The input of contaminants into soil and groundwater may lead to a persistent pollution. The methods for remediation of the environmental compartments contaminated with organic contaminants comprise, in addition to physical and chemical methods, also biological technologies. The technologies are subdivided into ex situ and in situ methods. Ex situ methods include excavation of the soil and subsequent treatment at the site (on-site) or elsewhere (off-site). In situ means that the soil remains in its natural condition during treatment. Generally, in situ technologies also include ex situ components, e.g., water- or gas-treatment plants.
The goal of in situ technologies is to mineralize the contaminants microbiologi-cally to form harmless end products. Many biodegradation processes, especially of nonchlorinated compounds (e.g., mineral oil hydrocarbons), require aerobic conditions or denitrification. Although these contaminants may also be degraded under other conditions (e.g., iron reduction, sulfate reduction), these bioprocesses are not very effective and are therefore not used in enhanced bioremediation. These processes are productive, i.e., the contaminants serve as sources of carbon and energy. Chlorinated hydrocarbons usually require anaerobic conditions. They may be degraded productively (during dehalorespiration, chlorinated hydrocarbons serve as obligate electron acceptors) or co-metabolically. In the latter, microorganisms cannot grow on these contaminants but need an additional substrate for growth and biodegradation. Additional nutrient elements such as nitrogen and phosphate as well as electron acceptors are often lacking. In addition to such mineralization, other biochemical processes that result in a reduction in toxicity can also be applied in in situ remediation processes. These are
• cometabolic aerobic or anaerobic transformation
With the use of these processes, a wide variety of contaminants seem to be treatable in situ, including
• mineral oil hydrocarbons
• monoaromatic and polyaromatic compounds
• chlorinated or nitrated aliphatics and aromatics
• inorganic ions, including simple and complex cyanides
However, until now only a limited number of possible techniques have been developed for practical application. The main reason for the actual state of development is that the in situ degradability of contaminants is limited by numerous factors (Fig. 12.1). Most of these factors, like low solubility, strong sorption on solids, sequestration with high molecular weight matrices, diffusion into macropores of soils and sediments, scavenging by insoluble and lipophilic phases, lead to a limited mass transfer. This lack of bioavailability may prevent sufficient degradation and result in persistence of the contaminants.
While planning an in situ remediation process, one has to consider that the technology covers processes working on various scales (from nanometers to kilometers) (Fig. 12.2). Import and export of contaminants by bacterial cells and induction of the degradative enzymes occurs on the nanometer scale. Surface processes occur on the micrometer scale. In the range of micrometers to millimeters diffusion processes (soil micro pores) as well as micro-inhomogeneities occur. The meter scale represents small inhomogeneities, e.g., silt lenses in a sandy aquifer. Finally, the kilome-
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