The vacuum extraction probe is always placed in the vadoze zone. The success of the method depends on the volatilization of VOC from water into air present in voids. An injecting medium is used to extract soil-water and/ or soil-air. When oxygen is used instead of nitrogen as the injecting medium, it enhances aerobic biodegradation.
           Soil structure influences a lot on the passage of extracted water and vapour and hence on the success of vacuum extraction technique. It is not only important that the injecting medium is delivered efficiently but also the extracted product reaches the exit with less hindrance. Granular soils provide better passage where as the presence of clay and organic matter impedes the transmission of both fluid and vapour. Organic matter provides high retention leading to less volatilization. High density and water content also minimize transmissivity. Apart from soil, the VOC properties such as solubility, sorption, vapour pressure, concentration etc. also influence the extraction process.  
Solidification and stabilization      
           This is the process of immobilizing toxic contaminants so that it does not have any effect temporally and spatially. Stabilization-solidification (SS) is performed in single step or in two steps. In single step, the polluted soil is mixed with a special binder so that polluted soil is fixed and rendered insoluble. In two step process, the polluted soil is first made insoluble and non-reactive and in the second step it is solidified. SS process is mostly justified for highly toxic pollutants. In-situ SS process is mostly influenced by the transmissivity characteristics of the soil, viscosity and setting time of the binder. Well compacted soil, high clay and organic content do not favour in-situ SS.
           In ex-situ methods, polluted soil is first grinded, dispersed, and then mixed with binder material. The resultant SS material need to be disposed in a well contained landfill. It is essential that the resultant SS product does not undergo leaching. The common binders used in practice include cement, lime, fly ash, clays, zeolites, pozzolonic products etc. Organic binders include bitumen, polyethylene, epoxy and resins. These organic binders are used for soil contaminated with organic pollutants.  
Chemical decontamination  
           This method is mostly applicable for those soils which have high sorbed concentration of inorganic heavy metals (IHM). The first process in this method is to understand the nature of bonding between the pollutant and the soil surface. A suitable extractant need to be selected for selective sequential extraction (SSE) of IHM from the soil mass. The extractants include electrolytes, weak acids, complexing agents, oxidizing and reducing agents, strong acids etc. The use of these extractants in single or in combination will depend upon the concentration of IHM and nature of the soil mass.
           In-situ application (as depicted in Fig. 4.4) of extractants would remove IHM from the soil surface and enter into the pore water. The pore water is pumped and treated (pump and treat method) on the ground. While treating the pumped water, both extractants and IHM are removed.

Fig. 4.4 A schematic diagram for in-situ chemical decontamination

           Another method is to allow the contaminated pore water to flow through a permeable reactive barrier (PRB). Hence the placement of the barrier is determined by the direction of flow of ground water. The material packed in the barrier will retain IHM by exchange (sorption), complexation or precipitation reaction. The transmission and the reaction time determine the thickness of the reactive barrier to be provided. The material to be provided in the barrier is influenced by the knowledge of IHM to be removed. This is mainly due to the fact that the above mentioned reaction occurs differently when IHM is present as single or as multiple species.
           The successful use of PRB or treatment wall (TW) depends upon its location such that majority of the contaminated groundwater flows through it. It is essential to have a good knowledge on the hydrogeological conditions where such barriers need to be placed. In some cases, sheet pile walls are used to confine the flow towards the permeable barrier. Some of the materials used in such PRBs are exchange resins, activated carbon, zeolites, various biota, ferric oxides, ferrous hydroxide etc. Hydraulic conductivity of the PRB should be greater than or equal to the surrounding soil for proper permeation to occur. Further, reaction kinetics and permeability of the barrier would determine the thickness of the wall to be provided such that enough residence time is achieved for the removal reaction to occur.