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Acid a substance that has a pH of less than 7, which is neutral. Specifically, an acid has more free hydrogen ions (H+) than hydroxyl ions (OH-).

Acute Short-term, in relation to exposure or effect. In experimental toxicology, "acute" refers to studies of two weeks or less in duration (often less than 24h).

Algae Microscopic plants which contain chlorophyll and float or suspend in water. Excess algae growths can impact tastes and odors to potable water. Their biological activities affect the pH and dissolved oxygen of the water.

Alkaline sometimes water or soils contain an amount of alkali (strongly basic) substances sufficient to raise the pH value above 7.0 and be harmful to the growth of crops.

Alkalinity the capacity of water for neutralizing an acid solution.

Anion a negatively charged ion. Nitrate (NO3-) and chloride (Cl-) are examples of anions.

Anion exchange the chemical process where negative ions of one chemical are preferentially replaced by negative ions of another chemical. In water treatment, the net effect is the removal of an unwanted ion from a water supply. For example, some water treatment facilities or homeowners install an anion exchange system to remove nitrate (NO3-) from their water supplies.

Antibiotic A chemical agent that is produced by one organism that is harmful to another organism.

Antimicrobial agent An agent that kills or inhibits microbial growth.

Assay A test for a particular chemical or effect.

Assimilable Organic Carbon (AOC) The fraction of total organic carbon in water that can be used for microbiological growth. It is measured by a growth potential assay.

Aquifer A natural underground layer, often of sand or gravel, that contains water. 

Base a substance that has a pH of more than 7, which is neutral. A base has less free hydrogen ions (H+) than hydroxyl ions (OH-).

Bioavailability Extent to which a substance to which the body is exposed (by ingestion, inhalation, injection, or skin contact) reaches the systemic circulation, and the rate at which this occurs.

Biochemical Oxygen Demand (BOD) The amount of oxygen (measured in mg/L) that is required for the decomposition of organic matter by single-cell organisms, under test conditions. It is used to measure the amount of organic pollution in wastewater.

Biocide / Disinfectant An agent that is capable of killing microorganisms; however, it may not be 100% effective. In biofilm usage: A chemical used to disinfect or remove biofilm in order to control a detrimental effect of the biofilm.

Biodegradable pollutants Pollutants that are capable of decomposing under natural conditions.

Biofilm Population of various microrganisms, trapped in a layer of slime and excretion products, attached to a surface.

Biological contaminants Living organisms such as viruses, bacteria, fungi, and mammal and bird antigens that can cause harmful health effects to humans.

Biological monitoring (or biomonitoring) The use of a biological entity as a detector and its response as a measure to determine environmental conditions. Toxicity tests and biological surveys are common biomonitoring methods.

Cation a positively charged ion. For example, calcium (Ca+2) and magnesium (Mg+2) are cations.

Cation exchange a process where positively charged ions of one chemical are preferentially replaced by positive ions of another chemical. For example, water softeners replace Ca+2 and Mg+2 ions with sodium (Na+2) ions.

Chemical Oxygen Demand (COD) The amount of oxygen (measured in mg/L) that is consumed in the oxidation of organic and oxidasable inorganic matter, under test conditions. It is used to measure the total amount of organic and inorganic pollution in wastewater. Contrary to BOD, with COD practically all compounds are fully oxidized.

Chemical pollution Introduction of chemical contaminants into a water body.

Chloramines A chemical complex that consists of chlorine and ammonia. It serves as a water disinfectant in public water supplies in place of chlorine because chlorine can combine with organics to form dangerous reaction products. In which forms chloramines exist depends on the physical/ chemical properties of the water source.

Chlorinated hydrocarbons Hydrocarbons that contain chlorine. These include a class of persistent insecticides that accumulate in the aquatic food chain. Among them are DDT, aldrin, dieldrin, heptachlor, chlordane, lindane, endrin, Mirex, hexachloride, and toxaphene.

Chlorinated pesticides 
Commonly used agricultural pesticides. Some people who drink water contaminated with these compounds could experience problems or damage to the eyes, liver, kidneys, or spleen and may have an increased risk of getting cancer.

Chronic Health Effect The possible result of exposure over many years to a drinking water contaminant at levels above its MCL.

Coliform bacteria A group of nonpathogenic organisms (Colon bacilli) usually found in the digestive tract of all warm-blooded animals and humans. The presence of coliform bacteria in water is an indicator of possible pollution by fecal material and the presence of pathogenic bacteria that can cause diseases such as intestinal infections, dysentery, hepatitis, typhoid fever and cholera. Generally reported as colonies or colony forming units (cfu) per 100 milliliters (ml) of sample. (see fecal coliform)

Concentration The amount of a given chemical or pollutant in a particular volume or weight of air, water, soil, or other medium - often expressed as milligrams per liter (mg/l) or parts per million (ppm).

Colony A population of cells growing on solid medium (plates) arising from a single colony forming unit.

Colony Forming Unit (CFU) The minimum number of separable cells on the surface of or in semi-solid agar medium which gives rise to a visible colony of progeny on the order of tens of millions of cells in number. CFUs may consist of pairs, chains and clusters as well as single cells.

Chronic Health Effect The possible result of exposure over many years to a drinking water contaminant at levels above its MCL.

Coliform A group of related bacteria whose presence in drinking water may indicate contamination by disease-causing microorganisms.

Contaminant An undesirable substance not normally present, or an usually high concentration of a naturally-occurring substance, in water, soil, or other environmental medium. In more restricted usage, a substance in water that may be harmful to human health.

Desalination The removal of salts from saline water to provide freshwater. This method is becoming a more popular way of providing freshwater to populations.

Disinfection By-Product (DBP) A chemical by-product of the disinfection process. Disinfection by-products are formed by the reaction of the disinfectant, natural organic matter, and the bromide ion (Br–). Some disinfection by-products are formed through halogen (e.g., chlorine or bromine) substitution reactions; i.e., halogen-substituted by-products are produced. Other disinfection by-products are oxidation by-products of natural organic matter (e.g., aldehydes—RCHO). Concentrations are typically in the microgram-per-litre or nanogramper-litre range.

Dissolved oxygen The amount of oxygen dissolved in water at a certain time, expressed in ppm mg/L.

Environmental monitoring Continuous or repeated measurement of agents in the environment to evaluate environmental exposure and possible damage to living organisms. Measurements obtained are compared with appropriate reference values based on knowledge of the probable relationships between ambient exposure and resultant adverse effects.

Escherichia coli (E. coli) A gram-negative, facultatively anaerobic, nonspore-forming bacillus commonly 
found in the intestinal tracts of humans and other warm-blooded animals. In sanitary bacteriology, Escherichia 
coli is considered the primary indicator of recent fecal pollution.

Effluent Water that flows from a sewage treatment plant after it has been treated.

Fecal coliform A portion of the coliform bacteria group originating in the intestinal tract of warm-blooded animals that passes into the environment in feces. Fecal coliform often is used as an indicator of the bacteriological safety of a water supply. Generally reported as colonies or colony forming units (cfu) per 100 milliliters (ml) of sample.

Finished Water Water that has been treated and is ready to be delivered to customers.

Flocculation A step in water filtration in which alum is added to cause particles to clump together.

 Granulated activated carbon, an inexpensive treatment technology by which water passes through grains of carbon. Compared with more expensive solid-block models, GAC filters sometimes affect mostly the taste and smell of water.

Giardia lamblia A microorganism frequently found in rivers and lakes, which, if not treated properly, may cause diarrhea, fatigue, and cramps after ingestion.

Groundwater Water that can be found in the saturated zone of the soil; a zone that consists merely of water. It slowly moves from places with high elevation and pressure to places with low elevation and pressure, such as rivers and lakes.
Haloacetic Acid (HAA) (CX3COOH, where X = Cl, Br, H in various combinations) A class of disinfection by-products formed primarily during the chlorination of water containing natural organic matter. When bromide (Br–) is present, a total of nine chlorine-, bromine-and-chlorine-, or bromine-substituted species may be formed. Trihalomethanes and haloacetic acids are the two most prevalent classes of by-products formed during chlorination; and subject to regulation under the Disinfectant/Disinfection by-products rule

Hardness (water) A measure of the amount of dissolved calcium (Ca) and magnesium (Mg) in water. Hardness is often reported as ppm or milligrams per liter (mg/l) of CaCO3. If hardness is reported in grains per gallon, one grain per gallon of hardness equals 17.1 ppm of hardness as CaCO3. Hard water can produce scale deposits in pipes and water heaters. Additionally, compared to soft water, hard water requires that additional soaps and detergents be used to produce foam or lather. Hardness scale: Soft - 0-60 mg/l; Moderately Hard - 61-120 mg/l; Hard - 121-180 mg/l; and Very Hard- over 180 mg/l.

Heavy metals Metals that have a density of 5.0 or higher and a high elemental weight. Most are toxic to humans, even in low concentrations.

Heterotrophic bacteria Those bacteria that require an organic carbon source for growth; i.e. derive energy and carbon from organic compounds.

Heterotrophic Plate Count Bacteria (HPC) Those bacteria that will grow on nonselective heterotrophic medium plates.

Indicator organisms Microrganisms, such as coliforms, whose presence is indicative for pollution or for the presence more harmful microrganisms.

Infiltration The downward entry of water through the soil surface.

Influent The stream of water that enters any system or treatment unit.

Inorganic Contaminants Mineral-based compounds such as metals, nitrates, and asbestos. These contaminants are naturally-occurring in some water, but can also get into water through farming, chemical manufacturing, and other human activities.

Leachate Water that contains solute substances, so that it contains certain substances in solution after percolation through a filter or soil.

Membrane A thin barrier that allows some compounds or liquids to pass through, and troubles others. It is a semi-permeable skin of which the pass-through is determined by size or special nature of the particles. Membranes are commonly used to separate substances.

Maximum Contaminant Level (MCL) The highest level of a contaminant that EPA allows in drinking water. MCLs ensure that drinking water does not pose either a short-term or long-term health risk. EPA sets MCLs at levels that are economically and technologically feasible.

Maximum Contaminant Level Goal (MCLG) The level of a contaminant at which there would be no risk to human health. This goal is not always economically or technologically feasible, and the goal is not legally enforceable

Metabolism Sum total of all physical and chemical processes that take place within an organism; in a narrower sense, the physical and chemical changes that take place in a given substance within an organism. Metabolism includes the uptake and distribution within the body of chemical compounds, the changes (biotransformation) undergone by such substances, and the elimination of the compounds and of their metabolites from the organism.

Microbial growth The multiplication of microrganisms such as bacteria, algae, diatoms, plankton, and fungi.

Microorganisms Tiny living organisms that can be seen only with the aid of a microscope. Some microorganisms can cause acute health problems when consumed in drinking water. Also known as microbes.

Monitoring Testing that water systems must perform to detect and measure contaminants. A water system that does not follow EPA's monitoring methodology or schedule is in violation, and may be subject to legal action.

Mineral Water Contains large amounts of dissolved minerals such as calcium, sodium, magnesium, and iron. Some tap waters contain as many or more minerals than some commercial mineral waters. There is no scientific evidence that either high or low mineral content water is beneficial to humans.

Nephelometric turbidity unit (NTU) Unit of measure for the turbidity of water. Essentially, a measure of the cloudiness of water as measured by a nephelometer. Turbidity is based on the amount of light that is reflected off particles in the water.

Nutrient (1) An element or compound essential to life, including carbon, oxygen, nitrogen, phosphorus, and many others; (2) as a pollutant, any element or compound, such as phosphorus or nitrogen that in excessive amounts contributes to abnormally high organic growth in aquatic ecosystems.

Organic Contaminants Carbon-based chemicals, such as solvents and pesticides, which can get into water through runoff from cropland or discharge from factories. EPA has set legal limits on 56 organic contaminants.

Osmosis Water molecules passing through membranes naturally, to the side with the highest concentration of dissolved impurities. the movement of water molecules through a thin membrane. The osmosis process occurs in our bodies and is also one method of desalinating saline water.

Parts per billion Expressed as ppb; a unit of concentration equivalent to the µg/l.

Parts per million Expressed as ppm; a measure of concentration. One ppm is one unit weight of solute per million unit weights of solution. In water analysis the ppm is equivalent to mg/l.

Pasteurisation The elimination of microrganisms by heat applies for a certain period of time.

Pathogen A disease-causing organism.

Persistence Attribute of a substance that describes the length of time that the substance remains in a particular environment before it is physically removed or chemically or biologically transformed.

pH The value that determines if a substance is acid, neutral or basic, calculated from the number of hydrogen ions present. It is measured on a scale from 0 to 14, on which 7 means the substance is neutral. pH values below 7 indicate that a substance is acidic and pH values above 7 indicate that it is basic.

Pollutant A contaminant at a concentration high enough to endanger the life of organisms.

Polychlorinated biphenyls (PCBs) A group of synthetic, toxic industrial chemical compounds once used in making paint and electrical transformers, which are chemically inert and not biodegradable. PCBs were frequently found in industrial wastes, and subsequently found their way into surface and ground waters. As a result of their persistence, they tend to accumulate in the environment. In terms of streams and rivers, PCBs are drawn to sediment, to which they attach and can remain virtually indefinitely. Although virtually banned in 1979 with the passage of the Toxic Substances Control Act, they continue to appear in the flesh of fish and other animals.

POP's Persistent Organic Pollutants, complex compounds that are very persistent and difficultly biologically degradable.

Potable water Water that is safe for drinking and cooking.

Process water Water that serves in any level of the manufacturing process of certain products.

Protozoans are organisms, such as cryptosporidium, that have a life-cycle with several stages, some of which allow them to pass through a chlorine disinfection process unharmed.

Qualitative water assessment Analyses of water used to discribe the visible or aestetic charcteristics of water.

Quantitative water assessment Use of analyses of water properties and concentrations of compounds and contaminants in order to define water quality.

Reclaimed wastewater Treated wastewater that can be used for beneficial purposes, such as irrigating certain plants.

Reverse osmosis (Desalination) The process of removing salts from water using a membrane. With reverse osmosis, the product water passes through a fine membrane that the salts are unable to pass through, while the salt waste (brine) is removed and disposed.

Risk assessment Identification and quantification of the hazard resulting from a specific use or occurrence of a chemical or physical agent. Risk assessment considers any possible harmful effects on individual people or on society of using the chemical or physical agent in the amount and manner proposed and via all possible routes of exposure. Quantification ideally requires the establishment of dose-effect and dose-response relationships in likely target individuals and populations.

Run-Off The part of precipitation water that runs off the land into streams or other surfacewater.

Safe water Water that does not contain harmful bacteria, toxic materials, or chemicals, and is considered safe for drinking.

Saline water Water that contains significant amounts of dissolved solids.

Source Water Water in its natural state, prior to any treatment for drinking.

Surface water Water that is on the Earth's surface, such as in a stream, river, lake, or reservoir.

Synergism The combined action of several chemicals, which produces a total effect greater than the effects of the chemicals separately.

Total chlorine residual The total amount of chlorine residual present after a given contact time in a water sample, regardless of the type of chlorine. See also residual chlorine; total chlorine.

Total Organic Carbon (TOC) A measure of the concentration of organic carbon in water, determined by oxidation of the organic matter into carbon dioxide (CO2). TOC includes all the carbon atoms covalently bonded in organic molecules. Most of the organic carbon in drinking water supplies is dissolved organic carbon, with the remainder referred to as particulate organic carbon. In natural waters, total organic carbon is composed primarily of nonspecific humic materials. Total organic carbon is used as a surrogate measurement for disinfection by-product precursors, although only a small fraction of the organic carbon will react to form these by-products.

Toxicity Adverse effects of a substance on a living organism, defined with reference to the quantity of substance administered or absorbed, the way in which the substance is administered (inhalation, ingestion, topical application, injection) and distributed in time (single or repeated doses), the type and severity of injury, the time needed to produce the injury, the nature of the organism(s) affected, and other relevant conditions.

Toxicity test Experimental study of the adverse effects of exposure of a living organism to a substance for a defined duration under defined conditions.

Toxic water pollutants Compounds that are not naturally found in water at the given concentrations and that cause death, disease, or birth defects in organisms that ingest or absorb them.

Toxin Any of various substances produced by certain plant and animal cells, including bacterial toxins, phytotoxins, and zootoxins

THM Trihalomethanes. Toxic chemical substances that consist of a methane molecule and one of the halogen elements fluorine, bromine, chlorine and iodine attached to three positions of the molecule. They usually have carcinogenic properties.

Trace metals
 Trace metals may come from industrial contamination or natural deposits. They can cause increased cancer risk, damage to organs and changes in blood chemistry.

Triazines are herbicides, or weed killers, that are used on a variety of U.S. crops, including grains and citrus. Triazines are currently present in both ground and surface waters. They're banned in Europe.

Turbidity The cloudy appearance of water caused by the presence of tiny particles. High levels of turbidity may interfere with proper water treatment and monitoring.

UV Ultra Violet. Radiation that has a wavelength shorter than visible light. It is often used to kill bacteria and destroy ozone.

VOC Volatile Organic Compound. Synthetic organic compounds which easily vaporize and are often carcinogenic.

Wastewater The spent or used water from a home, community, farm, or industry that contains dissolved or suspended matter.

Water monitoring The process of constant control of a body of water by means of sampling and analyses.

Water pollution The presence in water of enough harmful or objectionable material to damage water quality.

Water quality The condition of water with respect to the amount of impurities in it.

Watershed--the land area that drains water to a particular stream, river, or lake. It is a land feature that can be identified by tracing a line along the highest elevations between two areas on a map, often a ridge.

Water solubility The maximum possible concentration of a chemical compound dissolved in water.