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H
hardpan - (3) a hardened soil layer in the lower A horizon
or the B horizon caused by cementation of soil particles with organic
matter or with materials such as silica or calcium carbonate.
hazardous substance - (2) CERCLA hazardous substances,
pollutants, and contaminants as defined in CERCLA Sections 101(14)
and 101(33).
headframe - (2) the vertical steel or timber frame at the
top of a shaft, which carries the sheave or pulley for the hoist.
heap - (3) a large, engineered pile of ore over which chemical
agents such as cyanide are sprinkled in in the process of extracting
metals by heap leaching.
heap leaching - (3) a low-cost technique for extracting
metals from ore by percolating leaching solutions through heaps
of ore placed on impervious heap leach pads. This method
is usually used on low-grade ores. (4) an extraction process in
which stockpiled ore is leached to remove target metals. Leaching
solutions, generally weak acids or alkaline cyanide, are percolated
through heaps of ore. Leachate is collected and metals contained
in the leachate are extracted chemically or electrochemically. Typically,
the particle size of the ore to be leached is reduced to increase
surface exposure of metal-containing minerals. Despite the reduced
particle size after leaching ceases, the heap typically has many
properties in common with a waste rock dump. See also cyanidation.
heap leach pad - (3) a large, impermeable foundation or
pad used as a base for the placement of ore during heap leaching
heavy metal - (2) principally the metals zinc, copper, cobalt
and lead; however, may include one or more of the following metals:
bismuth, cadmium, gold, indium, iron, manganese, mercury, nickel,
palladium, silver, thallium, and tin (often included, though not
a metal).
highwall - (2)(3) (a) the unexcavated face of exposed overburden
and coal or ore in an open-cast mine or (b) the face or bank on
the uphill side of contour strip mine excavation.
horizon - (3) See also soil horizon.
humidity test cell - (2) a kinetic test procedure used primarily
to measure rates of acid generation and neutralization
in sulfide-bearing rock.
hydrated lime - (4) calcium hydroxide [Ca(OH)2]. Produced
from calcium oxide (CaO) or quick lime. Used as a neutralizing
agent. See also lime and quick lime.
hydration - (4) the incorporation or presence of water within
a chemical structure. See also anhydrous.
hydraulic conductivity - (3) The property of a soil that
governs the rate of ground water flow within it. The hydraulic conductivity
of a particular soil is defined by the size, abundance, and geometry
of its open pores; the rate of ground water flow increases with
increasing hydraulic conductivity. Fine-grained soils such as clay
and silt have very small pores and have much lower hydraulic conductivities
than coarse-grained soils such as sand and gravel. (4) a measure
of the ability of a fluid to move through the interconnected void
spaces in a sediment or a rock. Flow through a porous medium in
response to a unit potential gradient. Hydraulic conductivity depends
on both permeability and properties of the fluid such as viscosity
and density.
hydraulic gradient - (3) the slope of the free surface of
water in a stream flowing in an open channel.
hydrocarbons - (3) any of a vast family of compounds containing
carbon and hydrogen in various combinations, found especially in
fossil fuels. Hydrocarbons in the atmosphere resulting from evaporation
and incomplete combustion may be a source of air pollution.
hydrologic balance - (3) an accounting of water inflow to,
outflow from, and storage in a hydrologic unit such as a drainage
basin, aquifer, lake or reservoir; the relationship between evaporation,
precipitation, runoff, and change in water storage.
hydrolysis - (4) a chemical reaction of a compound or ion
with water in which water is split into H+ and OH-.
hydrostatic pressure - (3) the pressure exerted by the water
at any given point in a water body at rest. The hydrostatic pressure
of ground water is generally due to the weight of water at higher
levels in the zone of saturation.
hydrothermal - (4) heated aqueous-rich solutions, and the
processes (hydrothermal alteration) in which they are involved.
hyporheic zone - (3) the area where water in a stream channel
has moved back into the subsurface sediments. The hyporheic zone
may occur under or next to the streambed.
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