Underground Storage Tank (UST) Glossary

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abiotic: not biotic; not formed by biologic processes.

absolute viscosity: A measure of a fluid’s resistance to tangential or shear stress. Also referred to as dynamic viscosity; see also viscosity. Units are usually given in centipoise.

absorption: the penetration of atoms, ions, or molecules into the bulk mass of a substance.

Actinomycetes: any of numerous, generally filamentous, and often pathogenic, microorganisms resembling both bacteria and fungi.

adsorption: the retention of atoms, ions, or molecules onto the surface of another substance.

advection: the process of transfer of fluids
(vapors or liquid) through a geologic formation in response to a
pressure gradient that may be caused by changes in barometric pressure,
water table levels, wind fluctuations, or infiltration.

aeration: the process of bringing air into contact
with a liquid (typically water), usually by bubbling air through the
liquid, spraying the liquid into the air, allowing the liquid to
cascade down a waterfall, or by mechanical agitation. Aeration serves to
(1) strip dissolved gases from solution, and/or (2) oxygenate the
liquid. The rate at which a gas transfers into solution can be
described by Fick’s First Law.

aerobic: able to live, grow, or take place only when free oxygen is present.

afterburner: an off-gas posttreatment unit for
control of organic compounds by thermal oxidation. A typical
afterburner is a refractory-lined shell providing enough residence time
at a sufficiently high temperature to destroy organic compounds in the
off-gas stream.

aggregate: coarse mineral material (e.g., sand,
gravel) that is mixed with either cement to form concrete or tarry
hydrocarbons to form asphalt.

algae: chiefly aquatic, eucaryotic one-celled or
multicellular plants without true stems, roots and leaves, that are
typically autotrophic, photosynthetic, and contain chlorophyll. Algae
are not typically found in groundwater.

aliphatic: of or pertaining to a broad category
of carbon compounds distinguished by a straight, or branched, open
chain arrangement of the constituent carbon atoms. The carbon-carbon
bonds may be either saturated or unsaturated. Alkanes, alkenes, and
alkynes are aliphatic hydrocarbons.

alkanes: the homologous group of linear saturated
aliphatic hydrocarbons having the general formula C(n)H(2n+2). Alkanes
can be straight chains, branched chains, or ring structures. Also
referred to as paraffins.

alkenes: the group of unsaturated hydrocarbons
having the general formula C(n)H(2n) and characterized by being highly
chemically reactive. Also referred to as olefins.

alkynes: the group of unsaturated hydrocarbons with a triple Carbon-Carbon bond having the general formula C(n)H(2n-2).

ambient: surrounding; the surrounding environment and conditions.

anaerobic: able to live, grow, or take place where free oxygen is not present.

analog: in chemistry, a structural derivative of a parent compound.

anisotropic: the condition in which hydraulic properties of an aquifer are not equal when measured in all directions.

anoxic: total deprivation of oxygen.

aqueous solubility: the extent to which a
compound will dissolve in water. The log of solubility is generally
inversely related to molecular weight.

aquifer: a geologic formation capable of transmitting significant quantities of groundwater under normal hydraulic gradients.

aquitard: a geologic formation that may contain
groundwater but is not capable of transmitting significant quantities
of groundwater under normal hydraulic gradients. In some situations
aquitards may function as confining beds.

aromatic: of or relating to organic compounds
that resemble benzene in chemical behavior. These compounds are
unsaturated and characterized by containing at least one 6-carbon
benzene ring.

asymptote: a line that is considered to be the
limit to a curve. As the curve approaches the asymptote, the distance
separating the curve and the asymptote continues to decrease, but the
curve never actually intersects the asymptote.

attenuation: the reduction or lessening in amount
(e.g., a reduction in the amount of contaminants in a plume as it
migrates away from the source).

Atterberg limits: the moisture contents which define a soil’s liquid limit, plastic limit, and sticky limit.

auger: a tool for drilling/boring into
unconsolidated earth materials (soil) consisting of a spiral blade
wound around a central stem or shaft that is commonly hollow
(hollow-stem auger). Augers commonly are available in flights
(sections) that are connected together to advance the depth of the

autoignition temperature: the temperature at
which a substance will spontaneously ignite. Autoignition temperature
is an indicator of thermal stability for petroleum hydrocarbons.

autotrophic: designating or typical of organisms
that derive carbon for the manufacture of cell mass from inorganic
carbon (carbon dioxide).

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bacteria: unicellular microorganisms that exist
either as free-living organisms or as parasites and have a broad range
of biochemical, and often pathogenic, properties. Bacteria can be
grouped by form into five general categories: cocci (spherical), bacilli
(rod-shaped), vibrio (curved rod-shaped), spirilla (spiral), and
filamentous (thread-like).

baghouse: a dust-collection chamber containing
numerous permeable fabric filters through which the exhaust gases pass.
Finer particulates entrained in the exhaust gas stream are collected
in the filters for subsequent treatment/disposal.

ball valve: a valve regulated by the position of a free-floating ball that moves in response to fluid or mechanical pressure.

Bentonite: a colloidal clay, largely made up of
the mineral sodium montmorillonite, a hydrated aluminum silicate.
Because of its expansive property, bentonite is commonly used to
provide a tight seal around a well casing.

berm: a sloped wall or embankment (typically
constructed of earth, hay bales, or timber framing) used to prevent
inflow or outflow of material into/from an area.

bioassay: a method used to determine the toxicity
of specific chemical contaminants. A number of individuals of a
sensitive species are placed in water containing specific
concentrations of the contaminant for a specified period of time.

bioaugmentation: the introduction of cultured
microorganisms into the subsurface environment for the purpose of
enhancing bioremediation of organic contaminants. Generally the
microorganisms are selected for their ability to degrade the organic
compounds present at the remediation site. The culture can be either an
isolated genus or a mix of more than one genera. Nutrients are
usually also blended with the aqueous solution containing the microbes
to serve as a carrier and dispersant. The liquid is introduced into
the subsurface under natural conditions (gravity fed) or injected under

bioavailability: the availability of a compound
for biodegradation, influenced by the compound’s location relative to
microorganisms and its ability to dissolve in water.

biocide: a substance capable of destroying (killing) living organisms.

biodegradability (or biodegradation potential):
the relative ease with which petroleum hydrocarbons will degrade as the
result of biological metabolism. Although virtually all petroleum
hydrocarbons are biodegradable, biodegradability is highly variable and
dependent somewhat on the type of hydrocarbon. In general,
biodegradability increases with increasing solubility; solubility is
inversely proportional to molecular weight.

biodegradation: a process by which microbial
organisms transform or alter (through metabolic or enzymatic action)
the structure of chemicals introduced into the environment.

biomass: the amount of living matter in a given area or volume.

boiling point: the temperature at which a
component’s vapor pressure equals atmospheric pressure. Boiling point
is a relative indicator of volatility and generally increases with
increasing molecular weight.

Btu: “British Thermal Unit”; the quantity of heat
required to raise the temperature of one pound of water one degree
Fahrenheit at 39 degrees F; used as the standard for the comparison of
heating values of fuels.

bubble radius: the maximum radial distance away
from a biosparging well where the effects of sparging are observable.
Analogous to radius of influence of an air sparging well.

bulk density: the amount of mass of a soil per
unit volume of soil; where mass is measured after all water has been
extracted and total volume includes the volume of the soil itself and
the volume of air space (voids) between the soil grains.

butterfly valve: a shut-off valve usually found
in larger pipe sizes (4 inches or greater). This type of valve can be
used for non-critical flow control.

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capillary fringe: the zone of a porous medium
above the water table within which the porous medium is saturated by
water under pressure that is less than atmospheric pressure.

capillary suction: the process whereby water
rises above the water table into the void spaces of a soil due to
tension between the water and soil particles.

catalytic oxidizer: an off-gas posttreatment unit
for control of organic compounds. Gas enters the unit and passes over a
support material coated with a catalyst (commonly a noble metal such
as platinum or rhodium) that promotes oxidation of the organics.
Catalytic oxidizers can also be very effective in controlling odors.
High moisture content and the presence of chlorine or sulfur compounds
can adversely affect the performance of the catalytic oxidizer.

chemotrophs: organisms that obtain energy from oxidation or reduction of inorganic or organic matter.

coefficient of permeability: see hydraulic conductivity.

cometabolism: the simultaneous metabolism of two
compounds, in which the degradation of the second compound (the
secondary substrate) depends on the presence of the first compound (the
primary substrate). For example, in the process of degrading methane,
some bacteria can degrade hazardous chlorinated solvents that they
would otherwise be unable to attack.

complexation: a reaction in which a metal ion and
one or more anionic ligands chemically bond. Complexes often prevent
the precipitation of metals.

condensate: the liquid that separates from a vapor during condensation.

conductivity: a coefficient of proportionality
describing the rate at which a fluid (e.g., water or gas) can move
through a permeable medium. Conductivity is a function of both the
intrinsic permeability of the porous medium and the kinematic viscosity
of the fluid which flows through it.

cone of depression: the area around a discharging
well where the hydraulic head (potentiometric surface) in the aquifer
has been lowered by pumping. In an unconfined aquifer, the cone of
depression is a cone-shaped depression in the water table where the
media has actually been dewatered.

confined aquifer: a fully saturated aquifer
overlain by a confining layer. The potentiometric surface (hydraulic
head) of the water in a confined aquifer is at an elevation that is
equal to or higher than the base of the overlying confining layer.
Discharging wells in a confined aquifer lower the potentiometric surface
which forms a cone of depression, but the saturated media is not

confining layer: a geologic formation characterized by low permeability that inhibits the flow of water (see also aquitard).

conservative: (a) in the case of a contaminant,
one that does not degrade and the movement of which is not retarded; is
unreactive. (b) in the case of an assumption, one that leads to a
worst-case scenario, one that is most protective of human health and the

constituent: an essential part or component of a
system or group (e.g., an ingredient of a chemical mixture). For
instance, benzene is one constituent of gasoline.

cyclone: a type of separator for removal of
larger particles from an exhaust gas stream. Gas laden with
particulates enters the cyclone and is directed to flow in a spiral
causing the entrained particulates to fall out and collect at the
bottom. The gas exits near the top of the cyclone.

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Darcy’s Law: an empirical relationship between
hydraulic gradient and the viscous flow of water in the saturated zone
of a porous medium under conditions of laminar flow. The flux of vapors
through the voids of the vadose zone can be related to a pressure
gradient through the air permeability by Darcy’s Law.

degradation potential: the degree to which a substance is likely to be reduced to a simpler form by bacterial activity.

denitrification: bacterial reduction of nitrite to gaseous nitrogen under anaerobic conditions.

density: the amount of mass per unit volume.

diffusion: the process by which molecules in a
single phase equilibrate to a zero concentration gradient by random
molecular motion (Brownian motion). The flux of molecules is from
regions of high concentration to low concentration and is governed by
Fick’s Second Law.

dispersion: the process by which a substance or chemical spreads and dilutes in flowing groundwater or soil gas.

dissolution: dissolving of a substance in a liquid solvent (e.g., water).

downgradient: in the direction of decreasing static head (potential).

drawdown: lowering the water table due to withdrawal of groundwater as from a well.

dynamic viscosity: a measure of a fluid’s resistance to tangential or shear stress.

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effective porosity: the amount of interconnected
pore space in a soil or rock through which fluids can pass, expressed
as a percent of bulk volume. Some of the voids and pores in a rock or
soil will be filled with static fluid or other material, so that
effective porosity is always less than total porosity.

effluent: something that flows out, especially a liquid or gaseous waste stream.

electron acceptor: a chemical entity that accepts
electrons transferred to it from another compound. It is an oxidizing
agent that, by virtue of its accepting electrons, is itself reduced in
the process. See also terminal electron acceptor and

electron donor: a chemical entity that donates
electrons to another compound. It is a reducing agent that, by virtue
of its donating electrons, is itself oxidized in the process. (see
also electron acceptor and oxidation-reduction.)

empirical: relying upon or gained from experiment or observation.

entrained: particulates or vapor transported along with flowing gas or liquid.

enzyme: (a) any of numerous proteins or
conjugated proteins produced by living organisms and functioning as
biochemical catalysts. (b) a protein that a living organism uses in
the process of degrading a specific compound. The protein serves as a
catalyst in the compound’s biochemical transformation.

eucaryotes: an organism having one or more cells with well-defined nuclei.

evaporation: the process by which a liquid enters the vapor (gas) phase.

ex situ: moved from its original place; excavated; removed or recovered from the subsurface.

extraction well: a well employed to extract
fluids (either water, gas, free product, or a combination of these)
from the subsurface. Extraction is usually accomplished by either a
pump located within the well or suction created by a vacuum pump at the
ground surface.

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facultative anaerobes: microorganisms that can
grow in either the presence or the absence of molecular oxygen. In the
absence of oxygen these microorganism can utilize another compound
(e.g., sulfate or nitrate) as a terminal electron acceptor.

facultative: used to describe organisms that are
able to grow in either the presence or absence of a specific
environmental factor (e.g., oxygen). See also facultative anaerobe.

Fick’s First Law: an equation describing the rate
at which a gas transfers into solution. The change in concentration of
gas in solution is proportional to the product of an overall mass
transfer coefficient and the concentration gradient.

Fick’s Second Law: an equation relating the
change of concentration with time due to diffusion to the change in
concentration gradient with distance from the source of concentration.

field capacity: the maximum amount of water that a
soil can retain after excess water from saturated conditions has been
drained by the force of gravity.

flow tube: a calibrated flow measuring device made for a specific range of flow velocities and fluids.

flux: the rate of movement of mass through a unit
cross-sectional area per unit time in response to a concentration
gradient or some advective force.

free product: a petroleum hydrocarbon in the liquid (“free” or non-aqueous) phase (see also non-aqueous phase liquid, NAPL).

friable: easily crumbled, not cohesive or sticky.

fungi: aerobic, multicellular, nonphotosynthetic,
heterotrophic microorganisms. The fungi include mushrooms, yeast,
molds, and smuts. Most fungi are saprophytes, obtaining their
nourishment from dead organic matter. Along with bacteria, fungi are the
principal organisms responsible for the decomposition of carbon in the
biosphere. Fungi have two ecological advantages over bacteria: (1)
they can grow in low moisture areas, and (2) they can grow in low pH

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gate valve: a valve regulated by the position of a circular plate.

globe valve: a type of stemmed valve that is used
for flow control. The valve has a globe shaped plug that rises or
falls vertically when the stem handwheel is rotated.

gradient: the rate of change in value of a
physical or chemical parameter per unit change in position. For
example, hydraulic gradient is equal to the difference in head measured
at two points (usually wells) divided by the distance separating the
two points. The dimensions of head and distance are both lengths,
therefore the gradient is expressed as a dimensionless ratio (L/L).

groundwater: the water contained in the pore spaces of saturated geologic media.

grout: a watery mixture of cement (and commonly
bentonite) without aggregate that is used to seal the annular space
around well casings to prevent infiltration of water or
short-circuiting of vapor flow.

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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

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: the relationship between the partial
pressure of a compound and the equilibrium concentration in the liquid
through a proportionality constant known as the Henry’s law constant.

heterogeneous: varying in structure or composition at different locations in space.

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

homogeneous: uniform in structure or composition at all locations in space.

hose barb: a twist-type connector used for connecting a small diameter hose to a valve or faucet.

hydraulic conductivity: a coefficient of
proportionality describing the rate at which water can move through a
permeable medium. Hydraulic conductivity is a function of both the
intrinsic permeability of the porous medium and the kinematic viscosity
of the water which flows through it. Also referred to as the
coefficient of permeability.

hydraulic gradient: the change in total
potentiometric (or piezometric) head between two points divided by the
horizontal distance separating the two points.

hydrocarbon: chemical compounds composed only of carbon and hydrogen.

hydrogen peroxide: H(2)O(2). 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 [H(2)O] 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

hydrophilic: having an affinity for water, 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. A
property exhibited by non-polar organic compounds, including the
petroleum hydrocarbons.

hypoxic: a condition of low oxygen concentration, below that considered aerobic.

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in situ: in its original place; unmoved; unexcavated; remaining in the subsurface.

in-line rotameter: a flow measurement device for
liquids and gases that uses a flow tube and specialized float. The
float device is supported by the flowing fluid in the clear glass or
plastic flow tube. The vertical scaled flow tube is calibrated for the
desired flow volumes/time.

indigenous: living or occurring naturally in a specific area or environment; native.

infiltration gallery: an engineered structure
that facilitates infiltration of water into the subsurface.
Infiltration galleries may consist of one or more horizontal or vertical
perforated pipes, a single gravel-filled trench or a network of such
trenches, or a combination of these.

infiltration: the downward movement of water through a soil in response to gravity and capillary suction.

injection well: a well used to inject under pressure a fluid (liquid or gas) into the subsurface.

inlet well: a well through which a fluid (liquid or gas) is allowed to enter the subsurface under natural pressure.

inoculate: to implant microorganisms onto or into a culture medium.

intergranular: between the individual grains in a rock or sediment.

intrinsic permeability: a measure of the relative
ease with which a permeable medium can transmit a fluid (liquid or
gas). Intrinsic permeability is a property only of the medium and is
independent of the nature of the fluid.

isotropic: the condition in which hydraulic properties of an aquifer are equal when measured in any direction.

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kinematic viscosity: the ratio of dynamic
viscosity to mass density. Kinematic viscosity is a measure of a
fluid’s resistance to gravity flow: the lower the kinematic viscosity,
the easier and faster the fluid will flow.

kow: see octanol/water partition coefficient.

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liquid limit (LL): the lower limit for viscous flow of a soil.

liquidity index (LI): quantitative value used to
assess whether a soil will behave as a brittle solid, semisolid,
plastic, or liquid. LI is equal to the difference between the natural
moisture content of the soil and the plastic limit (PL) divided by the
plasticity index (PI).

lithology: the gross physical character of a rock or rock types in a stratigraphic section.

lower explosive limit (LEL): the concentration of
a gas below which the concentration of vapors is insufficient to
support an explosion. LELs for most organics are generally 1 to 5
percent by volume.

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magnehelic gauge: a sensitive differential
pressure or vacuum gauge manufactured by Dwyer Instrument Co. that uses
a precision diaphragm to measure pressure differences. This gauge is
manufactured in specific pressure or vacuum ranges such as 0 to 2 inches
of water column. Magnehelic gauges are typically used to measure SVE
system vacuums.

manifold: a pipe with several apertures for making multiple connections.

manometer: an instrument for measuring fluid
pressure. Typically a U-shaped tube in which opposing fluid pressures
reach an equilibrium. The pressure is equal to the differences in the
levels of the fluid on either side of the tube.

metabolism: a term that encompasses all of the
diverse reactions by which a cell processes food material to obtain
energy and the compounds from which new cell components are made.

methanogenic: referring to the formation of methane by certain anaerobic bacteria during the process of anaerobic fermentation.

microaerophilic: obligate aerobes that function best under conditions of low oxygen concentration.

microcosm: a diminutive, representative system
analogous to a larger system in composition, development, or
configuration. As used in biodegradation treatability studies,
microcosms are typically constructed in glass bottles or jars.

microorganisms: microscopic organisms including bacteria, protozoans, yeast, fungi, mold, viruses, and algae.

mineralization: the release of inorganic chemicals from organic matter in the process of aerobic or anaerobic decay.

moisture content: the amount of water lost from a
soil upon drying to a constant weight, expressed as the weight per
unit weight of dry soil or as the volume of water per unit bulk volume
of the soil. For a fully saturated medium, moisture content equals the

molecular diffusion: process whereby molecules of various gases tend to intermingle and eventually become uniformly dispersed.

molecular weight: the amount of mass in one mole
of molecules of a substance as determined by summing the masses of the
individual atoms which make up the molecule.

monoaromatic: aromatic hydrocarbons containing a single benzene ring.

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non-aqueous phase liquid (NAPL): contaminants that remain as the original bulk liquid in the subsurface (see also free product).

nutrients: major elements (e.g., nitrogen and
phosphorus) and trace elements (including sulfur, potassium, calcium,
and magnesium) that are essential for the growth of organisms.

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obligate aerobes: organisms that require the presence of molecular oxygen ([O(2)] for their metabolism.

obligate anaerobes: organisms for which the
presence of molecular oxygen is toxic. These organisms derive the
oxygen needed for cell synthesis from chemical compounds.

occlude: to cause to become obstructed or closed and thus prevent passage either into or from.

octanol/water partition coefficient (Kow): a
coefficient representing the ratio of the solubility of a compound in
octanol (a non-polar solvent) to its solubility in water (a polar
solvent). The higher the Kow, the more non-polar the compound. Log Kow
is generally used as a relative indicator of the tendency of an organic
compound to adsorb to soil. Log Kow values are generally inversely
related to aqueous solubility and directly proportional to molecular

off-gas treatment system: refers to the unit
operations used to treat (i.e. condense, collect, or destroy)
contaminants in the purge gas from the thermal desorber.

olefins: see alkenes.

orifice plate: a flow measurement device for
liquids or gases that uses a restrictive orifice plate consisting of a
machined hole that produces a jet effect. Typically the orifice meter
consists of a thin plate with a square edged, concentric, and circular
orifice. The pressure drop of the jet effect across the orifice is
proportional to the flow rate. The pressure drop can be measured with a
manometer or differential pressure gauge.

oxidation-reduction (redox): a chemical reaction
consisting of an oxidation reaction in which a substance loses or
donates electrons, and a reduction reaction in which a substance gains
or accepts electrons. Redox reactions are always coupled because free
electrons cannot exist in solution and electrons must be conserved.

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paraffins: see alkanes.

partial pressure: the portion of total vapor pressure in a system due to one or more constituents in the vapor mixture.

permeability: a qualitative description of the
relative ease with which rock, soil, or sediment will transmit a fluid
(liquid or gas). Often used as a synonym for hydraulic conductivity or
coefficient of permeability.

pH: a measure of the acidity of a solution. pH is
equal to the negative logarithm of the concentration of hydrogen ions
in a solution. A pH of 7 is neutral. Values less than 7 are acidic, and
values greater than 7 are basic.

phototrophs: organisms that use light to generate energy (by photosynthesis) for cellular activity, growth, and reproduction.

pilot test: operation of a small-scale version of
a larger system to gain information relating to the anticipated
performance of the larger system. Pilot test results are typically used
to design and optimize the larger system.

pitot tube: a device used to measure the total
pressure of a fluid stream that is essentially a tube attached to a
manometer at one end and pointed upstream at the other.

plastic limit (PL): the lower limit of the plastic state of a soil.

plastic soil: one that will deform without
shearing (typically silts or clays). Plasticity characteristics are
measured using a set of parameters known as Atterberg Limits.

plasticity index (PI): the range of water content
in which soil is in a plastic state. PI is calculated as the
difference between the percent liquid limit and percent plastic limit.

polyaromatic hydrocarbon: aromatic hydrocarbons containing more than one fused benzene ring. Polyaromatic hydrocarbons are commonly designated PAH.

polynuclear aromatic hydrocarbon: synonymous with polyaromatic hydrocarbon. Designated PNA.

pore volume: (1) the total volume of pore space
in a given volume of rock or sediment. Pore volume usually relates to
the volume of air or water that must be moved through contaminated
material in order to flush the contaminants. (2) the volume of water (or
air) that will completely fill all of the void space in a given
volume of porous matrix. Pore volume is equivalent to the total
porosity. The rate of decrease in the concentration of contaminants in
a given volume of contaminated porous media is directly proportional
to the number of pore volumes that can be exchanged (circulated)
through the same given volume of porous media.

porosity: the volume fraction of a rock or
unconsolidated sediment not occupied by solid material but usually
occupied by water and/or air.

pressure gradient: a pressure differential in a
given medium (e.g., water or air) which tends to induce movement from
areas of higher pressure to areas of lower pressure.

procaryotes: a cellular organism in which the nucleus has no limiting membrane.

protozoa: single-celled, eucaryotic
microorganisms without cell walls. Most protozoa are free-living
although many are parasitic. The majority of protozoa are aerobic or
facultatively anaerobic heterotrophs.

psi (pounds per square inch): a unit of pressure
or pressure drop across a flow resistance. One psi is equivalent to the
pressure exerted by 2.31 feet of water column.

psig (pounds per square inch (gauge)): 0 psig = 14.696 psia (psi absolute) = 1.0 atmosphere.

pugmill: a chamber in which water and soil are
mixed together. Typically mixing is aided by an internal mechanical
stirring/kneading device.

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radius of influence: the maximum distance away
from an air injection or extraction source that is significantly
affected by a change in pressure and induced movement of air.

reagent: a substance or solution used in a
chemical reaction, especially those used in laboratory work to detect,
measure, or produce other substances.

recalcitrant: unreactive, nondegradable; refractory.

redox: short for oxidation-reduction.

refractory index: a measure of the ability of a
substance to be biodegraded by bacterial activity. The lower the
refractory index, the greater the biodegradability.

retardation: preferential retention of
contaminant movement in the subsurface resulting from adsorptive
processes or solubility differences.

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saturated zone: the zone in which all the voids
in the rock or soil are filled with water at greater than atmospheric
pressure. The water table is the top of the saturated zone in an
unconfined aquifer.

sentinel well: a groundwater monitoring well
situated between a sensitive receptor downgradient and the source of a
contaminant plume upgradient. Contamination should be first detected in
the sentinel well which serves as a warning that contamination may be
moving closer to the receptor. The sentinel well should be located far
enough upgradient of the receptor to allow enough time before the
contamination arrives at the receptor to initiate other measures to
prevent contamination from reaching the receptor, or in the case of a
supply well, provide for an alternative water source.

septa fitting: a special fitting used to seal
vials (a liner for a threaded cap) or gas chromatographs (GCs) to
provide closure. Septas can be manufactured in single, double, or
triple layers of silicone rubber and other plastic materials. A syringe
with a measured quantity of contaminant can be injected through a septa
closure and into a GC column for separation analysis.

sequester: to undergo sequestration.

sequestration: the inhibition or stoppage of
normal ion behavior by combination with added materials, especially the
prevention of metallic ion precipitation from solution by formation of
a coordination complex with a phosphate.

SESOIL: a one-dimensional model for estimating
pollutant distribution in an unsaturated soil column. SESOIL results
are commonly used to estimate the source term for groundwater transport
modeling of the saturated zone.

short circuiting: the entry of ambient air into
an extraction well (used for SVE and bioventing) without first
passing through the contaminated zone. Short circuiting may occur
through utility trenches, incoherent well or surface seals, or layers
of high permeability geologic materials.

soil moisture: the water contained in the pore spaces in the unsaturated zone.

solubility: the amount of mass of a compound that will dissolve in a unit volume of solution.

sorbent canisters: gas-tight canisters typically
filled with activated carbon (charcoal) for collection and transport of
vapor samples. In the laboratory the vapors are desorbed and analyzed
to identify the organic compounds and quantify their concentration.

sorbent tubes: glass tubes filled with a sorbent
material that reacts chemically with specific organic compounds. Based
on the nature of the sorbent and the extent of the chemical reaction,
organic compounds can be identified and their concentration quantified.

sorption: a general term used to encompass the processes of absorption, adsorption, ion exchange, and chemisorption.

sparge: injection of air below the water table to
strip dissolved volatile organic compounds and/or oxygenate the
groundwater to facilitate aerobic biodegradation of organic compounds.

specific gravity: the dimensionless ratio of the
density of a substance with respect to the density of water. The
specific gravity of water is equal to 1.0 by definition. Most petroleum
products have a specific gravity less than 1.0, generally between 0.6
and 0.9. As such, they will float on water–these are also referred to
as LNAPLs, or light non-aqueous phase liquids. Substances with a
specific gravity greater than 1.0 will sink through water–these are
referred to as DNAPLs, or dense non-aqueous phase liquids.

sticky limit: the limit at which a soil loses its ability to adhere to a metal blade.

stratification: layering or bedding of geologic materials (e.g., rock or sediments).

stratum: a horizontal layer of geologic material
of similar composition, especially one of several parallel layers
arranged one on top of another.

sump: a pit or depression where liquids drain, collect, or are stored.

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Tedlar bags: gas-tight bags constructed of non-reactive material (Tedlar) for the collection and transport of gas/vapor samples.

terminal electron acceptor (TEA): a compound or
molecule that accepts an electron (is reduced) during metabolism
(oxidation) of a carbon source. Under aerobic conditions molecular
oxygen is the terminal electron acceptor. Under anaerobic conditions a
variety of terminal electron acceptors may be used. In order of
decreasing redox potential, these TEAs include nitrate, manganic
manganese, ferric iron, sulfate, and carbon dioxide. Microorganisms
preferentially utilize electron acceptors that provide the maximum free
energy during respiration. Of the common terminal electron acceptors
listed above, oxygen has the highest redox potential and provides the
most free energy during electron transfer.

thermal desorber: describes the primary treatment
unit that heats petroleum-contaminated materials and desorbs the
organic materials into a purge gas or off-gas.

thermal desorption system: refers to a thermal
desorber and associated systems for handling materials and treated
soils and treating offgases and residuals.

total petroleum hydrocarbons (TPH): a measure of
the concentration or mass of petroleum hydrocarbon constituents present
in a given amount of air, soil, or water. The term total is a
misnomer, in that few, if any, of the procedures for quantifying
hydrocarbons are capable of measuring all fractions of petroleum
hydrocarbons present in the sample. Volatile hydrocarbons are usually
lost in the process and not quantified. Additionally, some
non-petroleum hydrocarbons may be included in the analysis.

total recoverable petroleum hydrocarbons (TRPH):
an EPA method (418.1) for measuring total petroleum hydrocarbons in
samples of soil or water. Hydrocarbons are extracted from the sample
using a chlorofluorocarbon solvent (typically Freon-113) and quantified
by infrared spectrophotometry. The method specifies that the extract
be passed through silica gel to remove the non-petroleum fraction of
the hydrocarbons.

travel time: the time it takes a contaminant to travel from the source to a particular point downgradient.

tripolyphosphates: Salts with P(3)O(10)[-5 charge] anion. Most common is sodium tripolyphosphate [Na(5)P(3)O(10)].

turbine wheel: a rotor designed to convert fluid
energy into rotational energy. Hydraulic turbines are used to extract
energy from water as the water velocity increases due to a change in
head or kinetic energy at the expense of the potential energy as the
water flows from a higher elevation to a lower elevation. The fluid
velocity tangential component contributes to the rotation of the rotor
in a turbomachine.

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unconfined aquifer: an aquifer in which there are
no confining beds between the capillary fringe and land surface, and
where the top of the saturated zone (the water table) is at atmospheric

unsaturated: the characteristic of a carbon atom in a hydrocarbon molecule that shares a double bond with another carbon atom.

unsaturated zone: the zone between land surface
and the capillary fringe within which the moisture content is less than
saturation and pressure is less than atmospheric. Soil pore spaces
also typically contain air or other gases. The capillary fringe is not
included in the unsaturated zone.

upgradient: it the direction of increasing potentiometric (piezometric) head.

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vacuum draft tube: a narrow tube lowered into an
extraction well through which a strong vacuum is pulled via a suction
pump at ground surface. Fluids (gas, water, and/or free product) are
drawn into the draft tube and conveyed to the surface for treatment or
disposal. Depending upon the configuration of the extraction system,
the inlet of the draft tube may be either above or below the static
level of the liquid in the well.

vadose zone: the zone between land surface and
the water table within which the moisture content is less than
saturation (except in the capillary fringe) and pressure is less than
atmospheric. Soil pore spaces also typically contain air or other gases.
The capillary fringe is included in the vadose zone.

vapor density: the amount of mass of a vapor per unit volume of the vapor.

vapor pressure: the force per unit area exerted
by a vapor in an equilibrium state with its pure solid, liquid, or
solution at a given temperature. Vapor pressure is a measure of a
substance’s propensity to evaporate. Vapor pressure increases
exponentially with an increase in temperature.

venturi: a short tube with a constricted throat
for determining fluid pressures and velocities by measuring
differential pressures generated at the throat as a fluid traverses the

viscosity: a measure of the internal friction of a
fluid that provides resistance to shear within the fluid. The greater
the forces of internal friction (i.e. the greater the viscosity), the
less easily the fluid will flow.

volatilization: the process of transfer of a
chemical from the aqueous or liquid phase to the gas phase. Solubility,
molecular weight, and vapor pressure of the liquid and the nature of
the gas-liquid interface affect the rate of volatilization.

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water table: the water surface in an unconfined aquifer at which the fluid pressure in the pore spaces is at atmospheric pressure.

weathering: the process during which a complex
compound is reduced to its simpler component parts, transported via
physical processes, or biodegraded over time.

wellhead: the area immediately surrounding the top of a well, or the top of the well casing.

windrow: a low, elongated row of material left uncovered to dry. Windrows are typically arranged in parallel.

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