Abiotic: Occurring without the involvement of living microorganisms.

Advection: Transport of a solute by the bulk motion of flowing groundwater.

Aerobic: Conditions for growth or metabolism in which the organism is sufficiently supplied with molecular oxygen.

Aerobic Respiration: Process whereby microorganisms use oxygen as an electron acceptor to generate energy.

Aliphatic Compounds: Acyclic or cyclic, saturated or unsaturated carbon compounds, excluding aromatic compounds.

Amendment: Substrate introduced to stimulate the in situ microbial processes (vegetable oils, sugars, alcohols, etc.).

Anaerobic: Environmental conditions requiring the absence of molecular oxygen.

Anaerobic Respiration: Process whereby microorganisms use a chemical other than oxygen as

Biostimulation involves the addition of nutrients, oxygen, or other electron donors and acceptors to an ecosystem in order to enhance indigenous microorganisms available for degradation of contaminants.

Chlorinated Solvent: Organic compounds with chlorine substituents that commonly are used for industrial degreasing and cleaning, dry cleaning, and other processes.

Chlorinated Ethene: Organic compounds containing two double-bonded carbons and possessing at least one chlorine substituent.

Cometabolism: A reaction in which microorganisms transform a contaminant even though the contaminant cannot serve as an energy source for growth. The microorganisms require the presence of other compounds (primary substrates) to support growth.

Dense, Nonaqueous-Phase Liquid (DNAPL): A water-immiscible organic liquid that is denser than water

Electron Acceptor - "Microorganisms gain energy by catalyzing energy-producing chemical reactions that involve breaking chemical bonds and transferring electrons away from the contaminant. The type of chemical reaction is called an oxidation-reduction reaction: the organic contaminant is oxidized, the technical term for losing electrons; correspondingly, the chemical that gains the electron is reduced. The contaminant is called the electron donor, while the electron recipient is called the electron acceptor. The energy gained from these electron transfers is then "invested," along with some electrons and carbon from the contaminant, to produce more cells." - National Research Council, 1993

Electron Donor: “Microorganisms, such as bacteria, obtain energy to grow by transferring electrons from an electron donor to an electron acceptor. An electron donor is a compound that gives up or donates an electron during cellular respiration, resulting in the release of energy. The microorganism through its cellular machinery collects the energy for its use. The final result is the electron is donated to an electron acceptor. During this process (electron transport chain) the electron donor is oxidized and the electron acceptor is reduced. Petroleum hydrocarbons, less chlorinated solvents like vinyl chloride, soil organic matter, and reduced inorganic compounds are all compounds that can act as electron donors. These reactions are of interest not only because they allow organisms to obtain energy, but also because they are involved in the natural biodegradation of organic contaminants. When clean-up professionals use monitored natural attenuation to clean up contaminated sites, biodegradation is one of the major contributing processes.” - USGS, Toxic Substances Hydrology Program, http://toxics.usgs.gov

Electron shuttles are chemical compounds that stimulate the biodegradation of contaminants by facilitating the transfer of electrons to and from bacteria (Benz and others, 1998; Bradley and others, 1998; Finneran and Lovley, 2001; Hernandez and Newman, 2001; Lovley and others 1996; Lovley and others, 1998; Newman and Kolter, 2000, Scott and others, 1998). Humic acids can act as electron shuttles because their double bonds (quinone groups) can act like the "on/off" switches in computer memories. That is, they can be reduced or oxidized, and thus can "shuttle" electrons between the ultimate electron donor (organic carbon) and the ultimate electron acceptor (ferric iron Fe(III)). Another analogy would be a catalyst that is not consumed in the overall chemical reaction, and which can be used over and over again. Since the electron shuttles are not consumed, high concentrations are not needed to speed up the overall reaction. For example, humic acids can accept electrons from bacteria that are oxidizing contaminants such as vinyl chloride (Bradley and others, 1998) and subsequently donate those electrons to a terminal electron acceptor such as ferric iron. Therefore, electron shuttles can facilitate oxidative and potentially reductive modes of biodegradation. Examples of electron shuttles are humic acids, quinones such as the humic acid analogue anthraquinone disulfonic acid (AQDS), phenazines, and thiol-containing molecules such as cysteine (Hernandez and Newman, 2001).” - USGS, Toxic Substances Hydrology Program, http://toxics.usgs.gov

Ganglia: DNAPLs that are cut off and disconnected from the main continuous DNAPL body.

Passive soil vapor extraction (PSVE): "PSVE is an enhanced attenuation (EA) approach that removes volatile contaminants from soil. The extraction is driven by natural pressure gradients between the subsurface and atmosphere (Barometric Pumping), or by renewable sources of energy such as wind or solar power (Assisted PSVE). The technology is applicable for remediating sites with low levels of contamination and for transitioning sites from active source technologies such as active soil vapor extraction (ASVE) to natural attenuation." SRNL-STI-2009-00571



ITRC (Interstate Technology & Regulatory Council). 2008. In Situ Bioremediation of Chlorinated Ethene: DNAPL Source Zones. BioDNAPL-3. Washington, D.C.: Interstate Technology & Regulatory Council, Bioremediation of DNAPLs Team. www.itrcweb.org.
USGS, Toxic Substances Hydrology Program, http://toxics.usgs.gov