General Considerations

Figure 12.4 shows the typical characteristics of long-term pollution. Contaminants enter the soil and migrate downwards. The amount of contaminants remaining in the unsaturated zone is controlled by sorption, diffusion into soil pores, and retention by capillary forces. Generally, most contaminants have low solubility; hence, high amounts of contaminants may appear - when they reach the groundwater table. Depending on their buoyant density, a separate phase on top of the groundwater, called LNAPL (light nonaqueous-phase liquid), or at the base of the aquifer (groundwater zone), called DNAPL (dense nonaqueous-phase liquid) may occur. Minor amounts of contaminants are dissolved in the groundwater and are transported with the natural groundwater flow, forming a contaminant plume. The spatial extent of the plume and the contaminant concentrations depend on the duration of the pollution, the sorption and transport characteristics, and the efficiency of the natural biotic and abiotic degradation processes.

Depending on the characteristics of the contaminants and on the site conditions, different technologies have to be chosen. The technologies are divided into technol-

Fig. 12.4 Localization of different microbial in situ technologies.

ogies for treatment of the unsaturated and saturated soils. A further subdivision comprises active and passive technologies. All these technologies can be combined with bioaugmentation, which is the addition (i.e., infiltration) of specific contaminant-degrading bacteria previously isolated and propagated in the laboratory. However, bioaugmentation is still controversial, because the majority of the infiltrated cells become attached to the soil within a few centimeters. Furthermore, establishing added microflora in an environmental compartment requires highly specific conditions [2]. Usually, the infiltration of nutrients during the remediation causes the added bacteria to be overgrown by the autochthonous microflora. Most technologies include not only degradation of the contaminants in situ but also physical removal of the contaminants (e.g., together with exhausted soil vapor or pumped groundwater), which requires an additional treatment step.

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