Water Glossary: H

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	A characteristic of water determined by the levels of calcium and magnesium.
heavy metals	Metallic elements, e.g., cadmium, chromium, copper, lead, mercury, nickel, and zinc, that are used to manufacture products; they are present in some industrial, municipal, and urban runoff.
heterotroph	An organism that is not capable of making its own food
heterotrophic plate count (HPC)	A bacterial enumeration procedure used to estimate bacterial density in an environmental sample, generally water. Other names for the procedure [within the water industry] include total plate count, standard plate count, plate count, and aerobic plate count.*
hydrocarbons	Substances containing only hydrogen and carbon, such as methane, alkane, or ethylene
hydroelectric plant	A power plant that produces electricity from the power of rushing water turning turbine-generators.
hydrogen sulfide (H2S) gas	A flammable, toxic, colorless gas with an offensive odor (similar to rotten eggs)
hydrology	the scientific study of the behavior of water in the atmosphere, on the Earth's surface and underground.
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