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Natural Attenuation

Natural attenuation is the reduction in concentration, mass, or mobility of a COC with distance and time due to naturally occurring processes in the groundwater system. These processes can be classified as physical (such as dispersion, diffusion, and dillution), sorption, and biodegradation. The physical and sorption processes result in reduction of concentration and mobility of a contaminant but not its total mass, and are referred to as "nondestructive" mechanisms. The chemical and biological reactions result in the reduction of the total mass of contaminants in the aquifer, and are referred to as "destructive" mechanisms. For hydrocarbons in the groundwater, biological degradation is found to be the most important process in the reduction of mass.
Depending upon the dissolution of contaminants and the properties of the aquifer, a plume will expand until it reaches equilibrium where the rate of contaminant release from the source is balanced with the rate of natural attenuation. At equilibrium the plume stabilizes. When the source area is depleted to the point that the rate of natural attenuation exceeds the source input, the result will be a shrinking plume over time.

Aerobic Biodegradation

Microbial degradation takes place in aerobic and anaerobic forms at the Popile site. The dominant degradation mechanism is found to be an aerobic process, in which micro-organisms gain energy from PCP and PAHs through a series of oxidation-reduction reactions, for example, PCP and PAHs are oxidized (lose electrons), and oxygen is reduced (gain electrons). Major byproducts of the aerobic degradation are carbon dioxide, chloride, and water. Field evidence of aerobic biodegradation at the Popile site includes increased carbon dioxide and chloride concentrations, and decreased oxygen levels when compared with background levels outside the zone of contamination.

Observed Concentrations (microgram per liter)

Areas ⁽¹⁾	Dissolved Oxygen	Carbon Dioxide	Chloride
Background	4,500	67	200
Reaction Area	1,250	88	575
Source Area	400 ⁽²⁾	100	1,200

Note:
⁽¹⁾ Areas are color coded in this table and the figure below.
⁽²⁾ Area where anaerobic biodegradation might take place.

Approximate Areas of Biological Reactions

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Natural Attenuation

Aerobic Biodegradation

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