Acute toxicity -- The discernible adverse effects induced in an organism within a short period of time (days) of exposure to a chemical. For aquatic animals, this usually refers to continuous exposure to the chemical in water for a period of up to four days. The effects (lethal or sub-lethal) occurring may usually be observed within the period of exposure with aquatic organisms. Acute toxicity is usually defined as TUa = 100/LC50. Note that acute means short, not mortality.

Acute Toxicity test -- A method used to determine the concentration of a substance that produces a toxic effect on a specified percentage of test organisms in a short period of time (e.g., 96 hours). As a general guideline, death is the measure of toxicity.

Ambient Toxicity -- Toxicity found in the "unaffected" portion of an effluent's receiving waters.

Bioassay -- A test used to evaluate the relative potency of a chemical by comparing its effect on a living organism with the effect of a standard preparation on the same type of organism. The term "bioassay" is commonly, though not technically correct, used interchangeably with the term "toxicity test".

Chronic toxicity -- An adverse effect that lingers or continues for a relatively long period of time. A chronic effect can be lethality, growth, reduced reproduction, etc. Chronic toxicity is defined as TUc = 100/NOEC or TUc = 100/ECp (or 100/ICp). Note: chronic means long.

Chronic toxicity test -- A method used to determine the concentration of a substance in water that produces an adverse effect on a test organism over an extended period of time. Reductions in reproduction or growth are measures of chronic effects.

Coefficient of Variation (CV) -- The standard statistical measurement of the relative variation of a distritubtion or set of data, defined as the standard deviation divided by the mean. Coefficient of variation is a measurement of precision within and and among laboratories.

Confidence limits or interval (CI) -- The limits or interval within which, at some specified level of probability, the true value of a result lies. Typically LC50 values are reported with a 95 % confidence limits.

Control -- An exposure of test organisms to dilution water only (no toxicant is added). Dilution water may consist of laboratory or client supplied receiving waters.

Critical life stage -- The period of time in an organism's life span in which it is the most susceptible to adverse effects caused by exposure to toxicants, usually during early development (egg, embryo, larvae). Chronic toxicity tests are often run on critical life stages to replace long duration, life cycle tests since the toxic effect occurs during the critical life stage.

Death -- Defined as the lack of movement or reaction even after gentle prodding,

Definitive bioassay -- A bioassay designed to establish concentration at which a particular end point occurs. Exposures for these tests are longer than for screen or range finding assays and usually incorporate multiple replicates.

Dilution allowance -- Allowance given to account for initial dilution of effluent into receiving waters. Becomes an important factor when calculating proper test concentrations for toxicity testing.

Dilution water -- The water to which the test substance is added (diluted) and in which the organisms undergo exposure.

Dose Response Curve -- A mathematical representation of the response of test organisms to different concentrations of a toxicant/effluent.

Effective Concentration (EC) -- A point estimate (statistically derived) of the toxicant concentration that would cause an quantal ("all or nothing") effect, such as death or lack of fertilization, in a given time, for example, 96hr EC50.

EC50 -- The concentration of test substance in dilution water that is calculated to effect 50 percent of a test population during continuous exposure over a specified period of time.

Exposure time -- Length of time a test organism is exposed to a test solution.

Flow-through tests -- Refers to the continuous or very frequent passage of fresh test solution through a test chamber with no recycling. Because of the large volume (often 400 L/day) of effluent normally required for flow-through tests, it is generally considered too costly and impractical to conduct these tests off-site at a central laboratory.

Hazardous Waste Test -- A test to determine whether or not a particular sample exceeds state toxicity guidelines and is therefore classified as hazardous waste.

Hypothesis Testing -- A technique that determines what concentration is statistically different from the control. Endpoint determined from hypothesis testing are NOEC and LOEC.

Inhibition Concentration (IC) -- A point estimate (statistically derived) of the toxicant concentration that would cause a given percent reduction in a non-quantal biological measurement such as fecundity or growth. For example, an IC25 would be the estimated concentration of toxicant that would cause a 25% reduction in mean young produced or in growth.

Lethal Concentration (LC) -- toxicant concentration producing death of test organism. For example, a 96 hr LC50 would be the test concentration killing 50% of exposed organisms after 96 hours of exposure.

LC50 -- Lethal concentration of a substance killing 50 percent of an exposed organisms at a specific time interval. Also referred to as the median lethal concentration (MLC).

LOEC (Lowest Observed Effect Concentration) -- The lowest toxicant concentration of an effluent or a toxicant in a chronic bioassay that caused an adverse effect statistically different from the control. Also referred to as the LOEL (Lowest Observed Effect Level).

Major Permit -- Any permit(ee) with a design flow of 1.0 MGD (million gallons per day) or greater (municipal). Any permit(ee) which scores 80 or greater on the major/minor permit classification scale (industrial).

MATC (Maximum Acceptable Toxicant Concentration) -- Toxicant concentration that may be present in a receiving water without causing significant harm to productivity or other uses. MATC is determined by long term tests of either partial life cycle with sensitive life stages or a full life cycle of the test organism.

MGD -- Million Gallons per Day of discharge.

Minimum Significant Difference (MSD) -- This is the magnitude of difference from the control where the null hypothesis (the effluent is not toxic) is rejected in a statistical test comparing a treatment (effluent concentrations) and a control. MSD is based on the number of replicates, control performance and the power of the test.

Mixing Zone -- An allocated impact area in a water body where numeric water quality criteria can be exceeded as long as acutely toxic conditions are prevented. Also referred to as ZID (Zone of Initial Dilution).

Neonate -- Recently hatched cladoceran (water flea)(ie., Ceriodaiphnia Dubia).

NOEC (No Observed Effect Concentration) -- The highest concentration of an effluent or a toxicant in a chronic bioassay that did not cause adverse effect statistically different from the control.

NOEL (No Observed Effect Level) -- The highest measured continuous concentration of an effluent or other toxicant that causes no observed effect on a test organism.

NPDES -- The National Pollutant Discharge Elimination System prescribed by Section 402 of the Clean Water Act.

Point Estimate Techniques -- Statistical techniques, such as Probit Analysis, Interpolation Method, or Trimmed Sperman Karber Method, that are used to determine the effluent concentration at which adverse effects occurred. For example, the LC50 is the concentration at which 50% mortality occrred. These interpulative methods do not require that the desired endpoint concentration concntration be actually tetsed as in hypothesis testing.

Point source -- A discrete conveyance such as a pipe, ditch, etc. contributing pollutants to the environment.

Pollutant -- A contaminant introduced into a receiving water which is subject to technology-based or water-quality based effluent limitations in the permit.

POTW -- Publicly Owned Treatment Works, usually consisting of primary and secondary (biological) treatment.

Reference Toxicant -- A chemical used to access the constancy of response of a given species of test organisms to that chemical. It is assumed that any change in sensitivity to the reference substance will indicate the existence of some similar change in degree of sensitivity to other chemicals/effluents whose toxicity is to be determined.

Renewal test -- A test without continuous flow of solution, but with occasional renewal of test solutions after prolonged periods, e.g., 24 hours.

Replicate -- Two or more duplicate tests, samples, organisms, concentrations, or exposure chambers.

Response -- The measured biological effect of the material tested. In acute toxicity tests this is usually death. In biostimulation tests this is usually biomass increase. In chronic toxicity tests this can be reductions in reproduction, growth as well as death.

Screening test -- An abbreviated toxicity test with one or two toxicant concentrations. Some Regional and State effluent biomonitoring programs stipulate its use. If lethality is observed in the screening test, a definitive test may be required.

Static tests -- Toxicity tests with aquatic organisms in which no flow of test solution occurs. Solutions may remain unchanged throughout the duration of the test. Types include: (i) nonrenewal - the test organisms are exposed to the same effluent solution for the duration of the test; and (ii) renewal - the test organisms are exposed to a fresh solution of the same concentration of effluent every 24 hours or other prescribed interval, either by transferring the test organisms from one test chamber to another, or by replacing all . 1 or a portion of the effluent solution in the test chambers.

Static renewal test -- A test method in which the test solution is periodically replaced at specific intervals during the test.

Toxicity -- Adverse effect to a test organism caused by "pollutants. " Toxicity is a resultant of concentration and time, modified by variables such as temperature, chemical form, and availability.

Toxicity test -- A measure of the toxicity of a chemical or an effluent using living organisms by determining the degree of response (survival, reproduction, growth, etc.) of an exposed organism to the chemical or effluent.

Toxic Units (TUs) -- A measure of toxicity in an effluent as determined by the acute toxicity units or chronic toxicity units. Higher TUs indicate greater toxicity.

Toxic Unit Acute (TUa) -- A mathematical conversion of LC50 into a relatable value. TUa = 100/LC50.

Toxic Unit Chronic (TUc) -- A mathematical conversion of an NOEC or NOEL into a relatable value. TUc = 100/NOEC.

Toxicity Identification Evaluation (TIE) -- A set of procedures used to identify the specific chemical(s) responsible for effluent toxicity. TIEs are a subset of the TRE.

Toxicity Reduction Evaluation (TRE) -- A site-specific study conducted in a stepwise process designed to identify the causative agents of effluent toxicity, isolate the sources of toxicity, evaluate the effectiveness of toxicity control options, and then confirm the reduction in effluent toxicity. TREs may include TIE testing as part of this process.

Whole Effluent Toxicity (WET) -- The total toxic effect of an effluent measured directly with a toxicity test.

Zone of Initial Dilution (ZID) -- An allocated impact area, or mixing zone, in a water body where numeric water quality criteria can be exceeded as long as acutely toxic conditions are prevented.