Vapor Extraction and Biodegradation

A portion of volatile LNAPL in the subsurface vaporizes into air-filled pore spaces until the vapor-liquid equilibrium concentrations are established. If the soil zone is naturally permeable from the fluid surface to the soil surface, a concentration gradient is established. Eventually, most of the volatile mass can be transferred from the LNAPL to the atmosphere, minus that portion retained by sorption on the soil, or biologically degraded. 

Several attempts have been reported by recovery teams to recover LNAPL by the use of vacuum wells set above the fluid surface. While these efforts have attained marginal success, several factors have been seen to interfere. Vacuum wells set in the vadose zone tend to encourage airflow from the surface downward through the soil, as well as to extract vapors. The result is often the enhancement of biological degradation near the wells, which tends to cause the well screens to become plugged with biomass. Routine maintenance of the wells is required to keep them functioning properly. 

The major disadvantage to recovery of LNAPL by vapor extraction is the large quantity of energy required to maintain rapid volatilization of the product and airflow to transport the diluted vapors to the surface. The remediation time for this type of system tends to be quite long, often in terms of years. 

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