Environmental Engineering Reference
In-Depth Information
FIGURE 11.3
Electronic valves control fresh air circulation in a treatment cell. (Photo by Kent S. Murray.)
• Contaminants must be biodegradable. Many light nonaqueous phase liquid
(LNAPL) compounds such polynuclear aromatic hydrocarbons (PAHs) can only
be degraded through the use of active bioremediation techniques.
• A suitable temperature range exists for the natural processes to begin and sustain
themselves.
• Nutrients (fertilizers) are present to support microbial growth.
• Enough oxygen is present to assist with the decomposition process.
When these conditions are not satisfied or are not optimal, the microbes do not grow, grow
slowly, or die. One way to improve unfavorable conditions is to pump air, nutrients, or
other substances (such as a carbohydrate like molasses) into the impacted soil where the
microbes have been placed to enhance degradation (Murray and Clark 1993; USEPA 2001a).
11.3.3 Monitored Natural Attenuation
Monitored natural attenuation relies on natural processes to attenuate (render less harm-
less) contaminants in soil. Natural attenuation occurs at most contaminated sites; however,
favorable conditions must exist to achieve successful remediation in a reasonable amount
of time (USEPA 2001b). Impacted soil is tested periodically to ensure that the attenuation of
contaminants is occurring at a satisfactory rate. Natural attenuation of LNAPL compounds
occurs much more quickly and efficiently than dense nonaqueous phase liquid (DNAPL)
compounds, which require much more evaluation (USEPA 1998). Figure 11.4 shows an
example of natural attenuation; a contaminant present in the soil evaporates from the soil
into the atmosphere where it is destroyed by sunlight through photolysis.
This technique is often used when the source and the majority of the contamination
have been removed, and it is not practical to use more costly remediation methods to
remove the remaining residual concentrations (USEPA 2001b).
11.3.4 Mechanical Soil Aeration
Mechanical soil aeration utilizes photolysis to remediate contaminants in near surface soil.
This technique is most effective when the contaminants of concern have a high relative
vapor pressure and are degradable by photolysis. The process is essentially similar to the
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