Contaminated and Hazardous Waste Site Management

Glossary H

Half-life: Radioactively, half-life is the time required for half of a given quantity of material to decay. Chemically, it is the time required for half of a given quantity of material to undergo a chemical reaction.

Hardness: When hard water is used with soap it will form an insoluble residue and hard water will form a scale in utensils in which the water has been allowed to evaporate. Hardness is mainly caused by calcium and magnesium ions. Hardness is generally expressed in mg/L calcium carbonate (Ca CO3).

Hazardous Waste: By-products of society that can pose a substantial present or potential hazard to human health and the environment when improperly managed. It possesses at least one of four characteristics (ignitabiliy, corrosivity, reactivity, or toxicity), or appears on special EPA lists.

Headspace: The empty volume in a container between the cap and the water level of the sample.

Headwaters: The source and upper part of a large stream or river, including the drainage basin.

Heat Capacity: The quantity of energy that must be supplied to raise the temperature of a substance. For contaminated soils heat capacity is the quantity of energy that must be added to the soil to volatilize organic components. The typical range of heat capacity of soils is relatively narrow, therefore variations are not likely to have a major impact on application of a thermal desorption process.

Heavy Metals: Metallic elements, some of which are required in trace concentrations for plant and/or animal nutrition, but which become toxic at higher concentrations (e.g. lead, mercury).

Heavy Oils: Petroleum products dominantly composed of large molecular weight, structurally-complex compounds.

Henry's Law: An empirical law which states that the quantity of gas/vapour dissolved in water is proportional to the pressure of the gas/vapour in contact with the water at a specified temperature, i.e. as gas pressure increases, gas concentration in the water increases.

Henry's Law Constant: The ratio of the concentration of a compound in air (or vapor) to the concentration of the compound in water under equilibrium conditions. Henry's Law Constants are temperature dependent.

Heterogeneity: The part of the subsurface that is different in some property (hydraulic conductivity, for example).

Heterogeneous: Composed of non-uniform constituents whose material properties vary in space. All geological material is heterogeneous, but the property of interest (porosity, for example) may be sufficiently uniform for the material to be treated as homogeneous in terms of that property.

Heterotrophic: Designating or typical of organisms that derive carbon for the manufacture of cell mass from organic matter.

Holding Time: The elapsed time expressed in days from the date of receipt of the sample by the contractor until the date of its analysis. Holding time = (sample analysis date - sample receipt date)

Homogeneity: Characteristic of a medium in which material properties are identical throughout. Although heterogeneity, or non-uniformity, is the characteristic of most aquifers, assumed homogeneity, with some other additional assumptions, allows use of analytical models as a valuable tool for approximate analyses of groundwater movement.

Homogeneous: Composed of uniform constituents throughout. That is, having material properties (e.g., hydraulic conductivity) which do not vary in space.

Hydraulic Conductivity: A measure of the ability of a fluid to flow through a porous medium determined by the size and shape of the pore spaces in the medium and their degree of interconnection and also by the viscosity of the fluid. Hydraulic conductivity can be expressed as the volume of fluid that will move in unit time under a unit hydraulic gradient through a unit area measured at right angles to the direction of flow.

Hydraulic Gradient: The slope of the groundwater level or water table.

Hydraulic Head: Height above a datum plane (such as mean sea level) of the column of water that can be supported by the hydraulic pressure at a given point in a groundwater system. Equal to the distance between the water level in a well and the datum plane.

Hydrocarbon: Chemical compounds composed only of carbon and hydrogen.

Hydrogen Peroxide: H2O2. Hydrogen peroxide is used to increase the dissolved oxygen content of groundwater to stimulate aerobic biodegradation of organic contaminants. Hydrogen peroxide is infinitely soluble in water, but rapidly dissociates to form a molecule of water (H2O) and one-half molecule of oxygen (O). Dissolved oxygen concentrations of greater than 1,000 mg/L are possible using hydrogen peroxide, but high levels of D.O. can be toxic to microorganisms.

Hydrogeological Model: A representation, often simplified and perhaps conceptual, of the hydrogeological flow system. The aspects important for the site are emphasized. See also model.

Hydrogeology: The geology of groundwater, with particular emphasis on the chemistry and movement of water.

Hydrograph: A graphical plot of changes in elevation of water or flow of water with respect to time.

Hydrologic Cycle: The continued circulation of water between the ocean, atmosphere and land is called the hydrologic cycle.

Hydrophilic: Having an affinity for water ("water-loving"), or capable of dissolving in water; soluble or miscible in water.

Hydrophobic: Tending not to combine with water, or incapable of dissolving in water; insoluble or immiscible in water ("water-fearing"). A property exhibited by non-polar organic compounds, including the petroleum hydrocarbons.

Hypoxic: A condition of low oxygen concentration, below that considered aerobic.

Hydrostratigraphic Unit: A formation, part of a formation, or a group of formations which have similar hydrologic characteristics.

Hysteresis: Phenomenon in which properties such as capillary pressure or relative permeability may differ depending on whether a fluid-fluid interface is advancing (imbibition) or receding (drainage).