National Water-Quality Assessment (NAWQA) Program

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z |

A

Acidification

An increase in the concentration of anions associated with the production of acids, the presence of which can decrease the acid neutralizing capacity of the water body (see Leydecker and others, 1999 for further discussion).

Activity

The amount of radioactivity in a given volume of material such as water or air. For water, activity is generally expressed in units of picocuries per liter.

Alpha radiation

Alpha radiation is composed of a particle, consisting of two protons and two neutrons, spontaneously emitted from the nucleus of a subset of radioactive elements (mostly the heaviest elements) during radioactive decay. Alpha radiation is ionizing radiation, meaning that it strips electrons from adjacent atoms as it passes. Alpha radiation cannot penetrate skin; thus, an alpha-particle emitting radionuclide must be ingested in order to contact internal tissue. Because of the large size, alpha particles are likely to collide with cell tissue, causing tissue damage. An accumulation of tissue damage in the cell nucleus could lead to cell mutation and potential cancer formation (risk).

Alpha radioactivity, gross

A laboratory measurement of total alpha radioactivity emitted by a sample. This measurement includes alpha-particle radioactivity emitted by isotopes of naturally occurring uranium, thorium, radium, and progeny such as polonium, as well as alpha particles emitted from isotopes of plutonium, which is manmade (not naturally occurring). Long-term measurement, usually conducted 20 to 30 days after sample collection, measures only the amount of alpha radiation present from long-lived radionuclides, such as naturally occurring uranium-238, radium-226 (half-life, 1,602 years), or the long-lived isotopes of plutonium. Short-term measurements, usually made within 24, 48, or 72 hours after sample collection indicate the alpha-particle decay of radionuclides with short half-lives.

Alpha-radioactivity count

An analytical technique that specifies alpha radioactivity emitted by a chemically purified sample that contains only one radionuclide of interest. The alpha-particle activity is counted in a low-background gas proportional counter.

Alpha spectrometry

An analytical technique that specifies the amount of alpha radiation emitted at specified energy levels, thus allowing determination of individual radionuclide concentrations (from known energy levels of alpha particles unique to each radionuclide). The gridded-pulse-ionization chamber is the most sensitive and most commonly used instrument.

B

Beta radioactivity

Beta radiation is composed of a particle, consisting of an electron, spontaneously emitted from the nucleus of a subset of radioactive elements during radioactive decay. Beta radiation, like alpha radiation, is ionizing radiation—it strips electrons from adjacent atoms as it passes. Beta radiation can only penetrate the surface layer of skin; thus, a beta-particle emitting radionuclide must be ingested in order to contact internal organs or tissues. An accumulation of tissue damage in the cell nucleus could lead to cell mutation and potential cancer formation.

Beta radioactivity, gross

A laboratory measurement of total beta radioactivity emitted by a sample. This measurement includes radioactivity emitted by naturally occurring progeny of uranium and thorium, such as radium-228 and lead-210, and numerous other naturally occurring beta-particle-emitting radioactive isotopes.

Beta radioactivity count

An analytical technique that specifies total beta radioactivity emitted by a chemically purified sample that contains only one radionuclide of interest. The beta-particle activity is counted in a low-background gas proportional counter.

C

Contaminant

As defined in the Safe Drinking Water Act, any physical, chemical, biological, or radiological substance or matter in water.

D

Decay product of radiation

The isotope remaining after radioactive decay.

G

Gamma radioactivity

Gamma radiation is composed of a packet of energy, also known as a photon or photon particle, spontaneously emitted from the nucleus of most radioactive elements during radioactive decay. Gamma radiation is ionizing radiation meaning that it strips electrons from adjacent atoms as it passes. Gamma radiation can penetrate through skin into internal tissues as opposed to alpha and beta radiation, which cannot. An accumulation of tissue damage in the cell nucleus from gamma radiation could lead to cell mutation and potential cancer formation.

I

Isotope

Various forms of a single element differing in the number of neutrons in the nucleus. Unstable isotopes of an element decay through the emission of a form of radioactivity.

L

Laboratory reporting level (LRL)

The laboratory reporting level (LRL) is a minimum concentration pre-determined by laboratory personnel before the project begins, which generally is based on typical counting results under routine operating conditions in a given sample matrix by using known standards (American Society of Testing and Materials, 1999) that the laboratory personnel define as a level of detection they can routinely achieve. It generally is defined by the requirement that the counted radioactivity must differ from the background count by three times the standard deviation of the background count. The LRL was targeted at 0.5 to 1 pCi/L for the individual radionuclide concentrations determined in this study.

M

Maximum contaminant level (MCL)

A maximum contaminant level (MCL) is a legally enforceable USEPA drinking-water standard. It sets the maximum permissible level of a contaminant in water that is delivered to users of a public water system. The USEPA sets MCLs as close as is feasible to the maximum contaminant level goal (MCLG), which is the maximum level of a contaminant in drinking water that has an adequate margin of safety and has not been shown or proved to cause adverse health effects. When establishing MCLs, the USEPA takes into account the best available analytical and treatment technologies and cost considerations. The MCL for Ra-226, Ra-228, uranium, and gross alpha-particle activity in community water systems is as follows:
• Sum of Ra-226 and Ra-228, 5 pCi/L
• Uranium, 30 µg/L
• Gross alpha-particle activity (including Ra-226 but excluding radon and uranium), 15 pCi/L

P

pH

pH is a measure of the hydrogen ion activity in a solution. Water is said to be neutral, at a pH of 7. A pH less than 7 is considered to be acidic, and a pH greater than 7 is considered basic or alkaline.

Picocurie per liter

The measurement unit that expresses the amount of radioactivity in water. One picocurie per liter (pCi/L) equals 2.2 radioactive disintegrations per minute per liter of water.

Precision estimate (PE) or combined standard uncertainty (csu)

A calculated measure of uncertainty of the laboratory analysis. Determination of the precision estimate for radionuclides evaluates many sources of error, some that are unique to radionuclides. Because radioactive elements decay randomly at any given instant of time, any measurement of radioactivity has an associated uncertainty (also called “counting error”) independent of, and in addition to, laboratory sources of analytical uncertainty. Uncertainty can be reported in a variety of ways; the most common include (1) the sum of the laboratory and counting uncertainty, known as the“precision estimate” or the “combined standard uncertainty (CSU)” (or less frequently as the “total propagated uncertainty”), and (2) a counting error or counting uncertainty only. There is a 67 percent or 95 percent probability (based on one or two standard deviations, respectively, of the radioactivity count) that the true value of the radionuclide concentration is within the range of the reported measured value plus or minus the precision estimate or uncertainty. The precision estimate or uncertainty terms generally are less than the measured value except when the measured values are low concentrations (near the LRL). The precision estimates associated with the various radionuclide concentrations determined by the alpha spectrometry measurement technique and presented in this report are given as the two standard deviation propagated precision estimates.

R

Radionuclide

An isotope of an element that emits radiation.