Glossary

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HALF LIFE:

The time it takes certain materials such as persistent pesticides to become chemically altered.

HALOGENATED ORGANIC COMPOUND:

A compound containing molecules of chlorine, bromine iodine, and fluorine. Halogenated organic compounds were used in high-voltage electrical transformers because they conducted heat well while being fire resistant and good electrical insulators. Many herbicides, pesticides, and degreasing agents are made from halogenated organic compounds.

HAND AUGER:

Hand held earth boring tool (see AUGER).

HAZARD RANKING SYSTEM (HRS):

The primary screening tool used by EPA to assess the risks posed to human health or the environment by abandoned or uncontrolled hazardous waste sites. Under the HRS, sites are assigned scores on the basis of the toxicity of hazardous substances that are present and the potential that those substances will spread through the air, surface, water, or groundwater, taking into account such factors as the proximity of the substance to nearby populations. Scores are used in determining which sites should be placed on the NPL.

HAZARDOUS AND SOLID WASTE AMENDMENTS (HSWA):

1984 amendments to RCRA which required phasing out land disposal of hazardous waste and added minimum technology requirements.

HAZARDOUS SUBSTANCE:

As defined under CERCLA, a hazardous substance is any material that poses a threat to public health or the environment. The term also refers to hazardous wastes as defined under RCRA. Typical hazardous substances are materials that are toxic, corrosive, ignitable, explosive, or chemically reactive. If a certain quantity of a hazardous substance, as established by EPA, is spilled into the water or otherwise emitted into the environment, the release must be reported. Under the legislation cited above, the term excludes petroleum, crude oil, natural gas, natural gas liquids, or synthetic gas usable for fuel.

HEAVY METAL:

A group of toxic metals including arsenic, chromium, copper, lead, mercury, silver, and zinc. Heavy metals often are present at industrial sites at which operations have included battery recycling and metal plating.

HERBICIDE:

A chemical pesticide designed to control or destroy plants, weeds, or grasses.

HETEROGENEITY:

The quality of having different characteristics in different locations.

HYDRAULIC CONDUCTIVITY:

A coefficient of proportionality describing the rate at which water can move through a permeable medium.  The density and kinematic viscosity of the water must be considered in determining hydraulic conductivity.

HYDRAULIC GRADIENT:

In a closed conduit, it is the slope of the hydraulic grade line. In open channels it is the slope of the water surface. In porous media it is measure of the decrease in head per unit distance in the direction of flow. A hydraulic gradient of 100-percent means a one-foot drop in head in one foot of flow distance. 

HYDRAULIC HEAD:

The sum of the elevation head, the pressure head, and the velocity head at a given point in an aquifer.  Also known as the “total head.”

HYDROCARBON:

Chemical compounds composed only of carbon and hydrogen.

HYDROGEOLOGY:

Scientific consideration relating to geological formations, soil, surface water, and groundwater.

HYDROLOGIC CYCLE:

The cyclic transfer of water vapor from the oceans via evapotranspiration into the atmosphere, from the atmosphere via precipitation to earth, and through runoff into streams, rivers, and lakes, and ultimately back into the oceans.

HYDROLOGY:

The science that deals with the properties, movement, and effects of water found on the earth's surface, in the soil and rocks beneath the surface, and in the atmosphere.