its name from the rotating action of rows of cilia near its mouth.
rotor The rotating part of a machine. The rotor is surrounded by the stationary (nonmoving) parts (stator) of the machine.
rotten ice Ice whose mechanical strength has been reduced by warming and percolation of water.
rough fish A nonsport fish, usually omnivorous in food habits, but not prized due to poor flavor, excessively bony flesh, or inadequate cooperation with anglers.
roughing filter A contactor consisting of high porosity media operated at a high loading rate to afford preliminary or intermediate wastewater treatment.
route of exposure The avenue by which a chemical comes into contact with an organism (e.g., inhalation, ingestion, dermal contact, injection).
routing 1) The operation of translating a volume of water from one forecast point through a reach to another forecast point. 2) 3)
rubbish Solid waste, excluding food waste and ashes, from homes, institutions, and work places.
ruderal A weed; an introduced plant species growing under disturbed conditions, in waste places or among rubbish.
runoff The total amount of water flowing into a body of water. It includes overland flow, return flow, and interflow.
rural water use A term used to describe water used in suburban or farm areas for domestic and livestock needs. The water generally is self-supplied, and includes domestic use, drinking water for livestock, and other uses, such as dairy sanitation, evaporation from stock-watering ponds, and cleaning and waste disposal. See also domestic use, livestock water use, and self-supplied water.
rusting Corrosion of iron or an iron based alloy to form a reddish-brown product which is primarily hydrated ferric oxide.